Berinder Brar

Optical investigations of the dynamic behavior of GaSb/GaAs quantum dots

Time‐resolved radiative recombination measurements on GaSb quantum dots have been performed. The GaSb quantum dots are grown by molecular beam epitaxy on (100) GaAs through a self‐assembly process. Time‐resolved measurements show that, after a rapid hole capture process, the photoluminescence decays with a fast and a slow component. The fast component is shortened significantly with higher excitation intensity while the slow component is roughly constant. The radiative lifetimes are much longer than the lifetimes of ordinary GaSb quantum wells with a straddling band lineup. These results support a staggered band lineup and space charge induced band‐bending model.

Surface donor contribution to electron sheet concentrations in not‐intentionally doped InAs‐AlSb quantum wells

The electron concentration in not‐intentionally doped InAs/AlSb quantum wells is found to depend sensitively on the top AlSb barrier thickness even for barriers as thick as 100 nm. The carrier concentration increases as the thickness of this barrier is decreased. The analysis of the dependence of concentration on top barrier thickness indicates that the Fermi level is pinned at the surface of the sample, 850±50 meV below the conduction band edge of the AlSb top layer. Surface donors are the main contribution to the high carrier concentrations in these not‐intentionally doped wells.

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.

INTERBAND IMPACT IONIZATION BY TERAHERTZ ILLUMINATION OF INAS HETEROSTRUCTURES

Experimental studies of InAs heterostructures illuminated by far‐infrared (FIR) radiation reveal an abrupt increase in the charge density for FIR intensities above a threshold value that rises with increasing frequency. We attribute this charge density rise to interband impact ionization in a regime in which ωτm∼1, where τm is the momentum relaxation time, and f=ω/2π is the FIR frequency. The dependence of the density rise on the FIR field strength supports this interpretation, and gives threshold fields of 3.7–8.9 kV/cm for the frequency range 0.3–0.66 THz.

Zero-field spin splitting in InAs-AlSb quantum wells revisited

We present magnetotransport experiments on high-quality InAs-AlSb quantum wells that show a perfectly clean single-period Shubnikov-de Haas oscillation down to very low magnetic fields. In contrast to theoretical expectations based on an asymmetry induced zero-field spin splitting, no beating effect is observed. The carrier density has been changed by the persistent photo conductivity effect as well as via the application of hydrostatic pressure in order to influence the electric field at the interface of the electron gas. Still no indication of spin splitting at zero magnetic field was observed in spite of highly resolved Shubnikov- de Haas oscillations up to filling factors of 200. This surprising and unexpected result is discussed in view of other recently published data.

Influence of the interface composition of InAs/AlSb superlattices on their optical and structural properties

A series of five short‐period (InAs)6/(AlSb)6 superlattices, grown either with AlAs‐like, InSb‐like, or alternating interfaces, were studied by means of x‐ray diffraction, high resolution transmission electron microscopy (HRTEM), Raman spectroscopy, photoluminescence and ellipsometry. The combination of these techniques allows us to explain the pronounced differences in the optical and structural properties of both types of interfaces. In samples with an AlAs‐like bottom interface x‐ray, HRTEM and Raman results demonstrate the differing structural quality to be related to inhomogeneous strain relaxation and As intermixing. The energies of the critical points E0, E1 and E1+Δ1 of the samples with pure AlAs‐like interfaces are shifted by more than 100 meV to higher energies with respect to those of the samples with InSb‐like interfaces. These differences can be understood on the basis of the different interfacial atomic structure and strain in the samples.

Spiral growth of GaSb on (001) GaAs using molecular beam epitaxy

Atomic force microscopy is employed to obtain images of the surface of GaSb epilayers grown on (001) GaAs using molecular beam epitaxy. The images reveal a surface that consists of micron size mounds that are approximately 4 nm high. A stepped surface is clearly observed on the mounds with a single step edge that originates from a screw dislocation at the center of the mound and moves out to the edge in a spiral fashion. The surface structure of the spiral mounds is observed to depend on the growth temperature of the GaSb epilayer, presumably as a result of a shorter diffusion length of the group III adatoms for lower substrate temperatures.

Are there Tamm-state donors at the InAs-AlSb quantum well interface ?

An explanation of the temperature dependence of the electron concentration in InAs/AlSb quantum wells, along with the high electron mobilities found, calls for a model involving a high concentration of interface donors whose electron scattering cross section is much less than that of conventional point defect donors. We postulate that the interface donors are in fact not point defects, but a Tamm‐state‐like band of de‐localized interface states not associated with defects, but with the strong discontinuity in the periodic potentials on the two sides of the interface. Heuristic arguments supporting this hypothesis are given.

Surface‐layer modulation of electron concentrations in InAs–AlSb quantum wells

A strong dependence of the electron concentration in not‐intentionally doped InAs–AlSb quantum wells on the nature of the surface layer is demonstrated. We find that quantum wells capped with a GaSb layer result in much higher electron concentrations than wells capped with an InAs layer. The difference in concentration increases as the top barrier thickness of the well decreases. These changes are due to a shift in the pinning position of the Fermi level at the surface. For InAs surface layers, the Fermi‐level pinning position also depends sensitively on the presence of residual Sb, which is found to move the pinning position further into the conduction band of InAs. The Fermi level is found to be pinned above the conduction band edge of bulk InAs by 80 meV for as‐grown InAs surface layers, and 300 meV for etch‐exposed InAs surface layers.

Quality of AlAs‐like and InSb‐like interfaces in InAs/AlSb superlattices: An optical study

Two short‐period InAs/AlSb superlattices, grown with an AlAs‐like interface and an InSb‐like interface, respectively, were studied with Raman spectroscopy, x‐ray diffraction, and ellipsometry. Our measurements show that the InSb‐like interface grows perfectly pseudomorphically, whereas the sample with the AlAs‐like interface shows indications of relaxation and As interdiffusion. This different interface quality seems to be a fundamental problem, rather than the result of the growth technique.