Y. Adachi, N. Ohashi, T. Ohgaki, T. Ohnishi, I. Sakaguchi, S. Ueda, H. Yoshikawa, K. Kobayashi, J.R. Williams, T. Ogino and H. Haneda, “Polarity of heavily doped ZnO films grown on sapphire and SiO2 glass substrates by pulsed laser deposition,” in Thin Solid Films, Vol. 519, No. 18, pp. 5875–5881, 2011, ISSN: 0040-6090.