Critical increase in Na-doping facilitates acceptor band movements that yields similar to 180 meV shallow hole conduction in ZnO bulk crystals

Abstract

Stable p-type conduction in ZnO has been a long time obstacle in utilizing its full potential such as in opto-electronic devices. We designed a unique experimental set-up in the laboratory for high Na-doping by thermal diffusion in the bulk ZnO single crystals. SIMS measurement shows that Na concentration increases by 3 orders of magnitude, to similar to 3 x 10(20) cm (3) as doping temperature increases to 1200 degrees C. Electronic infrared absorption was measured for Na-acceptors. Absorption bands were observed near (0.20-0.24) eV. Absorption bands blue shifted by 0.04eV when doped at 1200 degrees C giving rise to shallow acceptor level. Na-Zn band movements as a function of doping temperature are also seen in Photoluminescence emission (PL), Photoluminescence excitation (PLE) and UV-Vis transmission measurements. Variable temperature Hall measurements show stable p-type conduction with hole binding energy similar to 0.18eV in ZnO samples that were Na-doped at 1200 degrees C.