Growth and characterization of undoped ZnO films for single crystal based device use by radical source molecular beam epitaxy (RS-MBE)

The integrated use of (1 1 2̄ 0) a-plane sapphire substrates and high temperature growth with low temperature buffer layers have led to high quality undoped ZnO epitaxial films with mobilities as high as 120 cm²V^-1 s^-1 and residual carrier concentrations as low as 7.6 × 10¹⁶ cm^-3. Pole figure measurements reveal that a-plane sapphire substrates are effective for the elimination of 30° rotation domains, which usually appear using c-plane sapphire substrates. In particular, when using c-plane sapphire substrates annealing at 1000°C in O₂ with c/2-height surface steps, the X-ray diffraction pole figure peak intensity related to these rotation domains increased. The use of low temperature buffer layers allow high temperature ZnO growth on sapphire, as initial ZnO growth does not occur at high initial growth temperature.