Nanoscience II: Semiconductor nanostructures

Transcription

1 Nanoscience II: Semiconductor nanostructures Markku Sopanen MICRONOVA Department of Micro- and Nanosciences Aalto University School of Science andtechnology Acknowledgments: Prof. Harri Lipsanen, Dr. Mikael Mulot, Dr. Marco Mattila, Dr. Teppo Hakkarainen page 1

2 Outline 1 Semiconductor nanostructures 2 Quantum dots 3 Photonic crystals page 2

3 1 Semiconductor nanostructures page 3

4 What is a semiconductor nanostructure? Obviously a structure containing at least one semiconductor material and having at least one dimension in nanometer scale. However, usually one-dimensional structures are not considered as nano. Classification by properties Electronic tailoring (quantum dots, wires) Optical tailoring (photonic crystals) Classification by nanostructure dimensionality 1D (quantum wells, superlattices, Bragg mirrors) 2D (quantum wires, nanowaveguides, planar photonic crystal) 3D (quantum dot, nanoparticle, photonic crystal) [Charge carrier system dimensionality is the opposite way.] Semiconductors do not usually play a crucial role in metamaterials. page 4

5 Covalent bonds in semiconductors Electronic structure of Si: 1s 2 2s 2 2p 6 3s 2 3p 2 4 valence electrons, 4 electrons missing to fill the outer shell Electronic structure of Ga: 1s 2 2s 2 2p 6 3s 2 p 6 3d 10 4s 2 4p 1 3 valence electrons, 5 electrons missing Electronic structure of As: 1s 2 2s 2 2p 6 3s 2 p 6 3d 10 4s 2 4p 3 5 valence electrons, 3 electrons missing Ga As Ga As As Ga As Ga Ga As Ga As Some semiconductors have more ionic bonds (II-VI, etc.). Electrons involved in the bonds are trapped in the bonds, and are not available for conduction. Pure semiconductor is a poor conductor But free carriers can be easily created by doping. page 5

6 Doping E c E d E g Filled valence band E v Phosphorus impurity atom (extra valence electron ) in silicon lattice: the extra valence atom is weakly bond: an energy E c - E d << E g is required to create a free electron. This type is called donor defect/impurity => n-type semiconductor E c Boron impurity atom: acceptor defect/impurity => p- type semiconductor E g Filled valence band E a E v page 6

7 Diamond structure Diamond structure = FCC lattice + 2 identical atoms in the primitive cell: (0,0,0) and (a/4, a/4, a/4) Examples: Si, Ge and diamond Zinc-blende lattice = FCC lattice + 2 different atoms in the primitive cell Examples: GaAs, InP, GaP, GaSb, InSb, ZnS, ZnSe, (GaN, SiC and ZnO are difficult to manufacture in zinc-blende structure) Crystal viewer (diamond and Zinc blende structure): page 7

8 Semiconductor band structure Electronic structure of Si: 1s 2 2s 2 2p 6 3s 2 3p 2 N Si atoms: 2N electrons in 3s orbital, 2N electrons in 3p orbitals Empty upper bands Energy 3p 3s 2N electrons 2N electrons 4N electrons Conduction band Valence band Filled lower bands N isolated Si atoms N Si atoms in crystal form Energy states of Si atoms expand into the energy bands of Si crystal The lower bands are filled and higher bands are empty The highest totally filled band is the valence band The lowest empty band is the conduction band page 8

9 GaAs band structure (E-k diagram) L-valley X-valley E g Conduction band Valence band page 9

10 Direct and indirect bandgap Direct band gap: The conduction band is formed only by overlap of s- orbitals Indirect band gap: The conduction band is a mix of p- and s-orbitals page 10

11 Quantum well z y x InP InAs 0.65 P 0.35 (5nm) L z InP L y Cross-sectional TEM picture of a GaInNAs QW grown on GaAs. L x Quantum well: a thin semiconductor layer (L z <20nm) embedded between two semiconductors with larger bandgaps. Electrons and holes trapped in the well are free to move in the x-y plane, but are strongly confined in the z-direction = 2D electron gas. page 11

