Temperature Dependant Annealing of ZnO Nanowires in Hydrogen and Hydrogen Donor Formation Energy

We present experiments and analysis used to probe the temperature dependence of the annealing zinc-oxide nanowires in a hydrogen gas ambient and a calculation of the formation energy of the hydrogen dopant and the hydrogen dopant ionization energy. The nanowires were grown using a vapor-liquid-solid technique with diameters from 30 to 400 nm and lengths up to 5 mm. The experiments were performed using sequentially elevated annealing temperatures to identify the threshold energy for the incorporation of hydrogen as a dopant into a mat of ZnO nanowires. It was observed that above a temperature of about 545 K the conductivity of the nanowires increased dramatically. This conductivity increase was robust to the removal of the hydrogen gas ambient and the return of the sample to room-temperature. Using the data collected, the formation energy (energy of incorporation) of the hydrogen into the lattice of the ZnO nanowires was calculated. It was found that this value was about 30% lower than the theoretical predictions and the experimentally derived value for bulk ZnO. These results will be contrasted and compared to previous results of similar experiments and analysis on ZnO nanoparticles and bulk ZnO samples.