p-type doping of ZnO films and growth of tenary ZnMgO and ZnCdO: application to light emitting diodes and laser diodes

ZnO and the related ZnMgO and ZnCdO ternary solid solutions are possible alternatives to GaN-based compounds for the fabrication of ultraviolet (UV)/blue light emitting diodes (LEDs) and injection laser diodes. The ZnO materials system has advantages in terms of the higher binding energy of excitons (60 meV), which leads to efficient electron-hole pair-to-photon conversion at the elevated operating temperatures likely for such devices, the availability of commercially available high-quality ZnO substrates, lower growth temperatures (by at least 200°C) and simplicity of selective wet etching processes for mesa formation in devices. Progress in development of ZnO LEDs has been disappointing due to the difficulty of achieving robust p-type doping and the low crystal quality of heterojunctions and quantum wells. We critically review reports of p-type doping using group V impurities as well as progress in growing ternary ZnMgO and ZnCdO films. We also summarise recent progress and prospects for further advancement of ZnO-based light emitters.