Influence of lateral electric field in InAs-Quantum Dots

Transcription

1 Influence of lateral electric field in InAs-Quantum Dots Dissertation zur Erlangung des Grades eines Doktors der Naturwissenschaften in der Fakultät für Physik und Astronomie an der Ruhr-Universität Bochum von Victorina Poenariu aus Alexandria Bochum 2005

2 Dissertation eingereicht am: Tag der mündlichen Prüfung: Berichterstatter: Prof. Dr. A. D. Wieck Prof. Dr. M. Bayer

3 CONTENTS 3 Contents 1 Introduction 5 2 Basics of Quantum Information Processing Qu-bits Quantum Cryptography A Quantum Dot Single Photon Source Physical properties of QD Quantum Dots and Confined Excitonic States Two - Dimensional Harmonic Oscillator Quantum Dots in an Electric Field Lateral p-i-n and n-i-n Diodes Experimental details Molecular Beam Epitaxy (MBE) System InAs/GaAs-Self-assembled Growth Process Device fabrication Rapid Thermal Annealing Technique Photoluminescence Measurements on Quantum Dots Electrical Characterization Time-Resolved and Single Dot Photoluminescence on InAs- Quantum Dots Thermal Processing of InAs/GaAs-Quantum Dots Photoluminescence Measurements Blueshift of Energy Levels by Annealing Time-Resolved Spectroscopy State of the Art Characterization of the High Density InAs-Quantum Dots Time-Resolved Measurements under an in-plane Electric Field Radiative Lifetime in in-plane Electric Field Single dot spectroscopy State of Research Characterization of Low Density InAs-Quantum Dots Lateral Quantum Confined Stark Effect

4 4 CONTENTS 7.4 Photoluminescence Measurements under a Lateral Electric Field Red Shift of Energy levels by Lateral Electric Field Summary 75 References 77 List of figures 85 List of symbols 88 List of Tables 90 A Heterostructures 91 B Processing parameters 94 C List of publications 97

5 1 Introduction Self-assembled InAs quantum dots (QDs) have been studied intensively in the past because of the strong three-dimensional carrier confinement, and the promising applications making use of the characteristic δ-like density of states and quantization energies of the order or greater than k B T at room temperature. Recent studies include single quantum dots (SQDs) for future devices like photon sources [1], single photon turnstiles [3], or quantum computing [4], [5]. Also, the quantum dot is one such structure which can be successfully used in technological applications such as QD laser diodes and QD infrared photodetectors (QDIP) [7], [8], [9]. The main advantage of QDIP is that light can be directly coupled to the electrons in the normal incidence geometry due to the effect of QD confinement in directions perpendicular to the growth axis. So, the dark current used in application like infrared detectors, is related to the electron escape rate (inverse of the tunneling time) in QDs under an electric field. QDs can be used to realize a quantum-controlled logic gate under an external static electric field, which can be used for quantum computing. From the application point of view, the QDs in an external electric field are very interesting to study, for instance, the Stark effect can be used to produce ultrafast electro-optical modulator and self-electro-optic effect devices. Recently, theoretical investigations of the biexciton effect on the Stark effect in GaAs and CdS quantum dots shows that the red-shift changes to a blue-shift when the biexciton effects are taken into account. This opens the possibility to employ such effects in development of optoelectronic device such as optical memories and switches. The present dissertation is dealing with the fabrication of a lateral p-i-n structure and investigation of lateral electric field on self-assembled InAs-quantum dots by means of a combinative patterning technique based on electron beam lithography (EBL), focused ion beam implantation (FIB) and standard optical lithography. The work is organized as follows: Chapter 2 brings together ideas from classical information theory, computer science, and quantum physics. From a theoretical point of view, the fundamental concept of quantum computing is demonstrated, but the practical realization is still at the very beginning. Through this thesis we will attempt to get a closer look on the realization of the quantum computing by involving InAs quantum dots and lateral electric field. In Chapter 3, the physical properties of quantum dots such as atom-like density of states, leading to their description as an artificial-atom are described. In Chapter 4, an 5

6 6 1 INTRODUCTION overview of the sample design along with measurement techniques are presented. A thermal treatment technique and its effects on the quantum dots are described in Chapter 5. The radiative lifetime spectra in a p-i-n as well as in a n-i-n structure as a function of the applied voltage are investigated in Chapter 6. First of all an electrical characterization of the investigated devices is described afterwards, followed by details of time-resolved photoluminescence spectra. In Chapter 7, the spectroscopy of single quantum dots under the control of an external parameter, is considered in detail. This allows the inhomogeneous broadening presenting in ensemble studies to be removed. With increasing bias, a red-shift of the energy peak, the fingerprint of the quantum confined Stark effect is expected. From the confinement, the shift to the lower energy is expected to be bigger in lateral direction comparative to the ones of applying electric field in vertical direction. Single-QD spectroscopy has proven, by many experiments, to be a powerful tool in the study of few-particle effects [6]. It allows us to investigate emission spectra without having to reduce the influence of inhomogeneous broadening. Finally, in Chapter 8, a summary of these studies, is presented.

7 2 Basics of Quantum Information Processing The following chapter gives a short introduction to the theoretical fundamentals of quantum information processing [2]. It is a somehow interesting subject, because there are many ways and levels of explaining it. Quantum physics concepts are the basis for quantum computing in general. The quantum devices become smaller and smaller and reach the limit of classical physics. People wish the computer to be faster and faster, but the limit is how fast the conventional computer can become. This is because computers process information in a step-by-step mode and to make a really fast one, we need to build a quantum computer. But, first of all, we have to understand how classical computation works. The computers process the information in binary form, that means it represents everything as zeros and ones. And it contains tiny circuit elements that can be off or on representing the 0 state and 1 state, respectively. These circuit elements are called binary digits, or bits and every data is processed in form of these bits. In a quantum computer, the elements representing the data are called qu-bits and can be a combination or a superposition of both state 0 and state 1 at once. That means all states can be computed simultaneously in one step. One potential candidate for qu-bit is an electron in a quantum dot. It behaves like a tiny magnet which can operate only in two opposite directions: one direction is down or binary 0 and the other is up or binary 1. This means it could be used as a qu-bit, and its quantum states can be controlled by a magnetic field. The electric charge of the electron can also be used as basis for qu-bit. The procedure which creates an interaction between qu-bits is called a gate and can be used to operate a quantum device. A controllable interaction is introduced between the quantum bits, therefore when the bits are required to perform a calculation, they are not affected individually like in a classical computer. This enables a quantum computer to perform several operations at the same time. A quantum mechanical algorithm for factorization was discovered by Shor in 1994, and revealed it self to be exponentially faster than any known classical algorithm. A few years later, in 1997, Grover showed that by using the same amount of hardware as in the classical case, but having the input and output in superpositions of states, is was possible to find an object in a O( N) quantum mechanical step instead of a O(N) classical step. 7

8 8 2 QUANTUM DOTS 2.1 Qu-bits A single qu-bit can be described as a two-state system. It can be written as a unit vector in a two dimensional complex vector space for which a particular basis has been fixed: 0 and 1. Considering this, the qu-bit can be in a superposition of 0 and 1 such as α 0 + β 1 where α and β are complex numbers such that α 2 + β 2 = 1. The orthonormal basis 0 and 1 can correspond to the and polarizations of a photon respectively, or to the polarizations ր and տ. Another example is that 0 and 1 could correspond to the spin-up and spin-down states of an electron, or to the ground and excited states of a single atom. A quantum system has n qu-bits and so has 2 n available mutually orthogonal quantum states [14]. In general, 2 n orthogonal eigenstates of n qu-bits can be written as i, where i is a n-bit binary number. For instance, for three qu-bits we have 000, 100, 010, 001, 110, 101, 011, 111. Quantum information processing operates on both values stored in a qu-bit at the same time. As aforementioned, n qu-bits encode 2 n values, thus a quantum computer can compute all values at once. This phenomenon is called quantum parallelism. But the reality shows that the quantum superpositions of multiple particles presents a strange correlation with no classical analogs, for instance correlation between individual atoms or photons, which are the quanta of electromagnetic waves. Such correlated objects are named entangled. 2.2 Quantum Cryptography The quantum information is not complete without quantum cryptography. The following scheme illustrates schematically a communication channel between A and B places. In general, the security of an encrypted text depends on the undercover of the encrypting and decrypting procedure. With current procedure, the code with the algorithm of encrypting and decrypting could be revealed without compromising the security of the cryptogram. Such codes contain a set of specific parameters (keys) which are used together with the message as an input A Encode channel Decode B Figure 1: A typical communication channel: The source A produces information which is manipulated (encoded) and then sent over the channel. At the receiver B, the message is decoded and the information extracted.

9 2.3 A Quantum Dot Single Photon Source 9 to the encrypting process, and together with the cryptogram as an input to the decrypting procedure. We can write the following way: Encryption:E k (P) = C; Decryption:D k (C) = P, where P is the message, C is the cryptogram, k is the name of the key, E for encryption and D for decryption operation. In this moment, we have the control of the algorithms and therefore the security of the cryptogram depends only on the security of the key. At a certain stage of communication the key must be shared via a very secured channel. But any classical key distribution can always be passively monitored by any eavesdropper. In contrast quantum cryptography includes various ideas which main goal being is a secure quantum key distribution. This is a method in which quantum states are used to establish a random secret key for cryptography. A and B transmit single or entangled quanta (carriers of information) via the channel and the eavesdropper performs a set of physical measurements on the transmitted quanta. In accord with quantum mechanics, the transmitted quanta are modified by the measurements and A and B can find it out in a subsequent public communication. So we need for quantum key distribution a quantum channel because the public channel is accessible to anybody. A solution could be that the quantum key to be encoded by a set of non-orthogonal quantum states of a single particle, because non-orthogonal states cannot be cloned. For example, polarized photons might be used for quantum key distribution [12] as well as the time-energy entanglement of a photon pair produced by parametric down-conversion [13]. The scientists hope to fabricate quantum devices on a solid state basis, which would be the next step future of the extremely successful microfabrication technique for classical computers. Moreover, it is not enough to have two different states 0 and 1 which are themselves stable, we require for available quantum computation, superpositions such as that conserve their phase. At present, there are three candidate systems for quantum computation: (a) charged atoms confined in an ion trap, (b) methods of nuclear magnetic resonance performed on atoms and (c) charge carriers or/and their spins in semiconductor quantum dots. 2.3 A Quantum Dot Single Photon Source Another major goal in the field of quantum information science is the realization of an efficient single photon source [15]. This source is like a reservoir, able to generate photons on demand. Such a source allows the quantum control of the photon generation process, for example, single photons can be generated within a short time. This property makes possible to encode information on a simple photon level. The single-photon source used now a day involves highly attenuated laser pulses, but this way possesses a serious disadvantage since photons are created randomly following Poissonian photon statistics. For this, the average photon number per pulse has to be kept as low as 0.1 in order to maintain a

10 10 2 QUANTUM DOTS low multi-photon emission probability. In order to realize a practical single photon source is necessary to imply three main elements: a single quantum emitter, a regulation of the excitation and/or the recombination process and an efficient output coupling of the single photons. A first condition is that the active emitter has to emit one photon and to possess a high quantum efficiency, approximatively η 1. An ideal candidate which exhibits this small quantum efficiency are epitaxially self-assembled InAs quantum dots. Spontaneous emission of a quantum emitter is generally emitted in all directions (due to full solid angle) and is therefore hard to capture efficiently. For this reason, in practice is useful to couple the emitter to a cavity mode with directional field profile. Self-assembled QDs can be easily embedded into an appropriate microcavity, microdisc or into a micropillar. In order to achieve a high photon collection efficiency, the emission energy of the QD should be in resonance with a fundamental mode of a microcavity. It is well known that the transition energy of an exciton in a QD is only defined within the broad inhomogeneous linewidth ( mev ). From this point of view, single donor or acceptor bound excitons are advantageous as active emitters in microcavities since their emission energies are at well defined energies. So, a tuning of the exciton transition versus the mode resonance is necessary. The energetic resonance can be achieved by temperature tuning or by electric field tuning via the quantum-confined Stark effect. From a practical application point of view, a few experiments have already been done [16], using as single-photon source a single quantum dot, embedded into an intrinsic region of a p-i-n junction. Performing photoluminescence and electroluminescence measurements on this p-i-n junction, it was demonstrated that it was able to act as an electrically driven single-photon source. One problem still exists, is that single electron and hole charging energies must be large compared to the thermal background energy to ensure single photon emission. Therefore, this device can only be operated at ultra-low temperatures (T 1 K). At room temperature, the operation could be achieved by using QDs with higher confinement potential to suppress non-radiative carrier losses into the barrier. Promising results with CdSe/Zn QDs have been recently reported up to temperatures of 200 K [19]. Furthermore, InAs QDs can be tailored to possess much deeper carrier confinement potential [17]. These QDs emit in the 1.3 µm wavelength range and are thus well suited for fiber optic communications. It was also theoretically proposed that a single quantum dot could be used, not just for emission of single photons but also for generation of entangled pair of photons [18], [20], [21].

11 3 Physical properties of Semiconductors Quantum dots This chapter will give an overview of the physical properties of quantum confined semiconductor structures. These are artificial structures in which the electrons and holes are confined in one or more directions. We will concentrate mainly on quantum dots structures in which the carriers are confined in all three space directions. Fermi and de Broglie Wavelength One important parameter which characterizes the mesoscopic systems is the Fermi wavelength which is defined as: λ F = 2π k F (1) where k F is the Fermi wave number. At low temperature, the electrons in a microscopic system have the energy in the order of the Fermi energy. The de Broglie wavelength depends on the effective mass m of the carriers and on the temperature, it has also an important role in the description of quantum structures: λ = h p = h 3m kt = 1.22 [nm] Ec [ev ] (2) where m is the effective mass of the electron effective mass m e and hole effective mass m h, m = m e + m h. If the size of one structure is close to the de Broglie wavelength ( 10 nm), then quantization effects like energy will appear. The Energy of a Photon From Einstein theory we can write the energy as: E = ω = hν (3) and combining the energy with the frequency ν = c, the energy will become: λ E = hc λ = 1.24 ev λ[µm] (4) 11

12 12 3 PHYSICAL PROPERTIES OF QD 3.1 Quantum Dots and Confined Excitonic States In a QD, the mobility of electrons is blocked in one or more spatial directions. In a bulk semiconductor material, electrons are free to move in any of the three spatial directions. A confining structure may be made by embedding a limited region of one semiconductor within another with different band gaps. The difference between allowed electronic states for the two materials forms a barrier for free electron movements. If any dimension of the structure approaches the de Broglie wavelength of an electron, quantum effects will arise. For a free particle, which has a mass m, the confinement in the x direction will give it an additional kinetic energy of magnitude: E confinment = ( p x 2 ) 2m 2 2m ( x) 2 (5) which becomes relevant if it is comparable to, or greater than the kinetic energy of the particle due to its thermal motion in the x direction. If this condition is taken into account, the confinement energy can be written: E confinment 2 2m ( x) 2 > 1 2 k BT (6) where k B is the Boltzmann constant, p x is the momentum given by: p x = / x. This equation tells us that quantum size effects will be important if x 2 m k B T This is equivalent to say that x must be of the same order of magnitude as (7) Structure Bulk Quantum well Quantum wire Quantum dot Quantum confinement None 1-D 2-D 3-D Number of free dimensions Table 1: Number of degrees of freedom tabulated against the dimensionality of the quantum confinement.

