Epitaxie von Halbleiternanostrukturen

Transcription

1 Hauptseminar des IHFG: Nanooptik & Nanophotonik SS 2016 Epitaxie von Halbleiternanostrukturen April 2016

2 Modern Epitaxy: Application on Quantum Dots Thomas Klumpp and Supervisor: Dr. Michael Jetter University of Stuttgart and IHFG (April 7, 2016) Epitaxial thin films have attracted considerable attention in recent years in the search for new materials and represent the basis for the fabrication of semiconductor heterostructures and devices. Thereby epitaxy, namely, the growth process of a solid film on a crystalline substrate in which the atoms of the growing film mimic the arrangement of the atoms of the substrate, is one of the most important issues in thin film technology. Since it offers the opportunity to create metastable structures, with novel physical and chemical properties. The structure and the properties of the film can be manipulated by the geometric and electronic structure of the substrate. Especially important is the case of heteroepitaxy, i.e., the epitaxial growth of a solid film differing from the substrate crystal with respect to its chemical structure. The importance of epitaxy concerns both, fundamental research on thin film growth processes and the application of these procedures to grow high quality layers from different materials for the realization of technically important functions. This concerns also the development of a series of epitaxial growth techniques applied in different branches of solid state electronics, optoelectronics and photonics in manufacturing processes of discrete as well as integrated devices. Introduction Starting in the 19th century, where mineralogists noticed that two different naturally occurring crystal species sometimes grew together with some define and unique orientation relationship, as revealed by their external forms. These observations led to attempts to reproduce the effect artificially, the first recorded successful attempt was reported in 1836 by Frankenheim [1], followed from early studies of alkali-halide overgrowth in the beginning of the 20th century, basic concepts for lattice match between layer and substrate were developed in the late 1920ies. The discovery of X-ray diffraction (1912) and electron diffraction (1927) by crystals had a strong impact on the knowledge about crystal structure. Based on the reviews on natural overgrowth phenomena and structural data from X-ray diffraction studies, Louis Royer established in 1928 the conditions for orientated overgrowth, defining the term epitaxy, which has the Greek roots επι (epi) meaning upon or above, and τ αξισ (taxis) meaning an ordered manner and concluded general rules for epitaxy. It could also be translated as arrangement on. Royer formulated the following rule of epitaxy: epitaxy occurs only when it involves the parallelism of two lattice planes that have networks of identical or quasi-identical form and of closely similar spacings [1]. Followed by the theory of misfit dislocations introduced about 1950, experimental data gained later, indicated that epitaxy occurs if the lattice misfit (or, lattice mismatch) f, defined as f = a s a l a l, (0.1) where a s and a l are the corresponding network spacings (lattice constants) in the substrate and film (layer), respectively, is not larger than 15 %. This geometrical approach to the understanding of epitaxy, introduced by Royer, has remained prominent to the present day [1], though it was established later that epitaxy may also occur for much larger misfits [2]. In order of completeness it should be noted, that there are also alternative definitions of the lattice mismatch f found in the literature, particularly these are and f alternative1 = a l a s a s, (0.2) f alternative2 = a l a s a l. (0.3) Note the change in sign of the alternative relations with respect to eq. (0.1). In the 1930ies, G.I. Finch and A.G. Quarrell concluded from a study of zinc oxide on sputtered zinc, that the initial layer is strained in order to attain lattice matching parallel to the interface. The layer latticeparameter vertical to the interface was also considered to be changed to maintain approximately the bulk density. They named this phenomena pseudomorphism [2]. Though later the experimental evidence was pointed out to be by no means conclusive [3], the concept of pseudomorphic layers proved to be of basic importance for epitaxial structures [2]. Major progress in epitaxy was achieved by technical improvements of the growth techniques, namely, liquid phase epitaxy in the early, and molecular beam epitaxy (MBE) and metalorganic vapor phase epitaxy (MOVPE) 1

