Subbandgap absorption from photocurrent spectra in Al0.18GaN/GaN heterostructures

We have measured optical absorption in Al0.18GaN/GaN heterostructures, prep ared by MOCVD on sapphire substrates, by the constant photocurrent method ( CPM) between 300 and 500 K. The constant photocurrent mode is appropriate s ince the response time decreases when the incident light energy crosses fro m below to above the bandgap energy. The films were further characterized b y temperature-dependent dark conductivity and Hall measurements. We have fo und exponential band tails exceeding thermal broadening in the absorption s pectra below the bandgap energy indicating the presence of disorder and/or deep defects. (C) 2001 Elsevier Science B.V. All rights reserved.