  We have investigated the initial growth behavior and resulting
microstructural properties of heteroepitaxial ZnO thin films prepared by pulsed
laser deposition on sapphire (0001) substrates. High-resolution x-ray
diffraction and transmission electron microscopy studies revealed that the
initial growth behavior and the microstructure of the films significantly
depend on the growth parameters. ZnO films grown at 700 oC with 20 mTorr O2
partial pressure initiated with a columnar growth mode and contained two kinds
of domains. These domains were in-plane orientated either
ZnO[112-0]//Al2O3[101-0] or ZnO[101-0]//Al2O3[101-0], and were surrounded by
highly defective domain boundaries with threading dislocations. The films grown
at 800 oC with 1 mTorr O2 showed 2-dimensional layered growth with only one
in-plane epitaxial relationship, ZnO[112-0]//Al2O3[101-0]. Most of the defects
in the layered grown films were basal plane stacking faults near the interface
between ZnO and the substrate. The formation mechanism of the 30o-twisted
domains with the in-plane orientation of ZnO[101-0]//Al2O3[101-0] is discussed.