3 FIG. 2. Schematic of a heterostructure, consisting of two layers with a common interface, where a is the common lateral lattice constant, a 1 and a 2 denote the vertical lattice constants of the strained layer 1 and 2. FIG. 1. The two epitaxy types: homoepitaxy and heteroepitaxy. While a s and a l are the corresponding lattice constants in the substrate and epitaxial layer. Shown are latticematched growth on the left, and strained coherent growth on the right. in the late 1960ies. Current tasks for epitaxial growth are often motivated by needs for the fabrication of advanced devices, aiming to control carriers and photons [2]. An overview of the different growth techniques is given later, in the Epitaxy Methods section. Most semiconductor devices that are fabricated today, are made out of a thin stack of layers, with a typical total thickness of only a few µm including stuctures of reduced dimensionality on a nanometer scale. Layers in such stack differ in material composition and may be as thin as a single atomic layer. All layers are to be grown with high perfection and composition control, on a bulk crystal used as a substrate. The growth technique employed for coping with this task, is termed epitaxy [2]. Semiconductor devices control the flow and confinement of charge carriers and photons. To fulfil its function, a device is composed of crystalline layers and corresponding interfaces with different physical properties. Epitaxy is employed to assemble such layer structure. If an epitaxial film (also called epitaxial layer or active layer) is deposited on a substrate of the same composition, the process is called homoepitaxy, otherwise if the active layer differs with his lattice constant from the lattice constant of the substrate it is called heteroepitaxy. In more detail, homoepitaxy is a kind of epitaxy performed with only one material, in which a crystalline film is grown on a substrate or film of the same material, it is also referred as lattice-matched growth. This technology is used to grow a film which is more pure than the substrate and to fabricate layers having different doping levels. While heteroepitaxy is a kind of epitaxy performed with materials that are different from each other. In heteroepitaxy, a crystalline film grows on a crystalline substrate or film of a different material, which forces the epitaxial layer lattice to be strained. This technology is often used to grow crystalline films of materials for which crystals cannot otherwise be obtained and to fabricate integrated crystalline layers of different materials. Examples include silicon on sapphire, gallium nitride (GaN) on sapphire, aluminium gallium indium phosphide (AlGaInP) on gallium arsenide (GaAs) or diamond or iridium. FIG. 1 depicts the two different epitaxy types. Properties of Heterostructures Looking closer on the properties of heterostructures one can observe the so called pseudomorphic heterostructures. If we consider a free standing crystalline structure consisting of two layers, which have a common interface as delineated in FIG. 2 and we assume the layers to have the same cubic crystal structure, but (in absence of the common interface) different unstrained lattice constants a and thickness, labelled a 1 and a 2 respectively. If the difference in lattice constants is not too large (say, below 1 % [2]), the layers may form an interface without structural defects and adopt a common in-plane lattice constant a parallel to the interface, with an intermediate value a 1 > a > a 2. Since a is smaller than the relaxed (unstrained) lattice parameter a 1, layer 1 is compressively strained in lateral direction (i.e., parallel to the interface) by the contact to layer 2. Layer 1 consequently experiences a distortion also in the vertical direction to approximately maintain its bulk density. The vertical lattice constant a 1 of the strained layer 1 is hence larger than the unstrained value a 1. Vice versa layer 1 exerts a laterally tensile stress on layer 2, leading likewise to a vertical strain a 2 < a 2. As mentioned before such a heterostructure is called pseudomorphic, and the layers are designated coherently strained. The lattice misfit f may have either sign, as illustrated in FIG. 3 by comparing a compressively and a tensely strained layer. 2