12 Energy levels for electrons E E C E C e4 (l=4) e3 (l=3) e2 (l=2) e1 (l=1) E g,1 E g,2 L z L z E V z In the infinite well approximation, the energy levels are given by: E e = 2 π me Lz Electron energy: E = E C + E e + 2 ( k 2 x + 2m e k 2 y ) page 12

13 Energy levels for holes E E C E V hh1 (l=1) lh1 (l=1) hh2 (l=2) lh2 (l=2) E g,1 E g,2 L z L z E V z In the infinite well approximation, the energy levels are given by: E hh = 2 π m 2 L hh z E lh = 2 π mlhLz Heavy hole energy: E = E V + E hh + 2 ( k 2 x 2m + hh k 2 y ) page 13

14 Density of states (electrons): 2D vs. 3D D(E) 3D 2D E C E e1 E e2 E e3 E * m D( E) = e2 H ( E El ) de π l ( E ) H E l 1, when E El = 0, when E < E l page 14

15 Superlattices Superlattice structure Intersubband emission Superlattice consists of two (or more) different materials in alternating layers. The periodicity induces subbands within the conduction band and the valence band. For electronic effects layer thicknesses are 1-10 nm and for optical effects nm. page 15

16 Microelectronics and -photonics Transistor + pin-photodiode Microcavity LED There are already nm-scale layers in present devices. Integrated optics E.g., the QW s are 2-3 nm thick in white LEDs. page 16

17 Quantum wire z y x InP InAs 0.65 P 0.35 L z L x InP L y (110) cross-section TEM picture of stacked InAs QWires in InAlAs matrix lattice matched to InP. Quantum wire: 1D electronic system (confinement in 2D) Electrons and holes trapped in the wires are free to move only along the y-direction page 17

18 Density of states: 3D, 2D and 1D D(E) 1D 3D 2D E C E 1e E 2e E 3e E Note: At the absorption edge, the density of states is 0 in the bulk (3D) case. However, it is very large in quantum wires (1D). page 18

19 Fabrication of quantum wires Top-down methods: wires, e.g., defined by lithography and consequent etching Bottom-up methods: wires, e.g., grown by VLS (vapor-liquidsolid) method using metal particles as seeds page 19

20 Example: InP nanowires on InP by MOVPE VLS growth of InP using In droplets SEM image of InP nanowires on InP TEM image of InP nanowires: the metal droplet can be seen at the end of the wire page 20

21 Applications of quantum wires - Nanowire transistors, logic elements, electronic waveguides - Optical waveguides, optical emitters - Sensors utilizing functionalized surface page 21

22 Density of states in 3-dimensional (bulk), 2-dimensional (well), 1-dimensional (wire) and 0-dimensional (dot) semiconductors Density of states in QDs page 22

23 2 Quantum dots page 23

24 QD classification Quantum dots (QDs): nanosize structures of crystalline nature, confined in three dimensions Classification of quantum dots by various criteria: Classification by structure Particles Composites Single crystals Classification by fabrication Homogeneous nucleation Heterogeneous nucleation Kinetically confined synthesis Physical techniques (lithography, nanoimprinting, etc.) Classification by confinement potential Strongly confined Weakly confined page 24

25 QD nanoparticles QD band gap is effectively shifted in proportion to 1/R 2. The size causes different colors in optical absorption and emission. Fluorescence (emission) of CdTe quantum dots in solution. Color variation is due to diameter from 2 nm (green) to 5 nm (red). page 25

26 Core-shell QD - core-shell structure has a core QD surrounded by a thin shell of another material - surface consists of a large fraction of the atoms in the quantum dot => surface structure important factor for the properties, e.g. biotin activated quantum dots (Evident Technologies) page 26