13 3.1 Quantum Dots and Confined Excitonic States 13 Figure 2: Schematic comparison of typical dimensions of bulk material, quantum well and quantum dots. the de Broglie wavelength for the thermal motion and give us an idea of how small the structure must be in order to observe quantum confinement effects. Table 1 summarizes the three basic types of quantum confined structures and the number of degrees of freedom associated with the type of quantum confinement. The structures are classified whether the electrons are confined in one, two or tree dimensions (Fig.2). Beginning with a bulk, 3-D material, the electrons and holes are free to move within their respective band in all three directions, and hence they have three degrees of freedom. Making the structure thin along one axis results in a 2-D layer, called a quantum well. If thinned along a second axis, a 1-D quantum wire is produced. Thinning along all three axis yields a 0-D structure called a quantum dot. This means that the motion of the electrons and holes is quantized in all three directions, so they are completely localized in the quantum dot. For every following discussion we will assume as growth direction the z-direction. Density of States First, we will consider the density of the state (DOS), D(E) for different dimensionalities. The DOS that gives the number of the states in the interval E E + de, assuming parabolic band dispersion and infinite potential barriers, is given by: D 3D (E) = 8πm 2m E h 3 (8)

14 14 3 PHYSICAL PROPERTIES OF QD Figure 3: The density of states as a function of energy for an effective material dimensionality of a) 3 (bulk), b) 2 (well), c) 0 (dot). D 0D (E) = 2N D D 2D (E) = 4πm h 2, (9) n x,n y,n z δ(e E nx E ny E nz ), (10) The 0D-DOS has the shape of δ peaks. Where m is the effective mass of the particle, Θ(E) is the Heaviside step function, δ-dirac function, N D volume density of quantum dots and E nx, E ny, E nz are the quantization energy corresponding to each direction x, y, z. In Fig. 3, the quantization of energy, i.e. the reduction of the dimensionality of the system is revealed in form of the dependence of the density of state on energy. At low enough temperatures the energy of photons is too low to excite the electrons and the strong quantization of energy determines the electronic properties of quantum dots. To calculate the DOS, the energies for different cases are given by: E 3D = 2 k 2 2m (11) E 2D = 2 2m [k (II)2 + ( nzπ L z ) 2 ] (12) E 0D = 2 2m [( nxπ L x ) 2 + ( nyπ L y ) 2 + ( nzπ L z ) 2 ], (13) where k is the wavevector, in the 2D case, k II is the component in the plane of the quantum well and, the quantum numbers n x, n y, n z specify the quantized levels in each direction. Thus, with the reduction of the dimensionality, the effect

15 3.2 Two - Dimensional Harmonic Oscillator 15 of the confinement on the DOS is large, leading to effects not present in the bulk material. The energy of an electron confined in a small area such as quantum dot is strongly quantized and the energy spectrum is discrete, analogous to the energy spectrum of atoms. The difference is, that we can tune the position of the energy levels by altering the size of the QDs. In a quantum dot structure the confined carriers have no degrees of freedom at all. 3.2 Two - Dimensional Harmonic Oscillator In Fig. 4 the potential V eff (r) describes the lateral modulation of the subband edges. To the beginning, in accordance with [6], the In content is assumed to be maximum at the top in the center of the quantum dots. Figure 4: The adiabatic approximation for electrons is sketched here. The lateral confinement of carriers in InAs quantum dots can be described by a two dimensional potential: V (x,y) = V m ω 0 (x 2 + y 2 ) (14) The classical Hamiltonian operator from the Schrödinger equation Hψ = Eψ, is defined in the 2D-harmonic oscillators as: H = (p2 x + p 2 y) 2m m ω 0 (x 2 + y 2 ) (15)

16 16 3 PHYSICAL PROPERTIES OF QD i.e the eigenenergies are given by: E nx,n y = (n x + n y + 1) ω 0,n x,n y = 0, 1, 2... (16) where m is the effective mass and p x, p y are the components of the impulse momentum p. The effective mass is isotropic and constant. Therefore, the energy of the harmonic oscillator is quantized. Note also, that the smallest value (the ground state) is not zero, but ω/2. In a quantum dot with lens shape the ground state wavefunction has cylindrical symmetry. As proposed by Wois et. al [36] from the quantum dot geometry, it is appropriate to separate the growth z-direction from the lateral coordinates. A single electron confined in the (x, y) plane by a harmonic isotropic potential V = m ω (x 2 + y 2 ) and by a magnetic field perpendicular to this plane, described by a vector potential A = B ( y,x, 0) can be studied using the Hamiltonian [42]: 2 H i = 1 2m (p i + ea i ) 2 + m 2 ω 0 2 (x i 2 + y i 2 ) (17) It is convenient to introduce Ω 2 defined as: Ω 2 = ω0 2 + ωl 2 where, ω L = 1ω 2 c is the Larmor frequency and ω c = eb is the cyclotron frequency. In this case, the m eigenstates of H i analyzed for the first time by Fock [79] and Darwin [80], and called also Fock-Darwin states become: E nm = Ω(2n + m + 1) + m ω L (18) n = n + n + (19) m = n + + n (20) where m is the angular momentum and n the quantum number. When magnetic field is zero (ω L = 0, Ω = ω 0 ) the eigenstates show a degeneracy. 3.3 Quantum Dots in an Electric Field In this subsection the influence of the electric field on the electron-hole pair states of three-dimensional confined quantum structures will be presented. Quantum confined Stark effects (QCSE), i.e. the effect of an applied electric field on confined carriers and excitons are investigated here.

17 3.3 Quantum Dots in an Electric Field 17 Lateral Field-parallel to Epitaxial Layer We will use the 2D-harmonic oscillator to study the in-plane potential V (x, y). In a harmonic oscillator, a particle of mass m has a potential energy [38]: V (x) = 1 2 mω2 (x 2 + y 2 ) (21) If we assume that this particle has a charge q and that it is placed in an uniform electric field F parallel to x direction, this classical potential will becomes: w(f) = qfx (22) With this equation, to obtain the Hamiltonian operator H(F) in the presence of the field F, it is necessary to add the term: W(F) = qf(x + y) (23) to the potential energy V(x) of the harmonic oscillator. This gives the Hamiltonian: H(F) = p2 2m m ω 2 (x 2 + y 2 ) qf(x + y) (24) If we define ϕ to be an eigenvector of the Hamiltonian H(F) the Schrödinger equation will be given by: H(F) ϕ = E ϕ (25) Introducing eq. 24 in eq. 25 with the respect of the acceptable value for E, one can observe that in the presence of the electric field F, the energies of the stationary states of the harmonic oscillator are modified: E(F) = (n x + n y + 1) ω q2 F 2 2m ω 2 (26) Therefore, in Fig. 5 the entire spectrum of the harmonic oscillator is shifted by the quantity q 2 F 2 /2m ω 2. So, an other shift of qf/m ω 2 is observed. This translation comes from the fact that the electric field exerts a force on the particle. If the product qf is positive, the translation is performed in the positive x-direction, which is indeed the direction of the force exerted by F. The only effect of the electric field is to change the x-origin and the energy origin. From Fig. 5, it is easily observed that while F is zero, the potential energy V (x) is represented by a parabola. On the other side, if F is not zero, it is necessary to add to this potential energy the quantity - qfx. It is important to mention that in the

18 18 3 PHYSICAL PROPERTIES OF QD W F F Figure 5: The presence of an uniform electric field has the effect of adding a linear term W to the potential energy V of the harmonic potential. presence of an electric field, we still have an harmonic oscillator. In this case, the 2D-oscillator-function Φ n,m can be derived analytically from quantum mechanics, with the quantum number n and the angular momentum m. The eigenenergies E nx,n y = ω(1 + n x + n y ) (27) are (2n + 1)-fold degenerate with respect to m, with m = n, n + 1,..., n.. In Fig. 6 the degenerate spectrum of a 2D-oscillator for electrons and holes together with the allowed optical transitions is schematically represented. As a consequence (from QCSE), an overall red shift is expected with increasing field for all transitions. This red shift might be compensated by the complementary decrease in the excitonic binding energy as electron and holes are shifted apart [8]. Therefore, the electric field depends strongly on distinct directions (e.g. in the real QDs, the asymmetry of the potential can be influenced by the crystal directions [110] and [-110]) and on the ratio of asymmetry. In this work, structures have been developed, which allow to apply electric field along these crystal orientations. Vertical Field - parallel with Growth Direction A vertical electric field to the QD layer plane F z in which usually the field is directed along the z-direction can be taken into account by replacing W i (z) in the Hamiltonian equation with: W i (z) W i (z) ± qfz (28)

19 3.3 Quantum Dots in an Electric Field 19 Figure 6: Energy spectrum of an 2D harmonic oscillator for electrons and holes. Using second-order perturbation theory, the change in energy can by expressed as E 1 = 1 24π 2(15 π F 2 m e L 4 2 1)e2 (29) 2 This is equivalent to say that the energy of the lowest subband decreases quadratically with the applied electric field. With increasing field a parabolic decrease of the ground state energy of the electron and the hole is expected. This decrease is emphatically the decrease in electron-hole overlap with F. A lot of studies have been carrier out in both, experimental and theoretical modes. The apex of the parabola was found at finite electric field that involves a gradient of the In-content [37], [43], [44], [45]. From Fig. 7 it is easy to see that in the triangular Figure 7: Band diagram for a quantum well with flat bands(a) and bands tilted by an electric field, which lowers the energy of both bound states(b).the envelope wavefunction of an electron and heavy-hole ground state is shown.

20 20 3 PHYSICAL PROPERTIES OF QD p i n V 0 l i Figure 8: A sketch of a p-i-n diode diagram. The voltage V 0 is applied to the p-region, so that positive and negative V 0 correspond to forward and reverse bias respectively. potential (in the case of QW), the electrons and holes are pushed towards the same edge. Due to the different effective masses, the heavy hole wavefunction is strongly affected and results in a built-in dipole moment. Also, the exciton binding energy is expected to decrease. 3.4 Lateral p-i-n and n-i-n Diodes In semiconductor optoelectronic devices the p-i-n structure is largely used, for example as photodiode solar cell, light emitting diodes and optical modulators. It consists of a n-doped and a p-doped layer that are separated by an intrinsic region of thickness l i incorporated at the junction. The i-region is the optically active part of the diode. The doped regions form contacts, that is to control the number of electrons and holes injected into the active region, and to permit the application of strong electric fields (higher than 10 4 V/m). The p-i-n structure is shown schematically in Fig. 8. An important feature of the p-i-n structure is the formation of the depletion region, because the external voltage drops across i- region and this region. This happens because of a very high resistance compared to the heavily p- and n- region. The width of this region is determined for a given voltage, by the doping levels in p- and n- region. In a p-i-n structure the residual doping level in the i-region is very small, and so the depletion region can extend across the whole i-region. This extension is very small in comparison with l i because of the heavy doping level in the contacts. This means, that any external applied voltages will drop almost completely across the i-region. Fig. 9 shows the band diagram of a p-i-n structure; in a) the band alignments at zero

21 3.4 Lateral p-i-n and n-i-n Diodes 21 E p i n E p i n E g E g ev 0 l i l i z z (a) V 0 = 0 (b) Reverse bias applied Figure 9: Band alignments in a p-i-n diode structure with an i-region thickness of l i. a) Voltage = 0. b) Reverse voltage applied.the thick dashed lines indicate the Fermi levels. E g is the band gap of the semiconductor used for the p- and n- regions. voltage and in b) the band alignments with an external voltage applied. At zero voltage the Fermi levels of the p - and n - regions align with each other. If a voltage is applied, the energy difference between the regions will be equal to ev 0. That is when a negative voltage is applied to the p - region. Reverse bias tends to increase the voltage drop across the i - region, while forward bias tends to reduce it. To calculate the electric field along the i-region, it is necessary to solve Poissons equation: 2 V = ǫ r ǫ 0 (30) where V is the voltage and is the electric charge density. Because of symmetry reason it is assumed, that derivatives in the x and y planes must be zero and Poissons equation becomes: δ 2 δz2v (z) = (z) (31) ǫ r ǫ 0 We can assume that = 0 in the i-region because it is undoped and it is fully depleted of all free carriers. The electric field strength can be calculated using equation: F = V (32)

22 22 3 PHYSICAL PROPERTIES OF QD This will give: F = dv dz = C 1 (33) which expresses, that the electric field is constant in i-region. C 1 is a constant. From Fig. 9 b) we can see that the magnitude of the voltage drop across the i-region is approximately equal to V bi V o.the value of the constant electric field in the i-region is therefore given by: F = V bi V o l i (34) That shows that the negative bias increases the field across the i-region, while a small forward bias reduces the field. The field is zero for a forward bias of V 0 = V bi and at a zero bias the field is equal to V bi /l i. In case of GaAs, V bi is 1.5 V and for a width of intrinsic region of l i = 1µ the field at zero bias becomes Vm 1.

23 4 Experimental details In this chapter we will take a close look on the technological process used to define the devices, as well as the investigation methods like time-resolved and single dot photoluminescence spectroscopy. 4.1 Molecular Beam Epitaxy (MBE) System Molecular beam epitaxy (MBE) was developed in the early 1970s and it evolved primarily as a technique for the growth of III/V compound semiconductors as well as several other materials. Today it is possible to grow a wide range of very high-purity semiconductors materials with excellent optical and electrical properties. MBE can produce layers with very abrupt interfaces and good control of the thickness, doping, and composition. Because of the high degree of control possible with MBE, it is a useful tool in the development of sophisticated electronic and optoelectronic devices. A schematic diagram of a MBE machine is shown in Fig. 10. A typical MBE system consists of three vacuum chambers: a growth chamber, a buffer chamber, and a load lock chamber, of which the growth chamber is the most important. The buffer section is involved in the preparation and storage of the wafers before entering the growth chamber. The load lock part is used to bring samples in and out of the vacuum environment while maintaining the integrity of the vacuum in the other two chambers. The Epitaxy materials are stored in some heated cells, called effusion cells. Each source is independently heated, until atoms of the source material are able to escape by thermal emission. An advancement of MBE system uses room temperature gases for the source materials, thus avoiding significant contamination problems and the necessary higher substrate temperatures that can cause segregation of dopant atoms. In our MBE, elements as Ga, In, As, Al, Si as n-type dopant, C as p-type dopant, are evaporated from the effusion cells (hot ovens) in the form of molecular beams onto a heated substrate. This takes place in ultra high vacuum (UHV) so that the beams are not scattered due to the large mean free path of the molecules at UHV. Atoms from the sources are able to travel in a straight line until they collide with the substrate material. A computer remotely operates the shutter controls, allowing the emission of different species of atoms to be directed at the substrate. The typical rate of growth in our MBE is around a 1ML/s. The slow rate allows abrupt changes in material composition. Under the right conditions, the beam of atoms will attach to the substrate material and an epitaxial layer will begin to form. One of the most useful tools for in situ characterization of the MBE growth is RHEED (Reflection High Energy Electron). It can be used to calibrate growth rates, determine the proper arsenic overpressure, calibrate the substrate temperature, give feedback on surface morphology, and provide information about growth kinetics. The RHEED gun emits about 10 KeV electrons 23

24 24 4 EXPERIMENTAL Figure 10: Schematic diagram of III/V-MBE system. After Sze [41]. which strike the surface at a superficial angle between 1 and 3 degrees. RHEED intensity oscillations can be used as an accurate, direct measure of the growth rates in MBE. When growth is started, for example on a smooth GaAs surface, the intensity of the RHEED pattern, especially the specular reflection, begins to oscillate. The oscillation frequency corresponds to the monolayer growth rate, where a monolayer is the thickness of one full layer of Ga and one full layer of As atoms. Some of semiconductor materials which can be grown with MBE system from Ruhr-Uni-Bochum are: HEMT structures for high mobility 2DEG, GaAs/Al x Ga 1 x As quantum wells for optical applications, self-assembled InAs quantum dots, regrowth on ion implanted material. From this point we will focus on the fabrication of self-assembled QD. To the beginning the QD was grown using lithography technique. This implies multistep processing which degrades the surface of the sample and the grown QDs. Using MBE system avoids these complex processing steps, and is also very time-efficient with a complete growth sequence as short as one hour. Thus, as a conclusion: in MBE system QD can be grown very efficiently due to the self-organization effect. The Stranski Krastanov (SK) growth method is used to fabricate defect-free quantum dots QDs in lattice mismatched systems of about less than 10 nm diameter. Some growth parameters like growth temperature and As pressure, can affect the growth process of QDs. The growth temperature of QDs plays an important role in determining the growth distribution of InAs-QDs. The lattice mismatch of 7 % between InAs (0,605) and GaAs (0,565) is high enough to induce the SK growth. At low temperatures, 500 C, SK growth dominates the growth and results in unresolved QD patterns. At high growth temperatures namely 650 C, the InAs is unstable and will evaporate from the surface (no dot growth possible). For medium growth temperatures, 550 C, the QDs density equals the defect surface density of the