4 toward the growth front (or crystallization interface). In order to keep the crystallization process running, the driving force of crystallisation, i.e., the local supersaturation of the metastable phase, should be ensured in the area of this interface. Youngs Relation & Wetting Angle φ The local supersaturation of the metastable phase leads to contact angle with the substrate. This angle is the so called wetting angle φ, which depends on the interface tensions respectively surface energies γ. We assume a nucleus with the shape of a spherical cap with radius r on a solid substrate, as shown in FIG. 4. The balance FIG. 3. Biaxially strained layers (red atoms) on substrate (blue atoms) with another lattice constant a s. On the left side the unstrained lattice constant of the layer a l is larger than a s, and the layer is compressively strained in lateral direction; while on the right side the layer is tensely strained. Theoretical Basics The leading question is, how the homo- and heteroepitaxial crystallisation phenomena works. Epitaxy, in common with all forms of crystal growth, is in fact a well controlled phase transition, which leads to a single crystalline solid [3]. Consequently, formation of an epitaxially grown deposit constitues the creation of a new phase [4]. This is accomplished through a nucleation and growth relationship between two crystalline phases, which makes it possible for a crystalline phase e (epilayer) to grow in a structure-dependent manner onto a crystalline phase s (substrate) of given structure. In general, an interracial region which is chemically and structurally inhomogeneous is then developed. In principle, in the completed epitaxial growth reaction, there exists a two-phase system consisting of two adjacent heterochemical (heteroepitaxy) or isochemical (homoepitaxy) epitaxial partners, i.e., the epilayer e and the substrate s [1]. The phenomenon of epitaxial crystallization is based on a few key processes, which lead to parallel oriented growth of a single crystalline layer on a crystallographically oriented single crystal surface [1]. First, and most general, is the phase transition between the metastable phase and the epilayer which has to be grown. This process is usually related to mass transport of the constituent species from the bulk of the metastable phase FIG. 4. Nucleus created on a substrate. The balance of surface energies γ lead to a wetting angle φ. of interface tensions, at the line of contact between the three phases metastable ambient phase (index a) solid nucleus (n) solid substrate (s) is given by the three quantities, which represent the energies needed to create the unit area of each of the three interfaces. From FIG. 4 we read Youngs relation, for the absolute values of tensions in balance. γ as = γ ns + γ an cos(φ) (1.1) The balance of the surface energies γ lead to the wetting angle φ. cos(φ) = γ as γ ns γ an (1.2) The wetting angle φ may vary between 0 and 180 depending on the degree of wetting, corresponding to the affinity of nucleus and substrate materials. Hence φ determines the shape of the nucleus. The surface energies leading to Youngs relation (eq. (1.2)) affect the initial stage of layer deposition on a substrate of different material. Epitaxy usually aims at depositing a layer with 3

5 a smooth growth surface. This would correspond to a wetting angle of 0 in Youngs relation, thus arises cos(0) = γ as γ ns γ an γ as = γ ns + γ an. If this condition applies, or γ as exceeds the sum of the two other interface energies we obtain complete wetting of the layer on the substrate. This condition also implies that layer atoms are more strongly attracted to the substrate than to themselves. Growth may then proceed in an atomically flat layer-by-layer mode referred to as Frank-van der Merwe growth mode, which will be discussed later in more detail. Low or non-wetting of the layer on the substrate will occur when the layer atoms are more strongly attracted to themselves than to the substrate, this situation is expressed in Youngs relation by a wetting angle of π, (a) cos(π) = γ as γ ns γ an γ as = γ ns γ an. One can see the change of sign in Young s relation between these two different wetting situations. Going back from this fluid model to the molecular or atomic picture of the phase transition, one can see that the growth process in epitaxy happens due to mass transport towards the crystallization interface and nucleation of the epilayer. In the atomic picture the growth process is by definition related to atomic ordering, which leads to creation of the first atomic or molecular monolayer of the growing film. Atomic ordering is a surface kinetic process, which is