Impurity conduction in silicon carbide

We report on admittance spectroscopy (AS) investigations taken on aluminum (Al)doped 6H-SiC crystals at low temperatures. Admittance spectra taken on Schottky contacts of highly doped samples (N-A >= 7.2 x 10(17) cm(-3)) reveal two series of conductance peaks, which cause two different slopes of the Arrhenius plot. The steep slope is attributed to the At acceptor, while the flatter one - obtained from the low temperature peaks - is attributed to the activation energy epsilon(3) of nearest neighbor hopping. We propose a model, which explains the unexpected sharpness of the low temperature conductance peaks and the disappearance of these peaks for low acceptor concentrations. The model is verified by simulation, and the AS results are compared with corresponding results obtained from resistivity measurements taken on 4H- and the identical 6H-SiC samples.