27 Examples of the fabrication methods of quantum dots Physical technique: patterning of heterostructures - e-beam lithography - maskless FIB lithography AlGaAs GaAs AlGaAs Homogeneous nucleation: nanoclusters in glass Mask -e CdSe etching Etsning large surface/volume ratio ~20 nm GaAs QD Kvantpunkt 8 nm => degradation of optical properties due to processing steps SiO 2 (insulator) => optical color filters Heterogeneous nucleation: self-assembled growth Smält kiseldioxid As 2 In Självorganiserad no artificial patterning! tillväxt InAs GaAs => defectfree structure page 27

28 Colloidal growth (kinetically controlled synthesis) - monodisperse nanocrystals (diameter variation <5%) needed - chemical synthesis (fig.): reagents are rapidly injected into hot solvent, colloids are formed in the supersaturated solution page 28

29 Group II-VI semiconductor nanocrystals - group II-VI semiconductors ME, where M = Zn, Cd, Hg and E = S, Se, Te are the most common nanocrystals due to their ease of chemical synthesis (CdSe, ZnS...) - more complex coated nanocrystals, such as CdSe/ZnS core-shell structure important (Evident Technologies) page 29

30 Group III-V semiconductors - group III-V semiconductor nanocrystals such as InP and InAs can be produced similarly as the II-VI structures - not very useful in applications Epitaxial growth: Fabrication of nanocrystals on surface by epitaxy (layer growth) - growth from vapor phase (CVD), molecular beam epitaxy (MBE), laser ablation etc. - good control of growth conditions required (amount of material, choice of materials, temperature) - typically mismatch of lattice constants between deposited thin layer and substrate causes nucleation into nanoscale islands (quantum dots) page 30

31 Modern epitaxial techniques - good control of layer thickness d ( d < 1Å) and composition needed MBE (molecular beam epitaxy) - ultra-high vacuum - like vacuum evaporation - often solid sources - several systems in Tampere, one in Micronova In A s MOVPE or MOCVD (metalorganic vapor phase epitaxy) - sources: vapors or gases - two systems in Optoelectronics Lab., Micronova As P Ga Al In page 31

32 Growth modes in epitaxy Frank-van der Merwe (2-d) Volmer-Weber (3-d) Stranski-Krastanow (2-d + 3-d) Transition to 3-d growth after ultrathin strained wetting layer page 32

33 Coherent Stranski-Krastanow growth mode Ge islands on Si not dislocated Eaglesham, Cerullo, Phys. Rev. Lett. 64, (1990) TEM image of Ge island on Si Stress is not released by dislocation formation. Strain energy is accumulated both in the island and in the substrate. page 33

34 Self-assembled growth of III-V QDs Stranski-Krastanow growth mode AFM E.g., InAs island formation on GaAs surface - InAs has 8% larger lattice constant than GaAs - after deposition of >1.7 monolayers of InAs, small islands (~10 nm wide) are formed (energetically favorable) on a very thin 2D layer (wetting layer) - islands are defect-free and act as quantum dots with a high density (~10 10 cm -2 ) page 34

35 Example: self-assembled InP islands on GaAs - from vapor phase or molecular beam at C P In ultrathin strained layer, ~3 ML InP on GaAs InP GaAs Tg=635 C AFM images of InP nanocrystals on GaAs surface. InP layer thickness is 3 monolayers (~0.9 nm). Density of 20 nm high nanocrystals is about 10 9 cm -2. page 35

36 Shape engineering of quantum dots - nanocrystals can be capped (e.g. with GaAs) to form buried quantum dots - the shape can be altered either by the capping process or by annealing TEM cross section of InAs nanocrystal on GaAs surface. AFM image of InAs(P) quantum rings fabricated at our laboratory. Annealing of InAs dots in P atmoshere results in shape change. page 36

37 Stacked quantum dots Multilayer stacks of quantum dots can also be grown - the quantum dots have laterally statistical distribution in position - vertical coupling due to strain fields causes vertical ordering - size and shape of dots can be tuned by GaAs barrier layer thickness TEM cross section of stacked InAs quantum dots. page 37

