Quantum Dots.

Quantum Dots

What is a quantum dot? In two words, a semiconductor nanocrystal.
Easily tunable by changing the size and composition of the nanocrystal

Gallium Arsenide Quantum Dots
Gallium arsenide is a III-V semiconductor Higher saturated electron velocity and higher electron mobility than silicon Gallium arsenide can emit and absorb light, unlike silicon No silicon laser is possible (or has been made yet)

Energy Band Levels Electrons exist in discrete energy levels in bulk semiconductor material. There exists a forbidden range of energy levels in any material called the band gap.

By absorbing some sort of stimulus (in light or heat form), an electron can rise to the conduction band from the valence band. This action leaves behind a “hole” in the valence band. The hole and the electron together are called an exciton.

The average distance between an electron and a hole in a exciton is called the Excited Bohr Radius.
When the size of the semiconductor falls below the Bohr Radius, the semiconductor is called a quantum dot.

Tuning Quantum Dots By changing size, shape, and composition, quantum dots can change their absorptive and emissive properties dramatically

Manufacturing methods
Electron beam lithography Molecular beam epitaxy

Electron Beam Lithography
Electrons are accelerated out of an electron gun and sent through condenser lens optics directly onto a wafer λ = (12.3 Å / √V) Advantages: generation of micron and submicron resist geometries greater depth of focus than optical lithography masks are unnecessary Optical diffraction limit is not a real concern

Electron Beam Lithography
Disadvantage(s): The lithography is serial (masks aren’t used; instead the beam itself sweeps across the wafer) => Comparatively low throughput ~5 wafers per hour at less than 1 micrometer resolution The proximity effect: Electrons scatter because they are relatively low in mass, reducing the resolution. Heavy ion lithography has been proposed, but still is in development stages

Molecular Beam Epitaxy
Molecular beam epitaxy (MBE) is the deposition of one or more pure materials onto a single crystal wafer one layer of atoms at a time in order to form a perfect crystal This is done by evaporating each of the elements to combine, then condensing them on top of the wafer. The word “beam” means that the evaporated atoms only meet each other on the wafer

Artificial Atom Double Barrier Heterostructure Dot: In0.05Ga0.95As
Source &Drain : GaAs 2D Electron Gas Confine with gate bias D ~ Fermi wavelength → Discrete energy levels

Adding Electrons, changing Vgate
2D-Harmonic Oscillator Shell structure as in atoms Magic Numbers: 2, 6, 12... To add “even” electron requires only additional Coulomb energy

Comparison with Hydrogen
Artificial Atom: Energy levels ~ 1meV Size ~ 10μm Weak magnetic fields can affect energy levels Hydrogen: Energy levels ~ 1eV Size ~ 1Å Only strong magnetic fields can perturb energy levels Factor

Tuning the Quantum Dot Tune so we have one valence electron
Initial state can be set by applying homogeneous magnetic field → |0> Low temperature: kT < ΔE (state gap) Now we have defined our single qubit Energy Unoccupied state Gate bias Spin up - electron position

The Physical System: Excitons Trapped in GaAs Quantum Dots
Exciton - a Coulomb correlated electron-hole pair in a semiconductor, a quasiparticle of a solid. Often formed when photons excite electrons from the valence band into the conduction band. Wavefunctions are “hydrogen-like” i.e. an “exotic atom” though the binding energy is much smaller and the extent much larger than hydrogen because of screening effects and the smaller effective masses Decay by radiating photons. Decay time ~50ps-1ns Hence can define the computational basis as absence of an exciton |0>, or existence of an exciton |1>

Stufler et al. Large wafer containing InGaAs QD was placed between a bias voltage and exposed to ultrafast laser pulses. Cos(Θ/2)|0>+Sin(Θ/2)|1> |1> => electric charge =>Photocurrent (PC) PC~Sin2(Θ/2) π-pulse corresponds to a population inversion

Saint-Petersburg State University
Quantum Dots. Optical and Photoelectrical properties of QD of III-V Compounds. Alexander Senichev Physics Faculty Department of Solid State Physics

Introduction If the size of semiconductor crystal is reduced to tens or hundreds of inter-atomic spacing, all major properties of material change because of size quantization effects.

Introduction Quantum Well Quantum Dots
The extreme case of size quantization is realized in semiconductor structures with confinement of carriers in three directions – they are Quantum Dots.

