Index of content:
Volume 95, Issue 6, 15 March 2004
- ELECTRONIC STRUCTURE AND TRANSPORT (PACS 71-73)
95(2004); http://dx.doi.org/10.1063/1.1646451View Description Hide Description
Using a full-potential band structure approach, we have investigated the work function of barium oxide coatedtungstencathodes in low pressure discharge lamps. The main objective of this work is to understand why the work function for such cathodes is lower than that of the uncoated tungsten. The model studied in this work is based on a well known supposition that the source of thermionic electrons is the barium atoms released from the barium oxide coating due to a chemical reaction with the underlying metallic tungsten. For the unrelaxed seven-layer model of (100) surface of barium on barium oxide, the work function is calculated to be 2.22 eV, which is lower than that of BaO, Ba, and W metals separately. For a fully relaxed nine-layer surface, it becomes 1.36 eV. Although this value of the work function is lower than those estimated for the fluorescent cathodes by electrical measurements, which averages contributions from surfaces in all possible random orientations, this model provides a satisfactory explanation of the lowering of the work function of tungsten based cathodes in low pressure fluorescent lamps.
95(2004); http://dx.doi.org/10.1063/1.1647259View Description Hide Description
Ultrathin oxygenated amorphous CdTefilms are prepared by rf sputtering of CdTe in a background of argon or argon/nitrogen/oxygen mixtures. Atomic force microscopy(AFM) is used to characterize the films and shows that they have an island structure typical of most sputteredthin films. However, when sufficiently low powers and deposition rates are employed during sputtering, the resulting films are remarkably smooth and sufficiently thin for use as barrier layers in inelastic electron tunneling (IET) junctions. Four terminal current–voltage data are recorded for tunnel junctions and conductance–voltage curves are derived numerically. WKB fits to the conductance–voltage curves are obtained using a two-component trapezoidal plus square (TRAPSQR) model barrier potential to determine values for the tunnel barrier parameters (height, shape, and width); these parameters are consistent with AFM topological measurements and values from similar devices reported in the literature. IET spectra are presented which confirm that electrons tunnel through ultrathin regions of the films, which contain aluminum oxide subregions in a manner consistent with the TRAPSQR barrier model. Because tunneling occurs predominantly through these ultrathin regions, IET spectroscopic data obtained are representative of states at, or within a few tenths of nanometers from, the surface and confirm that the surface stoichiometry is very sensitive to changes in the argon/oxygen/nitrogen concentration ratios during film growth. Full IET spectra, current–voltage, and conductance–voltage data are presented together with tunnel barrier parameters derived from (WKB) fits to the data. The results presented here indicate that inelastic electron tunnelingspectroscopy is a useful tool for characterizing the surface states of and possibly other photovoltaic materials.
95(2004); http://dx.doi.org/10.1063/1.1649458View Description Hide Description
We develop a self-consistent solution of the Schrödinger and Poisson equations in semiconductorheterostructures with arbitrary doping profiles and layer geometries. An algorithm for this non-linear problem is presented in a multiband k⋅P framework for the electronic band structure using the finite element method. The discretized functional integrals associated with the Schrödinger and Poisson equations are used in a variational approach. The finite element formulation allows us to evaluate functional derivatives needed to linearize Poisson’s equation in a natural manner. Illustrative examples are presented using a number of heterostructures including single quantum wells, an asymmetric double quantum well,superlattices, and trilayer superlattices.