A. Pinczuk

Raman scattering from GexSi1−x/Si strained‐layer superlattices

Raman spectroscopy has been used to determine built‐up deformation in GexSi1−x/Si strained‐layer superlattice grown by molecular beam epitaxy. By comparing peak positions in commensurate superlattices and single layers with those from incommensurate thick layers of the same composition we can obtain a quantitative determination of strain. Linewidths are affected by the presence of inhomogeneous strain, dislocations, and disorder. Lines are always narrower in superlattice samples, indicating better crystalline quality. In particular, the Raman line from the Si layers of the strained‐layer superlattices is indistinguishable from that from single‐crystalline Si in both linewidth and frequency. This is consistent with the expectation that the entire lattice mismatch is accommodated as a homogeneous tetragonal strain in the alloy layers only.

Compositional dependence of band‐gap energy and conduction‐band effective mass of In1−x−yGaxAlyAs lattice matched to InP

The band‐gap energy and the electron effective mass of In1−x−yGaxAlyAs lattice matched to InP have been determined as a function of Al content. From photoluminescence measurements we obtain Eg(eV) = (0.76±0.04)+(1.04±0.10)y+(0.87±0.13)y2. The electron effective mass is determined from the plasma frequencies measured with Raman scattering in n‐type samples. Its compositional dependence is given by m* = (0.0427±0.0015)+(0.0683±0.0007)y.

Influence of an undoped (AlGa)As spacer on mobility enhancement in GaAs‐(AlGa)As superlattices

The introduction of an undoped (AlGa)As spacer enhances significantly the low‐temperature mobility in modulation‐doped GaAs‐(AlGa)As superlattices. Mobilities increase monotonically with spacer thickness. This indicates that ionized impurity scattering can be further suppressed by increasing the separation between carriers and their parent donors. Hall mobilities of 93 000 cm2/V sec were observed for average Hall densities of 4.9×1016 cm−3 at 4.2 K.

Lattice vibrations of In1−xGaxAsyP1−y quaternary compounds

We report a Raman study of the lattice vibrations of the quaternary compound In1−xGaxAsyP1−y over the whole range of compositions lattice‐matched to InP. The major bands in the spectra show a pseudo‐two‐mode behavior involving the optical phonons of InGaAs and InP. A low‐frequency structure is attributed to disorder‐induced scattering by acoustical vibrations. Weaker bands are assigned to Ga‐P and In‐As pair vibrations.

Optical processes of 2D electron plasma in GaAs-(AlGa)As heterostructures

Abstract We observed the characteristic behavior of a 2D electron plasma in optical emission and absorption across the energy gaps in modulation-doped GaAs-(AlGa)As quantum-well heterostructures. A renormalization of the energy gap with magnitude comparable to the Hartree energy is found in emission spectra. Large breakdown of the parity selection rule of the optical matrix element in quantum-wells is also observed. In absorption there is evidence of final-state electron-hole interactions even at plasma densities n≳ 4 × 1011 cm−2.

Observation of quantum wire formation at intersecting quantum wells

We report the first observation of a quantum bound state formed at the junction of two intersecting quantum wells in the shape of a T. The atomically precise T junctions are fabricated by a novel cleaved edge overgrowth process in the AlGaAs/GaAs system. The identification of bound states with energies in excess of 20 meV is made by optical emission and absorption spectroscopy. Such quantum wire states are caused by the unique confinement of the lowest state wave function to the region of the T junction.

Inelastic light scattering by charge carrier excitations in two-dimensional plasmas: Theoretical considerations

Abstract We discuss the single particle and collective charge carrier excitations of the two-dimensional plasmas that occur in non-polar semiconductors and the microscopic mechanisms for the resonant inelastic light scattering by the single particle and collective inter-subband excitations. Two limiting cases are analyzed, the “flat-band” model which is a rough approximation to the configuration used by Pinczuk et al., and the “bent-band” model in which Franz—Keldysh effects play an important role, that is an approximation to the configuration used by Abstreiter and Ploog. Unlike Raman scattering by optical phonons, the dominant contributions to the light scattering by the single particle and collective charge carrier excitations come from scattering processes at optical energy gaps, such as the E0+Δ0 gap, that directly involve the charge carriers, particularly in non-polar semiconductors. In III–V compound semiconductors sizeable contributions to the inelastic scattering by coupled LO phonon-collective inter-subband excitation modes may be expected to come from electro-optic, deformation potential and Frohlich scattering processes at the E1 and E1+Δ1 gaps.

Observation of intersubband excitations in a multilayer two dimensional electron gas

Abstract We report the observation, by resonant inelastic light scattering, of intersubband excitations of the multilayer two dimensional electron gas, in modulation doped GaAsAlGaAs heterojunction superlattices. These are the first measurements of these transitions by any technique, and furnish intersubband energies in good agreement with calculated values. The spectral bands are broad, and nearly Lorentzian in shape: the implied relaxation rates scale linearly with band energy and are significantly faster than transport relaxation rates. Finally, the polarized spectra reveal differences between spin-flip and non spin-flip excitations which are unique to multilayer two dimensional electron gases.

Light scattering by two-dimensional electron systems in semiconductors

Abstract We discuss recent work on resonant inelastic light scattering by multilayer and single-layer two-dimensional electron gases in semiconductors. The multilayer systems occur in doped multiple -quantum-well GaAs(AlGa)As heterostructures. The single layers are at doped (GaAs-(AlGa)As heterojunctions; and at InAsMOS interfaces. Light scattering spectra display a wealth of information related to intersubband excitations. We consider in detail the depolarization field effects of collective electron-electron interactions: and correlations between spectral line shapes and transport properties.

Electric field induced heating of high mobility electrons in modulation‐doped GaAs‐AlGaAs heterostructures

We report the simultaneous determination of the temperature and mobility of electrons in modulation‐doped multiple quantum well GaAs‐AlGaAs heterostructures as a function of applied electric field up to 150 V/cm. The decrease of high field mobility with increasing electron temperature is found to be much more rapid than the decrease of the low field mobility with the lattice temperature.