Density-of-states distribution in AlGaN obtained from transient photocurrent analysis

Dark conductivity and photoconductivity in AlxGa1-xN (x=0.025, 0.18) films prepared by MOCVD on a sapphire substrate with a GaN bottom layer have been investigated. In the temperature range 300–500 K, the barrier at the heterostructure interface appears to play an important role in the thermal activation of the photocurrent after pulsed laser excitation, for high aluminium concentrations. Transient photoconductivity measurements on Al0.025GaN films have been made and are interpreted in terms of multiple trapping and release of carriers in localised states. The current transient is characterised by an initial rapid decay in the sub-microsecond regime, followed by a much slower power law decay out to tens of milliseconds. These features, which occur for both sub- and super-gap excitation, are consistent with the presence of a steep exponential tail of states at the band edge, followed by a broad peak centered at approximately 0.4 eV below Ec.