Effects of low temperature ZnO and MgO buffer thicknesses on properties of ZnO films grown on (0001) Al2O3 substrates by plasma-assisted molecular beam epitaxy

We investigated the structural properties of Zn-polar ZnO films with low temperature (LT) ZnO and MgO buffer layers grown by plasma-assisted molecular beam epitaxy on (0001) c-Al2O3 substrates using X-ray diffraction and transmission electron microscopy (TEM). The effects of MgO buffer layer thickness and LT ZnO buffer layer thickness were also examined. The optimum thicknesses for better crystal quality were 8 and 40 nm. One-pair and two-pair LT ZnO/MgO buffer layers were employed, and the changes in the structural properties of the high-temperature (HT) ZnO films using such buffer layers were studied. Contrary to the general tendency of c-ZnO films, the HT ZnO films on the LT ZnO/MgO buffer layers showed higher full width at half maximum (FWHM) values for X-ray rocking curves (XRCs) with (0002) reflection than those with (10 (1) over bar1) reflection. Compared with the one-pair LT ZnO/MgO buffer layers, the FWHM values of (0002) XRCs markedly decreased, whereas those of (10 (1) over bar1) XRCs slightly increased due to the insertion of one more pair of LT ZnO/MgO buffer layers into the previous film with one-pair LT ZnO/MgO buffer layers. The cross-sectional TEM observations with the two-beam condition confirmed that the screw dislocation was the dominant threading dislocation type-a finding that agreed well with the XRC results. On the basis of the plan-view TEM observations, the densities of the total threading dislocations for the HT ZnO films with the one- and two-pair LT ZnO/MgO buffer layers were determined as 2.3 x 10(9) cm(-2) and 1.6 x 10(9) cm(-2), respectively. The results imply that the crystal quality of Zn-polar ZnO films can be improved by two-pair LT ZnO/MgO buffer layers, and types of threading dislocations can be modified by adjusting the buffer system. (C) 2010 Elsevier B.V. All rights reserved.