25 4.2 InAs/GaAs-Self-assembled Growth Process 25 GaAs buffer layer. Usually used substrate temperature for GaAs and AlAs is 600 C and for InAs is 525 C. The stability of an array of InAs dots depends in principle on the arsenic pressure [25]. An optimal mediate As pressure must be chosen to keep proper surface mobility of atoms and to provide sufficient As into growing crystal. A change up to 50 % in the standard MBE arsenic pressure (P As Torr), the typical equilibrium array of dots of high density ( cm 2 ) develops. For an increase of the arsenic pressure by a factor of 3, the dot size is reduced and a large concentration of dislocated InAs clusters (50 100) nm in size appears, for a reduction of the As pressure the dots disappear completely. The place of dots is taken by macroscopic two-dimensional InAs islands. Usually the As pressure used for growth of GaAs and AlAs is Torr and for InAs Torr. The realization of quantum dots (QDs) has opened amazing new areas of physical research and technological applications that deal with atomic scales. The MBE was the main motor for the developments of devices that used band gap engineering and quantum confinements. 4.2 InAs/GaAs-Self-assembled Growth Process Semiconductor devices are produced by depositing thin crystalline films on a single-crystal substrate in a process called epitaxy. The two most common methods of performing epitaxy are chemical vapor deposition and molecular beam epitaxy. If the layer is of the same material as the substrate, then it is called homoepitaxial (GaAs/GaAs); if the layer is of a different material it is called heteroepitaxial (InAs/GaAs, AlGaAs/GaAs). In Fig. 11 are depicted three possible growth modes. The first method Franck-van der Merwe, is simply the successive addition of 2-D layers to the substrate crystal. The second mode, Volmer-Weber, will occur if the added material can minimize its free energy by trading increased surface area for decreased interface area, forming an island structure like water droplets on glass. A third possibility can arise if the lattice spacing of the added material mismatches the substrate. This third mode is called Stranski- Krastanow. If the lattice constants of the substrate and the crystalline material differ considerably, only the first deposited monolayer crystallizes in the epitaxial way. Dislocations generated by lattice-mismatch stain are primarily responsible for the restrictions of combinations materials with similar crystal structure and lattice spacing. A significant strain occurs in the layer when a critical thickness is exceeded. This leads to a spontaneous creation of randomly distributed islands of regular shape. The shape and the average size of islands depend mainly on the strain intensity in the layer, the temperature at witch the growth occurs and

26 26 4 EXPERIMENTAL Frank-van der Merwe Volmer-Weber Stranski-Krastanow 2 1 layer-by-layer trade surface for interface dislocate in island Figure 11: Three possible growing modes are sketched. Frank-van der Merwe, Volmer-Weber, and Stranski-Krastanow. the growth rate [31], [32]. The transition from epitaxial structure to the random arrangements of islands is called Stranski - Krastanow transition and it occurs between monolayer deposition. InAs growth on GaAs (100) is the pair of compounds model for Stranski - Krastanow growth method and island formation. The strain in thin film is larger than for other semiconductor systems, due to the large lattice mismatch of 7 % between InAs (a InAs = nm) and GaAs (a GaAs = nm). A simple rule says [22], that if due to segregation an In concentration at the growing surface in excess of 85 % is reached, island formation starts [22]. Here, growth starts with a strained 2-D wetting layer, but islands form after a few monolayers. The Stranski - Krastanow growth method of InAs depends on the substrate orientation, the islands occur only on (001) and (111) B surfaces, not on (110) and (111) A surface. However, InAs quantum dots can also be obtained on (110) surfaces if a thin AlAs layer is deposited before the island growth. Also, the chemical composition of the islands depends on the deposition rate and overgrowth temperature. Typically, the islands contain Ga but, for slow deposition rate, less Ga is incorporated, whereas for high growth rates the deal of Ga into the islands is larger. The quantum dots formed in the Stranski - Krastanow phase transition are called self - organized or self - assembled dots (SAD). Numerous investigations established that the growth surface of the strained layer on the lattice-mismatched substrate is initially flat, and the so-called wetting layer (WL) is formed. However, when a certain critical thickness is reached, the planar front of growth is transformed into an array of

27 4.2 InAs/GaAs-Self-assembled Growth Process 27 3D nanoislands on the WL surface [33]. The transition from 2D to 3D depends on the main growth parameters: substrate temperature, arsenic pressure, and film thickness. The lattice-mismatch strain relaxation of InAs on GaAs has long been studied [25], [26], [27], [28], [29], [30] but it is still unclear whether ultimate limitations exist to obtain perfectly uniform size distribution. It was seen that the most uniform SAD sizes are observed only at the very initial stages of their formation at approximatively 1.6 ML and that the strain, induced by increasing the total amount of deposited InAs, is a more critical growth parameter for the tuning of SAD size and density. The strain induced by further coverage with InAs causes no increase in the diameter or height of self-assembled QD, but increases the number of dots. This suggests the existence of an energy barrier to dots growth which may simply be the energy barrier for the nucleation of a misfit dislocation. This energy must be overcome to form dots. With increasing self-assembled QD density, most of the InAs previously on the substrate is incorporated into SAD. This can only take place with a corresponding decrease in the thickness of the two-dimensional Stranski-Krastanow wetting layer, involving surface mass transport of In. D. Leonard et al. found, that with InAs total coverage as the critical parameter, the critical thickness for elastic relaxation is of 1.5 ML. Another study shows that the critical thickness decreases when the arsenic pressure increases [33]. However, for various device applications, it is necessary to control the parameters of QD array and individual islands, such as the surface density, array uniformity, size and shape of an island. 1 µm 2.00 kv Figure 12: Scanning Electron Microscopy (SEM) image of an array of self-assembled InAs/GaAs quantum dots (sample 11376). For the microscopic investigations the dots were grown on the sample surface. A quantum dot of (30-50) nm in diameter is extracted.

28 28 4 EXPERIMENTAL Topographic Characterization For a full understanding of the complex formation process of self-assembled islands, knowledge about form, height and thickness is the relevant factors. These magnitudes extend to the first information about the electronic structure of quantum dot. The dots were grown on the surface of a GaAs/AlAs heterostructure, to investigate these properties. We used Scanning Electron Microscope (SEM) as tool to get a view on morphologic and electronic properties of the islands. The SEM, analysis the surface of solid objects, producing images of higher resolution than optical microscopy. SEM produces representations of three-dimensional samples from a diverse range of materials. In Fig. 12 information about the size of quantum dot is achieved. The picture was taken with an acceleration voltage of 2 kv and by a 1 µm work area. The diameter of quantum dots was found to be between (30-50) nm and height in the range of (5-8) nm. We assume that our dots have a lens shape (a TEM image from literature of a such shape are shown in (Fig. 13)) and the average dot density is about cm 2. For optical studies, the dot has to be covered by a GaAs cap layer. Figure 13: TEM picture of lens-shaped quantum dot. 4.3 Device fabrication Heterostructures All the samples used in this work are GaAs/AlAs heterostructures containing InAs-QD layers grown by solid source MBE (Molecular Beam Epitaxy). The detailed layer sequences are provided in Appendix A. Here, the materials are introduced together with some relevant properties. The compounds GaAs and InAs crystals constitute in the zinc-blende structure. A other important property of a semiconductor is its energy gap. Some important band structure parameters

29 4.3 Device fabrication 29 Material GaAs InAs Lattice constant 5.65 A 6.06 A Energy gap E g 1.43 ev 0.36 ev Electron effective mass m / m n Table 2: Basic properties of GaAs and InAs at 300 K. of the used compound semiconductors are listed in Tab.2. The band structure can be controlled through the growth of alternate layers of semiconductors. Appropriate choice of two semiconductors in such heterostructures can drastically change the properties of the resulting material. Growing a thin dot layer of material with a band gap smaller than that of the surrounding material (the barrier) will results in the confinement of carriers to this inner layer. If the size of the dot is comparable to the de Broglie wavelength of the carriers then quantization effects come in. Generally the carrier mobility is affected by two factors: the effective mass of the charge carrier and its interaction with the lattice. In case of III-V compounds the lattice is more closely bound because of a small amount of ionic character in the binding which could result in a weaker interaction between carrier and lattice and consequently in a larger mobility. Because both, the minimum of the conduction band and the maximum of the valence band occur at the center of the Brillouin zone, the InAs islands are well advisable for optical applications. It is well known that the direct energy gap for InAs is 0.42 ev (77 K) to 0.36 ev (300 K) comparative with the GaAs energy gap, 1.52 ev (77 K) to 1.42 ev (300 K). The spontaneous alignment of the InAs quantum dots and by this the spatial control of the position of the growth islands is important for some device structures, for example, for modulating the gain in laser structures. GaAs/AlAs Heterostructure with InAs Quantum Dot Layer Here we discuss structures that contain one InAs quantum dot layer embedded in GaAs matrix (Tab. 3). The relevant part has the following layer sequence: after a 400 nm GaAs buffer layer a single layer of InAs QDs was grown. The QDs were followed by 120 nm GaAs and 30 periods of a 2 nm AlAs/2 nm GaAs supperlatice. Then, 10 nm GaAs, 3 nm AlAs and finally 50 nm GaAs were deposited. The QDs were prepared by depositing a nominal coverage of 1.8 ML InAs at a substrate temperature of 530 C without rotating the substrate. This results in a gradient in dot density from zero to cm 2. By choosing the appropriate part of the wafer, we were able to fabricate either devices with

30 30 4 EXPERIMENTAL Material InAs layer depth 243 nm 243 nm 243 nm 243 nm 243 nm density of QD cm x 10 cm x 10 cm x 10 cm cm crystallographic direction of the electric field applied [0, 1, 1] [0, -1, 1] [0, 1, 1] [0, -1, 1] [0, -1, 1] device p-i-n n-i-n n-i-n p-i-n p-i-n type measurements single dot spectroscopy time-resolve time-resolve time-resolve single dot spectroscopy Table 3: Properties of the InAs/GaAs quantum dot of material The mentioned depth is the distance between surface and the quantum dot layer. high dot density for time-resolved measurements or with low density for single dot spectroscopy. Multi-Layers Heterostructure Another used layer structure starts with a 100 nm thick GaAs buffer followed by a sequence of 10 InAs-QD layers separated by 100 nm thick GaAs and embedded in AlAs/GaAs short-period superlattices. The QD growth temperature was 520 C and the density cm 2. The islands were overgrown with 8 nm GaAs at a reduced substrate temperature of 505 C. This multi-layer sample structure was chosen to facilitate the absorbtion measurements performed in the group of Prof. U. Woggon (Uni-Dortmund). These samples had been annealed at different temperatures and were investigated by photoluminescence (PL) measurements. Focus Ion Beam (FIB) Implantation The fabrication of diodes, transistors and integrated circuits requires a very precise control over the spatial doping of the wafer to form the devices. Structures with submicron dimensions can be written due to the very fine diameter of the focused beam - down to 30 nm. This makes it possible to write narrow insulating lines and narrow conductive regions. So, a large variety of ion sources can be chosen depending on the aimed type of doping and the target material. In the framework of this thesis we have used lateral implantation of regions p - type and n- type dopants to achieve a p-i-n diode structure. The whole process takes place in ultrahigh vacuum which is totally compatible with MBE growth conditions. Subsequent overgrowth after FIB implantation can be performed. The

31 4.3 Device fabrication 31 crystal growth direction z AlGaAs GaAs InAs-QD GaAs GaAs InAs-QD GaAs AlGaAs BC Material No. of InAs QD layers E g GaAs E g InAs E g AlGaAs Wavelength emission 1250 nm BV Figure 14: Schematic diagram and properties of the InAs/GaAs quantum dot of material The quantum dots are formed in the thin InAs layers sandwiched between GaAs layers which have large band gap. The mentioned wavelength in the table is the PL emission wavelength of QDs. operation principle of an EIKO 100 column used in the framework of this thesis is illustrated in Fig. 15. The focused ion beam system is composed of three main parts: the ion source, the optics column and the sample movements table. In an ultra high vacuum chamber (with a pressure of about Torr), an implantation sample is placed on a movable stage. The position of the table is controlled by a laser interferometer. The ion source is a liquid metal ion source. The ion source consists of a metallic alloy, that is put in a heatable container with a sharp needle sticking out of it. The molten metal wets the needle and flows to the tip. A voltage of a few kv is applied between the tip and an aperture close to it, called extractor, achieving a high electric field at the tip. The liquid metal is pulled into a sharp cone, called Taylor cone, and the ions are emitted from the tip of this cone. The source, which was used in this work is an eutectic compound of Au-Si-Be. The different ions are selected with a E B mass separator, a so called Wien filter, formed by crossed electric and magnetic fields. In this way it is possible to implant Si, Be, Au ions separately into the sample. The ions are collimated onto the sample by a system of electrostatic lenses and apertures. The lenses have a cylindrical symmetry around the beam axis. The main limitation to the performance of the focused ion beam consist in the chromatic aberrations, due to the energy spread of the emitted ions. The spherical aberrations arise from the nonideal radial dependence of the focusing field of the lens and can be corrected with the stigmator array. Small misalignments from the mechanical column prealignement are corrected with a system of plates near the objective and the condensor lenses. The FIB system EIKO 100 with AuSiBe ion source can achieve a good focus below 100 nm and 100 kev acceleration voltage. With

32 32 4 EXPERIMENTAL Focussed Ion Beam column (EIKO 100E) V Acc LIMS - EMITTER V Ext EXTRACTOR CONDENSOR LENS (CL) 1 CL ALIGNMENT CONDENSOR LENS (CL) 2 CL APERTURE APERTURE BEAM SWITCH E x B MASS FILTER OL APERTURE APERTURE FARADY CUP OL ALIGNMENT STIGMATOR OBJECTIVE LENS (OL) V Obj DEFLECTOR SAMPLE TABLE Figure 15: Schematic design of the Focus Ion Beam system EIKO 100. the help of PDS (Pattern Drawing System) software, almost any implantation pattern can be programmed. The geometrical writing field is 0.5 mm 0.5 mm. The beam current can be measured with a Faraday cup and the implanted dose can be calculated with the help of this current in the following way: D = B cti 2 ca (35) where B c is beam current, t is the time, I is the increment, A is the area, c denoted the charge. Some of the advantage offered by ion implantation to device fabrication are listed below [40]: 1. wide range of dopants 2. precise control over the ion dose 3. fine control over lateral dimensions

33 4.3 Device fabrication 33 Figure 16: The implantation depth profile determined by a STRIM simulation. 4. precision alignment 5. the implantation environment is clean. With the help of computer program STRIM (Simulation Transport of Ions in Matter) we can simulate the real depth profile of the ions implanted as a function of the material and the material thickness. The theoretical distribution of the implanted impurity atoms is a Gaussian distribution. According with the growth report for sample no , which was used to fabricate p-i-n and n-i-n devices, the corresponding depth profiles for Si 2+ and Be + ions implanted are depicted in Fig. 16. The profiles are calculated for an implanted dose, in case of Si 2+ of cm 2 and cm 2 in case of Be +. The characteristic energies used are 100 kev and 60 kev for Si 2+ and Be +, respectively. The implantation energies and the distance between QDs and surface are matched in such a way that the depth distribution is almost similar for Si and Be and that the maximum of the distribution is exactly at the position of the QD layer. With the help of Van-der-Pauw-Method [34] it is possible by Hall measurements to obtain information about the resistivity, carriers mobility, and carriers concentration of the implantation domain.

