1 Semiconductor nanostructures

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1 Outline Nanoscience II: Semiconductor nanostructures Semiconductor nanostructures Quantum dots 3 Photonic crystals 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 page 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. 1 Semiconductor nanostructures Classification by properties Electronic tailoring (quantum dots, wires) Optical tailoring (photonic crystals) Classification by nanostructure dimensionality 1D (quantum wells, superlattices, Bragg mirrors) D (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 3 page 4

2 Covalent bonds in semiconductors Doping Electronic structure of Si: 1s s p 6 3s 3p 4 valence electrons, 4 electrons missing to fill the outer shell Electronic structure of Ga: 1s s p 6 3s p 6 3d 10 4s 4p 1 3 valence electrons, 5 electrons missing Electronic structure of As: 1s s p 6 3s p 6 3d 10 4s 4p 3 5 valence electrons, 3 electrons missing E g E c E d Ga As Ga As Ga As Ga As Ga As Ga As Filled valence band 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 v 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 Boron impurity atom: acceptor defect/impurity => p- type semiconductor E g Filled valence band E c E a E v page 6 Diamond structure Diamond structure = FCC lattice + 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 + different atoms in the primitive cell Examples: GaAs, InP, GaP, GaSb, InSb, ZnS, ZnSe, Semiconductor band structure Electronic structure of Si: 1s s p 6 3s 3p N Si atoms: N electrons in 3s orbital, N electrons in 3p orbitals Energy 3p 3s N electrons N electrons 4N electrons Empty upper bands 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 (GaN, SiC and ZnO are difficult to manufacture in zinc-blende structure) Crystal viewer (diamond and Zinc blende structure): page 7 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

3 GaAs band structure (E-k diagram) Direct and indirect bandgap L-valley X-valley E g Conduction band Valence band 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 9 page 10 Quantum well Energy levels for electrons E z y x InP InAs 0.65 P 0.35 (5nm) E C E C e4 (l=4) e3 (l=3) e (l=) e1 (l=1) L z L x InP L y Cross-sectional TEM picture of a GaInNAs QW grown on GaAs. Quantum well: a thin semiconductor layer (L z <0nm) embedded between two semiconductors with larger bandgaps. E g,1 E g, L z E V z E e L z In the infinite well approximation, the energy levels are given by: π = me Lz Electrons and holes trapped in the well are free to move in the x-y plane, but are strongly confined in the z-direction = D electron gas. page 11 Electron energy: E = E C + E e + ( k x m + k ) e y page 1

4 Energy levels for holes Density of states (electrons): D vs. 3D E E C E V hh1 (l=1) lh1 (l=1) hh (l=) lh (l=) D(E) 3D D E g,1 E g, L z L z E V z Heavy hole energy: In the infinite well approximation, the energy levels are given by: E hh π = m E = E L hh z V + E hh + E lh ( k π = x mhh mlhlz + k ) y page 13 E C E e1 E e E e3 * m D( E) = e H ( E El ) de π l ( E ) H E l E 1, when E El = 0, when E < E l page 14 Superlattices Microelectronics and -photonics 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. Transistor + pin-photodiode Microcavity LED There are already nm-scale layers in present devices. For electronic effects layer thicknesses are 1-10 nm and for optical effects nm. page 15 Integrated optics E.g., the QW s are -3 nm thick in white LEDs. page 16

5 Quantum wire Density of states: 3D, D and 1D z y x InP D(E) 1D 3D D InAs 0.65 P 0.35 L z L x InP Quantum wire: 1D electronic system (confinement in D) Electrons and holes trapped in the wires are free to move only along the y-direction L y (110) cross-section TEM picture of stacked InAs QWires in InAlAs matrix lattice matched to InP. E C E 1e E e E 3e 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). E page 17 page 18 Fabrication of quantum wires Example: InP nanowires on InP by MOVPE VLS growth of InP using In droplets 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 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 19 page 0

6 Applications of quantum wires - Nanowire transistors, logic elements, electronic waveguides Density of states in 3-dimensional (bulk), -dimensional (well), 1-dimensional (wire) and 0-dimensional (dot) semiconductors Density of states in QDs - Optical waveguides, optical emitters - Sensors utilizing functionalized surface page 1 page QD classification Quantum dots 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 3 page 4

7 QD nanoparticles QD band gap is effectively shifted in proportion to 1/R. The size causes different colors in optical absorption and emission. 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) Fluorescence (emission) of CdTe quantum dots in solution. Color variation is due to diameter from nm (green) to 5 nm (red). page 5 page 6 Examples of the fabrication methods of quantum dots Colloidal growth (kinetically controlled synthesis) 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 ~0 nm GaAs Kvantpunkt QD => degradation of optical properties due to processing steps 8 nm SiO (insulator) => optical color filters - monodisperse nanocrystals (diameter variation <5%) needed - chemical synthesis (fig.): reagents are rapidly injected into hot solvent, colloids are formed in the supersaturated solution Heterogeneous nucleation: self-assembled growth Smält kiseldioxid As In Självorganiserad no artificial patterning! tillväxt InAs GaAs => defectfree structure page 7 page 8

8 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) 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 9 page 30 Modern epitaxial techniques Growth modes in epitaxy - 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 MOVPE or MOCVD (metalorganic vapor phase epitaxy) - sources: vapors or gases - two systems in Optoelectronics Lab., Micronova As P Ga Al In In A s Frank-van der Merwe (-d) Volmer-Weber (3-d) Stranski-Krastanow (-d + 3-d) Transition to 3-d growth after ultrathin strained wetting layer page 31 page 3