Introduction Generally, electronic spectrum of the ideal quantum dots is a set of discrete levels. Qualitative behavior of Density of States in: a) Bulk semiconductor b) Quantum Wells c) Quantum Wires d) Quantum Dots E а) b ) с) d

Device application of QDs
Lasers with active area based on QDs Light-Emitting Device (LED) based on QDs Quantum Dots Solar Cells

Technology of QDs Formation
The base of technologies of QDs formation is self-organizing phenomenon. There are three types of initial stage of epitaxial growth: 2D growth of material A on surface of substrate B ; (Frank-van der Merve) 3D growth of material A on surface of substrate B ( Volmer-Weber method); Intermediate mode of growth – the Stranski-Krastanow mode. 2D growth 3D growth Stranski-Krastanow

Molecular Beam Epitaxy (MBE) MBE may be defined as the deposition of epitaxial films onto single crystal substrates using atomic or molecular beams. MBE involves elementary processes: Adsorption of atoms and molecules; Thermal desorption; 3) Diffusion of adatoms on surface of substrate; 4) Nucleation; 1 2 4 3 Solid substrate

Molecular Beam Epitaxy (MBE) MBE system consist of: a growth chamber a vacuum pump a effusion (Knudsen) cells a manipulator and substrate heater an in-situ characterization tool – RHEED (reflection high energy electron diffraction) The typical rate of MBE growth is about 1 ML/s.

Molecular Beam Epitaxy (MBE) The oscillation of the RHEED signal exactly corresponds to the time needed to grown a monolayer. The diffraction pattern on the RHEED windows gives direct indication of the state of the surface.

Metal organic chemical vapor deposition (MOCVD) Metal organic chemical vapor deposition is a technique used to deposit layers of materials by vapor deposition process. MOCVD system contains: the gas handling system to meter and mix reagents the reactor the pressure control system the exhaust facilities

Metal organic chemical vapor deposition (MOCVD) The basic chemistry equation of this reaction is as follows: Group III sources are trimetilgallium (TMGa), TMAl, TMIn. Group V sources are typically hydride gases such as arsine, phosphine. Growth rate and composition is controlled by partial pressures of the species and by substrate temperature

Dependence of QDs morphology on growth conditions
The basic control parameters in the case of MBE growth: the substrate temperature; the growth rate; the quantity InAs, ratios of III/V materials; Exposure time in As stream; As research shows, morphology of QDs ensembles strongly depends on temperature of substrate and growth rate.

Dependence of QDs morphology on growth conditions

Optical properties of QDs
Photoluminescence spectra of various ensembles of QDs:

The major processes which explain the temperature behavior of QDs PL-spectra: Thermal quenching of photoluminescence Thermal quenching is explained by thermal escape of carriers from QD into the barrier (or wetting layer) “Red shifting” As experiment shows, at the temperature, when thermal quenching begins, we can see a following change: the maximum of PL line is shifting in the “red region”. Such behavior of PL spectrum is explained by thermal quenching of carriers and their redistribution between small and large QDs.

Thermal broadening of PL-spectrum. The one of the major factors which defines PL-line width is size dispersion of QDs, i.e. statistic disregistry in ensembles of QDs. Other process which affects on PL-line width is the electron-phonon interaction. Tunnel processes Tunneling of carriers between QDs competes with escape of carriers from QDs in all temperature range. Probability of tunneling increases with temperature growth. Tunneling processes can affect on high-temperature component of photoluminescence spectrum.

Photoelectrical properties of QDs
Photoluminescence spectra at 10 K as a function of bias excited at (a) eV above the GaAs band gap, (b) eV resonant with the wetting layer, and (c) eV resonant with the second dot excited state. Schematic excitation, carrier loss, and recombination processes are indicated for the three cases. Photocurrent spectra as a function of bias at 10 K. Quantum-dot features are observed for biases between -3 and -6 V. The inset shows photocurrent from two-dimensional wetting-layer transition, observed to its full intensity at biases of only ~ -0.5 V.

Semiconductor Quantum Dots
Justin Galloway Department of Materials Science & Engineering

Introduction Effective Mass Model Reaction Techniques Applications
Outline Introduction Effective Mass Model Reaction Techniques Applications Conclusion

How Quantum Dots Semiconductor nanoparticles that exhibit quantum confinement (typically less than 10 nm in diameter) Nanoparticle: a microscopic particle of an inorganic material (e.g. CdSe) or organic material (e.g. polymer, virus) with a diameter less than 100 nm More generally, a particle with diameter less than 1000 nm 1. Gaponenko. Optical properties of semiconductor nanocrystals 2.