strongly dependent on the structure and the chemical activity of the substrate surface [1]. A schematic illustration of the geometrical configuration of the epitaxial growth system, in different time periods, is shown in FIG. 6. (b) FIG. 6. Schematic illustration of the geometrical configuration of the epitaxial growth system in different time periods of crystallization. (a), it is shown the nucleation period related to growth of the first monolayer. (b), early stage of epitaxy, when a thin epilayer has already been grown. processes that take place directly on the surface, can be broken down as the following: adsorption, migration on the substrate surface, formation of new growth nuclei (nucleation), Installation of atoms in the crystal lattice (through incorporation, diffusion) desorption. FIG. 5. Microscopic processes during the epitaxial growth on the surface. [5] Apart from the dissociation and the diffusion of the starting substances, there can be a variety of chemical processes in the gas phase. Above all, the processes on the substrate surface are important, which can be described by a microscopic model, which takes into account only the movement and the balance of the atoms. The FIG. 5 illustrates the microscopic processes during the epitaxial growth on the surface. Due to the less strong bond of group-v elements and their oversupply necessary for stable growth conditions, the behaviour of group-iii elements is the determining factor. The surface morphology that forms is determined by the type of installation of adsorbates, therefore there are essentially two ways. For an ideal two dimensional layer growth the incorporation is preferably carried out at the steps of the substrate. The incorporation efficiency depends on the number of free bonds, which are provided by the step or neighbouring atoms and the underlying layer. Thus, this is dependent on the disorientation of the substrate. The other possibility for atom implant is 4

6 realized by forming nucleation or growth nuclei. Here diffusing atoms on the surface build two dimensional islands together, which are in competition to the steps offered by the substrate. Diffusion Constant The average distance λ, that an atom on the substrate surface can cover, is called diffusion length and is defined as λ = Dτ, (1.3) while τ is the mean lifetime (throughout installation in the semiconductor crystal and the desorption, the average lifetime τ is limited). Using an Einstein-model, which includes adsorption, nucleation and desorption, the behaviour of the surface diffusion constant D can be observed and it can be shown that the following proportionality applies D 2k BT h ( exp E ) A, (1.4) k B T where T is the temperature, E A the activation energy for transportation, h the Planck constant and k B the Boltzmann constant. One can see, if the growth temperature is increasing and therefore the diffusion length (according to eq. (1.4) and (1.3)), one can obtain almost planar surfaces. Thus the ratio of the step density to the diffusion length is largely responsible for the surface morphology [6]. In FIG. 7 the situation is depicted. However, when Growth Rate A low diffusion length λ can also be the result of a large growth rate R [10]. All of the aforementioned processes are associated with specific time constants, which determine the growth rate R = ML t, (1.5) where ML are monolayers, and t stands for the time. As already mentioned, a larger growth rate reduces the average lifetime τ and hence the diffusion length λ. R 1 τ R D λ 2 That is reasonable, because with a high density of adsorbates on the surface, the probability of a collision of several atoms increases, which reduces the average lifetime τ (respectively λ, see eq. (1.3)). Through very high group-v/iii element ratio, the diffusion can be reduced on the surface. Depending on the nature of the atom the activation energy E A also varies for a course change, and thus, the diffusion constant D (see eq. (1.4)). For example, gallium and indium have much lower activation energies and thus large diffusion lengths as opposed to aluminum. However, the growth rate is highly dependent on the temperature, as illustrated in FIG. 8. At low temperatures, FIG. 7. Schematic illustration of the two layer-by-layer growth modes, where λ is the diffusion length and l is the terrace width. Furthermore the relation between the surface morphology and the RHEED[7]/RAS[8]/GIXS[9] intesity behavior is shown in the small graph on top. [6] the mean free path λ drops below the distance between steps (λ l), rise of nucleation on free surface is obtained (island nucleation). This is also possible by lowering the temperature (remember eq. (1.4)). On the other hand, if the diffusion length is larger than the terrace width l (λ l) incorporation in the steps is preferred. FIG. 8. Growth rate of GaN layers in the MOVPE depending on the growth temperature. [11, 12] the growth is limited by the rate of kinetic decomposition of the reactants (kinetically limited) ( R exp E ) A, (1.6) k B T and the surface reactions. With increasing temperature, the growth rate increases exponentially and gets indepen- 5

