Effect of elastic anisotropy on the strain fields and band edges in stacked InAs/GaAs quantum dot nanostructures

The effect of elastic anisotropy on the strain fields and confinement potentials in InAs/GaAs quantum dot (QD) nanostructures was investigated for an isolated dot and a stacked multi-layer dots using finite element analysis and model solid theory. The assumption of isotropy tends to underestimate especially hydrostatic strain that is known to modify confinement potentials in conduction band. Consideration of anisotropy results in a wider band gap and shallower potential well as compared with the isotropic model. Since the band gap and potential well depth would be related to opto-electronic properties of quantum dot systems via quantum mechanical effects, it is suggested that consideration of elastic anisotropy in the calculation of strains and band structures is necessary for the design of QD-based opto-electronic devices.     

Self-organized formation of shell-like InAs/GaAs quantum dot ensembles

Formation of a multimodal quantum dot (QD) ensemble by strained layer epitaxy of InAs on GaAs near the critical value for the onset of the 2D-3D transition is studied. Reflection anisotropy spectroscopy is employed to confirm that a smooth surface is maintained during strained layer growth prior to QD formation. Instantaneous capping after deposition leads to InAs quantum wells with some thickness flucuations. Multimodal QD InAs ensembles form after an at least short growth interruption prior to cap layer deposition. The QDs consist of pure InAs with heights varying in steps of complete InAs monolayers. Related exciton energies indicate a simultaneous increase of both height and lateral extension, i.e. a shell-like increase of sizes. The formation of the multimodal QD ensemble is described by a kinetic approach. A growth scenario is presented where QDs having initially shorter base length stop vertical growth at a smaller height, accounting for the experimentally observed shell-like sub-ensemble structure.     

A Photovoltaic InAs Quantum-Dot Infrared Photodetector

A photovoltaic quantum dot infrared photodetector with InAs/GaAs/AIGaAs structures is reported. The detector is sensitive to normal incident light. At zero bias and 78K, a clear spectral response in the range of 2-7μm has been obtained with peaks at 3.1, 4.8 and 5.7μm. The bandgap energies of GaAs and Al0.2Ga0.8As at 78K are calculated and the energy diagram of the transitions in the Quantum-Dot Infrared Photodetector （QDIP） is given out. The photocurrent signals can be detected up to 110 K, which is state-of-the-art for photovoltaic QDIP, The photovoltaic effect in our detector is a result of the enhanced band asymmetry as we design in the structure.     

Ordered InAs Quantum Dots with Controllable Periods Grown on Stripe-Patterned GaAs Substrates

GaAs （001） substrates are patterned by electron beam lithography and wet chemical etching to control the nucleation of lnAs quantum dots （QDs）. InAs dots are grown on the stripe-patterned substrates by solid source molecular beam epitaxy. A thick buffer layer is deposited on the strip pattern before the deposition of InAs. To enhance the surface diffusion length of the In atoms, InAs is deposited with low growth rate and low As pressure. The AFM images show that distinct one-dimensionally ordered InAs QDs with homogeneous size distribution are created, and the QDs preferentiMly nucleate along the trench. With the increasing amount of deposited InAs and the spacing of the trenches, a number of QDs are formed beside the trenches. The distribution of additional QDs is long-range ordered, always along the trenchs rather than across the spacing regions.     

Thermal Analysis of InAs/AlSb Short Wavelength Mid-IR Quantum Cascade Lasers

We present the effects of hetero-interfaces and major key parameters on the thermal behaviors and performance of short wavelength mid-IR InAs/AlSb quantum cascade lasers （QCLs）. We use a finite element method （FEM） with commercial software, ANSYS, to simulate the heat dissipation in QCLs in cw operation mode with an epilayer-down mounting package. The thermal performance is characterized by the temperature increase AT （self-heating effect） between the active region of QCLs and the heatsink. Results show that （1） the self-heating effects of InAs/AlSb QCLs are much less than those in AlInAs/GaInAs Q, CLs, （2） narrower ridges lead to significantly cooler active regions of InAs/AlSb QCLs due to poor heat transport in the cross-plane direction （across interfaces） and that most of the heat flows out of the active region in the lateral direction, and （3） the cavity length of the laser has little influence on the self-beating effect of the device, but the long cavity reduces mirror loss and threshold current density.     

GaAs Based InAs/GaSb Superlattice Short Wavelength Infrared Detectors Grown by Molecular Beam Epitaxy

InAs/GaSb superlattice （SL） short wavelength infrared photoconduction detectors are grown by molecular beam epitaxy on GaAs（O01） semi-insulating substrates. An interfacal misfit mode AISb quantum dot layer and a thick GaSb layer are grown as buffer layers. The detectors containing a 200-period 2 ML/S ML InAs/GaSb SL active layer are fabricated with a pixel area of 800×800 μm^2 without using passivation or antirefleetion coatings. Corresponding to the 50% cutoff wavelengths of 2.05μm at 77K and 2.25 μm at 300 K, the peak detectivities of the detectors are 4 × 10^9 cm·Hz^1/2/W at 77K and 2 × 10^8 cm·Hz1/2/W at 300K, respectively.     

