Growth of InAs-AlSb quantum wells having both high mobilities and high concentrations

Low-temperature mobilities in InAs-AlSb quantum wells depend sensitively on the buffer layer structures. Reflection high energy electron diffraction and x-ray diffraction show that the highest crystalline quality and best InAs transport properties are obtained by a buffer layer sequence GaAs → AlAs → AlSb → GaSb, with a final GaSb layer thickness of at least 1 μm. Using the improved buffer scheme, mobilities exceeding 600,000 cm²/Vs at 10 K are routinely obtained. Modulation δ-doping with tellurium has yielded electron sheet concentrations up to 8 × 10¹² cm⁻² while maintaining mobilities approaching 100,000 cm²/Vs at low temperatures.     

Investigation of the influence of the well and the barrierthicknesses in GaSb/AlSb/GaSb/AlSb/InAs double-barrier interbandtunneling structures

The tunneling currents of GaSb/AlSb/GaSb/AlSb/InAs double-barrier interband tunneling (DBIT) structures were studied experimentally by varying the thickness of the well and the barrier layers systematically. The optimal thicknesses for the GaSb well and the AlSb barriers were found to be 6.5 and 1.0 nm, respectively, to obtain a high peak current density (19 kA/cm²), with a large peak-to-valley ratio of 4. The high peak current in the DBIT structure shows the strong effect of the resonant coherence of the wave function across the double barrier. For the case of a small GaSb well width (3 nm), a drastic reduction of the peak current was observed, an effect suggesting that the electron-wave function in the InAs couples primarily to the quantized light hole state in the GaSb well     

Intersubband Raman scattering in InAs/AlSb quantum wells

Raman spectroscopy has been used to study intersubband transitions in InAs/AlSb single quantum wells grown by molecular-beam epitaxy on (100) GaAs substrates using strain-relaxed AlSb or GaSb buffer layers. From the measured energies of the coupled longitudinal optical phonon-intersubband plasmon modes the single particle transition energies between the first and second confined electron subbands were deduced as a function of the width of the pseudomorphically strained InAs well. Subband spacings calculated including the effects of strain and nonparabolicity were found to be in agreement with the experimental transition energies. For a given well width, the two-dimensional electron concentration deduced from the Raman measurements was found to be lower than the concentration measured in the dark by Hall effect, but showed a significant increase with increasing optical excitation intensity.     

Resonant interband tunneling device with multiple negativedifferential resistance regions

The first observations of multiple negative differential resistance (NDR) regions in resonant interband tunneling devices are reported. In vertically integrated poly-type heterostructures of InAs/AlSb/GaSb, the peak voltages are reduced by a factor of 2 compared to the AlInAs/GaInAs material system, while high peak-to-valley ratios of 4:1 (17:1) at 300 K (77 K) are maintained. The InAs/AlSb/GaSb material system offers advantages for circuit applications of such devices, particularly operation at lower voltages     

In-situ monitoring, structural, and optical properties of ultrathin GaSb/GaAs quantum wells grown by OMVPE

We present a study of the growth of strained ultrathin GaSb quantum well (QW) layers in a GaAs host crystal by organometallic vapor phase epitaxy (OMVPE). We report surface anisotropy features observed by reflectance difference spectroscopy (RDS) during exposure of the GaAs (001) surface to trimethylantimony (TMSb) and during subsequent growth interruption. We demonstrate the formation of a floating layer of Sb during growth of GaAs over GaSb quantum well layers. The periodic nature of the RDS signal during growth of multiple quantum well (MQW) structures allows us to construct time-resolved RDS spectra, detailing the evolution of the surface anisotropy. We show how x-ray diffraction (XRD) data may be used to determine the graded compositional profile resulting from Sb segregation at the GaAs/GaSb interface. Photoluminescence (PL) spectra at 2 K from MQW structures exhibit two peaks below the GaAs bandgap. The lower-energy peak, which we attribute to a type-II transition at the GaSb/GaAs interface, shifts logarithmically with excitation power density. The higher energy peak shows no shift with excitation power, and is attributed to a transition occurring within the graded barrier layers.     

Investigation of negative differential resistance phenomena inGaSb/AlSb/InAs/GaSb/AlSb/InAs structures

The negative differential resistance (NDR) phenomena were observed in GaSb/AlSb/InAs/-GaSb/AlSb/InAs resonant interband tunnel structures. Electrons have resonantly achieved interband tunneling through the InAs/GaSb broken-gap quantum well. The InAs well width causes significant variations of the peak current density and NDR behaviors. The peak current density varies exponentially with the AlSb barrier thickness. The multiple NDR behavior was observed with appropriate InAs well and AlSb barrier thicknesses, e.g., 30 Å thick AlSb barrier and 240 Å wide InAs well. Only single negative resistance has, otherwise, been seen. The three-band model was used to interpret the effect of the InAs well and AlSb barrier on the current-voltage characteristics of GaSb/AlSb/InAs/GaSb/AlSb/InAs structures     

Terahertz electroluminescence from GaSb/AlSb quantum cascade laser

A phonon depopulation GaSb/AlSb quantum cascade laser was fabricated for the first time. The low longitudinal-optical phonon energy and small effective electron mass of GaSb enables a low electric field at the lasing threshold and a wide design range. The frequency of oscillation was designed to be 2.6 THz. With an increase of injection current, an abrupt increase of luminescence was observed.     