Properties of Quantum Dots Compared to Organic Fluorphores? Properties
High quantum yield; often 20 times brighter Narrower and more symmetric emission spectra times more stable to photobleaching High resistance to photo-/chemical degradation Tunable wave length range nm Properties CdSe CdTe J. Am. Chem. Soc. 2001, 123,

Excitation in a Semiconductor Excitation
The excitation of an electron from the valance band to the conduction band creates an electron hole pair Excitation Creation of an electron hole pair where h is the photon energy Band Gap (energy barrier) exciton: bound electron and hole pair usually associated with an electron trapped in a localized state in the band gap

Recombination of Electron Hole Pairs
Recombination can happen two ways: radiative and non-radiative Release recombination processes radiative recombination  photon non-radiative recombination  phonon (lattice vibrations)

Effective Mass Model Developed in 1985 By Louis Brus
Relates the band gap to particle size of a spherical quantum dot Model Band gap of spherical particles The average particle size in suspension can be obtained from the absorption onset using the effective mass model where the band gap E* (in eV) can be approximated by: Egbulk - bulk band gap (eV), h - Plank’s constant (h=6.626x10-34 J·s) r - particle radius e - charge on the electron (1.602x10-19 C) me - electron effective mass  - relative permittivity mh - hole effective mass 0 - permittivity of free space (8.854 x10-14 F cm-1) m0 - free electron mass (9.110x10-31 kg) Brus, L. E. J. Phys. Chem. 1986, 90, 2555

Term 2 The second term on the rhs is consistent with the particle in a box quantum confinement model Adds the quantum localization energy of effective mass me High Electron confinement due to small size alters the effective mass of an electron compared to a bulk material Model Consider a particle of mass m confined in a potential well of length L. n = 1, 2, … For a 3D box: n2 = nx2 + ny2 + nz2 Brus, L. E. J. Phys. Chem. 1986, 90, 2555

Term 3 The Coulombic attraction between electrons and holes lowers the energy Accounts for the interaction of a positive hole me+ and a negative electron me- Model Electrostatic force (N) between two charges (Coulomb’s Law): Work, w = F·dr Consider an electron (q=e-) and a hole (q=e+) The decrease in energy on bringing a positive charge to distance r from a negative charge is: Brus, L. E. J. Phys. Chem. 1986, 90, 2555

Term Influences The last term is negligibly small Term one, as expected, dominates as the radius is decreased Model Modulus Conclusion: Control over the particle’s fluorescence is possible by adjusting the radius of the particle

Quantum Confinement of ZnO & TiO2
ZnO has small effective masses  quantum effects can be observed for relatively large particle sizes Confinement effects are observed for particle sizes <~8 nm TiO2 has large effective masses  quantum effects are nearly unobservable Model

Formation of Nanoparticles
Varying methods for the synthesis of nanoparticles Synthesis technique is a function of the material, desired size, quantity and quality of dispersion The Making Synthesis Techniques • Vapor phase (molecular beams, flame synthesis etc… • Solution phase synthesis Aqueous Solution Nonaqueous Solution Semiconductor Nanoparticles II-VI: CdS, CdSe, PbS, ZnS III-V: InP, InAs MO: TiO2, ZnO, Fe2O3, PbO, Y2O3 Semiconductor Nanoparticles Synthesis: Typically occurs by the rapid reduction of organmetallic precusors in hot organics with surfactants some examples of in vitro imaging with QDs (

Nucleation and Growth The Making
Figure 1. (A) Cartoon depicting the stages of nucleation and growth for the preparation of monodisperse NCs in the framework of the La Mer model. As NCs grow with time, a size series of NCs may be isolated by periodically removing aliquots from the reaction vessel. (B) Representation of the simple synthetic apparatus employed in the preparation of monodisperse NC samples. Horizontal dashed lines represent the critical concentration for nucleation and the saturation concentration C. B. Murray, C. R. Kagan, and M. G. Bawendi, Annu. Rev. Mater. Sci. 30, 545, 2000.

