Investigation of source of n-type conductivity in bulk ZnO

In this study, we have investigated the source of the n-type conductivity in bulk ZnO. Four samples with Zn concentrations 52%, 53%, 54% and 55 % were prepared by molecular beam epitaxy (MBE). ZnO layers exhibited remarkable increase in carrier concentration (5.0 X 10/sup 16/ cm/sup -3/ to 2.2 X 10/sup 19/ cm/sup -3/) and decrease in resistivity (14.4 O cm to 0.009 O cm) as a function of Zn contents (52% to 55%) in the layers, respectively. We observed that with increasing thickness, the microstructure of films was improved. The thick films have less Zn-interstitials defects and consequently have lower carrier concentration and higher resistivity. The shifting of Raman E2 high mode of ZnO towards higher frequency with increasing Zn contents in the samples also confirmed the presence of excessive Zn-interstitials in the layer. (author)