Theoretical investigation of the lattice dynamics of GaAlSb superlattices

We investigate from first principles the effect of substitutional disorder on the off-resonance Raman activity of GaSb/AlSb superlattices (SLs) including strain and mixing effects. The method we use is based on a perturbation scheme which allows to take into account both the actual chemical configuration and local displacements from a given reference (virtual) system. We calculate the first-order Raman spectra of different (GaSb)_n(AlSb)_n superlattices grown in the [0 0 1] direction. The investigation of the effects of the interface modes on the Raman spectra allows the correct interpretation of the features found in the experiment.     

MBE方法在GaAs(001)衬底上外延生长GaSb膜

利用分子束外延方法(MBE)在GaAs(001)衬底上外延生长了GaSb薄膜,利用高分辨透射电子显微镜(HRTEM)、原子力显微镜(AFM)、Hall效应(HallEffect)和低温光荧光谱(LTPL)等手段对薄膜的晶体质量、电学性能和光学质量进行了研究。发现直接生长的GaSb膜表面平整,空穴迁移率较高。引入GaSb/AlSb超晶格可有效阻断进入GaSb外延层的穿通位错,对应的PL谱强度增强,材料的光学质量变好。     

Metallorganic chemical vapor deposition of InAs/GaSb superlattices on GaAs substrates using a two-step InAs buffer layer

InAs/GaSb type II superlattices (T2SLs) were grown heteroepitaxially, via metallorganic chemical vapor deposition (MOCVD), on GaAs substrates. The 7% lattice mismatch between the T2SL and GaAs substrate was accommodated through the use of a two-step InAs buffer layer. The periodicity of the grown structures was confirmed by X-ray diffraction (XRD). FTIR measurements of the structures yielded cutoff wavelength values from 4.9 μm to 8.7 μm, which, as expected, scaled with thickness of the InAs in the SL structure. Kronig-Penny modeling of the structures produced the general trend of the experimental data.     

Emission characteristics improvement in structures with InAs/GaAsN/InGaAsN superlattices

The influence of some parameters of nitrogen-containing heterostructures InAs/GaAsN/InGaAsN with strain-compensated superlattices (SCSL) on their emission characteristics has been studied. It is established that the net strain in the structure affects the photoluminescence (PL) linewidth, internal quantum efficiency, intensity, and wavelength. The maximum PL intensity and minimum full width at half maximum (FWHM) of the PL line were achieved with small strains (0–0.2%), whereas the maximum wavelengths (∼1.76 μm) observed for large strain (about +1%). By adding multilayer InAs inserts in the active InGaAsN quantum well in combination with using strain-compensated GaAsN/InGaAsN superlattices, it is possible to control the room-temperature emission wavelength in the range of 1.45–1.76 μm without significantly deteriorating the emissiion characteristics.     

Shape and size control of InAs/InP (113)B quantum dots by Sb deposition during the capping procedure

The role of Sb atoms present on the growth front during capping of InAs/InP (113)B quantum dots (QDs) is investigated by cross-sectional scanning tunnelling microscopy, atomic force microscopy, and photoluminescence spectroscopy. Direct capping of InAs QDs by InP results in partial disassembly of InAs QDs due to the As/P exchange occurring at the surface. However, when Sb atoms are supplied to the growth surface before InP capping layer overgrowth, the QDs preserve their uncapped shape, indicating that QD decomposition is suppressed. When GaAs(0.51)Sb(0.49) layers are deposited on the QDs, conformal growth is observed, despite the strain inhomogeneity existing at the growth front. This indicates that kinetics rather than the strain plays the major role during QD capping with Sb compounds. Thus Sb opens up a new way to control the shape of InAs QDs.     

Effects of a shell on the electronic properties of nanowire superlattices

We compute the structural and electronic properties of core-shell InAs/GaAs nanowire superlattices using Keating's valence force field and a sp3d5s* tight-binding model. We show that the GaAs shell limits strain relaxation but homogenizes the hydrostatic strain distribution in the InAs layers. This prevents the formation of a strain-induced well in the conduction band at the surface of the nanowires, which was shown to trap the electrons in thin InAs layers (Phys. Rev. B 2006, 74, 155304). The shell, however, enhances the piezoelectric field, which increases the separation between the electrons and holes in thick InAs layers. These results emphasize the intricate links between the structural and electronic properties of strained nanowire heterostructures.     

Molecular beam epitaxy of GaSb on GaAs substrates with AlSb/GaSb compound buffer layers

GaSb films with AlSb/GaSb compound buffer layers were grown by molecular beam epitaxy on GaAs (001) substrates. The crystal quality and optical properties were studied by high resolution transition electron microscopy and low temperature photoluminescence spectra (PL), respectively. It was found that the AlSb/GaSb compound buffer layers can restrict the dislocations into GaSb epilayers. The intensity of PL spectra of GaSb layer becomes large with the increasing the periods of AlSb/GaSb superlattices, indicating that the optical quality of GaSb films is improved.     