38 Cross-sectional scanning tunneling microscopy (STM) of cleaved InAs quantum dots shows structural and compositional information with atomic resolution (fig.) - the typical structure for capped dots is a truncated pyramide (below) Pyramidal InAs QDs 40x40 nm 2 cross-section STM current image of cleaved InAs quantum dot and the wetting layer. 5 nm high and 15 nm wide InAs quantum dot page 38

39 Optical properties of self-assembled quantum dots - density of state of quantum dots resemble that of atoms: sharp energy levels - modeling of the quantum dot can be approximately done by using a simple structure (fig.) Modeling of the self-assembled quantum dot potential using a hemispherical cap of InAs on top of an InAs wetting layer embedded in a GaAs substrate and cap layer. Schematic of the energy levels in an InAs/GaAs self-assembled quantum dot having 5 electron and hole shells (s, p, d, f, g) with a degeneracy (2,4,6,8,10 particles / energy). The shells here are partially filled (state-filling process). page 39

40 - ideal quantum dot system would give narrow lines in optical spectra - in real systems the size and shape fluctuation of the quantum dots broadens the spectra (fig. below) - typical photoluminescence (PL) spectra of >>10 3 dots consists of Gaussian peaks (note state-filling) PL spectra excited states ground state λ pump PL State-filling of the quantum dot shells with increasing excitation intensity in low temperature photoluminescence (PL) spectroscopy. The inset shows a Gaussian fit used to deconvolute the contributions from the various shells. page 40

41 Stressor quantum dot structure - strain field of a self-assembled island causes local decrease of bandgap of a quantum well just below the island. The quantum dot has nearly parabolic potential for electrons and holes. - almost perfect crystal structure BAND DIAGRAM => narrow intense PL peaks AFM (1 x 1 µm) CB VB self-assembled island quantum well ( a > a substrate ) QD PL SPECTRUM QW high excitatio n low excitation Energy (ev) page 41

42 QD applications Semiconductor quantum structures are already commonly used in optoelectronic applications such as telecom lasers, CD & DVD readwrite heads, light emitting diodes (LEDs) etc. QD structures are expected to improve performance, e.g, in near-infrared QD lasers ( nm), QD vertical cavity surface-emitting lasers (VCSEL), QD photodetectors. They might also enable new devices in, e.g., quantum computing. QD VCSEL page 42

43 Photonic crystals page 43

44 Natural photonic crystals a < 100nm Sea mouse a = 510nm page 44

45 Natural opals 2 μm page 45

46 Photonic crystal classification Photonic Crystals (PhCs) 1D PhCs Bragg, D PhCs 3D PhCs Yablonovitch et al., 1991 PhC fibers Russel et al., 1995 Planar PhCs Krauss et al., 1996 page 46

47 Bragg grating mirror (1D PhC) d L d H λ 0 n L n H Studied by Lord Rayleigh in 1887 Quarter wave layers:, d L = λ 0 4n L d H = λ 0 4n H page 47

48 Bragg grating mirror example: SiN/SiO 2 mirror Relfectivity λ «stop band» λ 0 Wavelength (nm) When the incidence angle decreases, the reflection band becomes narrower and eventually vanishes page 48

49 From Bragg mirrors to photonic crystals Joannopoulos et al., MIT Photonic crystal: generalization of the Bragg mirror concept to 2D and 3D periodic structures A 3D photonic crystal can have a full bandgap: it then reflects light for any incident angle. Full bandgap requires a large refractive index contrast in the structure. page 49

50 The Yablonovite Manufactured by the Yablonovitch group at MIT in 1991 First 3D PhC with a full photonic bandgap in microwave range Consists of a periodic pattern of holes drilled into plexiglas. Each hole is drilled three times in three different directions The obtained 3D pattern reproduces the diamond structure page 50

51 Artificial opals Material Institute of Madrid Vos et al. Nature Opal can be manufactured by sedimentation of SiO 2 spheres of controlled size (Left picture). Only inverted opals with refractive index above 2.2 exhibit a full photonic bandgap (Right picture). page 51