34 34 4 EXPERIMENTAL Electron Beam Lithography, Photolithography, Etching and, Ohmic contacts The fabrication of lateral p-i-n diodes developed in this thesis involves the combination between focus ion beam implantation, electron beam lithography (EBL) and standard optical lithography, because of the small intrinsic regions which are of the order of (2-4) µm. First of all the samples are submitted to a rapid thermal annealing step at 880 C temperature for 30 s in order to blue-shift the emission wavelength of QD from 1250 nm to 980 nm (1.26 ev) at 300 K. This step is useful to fit the sensitivity range of the available detectors. More details about RTA process are given in subsection 4.4. Electron beam lithography technique refers to a lithographic process that uses a focused beam of electrons to form the circuit patterns needed, in contrast with optical lithography which uses light for the same purpose. Electron lithography offers higher patterning resolution than optical lithography because of the shorter wavelength of the kev electrons that it uses. An EBL system simply draws the pattern over the resist on the wafer using the electron beam as its drawing pen. The resolution of optical lithography is limited by diffraction, but this is not a problem for electron lithography. The reason for this is the short wavelength ( angstroms) exhibited by the electrons in the energy range that they are being used by EBL systems. This step was performed in order to define the implantation windows. The procedure consists in covering the whole sample with an PMMA - resist and exposure to an electronic fascicle under a higher acceleration voltage of 20 kv. This is a thin polymer film that is sensitive to the electron impacts. The samples were subjected to the following technological work parameters: 200 Magnification, field size µm, and area dose µas/cm 2. The exposed resist becomes soluble in a developer and the mask pattern can be transferred onto the sample by resist development. Afterwards, by selective wet chemical etching, 50 nm GaAs from the implantation windows are removed; the intrinsic region forms ridge. After EBL, the lateral implantation of Si and Be step was performed as described in subsection 4.3. During the implantation process the samples were kept covered with this PMMA photoresist to prevent the intrinsic region against the side dose unavoidable in FIB implantation. Electrical activation of the implanted ions is achieved by a thermal treatment at 720 C for 30 s in a nitrogen atmosphere. To avoid As evaporation during the annealing process the samples were covered with a GaAs piece. In case of optical lithography we have used a positive resist (Shipley SP 2510) which is removed in the region exposed to UV - light. As a light source I have used a mercury lamp with an emission of 365 nm. The first optical lithography step defines the mesa shapes of the p-i-n diode. The chemical etching step removes the epitaxial structure around the desired area. I have used a nonselective etching solution consisting of H 2 SO 4 : H 2 O 2 : H 2 O with a mixing ratio of 1 : 1 : 50. The typical etching rate is 75 nm/min at room temperature. I used an etching depth around 300 nm below the sample surface

35 4.3 Device fabrication 35 in the heterostructure in order to isolate the mesas electrically from each other (see Fig. 16). In this way µm lateral mesa structures were defined. Electrical access to the lateral p-i-n device is achieved depositing ohmic contacts. Our metallization consists in five metal layers (Ni, Ge, Au, Ni, Au nm total thickness) for n-type ohmic contact and of three metal layers (Au, Zn, Au nm total thickness) in case of p-type ohmic contacts. The metal patterning was achieved by lift-off technique. This implies to spin a new resist film on the mesa surface and by lithography process, small windows are opened in this film L = 80µm Si-, Be-doped InAs QDs d = 2-4 µm ohmic contacts Figure 17: The pattern overview of lateral p-i-n device. in the region where the ohmic contacts should be deposited. Then, follows the metal deposition by thermal evaporation. The sample is dipped in a specific solvent, which penetrates through the uncovered areas, dissolves the resist and the undesired metal is lifted-off, leaving a clean metal pattern behind. This metallic contact is alloyed in an ambient gas atmosphere of 10% H % N 2 at a temperature of 400 C. Fig. 17 shows the pattern fabrication of such a device. The contact resistance is obtained using a transmission line structure. Measuring the resistance between contacts separated by increasing distances, the contact resistance can be extracted.

36 36 4 EXPERIMENTAL 4.4 Rapid Thermal Annealing Technique Annealing is the thermal treatment of a addle solid which results in the recovery of crystal structure and physical properties. The crystal lattice damage is for instance introduced, by an implantation process and the amount of the damage is strongly dependent on the temperature during implantation and the ion dose [40]. In this work Si and Be ions are introduced in the GaAs/AlAs heterostructure which contains a single InAs quantum dot layer. Silicon an element from group IV in the periodic system of elements and takes place as a donor (D) in our heterostructure, while beryllium, an element of group II, is introduced by implantation as an acceptor (A). The annealing process is used in this thesis, to realize the recrystallization of the heterostructure lattice and to electrically activate the implanted ions. On the other hand, the annealing step is also used to tune the energy levels in quantum dots. Experience has shown that it is necessary to anneal the implanted sample to electrically activate the implanted dopants. This thermal process is achieved ex-situ in a rapid thermal annealing (RTA) tool from firm AST 100. So, this step is available in-situ in MBE system following the implantation step in a high vacuum cavity, afterwards, a new growth step can take place. The sample, in ex-situ annealing step is introduced in a quartz-glass reactor and is located on a Si-wafer. The temperature is controlled by a pyrometer and by a calibrated thermocouple. Two halogen lamps situated above and below the reactor, heating the sample with a heating rate between ( ) C/s. The maximum temperature is 1000 C. The process takes place in a nitrogen atmosphere, usually for 30 s. To avoid the As loss from the surface, the samples are covered face-to-face with a GaAs piece. The annealing temperatures used for the sample are between 300 C and 900 C, depending on ion type, ion energy and ion dose. 4.5 Photoluminescence Measurements on Quantum Dots The quality of self-assembled InAs quantum dots, for instance their homogeneity and wavelength emission can be obtained with the help of photoluminescence measurements (PL). As a simple definition, the photoluminescence is the reemission of light after absorbing a photon of higher energy. The light emission is the result of the recombination of an electron and a hole, while the wavelength of the light is determined by the energy difference between the recombining electron and hole states minus the exciton binding energy. The PL setup uses as an excitation source, a GaAs-Laser diode with λ = 635 nm wavelength which corresponds to an energy of E = 1.95 ev. A schematic of the photoluminescence setup is presented in Fig. 18. The samples are fixed on a finger cryostate on the front side. The cryostate is mounted on a xy - table, which permits to move the sample in three directions to the intersection point of Laser beam and optical

37 4.5 Photoluminescence Measurements on Quantum Dots 37 lock-in COMPUTER InGaAs detector Laserdiode M 1 G Lenses Cryostate M 2 Monochromator Sample mirror Figure 18: Experimental setup used for the observation of photoluminescence (PL). The sample is excited with a laser with a photon energy greater than the GaAs band gap. axis. The Laser is focused on the sample by using a mirror and focusing lens. The laser is modulated with a frequency of 133 Hz to allow a signal filtering by a Lock-in-amplifier. Through the second lens the light is focused to the entrance of the monochromator (Jobin Yvon Spex 500M). The luminescence is detected by a nitrogen cooled InGaAs detector and converted into a signal, which is measured with a lock-in amplifier. The monochromator control and the data processing are realized with the help of a computer using a LabView programm. In Fig. 19 the photoluminescence process of an InAs-quantum dots is shown. The process starts with the absorbtion of a photon from the exciting light source. The created charge carriers may be captured by the dot, since they have a lower energy there. An important condition as the excitation source to injects electrons into the conduction band and holes into the valence band is that the chosen source must have the emission wavelength greater than E g. The capture is followed by relaxation to the lowest energy state of the lowest conduction band state for electrons and of the highest valence band for holes, whence they will recombine by emitting a photon. The carrier relaxation into the quantum dots is in accordance with Pauli principle: Since each dot level can be populated by a finite number of carriers only, several levels will be occupied [11]. The recombination process follows the selection rule, that electrons in a given conduction band state can recombine only with holes in valence band having the same quantum numbers. If the relaxation is fast compared with recombination rate, only the luminescence from the lowest state appears, provided the excitation power density is not too high. If the excitation power density increases, luminescence from excited states appears. In case of a not too fast relaxation rate, it is possible to observe emission from excited states, even at low excitation power density. The strength of these lines depends on the ratio between the recombination and relaxation time. Due to the usually high density of InAs-quantum dots cm 2, the emission

38 38 4 EXPERIMENTAL conduction band emitted photon valence band electron capture electron relaxation h hole relaxation hole capture radiative recombination mobile electron excitation + mobile hole absorbed photon Figure 19: Schematic diagram of the process occurring during photoluminescence in InAs-quantum dots. The exciting photon creates an electron-hole pair. h 1 wavelength is measured from an ensemble of quantum dots. Each quantum dot has an individual energy spectrum. Additionally, small differences in the form and parameters occur which result in an inhomogeneously broadened spectrum originating from recombination of the optically generated electron-hole pairs in the discrete dot shells. Fig. 20 shows a typical photoluminescence spectrum of an ensemble of InAs/GaAs quantum dots used in this work. Since the signal strength is proportional to the carrier density, the shape of the emission spectrum is independent of the way in which the carriers are excited. Three distinct emission features are observed from the PL spectrum which correspond to the energies E 0, E 1, E 2. The energy E 0 = ev raises from the ground state and E 1 = 1.05 ev is the energy of the first excited state. The separation energies between adjacent peaks is E = E 1 E 0 = 66 mev for the ground and first excited state. The energy difference of the first and second excited state is E = E 2 E 1 = 71 mev. It is easy to see, that the peaks are approximately equidistantly spaced in energy. This indicates that the in-plane confinement potential which has rotational symmetry due to the lens shape of the present dot, can be described by a parabola V (r) = 1 2 m Ω 2 r 2. A low effective thickness of the deposited InAs layer ( 1M L) results in narrow line in the photoluminescence spectra. By analyzing the PL spectrum we can obtain information about the discrete energy levels and thus the photoluminescence spectroscopy is an important tool in the study of semiconductors microstructures.

39 4.6 Electrical Characterization 39 PL-intensity [a.u.] energy [ev] Sample at T = 300 K E 2 E 0 E 1 E 2 E 1 E wavelength [nm] Figure 20: A typical PL spectrum of an InAs/GaAs quantum dot ensemble used for investigation in this thesis. The PL measurement is performed at room temperature and on a sample with high density of quantum dots. 4.6 Electrical Characterization To characterize the samples electrically, the I-V characteristics of the p-i-n and n-i-n diodes from this thesis has been measured with a Hewlett Packard 4156A DC parameter analyzer. The HP analyzer is composed of four source/monitor units and two voltage measurements units. The control of the measurements is realized by the help of a LabView programm. Most of the characteristics was done at T = 4.2 K temperature in a L-He cryostat. The time-resolved and single dot spectroscopy measurements were performed at 10 K. Low temperature conditions are necessary for the observation of quantum effects in nanoscalestructured materials. The I-V measurements of the p-i-n diodes were performed by applying a potential difference between two contacts, in this case n- and p- type, and measuring the current which flows through the device. It is also possible to apply a voltage between n- and n-contacts or p- and p-contacts in order to check the quality of the Ohmic contacts.

40 40 4 EXPERIMENTAL 4.7 Time-Resolved and Single Dot Photoluminescence on InAs-Quantum Dots For optical studies a bath cryostat was used to cool the sample down to 10 K. For the excitation, a tunable Ti:Saphire laser was used in CW-mode (continuous wave) for single dot photoluminescence, or in pulsed mode for time -resolved PL measurements. The laser pulses have a repetition rate of 75 MHz and a pulse duration of 100 fs. To suppress scattered laser light, the excitation for the first experiments was done by side-in-coupling under an angle of 50. With a focal length,f = 6.5 cm lens a focal spot of 100 µm diameter on the sample was achieved. For getting the needed spatial resolution, a microscope objective with a n/a = 0.5 and a working distance of 33 mm was used to gather the light. In the second series the laser was coupled through the same microscope objective. The quantum dot signal was then directed to a 0.5 m focal length monochromator, where the signal could be send to a liquid nitrogen cooled InGaAs-CCD detector for PL studies. In order to do time-resolved PL, a mirror could be flipped inside the monochromator, sending the signal to a second output where the streak camera was placed. Also, for time-resolved PL a xyz piezo table was used to move the microscope objective and enable us to search for positions where luminescence from QD arises. The overal temporal resolution of the streak camera is about 3 ps, which is increased by the monochromator to 20 ps.

41 5 Thermal Processing of InAs/GaAs-Quantum Dots Optical studies of high quality of semiconductor self-assembled quantum dots such as photoluminescence [46], [47], magnetocapacitance-spectroscopy [48], as well as the evidence of the single QD sharp lines [50] with their temperaturedependent behaviors [49] have been done with a great interest. It has been shown that thermal treatment of QD heterostructures can be a suitable method, to tailor the number of confined states, and to manipulate their energy levels and their intersublevel energy spacing. To control such QD parameters, it will be necessary to understand better the energy relaxation mechanisms of carriers from excited states, and also to fabricate detectors and emitters based on intersublevel transitions in zero - dimensional systems. The thermal annealing is used to decrease the strain and defects of lattice and also for electric activation of implanted ions. The whole process can be briefly described by the diffusion of atoms resulted from the thermal treatment. Therefore, high temperature annealing can introduce severe changes in the QD shape, size and composition due to a possible interdiffusion process. The aim here was to study the effects of rapid thermal annealing on the optical properties of self-assembled InAs/GaAs quantum dots by photoluminescence measurements. Diffusion Law For the higher annealing temperatures, some thermal diffusion occurs. The first diffusion Fick s law says, that the quantity of material diffusing per unit time is proportional to the concentration gradient and that the direction is towards the lower concentration [40], that is, J = D δn δx, (36) where J is the flux density, N is the impurity concentration and D the diffusion constant. From the transport equation δn δt = δj δx. (37) and by substitution with 36, the diffusion equation is derived as: δn δt = Dδ2 N δx 2. (38) The diffusion constant D is characteristic for each diffusant and is strongly dependent on temperature: D = D 0 exp( E a ), (39) kt 41

42 42 5 THERMAL PROCESSING OF INAS/GAAS-QUANTUM DOTS where E a is the activation energy. Sample Processing The thermal process takes place ex-situ in a RTA (rapid thermal annealing) tool from AST company. The investigated sample, (chapter 4.3), was grown by molecular beam epitaxy on a semi-insulating GaAs (100) substrate at a substrate temperature of 520 C. Stacks of InAs-quantum dots have been deposited at a nominal coverage thickness of 2.1 ML, resulting in InAs islands with a density of (2 3) cm 2. The islands were overgrown with 8 nm GaAs at a reduced substrate temperature of 510 C. A sequence of 10 QD layers, spaced by 100 nm thick GaAs, and embedded in an AlAs/GaAs short-period superlattice have been used. In Fig. 21, the layer structure of the InAs-QD stack embedded in a GaAs-matrix is sketched. The wafer was rotated during growth process to obtain uniformity throughout the wafer. After the growth, the RTA 2.1ML WL&InAs-QDs GaAs-cap layer Al Ga As-superlattice GaAs Al Ga As-superlattice GaAs-buffer 42nm 120nm 120nm 42nm 120nm ]10 InAs-QDs Figure 21: Layer structure of a sample with 10-InAs-QDs layers embedded in a GaAs-matrix. process starts with a 4 4 mm pieces cut out of the same wafer by a diamond tip. These pieces were tempered by rapid thermal annealing at different temperatures, (800 C, 840 C, 860 C, 880 C, 920 C, 940 C, 960 C) for 30 s in a nitrogen (N 2 ) atmosphere. To prevent Arsenic loss during the heat treatment, the samples were covered face-to-face with a piece of fresh GaAs wafer. 5.1 Photoluminescence Measurements After the RTA step, the samples have been investigated by photoluminescence (PL) measurements. A laser diode with 635 nm wavelength and 5 mw excitation power was used as excitation source. The condition that the Laser to excite the carriers in the semiconductor is that, his energy should be greater than E g (in this case the laser energy is 1.95 ev). The samples have been investigated

43 5.1 Photoluminescence Measurements 43 PL - Intensity [a.u] 1,20 1,15 1,10 1,05 1,00 0,95 0, nm 0, QD T = 300 K 0,6 0,5 0,4 0,3 0,2 0,1 0,0-0, nm Energy [ev] 1180 nm E = 65.1 mev Wavelength [nm] 31.3 mev FWHM Figure 22: The photoluminescence spectrum at 300 K for an as-grown sample with a 10-layers QD array. The wavelength in nm for the interband transitions is indicated at the corresponding peaks. at room temperature (300 K) as well as low temperature (77 K). The general principles of photoluminescence in semiconductors have been described in chapter 4.5. The optically injected electrons and holes rapidly relax to the bottom of their bands and from there, they recombine and send out photons. In a quantum dot, the lowest level available to the electrons and holes corresponds to the n = 1 confined state. Hence the low intensity luminescence spectrum consists of a peak of spectral width k B T at energy hυ = E g + E hh1 + E e1, and note that the quantization energies of the heavy hole are smaller than those of the electrons, because of their heavier effective mass. This shows that the emission peak is shifted by the quantum confinement of the electrons and holes to higher energy compared to the bulk semiconductor (in this case GaAs). Considering the PLemission at 300 K of the sample there are three emission peaks between 0.9 ev and 1.2 ev (shown in Fig. 22), and they are almost equidistant arranged. The strongest emission at E 0 = 0.99 ev corresponds to the recombination process of the electrons and holes in the ground state (s-s transition). The next peaks correspond to the p-p transition (the first excited state) E 1 = 1.05 ev and d- d transition (the second excited state) E 2 = 1.12 ev. The energy difference

44 44 5 THERMAL PROCESSING OF INAS/GAAS-QUANTUM DOTS

45 5.1 Photoluminescence Measurements 45

46 46 5 THERMAL PROCESSING OF INAS/GAAS-QUANTUM DOTS Figure 23: The photoluminescence spectra at 77 K for an as-grown sample with a QD stack (dot trace a)) and after RTA treatment (black traces b, c, d,...). The annealing temperatures are in the range of 800 C and 960 C. The wavelength in nm for the s-s transitions of individual samples is indicated at the corresponding peaks. The measurements have been performed using a SP 500M spectrometer and a laser diode with 635 nm wavelength as excitation source.

