L. L. Chang

Resonant tunneling in semiconductor double barriers

Resonant tunneling of electrons has been observed in double‐barrier structures having a thin GaAs sandwiched between two GaAlas barriers. The resonance manifests itself as peaks or humps in the tunneling current at voltages near the quasistationary states of the potential well. The structures have been fabricated by molecular beam epitaxy which produces extremely smooth films and interfaces.

Molecular‐beam epitaxy (MBE) of In1−xGaxAs and GaSb1−yAsy

Films of In1−xGaxAs and GaSb1−yAsy over the entire composition ranges have been grown on (100) GaAs, InAs, and GaSb substrates by MBE. In situ observations by high‐energy electron diffraction have revealed a variety of surface reconstructions and correlated the growth process with the lattice mismatch. The compositions are governed by the relative rates of In and Ga in In1−xGaxAs, but primarily by that of Sb in GaSb1−yAsy because of its dominant incorporation over As. In these alloys, Sn is found to be a donor throughout In1−xGaxAs but an amphoteric impurity in GaSb1−yAsy.

In1−xGaxAs‐GaSb1−yAsy heterojunctions by molecular beam epitaxy

Smooth films of n‐In1−xGaxAs and p‐GaSb1−yAsy were grown by molecular beam epitaxy. As a function of the compositions, x and y, the lattice constants vary linearly while the energy gaps show a downward bowing. Abrupt heterojunctions made of these alloys with close lattice matching exhibit a series of current‐voltage characteristics which change from rectifying to Ohmic as x and y are reduced. The relative location of the band‐edge energies of the two semiconductors at the interface is shown to account for the unusual characteristics observed experimentally.

Optical absorption of In1−xGaxAsGaSb1−yAsy superlattices☆

Abstract The formation of subbands in In 1−x Ga x AsGaSb 1−y As y superlattices has resulted in entirely different absorption characteristics from those of the host semiconductors. The measured absorption edges agree with the calculated energy gaps of the superlattices of various configurations, establishing that the conduction bandedge of InAs lies approximately 0.15eV below the valence bandedge of GaSb.

New III‐V diluted magnetic semiconductors (invited)

A new class of diluted magnetic semiconductor (DMS) based on a III‐V semiconductor is reviewed. The new DMS, (In,Mn)As, was made possible by low temperature molecular beam epitaxial growth. Magnetic measurements and x‐ray diffraction showed homogeneous incorporation of Mn in the films under certain growth conditions, and inclusion of a MnAs‐like phase if the conditions are not optimized. The films can be made either p‐ or n‐type by choosing the growth conditions and/or doping. Homogeneous n‐type (In,Mn)As layers were paramagnetic and showed negative magnetoresistance. On the other hand, remanent magnetization was observed in p‐type samples at low temperature and an anomalous Hall effect associated with it. The presence of such effects was most readily explained in terms of formation of bound magnetic polarons. A first result of anomalous Hall effect in a heterojunction is also presented.

Molecular beam epitaxy of alternating metal‐semiconductor films

Alternately repeated layers of metal epitaxy on semiconductor substrates and semiconductor epitaxy on metal substrates are grown in an ultra-high vacuum evaporation system by first depositing the metal film on the clean surface of the semiconductor substrate over the temperature range between room temperature and 400 DEG C; and then depositing the semiconductor film on the clean surface of the metal over the temperature range between 500 DEG C and 600 DEG C.

A new one‐dimensional quantum well structure

A new one‐dimensional quantum well structure is proposed. The structure is created by converting one side of the potential barrier of an asymmetric quantum well into a periodically indented potential. Both the electron and hole states are confined in the one‐dimensional channel adjacent to the indented region of the side potential barrier, making it possible to achieve enhanced exciton binding energies and oscillator strengths for optical properties.

Observation of semiconductor‐semimetal transition in InAs‐GaSb superlattices

The semiconductor‐semimetal transition in InAs‐GaSb superlattices is observed at a layer thickness in the vicinity of 100 A. The transition manifests itself in an increase in the measured carrier concentration as a result of electron transfer from GaSb to InAs when ground subbands of electrons and heavy holes cross each other. Shubnikov‐de Haas measurements confirm the carrier enhancement in the semimetallic state.

Lattice relaxation of InAs heteroepitaxy on GaAs

Abstract Single layers of InAs are grown on GaAs at different orientations by molecular beam epitaxy. They are evaluated by electron diffraction, supplemented by X-ray absorption and reflection measurements. The threshold in layer thickness for lattice relaxation is similar for both (100) and (100). The former, however, generally exhibits an extended transition region with details depending on the growth conditions, while such a transition is invariably abrupt in the latter situation. The (100) growth, in addition, retains the narrowly streaked diffraction patterns throughout the deposition in contrast to the (100) growth. These observations are attributed to different microscopic local atomic environments.

Two-dimensional electronic structure in InAs-GaSb superlattices☆☆☆

Abstract We have achieved by molecular-beam-epitaxy the new type of superlattice of InAs and GaSb whose energy gaps do not overlap. The observed Shubnikov-de Haas oscillations manifest the two-dimensional electronic subband structure, corroborating theoretical calculations. The deduced electron mass is enhanced primarily as a result of the strong nonparabolicity in the conduction-band of InAs.