P.J. Stiles

The channel conductivity in n-type Si inversion layers at very low electron densities☆

For electron densities less than 7 × 1011 cm−2 the channel conductivity is activated with a T-independent activation energy for T ⪸ 1.3 K. For 0.5 < T < 1.3 T K the activation energy is strongly T-dependent and for 0.03 < T < 0.5 K the conductivity becomes almost T-independent but not density independent. Variable range hopping does not appear to dominate the conductivity at low temperatures.

Optical spectroscopy of surfaces: Reflectance studies of chemisorption

A high precision technique has been used to study the small changes in near-normal incidence reflectance R caused by chemisorption of gases on an atomically clean metal surface maintained in ultrahigh vacuum. Examples are given for O2, CO, and H2 on Mo(100) in the photon energy range 1.9 <ℏω < 4.8eV. The relative reflectance change ΔRR is negative and reaches as much as 1%. Structure that appears in ΔR(e), where e=exposure (pressure × time), is attributed to different binding configurations of the adatoms as observed, e.g., in LEED experiments. The dependence of ΔRR on ℏω suggests the presence of adsorbate-induced surface states within a few eV of the Fermi energy. Possible approaches to determining the dielectric function of the surface region from the data are discussed.

Magnetocapacitance measurements in GaAs heterostructures

Abstract Measurements of the magnetocapacitance of a two-dimensional electron gas in high mobility GaAs(Ga, Al)As heterostructure confirm, for the first time, the relationship between the density of states and the capacitance of a two-dimensional electron gas. We present the first magnetocapacitance measurements in the fractionally quantized Hall regime.

Evidence for a superlattice at Si–SiOx interface

Evidence is presented for periodic structure at the Si–SiOx interface in MOSFET’s in which the interface is a vicinal plane of (100). The results are discussed in terms of their relevance to interface technology.

Optical reflectance studies of chemisorption on a clean metal surface

Abstract Changes in optical reflectance from a Mo (100) surface due to chemisorption of O2 and CO in amounts up to a monolayer show that adsorbate-dependent surface states are present near the Fermi energy and are sensitive to the adatom binding configuration.

Electrochemical potential oscillations of the two-dimensional electron gas in GaAs/AlGaAs heterostructures in high magnetic fields

Abstract We have extended our previous direct measurement of the electrochemical potential of the two-dimensional electron gas (2DEG) in GaAs/GaAlAs heterostructures relative to a reference material as a function of magnetic field using a “floating gate” technique. the analysis of the jumps in the electrochemical potential of the 2DEG indicates that there is a strongly temperature depenpendent non-zero density of states between the Landau levels. We have also observed the effect of eddy currents, induced in the 2DEG by changing magnetic field which form persistent currents in the quantum Hall regime.

The frequency dependence of the diagonal conductivity of a 2DEG in GaAs heterostructure in the quantum Hall regime

Abstract We have measured directly the diagonal conductivity of a 2DEG in GaAs/ A1GaAs heterostructure via a capacitively coupled structure with d.c. bias at magnetic fields up to 6 tesla. From the ungated region of a sample of Corbino geometry, the diagonal conductivity at the Landau gap regions was measured via capacitive coupling to a non-quantized 2DEG in the source and drain capacitors. A strong frequency dependence was observed over a range of frequencies from 100 Hz to 20 kHz at the temperature of 1.3 K. The result is compared to the calculations from the percolation theory. From the measurements of two different temperatures, 1.3 and 4.2 K, the activation energy for the conductivity was estimated to be about 10% of Landau level spacing.

Fine structure in the field effect mobility of MOS transistors

Abstract We have studied the fine structure in the field effect mobility of MOS transistors at 4.2 K. With applied substrate bias, the position in gate voltage of the structure is unchanged and the amplitude decreases. The position of the structure is also unchanged by normal magnetic fields of up to 150 kgauss. We have studied samples with four equivalent source-drain contacts. The structure depends strongly on the contacts. The structure in the field effect mobility appears to be due to the contacts and not a property of the inversion layer or surface states.

Modification of the magnetoconductance of a two-dimensional electron gas by altering the boundary conditions in the third dimension

Abstract Substrate bias can be used as a powerful diagnostic tool for studying the magneto-transport in an inversion layer. One can accurately determine the mobile carrier threshold by such a study. Marked changes in the mobility occur with increasing confinement to the surface; a decrease for high electrons densities and an increase for low electron densities. The former was expected while the latter was not.

Studies of the fractional quantum Hall effect in a silicon MOSFET

We have observed the fractional quantum Hall effect in high mobility (> 3.5 m2/V·s) Si MOSFET thus confirming that this effect is a universal property of the two-dimensional electron gas. We have studied this effect as a function of temperature down to 50 mK and have observed the competition between the integer and the fractional quantum Hall effects.