Chanh Nguyen

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.

High-transconductance InAs/AlSb heterojunction field-effect transistors with delta -doped AlSb upper barriers

The authors report the fabrication and temperature-dependent characterization of InAs/AlSb quantum-well heterojunction field-effect transistors (HFETs). Devices with electron sheet concentrations of 3.8*10/sup 12/ cm/sup -2/ and low-field electron mobilities of 21000 cm/sup 2//V-s have been realized through the use of Te delta -doping sheets in the upper AlSb barrier. One device with a 2.0- mu m gate length showed a peak extrinsic transconductance of 473 mS/mm at room temperature. Gate leakage current, operating current density, and extrinsic transconductance were found to decrease with decreasing temperature.<<ETX>>

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.

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.

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.

Methane/hydrogen‐based reactive ion etching of InAs, InP, GaAs, and GaSb

Reactive ion etching (RIE) of InAs, InP, GaAs, and GaSb using CH4/H2 mixtures has been studied to determine the resulting etch profiles and surface morphologies, as well as the dependence of etch rates on cathode temperature, chamber pressure, and electrode self‐bias. These materials are found to etch slowly and controllably, with etched samples having smooth surfaces and nearly vertical sidewalls. Our results demonstrate that CH4/H2 RIE is a promising technology for fabricating electronic devices using the newly emerging InAs/GaSb/AlSb material system as well as the better established InP material system.

InAs‐AlSb quantum well as superconducting weak link with high critical current density

We have constructed a superconductor‐semiconductor weak link using an InAs‐AlSb quantum well as the channel between Nb electrodes. The structure draws on (a) the barrier‐free nature of the Nb‐InAs interfaces and (b) the combination of high electron concentrations and very high electron mobilities in the InAs wells, which yield a large superconducting coherence length (3700 A) inside the channel. A large (lineal) critical supercurrent density of 32 mA/mm was obtained at 1.4 K, for a channel length of 0.6 μm. After temporary exposure to a magnetic field, the current‐voltage characteristic became hysteretic, with an increase in critical current density to 132 mA/mm. The latter value corresponds to an areal current density of 8.8×105 A/cm2.

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.

Energy-dependant cyclotron mass in InAs/AlSb quantum wells

The influence of conduction band non-parabolicity on the cyclotron resonance of a two-dimensional electron system in InAs quantum wells is investigated. We demonstrate that the experimentally determined dependence of the cyclotron mass on the carrier density in the well can be excellently described using a two-band k.p model. In contrast to previously studied systems our experimental results allow us to deduce quantitatively the quantization energy of the first electrical subband for wells of different width.

Effects of barrier thicknesses on the electron concentration in not‐intentionally doped InAs–AlSb quantum wells

We present the results of a study of the dependence of electron concentration in molecular‐beam‐epitaxy‐grown not‐intentionally doped InAs–AlSb quantum‐well structures on the top barrier thickness in single‐well structures and on the internal barrier thickness in multiwell structures. A contribution from surface donors was found to be dominant in single wells with thin top barriers. The bulk donor concentration in the AlSb barriers varied from 2 × 1015 to 1016 cm−3. Interface donors at the InAs–AlSb heterointerface were also found, with an energetic location below the Fermi level in the wells. They are primarily responsible for the increase of the carrier concentration with increasing temperature.