Structure and electronic properties of sputtered amorphous gap films.

Stoichiometric amorphous GaP films have been prepared by RF sputtering on to substrates held at temperatures from 20°C to 200 °C. Some room-temperature deposited samples were annealed at higher temperatures (200-400 °C) in vacuum. Structural, optical and electrical characterisation of these films was carried out by means of a variety of experimental techniques. Electron microscopy experiments, using both transmission electron microscope (TEM) and scanning electron microscope together with energy dispersive X-ray analyser (SEM-EDAX) have revealed that, while on the macroscopic scale the samples are homogeneous, continuous and smooth, on the microscopic scale they contain structural inhomogeneities, namely voids, and a small degree of compositional non-uniformity. Extended X-ray absorption fine structure (EXAFS) spectroscopy has provided information on the atomic structure of the films, in particular on the short-range order around both Ga and P atoms. It was found that the a-GaP network is four-fold coordinated and chemically ordered, i.e. wrong bonds, Ga-Ga and P-P bonds, do not exist. In addition the technique revealed very little disorder in the bond length but wide distributions in the bond angles, the latter being more pronounced around P atoms than around Ga atoms. Information on the short-range atomic structure has been obtained from infrared (IR) spectroscopy through the identification of the vibrational modes of the bonds. The main finding of these measurements was that the a-GaP network is chemically ordered, which is consistent with the EXAFS work. From a combination of conventional reflection-transmission (R-T) and the most recently developed photothermal deflection spectroscopy (PDS) techniques, the absorption coefficient in the range of 10 cm-l < 10.;5 cm-1, the refractive index and values ofthe optical gap were determined. The results are explained in terms of the nature and distribution in energy of the density of states (DOS) in the forbidden gap. The DOS in the conduction band is also extracted from the X-ray absorption near edge structure (XANES) data, which is combined with the DOS-valence band information taken from the literature to obtain full details of the electronic structure of a-GaP. D.c conductivity measurements at high- and low-temperatures showed a continuously varying activation energy. This was interpreted as a transport mechanism taking place in a band tail but in which the conduction path moves downwards in the tail as T is lowered.