Carrier transport and capture in GaN single crystals and radiation detectors and effect of the neutron irradiation

Abstract

Carrier transport and trapping were investigated in GaN single crystals and semi-insulating epitaxial MOCVD layers by thermally stimulated current (TSC) and thermally stimulated depolarization (TSD) spectroscopy. Effect of irradiation by 100 keV reactor neutrons with fluencies of up to 10 16 n/cm 2 was identified. We demonstrate that in the unirradiated samples TSC spectra might be caused not by carrier generation from traps, but it rather originates from the thermal mobility variation. The numerical analysis revealed that mobility limited by ionized impurities varies as ∼ T 2.8 and lattice scattering causes the dependence ∼ T −3.5. The highest mobility values were up to 1550 cm 2/Vs at 148–153 K, indicating relatively high quality of the unirradiated samples. The irradiation, depending on the doses, induced complex changes of the carrier transport. The growth of the sample resistivity took place because of the more intense carrier trapping and scattering accompanied by appearance of the percolation transport phenomena. The whole ensemble of the defect traps was identified by the TSC in multiple heating regime and by measurements at different applied biases. In the samples irradiated by 10 16 n/cm 2 the following activation energy values were found: 0.16–0.2, 0.27–0.32, 0.36–0.45 and 0.73–0.74 eV. Effects associated with the percolation transport in a disordered media were observed in the samples irradiated by 5×10 14–10 16 n/cm 2, e.g., current instabilities and decrease of the effective thermal activation energy values by heating in the temperature region below 150 K. A model of carrier percolation transport is presented, in which effect of potential fluctuations due to crystal inhomogeneities is involved. The TSD measurements had confirmed the intensifying effect of inhomogeneities upon irradiation.