Metastable Changes in the Dark Conductivity of Undoped and Carbon-Doped β-Ga2O3

Measurements of the electrical dark conductivity sigma(D) of undoped and carbon-doped beta-Ga2O3 thin films reveal a metastable increase upon illumination with sub-bandgap light. In the relaxed state, sigma(D) shows activated behavior for T > 250 K with an activation energy of 0.68 and 0.63 eV for undoped and C-doped beta-Ga2O3, respectively. For T < 250 K, sigma(D) approaches a constant value of approximate to 6 x 10(-12) Omega(-1) cm(-1). Illumination with sub-bandgap light results in an increase of the dark conductivity by up to 6 orders of magnitude. This state is metastable and relaxes with time. The data indicate the presence of a broad distribution of localized states in the bandgap. The time and temperature dependence of the relaxation is investigated for doped and undoped samples. The data can be described by a sum of two and three stretched exponential decays for undoped and carbon-doped beta-Ga2O3, respectively. From the time constants of the decays, the energetic depth of the localized defect states is estimated.