Growth of undoped ZnO films with improved electrical properties by radical source molecular beam epitaxy

High-quality undoped ZnO epitaxial films with mobilities as high as 120cm²V^-1s^-1 and carrier concentrations as low as 7.6 × 10¹⁶ cm^-3 have been grown on (112̄0) a-sapphire substrates using low-temperature buffer layers, a slow substrate cooling process and a modified oxygen radical cell. Pole figure measurements reveal that a-plane sapphire substrates are effective for the elimination of 30° rotation domains, which usually appear in the case of ZnO growth on c-sapphire. The low-temperature buffer layers allow high-temperature growth, because initial ZnO growth does not occur with high initial growth temperature. The use of slow substrate cooling prevents the deterioration of the electrical properties of the ZnO films. Use of quartz insulators in the oxygen radical cell eliminates aluminum contamination, which is a serious problem when using conventional alumina insulators.