A study of the properties of hydrogenated amorphous germanium produced by r.f. glow discharge as the electrode gap is varied the link between microstructure and optoelectronic properties

Abstract

Hydrogenated amorphous germanium films were deposited by r.f. glow discharge on the powered electrode of a diode-type deposition system. By varying the electrode gap D and keeping all other deposition parameters constant, a continuous monotonic change in both the optoelectronc and the structural properties of the films was observed. As D was decreased from 3.2 to 1.2 cm the product nμτ of the quantum efficiency n, mobility μ and lifetime τ increased from 2.1 × 10⁻⁹ to 1.7 × 10⁻⁷cm² V⁻¹, and the photoluminescence intensity increased. Other measurements taken indicate that changes in the electronic density of states cannot account for the observed improvement in the phototransport. Rather, we implicitly link this improvement to the observed changes in film structure. As D was decreased, the structure varied continuously from being heterogeneous of the order of 200 A and porous, to being homogeneous and non-porous. This change in structure was inferred from measurements of film stress, transmission electron microscopy, infrared transmission spectroscopy, gas evolution during controlled annealing and differential scanning calorimetry. The heterogeneous structure is interpreted in terms of a two-phase ‘island-tissue’ model that accounts for the observed changes in all the structural measurements as well as the deterioration in the optoelectronic properties. Such an island-tissue structure is probably caused by low surface mobility of adatoms arriving at the growing surface. Mechanisms involved in determining the surface mobility, and how they may change with D, are discussed. It is suggested that the ion bombardment plays a crucial role in eliminating the heterogeneities when considered with other deposition mechanisms. Measurements taken of the electrode bias voltages suggest that ion bombardment decreases with decreasing D.