Effect of ion damage on optical properties of ZnO films grown by plasma-assisted MBE

The effect of plasma-induced ion damage on the optical properties of ZnO films grown by plasma-assisted molecular beam epitaxy on a-sapphire substrates and GaN(0001)/c-sapphire templates prepared has been studied using steady-state and time-resolved photoluminescence. We observed that the deflecting the ions produced by the RF oxygen plasma away from substrate results in improved excitonic emission and modification of the defect-related PL spectrum. The intensity of the near-band-edge lines in the photoluminescence spectra from the layers grown with the ion deflection was found to increase by factors 7 to 20 for the layers grown on GaN(0001)/c-sapphire at a plasma power of 350 W and by 3 to 4 times for ZnO grown on a-sapphire substrates at a plasma power of 265 W as compared to the controls grown without the ion deflection. The yellow-green spectral range was dominated by different defect bands in the films grown with and without ion deflection. The effect of RF power on peak positions of the defect band was studied for the films grown without ion deflection. For the ZnO films grown on a-plane sapphire substrates, time-resolved photoluminescence showed a significant increase in luminescence decay times both at RT and 89 K. However, for ZnO on GaN(0001)/csapphire substrates, virtually no improvement in decay time was found at 89 K with only a moderate increase in decay constant at room temperature.