Electrical properties of undoped bulk ZnO substrates

Undoped bulk ZnO crystals obtained from Tokyo Denpa show either resistive behavior [(5×104)−(3×105) Ohm cm) or low n-type conductivity (n ⋍1014 cm−3) with mobilities in the latter case of 130–150 cm2/V sec. The variation in resistivity may be related to the thermal instability of Li that is present in the samples. The Fermi level is pinned by 90-meV shallow donors that are deeper than the 70 meV and hydrogen-related 35-meV shallow donors in Eagle Pitcher and Cermet substrates. In all three cases, 0.3-eV electron traps are very prominent, and in the Tokyo Denpa material they dominate the high-temperature capacitance-frequency characteristics. The concentration of these traps, on the order of 2×1015 cm−3, is about 20 times higher in the Tokyo Denpa ZnO compared to the two other materials. The other electron traps at Ec −0.2 eV commonly observed in undoped n-ZnO are not detected in conducting Tokyo Denpa ZnO samples, but they may be traps that pin the Fermi level in the more compensated high-resistivity samples.