Non-catalytic growth of high aspect-ratio ZnO nanowires by thermal evaporation

High-density and high aspect-ratio ZnO nanowires were grown on Si(100) substrates by the thermal evaporation of metallic zinc powder without the use of metal catalysts or additives. The as-grown nanowires had diameters in the range of 60-100 nm with lengths 5-15 μm. Detailed structural characterization indicated that the obtained nanowires are single-crystalline with a perfect hexagonal facet and surfaces. The room temperature PL spectrum exhibited strong UV emission, affirming that the as-grown products have good optical properties. The possible growth mechanism for the formation of hexagonal-faceted and perfect surface ZnO nanowires is also discussed.     

Growth and Auger electron spectroscopy characterization of donut-shaped ZnO nanostructures

Zinc oxide (ZnO) nanodonuts have been obtained by vapor phase transport process utilizing a mixture of ZnO, graphite and erbium oxide powder as the evaporation source. ZnO nanodonuts prepared under various thermal processes indicate that ZnO nanodonuts start forming during the initial thermal ramp up stage. A subsequent holding of the growth temperature at 1000 °C causes the nanodonut to evolve into perfectly donut-shaped nanostructure. Additional deposition of ZnO on top of the nanodonut during the holding of the furnace temperature at 1000 °C result in partially filled nanodonuts or hemispherical nanostructures, or donuts that are completely buried beneath ZnO film. Auger electron spectroscopy depth profile analysis indicates that the deposited ZnO film is stoichiometric, whereas the nanodonuts and the completely filled hemispherical nanostructures are porous and are oxygen deficient. The volume density of the nanodonut is estimated to be 20% that of the background ZnO film.     

Interfaces in InAs/GaSb Superlattices Grown by Molecular Beam Epitaxy

Short period InAs（4ML）/GaSb（SML） superlattices （SLs） with InSb- and mixed-like （or Ga1-xInxAs1-ySby- like） interfaces （IFs） are grown by molecular-beam epitaxy （MBE） on （001） GaSh substrates at optimized growth temperature. Raman scattering reveals that two kinds of IFs can be formed by controlling shutter sequences. X-ray diffraction （XRD） and atomic force microscopy （AFM） demonstrate that SLs with mixed-like IFs are more sensitive to growth temperature than that with InSb-like IFs. The photoluminescence （PL） spectra of SLs with mixed-like IFs show a stronger intensity and narrower line width than with InSb-like IFs. It is concluded that InAs/GaSb SLs with mixed-like IFs have better crystalline and optical properties.     

Growth-Parameter Spaces and Optical Properties of Cubic Boron Nitride Films on Si（001）

Cubic boron nitride （c-BN） films were deposited on Si（001） substrates in an ion beam assisted deposition （IBAD） system under various conditions, and the growth parameter spaces and optical properties of c-BN films have been investigated systematically. The results indicate that suitable ion bombardment is necessary for the growth of c-BN films, and a well defined parameter space can be established by using the P/a-parameter. The refractive index of BN films keeps a constant of 1.8 for the c-BN content lower than 50%, while for c-BN films with higher cubic phase the refractive index increases with the c-BN content from 1.8 at χc =50% to 2.1 at χc = 90%. Furthermore, the relationship between n and p for BN films can be described by the Anderson-Schreiber equation, and the overlap field parameter γ is determined to be 2.05.     

Spatial imaging of valence band electronic structures in a GaSb/InAs quantum well

We measure local density of states (LDOS) for GaSb/InAs heterostructures with quantum wells in the valence band by scanning tunneling spectroscopy (STS) on the cleaved surface. Clear standingwave patterns of LDOS corresponding to the holes confined in the quantum wells are observed.     

Transmission electron microscopy study of vertical quantum dots molecules grown by droplet epitaxy

The compositional distribution of InAs quantum dots grown by molecular beam epitaxy on GaAs capped InAs quantum dots has been studied in this work. Upper quantum dots are nucleated preferentially on top of the quantum dots underneath, which have been nucleated by droplet epitaxy. The growth process of these nanostructures, which are usually called as quantum dots molecules, has been explained. In order to understand this growth process, the analysis of the strain has been carried out from a 3D model of the nanostructure built from transmission electron microscopy images sensitive to the composition.     

