STABLE AMORPHOUS GERMANIUM FILMS PREPARED IN ULTRA HIGH VACUUM AND MEASURED IN-SITU : STRUCTURE AND ELECTRONIC PROPERTIES

Abstract
Stable, homogeneous germanium films have been prepared by slow evaporation onto heated sapphire substrates in ultra high vacuum. Their properties do not change on annealing until they crystallise at 250°C (1,2). The structure of these films is characterised by low density, by a shift of the first diffraction peak to higher scattering angle, with respect to the crystal and by the existance of a diffraction pre-peak. The optical absorption edge is relatively sharp, has a shoulder at 0.85 eV, and the low photon energy refractive index has a lower value than for the crystal. The temperature T dependence of the d.c. electrical conductivity σ is characterised by an "S" shaped curve, when plotted as log σ versus T^-¼. The photoconductivity shows marked long time ( t > 10³ sec) relaxation effects at 77K and exhibits photoconductivity fatigue at 5K. These results are compared with amorphous germanium (a-Ge) films prepared under different conditions and with other amorphous and glassy materials. Some common trends in the structure and electronic properties are pointed out.