The thermoluminescence and thermally stimulated conductivity of n-type and p-type monocrystals were studied at temperatures ranging from 12 to 300K. In order to describe the underlying processes, a classical model was used which involved 1 species of trap and recombination center. The calculated glow curves could be fitted quite closely to experimental ones. An analysis of the main peaks of the glow curve yielded the thermal activation energies of the emptied traps. In the case of N-doped n-type crystals, thermal activation energies of 0.080 or 0.140eV were deduced for highly compensated samples. The Al-doped p-type samples revealed an activation energy of 0.220eV. On the other hand, B-containing p-type samples revealed an activation energy of 0.330eV. This corresponded to the well-known ionization energies of shallow dopants (N on cubic sites and hexagonal lattice sites). In order to estimate the energy which was required for charge transfer of the impurity, the thermoluminescence was monitored as a function of the excitation wavelength. It was found that an excitation energy of between 1.65 and 1.9eV was necessary in order to recharge the recombination center. The recombination center that was responsible for the thermoluminescence normally acted as a compensator in thermal equilibrium. In the case of p-type samples, it was possible to distinguish between the omnipresent N and a second compensating impurity which was energetically deeper than 0.240 to 0.300eV. The chemical nature of these centers remained unknown.

T.Stiasny, R.Helbig: Journal of Applied Physics, 1996, 79[8], 4152-6