Localized vibrational modes of lithium and lithium-defect pairs in silicon Final report, 1 Jul. 1967 - 30 Sep. 1968

Localized vibrational modes of lithium and lithium-defect pairs in silico

Localized vibration modes of defect pairs in silicon

Absorption bands and localized vibrational modes of silicon doped with boron compounds containing phosphorus, arsenic, antimony, or lithiu

Fermi Level Position at Semiconductor Surfaces

There have been several recent reports of barrier height studies on metal-semiconductor interfaces. Metals of widely different work functions evaporated onto Si and GaAs surfaces indicated that in each case the energy difference between the semiconductor conduction band edge and Fermi level at the interface,φ_(Bn), was essentially independent of the metal, which indicates that the Fermi level is fixed by surface states. In the present work barrier height measurements have been made on a number of zinc-blende semiconductors to determine (a) if the barriers are in all cases determined by surface states, and (b) the relation between the Fermi energy at the interface and the band gap E_g

Photoemission from Au and Cu into CdS

Many metal-semiconductor surface barrier rectifiers show photosensitivity for photon energies (hv) less than the semiconductor energy gap (E_g). Cases in the literature include metals evaporated or electrodeposited on elemental and III-V compound semiconductor surfaces. In these studies the source of the low-energy photocurrent, when hv < E_g, was shown to be the photoemission of carriers over the Schottky barrier between the metal film and the semiconductor. An extensive investigation has been reported for a series of metals, particularly Cu and Au, electroplated on n-type CdS with the conclusion that here also photoemission from the metal is responsible for most of the low-energy photovoltage. However, recent studies have questioned this conclusion for the CdS case. One study proposed that the origin of the low-energy photovoltaic response is electron photoexcitation from Cu impurities located in the CdS and within a diffusion length of the space charge region. Hole conduction probably in the 3d Cu levels was postulated for these samples, which had ≈ 30-ppm Cu. A second study interpreted the results as a p·n junction photovoltaic effect

Conduction Band Minima of Ga(As_(1−x)P_x)

Photoresponse of surface barriers on samples of Ga(As_(1−x_P_x) covering the range 0≤x≤1 has been measured. Thresholds corresponding to both direct and indirect band-to-band excitations within the semiconductor and also photoinjection from the metal have been identified. The threshold of the direct transition varies with composition from 1.37 eV in GaAs to 2.65 eV in GaP. The indirect transition was followed for x≳0.38 and again varied linearly from 2.2 eV in GaP to an extrapolated value in 1.62 eV in GaAs. The energy separation of the two conduction band minima in GaAs is in disagreement with previously reported values

Conduction Band Minima in AlAs and AlSb

The photoresponse of surface barrier rectifiers made by evaporating a metal such as gold or platinum on a cleaved surface of AlAs and AlSb has been measured in the front wall configuration. The photoresponse of such units for hv > E_g, where E_g is the energy gap, will be proportional to the absorption coefficient as long as the optical attenuation length is large compared to both the width of the space-charge region and the minority carrier diffusion length. The analysis is essentially the same as that for p-n junctions with the exception that the barrier is at the surface and hence more sensitive to photons of high absorption coefficient. Photoinjection of carriers from the metal into the semiconductor for photon energies where hv < E_g can also be observed

Fermi Level Position at Metal-Semiconductor Interfaces

The position of the Fermi level at a metal-semiconductor interface relative to the conduction band has been found to be a constant fraction of the semiconductor band gap for all but 3 of the 14 group IV or III-V semiconductors studied. In all cases, the position was essentially independent of the metal work function. This general result is not inconsistent with the limited theories of surface state energies now available. The three exceptional cases can be understood in terms of a first-order perturbation to the surface state energies correlated with a similar perturbation observed in the energy gap at the (111) zone edge. Experiments are also reported on Ga(As-P) alloys, and two II-VI materials showing distinctly different behavior

First exit times and residence times for discrete random walks on finite lattices

In this paper, we derive explicit formulas for the surface averaged first exit time of a discrete random walk on a finite lattice. We consider a wide class of random walks and lattices, including random walks in a non-trivial potential landscape. We also compute quantities of interest for modelling surface reactions and other dynamic processes, such as the residence time in a subvolume, the joint residence time of several particles and the number of hits on a reflecting surface.Comment: 19 pages, 2 figure

Stochastic Analysis of Dimerization Systems

The process of dimerization, in which two monomers bind to each other and form a dimer, is common in nature. This process can be modeled using rate equations, from which the average copy numbers of the reacting monomers and of the product dimers can then be obtained. However, the rate equations apply only when these copy numbers are large. In the limit of small copy numbers the system becomes dominated by fluctuations, which are not accounted for by the rate equations. In this limit one must use stochastic methods such as direct integration of the master equation or Monte Carlo simulations. These methods are computationally intensive and rarely succumb to analytical solutions. Here we use the recently introduced moment equations which provide a highly simplified stochastic treatment of the dimerization process. Using this approach, we obtain an analytical solution for the copy numbers and reaction rates both under steady state conditions and in the time-dependent case. We analyze three different dimerization processes: dimerization without dissociation, dimerization with dissociation and hetero-dimer formation. To validate the results we compare them with the results obtained from the master equation in the stochastic limit and with those obtained from the rate equations in the deterministic limit. Potential applications of the results in different physical contexts are discussed.Comment: 10 figure