Capping Quantum Dots Due to the extremely high surface area of a nanoparticle there is a high quantity of “dangling bonds” Adding a capping agent consisting of a higher band gap energy semiconductor (or smaller) can eliminate dangling bonds and drastically increase Quantum Yield The Making With the addition of CdS/ZnS the Quantum Yield can be increased from ~5% to 55% Synthesis typically consisted of lower concentrated of precursors injected at lower temperatures at slow speeds Shinae, J. Nanotechnology. 2006, 17, 3892

Quantum Dot Images Quantum dot images prepared in the Searson Lab using CdO and TOPSe with a rapid injection The Making 770000x 455000x 560000x

Quantum Dot Ligands Provide new Insight into erbB/HER receptor – Mediated Signal Transduction
Used biotinylated EGF bound to commercial quantum dots Studied in vitro microscopy the binding of EGF to erbB1 and erbB1 interacts with erbB2 and erbB3 Conclude that QD-ligands are a vital reagent for in vivo studies of signaling pathways – Discovered a novel retrograde transport mechanism Application QD’s Dynamics of endosomal fusion A431 cell expressing erbB3-mCitrine Nat. Biotechnol. 2004, 22;

Fluoresence data for all 4 toxin assays at high concentrations
Multiplexed Toxin Analysis Using Four Colors of Quantum Dot Fluororeagents Demonstrated multiplexed assays for toxins in the same well Four analyte detection was shown at 1000 and 30 ng/mL for each toxin At high concentrations all four toxins can be deciphered and at low concentrations 3 of the 4 Application QD’s Fluoresence data for all 4 toxin assays at high concentrations Cartoon of assay Anal. Chem. 2004, 76;

15 nm CdSe/ZnS TOPO/Polymer/PEG/target
Quantum Dot Imaging QDs with antibodies to human prostate-specific membrane antigen indicate murine tumors developed from human prostate cells 15 nm CdSe/ZnS TOPO/Polymer/PEG/target Application QD’s Gao et al., “In vivo cancer targeting and imaging with semiconductor quantum dots,” Nat. Biotechnol. 22, 969 (2004).

Magnetic Nanoparticles Biological Particles
Nano-sized magnetic particles can be superparamagnetic Widely Studied – Suggested as early as the 1970’s Offers control/manipulation in magnetic field Biological Particles Co has higher magnetization compared to magnetite and maghemite Science 291, 2001; J. Phys. D: Appl. Phys. 36, 2003; An Attractive Biological Tool

Magnetic Nanoparticles: Inner Ear Targeted Molecule Delivery and Middle Ear Implant
SNP controlled by magnets while transporting a payload Studies included in vitro and in vivo on rats, guinea pigs and human cadavers Demonstrated magnetic gradients can enhance drug delivery Application Magnetic Particles Magnetic Particles Perilymphatic fluid from the cochlea of magnet-exposed temporal bone Perilymphatic fluid samples from animals exposed to magnetic forces Audiol Neurotol 2006; 11:

Composite with A Novel Structure for Active Sensing in Living cells
Magnetic Quantum Dot Composite with A Novel Structure for Active Sensing in Living cells ① Cobalt core : active manipulation diameter : ~10 nm superparamagnetic NPs → manipulated or positioned by an external field without aggregation in the absence of an external field What is MQD ? Silica ZnS CdSe Co ② CdSe shell : imaging with fluorescence thickness : 3-5 nm visible fluorescence (~450 – 700 nm) ability to tune the band gap → by controlling the thickness, able to tune the emission wavelength, i.e., emission color ④ Silica shell : bio-compatibility & functionalization with specific targeting group thickness : ~10 nm bio-compatible, & non-toxic to live cell functions stable in aqueous environment ability to functionalize its surface with specific targeting group ③ ZnS shell : electrical passivation thickness : 1-2 nm having wider band gap (3.83 eV) than CdSe (1.91 eV) enhancement of QY → CdSe (5-10%)  CdSe/ZnS (~50%)

Conclusions The effective mass model give an excellent approximation of the size dependence of electronic properties of a quantum dot Recent synthesis advances have shown many quantum dot reactions to be robust, cheap, and safe then previously thought Quantum dots offer wide range electronic properties that make them an attractive tool for biological and medical work MQD’s improve afford in vivo manipulation expanded the applicability of quantum dots Rap-Up

Nanotechnology for the lazy: self-assembled semiconductor quantum dots
Gavin Bell University of Warwick

Outline Semiconductor quantum dots – what, why? InAs/GaAs SAQDs
Artificial versus self-assembled (SAQD) InAs/GaAs SAQDs Strained heteroepitaxy in MBE Analysing and controlling SAQD growth STM-MBE versus “STMBE” SEM, TEM, CAFM

Direct Gap Semiconductors
Direct gap semiconductors: lighting, optical communication, sensors, etc… For bulk material, the band gap controls the emission wavelength. Technology sets various ‘target’ wavelengths: CD player laser 780 nm, DVD 640 nm, Blu-ray and HD-DVD 405 nm. optical fibre transmission 1300 and 1550 nm.

