Convertibility of conductivity type in reactively sputtered ZnO thin films

Possible negative-U behavior of the V-O donor may explain rather unusual free carrier properties in as-sputtered ZnO thin films grown at extremely Zn-rich conditions. V-O can act as a source of free electron concentration at sample temperatures above 210 K. Below this temperature it changes its charge state to an inactive neutral charge state where it cannot act as a donor because of a thermal barrier with a threshold temperature of 170-210 K. The thermal barrier for an electron to go to neutral charge state from positive charge state is approximately 162 meV. Material can be converted to p-type by annealing V-O centers. O-rich growth conditions with low [V-O] may result in p-type conductivity.