47 5.2 Blueshift of Energy Levels by Annealing 47 between s-s and p-p transitions is E = E 1 E 0 = 65.1 mev. The intersublevel spacing E can be tuned also by adjusting the substrate temperature during the QD formation to change their equilibrium size [52], [53], [59]. The full width at half maximum (FWHM) is determined by the inhomogeneous broadening of the luminescence peaks and it is for the investigated sample around 31 mev. Fig. 23 shows the PL- spectra at low temperature (77 K) for the as-grown sample and the annealed samples. With increasing annealing temperature the energy difference between adjacent peaks E = E 2 E 1 becomes smaller, from 52.7 mev for the non-annealed sample down to 17.3 mev for an annealing temperature of 960 C, while the PL emission corresponding to the s-s transition shifts from 1176 nm (1.054 ev in energy) to 893 nm (1.388 ev in energy) in wavelength. This is easier to see in Fig. 24, which shows the effects of intermixing on the QD shell structure for sample Fig. 24 showns that the s-shell peak is blue shifted by almost 240 nm (335 mev) for the highest RTA temperature. Fig. 25 shows the spectra of the PL measurements performed at room temperature of such an annealed sample. From this graphs is visible shown that the s-shell blue-shifted by almost 330 mev for the highest RTA temperature, while the confinement to the wetting layer was reduced from 330 to 69 mev. The activation energy for the wetting layer (WL) emission equals to the energy difference between the GaAs band gap and the WL peak (Ea WL 61 mev for 860 C and 43 mev for 960 C annealing temperature), and it suggests, that the main source of non-radiative recombination at higher temperature is a carrier loss at the sample surface, while the activation energies of the lower QD shells (p, d) are most probably a consequence of carrier redistribution between the shells and the QDs. The PL-peak of the s-s transition shifts, in this case, from 1258 nm (for the not annealed sample) to 944 nm ( for 960 C annealing temperature) and the intersublevel energy spacing E = E 2 E 1 was changed from 67 mev to 25.5 mev by increasing the annealing temperature. Finally, the inhomogeneous broadening went from a full width at half maximum (FWHM) of 38 mev down to 8 mev as seen in Fig. 25-b). The FWHM appears to decrease linearly with RTA temperature. A FWHM as narrow as 8 mev is obtained for an intermixed temperature of 960 C for 30 s. 5.2 Blueshift of Energy Levels by Annealing The observed changes in the PL spectra upon annealing are attributed to a rearrangement of the atoms during the annealing process. The increase of the annealing temperature, results in the diffusion of Ga atoms into the quantum dot and the out-diffusion of the In atoms from the QD during annealing. This leads to a larger quantum dot with lower In content, which results in a shallower and broader confinement potential. Fig. 26 shows how the energy band changed by the annealing. The shallower confinement potential leads to the observed blue

48 48 5 THERMAL PROCESSING OF INAS/GAAS-QUANTUM DOTS Inter-sublevel [mev] T = 77 K Wavelength [nm] RTA Temperature [ C] 850 Figure 24: The effects of intermixing on the QD shell structure for sample The peak positions of the QD emission (triangle trace) and the intersublevel energy spacing (square trace) are plotted as a function of RTA temperature. shift in PL emission and the broader confinement results in a decreased level spacing, detected as reduced E. Combining the facts that the size of the QD is few nanometers in height and (20-30) nm in the in-plane diameter the thermally activated In diffusion influences mainly the In profile along the hight. The resulting reduction in the vertical confinement leads also to an increased of the in-plane extension of the excitonic wave function [58] and an increase of the coherence area. The strong energy shift of the luminescence line can be explained by the reduced confinement potential after annealing (see Fig. 26). The PL spectra reveal that the QD luminescence intensity decreases and the wetting layer and the GaAs matrix PL increase with higher annealing temperature. The observed wetting layer peak is due to the formation of the thin InAs quantum well which precedes the spontaneous island formation. The high temperature annealing induces also an increase of the QD size, while the shape is only weakly affected, and this is confirmed by TEM (transmission electron microscopy) studies [51]. Stronger and narrower emission peaks from QDs were also observed with even higher annealing temperature, which indicates, that the quantum dots retain their optical quality and their zero-dimensional density of states after the change of the potential. Also, the asymmetric distribution of In concentration along the growth direction created during the growth process is symmetrized by the diffusion during anneal-

49 5.2 Blueshift of Energy Levels by Annealing 49 a) FWHM T = 300 K FWHM [mev] b) RTA Temperature [ o C] Figure 25: a) Non-resonantly excited PL spectra at 300 K for an asgrown sample (red trace) and after RTA treatment(black traces). The PL intensity was normalized to 1 and the individual traces have been set-off for clarity. The inserts show the expected QD form. b) The full width at half maximum (FWHM) of the s-s peaks is plotted in.

50 50 5 THERMAL PROCESSING OF INAS/GAAS-QUANTUM DOTS GaAs QD-InAs QD-InAs GaAs GaAs GaAs 1.42 ev 0.98 ev 1.05 ev o RTA: 960 C/30s 1.31eV 1.34eV Figure 26: The InAs/GaAs-QD band diagram at 300 K, before and after 960 C annealing temperature. ing. As a conclusion, the rapid thermal annealing treatment of InAs quantum dots can be used to induce a blue shift of the emission energy and to tune the level spacing within the QD. As an application, the thermal treatment can be used to select the needed wavelength in, integrated optoelectronic devices. These multi-layer samples were chosen to facilitate absorption measurements performed in group of Prof. U. Woggon (Uni-Dortmund) [54], [55], [56], [57]. It was found, that by annealing treatment one can control the fine-structure splitting, biexciton binding energy and radiative lifetime in InGaAs quantum dots. Strong reductions of the fine-structure splitting from around 100 µev to below 10 µev and the radiative lifetimes from 1ns for unannealed samples down to about 200 ps for annealed samples were found for the smallest confinement energy. The biexciton binding energy is relatively weakly affected by annealing, varying between 2.5 mev and 3.7 mev. The change in biexciton binding energy can be explained as follows. The In distribution of the as-grown QDs is asymmetric due to the directionality of the growth process. Thus, the electron-hole charge separation results in a repulsive exciton-exciton interaction [60], which reduces the biexciton binding energy. It was demonstrated that the In distribution tends to a Gaussian shape for annealed samples, reducing the repulsive interaction and thus increasing the binding energy. From this, the annealing leads to a strong reduction of the inhomogeneous distribution of the biexciton binding energy, which is attributed to the symmetrization of the carrier confinement in the growth direction. In semiconductor quantum dots, the energetically lowest exciton states have the longest coherence times in comparison with the higher states. The magnitude of the splitting of these states and the energy renormalization of the two-exciton state (biexciton binding energy) are of certain interest for applications as sources of entangled photons, in quantum cryptography as well as in quantum information processing.

51 6 Time-Resolved Spectroscopy 6.1 State of the Art The progress in the molecular beam epitaxy growth falsified, some theoretical studies which have proposed, that the photoluminescence efficiency of quantum dots is intrinsically poor due to the severe restrictions on the energy and the relaxation of the carriers [81], [82]. Now, high quality defect-free QD samples which offer the opportunity for detailed experimental studies, such as recombination mechanisms in zero-dimensional quantum structures are available. The photoluminescence PL spectroscopy is the most widely used optical characterization method for semiconductor quantum dots. The PL spectrum contains an important number of informations, like: the direct band gap and the dynamics of relaxation and recombination. These PL characteristics are of relevant importance for optical applications like light detectors, emitters, modulators or lasers [88]. Investigations of the carrier dynamics in QDs, as well as in quantum wells (QWs) have been done by measuring the temperature dependence of the carrier lifetime [84]. A linear temperature dependence below 30 K exhibits the carrier lifetime of QWs, which is a typical characteristic of the free exciton [83], while at higher temperature the carrier lifetime decreases. In case of QDs, a flat temperature dependence of the exciton lifetime is observed at low temperature and further, the lifetime increases if the temperature increases. This increase is attributed to thermionic emission of carriers from the QW. The lifetime has been investigated on a series of annealed InAs/GaAs self-assembled samples by S. Malik et. al. [87]. A decrease in the lifetime has been observed, when the annealing temperature increases. This is due to the changes in the electron and hole wavefunction overlap, as the dot size increases with the annealing. Also, experimental investigations of the PL decay time at low temperatures have been carried out and show a strong decrease with the increasing of the QD size in agreement with theory [85], [86]. In this chapter, the lateral electric field dependence of the carrier lifetime has been investigated. 6.2 Characterization of the High Density InAs-Quantum Dots The wafer is a GaAs/AlAs heterostructure, containing a single InAs QD layer, from which were cut-off pieces with high dot density cm 2, namely, samples (p-i-n), and (n-i-n)(see table 3 for more information). Using such a base material, lateral p-i-n and n-i-n diodes were fabricated. To allow injection of carriers, a layer of SAQD was embedded into the intrinsic region of a p-i-n and n-i-n diode heterostructure. A crucial condition 51

52 52 6 TIME-RESOLVED SPECTROSCOPY WL Figure 27: The photoluminescence spectra at 300 K for a QD sample grown without rotation resulting in a density gradient. The smallest intensity PL peak corresponds to a low density dot (the black trace). The arrow is inverse to the In gradient (a, e, f, b, d: high QD density, c low QD density). WL denotes the emission of the wetting layer. a) b) Si, Be doped Ohmic contacts intrinsic region d=2-4 µm InAs-QDs Figure 28: Schematic complete p-i-n and n-i-n device in a) and a microscope image of the identical device in b).

53 6.2 Characterization of the High Density InAs-Quantum Dots 53 for eliminating the screen field effect in QDs, is to have a device with a optical active area of a few microns size. This is the reason for that these devices are fabricated with an intrinsic region width, which varies between (2 4) µm, as previously described in chapter 4.3. A general review of the complete device layout is displayed in Fig. 28 schematically and in the form of a photograph. The fabrication process combines electron beam lithography (EBL), focused ion beam (FIB) implantation and standard optical lithography. In Fig. 27 are depicted the PL emission spectra at different places of a whole wafer, measured with respect of the In cell position during growth. The small intensity PL peak, the increased WL-signal and increased band gap, belong to the region with a low density of quantum dots ( cm 2 ). In this case, the PL-peaks correspond to the s-s and p-p transitions, appear at 1255 nm and 1170 nm wavelength respectively, and the energy difference E = E 1 E 0 between the ground state and the first excited state is about of 70 mev. With the used PL spectroscopy setup, the focused laser beam attains a diameter of 20 µm, which for high dot densities of cm 2, results in the simultaneous excitation of 10 5 cm 2 quantum dots. As a consequence, the resultant spectra are inhomogeneously broadened, owing unavoidable fluctuations in dot size, shape, and composition. Typical inhomogeneous line widths are between (50 100) mev, compared with something of about 40 mev in our samples. Current-Voltage Characteristics The influence of an in-plane electric field on quantum dots and a p-i-n device was shortly addressed in sections 3.3 and 3.4, as well as the measurements set-up of diode I-V characteristics, in section 4.6. All measurements are performed at liquid helium temperature (4.2 K). Assuming 1.5 V (p-i-n) or 0 V (n-i-n) respectively, for the built-in voltage V bi, I have found that in these devices lateral electric fields of more than 100 kv/cm (p-i-n) and 10 kv/cm (n-i-n), respectively, can be achieved at low temperatures. The electrical field F is calculated according to: F = V bi V d (40) In electrical investigation of the high density QD p-i-n devices shows that, the maximum applicable negative voltage found here is 50 V, which corresponds to an electric field of 170 kv/cm. In this case, the electric field depends on the effective voltage (U eff = U applied I leakage R diode ), which is almost constant for an applied voltage of about 30 V and even decreases when ramping the applied voltage further. If a forward bias is applied to the p-i-n structure, the breakdown is observed at 1.4 V. A brief remark concerning leakage current should be mentioned. An interfering leakage current in reverse bias within our electric field range is detected, rather small (< 1nA at 10V ) even at zero bias field. This can be deduced from Fig. 29 a), where a representative current-voltage relation is

54 54 6 TIME-RESOLVED SPECTROSCOPY a) current [µa] 60,0µ 50,0µ 40,0µ 30,0µ 20,0µ 10,0µ 500,0n 0,0-500,0n -1,0µ -1,5µ -2,0µ p-i-n device d = 3 µm T = 4.2 K E-field II [0-1 1] electric field [kv/cm] applied voltage [V] b) 9.0µ 6.0µ electric field [kv/cm] n-i-n device d = 4 µm T = 4.2 K E-field II [0-1 1] current [µa] 3.0µ µ -6.0µ c) -9.0µ applied voltage [V] electric field [kv/cm] current [µa] 1,0µ 500,0n 0,0-500,0n n-i-n device d = 3 µm T = 4.2 K E-field II [0 1 1] -1,0µ voltage [V] Figure 29: I-V characteristics of the investigated a) p-i-n (2 µm) and n-i-n, b) (3 µm), c) (4 µm) structures. A diode-like behaviour has been observed. All the curves are measured at 4.2 K. The crystallographic directions are indicated for each graph apart.

55 6.3 Time-Resolved Measurements under an in-plane Electric Field 55 displayed for the high density p-i-n diode with 3 µm width of intrinsic region. In case of Fig. 29 b) and c), the same curves are depicted for the high density QD lateral n-i-n structures with 4 µm and 3 µm intrinsic region widths. The direction of the electric field with respect to the crystallographic axis is different for n-i-n devices (see table 3). For the structure with 4 µm width (b), the electric field is along the [0-1 1] direction, while for the device with 3 µm width (c) the electric field follows the [0 1 1] direction. For the n-i-n structures, the electric field can be expressed as: F = V/d, where d is the intrinsic region width, and V is the applied voltage. By analyzing the n-i-n I-V characteristics, a few tendencies are evident. Thus, the field can be reversed, the breakdown voltage is small but, the electric field is rather strong ( 2V 10 kv/cm) and last but not the least, zero field is possible. For both n-i-n devices an approximatively symmetric behaviour is measured. In the positive part, the breakdown appears at 3.3 V which corresponds to an electric field of 8.25 kv/cm for the device with 4 µm, while for the device with 3 µm width, when the electric field follows the [0 1 1] direction, the breakdown occurs at 1.5 V (5 kv/cm). Looking onto the negative region, the curves reveal a change at 2 V for both, which corresponds to fields of 5 kv/cm and 6.6 kv/cm, respectively. 6.3 Time-Resolved Photoluminescence Measurements under an in-plane Electric Field The samples described in aforementioned section have been investigated by timeresolved photoluminescence in group of Prof. M. Bayer (Uni-Dortmund), using the streak camera system. In principle, the experimental setup used for these measurements is similar to the one described in section 4.7, except for the excitation and collection of the luminescence. The optical resonant and non-resonant excitation, were done with a tuneable Ti:Sapphire laser in pulsed mode. Attenuators allow for changing the excitation power in a wide range (µw to mw). The fabricated devices are inserted in a He-flow cryostat at T = 10 K. The electric field is provided by a current-voltage tool at room temperature, allowing to apply voltages up to ± 35V. Time-resolved photoluminescence measurements are usually understood in terms of photoluminescence spectrum collected as a function of time. So, at each 12 ns, the laser emits a pulse of 1 ps length during which the number the counting of emitted photons at that time are collected. In the streak camera, the spectrally resolved PL intensity is plotted versus the delay time as it is shown in Fig. 30. By subtracting the background, normalizing the data and plotting on a logarithmic scale, one can extract a straight line with a monoexponential decay. A linear fitting procedure has been applied to determine the slope of the decay. In this way, we have investigated a p-i-n device as well as a n-i-n device with respect to the crystallographic orientation.