Molecular Beam Epitaxy of GaSb on GaAs Substrates with AlSb Buffer Layers

We investigate the molecular beam epitaxy growth of GaSb films on GaAs substrates using AlSb buffer layers. Optimization of AlSb growth parameter is aimed at obtaining high GaSh crystal quality and smooth GaSh surface. The optimized growth temperature and thickness of AlSb layers are found to be 450℃ and 2.1 nm, respectively. A rms surface roughness of 0.67 nm over 10 × 10 μm^2 is achieved as a 0.5 μm GaSh film is grown under optimized conditions.     

High Quality AlN with a Thin Interlayer Grown on a Sapphire Substrate by Plasma-Assisted Molecular Beam Epitaxy

We report an AlN epi-layer grown on sapphire by plasma-assisted molecular beam epitaxy with a thin interlayer structure. The effects of growth mode on threading dislocations (TDs) and surface morphology are studied. Then an interlayer structure grown under a V/Ⅲ ratio of 1 is adopted to improve the AlN crystalline quality. By optimizing the thickness of the interlayer, the TD density and surface roughness can be reduced simultaneously.     

Continuous Wave Performance and Tunability of MBE Grown 2.1μm InGaAsSb/AlGaAsSb MQW lasers

InCaAsSb/AlGaAsSb multi quantum well ridge waveguide lasers at 2.1 μm wavelength are fabricated by using molecular beam epitaxy. Continuous wave performance and tunability of the lasers are evaluated in a wide temperature range extend to 80℃. Output power of the laser at 30℃ exceeds 30 m W/facet at driving current of 0.5 A, the characteristic temperature To is 89K in 0-50℃ range. No fast degradation is observed in accelerated aging test at 90℃ for those lasers with lower Al content in cladding layers. Temperature tunability of the lasers is 1.36 nm/K. Single-mode output with side mode suppression ratios greater than 20 dB is achieved in a certain driving current region; current tunability is 8 × 10^-3 nm/mA regardless of mode hopping.     

Cr-Doped InAs Self-Organized Diluted Magnetic Quantum Dots with Room-Temperature Ferromagnetism

Cr-doped InAs self-organized diluted magnetic quantum dots （QDs） are grown by low-temperature molecularbeam epitaxy. Magnetic measurements reveal that the Curie temperature of all the InAs：Cr QDs layers with Cr/In flux ratio changing from 0.026 to 0.18 is beyond 400 K. High-resolution cross sectional transmission electron microscopy images indicate that InAs：Cr QDs are of the zincblende structure. Possible origins responsible for the high Curie temperature are discussed.     

Effects of In and Sb mono-layers to form rotated InSb films on a Si(1 1 1) substrate

A new method for InSb heteroepitaxial growth on a Si substrate was introduced in our previous work, in which an InSb film was formed via an InSb bi-layer. In the present work, to study the effects of In and Sb individual layers on the InSb film quality, InSb was deposited onto an InSb bi-layer, In mono-layer, and Sb mono-layer on a Si substrate. It was found that both In and Sb layers (in other words, InSb bi-layer) were essential to form a fine InSb film.     

Island dissolution during capping layer growth interruption

A possible scenario for the dissolution of partially capped quantum dots was investigated. This model is based on the consideration of the total free energy being a sum of elastic and surface energies as the quantum-dot material redistributes itself as a second wetting layer on top of the capping layer. Quantitative results were obtained for the case of InAs/GaAs quantum dots that are partially capped by GaAs. We compare our results with supporting experimental evidence. Received: 29 January 2001 / Accepted: 30 January 2001 / Published online: 3 April 2001     

Sb-mediated growth of high-density InAs quantum dots and GaAsSb embedding growth by MBE

Self-assembled InAs quantum dots (QDs) with high-density were grown on GaAs(0 0 1) substrates by antimony (Sb)-mediated molecular beam epitaxy technique using GaAsSb/GaAs buffer layer and InAsSb wetting layer (WL). In this Sb-mediated growth, many two-dimensional (2D) small islands were formed on those WL surfaces. These 2D islands provide high step density and suppress surface migration. As the results, high-density InAs QDs were achieved, and photoluminescence (PL) intensity increased. Furthermore, by introducing GaAsSb capping layer (CL), higher PL intensity at room temperature was obtained as compared with that InGaAs CL.     

Growth of highly disordered InGaP on (100) GaAs by molecular beam epitaxy with a GaP decomposition source

High-quality, lattice-matched InGaP on exact (100) GaAs was successfully grown by molecular beam epitaxy with a GaP decomposition source. The ordering parameter (η) of the InGaP is investigated as a function of the growth temperature. η is as low as 0.22 and almost insensitive to the growth temperature below 460 °C. It increases abruptly around 475 °C and has a maximum value of 0.35 at ≈490 °C. Double crystal X-ray diffraction and a low-temperature photoluminescence spectrum reveal that the present growth method is robust and provides better quality InGaP compared to other state-of-the-art growth technologies. Received: 20 November 2000 / Accepted: 27 January 2001 / Published online: 21 March 2001