We report on the results of a systematic series of experiments aimed at improving the stability of amorphous silicon (a-Si:H) films. We find that very low levels (0.2–0.4 ppm) of compensation by boron, when combined with growth conditions that favor a robust microstructure in the material, results in films with significantly improved stability and very low defect density. The films were grown using a reactive plasma beam technique using a remote plasma beam from an electron-cyclotron-resonance source. The improvement in stability is seen under both short-term (a few minutes) light soaking, and also under light soaking under high intensity (10×sun) illumination conditions. We conclude that the improvement in initial stability is a result of compensation of native donors (probably oxygen) in the material by boron. The surprising reduction in long term instability and corresponding defect density upon compensation implies that the microstructure and structural changes around the native impurities may play a role in the long-term instability of the material.

Transmission of electromagnetic waves propagating in planar, cylindrical, and spherical single dielectric layer, double-barrier resonant tunneling and multiple period Bragg reflector systems have been systematically studied by the transfer matrix method. Interesting features are found in the transmission spectra of the systems with cylindrical and spherical geometries, due to the coherence of the propagation waves in these systems. In particular, an increase of the resolution power (sharper transmission peaks) in the double-barrier resonant tunneling system is demonstrated in the cylindrical and spherical structures compared to that of the planar structure. Universal behavior, that the transmission at zero frequency limit decreases substantially as the propagation wave changes from plane to cylindrical wave and to spherical wave, has been shown in the spectra of these systems. Potential applications in reducing the radiation energy loss for transferring large density alternative current are discussed.

No abstract

Exciton binding energies of heavyand light-hole excitons affected by charge-carrier screening in GaAs-Al"Ga& As quantum wells are calculated by the variational-perturbation method. The exciton binding energies are found to decrease rapidly when the screening length is less than 30az (effective exciton Bohr radius). This screening length corresponds to a carrier density of 7.0X 10"/cm at T=10 K. In the calculation, the Debye screening model is used for charge carriers. The exciton binding energies as functions of the screening length, carrier density, and quantum-well parameters have been calculated. The critical carrier densities, above which no excitons can be formed, are obtained at different well thickness. The effects of charge-carrier screening to the exciton photoluminescence are also discussed.

No abstract