7 dent of temperature. In this plateau region, the growth rate is limited only by the supply of starting materials. The growth in the MOVPE, takes normally place in this transport limited region. At still higher temperatures, the growth rate decreases again, because of increased desorption of the atoms on the surface. In addition to temperature, the growth also depends on the prevailing pressure in the reactor and on the source materials that are used. Remembering the targeted nucleation on the surface (which is dependent on λ), that we want to use during epitaxy, one chooses growth temperature, group-v/iii element ratio, and misorientation of the substrate according to its application. Thus, a laser active region of a semiconductor requires high nuclei- or quantum dot (QD) densities (in order to achieve the required space factor), whereas for single-photon emitters a low nucleation or QD density is desirable. Finally, one wants to be capable of controlling the QD density over adjustable parameters in a manageable process. This can be realized throughout three well understood growth modes. Growth Modes in Epitaxy The growth process of thin epitaxial films is essentially the same as that of bulk crystals, except for the influence of the substrate at the initial stages. This influence comes from the misfit and thermal stress, from the defects appearing at the crystal-film interface and from the chemical interactions between the film and the substrate including segregation of the substrate elements towards the film surface [1]. Three common modes of crystal growth may be distinguished in epitaxy, as illustrated in FIG. 9. These are, the Volmer-Weber mode (VW-mode), the Frank-van der Merwe mode (FM-mode), and the Stranski-Krastanov mode (SK-mode). The mode by which the epitaxial film grows depends, as mentioned before, upon the lattice misfit between substrate and film, the supersaturation of the crystallizing phase, the growth temperature and the adhesion energy. In the VW-mode, or 3D island growth mode, small clusters are nucleated directly on the substrate surface and then grow into islands of the condensed phase. This surface morphology of the layer is observed if the layer atoms are more strongly attracted to each other than to the substrate (this situation is expressed in Youngs relation by a wetting angle of π, or γ as γ ns γ an ). If this condition applies or γ ns is even larger, then the layer does not wet the substrate surface. The surface energy of layer plus substrate is minimized if a maximum of the substrate surface (with a low surface energy γ as ) is not covered by layer material (which has a large surface energy γ an ). In FIG. 9 one can see, that the layer material is deposited in form of islands, which for thicker deposits eventually coalesce. Here nucleation proceeds by 3D nuclei. The FM-mode displays the opposite characteristics. Because the atoms are more strongly bound to substrate than to each other, the first atoms to condense from a complete monolayer on the surface, which becomes covered with a rather less tightly bound second layer. Growth may then proceed in an atomically flat layer-bylayer mode. FIG. 9 illustrates the initial stages of such two-dimensional layer growth for different thickness of deposited material. Nucleation in this case proceeds by the formation of 2D islands or step advanced as outlined earlier. FIG. 9. Schematic of the three growth modes, illustrated as a function of approximately equal coverage Θ given in units of monolayers (ML). 6