Room-temperature optically-pumped InGaSb quantum well lasers monolithically grown on Si(100) substrate

Room-temperature optically-pumped In/sub 0.2/Ga/sub 0.8/Sb quantum well lasers on Si are reported. The defect-free monolithic epistructure growth on a Si(100) substrate is initiated by an AlSb quantum dot nucleation layer followed by an AlSb/GaSb superlattice. The 13% mismatch between the AlSb and Si lattice is accommodated by misfit dislocations and associated crystallographic undulations in the AlSb buffer. The nucleation layer and buffer are characterised by atomic force microscopy and transmission electron microscopy. The lasing spectrum is characterised as a function of pump power and polarisation analysis.     

Effect of the Crystallographic Orientation of GaSb Films on Their Structural Properties during MBE Heteroepitaxy on Vicinal Si(001) Substrates

Semiconductors - GaSb films are grown by molecular-beam epitaxy using AlSb/As/Si transition layers on vicinal Si(001) substrates miscuted by 6° in (111) plane direction. The effect of GaSb...     

Topological Insulator Superlattices

HgTe/CdTe and InAs/GaSb/AlSb superlattices both exhibit a topological insulator transition. In each case, there is an inversion of the s- and p-band ordering for layer thicknesses above a critical value. The resulting topological phase is a 2D bulk insulator at zero temperature, with edges that conduct massless carriers whose direction of motion is locked to their direction of spin. These 1D edge states exhibit essentially dissipationless transport over coherence lengths greater than one micron, with a quantized conductance of e²/h per edge. When a current passes, opposite spins are separated to the two sample edges, giving rise to the so-called quantum spin Hall effect. Effects such as these may be exploited in future low temperature spintronic devices. The edge states in HgTe/CdTe differ from those in InAs/GaSb/AlSb in several ways, due to the type II band alignment and weaker electron–hole hybridization of the III-V superlattice. The former exhibit a simple exponential decay over thousands of Angstroms, while the latter are more strongly confined to the edge, with an oscillating wave function whose period increases with the edge state momentum. In any calculation, the edge state dispersion and the nature of the wave-function depend strongly on the boundary conditions used. A k · p model is presented using standard boundary conditions for the wave function and its derivative, which yields spin polarized edge states with a finite amplitude at the sample edge. The interaction between states at opposite sample edges is also considered.     

New system of self-assembled GaSb/GaP quantum dots

The atomic structure and energy spectrum of self-assembled GaSb/GaP quantum dots are discussed. It is shown that the quantum dots consist mainly of fully relaxed GaSb and have type-I band alignment with the ground electron state at the indirect valley of the GaSb conduction band.     

Effects of layer design on the performance of InAs/AlSb/GaSb resonant interband tunneling diodes on GaAs substrates

The room temperature current-voltage characteristics of InAs/AlSb/GaSb resonant interband tunneling diodes (RITDs), grown by chemical beam epitaxy on GaAs substrates, are reported as a function of the InAs buffer layer thickness and different interface configurations. The peak-to-valley current ratio (PVCR) improved from 1 to 12 as the buffer thickness was increased from 0 to 500 nm and the density of dislocations caused by the lattice mismatch of ∼7% decreased. No significant improvement was seen for a buffer thickness beyond 500 nm. Dislocation-free RITDs, grown lattice-matched on InAs substrates, show PVCRs of approximately 16. The InAs/AlSb Interfaces in these structures can be either InSb-like or AlAs-like and the interface can have a very strong effect on the diode performance. Unlike the case in InAs/AlSb field effect transistor structures, an AlAs-like interface results in better PVCRs in the diodes. Details of the results of this study are presented.     

基于有限差分的带间级联结构理论研究

利用前后向有限差分算法,对锑化物带间级联激光器有源区结构InAs/GaSb/AlSb的电子态进行了理论计算与分析。研究了哈密顿量算符序、有效质量参数修正、内界面态对结构能态及波函数的影响。分析表明,导带有效质量参数取正值可以在两种算符序下有效抑制伪解产生,Burt-Foreman算符序下的跃迁能量更为合理。对于内界面采用缓变假设后,其跃迁能量计算值略高于陡变界面下的计算结果,二者的波函数在界面附近差异不明显。     

Electron-electron interaction and spin-orbit coupling in InAs/AlSb heterostructures with a two-dimensional electron gas

The effect of electron-electron interaction on the spectrum of two-dimensional electron states in InAs/AlSb (001) heterostructures with a GaSb cap layer with one filled size-quantization subband. The energy spectrum of two-dimensional electrons is calculated in the Hartree and Hartree-Fock approximations. It is shown that the exchange interaction decreasing the electron energy in subbands increases the energy gap between subbands and the spin-orbit splitting of the spectrum in the entire region of electron concentrations, at which only the lower size-quantization band is filled. The nonlinear dependence of the Rashba splitting constant at the Fermi wave vector on the concentration of two-dimensional electrons is demonstrated.     

