The trap emptying process of low temperature thermoluminescence of 6H–SiC single crystals

Abstract

Thermoluminescence (TL) and thermally stimulated conductivity (TSC) of SiC n- and p-type single crystals were measured in the temperature region 12–300 K. To describe the underlying process, we assumed the classical model consisting of one species of trap and recombination center, respectively. The calculated glow curve can be fitted to the experimental one with good conformity. The analysis of the glow curve main peaks leads to the thermal activation energy of the emptied traps. For nitrogen (N) doped n-type crystals, we obtain thermal activation energies of 80 meV or 140 meV for highly compensated samples. Aluminum (Al) doped p-type samples show an activation energy of 220 meV, boron (B) containing p-type samples one of 330 meV. This corresponds to the well known ionization energies of the shallow doping (N on cubic and hexagonal lattice site for n type and Al, B for p type). To obtain the energy necessary for a charge transfer of the impurity, we studied TL as a function of the excitation wavelength. Here, an excitation energy between 1.65 and 1.9 eV turned out to be necessary to recharge the recombination center. The recombination center responsible for the TL normally acts as compensation in thermal equilibrium. For p-type samples, we can distinguish between the omnipresent N and a second impurity as compensation, which is energetically deeper than 240/300 meV. The chemical nature of these centers is still unknown.