56 56 6 TIME-RESOLVED SPECTROSCOPY a) 0 V b) -35 V Delay time Wavelength Figure 30: The photon counting dependence of the negative applied voltage. Lateral p-i-n Devices Lateral p-i-n devices have been investigated at low temperature (T = 10 K) by time-resolved PL measurements dependent on the intrinsic region width. First of all, we have looked at the bias dependence of the exciton lifetime in a p-i-n structure with 3 µm intrinsic region width. The laser wavelength was set to 835 nm, which corresponds to an energy of 1.49 ev. The corresponding delay time vs. applied voltage are presented in Fig. 31. The excitation power was set to 200 µw however only 4 % arrive to excite the sample. In this case, the effective excitation power is in the range of 8 µw. The excitation power density determines the number of photoexcited electron-hole pairs in the dot. At a certain positive gate voltage of 1.5 V, just before the onset of electroluminescence (EL), we find a radiative decay time of about 460 ps. Furthermore, a slight reduction to 450 ps, corresponding to 0 V, is observed as the applied voltage decreases. In the negative region, an increased lifetime to 480 ps is measured at an electric field of 105 kv/cm. The overall effect of 7 % in the lifetime variation is therefore not very pronounced in this device. A complete absence of the effect has been found, when higher pump intensities were used or when the laser is tuned above the GaAs band gap. The missing effect is attributed to the screening of the field by carriers inside the intrinsic region when working with higher excitation densities. As more free carriers are created when pumping above the GaAs energy gap, the effective field is reduced by them. Taking into account the last consideration, the focus will now be turned to the devices with smaller intrinsic region. Increasing the integration time and further improvements of the setup, made possible even lower pump intensities. Fig. 32 presents the decay dependence of applied voltage for a p-i-n device with 2 µm intrinsic region width as well as the the PL intensity in a logarithmical scale versus delay at four representative applied voltages (1 V, -2.5 V, -15 V, -35 V). To overcome the screening effect of the field, the laser

57 6.3 Time-Resolved Measurements under an in-plane Electric Field 57 electric field [kv/cm] T rad [ps] p-i-n diode 3 mwidth laser: 835 nm exc. power: 8 W applied voltage [V] Figure 31: Electric field dependence of the radiative lifetime in a p- i-n structure with 3 µm intrinsic width measured at T = 10 K. The excitation wavelength of 835 nm was chosen to avoid the screening effect from the GaAs carriers. wavelength was chosen to be 800 nm and the excitation power density to be 8 µw to prevent the p-shell contribution. The change in the lifetime as a function of the external electric field applied is drastically increased. Starting from 660 ps at a positive bias of 1.5 V, the decay gets faster and drops linearly down to 480 ps. With increasing the negative voltage up to 35 V, the radiative lifetime increases again up to more than 625 ps for an electric field of 182 kv/cm. The relative change in lifetime, for this device is about 30 %, is a strong effect compared with the one expressed before. A remark necessary to be mentioned is that some irregularities occur in the negative region of the curve. Thus, the increase in the lifetime is almost linear but the presence of a flat dependence can not be neglected. Indeed, in the ranges (20-25) V, and (1-3.5) V a flat behaviour is found. This dependence is no longer symmetric due to the built-in electric field, which is also the cause for the fact that the minimum lifetime is not longer at U = 0 V. Besides these afore-mentioned features, one more observation is evident. Namely, a further increase in reverse bias results in a further decrease in line intensity for all the structures investigated in this work.

58 58 6 TIME-RESOLVED SPECTROSCOPY a) electric field [kv/cm] T rad [ps] b) p-i-n diode 2 µmwidth : 800 nm laser exc. power: 8 µw applied voltage [V] Normalised Int. [log. scale] =558 ± 15 ps Trad V = +1 V Normalised Int. [log. scale] =500 ± 10 ps Trad V = -2.5 V Delay [ps] Delay [ps] Normalised Int. [log. scale] =552 ± 14 ps Trad V = -15 V Normalised Int. [log scale] =625±15 ps Trad V = -35 V Delay [ps] Delay [ps] Figure 32: a) The radiative lifetime determined by time-resolved photoluminescence measurements for a p-i-n diode with 2 µm wide intrinsic region measured at T = 10 K. The delay dependence of the fundamental transition at the 8 µw excitation power for four selectively applied voltage (+1 V, -2.5 V, -15 V, -35 V).

59 6.3 Time-Resolved Measurements under an in-plane Electric Field 59 Lateral n-i-n Devices Due to the doping, the p-i-n sample has an in-built electric field, where the minimum lifetime is located at non-zero external applied field. The effects of the intrinsic field can be minimized by using a n-i-n structure instead of a p-i-n structure. In this case, the lateral electric field applied to the n-i-n devices can be expressed as: F = V d (41) On one hand, we have studied the effects of the lateral electric field on these structures and, on the other hand, what happens with the radiative lifetime, when the electric field is applied with respect to different crystallographic directions. The sample was prepared, implanting Si instead of Be (more details in sections 4.2, 4.3) resulting in a n-i-n device. We begin with a n-i-n device, in which the electric field is applied along [0-1 1] direction. The device with a 4 µm width intrinsic region has been excited with a laser wavelength of 800 nm below the band gap wavelength in order to avoid excitation of carriers from GaAs. Like in the case of the p-i-n device, only the ground state has been investigated. To realize this, an excitation power as small as possible, namely 8 µw, was used to eliminate the p- shell contribution. The sample exhibits similar strong level-filling behaviour with equally spaced excited states (see Fig. 35). To excite only the the electron-hole pairs in the intrinsic region, the excitation and detection were made by sending the laser and the QD signal through a microscope objective from the front. In this way, we could reduce the spot size and excitation power to lower values. Fig. 33 shows the low-temperature radiative lifetime as a dependence of the applied voltage, as well as PL decay transients corresponding to 0 V, 2 V, 4 V applied voltage. The spectra reveal a similar behaviour but, can be distinguished that one of the direction supports a large amount of applied voltage than the other. By analyzing the graph, the decay is observed to change similar up to 2.5 V but then a contribution of electroluminescence (EL) for negative voltage makes the determination of the decay, (fitting range and constant background) to be hard. For positive voltage, the decay could be analyzed up to 4.5 V but then, a strong contribution of electroluminescence is observed. So, in the forward direction, when a positive voltage is applied, the radiative lifetime starts at zero electric field (which corresponds to zero bias) from 714 ps and increases up to 990 ps for 4.5 V which corresponds to an electric field of 11.2 kv/cm. As it was mentioned before, when the field values increased, the device emits light continuously through EL, making time-resolved measurements impossible. The increasing in the lifetime is almost linear, with a small tendency to the flat dependence in the range of 2.5 V - 3 V. In the negative part, a further increase in the lateral electric field up to 6.2 kv/cm results in a further increase of the radiative lifetime about 850 ps. The changes in the decay are in accordance with the I-V characteristics of the respectively device. Therefore, the change in the lifetime is in the range of 38 %

60 60 6 TIME-RESOLVED SPECTROSCOPY 1050 electric field [kv/cm] T rad [ps] n-i-n device 750 d = 4 m 700 laser = 800 nm exc. power 25 W applied voltage [V] Delay (ps) Norm. Intensity(log. scale) 0 V 2 V 4 V = 715 ps T1 T 1 = 905 ps T 1 = 805 ps Figure 33: The radiative lifetime dependence of the applied voltage a) and b) the time-resolved PL recorded on a n-i-n structure with 4 µm intrinsic region width measured at T = 10 K.

61 6.3 Time-Resolved Measurements under an in-plane Electric Field electric field [kv/cm] n-i-n device d = 3 m laser = 800 nm exc. power 10 W T rad [ps] applied voltage [V] Figure 34: Electric field dependence of the radiative lifetime in a p-i-n structure with 3 µm measured at T = 10 K. The excitation wavelength was chosen to avoid the screening effect from the GaAs carriers. for positive bias at very low external applied field. The same effects are observed in a n-i-n device with 3 µm intrinsic region width, but respecting now the [0 1 1] crystal direction. Fig. 34 displays radiative lifetime curve as a function of the applied voltage as well as the electric field. Similar to the above n-i-n device, the laser was set at 800 nm excitation wavelength and the excitation power fixed at 10 µw. The reasons are the same as expressed before for the [0-1 1] n-in device. Analyzing the decay time, the graph shows that two tendencies are evident. First, the radiative lifetime increases almost linear with the increase in the lateral electric field in both directions.the same flat behaviour seems to happen, but for a different voltage range. Thus, for positive applied bias, the flat tendency is revealed between (2.5-4) V, while in the negative region this behaviour arises in the (1-3.5) V voltage range. Secondly, a common feature in both applied voltage directions is, that a further increase in the high electric field results in the arising of the electroluminescence effect. As a consequence, a saturation in the carriers lifetime is confirmed. For positive applied voltages the lifetime starts from 375 ps at zero-bias and increases rapidly up to 585 ps at 8.5 V, which corresponds to an electric field of 28.3 kv/cm. In the negative region the same behaviour is noted, but the increase in the lifetime is stopped by the apparition of the EL effect at 515 ps corresponding to a negative bias of

62 62 6 TIME-RESOLVED SPECTROSCOPY 5 V. A significant information that can be extracted from this measurements is that, when a voltage is applied, the lifetime increases by more than 40 % of the flat-band value. This is a relative larger change as the 30 % variation measured at the 4 µm device. The effect can be explain by the individually annealing process performed on the samples. The exciton lifetime in an as-grown sample is about 1 ns. The thermal annealing leads to an increase of the dot size, which determines the exciton coherence volume, and therefore of the exciton oscillator strength. The increase in the dot size results in a lifetime reduction, which, might vary, however, for individual samples due to differences in the QD size caused by unintentional variations in annealing process. This increasing in the dot size due to the annealing, is confirmed by the reduced exciton lifetime below 400 ps at U = 0 V for the n-i-n device with 3 µm. Thus, this stronger change is expected from the larger dot size, which potentially allows for a stronger separation of electrons and holes. 6.4 Radiative Lifetime in in-plane Electric Field This subsection describes the effects displayed above. By confining the carriers in all three dimensions in a QD, it is increase the electron-hole wave function overlap and thus, increase the radiative quantum efficiency. The electric field tilts the band and displaces the electrons and holes in opposite directions. This leads to a decreased overlap of the wavefunction which is supposed to result in a decrease of the probability of recombination. On the other hand, the lifetime is expected to increase with higher electric field. Therefore, it is a proof that the field is situated at the position of the QDs. The effect, induced by the electric field, depends on the excitation conditions. This is supported by the analysis done in the previous subsection. Concerning excitation into GaAs, the spectroscopy measurements almost did not show a change of the exciton emission, which is attributed to the creation of free carriers leading, effectively to a short-circuit between gates and a screening of the applied field. The excitation energy was tuned below the GaAs band gap. In this regime, an insensitivity to the excitation power confirmed a negligible influence on the optical excitation in the GaAs bulk. It is remarkable, that all the investigated structures present deviations compared to the theoretical expectation. Thus, taking into account the electric field expression for a p-i-n structure (see eq. 40) the positive applied voltage should decrease the field, while the negative bias should lead to an increase of the electric field. However, in the investigated structures, the electric fields shows, in both applied voltage polarities, an increasing behaviour. An evident feature is, that the peak intensity is observed to quenching with the increase of the electric field. One can understand that the presence of an electric field tilts the band structure, (the valence and conduction bands) leading to changes in the energies of electrons and holes [71]. Thus the transitions

63 6.4 Radiative Lifetime in in-plane Electric Field 63 become more and more spatially indirect, leading to an increases of the radiative lifetime. One explanation of the increased carrier lifetime is attributed to the reduced radiative recombination probability due to the reduced electron-hole wave-function overlap. In the presence of the electric field, the electron and hole are pushed in opposite directions, reducing the band gap. This effect requires, single dot spectroscopy, which is discussed in the next chapter.

64 7 Single dot spectroscopy on a lateral InAs - QD p-i-n diode 7.1 State of Research In the most, theoretical and experimental, studies of the quantum confined Stark effect (QCSE) the electric field has been applied along the growth direction with spectacular results [63], [89], [90], [91], [92], [93], [94]. Only a few experimental papers were published concerning the lateral direction [70], [71]. For these, different spectroscopic techniques, like electroluminescence (EL) [75], electroreflectance [76] or transmission modulation measurements were used [77]. QCSE is the effect of an applied electric field to confined carriers and excitons. It results in a shift of the emitted light to lower energy i.e. higher wavelength. The results, reported by Bacher et al., who studied the Stark effect on a CdSe/ZnSe QD structures grown on an insulating GaAs substrate, show an energy shift in PL emission of about 1 mev by applying a lateral electric field in a capacitor-like geometry [67]. In the attempt to apply a lateral electric field for QDs formed by monolayer fluctuations in quantum well (which have much weaker confinement than the InAs QDs discussed here), small Stark shifts at maximum 0.5 mev have been observed [70]. Recently, the effect of biexcitons on the Stark effect in GaAs and CdS quantum dots has been theoretically investigated. The results show, that the red-shift, which usually occurs in QD energy levels, changes to a blue-shift, when biexciton effects are taken into account. Also, in the frame of this study, a comparison between Stark effect in GaAs and Stark effect in CdS reveals that, the Stark effect in GaAs is stronger than in CdS quantum dots. The explanation is attributed to the difference in biexciton binding energies of the two materials [66]. Another experiment recently reported, which has been done on an artificial device, revealed a small red shift in the range of 70 µev. This device is formed by depositing a n-type ohmic contact and a Schottky contact on the surface. The contacts are parallel to each other and are separated by a 2 µm-wide channel providing the desired in-plane field geometry [68]. Theoretical investigations show, that the amount of the Stark shift is much larger for large QDs, comparatively with one of small QDs and that it is very difficult to observe it, if a broad QD size distribution exists [78]. 7.2 Characterization of Low Density InAs-Quantum Dots In this chapter, lateral p-i-n devices with an InAs/GaAs quantum dot layer, is situated at 245 nm (see Fig. 16) below the sample surface, were fabricated. The investigated sample was grown on a semi-insulating GaAs (100) substrate using Stranski-Krastanow method. The QDs were prepared by depositing a nominal coverage of 1.8 ML InAs at a substrate temperature of 530 C without rotating the substrate. This resulted in a gradient in dot density from zero to 64