Low Dimensional Semiconductors
Carriers in electrons can be confined by potential barriers (e.g. in a low gap material surrounded by higher gap material). Confinement can be in one dimension only – a 2D quantum well. Confinement in all three directions gives a zero-dimensional quantum dot (QD). Quantum dot density of states g(E) just discrete levels (no continuum of states – just ‘particle-in-a-box’ energy levels). Confinement sizes are a few nanometres.

Semiconductor Lasers and LEDs
Various flavours of quantum well lasers – well established technology. Arakawa and Sakaki (1982) predicted that quantum dot lasers should be more efficient

Real Quantum Dot Lasers
Innolume GmbH QD lasers 1064 – 1320 nm QD Laser Inc., Japan QD lasers 1.3 and 1.55 micro

Artificial Quantum Dots
Nano-fabricated QDs Position and size control. Electrical contacting. Poor quality for optical applications (surfaces and defects cause strong non-radiative recombination). Hard work! How about self-assembled QDs?

Molecular Beam Epitaxy
Fire beams of material at substrate crystal in vacuum. Perfect for growing layer-by-layer (2D) structures. Can we grow 3D / 1D / 0D structures by MBE? 3D structure (or ‘0D’ if small) 2D structure

Strained Heteroepitaxy
InAs on GaAs leads to 6.7% compressive strain. 6.7% strain is huge! Epitaxial stress 5.3 GPa for the (001) surface. Strain energy in growing layer must be relieved. Normally: dislocations.

Transfer pristine surface to STM for atomic-resolution imaging.
STM-MBE Asx ~10-4 Pa STM-MBE Interrupt MBE growth. Transfer pristine surface to STM for atomic-resolution imaging. UHV ~10-8 Pa

InAs on GaAs(110) 0.5 ML 0.5 ML 2 ML 2 ML STM-MBE (vacuum transfer)
200 nm images. 2D islands then dislocations in 2D layer-by-layer growth. 5 ML

InAs on GaAs(001) 1 ML Regular layer-by-layer growth 2 ML 3D islands
Height ~ 3 – 8 nm Width ~ 10 – 20 nm l ~ 10nm, so 0D?

Strain Relief ε -6.7 % σ 5.3 GPa < 1.7 ML
Wetting layer (WL) WL + QDs ε % σ 5.3 GPa < 1.7 ML > 1.7 ML (very rapid transition) In-plane strain (epitaxial stress) relieved by 3D islands. Lattice planes ‘bulge out’. No dislocations – QDs are coherent. Balance surface energy against elastic energy.

Control of self-assembly?
Average size & size distribution. Affects emission wavelength and sharpness. Density. Affects gain of QD laser. Position? Lateral ordering after many layers. Don’t need for independent QDs in lasers. Need to understand growth process.

QD Nucleation GaAs homoepitaxy InAs-GaAs heteroepitaxy islands adatoms
3D island density jumps suddenly at the critical thickness.

In vacuo STM-MBE: Quenching
GaAs(001)-(2x4) homoepitaxy 100nm images. ‘Rapid’ quench vs. 10s anneal at the growth temperature: island density drops by 2/3. Surface rearrangement during quench process? Cannot return tip to a particular feature to observe its development. Cannot capture surface dynamics. QD nucleation extremely rapid – problems?

STMBE versus STM-MBE STM-MBE STMBE Asx ~10-6 mbar UHV ~10-10 mbar
The rest of the world! Relatively easy to operate compared to STMBE. STMBE Anan NCT, Japan: Prof. Shiro Tsukamoto.

QD Nucleation – STMBE 1.75 ML B A 1.70 ML a b c Images obtained during growth: 200s per image, corresponds to 0.05 ML coverage interval ( ML / s). Tip raster direction

Quantum Dot Nucleation – Up Close
B C Blue arrows: 1 ML height Red arrows: 3-fold lattice space (1.2 nm) 3D islands develop over a coverage interval of 0.01 ML. Total volume in 3D islands jumps from zero to 0.49 ML over a coverage interval of 0.05 ML. 0.44 ML ‘extra’ In quickly available for QDs.

