Comparison of structural, optical and electrical properties of undoped ZnO thin films grown on r- and c- Al2O3 substrates using pulsed laser deposition

The structural, optical and electrical properties of undoped ZnO thin films grown by pulsed laser deposition on r- and c-planes of sapphire substrates are compared. From the X-ray diffraction data, ZnO samples are found to be epitaxially grown and (112̄0) and (000l) oriented, when deposited on r- Al2O3 and c- Al2O3, respectively. Thus, the ZnO c-axis either lies in the substrate plane or is perpendicular to it, respectively, constituting the main structural difference between these two samples. Very different microstructures are also evidenced by atomic force microscopy. In the case of r- Al2O3, the sample is completely striated, the striae being oriented along one direction. In the case of c- Al2O3, no columnar growth is observed, in contrast with some previous reports in the literature. The low temperature photoluminescence is sharper for the sample grown on r- Al2O3 than for the c- Al2O3 one. In the first case, two donor bound exciton lines are clearly seen, with a linewidth down to 1.7 meV. In the second case, the main excitonic feature is broader, with a linewidth of approximately 4.3 meV. The activation energy of band edge emission quenching is found to be approximately 2 meV in each case. Finally, both the resistivity and electron mobility of the two samples are found to be very different. The microstructure of ZnO layers is believed to be the main cause of these optical and electrical differences.