The improvement of GaN p-i-n UV sensor by 8-pair AlGaN/GaN superlattices structure

GaN with pairs of AlGaN/GaN superlattices (SLs) structure for p-i-n UV photo detector are fabricated on sapphire by metal organic chemical vapor deposition (MOCVD). For 8-pair AlGaN/GaN SLs not only eliminates cracking through this strain management, but it also significantly decreases the threading dislocation density by acting itself as an effective dislocation filter. The related structure has exhibited excellent film qualities such as enhanced crystallinity, lower specific contact resistance, lower etching pit density or mean roughness in the film. GaN p-i-n diode fabricated with 8-pair SLs, the dark current of device is reduced by two orders of magnitude than that without SLs structure at reverse bias of −3 V. Moreover, the peak UV responsivity is 0.12 A/W, which is higher than that without SLs is 0.07 A/W at 360 nm. The rejecting ratio is also by two orders of magnitude higher than that without SLs structure.     

Molecular beam epitaxy growth methods of wavelength control for InAs/(In)GaAsN/GaAs heterostructures

We discuss the molecular beam epitaxy (MBE) growth methods of emission wavelength control and property investigations for different types of InAs/(In)GaAsN/GaAs heterostructures containing InGaAsN quantum-size layers: (1) InGaAsN quantum wells deposited by the conventional mode in a GaAs matrix, (2) InAs quantum dots deposited in a GaAsN matrix or covered by an InGaAs(N) layer, and (3) InAs/InGaAsN/GaAsN strain-compensated superlattices with quantum wells and quantum dots. The structures under investigation have demonstrated photoluminescence emission in a wavelength range of ～1.3-1.8?μm at room temperature without essential deterioration of the radiative properties.     

InAs/GaInSb应变层超晶格材料的界面结构

介绍了InAs／GaInSb应变层超晶格材料的界面结合类型及其对材料界面结构和光电性能的影响,分析了在超晶格界面处的原子置换和扩散现象。同时,总结了分子束外延生长过程中控制InAs／GaInSb超晶格界面结构的生长工艺,指出采用原子迁移增强（MEE）外延生长技术,可以有效地抑制界面处原子的置换和扩散现象。     

Improvements in (112?2) semipolar GaN crystal quality by graded superlattices

We report on the use of graded superlattices (SLs) for defect reduction in semipolar (112?2) GaN films, grown by metal-organic chemical vapor deposition. High-resolution x-ray diffraction analysis revealed that there was a great reduction in the full width at half maximum, both on-axis and off-axis, with the SLs. Atomic force microscopy images revealed a significant decrease in slate features which was associated with the basal-plane stacking faults. The transmission electron microscopy images showed that the threading dislocation was greatly reduced after the graded superlattices. Room temperature photoluminescence measurement revealed that the band-edge emission intensity increased with the insertion of the SLs, which suggested reduction in the nonradiative recombination centers.     

Lasing properties of strain-compensated InAs/InGaAsN/GaAsN heterostructures in 1.3–1.55 μm spectral range

We have studied the radiative properties of heterostructures comprising InAs/InGaAsN quantum wells in strain-compensated GaAsN/InGaAsN superlattices, which are intended for the active regions of lasers operating at 1.3–1.55 μm. Using such superlattices and additional InAs monolayer insertions, it is possible to control the wavelength of room-temperature emission from InGaAsN quantum wells within 1.3–1.55 μm. The InAs/InGaAsN/GaAsN laser heterostructures are obtained, whose laser generation spectrum at 85 K corresponds to room-temperature lasing at ～1.5 μm.     

Transients in the formation of nanowire heterostructures

We present results on the effect of seed particle reconfiguration on the growth of short InAs and InP nanowire segments. The reconfiguration originates in two different steady state alloy compositions of the Au/In seed particle during growth of InAs and InP. From compositional analysis of the seed particle, the In content in the seed particle is determined to be 34 and 44% during InAs and InP growth, respectively. When switching between growing InAs and InP, transient effects dominate during the time period of seed particle reconfiguration. We developed a model that quantitatively explains the effect and with the added understanding we are now able to grow short period (<10 nm) nanowire superlattices.     

Fabrication and characterization of epitaxial SrTiO3/Nb-doped SrTiO3 superlattices by double ECR ion beam sputter deposition

Epitaxial oxide superlattices (SLs) of SrTiO₃ (STO) and Nb-doped SrTiO₃(STNO) were fabricated on LaAlO₃ (LAO) (001) substrates by an ion beam sputter deposition (IBSD) system having double electron cyclotron resonance (ECR) ion guns. The [STO_x/STNO_y]₁₀ SLs were epitaxially grown at different stacking sequences (x = 6 nm, y = 1–6 nm) and maintained the periodicity z of 10. Structural properties and surface morphology are found to be strongly dependent on the STNO sublayer thickness (y). Highly strained SLs with two-dimensional growth mode is observed at smaller STNO sublayer thickness (y = 1 nm). With increasing to a critical thickness (y = 4 nm), the SLs are freely strained and transformed to three-dimensional growth mode. The results demonstrate that the double ECR-IBSD is a versatile technique for the growth of high-quality oxide SLs.     

