M.F. Kotkata

optical studies of disorder and defects in amorphous GexSe1−X films as a function of composition

Abstract The optical absorption edges of as-deposited evaporated a-Ge X Se 1−X films, with x = 0, 20, 33, 43 and 50 at%, have been accurately determined down to 0.6 eV from a combination of optical and PDS experiments. The results are analyzed as a function of composition in terms of chemical ordering, strain-induced disorder and coordination defects, taking the pure constituents and the stoichiometric GeSe 2 compound as references.

Studies of short-range order in amorphous Ge x Se100−x compounds by X-ray photoelectron spectroscopy

Abstract X-ray photoelectron spectroscopy experiments were performed on as-deposited evaporated a-Ge x Se100-x compound films, with x ranging from 20 to 50 at.%, in order to investigate the changes in the core level and valence band electron distribution as a function of composition. The results are interpreted in terms of short-range ordering, taking the stoichiometric a-GeSe2 compound as a reference. The valence band spectra are compared to theoretical predictions and discussed in relation to optical data obtained on the same samples. It is shown that 4:2 coordination is maintained over the whole composition range, and that chemically ordered structural units are favoured in all cases.

Preparation and characterization of co-evaporated a-GaAs films

Abstract Amorphous GaAs films were prepared from their constituents using the co-evaporated technique. The electrical and optical characterization of the prepared films have been carried out for the as-deposited (fresh), aged as well as for films annealed at 150 and 200 °C, Results of the temperature dependence of dc conductivity, in the range from −120 to 150 °C, are given and discussed. For nearly stoichiometric a-GaAs films and above RT the conduction mechanism is found to be thermally activated. In the low temperature range, the conduction is due to variable-range hopping of localized states at the Fermi level. A correlation between the optical and the electrical results, in view of the current structural models, brings about a shift in the valence band edge to high binding energies causing an increase in the optical gap of the as-deposited a-GaAs films upon annealing.

Studies of evaporated amorphous GeSe2 films as a function of annealing

The optical properties of evaporated amorphous GeSe 2 films have been determined before and after annealing by transmission and photothermal deflection spectroscopy measurements. The results are analyzed together with those of X-ray photoemission and X-ray diffraction experimemts. The annealing effects are related to changes in medium-range order. The low-energy optical absorption shows a defect state density varying with deposition conditions and annealing

Evidence for insulator–metal transition in amorphous chalcogenide Se–Ge–Te films

The temperature dependence of the dc conductivity and thermoelectric power was determined for five different amorphous chalcogenide Se–Ge–Te films, with Ge = 3.0–22 at.%, Se = 0–97 at.% and Te = 0–97 at.%. The films were prepared by thermal evaporation of GeSe4, GeTe4 and GeSe2Te2 quenched bulk materials. Values of the activation energy calculated from the temperature dependence of both electrical conductivity and thermoelectric power showed a decrease with increasing Ge content in the Se–Ge films as well as with replacement of Te for Se in the Se–Ge–Te films. The results showed an Anderson transition, with the conductivity showing insulating behaviour on the Ge–Se side to metallic behaviour at the binary composition Ge–Te. The radius of localization was obtained for the different compositions investigated. The wave function associated with the charge carriers at the composition Ge3.3Te96.7 is non-localized. A minimum metallic conductivity of 237 ± 5 (Ω cm)−1 was found.