Luminescence study of the disorder in polycrystalline InP thin films

InP films have been deposited on amorphous sapphire substrate by means of the metallorganic chemical vapour deposition technique with and without PH3/H-2 plasma preactivation. Polycrystalline materials, having average grain sizes of about 40 nm, which, however, vary according to the growth parameters, were obtained. The structural and compositional disorder strongly influence the optical and electrical properties. The most structurally disordered films present the largest photoluminescence (PL) efficiency and a blue-shift of the whole PL spectra. Radiative recombinations of carriers confined in crystallites having the smallest sizes are proposed to explain such an anomalous behaviour. The temperature dependence of the bandgap follows the well known phenomenological models except for the hydrogenated samples, in which a blue-shift of the intrinsic luminescence band is observed with increasing temperature. This anomalous effect is related to phonon-assisted transitions between nanoclusters with different sizes. Electrical conductivity measurements show different conductivity mechanisms in two different ranges of temperature. In particular, for temperatures lower than 200 K the conduction mechanism is due to a variable-range hopping process.