T. P. Smith

Landau-level broadening and scattering time in modulation doped GaAs/AlGaAs heterostructures

Abstract Single particle relaxation time and the scattering time of the 2DEG in modulation doped GaAs/AlGaAs heterostructure are measured by Ladau respectively, for samples with spacer layer thickness 1.5–500 nm. The results are compared with the recent calculations of Das Sarma and Stern and excellent agreement is obtained. When the excited subband is occupied under persistent photoconductive (PPC) conditions, the relaxation time for the excited subband was found to be nearly three times longer than that for the ground subbands. The results are interpreted in terms of screened remote impurity scattering.

Formation of stoichiometric SiGe oxide by electron cyclotron resonance plasma

Electron cyclotron resonance (ECR) plasma oxidation of SiGe alloys was investigated at temperatures from room temperature to 500 °C. Both Si and Ge are shown to be fully oxidized, forming SiO2 and GeO2. Auger depth profiling reveals that there is no Ge‐rich SiGe layer after oxidation. With increasing temperature up to 500 °C, the oxide is stoichiometric and it does not lose its GeO2 component. Oxidation has also been carried out at both positive and negative sample bias in order to identify the role of ions, electrons, and neutrals. From biasing experiments negative oxygen ions and atomic neutrals appear to be the major reaction species.

Sound velocity and index of refraction of AlAs measured by picosecond ultrasonics

We use picosecond optics techniques to generate and detect acoustic pulses in an epitaxially grown film of AlAs. From the round trip time of the acoustic pulse we find a sound velocity in the [100] direction of 6.4×105 cm s−1. We also show how measurements of this type can be used to determine the refractive index.

Magnetic flux commensurability in coupled quantum dots

We have studied the transition from isolated to coupled quantum dots in a lateral surface‐superlattice structure in the presence of a perpendicular magnetic field. The coupling between the dots can be tuned by changing the bias of the grid gate. Our magnetocapacitance measurements reveal three distinct regimes: isolated quantum dots where collective effects are not observed, a tight‐binding regime where the measurement results are sensitive to the rationality of flux quanta per unit cell, and a superlattice regime where commensurability effects between the magnetic orbits and the superlattice periodicity are observed.

Sidewall damage in n+‐GaAs quantum wires from reactive ion etching

Electron cyclotron resonance and radio frequency reactive ion etching have been used to fabricate narrow n+‐GaAs wires employing CCl2F2/He as the etch gas. A comparison of the induced sidewall damage is made using room‐temperature conductivity measurements of the etched structures and the effect of overetching is investigated. In addition, preliminary analysis of low‐temperature transport reveals that the amplitude of universal conductance fluctuations is extremely sensitive to sidewall damage.

Growth and transport properties of (Ga,Al)Sb barriers on InAs

We have investigated the transport properties of (Ga,Al)Sb–InAs heterostructures grown by molecular‐beam epitaxy with the Al composition in the range of 0.3–0.5 where the alloy serves as a potential barrier for electrons in InAs. Field effect capacitors using a thick (Ga,Al)Sb barrier demonstrate the modulation of carrier densities in InAs by the application of an electric field. In the region of thin (Ga,Al)Sb, where tunneling becomes dominant, a novel negative resistance appears in InAs/(Ga,Al)Sb/InAs single‐barrier structures due to the specific band alignment of this system. The results have been further elucidated under a magnetic field.

Comparison of damage in the dry etching of GaAs by conventional reactive ion etching and by reactive ion etching with an electron cyclotron resonance generated plasma

We have developed a high‐resolution etch for GaAs in an electron cyclotron resonance‐radio frequency (ECR‐rf) hybrid reactor using CCl2F2/He as the etch gas. Surface contamination from the gas and from the electrodes was found to affect the etching process as well as etched substrate. Surface damage studies using Schottky diode and x‐ray photoelectron spectroscopy analysis and sidewall damage characterization using room‐temperature conductance measurements of n+‐GaAs quantum wires indicate that very little damage is caused by ECR‐rf reactive ion etching compared with conventional rf reactive ion etching (RIE). In addition, the small residual damage after ECR‐rf RIE is measured to increase with etching time.

Fabrication and characterization of one‐ and zero‐dimensional electron systems

One‐ and zero‐dimensional electron systems confined in GaAs/GaAlAs have been fabricated. The starting material consisted of a modulation doped and a double barrier diode heterostructure grown by molecular‐beam epitaxy. Very high resolution electron beam lithography and reactive ion etching were used to pattern lines and dots with widths ranging from 100 to 400 nm. Two measurement techniques have been applied: capacitance measurements of density of states—a novel technique for observing quantum effects in these structures—and resonant tunneling measurements. We have observed oscillations in capacitance spectroscopy which reflect discrete energy levels associated with one‐ and zero‐dimensional electron systems. Preliminary tunneling measurements are presented.

Quantum confinement in few-electron systems

Abstract We review the physics of quasi-one-dimensional and zero-dimensional semiconductor heterostructures and discuss some of the developments which have led to the tremendous growth in this field. In particular, we focus on the measurement of the density of states in quantum dots using capacitance spectroscopy. We will discuss our results in light of related theoretical work and complementary experimental measurements.

Fractional states in few‐electron systems

We have observed fractional quantization of very few electrons confined in a semiconductor quantum dot using capacitance spectroscopy. The number of electrons per dot varies from 0 to about 40 as a function of bias on the quantum capacitors. The capacitance spectra have clear minima at gate voltages and magnetic fields, where the filling factors are 1/3 and 2/3. These measurements may allow direct comparison with few‐particle calculations.