Deep defect levels and the optical as well as thermal transitions of carriers from the levels into the corresponding bands were analyzed using Thermal and Optical Admittance Spectroscopy. High resistivity GaN-layers grown by MBE and heterostructures consisting of n-type GaN-layers grown with Low Pressure Chemical Vapor Deposition on 6H-SiC substrates are investigated. In the MBE-grown GaN layers we determine deep electron traps with thermal activation energies of EA=(0.45±0.04)eV and EA=(0.65±0.03)eV. Furthermore, three different kinds of optical transitions were distinguished by Optical Admittance Spectroscopy: near band gap transitions including the transition between the valence band and a shallow donor 50meV below the conduction band, a peak at 2.1eV associated with the yellow photoluminescence band and various deep level-band transitions in the infrared region.
The high sensitivity of the TAS to interface defect states was used to investigate GaN/SiC heterostructures. We found an interface defect state at 70 … 90meV. Furthermore, one level was obtained originating from the epitaxial GaN-layer having an activation energy of 63±3meV. A defect distribution was identified in the p-type SiC-substrate with activation energies between 160meV and 180meV.