65 7.2 Characterization of Low Density InAs-Quantum Dots 65 PL Intensity [a. u] energy [ev] RTA: 880C / 30s T = 77 K f d 5 mm p s 4.5 mm wavelength [nm] Figure 35: Low temperature PL spectra of InAs/GaAs-low density dot for a non-resonant excitation energy (1.95 ev). The PL emission lines at 1.45 ev originate from the InAs-wetting layer and the different peaks denotes by (s, p, d,..) belong to transitions from highly excited states in QDs cm 2. For single dot spectroscopy, an appropriate wafer pieces with low quantum dot density was chosen for the experiment (see Table 3). Fig. 35 depicts the PL emission spectra of such a sample cut off from the region on the wafer with low density of quantum dot ( cm 2 ). At an excitation of E = 1.95 ev, five emission peaks at higher energy, almost equidistantly spaced, appear. One of the luminescence peaks belongs to the ground state (E = 1.29 ev) and the further transitions are attributed to the higher dot levels and are interpreted as a successive filling of higher states in QDs. These lines appear at 1.32, 1.34, 1.37, and 1.40 ev. The strong emission line at 1.45 ev originates from the InAs-WL, which is a highly strained quantum film of about 1 ML thickness. In the frame of this thesis a new concept for lateral field generation was demonstrate, by using lateral p-i-n devices with embedded InAs QDs in the intrinsic region. The well-controllable lateral electric field was generated by in-plane p- i-n junctions, fabricated by implanting Si 2+ ions (n-type region) and Be + ions (p-type region) into a heterostructure containing InAs QDs. Before starting the

66 66 7 SINGLE DOT SPECTROSCOPY electric field [kv/cm] µ 50.0µ p-i-n device d = 2 µm T = 4.2 K current [µa] µ µ voltage [V] Figure 36: Current-Voltage characteristics at 4.2 K of the low dot density device. The intrinsic region of the p-i-n diode is here 2 µm wide. device fabrication process, the sample were annealed at 880 C for 30 s, to blueshift the QD emission to fit the sensitivity range of the available detectors. The detail of the processing was described in chapter 4). The width of the intrinsic region was (2-4) µm which allows the realization of a lateral electric fields higher than 10 5 V m 1. Current-Voltage Characteristics The probing of the current flowing through the junctions is a prerequisite procedure to test the functionality of a p-i-n diode. For this purpose, a voltage was applied to the lateral gates of the diodes and the current was measured at low temperature (4.2 K). In this way, the maximum electric field, which can be applied to the device was determined. A short presentation of the setup used for this kind of investigation is described in chapter 4.6. For the investigated device with low dot density in the optically active area, the current flows, if the n-type material is connected to the negative terminal of a power supply (forward bias). But, in the same time, from theoretical point of view, no current can flow until the contact-potential field is overcome. Therefore, no significant current occurs

67 7.3 Lateral Quantum Confined Stark Effect 67 in the forward direction until the external voltage source exceeds 1 volt (Fig. 36). Afterwards, any increase in externally applied voltage produces a large increase in current when the electrons and holes ride toward the junction. There, under the influence of the electric field they are swept across the junction, afterwards they recombine. If the n-type gate is connected to the positive terminal and p-type to the negative (reverse bias), the electric field drives the electrons and holes away from the junction, enhancing the field due to the contact potential. No essential current is observed, until a certain voltage is reached (breakdown voltage). The lateral p-i-n device with 2 µm wide intrinsic region reveals a welldefined current-voltage characteristic with breakdown voltages exceeding 20 V which corresponds to an electric field of about 110 kv/cm. Assuming that the complete applied voltage drops homogeneously over the intrinsic region, we have calculated the electric field in the intrinsic region according to F = (U bi U)/d. 7.3 Lateral Quantum Confined Stark Effect The basic idea of applying a lateral electric field starts from the physical point of view, that the dot height is usually much smaller than its lateral extension. Thus the dot states should be more sensitive to lateral electric fields than to vertical ones. Wolst and co-workers concluded from their measurements that the lateral Stark shift is very small and could not be detected in their ensemble measurements. In our self-assembled InAs/GaAs quantum dots, the height is around (5 8) nm and size between (30-50) nm. So that is obvious the difference between height and diameter. From the experimental point of view, a lateral field is much more difficult to realize than a vertical field, because the lateral patterning methods do not allow such a precise band gap engineering as the state of the art epitaxial technique. The lateral dimensions have to be in the range of a few µm because, otherwise, the field will be screened by carriers due to background impurities which are always present. The vertical and the in-plane motions of the carriers are influenced by geometrical quantization, so that they can be separated, and the energies of the single particle states are given by two contributions: E = E z + E in plane. From Fig. 35 we can see, that the shells are approximatively equidistantly spaced in energy, which indicates that the in-plane confinement potential can be described by a parabola V (r) = 1 2 mω2 r 2. Here the in-plane confinement potential is assumed to have rotational symmetry, because of the nearly circular base shape of the used quantum dots. The behavior of carriers in self-assembled quantum dots is determined in general by a variety of effects. Fortunately, for structures based on GaAs, these effects as a function of distinctive characteristic interaction energies manifest, as: exciton binding energy (25-50 mev), kinetic energy ( mev), and fine structure effects (1 mev). Also the energies of carriers with external electro-magnetic field belong to this category. Such effects are Zeeman interaction of the carrier spin with magnetic

68 68 7 SINGLE DOT SPECTROSCOPY field or the Stark shift of the excitonic levels in an electric field. The optical access to manipulate the charge inside a quantum dot is based on statistical emission or re-emission process of impurities in the semiconductor matrix embedding the dots. One way to obtain a direct control over the charge inside QD is the electrical injection of single carriers [61], [62]. Another way to control the occupation of the SQD (single quantum dot) with individual carriers is to apply an external electrical field. Here, is presented how the eigenstates in InAs quantum dots can be manipulated in a well controlled way by applying such an external electric field. So, the charge of the carriers in a SQD has two important consequences: first, the carriers in the dot interact with each other via the Coulomb interaction, resulting in the formation of biexcitons. Secondly, although electron-hole pairs are charge neutral, they can couple to external electric fields via their permanent or induced dipole moment. In the aim of the quantum confined Stark effect, the energy shift E of the QD emission in an external electric field F including both, the linear and the quadratic Stark effect is described as: E = p el F + βf 2, (42) where p el and β are the components of the permanent dipole moment and the polarizability in the direction of the electric field F. From the analysis of the fieldinduced energy shift of the PL signal, information about the permanent and the induced dipole moment of the recombining carriers in the QD can be extracted. Concerning, the investigated lateral p-i-n device, an internal electric field take place, so-called built-in voltage U bi so that the magnitude of the voltage, which drops across the intrinsic region, is approximately equal to U bi U. The field strength across the intrinsic region when a bias voltage is applied is therefore given by equation: F = U bi U, (43) d where d is the width of the intrinsic region, U bi the built-in voltage of the diode and U the applied voltage. From this formula one can see that the reverse bias tends to increase the voltage drop across the i-region, while the forward bias tends to reduce it. This is due to the fact that by applying a reverse bias (when a negative voltage is applied to the p-region and positive to the n-region) the electrons and holes are pushed away from each other, increasing the space charge region (2.2 µm in case of 2D p-n junction comparatively with 0.1 µm for 3D) [64]. Experimental results about the quantum-confined Stark effect in quantum wells [72], [73], [74] show shifts up to several zero-field exciton binding energies. If, in this case, the field is applied along z - direction, at the same time with forcing the electrons and holes in opposite direction, the barriers (several hundred mev hight) prevent the exciton from breaking apart. From this physical point of view, the excitons are stable up to very high field strengths. These quantum confined excitons interact with the field and the shift to lower energy is clearly higher than

69 7.4 Photoluminescence Measurements under a Lateral Electric Field 69 for one applied in-plane field. The shift of the quantum dot energy levels with the electric field can be calculated with the help of perturbation theory introduced in chapter 3.3. The result is analogous to the quadratic Stark effect in atomic hydrogen: the levels shift to lower energy in proportion to F Photoluminescence Measurements under a Lateral Electric Field The single dot photoluminescence spectroscopy is a suitable technique to investigate the electronic structure of a single QD. The measurements were carried out with a CCD camera with S-1 cathode at a sample temperature of 10 K. For the excitation, a tuneable Ti:Sapphire Laser was used in CW-mode (continuous wave) as described in section 4.7. A sample with 2 µm wide of intrinsic region was excited with a light wavelength, whose energy below the GaAs band gap of 760 nm (1.63 ev) and under an as small as possible low excitation power 60 µw from which only 4 % get on sample, namely 2.4 µw. Such a small excitation power allowed us to avoid the screening field when the dot is occupied by several excitons. It is desired to have the diode couple with a single or only few QDs. The emission energy is expected to depend on the applied field according to the Stark shift and that should be tuned to the high wavelength with the increasing voltage. The photoluminescence spectra for the afore mentioned sample with different lateral electric fields are shown in Fig. 37 a). Single-dot ground state emission lines can be seen from four different dots. From top to bottom, the lateral reverse bias was increased from 0 V to 6 V in steps of one volt. A strong decrease of the photoluminescence intensity was found at the reverse bias of 6 V, which corresponds to a lateral electric field of 37.5 kv/cm. In order to see the PL emission line behaviour, the emission peak around ev were shown in Fig. 37 (b) to make the Stark shift of the ground state visible. It is evident, (from Fig. 37) that a clear shift of the emission towards lower energy appearing with the increasing of the electric field. All lines exhibit a shift of about 0.5 mev before the signal vanished at a reverse applied bias of 6 V. After a more detailed analysis, the peak positions of several emission features and the energies of the Stark shift as a function of the applied voltage have been depicted in Fig. 38. A remarkable property of all lines is that at an electric field of 20 kv/cm the shift in energy is 0.25 mev. For the lines visible at higher fields, the relative shift varies strongly and has a large readout error due to the low PLintensity. Considering the difference in the shift for distinct lines, one can distinguish between three possible situations: a) a setup resolution is at its limit for the observed shift, leading to a pixel-wise readout of the lineshift, b) screening effects, for which the carriers around a special dot screen the field and thus the effective field at the particular dot is reduced, and c) the shift of the energy levels is largely compensated by the decrease in the exciton binding energy,

70 70 7 SINGLE DOT SPECTROSCOPY b) PL intensity [a.u] exc. power: 60 W laser: 760nm 0 V -1 V -3 V -6 V 1,3062 1,3064 1,3066 1,3068 1,3070 energy[ev] T = 10 K a) d = 2 µm 0 V PL-Intensity [a. u] - 1 V - 2 V - 3 V - 4 V - 5 V e n e r g y [ e V ] - 6 V Figure 37: Single-dot ground-state photoluminescence for a p-i-n structure with an intrinsic region of d = 2µm is shown in a). The emission lines from different dots are observed. b) The dot lines around ev, marked in a) with an arrow are shown.

71 7.4 Photoluminescence Measurements under a Lateral Electric Field 71 a) relative shift [mev] 0,5 0,4 0,3 0,2 0,1 electric field [kv/cm] dot 1 dot 2 dot 3 dot 4 0,0 T = 10 K applied voltage [V] b) peak position [nm] dot 1 dot 2 dot 3 dot applied voltage [V] Figure 38: The ground state energy shifts a) and peak positions b) as function of the applied voltage for four different single quantum dot.

72 72 7 SINGLE DOT SPECTROSCOPY accompanied by a blue shift of the line. A more thorough analysis of the field dependence of the emission energy shown in Fig. 39, and we got the empirical model E(F) = E 0 pf βf 2, were F is the electric field, p is the dipole moment, and β is the polarizability. The parameters p and β of different dots are given in Table 4. A numerical analysis reveals p to be in the order of 0.3 Å, which is a factor of 10, smaller than the those under vertical fields. Assuming the electron being located on the top of the QD, the laterally applied fields are perpendicular to the dipole moment of electron and hole, and therefore, p is found to be small [43]. Thus, we could neglect the second term of the model, which then becomes E(F) = E 0 βf 2 and the polarizability values are close to the previous ones. The polarizability is found to be 0.08 eå. (kv/cm) dot p only Å eå eå (kv/cm) (kv/cm) Table 4: The fitting parameters. 7.5 Red Shift of Energy levels by Lateral Electric Field The confining potentials in vertical direction are different for electrons and holes, which leads to a spatial separation between them. As a consequence of this separation, a permanent dipole moment p = e r is induced and the influence of an external electric field is measured by the polarizability, i.e the separation between electrons and holes. In applications like emitters and detectors it is desirable to have big polarizabilities because of a large tuning range of the emission, while for coherent high resolution spectroscopy, small polarizability is needed. Also, investigating single QD emission the polarizability needs to be small due to the Stark effect contribution to the linewidth, from the electric fields generated by fluctuating charge. The sign of the dipole was first interpreted as a evidence of a strong indium concentration gradient, with the apex indium-rich and the base indium-poor, and secondly as a truncation in the capped dot relative to uncapped dots [43]. The experimental data show that when an electric field is applied to the quantum dots, the electron and hole are pulled towards the opposite side of the dot, resulting in an overall reduction in exciton energy. At the same time,

73 7.5 Red Shift of Energy levels by Lateral Electric Field dot 1 Fit dot 2 Fit Energy [ev] Energy [ev] x x x x10 6 Electric field [V/m] x x x x x10 6 Electric field [V/m] dot 4 Fit Energy [ev] x x x x x10 6 Electric field [V/m] Figure 39: Electric field dependence of the energy peak position for different quantum dot lines. The solid lines are fitting curves based on a quadratic dependence. the mean-electron-hole distance along the electric field increases, resulting in a reduced overlap between electron and hole wave functions. These facts cause a redshift of the exciton emission energy and a simultaneous decrease in the emission intensity which indicates a decrease in oscillator strength with increasing lateral field. The drop in PL intensity occurs because the electron and hole become separated before they are binding into an exciton. It means that the field is around 10 4 V/cm, which cause the energy drop across one exciton comparable to the exciton binding energy. The redshift is expected due to the lateral Stark effect but the magnitude is rather small, about 0.5 mev for 6 V reverse bias voltage (37.5 kv/cm in electric field). This is due to the fact that the shift is partly compensated by a decrease in exciton binding energy [69]. In the presence of a lateral electric field, the valence and conduction bands become tilted, and along with a spatial separation between electrons results in a decrease of the energetic difference between the electron and hole levels. By these measurements,

74 74 7 SINGLE DOT SPECTROSCOPY we could verify the prediction by Wolst and co-workers [39] that the lateral Stark shift should be quite small because the shift of the energy levels is largely compensated by a decrease in the exciton binding energy due to the reduced wave function overlap. As can be seen in Fig. 39, the energy shift can be well described by a pure quadratic dependence on the applied field F with a polarizability of eå No indication of the significant contribution of the linear Stark (kv/cm) effect due to a permanent dipole moment was found. We didn t observe any increase in line width which is a consequence of a field-induced tunneling of the carriers out of the QD. In particular interband transitions, involving both electron and hole states, are sensitive to the structural properties of self-assembled QDs leading to asymmetric electron and hole wave functions [65]. When an intra-dot excitation is used instead of above-barrier (wetting layer) a much smaller decrease of PL intensity with applied voltage is achieved. This implies that in later case, photogenerated carriers are swept out by the field before being captured in the dot according to K. Kowalik et al. [68]. In conclusion both parameters are important, the permanent dipole moment influences the excitonic oscillator strength, while the polarizability determines the sensitivity of the exciton energy under an electric field.

