Enhancement of electron mobility in ZnO layers with applying ultrasonic spray-assisted MOVPE and buffer layers

Ultrasonic spray-assisted metalorganic vapor phase epitaxy (MOVPE), where the ultrasonically sprayed liquid source solution is used as the precursor of the vapor phase reaction, has been developed to grow single-crystalline ZnO and ZnMgO thin films on sapphire substrates. By applying RF sputtering-deposited ZnO buffer layers of 10-80 nm in thickness, the mobility reached 58-66 cm2/V s with the electron concentration of (4-9)×1017 cm−3 at the growth temperatures of 800-900 °C. The temperature dependence of mobility revealed that at room temperature the lattice scattering competed with the ionized impurity scattering, suggesting that the remarkable increase of the mobility is expected by reducing the residual impurities with purified sources. The results suggest the promising potential of the present growth method, with using safe and inexpensive source materials, as one of the MOVPE technologies contributing to mass production. On the other hand, the surface morphology and electrical properties were markedly degraded in ZnMgO, and these problems are still found to be difficult to be resolved.