52 Band diagram Normalized frequency a/λ Wavelength λ (μm) full bandgap, transmission forbidden in all directions Wavevector k no transmission in Γ L (111) direction reflectance maximum page 52

53 3D Photonic crystals Self-assembled opals Made by self-assembly of SiO 2, PMMA or polystyrene nanospheres. Structure must be inverted with Si to obtain a complete bandgap Typical sphere size for bandgap around 1.5µm: 900nm Possibily to sediment nanospheres onto Si patterned substrates. Material Institute of Madrid Difficult to insert defects in the lattice 2 µm VTT+Tyndall (Cork) page 53

54 3D Photonic crystals Lithography defined structures Time consuming and complex Sandia Nat. Lab or difficult to add defects M. Qi, H. Smith, MIT 10µm D. N. Sharp et al., Opt. Quant. Elec. 34, 3 (2002) page 54

55 Planar 2D PhCs n 1 n 2 > n 1 n 1 Vertical structure Confines light in the vertical direction 2D array of holes Controls light propagation in the plane 2D PhCs Relatively simple structure Have most of the properties of 3D PhCs Existing technologies can be directly applied or developed further Compatible with planar optoelectronics page 55

56 The InP/GaInAsP/InP system Provides light confinement in the vertical direction z y x E z TM H y H x InP GaInAsP TE H z Ey E x z (µm) 0-1 Field profile Air InP (n=3.17) GaInAsP (n=3.35) InP substrate -2 InP (n=3.17) 2 polarizations: Transverse Magnetic like (TM) H z ~ 0 Tranverse Electric like (TE) E z ~ 0 Active system Low index contrast system ( n = 0.18) Weak confinement in the core page 56

57 The Silicon-on-Insulator (SOI) system Provides light confinement in the vertical direction z y x E z TM H y H x Si SiO 2 TE H z Ey E x 0 Field profile Air Si (n=3.4) Si substrate 1 SiO 2 (n=1.45) 2 polarizations: Transverse Magnetic like (TM) H z ~ 0 Tranverse Electric like (TE) E z ~ 0 Passive system High index contrast system ( n = 1.95) Strong confinement in the core page 57

58 2D PhCs etched in InP membranes M. Mulot, M. Swillo, M. Qiu, M. Strassner, M. Hede, S. Anand, J. Appl. Phys. 95, p.5928, 2004 Facet view Top view W1 waveguide 300 nm InP Sample facet 600 nm InGaAs InP membrane = high index contrast system ( n = 2.17) improved light confinement compared to InP/GaInAsP/InP page 58

59 PhC waveguides 1 µm W1 waveguide W1 waveguide Line defects in PhCs can be used to guide light 1 line defect = W1 waveguide, 3-line defect = W3 waveguide PhC waveguides are essential building blocks of a PhC integrated circuit page 59

60 Filter combining cavity and waveguide Single defect resonant wavelength: λ i λ i λ 1, λ 2,...,λ i-1 λ 1, λ 2,...,λ i GaInAsP membrane Noda et al., Nature 2000 page 60

61 Point-defect cavity detector Detector signal (a.u.) bandgap Normalized frequency (a/λ) One hole removed = defect in the PhC lattice Simulation by 2D Finite Difference Time Domain method page 61

62 Point-defect cavity Detector signal (a.u.) Normalized frequency (a/λ) At the resonance wavelength, light is trapped in the defect The point-defect defect acts as a trap for photons. Light cannot escape the structure due to the surrounding bandgap. page 62

63 Single-cell photonic crystal laser Q = 2500 (measured) I th = 260 μa Max power: a few nw Hong-Gyu Park et al., Science 305, p (2004) page 63

64 Photonic crystal fibers Fabrication the stacking method Crystal Fibre A/S page 64

65 Photonic crystal fibers: Applications Large mode area fibers Nonlinear fibers Polarization maintaining fibers High numerical aperture fibers Double cladding active fibers Air-guiding fibers page 65