S. A. Chalmers

New quantum structures

Structures in which electrons are confined to move in two dimensions (quantum wells) have led to new physical discoveries and technological applications. Modification of these structures to confine the electrons to one dimension (quantum wires) or release them in the third dimension, are predicted to lead to new electrical and optical properties. This article discusses techniques to make quantum wires, and quantum wells of controlled size and shape, from compound semiconductor materials, and describes some of the properties of these structures.

Determination of tilted superlattice structure by atomic force microscopy

We have analyzed the structure of tilted superlattices on atomically stepped surfaces by using atomic force microscopy to detect ridges of GaAs formed by the selective oxidation and removal of intervening AlAs regions. Oxides were removed in a liquid cell of the atomic force microscope while scanning. We have demonstrated plan views which reveal the superlattice length and width uniformity, but the method is also in principle suited for cross‐sectional samples.

A reflection high‐energy electron diffraction study of (100) GaAs vicinal surfaces

The RHEED specular beam profiles produced by GaAs vicinal surfaces are examined and correlated with the surface topography. It is found that the shape of split peaks produced with the incident beam normal to step edges is a measure of surface orientation over lengths on the order of terrace widths, but is not sensitive to short range roughness on the terraces themselves. The profile produced with the incident beam parallel to step edges is sensitive to terrace roughness and can be correlated with RHEED intensity oscillation behavior. It is also found that reconstruction disorder can influence the specular beamwidth.

Step‐flow growth on strained surfaces: (Al,Ga)Sb tilted superlattices

We have demonstrated the molecular beam epitaxial growth of (Al,Ga)Sb tilted superlattices (TSLs) on 2° vicinal (100) GaSb and GaAs substrates. The TSLs grown on GaSb substrates exhibit good AlSb/GaSb separation and a uniform short‐range superlattice period. The TSLs grown on GaAs substrates are similar, except for the presence of threading dislocations and a decreased uniformity. The existence of TSLs proves that step‐flow growth can occur in this material system, and in the presence of strain. Lateral fluctuations in the tilt angle of the superlattice are observed and are found to be caused by a nonuniform adatom distribution which is correlated with the surface step density.

Real‐time simultaneous optical‐based flux monitoring of Al, Ga, and In using atomic absorption for molecular beam epitaxy

We have developed a multichannel atomic absorption measurement system for real‐time simultaneous monitoring of Al, Ga, and In molecular beam fluxes. In our configuration, distinct atomic emission lines from three hollow cathode lamps are combined into one beam, thus requiring only one pair of through view ports for the optical probe beam. Based on the dual beam optical configuration, the reference arm compensates for intensity drift of the light sources. In this work, we demonstrate the use of reflection high energy electron diffraction oscillations for calibrating the absorption signal.

A reflection high‐energy electron diffraction study of AlAs/GaAs tilted superlattice growth by migration‐enhanced epitaxy

We have used reflection high‐energy electron diffraction to investigate the influence of substrate temperature, arsenic pressure, and Al composition on the growth of tilted superlattices(TSLs) on atomically stepped surfaces. In the temperature range of 450–675 °C, we found that GaAs and TSLs grow smoother at high temperatures and low arsenic pressures. In the same range, AlAs grows smoother at low temperatures and high arsenic pressures. We also found that raising the AlAs composition of TSLs raises the substrate temperature required for pure step flow growth. We have also found new evidence that a low‐temperature, smooth‐surface growth regime for AlGaAs exists below 450 °C.

The growth of (Al,Ga)Sb tilted superlattices and their heteroepitaxy with InAs to form corrugated-barrier quantum wells

Abstract We have demonstrated the molecular beam epitaxial growth of (Al,Ga)Sb tilted superlattices (TSLs) on 2° vicinal (100) GaSb and GaAs substrates. The existence of (Al,Ga)Sb TSLs proves that step-flow growth can occur in this material system, and in the presence of strain. Lateral fluctuations in the tilt angle of the superlattice are observed and are found to be caused by a non-uniform distribution of incorporated adatoms which is correlated with the surface step density. The (Al,Ga)Sb TSL growth was also combined with InAs growth to form an InAs quantum well with “corrugated” barriers consisting of (Al,Ga)Sb TSLs. The electron mobilities in this structure exceeded 6×105 cm2/V⋯s.

Reflection high‐energy electron diffraction oscillations during molecular‐beam epitaxy growth of gallium antimonide, aluminum antimonide, and indium arsenide

We report the observation of reflection high‐energy electron diffraction (RHEED) intensity oscillations during the molecular‐beam epitaxy growth of gallium antimonide (GaSb), aluminum antimonide (AlSb), and indium arsenide (InAs). For GaSb, these oscillations were not observed at the normal substrate temperature used for growth of these compounds (500–550 °C), but only at lower temperatures. For continuous growth of AlSb at lower temperatures, the RHEED oscillations persist for ∼40 monolayers. For AlSb and GaSb, the recovery of RHEED oscillation amplitude after growth interruption is quite different from the recovery, if any, of specular spot intensity. The RHEED oscillations for AlSb were observed at 570 °C after growth of as little as 350 A of AlSb, which was nucleated directly on a GaAs substrate. Oscillations were seen in spite of the large lattice mismatch (7% with GaAs) and the large number of dislocations present (>109 cm−2). We have studied the effects of varying growth rates and flux ratios. The ...

Photoluminescence study of lateral carrier confinement and compositional intermixing in (Al,Ga)Sb lateral superlattices

We have compared the photoluminescence properties of an (Al,Ga)Sb lateral superlattice (LSL) quantum well to those of an (Al,Ga)Sb alloy quantum well, with respect to recombination energy and polarization dependence. From the results we have deduced the compositional intermixing and lateral carrier confinement present in the LSL structure. We found that the LSL well luminesces at 36 meV lower than the alloy well, and that emitted light from the LSL well is more than twice as intense when its electric field is polarized parallel versus perpendicular to the LSL ‘‘wires.’’ From these data we calculate that the lateral content of the LSL varies periodically between approximately 24% and 42% AlSb, and the maximum:minimum carrier density ratios are about 4:1 and 6:1 for electrons and heavy holes, respectively.

Anisotropic magnetotransport in an antiwire array inserted in a GaAs heterostructure

Abstract An array of antiwires is prepared in GaAs/AlGaAs heterostructures by using the tilted superlattice approach. A regular two-dimensional electron gas (2DEG) is grown on a 2o off axis (100) substrate. Half a monolayer of AlAs is inserted close to the GaAs/AlGaAs interface ordering itself along the step edges. Therefore an antiwire array is formed which directly influences the envelope wavefunction of the two-dimensional electrons. Low temperature magnetotransport measurements with current flow parallel and perpendicular to the wires reveal a pronounced anisotropy in the magnetoresistance ϱxx of the carriers. The value of ϱxx⊥(B=0) for current flow perpendicular to the antiwires is larger than g9xx|. The anisotropy increases with increasing carrier density in agreement with the formation of Bloch bands due to the periodicity of the antiwire array. For high magnetic fields B>2T the minima of the Shubnikov-de Haas oscillations for ϱxx⊥ and ϱxx| occur at the same fields indicating the homogeneity of the 2DEG. For low magnetic fields B≈0.5T we observe a maximum in ϱxx⊥ while ϱxx| stays constant. The Hall resistances ϱxy⊥ and ϱxy| are indistinguishable from each other for all ranges of B and Ns.