8 The SK-mode, or layer plus island growth mode, is an intermediate case. Here the condition γ as γ ns + γ an for FM-mode growth applies solely for the first deposited monolayer (or the first few monolayers). After exceeding some critical coverage thickness the growth changes to the VW-mode case where γ as γ ns γ an applies. Such change may be induced by the gradual accumulation of strain in the epitaxial layer [2]. The layer material then resumes growth in form of 3D islands, leaving a 2D wetting layer underneath. It should be noted that the critical thickness pointed out here lies usually below that required for the formation of misfit dislocations [2]. Stranski-Krastranov growth has gained much advertence in recent years, because it may be employed for growth of defect free quantum dots [2]. In heteroepitaxy we distinguish mainly between these three different growth modes, they arise from a thermodynamic consideration of the interface energies. Ostwald Ripening The previous discussion, about the simple viewing of energy minimization in thermodynamic equilibrium is often not enough to understand the structure of the quantum dots. The self-organized growth of quantum dots is influenced by the growth kinetics (SK-mode). The self- organized formation of the quantum dots can be divided into three phases: phase 0: layer growth with surfaces nucleation, phase 1: formation of 3D structures from the growth nuclei, phase 2: ripening of the quantum dots, phase 3: formation of dislocations and formation of relaxed islands, which are also represented in FIG. 10. The formation of the quantum dots is mainly at the locations at which nuclei formed and thus the strain energy is locally increased. Thus the density of the islands is highly dependent on the number of nuclei formed during the growth. But also smaller diffusion lengths due to low growth temperatures favour the formation of a high density of quantum dots with a small size. The size and density of the quantum dots is determined primarily by the amount of material in the quantum well and by the density of nuclei and not necessarily due to the minimization of energy. By surface diffusion, a subsequent ripening of the islands takes place. Due to a redistribution of adhering atoms to the substrate, the quantum dot size aligns to the energetically more favourable equilibrium. This process is also known as Ostwald ripening [5]. FIG. 11 shows an atomic force microscope (AFM) picture of a bunch of self-assembled QDs. FIG. 11. AFM record of self-assembling QDs from Ge on a Si substrate. [13] FIG. 10. The Ostwald ripening process for the formation of coherently strained quantum dots. After the transition from 2D to 3D growth, energetically more favourable larger quantum dots can be formed by surface diffusion or redistribution. For further material supply incoherent islands form a large number of dislocations. [5] 7

9 FIG. 12. Overview - Epitaxy Methods. A variety of different techniques is currently available, however this is only a brief overview over common methods from past, like liquid-phase epitaxy (LPE), to present, like metal-organic-vapour-phase epitaxy(movpe). Epitaxy Methods There are several epitaxial techniques, currently available for the growth of semiconductor materials, an overview is given in FIG. 12, there the abbreviations that are used in the following can be found as well. LPE, was used for much of the early research on group- III/V and II/VI semiconductors, the growth of excellent quality layers was extremely simple, low impurity and point defect levels could be realized, while the disadvantages were the scale of economics, and the inflexibility of the method. CVD, at the heated surface of a substrate, a solid component is deposited due to a chemical reaction from the vapour phase. Which leads to the subgroup, VPE, group- V or VI elements (for III/V materials, or for II/VI materials) is transported to the growth interface using the hydrides. Under the main topic of CVD methods, there are further more techniques like the MOVPE, HVPE, and ClVPE, which will be looked at in more detail later, in case of the MOVPE. PVD, unlike CVD, PVD uses physical process (such as heating or sputtering) to produce a vapour of material (gas phase), which is then deposited on the substrate. Sub categoric is here the IBAD and the famous MBE, which will be also looked at closer in the following. There are very specific mixed types of CVD and PVD excising as well, these are for example CBE and MOMBE. MBE and MOVPE are the main epitaxial methods used today. For a long time MOVPE was thought to be more of a production instrument, while MBE was the scientific tool. This happened because MBE could be dealt with straight forwardly with the available surface science know-how, while MOVPE was subject to the poor knowledge of the underlying chemistry, hydrodynamics and moreover surface analytical tools were not available [1]. But this