9 Coherent Stranski-Krastanow growth mode Self-assembled growth of III-V QDs Ge islands on Si not dislocated Eaglesham, Cerullo, Phys. Rev. Lett. 64, (1990) Stranski-Krastanow growth mode AFM E.g., InAs island formation on GaAs surface 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 - 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 D layer (wetting layer) - islands are defect-free and act as quantum dots with a high density (~10 10 cm - ) page 34 Example: self-assembled InP islands on GaAs Shape engineering of quantum dots - nanocrystals can be capped (e.g. with GaAs) to form buried quantum dots - from vapor phase or molecular beam at C P In ultrathin strained layer, ~3 ML InP on GaAs InP - the shape can be altered either by the capping process or by annealing GaAs Tg=635 C TEM cross section of InAs nanocrystal on GaAs surface. AFM image of InAs(P) quantum rings fabricated at our laboratory. AFM images of InP nanocrystals on GaAs surface. InP layer thickness is 3 monolayers (~0.9 nm). Density of 0 nm high nanocrystals is about 10 9 cm -. Annealing of InAs dots in P atmoshere results in shape change. page 35 page 36

10 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 Cross-sectional scanning tunneling microscopy (STM) of cleaved InAs quantum dots shows structural and compositional information with atomic resolution (fig.) Pyramidal InAs QDs - the typical structure for capped dots is a truncated pyramide (below) 40x40 nm cross-section STM current image of cleaved InAs quantum dot and the wetting layer. TEM cross section of stacked InAs quantum dots. page 37 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.) 5 nm high and 15 nm wide InAs quantum dot - 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 page 38 λ pump PL 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 (,4,6,8,10 particles / energy). The shells here are partially filled (state-filling process). page 39 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

11 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 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 Energy (ev) page 41 QD VCSEL page 4 Natural photonic crystals Photonic crystals a < 100nm page 43 Sea mouse a = 510nm page 44

12 Natural opals Photonic crystal classification Photonic Crystals (PhCs) 1D PhCs Bragg, 1887 D PhCs 3D PhCs Yablonovitch et al., 1991 μm PhC fibers Russel et al., 1995 Planar PhCs Krauss et al., 1996 page 45 page 46 Bragg grating mirror (1D PhC) Bragg grating mirror example: SiN/SiO mirror d L d H λ 0 λ «stop band» n L n H Studied by Lord Rayleigh in 1887 λ Quarter wave layers: d 0 L =, 4nL d H = λ 0 4n H page 47 Relfectivity λ 0 Wavelength (nm) When the incidence angle decreases, the reflection band becomes narrower and eventually vanishes page 48

13 From Bragg mirrors to photonic crystals 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 Joannopoulos et al., MIT Photonic crystal: generalization of the Bragg mirror concept to D 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 page 50 Artificial opals Band diagram Material Institute of Madrid Vos et al. Nature Opal can be manufactured by sedimentation of SiO spheres of controlled size (Left picture). Only inverted opals with refractive index above. exhibit a full photonic bandgap (Right picture). page 51 Normalized frequency a/λ Wavevector k no transmission in Γ L (111) direction reflectance maximum Wavelength λ (μm) full bandgap, transmission forbidden in all directions page 5

14 3D Photonic crystals 3D Photonic crystals Self-assembled opals Made by self-assembly of SiO, PMMA or polystyrene nanospheres. Structure must be inverted with Si to obtain a complete bandgap Lithography defined structures Time consuming and complex Typical sphere size for bandgap around 1.5µm: 900nm Possibily to sediment nanospheres onto Si patterned substrates. Material Institute of Madrid Sandia Nat. Lab or difficult to add defects Difficult to insert defects in the lattice Planar D PhCs µm VTT+Tyndall (Cork) page 53 M. Qi, H. Smith, MIT 10µm D. N. Sharp et al., Opt. Quant. Elec. 34, 3 (00) page 54 The InP/GaInAsP/InP system n 1 n > n 1 n 1 Vertical structure Confines light in the vertical direction D array of holes Controls light propagation in the plane z y x Provides light confinement in the vertical direction TM E z 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) D 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 InP substrate polarizations: Transverse Magnetic like (TM) H z ~ 0 Tranverse Electric like (TE) E z ~ 0 - Active system InP (n=3.17) Low index contrast system ( n = 0.18) Weak confinement in the core page 56

15 z y x The Silicon-on-Insulator (SOI) system Provides light confinement in the vertical direction TM E z H y H x Si SiO Si substrate TE H z Ey E x polarizations: Transverse Magnetic like (TM) H z ~ 0 Tranverse Electric like (TE) E z ~ Field profile Passive system PhC waveguides Air Si (n=3.4) SiO (n=1.45) High index contrast system ( n = 1.95) Strong confinement in the core page 57 Top view D PhCs etched in InP membranes M. Mulot, M. Swillo, M. Qiu, M. Strassner, M. Hede, S. Anand, J. Appl. Phys. 95, p.598, 004 Sample facet Facet view W1 waveguide 300 nm InP 600 nm InGaAs InP membrane = high index contrast system ( n =.17) improved light confinement compared to InP/GaInAsP/InP Filter combining cavity and waveguide page 58 1 µm W1 waveguide Single defect resonant wavelength: λ i λ i λ 1, λ,...,λ i-1 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 λ 1, λ,...,λ i GaInAsP membrane Noda et al., Nature 000 page 60

16 Point-defect cavity Point-defect cavity detector Detector signal (a.u.) bandgap Detector signal (a.u.) Normalized frequency (a/λ) Normalized frequency (a/λ) One hole removed = defect in the PhC lattice Simulation by D Finite Difference Time Domain method 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. Single-cell photonic crystal laser page 61 Photonic crystal fibers page 6 Q = 500 (measured) I th = 60 μa Max power: a few nw Fabrication the stacking method Crystal Fibre A/S Hong-Gyu Park et al., Science 305, p (004) page 63 page 64

17 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