Room-temperature negative photoconductivity in degenerate InN thin films with a supergap excitation

Negative photoconductivity (NPC), up to room temperature, has been observed in small band gap and degenerate (n-type) indium nitride (InN) thin films with superband-gap excitation of 2.3 eV. Samples investigated above 160 K showed bipolar behavior of photoconductivity with a fast positive response due to photogenerated electron and hole conduction in the valence and conduction band, respectively, followed by a slow relaxation below the dark-current background. However, below 160 K, the transient photoresponse was absolute negative with similar relaxation times. Hall measurements, under illumination, showed an increase in carrier density (n), but severe scattering in the charged recombination centers lowered the mobility (mu) and consequently a net n mu product controlling the PC. The NPC phenomenon in the degenerate system, not limited to InN, has been modeled on the basis of electronic scattering in the conduction band as against gap state transitions that controlled it in conventional nondegenerate semiconductors with subgap excitation.