Defect studies of ZnO films prepared by pulsed laser deposition on various substrates

The quality of ZnO thin films was characterized in this work by slow positron implantation spectroscopy (SPIS) combined with X-ray diffraction (XRD). The ZnO films were grown by pulsed laser deposition (PLD) on three different substrates: sapphire (0001) single crystal, MgO (100) single crystal and amorphous fused silica (FS). Films deposited on all substrates exhibit wurtzite ZnO structure and are characterized by an average crystallite size 20 - 100 nm.
Microstructure of ZnO films is strongly influenced by substrate. XRD investigations revealed that ZnO films deposited on sapphire (0001) and MgO (100) single crystalline substrates exhibit local epitaxy, i.e. a well-defined relation between film crystallites and the substrate. On the other hand, the film deposited on FS substrate exhibits (0002) fiber texture with random lateral orientation of crystallites in the plane of substrate. The films deposited on single crystalline substrates exhibit narrow XRD (0002) rocking curves with the half-widths (FWHM) of 1.3° and 1.2° for the film deposited on sapphire (0001) and MgO (100) substrate, respectively. On the other hand, the film deposited on amorphous FS substrate exhibits significantly wider XRD (0002) rocking curve having half width of 10°. Hence, the mosaic spread of planes is relatively small for the films grown on single crystalline substrates while it is substantial for the film grown on amorphous FS substrate. The half width of the XRD rocking curve is often used as a measure of quality of epitaxial thin films. However, half width of the rocking curve is a measure of the perfection of the structural relationship but is not directly correlated with the density of defects in the film. SPIS investigations revealed that ZnO films deposited on MgO and sapphire single crystalline substrates exhibit significantly higher density of defects than the film deposited on amorphous FS substrate despite the fact that half width of rocking curve exhibits opposite behaviour. In ZnO film deposited on the amorphous FS substrate the mismatch between atomic positions in the substrate and in the film is to some extend compensated for by many differing orientations (tilting) of the ZnO crystallites. As a consequence, the density of misfit dislocations in ZnO crystallites is relatively low and positrons are trapped predominantly at open volume misfit defects at the interfaces between the crystallites. On the other hand, ZnO films deposited on MgO and sapphire single crystalline substrates exhibit local epitaxy. In these cases, the lattice mismatch between the film and the substrate is accumulated mainly by misfit dislocations.