situation has changed and both methods can be utilized for growing structures well defined on the atomic level. MOVPE Metalorganic vapour phase epitaxy (MOVPE), also termed metalorganic chemical vapour deposition (MOCVD), is the most frequently applied CVD technique for semiconductor device fabrication. Industrial large scale reactors presently have the capacity for a simultaneous deposition on up to fifty 2-inch wafers, and a majority of advanced semiconductor devices is produced using this technique. Applications of MOVPE are not only restricted to semiconductors, but also include oxides, metals, and organic materials [2]. The MOVPE process starts with a gas mixture which contains the molecular compounds, termed precursors, necessary for growth, and a carrier gas. The latter is usually hydrogen (for special tasks also nitrogen), with an operating pressure between 10 3 Pa and atmospheric pres- 8

10 sure 10 5 Pa. For reasons of uniform deposition, usually nowadays low pressures are preferred [1]. This is a consequence of the more laminar flow pattern and the more homogeneous temperature field within the reactor. The choice of precursors depends obviously on the material to be deposited. For III-V semiconductors the standard precursors are, for example, metalorganic compounds of the group-iii elements, like trimethylgallium (Ga(CH 3 ) 3, in abbreviated form TMGa), and hydrides of the group- V elements, e.g., arsine (AsH 3 ) or phosphine (PH 3 ). In FIG. 13 the schematic structure of a MOVPE system is shown. There the group-iii metalorganics are Advantages: + Most flexible + Highest device yield & throughput + Ability to grow AlGaAs layers + In-situ monitoring Disadvantages: Expensive reactants Most parameters to control accurately Hazardous precursors MBE FIG. 13. Schematic structure of a MOVPE system (Aixtron AIX-200/4). [14] in high purity liquid or solid form, in so called bubblers held at very well-defined temperature. In this case, its used trimethylaluminum (Al 2 (CH 3 ) 6, TMAl), trimethylindium (In(CH 3 ) 3, TMIn) and trimethylgallium (TMGa). Also, high-purity hydrogen is then used as a carrier gas, it flows through the bubbler and goes then into the reactor. A typical reaction here would then look like Molecular beam epitaxy (MBE) is a physical vapour deposition (PVD) technique, which is widely applied in research labs and industrial production. The constituent elements of the crystalline solid are transported from the source(s) to substrate using molecular beams. A molecular beam is a directed ray of neutral atoms or molecules in a vacuum chamber. In MBE the beams are usually thermally evaporated from solid or liquid elemental sources. The characteristic feature of MBE is the mass transport in molecular or atomic beams. A vacuum environment is required to ensure that no significant collisions occur among the beam particles and between beam and background vapour. A schematic diagram of an MBE system is given in FIG. 14. The growth chamber is (CH 3 ) 3 Ga + AsH 3 GaAs + 3CH 4. (1.7) The gas mixture is fed into an open reactor where the heated substrate is placed as shown in FIG. 13. Many designs for MOVPE reactors have been proposed, tested, and are in use, e.g., the horizontal cold wall reactor, the vertical rotating disk reactor, the planetary reactor or the so called close coupled showerhead (CCS). The goal of such development work is the homogeneity of properties such as thickness, stoichiometry, and carrier concentration across the whole substrate surface [1]. MOVPE Specs [3] Typical growth rate R: 10 µm /h Growth temperature T : C Reactor pressure p: mbar FIG. 14. Schematic representation of a molecular beam epitaxy growth chamber. The circular arrow indicates the positioning of the gauge at the location of the substrate to calibrate the beam-equivalent pressure of the effusion cells, which contain different source materials. [2, 15] pumped to ultra-high vacuum (UHV), i.e., at a residualgas pressure below 10 7 Pa, by using a combination of different pumps like, e.g., ion pump, cryo-pump, turbomolecular pump, diffusion pump, or sublimation pump. 9