Optical properties of self-assembled lateral superlattices in AlInAs epitaxial layers and AlAs/InAs short-period superlattices

Characterization of self-assembled lateral superlattices in AlInAs epitaxial layers and AlAs/InAs short-period superlattices is presented. These structures are spontaneously generated during the epitaxial growth by metal–organic chemical vapor deposition and molecular beam epitaxy. Transmission electron microscopy reveals the structural details and electro-modulated reflectance is used to characterize the energy and anisotropy of the optical transitions in the lateral superlattices. We demonstrate several properties of these self-assembled structures: (a) the band gap energy can be changed by as much as 350 meV, (b) the polarization anisotropy of the lowest energy transition exceeds 90%, (c) the superlattice axis and the direction of the optical anisotropy can be oriented along two non-equivalent directions in the plane of the substrate, and (d) the valence band splitting between heavy- and light-hole transitions is significant. We discuss the difference between the samples from the two growth techniques. Finally, we theoretically model the electronic states in these lateral superlattices and we demonstrate that the difference in average InAs composition between the well and barrier can be as high as 35%.     

Splitting of Cyclotron Resonance Line in InAs/AlSb QW Heterostructures in High Magnetic Fields: Effects of Electron-Electron and Electron-Phonon Interaction

Cyclotron resonance in InAs/AlSb quantum well heterostructures in quantizing magnetic fields up to 13 T was studied. Effects of electron-electron and electron-phonon interactions were discovered.     

Effect of structural design on the optical properties of strain-compensated InAs/InGaAsN/GaAsN superlattices

We have studied the influence of structural design on the optical properties of heterostructures comprising InAs quantum wells (QWs) and quantum dots (QDs) in strain-compensated GaAsN/InGaAsN superlattices. It is established that, using such superlattices with various QW and barrier thicknesses and different numbers (from one to three) InAs inserts in the active region, it is possible to control the wavelength of room-temperature emission within 1.3–1.76 μ m without deteriorating the output radiation characteristics.     

The Influence of the Parameters of a Short-Period InGaAs/InGaAlAs Superlattice on Photoluminescence Efficiency

Technical Physics Letters - We have studied heterostructures based on short-period InGaAs/InGaAlAs superlattices (SLs) manufactured by molecular-beam epitaxy on InP substrates, intended for use as...     

Optical properties of strain-compensated InAs/InGaAsN/GaAsN superlattices

The optical properties of heterostructures comprising InAs/InGaAsN quantum wells in strain-compensated GaAsN/InGaAsN superlattices have been studied. It is demonstrated that, using such superlattices of various design and thickness and with additional InAs monolayer spacers, it is possible to control the wavelength of room-temperature emission from InGaAsN quantum wells within 1.3–1.6 μm without deteriorating the output radiation characteristics, which opens additional prospects for the development of lasers on GaAs substrates for telecommunication applications.     

Epitaxial growth of binary and ternary metallic strained superlattices and their magnetic properties

Since the structure at/near the interface of superlattices influences physical properties such as magnetic property, it is important to investigate details of the structure. The interface structure is characterized by the factors like atomic species, strain, mixing and roughness. The reflection high-energy electron diffraction (RHEED) system installed in our molecular-beam epitaxy (MBE) system enables us to observe, continuously, the change of the surface in-plane lattice constant, which is affected by atomic species, strain and/or mixing, on a realtime basis. Ternary superlattices consisting of three elements can clarify the effect of stacking sequence by comparison between the two types of superlattices with the reverse deposition sequences, since the effect caused by the combination of the same atomic species is cancelled out and the effect caused by the different stacking sequences remains. In the present paper, we review growth behaviors of binary and ternary metallic strained superlattices, especially magnetic ones, investigated mainly by our group, and summarize the discussion on their magnetic properties, mainly on the magnetic anisotropy, in terms of their structural characteristics. First, we introduce our RHEED system that works efficiently under a magnetic field arising from evaporation sources for low vapor-pressure materials. Then, MBE-grown binary strained superlattices, Co/Au, Co/Pt and Cu/Au, are discussed, with comparing to incoherent superlattices of Co/Ag and Cu/Ag having nearly the same lattice mismatch of constituents. Next, we review ternary strained superlattices with immiscible constituents with reverse deposition order, Au/Co/Ag and Ag/Co/Au superlattices, and Au/Co/Cu and Cu/Co/Au superlattices, in relation to the growth behaviors of binary superlattices. Finally, ternary strained superlattices containing both miscible and immiscible constituents, Pt/Co/Ag and Ag/Co/Pt superlattices, and Au/Ni/Ag and Ag/Ni/Au superlattices, are reviewed.