75 8 Summary Quantum dots (QDs) grown by self-assembled Stranski-Krastanow technique, have many favorable properties for both, new physics and device applications. These include high radiative efficiency, large confinement energies and energy level separations, high area densities, and their inclusion in a semiconductor matrix, permitting the efficient injection and extraction of carriers. Using an in-plane electric field, it is possible to tune the QD emission wavelength in resonance with a cavity, due to Stark effect. However, in a self-assembled quantum dot (SAQD) ensemble, there is a large inhomogeneous broadening of the energy spectrum due to the fluctuations of dot parameters, (50 100) mev. Therefore, to study excitonic phenomena, it is necessary to resolve the signal from an individual dot, namely using single-dot spectroscopy tools. Throughout this work, photoluminescence (PL) and time-resolved spectroscopy were used to investigate the effect of an in-plane (lateral) electric field applied to self-assembled InAs-quantum dots (QDs). The device fabrication combines electron beam lithography, focused ion beam implantation and standard optical lithography techniques. The distance between QDs and the surface of the sample was correlated with the energies for implantation so that the depth distributions are exactly at the position of the QD layer. The field was generated by in-plane p-i-n and n-i-n junctions, respectively, fabricated by implanting Si 2+ ions (n-type regions) and Be + ions (p-type regions) into a heterostructure containing InAs QDs. Afterwards, two pairs n-and p-type ohmic contacts have been used to apply an electric field in the plane of the quantum dot, located at 245 nm below the sample surface. In this way, various p-i-n as well as n-i-n devices have been achieved with the intrinsic region widths between (2 4) µm. Electrical investigations for p-i-n devices reveal well-defined diode I-V characteristics with breakdown voltage exceeding 20 V. As expected, an almost symmetric behaviour of the I-V characteristics for the n-i-n devices is found. The annealing effect of the sample has been investigated by photoluminescence measurements. The pieces were tempered at different temperatures (800 C, 820 C, 860 C, 880 C, 900 C, 920 C, 940 C and 960 C, ) for 30 s in a Nitrogen atmosphere. With increasing annealing temperature, a strong blue shift of the PL emission is observed but the intensity of the photoluminescence spectra decreases. The observed changes are attributed to a rearrangement of the atoms of which indium atoms are diffuse out while gallium atoms are diffuse into the quantum dots during the annealing process. This leads to a larger QD with lower indium content, i.e. a shallower and broader confinement potential. The shallower confinement potential leads to the blue-shift and the broader confinement results in a decreased level spacing detected as a reduced E. A study of single quantum dots in an external electric field was presented. Only the ground state (s-shell) of the QDs has been investigated. Increasing the electric field, two aspects are remarkable: First, the intensity drops with 75

76 76 8 SUMMARY increasing voltage and second, the PL shifts to the red. The reduction of the PL intensity can be explained by the separation of electron-hole pairs in a lateral field. This may decrease, the radiative recombination efficiency in the SQD, as well as the capture probability into the dot as the excitation was above the GaAs barrier bandgap. The overlap integral between electron and hole states decreases upon application of the electric field. A detailed inspection reveals a small frequency shift of approximately 0.5 mev, when the field is raised from 7.5 kv/cm to 37.5 kv/cm and a quadratic dependence on the electric field. The Stark shift is rather small because it is mostly compensated by a decrease in the exciton binding energy due to the reduced wave function overlap. There is no broadening of the line width due to the lateral carrier tunneling out of the dot. The magnitude of the Stark shift gives an estimation of the spatial separation of the electron and hole wave functions, e h = 0.3 Å, which is 10 times smaller than the value of 4 Å found for the vertical mode under flat-band conditions. In the literatures, the carriers lifetime has been investigated as a function of the temperature, annealing process, and QD size. This work presents the very first investigation of the radiative lifetime by applying a lateral electric field. An increase of the exciton lifetime by more than 30 % of the flat-band value is found when the voltage increases up to an electric field of about 180 kv/cm. This increased lifetime is attributed to a decrease in overlap of the electron and hole wave functions in the QDs. The field pushes the carriers in opposite directions, reducing, in this way, the wave function overlap and increasing the radiative lifetime. This increase in radiative lifetime is a clear proof that we really generate an electric field at the position of the quantum dots. This work demonstrates that the QD can be embedded in an optical resonator by changing the energy of the electronic transition from the conduction to valence band ground state of QD under the influence of an electric field. The spontaneous light emission can be controlled by bringing the transition in and out of resonance with a cavity photon mode. Such a device may be a central building block for single photon emitters which release a photon on demand, as required for quantum cryptography or quantum computing.

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85 List of Figures 1 A typical communication channel: The source A produces information which is manipulated (encoded) and then sent over the channel. At the receiver B, the message is decoded and the information extracted Schematic comparison of typical dimensions of bulk material, quantum well and quantum dots The density of states as a function of energy for an effective material dimensionality of a) 3 (bulk), b) 2 (well), c) 0 (dot) The adiabatic approximation for electrons is sketched here The presence of an uniform electric field has the effect of adding a linear term W to the potential energy V of the harmonic potential 18 6 Energy spectrum of an 2D harmonic oscillator for electrons and holes Band diagram for a quantum well with flat bands(a) and bands tilted by an electric field, which lowers the energy of both bound states(b).the envelope wavefunction of an electron and heavy-hole ground state is shown A sketch of a p-i-n diode diagram. The voltage V 0 is applied to the p-region, so that positive and negative V 0 correspond to forward and reverse bias respectively Band alignments in a p-i-n diode structure with an i-region thickness of l i. a) Voltage = 0. b) Reverse voltage applied.the thick dashed lines indicate the Fermi levels. E g is the band gap of the semiconductor used for the p- and n- regions Schematic diagram of III/V-MBE system. After Sze [[41] Three possible growing modes are sketched. Frank-van der Merwe, Volmer-Weber, and Stranski-Krastanow Scanning Electron Microscopy (SEM) image of an array of selfassembled InAs/GaAs quantum dots (sample 11376). For the microscopic investigations the dots were grown on the sample surface. A quantum dot of (30-50) nm in diameter is extracted TEM picture of lens-shaped quantum dot [68] Schematic diagram and properties of the InAs/GaAs quantum dot of material The quantum dots are formed in the thin InAs layers sandwiched between GaAs layers which have large band gap. The mentioned wavelength in the table is the PL emission wavelength of QDs Schematic design of the Focus Ion Beam system EIKO The implantation depth profile determined by a STRIM simulation The pattern overview of lateral p-i-n device

86 86 LIST OF FIGURES 18 Experimental setup used for the observation of photoluminescence (PL). The sample is excited with a laser with a photon energy greater than the GaAs band gap Schematic diagram of the process occurring during photoluminescence in InAs-quantum dots. The exciting photon creates an electron-hole pair A typical PL spectrum of an InAs/GaAs quantum dot ensemble used for investigation in this thesis. The PL measurement is performed at room temperature and on a sample with high density of quantum dots Layer structure of a sample with 10-InAs-QDs layers embedded in a GaAs-matrix The photoluminescence spectrum at 300 K for an as-grown sample with a 10-layers QD array. The wavelength in nm for the interband transitions is indicated at the corresponding peaks The photoluminescence spectra at 77 K for an as-grown sample with a QD stack (dot trace a) and after RTA treatment (black traces b, c, d,...). The annealing temperatures are in the range of 800 C and 960 C. The wavelength in nm for the s-s transitions of individual samples is indicated at the corresponding peaks. The measurements have been performed using a SP 500M spectrometer and a laser diode with 635 nm wavelength as excitation source The effects of intermixing on the QD shell structure for sample The peak positions of the QD emission (triangle trace) and the intersublevel energy spacing (square trace) are plotted as a function of RTA temperature a) Non-resonantly excited PL spectra at 300 K for an as-grown sample (red trace) and after RTA treatment(black traces). The PL intensity was normalized to 1 and the individual traces have been set-off for clarity. The inserts show the expected QD form. b) The full width at half maximum (FWHM) of the s-s peaks is plotted in The InAs/GaAs-QD band diagram at 300 K, before and after 960 C annealing temperature The photoluminescence spectra at 300 K for a QD sample grown without rotation resulting in a density gradient. The smallest intensity PL peak corresponds to a low density dot (the black trace). The arrow is inverse to the In gradient (a, e, f, b, d: high QD density, c low QD density). WL denotes the emission of the wetting layer Schematic complete p-i-n and n-i-n device in a) and a microscope image of the identical device in b)

87 LIST OF FIGURES I-V characteristics of the investigated a) p-i-n (2 µm) and n-i-n, b) (3 µm), c) (4 µm) structures. A diode-like behaviour has been observed. All the curves are measured at 4.2 K. The crystallographic directions are indicated for each graph apart The photon counting dependence of the negative applied voltage Electric field dependence of the radiative lifetime in a p-i-n structure with 3 µm intrinsic width measured at T = 10 K. The excitation wavelength of 835 nm was chosen to avoid the screening effect from the GaAs carriers a) The radiative lifetime determined by time-resolved photoluminescence measurements for a p-i-n diode with 2 µm wide intrinsic region measured at T = 10 K. The delay dependence of the fundamental transition at the 8 µw excitation power for four selectively applied voltage (+1 V, -2.5 V, -15 V, -35 V) The radiative lifetime dependence of the applied voltage a) and in b) the time-resolved PL recorded on a n-i-n structure with 4 µm intrinsic region width measured at T = 10 K Electric field dependence of the radiative lifetime in a n-i-n structure with 3 µm measured at T = 10 K. The excitation wavelength was chosen to avoid the screening effect from the GaAs carriers Low temperature PL spectra of InAs/GaAs-low density dot for a non-resonant excitation energy (1.95 ev). The PL emission lines at 1.45 ev originate from the InAs-wetting layer and the different peaks denoted by (s, p, d,..) belong to transitions from highly excited states in QDs Current-Voltage characteristics at 4.2 K of the low dot density device. The intrinsic region of the p-i-n diode is here 2 µm wide Single-dot ground-state photoluminescence for a p-i-n structure with an intrinsic region of d = 2µm is shown in a). The emission lines from different dots are observed. b) The dot lines around ev, marked in a) with an arrow are shown The ground state energy shifts a) and peak positions b) as function of the applied voltage for four different single quantum dot Electric field dependence of the energy peak position for different quantum dot lines. The solid lines are fitting curves based on a quadratic dependence The complete device fabrication step

88 List of symbols symbol meaning k F Fermi wave number λ F Fermi wavelength h Planck constant m effective mass k Boltzmann constant T Temperature p Impulse m angular momentum ω Pulsation ν Frequency 1 D One-Dimensionality system 2 D Two-Dimensionality system 3 D Three-Dimensionality system DOS Density of the state U bi built-in voltage U 0 applied voltage a lattice constant F electric field d intrinsic region width l i intrinsic region width µ el Permanent dipole moment α polarizability QDs Quantum dots SQDs Single quantum dot QCSE Quantum confined Stark effect WL Wetting layer M L Monolayer I V Current-Voltage DOS Density of state E Energy E Energy difference between states V (x) Potential energy E F Fermi energy E g Band gap energy SK Stranski-Krastanow E V Valence band Conduction band E C 88

89 SYMBOLVERZEICHNIS 89 symbol meaning R Resistance I Current electric U Voltage EBL Electron beam lithography FIB Focus ion beam implantation MBE Molecular beam epitaxy D Dose P L Photoluminescence V Volt RTA Rapid thermal annealing ev electron volt FWHM full width at half maximum GaAs Gallium Arsenide InAs Indium Arsenide mev milli electron volt λ wavelength µm micrometer QW quantum well L He liquid-helium

90 90 LIST OF TABLES List of Tables 1 Number of degrees of freedom tabulated against the dimensionality of the quantum confinement Basic properties of GaAs and InAs at 300 K Properties of the InAs/GaAs quantum dot of material The mentioned depth is the distance between surface and the quantum dot layer The fitting parameters

91 A Heterostructures The growth sheet reports of the samples used in this thesis are described in this annexe. The materials were grown in our MBE system. The wafer no is grown on a GaAs (100) substrate which contain 10 InAs- quantum dots layers embedded in a GaAs matrix. On this wafer were carried out the annealing investigations. The material no is a GaAs/AlAs heterostructure with a single InAs- quantum dots layer. The wafer was grown without rotating during the MBE growth process resulting in a dot density gradient that vary between 0 and cm 2. On this wafer were achieved time-resolved measurements and single dot photoluminescence spectroscopy Thickness Material Temperature Function 50 nm GaAs 660 C buffer 2 nm AlAs 660 C 2 nm 20x GaAs 660 C supper lattice 430 nm GaAs 660 C 1.8 ML InAs 598 C InAs-QDs 8 nm GaAs 588 C covering the ODs 112 nm GaAs 660 C 2 nm AlAs 660 C 2 nm 20x GaAs 660 C supper lattice 10 nm GaAs 660 C 3 nm AlAs 660 C 50 nm GaAs 660 C cap layer 1.8 ML InAs 598 C InAs-QDs for - topographic investigation 91

92 92 A HETEROSTRUCTURES first part Thickness Material Temperature Function 120 nm GaAs 635 C buffer 0.9 nm AlAs 635 C 1.2 nm 20x GaAs 635 C supper lattice 0.9 nm AlAs 635 C 4.9 nm 20x GaAs 635 C supper lattice 120 nm GaAs 635 C 1.8 ML InAs 545 C InAs-QDs 9.6 nm GaAs 535 C covering the ODs nm GaAs 635 C 1.8 ML InAs 545 C InAs-QDs 9.6 nm GaAs 535 C covering the ODs nm GaAs 635 C 1.8 ML InAs 545 C InAs-QDs 9.6 nm GaAs 535 C covering the ODs nm GaAs 635 C 1.8 ML InAs 545 C InAs-QDs 9.6 nm GaAs 535 C covering the ODs nm GaAs 635 C 1.8 ML InAs 545 C InAs-QDs 9.6 nm GaAs 535 C covering the ODs nm GaAs 635 C 1.8 ML InAs 545 C InAs-QDs 9.6 nm GaAs 535 C covering the ODs

93 93 second part Thickness Material Temperature Function nm GaAs 635 C 1.8 ML InAs 545 C InAs-QDs 9.6 nm GaAs 535 C covering the ODs nm GaAs 635 C 1.8 ML InAs 545 C InAs-QDs 9.6 nm GaAs 535 C covering the ODs nm GaAs 635 C 1.8 ML InAs 545 C InAs-QDs 9.6 nm GaAs 535 C covering the ODs nm GaAs 635 C 1.8 ML InAs 545 C InAs-QDs 9.6 nm GaAs 535 C covering the ODs nm GaAs 635 C 1.8 ML InAs 545 C InAs-QDs 9.6 nm GaAs 535 C covering the ODs nm GaAs 635 C 0.9 nm AlAs 635 C 1.2 nm 20x GaAs 635 C supper lattice 12 nm GaAs 635 C cap layer

94 B Processing parameters Standard cleaning put the sample into a beaker of acetone and put it into the ultrasoundbath: 2 min spin on and clean it ones again with acetone and isopropanol: 5 min drying with air Photolithography Standard cleaning Photoresist coating: spin on positive photoresist Shipley SP2510 for 30 s at 4200 rpm and dry it in the oven at 100 for 15 min spin on lift-off-resist (microposit-lift-off) at 3800 rpm for 30 s and dry it on the hotplate at 150 C for 5 min photoresist illumination: 40 s photoresist development: s spin on and clean with acetone and isopropanol dry with air Wet-chemical etching sulfuric acid solution: H 2 O : H 2 O 2 : H 2 SO 4 (50 : 1 : 1) The etch rate for large GaAs areas is about 75 nm/min. Evaporation of the metallization Evaporation Ohmic n-contact: 10 nm Ni, 60 nm Ge, 120 nm Au, 10 nm Ni, 100 nm Au Evaporation Ohmic n-contact: 40 nm Au, 40 nm Zn, 200 nm Au Sample cutting cutting of mm by a diamond tip Annealing step at 880 C for 30 s Alignment-marks definition for EBL standard cleaning 94

95 95 Photolithography Evaporation 60 nm Au Electron beam lithography definition of the implantation windows Focus ion beam implantation definition of the n- and p-type regions Annealing at 720 C for 30 s electrical activation of the implanted ions Mesa definition standard cleaning photolithography wet-chimical etching standard cleaning Definition of the ohmic contacts standard cleaning photolithography evaporation of the metallization first n-type afterwards p-type ohmic contacts lift-off standard cleaning Sample bonding fixation on chip carriers bonding with 25 µm thick Al wire

96 96 B PROCESSING PARAMETERS A summaries of the fabrication steps afore-mentioned. rapid thermal annealing C blue-shift of emission wavelength Bonding EBL - definition of the implantation windows - resist PMMA - acceleration voltage 20 kv Mesa etching - selective wet chemical etching - 50 nm GaAs in implantation window is removed (intrinsic region forms ridge) optical lithography - definition of mesas and contacts fabrication of the ohmic contacts in two steps: - n-type: NiGeAu - p-type: ZnAu FIB implantation Si n-type (5.5 x 10 cm, 200 kev) Be p-type (2.2 x 10 cm, 60 kev) - intrinsic region is protected by resist! rapid thermal annealing 720 o C electrical activation of implanted atoms Figure 40: The complete device fabrication step.