Persistent photoconductivity in InAs/AlSb heterostructures with double quantum wells

Effects of persistent photoconductivity in InAs/AlSb heterostructures with the cap GaSb layer and two-dimensional electron gas in the InAs double quantum wells at T = 4.2 K are studied. From Fourier analysis of the Shubnikov-de Haas oscillations, electron concentrations in each quantum well are determined at various wavelengths of illumination and pronounced asymmetry of the structure caused by the built-in electric field is demonstrated explicitly. The self-consistent calculations of the energy profile of the double quantum well are performed and the concentrations of ionized donors on both sides of the well are determined, which provided concretization of the previously suggested mechanism of bipolar persistent photoconductivity in such structures.     

1.54 lm GaSb/AlGaSb multi-quantum-well monolithic laser at 77 K grown on miscut Si substrate using interfacial misfit arrays

A GaSb quantum-well (QW) laser diode grown monolithically on a 5deg miscut Si (001) substrate is presented. The III-Sb epi-structure is grown monolithically on the miscut Si substrate via a thin (50 nm) AlSb nucleation layer. The 13% lattice mismatch between AlSb and Si is accommodated by a self-assembled 2D array of interfacial misfit dislocations (IMF). The 5deg miscut geometry enables simultaneous IMF formation and anti-phase domain suppression. The 1 mm times 100 mum GaSb QW laser diode operates under pulsed conditions at 77 K with a threshold current density of 2 kA/cm² and a maximum peak power of ~20 mW. Furthermore, the device is characterised by a 9.1 Omega forward resistance and a leakage current density of 0.7 A/cm² at -5 V.     

Novel GaAs/AlGaAs multiquantum-well Schottky-junction device andits photovoltaic LWIR detection

The authors have demonstrated photovoltaic detection for a multiple-quantum-well (MQW) long-wavelength infrared (LWIR) detector. With a blocking layer, the MQW detector exhibits Schottky I-V characteristics with extremely low dark current and excellent ideality factor. The dark current is 5×10^-14 A for a 100×100 μm² detector (designed for 10-μm response) at 40 K, nearly nine orders of magnitude lower than that of a similar MQW LWIR detector without the blocking layer. The ideality factor is ~1.01-1.05 at T=40-80 K. The measured Schottky-barrier height is consistent with the energy difference between first excited states and ground states, or the peak of spectral response. The authors also report a measured effective Richardson constant (A**) for a GaAs/AlGaAs heterojunction using this blocking layer structure. The A** is ~2.3 A/cm²/K ²     

Vertical transport in type-II heterojunctions with InAs/GaSb/AlSb composite quantum wells in a high magnetic field

Vertical transport in type-II heterojunctions with a two-barrier AlSb/InAs/GaSb/AlSb quantum well (QW) grown by MOVPE on an n-InAs (100) substrate is investigated in quantizing magnetic fields up to B = 14 T at low temperatures T = 1.5 and 4.2 K. The width of the QWs is selected from the formation condition of the inverted band structure. Shubnikov–de Haas oscillations are measured at two orientations of the magnetic field (perpendicular and parallel) relative to the structure plane. It is established that conduction in the structure under study is occurs via both three-dimensional (3D) substrate electrons and two-dimensional 2D QW electrons under quantum limit conditions for bulk electrons (B > 5 T). The electron concentrations in the substrate and InAs QW are determined. The g-factor for 3D carriers is determined by spin splitting of the zero Landau level. It is shown that the conductance maxima in a magnetic field perpendicular to the structure plane and parallel to the current across the structure in fields B > 9 T correspond to the resonant tunneling of 3D electrons from the emitter substrate into the InAs QW through the 2D electron states of the Landau levels.     

GaAs基GaSb体材料及InAs/GaSb超晶格材料的MBE生长

采用分子束外延方法在GaAs(100)衬底上生长GaSb体材料,以此GaSb为缓冲层生长了不同InAs厚度的InAs/GaSb超晶格,其10K光致发光谱峰值波长在2.0～2.6 μm.高分辨透射电子显微镜观察证实超晶格界面清晰,周期完整.     

Cyclotron resonance of dirac fermions in InAs/GaSb/InAs quantum wells

The band structure of three-layer symmetric InAs/GaSb/InAs quantum wells confined between AlSb barriers is analyzed theoretically. It is shown that, depending on the thicknesses of the InAs and GaSb layers, a normal band structure, a gapless state with a Dirac cone at the center of the Brillouin zone, or inverted band structure (two-dimensional topological insulator) can be realized in this system. Measurements of the cyclotron resonance in structures with gapless band spectra carried out for different electron concentrations confirm the existence of massless Dirac fermions in InAs/GaSb/InAs quantum wells.