Wetting Layer Growth STMBE ‘movie’ of WL growth.
Observe normal step-flow growth. Fill in pits. Terrace advances. Island grows and joins up with terrace. Each frame 200 s, 0.05 ML. Measure area of ‘new’ terrace developed in each frame. Can’t do this with STM-MBE. Amount of new terrace always less than amount of deposited InAs. About 0.5 ML In available for rapid QD nucleation as mobile, weakly bound adatoms.

STMBE: Does the ‘ST’ Interfere With the ‘MBE’?
Incommensurate RHEED patterns Direct Fourier transform of STMBE images follow RHEED intensity maxima quite nicely. (1x3) (1xa3) 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 350 400 450 500 550 600 C O E X I S T N G c(4x4) + (1x3) (2x4) (2xa3) DIFFUSE (4x2) increasing asymmetry c(4x4) ( 2 x 4 ) Substrate Temperature / °C InAs coverage / ML

QD Composition Very important in defining potential well (hence optical properties). STM can’t easily measure composition. Can infer from cross-sectional STM. Total QD volume too large for pure InAs QDs. Must be an InGaAs alloy. Which technique? Scanning Transmission Electron Microscopy (Super-STEM). Grazing incidence X-ray diffraction (synchrotron radiation source). Medium energy ion scattering (MEIS).

Scanning electron microscopy
SEM Z-contrast: brighter means more In-rich QD tops In-rich. QD bases Ga-rich. Aligned along steps. QDs nm across. 100 nm

Medium Energy Ion Scattering
UK MEIS facility at Daresbury Laboratories. Beamline: typically He+ or H+ at 100 keV primary energy. Toroidal electrostatic analyser records scattered ion energy and angular distributions simultaneously (energy resolution ~ 400 eV). Able to probe typical samples to a depth of several nm.

Introduction to MEIS surface
Align incident beam either along a crystal direction or ‘random’ angle (little channelling hence high counts). Measure the angular and energy spectra of scattered ions. Channelling (lots of low-angle collisions along a crystal direction) Electronic stopping – ion loses energy at a calculable rate (‘stopping power’); these energy losses can be converted to a depth scale. Elastic collision – ion recoils at energy dependent only on scattering angle and mass of target nucleus.

Sample MEIS Data Sample data from two Sb delta-layers in Si.
Angular data – crystallographic information (e.g. depth of channelling dip depends on crystalline perfection; e.g. surface structure) Energy spectra – energy resolution best for high mass species in a low mass matrix (e.g. In in GaAs): highest recoil energy. Convert energy scale to depth scale by: energy loss = path length x stopping power Hence produce depth profiles, with near-atomic layer depth resolution.

QD Energy Spectra Channelled spectra, [110] in and normal out, very similar for uncapped (red) and 1 nm GaAs cap (partially capped QDs). Thicker GaAs cap (5 nm): In signal merges into GaAs. Concentrate on uncapped QDs.

Modelling Discrete Structures
MEIS normally applied to 2D layered structures Every ion recoiling from a particular depth has travelled the same path length. This is no longer true for discrete structures. Need to model every possible path through the QD. Place an average QD on a grid of points and allow ions to enter at each point on the grid. Energy spectrum is sum over all possible paths. Ion entry point Shallower scattering Deeper scattering

Sample Results for Simulations
Simulations for constant In concentration through the QDs (size and density fixed by AFM). Composition is 100% In for the top curve, then 80%, 60%, 40% and 20%. Simulations of linear composition gradient QDs (curves are 0-100% In from bottom to top, %, %, % and % on each graph).

Best Fit for QDs Include contributions from QDs, WL and large 3D islands. Best fit is a linear In gradient from 20% at the QD base to 100% at the QD tops. The QD bases are rather Ga-rich. Complex growth kinetics: the WL plays a very important role.

Conclusions Semiconductor quantum dots.
Self-assembled versus fabricated. InAs-GaAs QDs easy to grow by MBE. Possible to tailor to important wavelengths (e.g nm). Study growth in detail with STM. STMBE ‘movies’ versus STM-MBE ‘snapshots’. Growth kinetics not fully understood. Study composition of discrete nanostructures with medium energy ion scattering (MEIS).

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