11 The need for such low pressure originates from the required purity of epitaxial semiconductors. The effusion cells usually mounted opposite to the substrate, produce beams of different species by evaporation. While the duration of exposure on the substrate is individually controlled by shutters, which are used for a rapid change of material composition or doping. The substrate is mounted on a heated holder and can be loaded and unloaded under UHV conditions by a preparation chamber. A gauge can be placed at the position of the substrate to measure and calibrate the beam-equivalent pressure (BEP) produced by individual sources. The UHV environment maintained during epitaxy, provides an excellent opportunity for in situ monitoring of the growth process. Usually RHEED[7] with an electron beam nearly parallel to the growth surface is applied, yielding structural information on the surface crystallography during surface preparation and during epitaxy. The location of the electron gun and the monitoring screen is also indicated in FIG. 14. What distinguishes MBE from other vacuum deposition techniques, is its significantly more precise control of the beam fluxes and growth conditions [1]. MBE Specs [3] Growth rate R: 1 µm /h (one atomic layer per sec) Growth temperature T : C Reactor pressure p: mbar Advantages: + High purity material can be grown + Abrupt change of chemical composition at interface (fast shutters) + High accuracy and uniformity + In-situ control of crystal growth by RHEED[7] Disadvantages: Expensive (capital) Low throughput Maintenance (UHV) Applications The properties of heterostructures, as presented in the previous sections, are already widely used in technical applications or constitute currently the basis for the design of novel semiconductor devices. The quantization of the electron energy in quantum wells enables for example, the increase in the efficiency of light emitting diodes and semiconductor lasers, the distributed Bragg reflectors (DBR) constitute, e.g., the function basis for the Vertical Cavity Surface Emitting Lasers (VCSEL). Further effects, with interesting applications for semiconductor heterostructures, arise from the possibilities of lateral microstructuring of such systems (or respectively, of the 2D electron gas they contain). This is possible among other things by various etching processes, e.g., plasma etching or wet chemical process, by ion implantation, and by depositing so-called gate electrodes. In this way, the electron system confined within the interface can further be limited to one dimension (quantum wire), or to a even smaller volume confined in three dimensions (QD). Summary Epitaxy has made vital contributions in the past decade, in certain areas of the technology of microelectronic devices. The MBE, with its UHV environment and the therefore available electron based surface sensitive techniques, enabled effective in situ control over the growth process. A similar control was later achieved by optical methods for the VPE techniques especially MOVPE. Epitaxy made also contributions in surface technology, where the preparation of substrate and thin film surfaces with specific orientation, reconstruction, and composition were enabled, which are in addition also effectively damage free. But the by far most widely exploited area, is related to heteroepitaxy, which is at present the technology of sophisticated heterostructures and quantum well structures with 1D, 2D and 3D quantization. TommyKlumpp@gmail.com m.jetter@ihfg.uni-stuttgart.de [1] M. Herman, W. Richter, and H. Sitter, Epitaxy: Physical Principles and Technical Implementation, Springer Series in Materials Science (Springer Berlin Heidelberg, 2013). [2] U. Pohl, Epitaxy of Semiconductors: Introduction to Physical Principles, Graduate Texts in Physics (Springer Berlin Heidelberg, 2013). [3] G. Stringfellow, Organometallic Vapor-Phase Epitaxy: Theory and Practice (Elsevier Science, 2012). [4] R. Kern and G. Metois, Current Topics in Materials Science. 3, 131 (1979). [5] R. Roßbach, InP-Quantenpunkte: Epitaxie und Charakterisierung, Hochschulschrift, Tönning (2007). [6] J. H. Neave, P. J. Dobson, B. A. Joyce, and J. Zhang, Applied Physics Letters 47, 100 (1985). [7] Reflection High Energy Electron Diffraction. [8] Reflectance Anisotropy Spectroscopy. [9] Grazing Incidence X-ray Scattering. [10] J. Johansson, W. Seifert, T. Junno, and L. Samuelson, J. Crystal Growth 195:546 (1998). 10

12 [11] A. Able, Untersuchungen zur metallorganischen Gasphasenepitaxie von Gruppe III Nitriden auf Silizium (111), Ph.D. thesis (2005). [12] O. Ambacher, Journal of Physics D: Applied Physics 31, 2653 (1998). [13] K. Eberl and N. Nestle, Heterostrukturen, (1998). [14] T. Schwarzbäck, Epitaxie AlGaInP-basierter Halbleiterscheibenlaser: Dauerstrichbetrieb, Frequenzverdopplung und Modenkopplung, Ph.D. thesis (2013). [15] Molecular_Beam_Epitaxy.png, accessed:

13 REVTEX 4.1 PDFL A TEX c 2016 Thomas Klumpp