Effect of buffer layer growth conditions on the secondary hexagonal phase content in cubic GaN films on GaAs(001) substrates

In this letter, we investigated the effect of the buffer layer growth conditions on the secondary hexagonal phase content in cubic GaN films on GaAs(0 0 1) substrate. The reflection high-energy electron diffraction (RHEED) pattern of the low-temperature GaN buffer layers shows that both the deposition temperature and time are important in obtaining a smooth surface. Four-circle X-ray double-crystal diffraction (XRDCD) reciprocal space mapping was used to study the hexagonal phase inclusions in the cubic GaN (c-GaN) films grown on the buffer layers. The calculation of the volume contents of the hexagonal phase shows that higher temperature and longer time deposition of the buffer layer is not preferable for growing pure c-GaN film. Under optimized condition, 47 meV FWHM of near band gap emission of the c-GaN film was achieved. (C) 2000 Elsevier Science B.V. All rights reserved.

Tunneling behaviors of photogenerated electrons in In0.15Ga0.85As/GaAs quantum well photoelectrodes

The photovoltaic spectral features and the behaviors of photocurrent versus the electrode potential for near surface In0.15Ga0.85As/GaAs quantum well electrodes have been investigated in nonaqueous solutions of ferrocene and acetylferrocene. The photovoltaic spectrum shows a sharp structure that reflects confined state-to-state exciton transition in the quantum well. Deep dips are observed in the photocurrent versus the electrode potential curves in both electrolytes at the different electrode potentials under the illumination of exciton resonance wavelength. These dips are qualitatively explained by considering the interfacial tunneling transfer of photogenerated electron within the quantum well.

Hydrogen contaminant and its correlation with background electrons in GaN

GaN epilayers grown by molecular beam epitaxy using NH3 as the nitrogen source were found to contain hydrogen. We further notice that the background electron concentration in GaN can be correlated with the amount of hydrogen contaminant. X-ray photoelectron spectroscopy (XPS) measurements of the N Is peak reveal that hydrogen is bound to nitrogen. This will make the corresponding Ga atom see insufficient N counterpart, as can be inferred from the XPS Ga 3d spectrum. We then think that nitrogen in the lattice terminated by hydrogen is an effective nitrogen vacancy and hence a donor accounting for the background electrons.

Performing Fast Addition and Multiplication by Transferring Single Electrons

This paper proposes novel fast addition and multiplication circuits that are based on non-binary redundant number systems and single electron (SE) devices. The circuits consist of MOSFET-based single-electron (SE) turnstiles. We use the number of electrons to represent discrete multiple-valued logic states and we finish arithmetic operations by controlling the number of electrons transferred. We construct a compact PD2,3 adder and a 12x12bit multiplier using the PD2,3 adder. The speed of the adder can be as high as 600MHz with 400nW power dissipation. The speed of the adder is regardless of its operand length. The proposed circuits have much smaller transistors than conventional circuits.

EFFECT OF IMAGE FORCES ON ELECTRONS CONFINED IN LOW-DIMENSIONAL STRUCTURES UNDER A MAGNETIC-FIELD

We consider the effect of image forces, arising due to a difference in dielectric permeabilities of the well layer and barrier layers, on the energy spectrum of an electron confined in a rectangular potential well under a magnetic field. Depending on the value and the sign of the dielectric mismatch, image forces can localize electrons near the interfaces of the well or in well centre and change the direct intersubband gaps into indirect ones. These effects can be controlled by variation of the magnetic field, offering possibilities for exact tuning of electronic devices.

TUNNELING ESCAPE TIME OF ELECTRONS FROM A QUANTUM-WELL WITH GAMMA-X MIXING EFFECT

By using the envelope function method we calculated the tunneling escape time of electrons from a quantum well. We adopted a simplified interface matrix to describe the GAMMA-X mixing effect, and employed a wave packet method to determine the tunneling escape time. When the GAMMA state in the well was in resonance with the X state in the barrier, the escape time reduced remarkably. However, it was possible that the wave functions in two different channels, i.e., GAMMA-GAMMA-GAMMA and GAMMA-X-GAMMA, could interfere destructively, leading the escape time greater than that of pure GAMMA-GAMMA-GAMMA tunneling.

COHERENT MODEL FOR RESONANT-TUNNELING OF ELECTRONS IN DOUBLE-QUANTUM WELLS UNDER ELECTRIC-FIELDS

Taking the inhomogenous broadening of the electron energy levels into account, a coherent model of the resonant tunneling (RT) of electrons in double quantum wells is presented. The validity of the model is confirmed with the experiments [M. Nido et al., Proc. SPIE 1268, 177 (1990)], and shows that the tunneling process can be explained by the simple coherent theory even in the presence of the carrier scattering. We have discussed the dependence of resonant tunneling on the barrier thickness L(B) by introducing the contrast ratio LAMBDA and the full width at half depth of the RT valley, and found that LAMBDA first increases with increasing barrier thickness, reaches a maximum, and then decreases with a further increase of L(B), in striking contrast to the Fabry-Perot model where a monotonic increase of the peak-to-valley ratio is predicted. We attribute the reduction of LAMBDA with large L(B) to the energy broadening resulting from the carrier scattering. A monotonic decrease of the full width at half depth of the RT valley with an increase of L(R) is also found.

A NEW METHOD FOR MEASURING THE EFFECTIVE DIFFUSION CONSTANT OF 2-DIMENSIONAL ELECTRONS IN THE PRESENCE OF MAGNETIC-FIELDS

The transient charge response Q(t) of a two-dimensional electron gas (2DEG) in GaAs/AlxGa1-xAs heterostructures to a small pulse of the gate voltage, applied between the top gate and source electrodes in a Corbino structure, was employed to directly measure the effective diffusion constant of a 2DEG in the quantum Hall regime. The measured diffusion constant D showed a drastic change as the magnetic field was swept through the integer fillings of the Landau levels.