S. J. Chung

Anisotropic structural and electronic properties of InSb/ AlxIn1-xSb quantum wells grown on GaAs (001) substrates

InSb quantum wells (QWs) with remotely doped Al/sub x/In/sub 1-x/Sb barriers are candidates for several novel device structures that rely on a long electron mean free path. Mesoscopic magnetoresistors that take advantage of the high electron mobility in InSb QWs at room temperature are currently being developed for read-head applications. The promise of InSb QWs for spin-transistor applications has been shown recently by experiments that demonstrate a large zero-field spin splitting and ballistic transport at temperatures as high as 185 K. Since a semi-insulating substrate is required for electronic applications, the InSb/Al/sub x/In/sub 1-x/Sb structures are grown on GaAs [001] substrates. The /spl sim/14% lattice mismatch between the epilayers and the substrate results in a high defect density that partially limits the electron mean free path. We will present a detailed characterization of these defects and elucidate their role in limiting electron mobility.

Determination of the concentration and temperature dependence of the fundamental energy gap in AlxIn1−xSb

We use transmission spectroscopy to determine the energy gap for the AlxIn1−xSb alloy system in the Al concentration range from 0% to 25% from cryogenic to room temperature. The samples are epitaxial layers grown by molecular beam epitaxy on GaAs substrates. Our room temperature results are compared to those from two earlier studies. In our Al concentration range, we find a linear change of energy gap with alloy lattice constant.

Band offset determination in the strained-layer InSb/AlxIn1−xSb system

We use interband exciton transitions in parabolically graded quantum wells to measure the band offset at the InSb/AlxIn1−xSb interface. The method we use is based on similar studies in the GaAs/AlxGa1−xAs system but modified to reflect the strong nonparabolicity and strain of the InSb/AlxIn1−xSb system. We find a conduction band offset ratio of 0.62±0.04 for Al concentrations in the range 2%–12%. The observed lack of variation of the offset with Al concentration suggests a lack of strain dependence in the InSb/AlxIn1−xSb system for practical Al concentrations.

Application of terahertz quantum-cascade lasers to semiconductor cyclotron resonance

Quantum-cascade lasers operating at 4.7, 3.5, and 2.3 THz have been used to achieve cyclotron resonance in InAs and InSb quantum wells from liquid-helium temperatures to room temperature. This represents one of the first spectroscopic applications of terahertz quantum-cascade lasers. Results show that these compact lasers are convenient and reliable sources with adequate power and stability for this type of far-infrared magneto-optical study of solids. Their compactness promises interesting future applications in solid-state spectroscopy.

Mobility anisotropy in InSb/AlxIn1−xSb single quantum wells

Three types of defects at the surface of InSb quantum well samples are identified: hillocks, square mounds, and oriented abrupt steps. The electron mobility in the quantum well correlates to the density of abrupt features, such that samples with a high density of anisotropic defects show anisotropy in the mobility. We propose that the dominant scattering mechanism associated with these abrupt features is a fluctuation in the quantum well morphology.

Effect of micro-twin defects on InSb quantum wells

We have investigated the effect of micro-twin defects on InSb quantum wells (QWs) grown on GaAs (001) substrates. Transmission electron microscopy analysis revealed that the QW layers are intersected by micro-twins that arise from lattice mismatch with the substrate. The intersected parts of the QWs lie near or in the {111} plane, which is tilted by 15.8° with respect to the (001) substrate. Hall effect measurements indicated that the degradation of electron mobility in the QWs is well correlated with the density of the micro-twins.

Exciton determination of strain parameters in InSb∕AlxIn1−xSb quantum wells

Excitons in semiconductors can be used as a tool to probe various material and structural properties. The authors studied strain-related materials parameters in InSb∕AlxIn1−xSb quantum well structures. By changing the Al concentration in the barrier layers (0.03<x<0.23), the strain in the quantum wells can be tuned continuously. Using infrared transmission measurements, the authors traced strain-induced shifts in the energies of the confined states. The different strain dependences of the light- and heavy-hole band edges allow us to determine deformation potentials α and β simultaneously.

Determination of deformation potentials in strained InSb quantum wells

We use interband exciton transitions in InSb∕AlxIn1−xSb multi-quantum-well samples to determine the heavy-hole and light-hole energy gaps as the strain is varied using Al concentrations up to 25%. The gaps are compared to deformation-dependent calculations of the energy gaps in the presence of biaxial strain to obtain a measure of the deformation potentials α and β.

Dependence of the Spin Coherence Length on Wire Width for Quasi‐1‐Dimensional InSb and InAs Wires and Bi Wire Surface States

In diffusive two‐dimensional electron systems (2DESs) with linear spin‐orbit interaction (SOI), quasi‐one‐dimensional (Q1D) confinement in narrow wires of width W is theoretically predicted to result in an enhancement of the spin relaxation length LS, such that LS∝1/W. We present our experimental data of the dependence of LS on W for three different systems: 1) ballistic InSb wires with specular boundary scattering and strong Dresselhaus SOI, 2) ballistic InAs wires with diffusive boundary scattering and Rashba SOI, and 3) diffusive bismuth wires with a large density of states at the surface. For all three systems, information on the spin relaxation is gathered from the weak‐antilocalization effect (WAL), a magnetotransport measurement sensitive to both the spin‐orbit coherence length and the phase coherence length Lφ. We find a dependence of LS on W, where LS increases with decreasing W in all three systems. However, the theory is valid for Q1D diffusive wires with linear SOI, which does not fit the prof...

Control and Probe of Carrier and Spin Relaxations in InSb Based Structures

Narrow gap semiconductors (NGS) offer several scientifically unique features important for the field of spintronics. In order to explore these features we are using standard pump-probe and magneto-optical Kerr effect (MOKE) spectroscopy at different excitation wavelengths, power densities, and temperatures. Our goal is measuring and controlling carrier/spin relaxation in a series of InSb-based quantum wells and films, and InMnSb ferromagnetic films. The dynamic effects observed in these structures demonstrate strong dependence on the photo-induced carrier density.