Impact of O2 flow rate on the growth rate of ZnO(0001) and ZnO(000-1) on GaN by plasma-assisted molecular beam epitaxy

Journal article, 2016

We studied the effects of a varying O$ flow rate on the growth of ZnO(0001) and ZnO(000-1) layers on GaN/Al2O3-templates by plasma-assisted molecular beam epitaxy. The O2 flow rate through the O-plasma source was varied between 0.25--4.5 standard cubic centimeters per minute corresponding to a growth chamber pressure between 3.0 x 10^-6 -- 5.0x10^-5 Torr. We found that the change of the O2 flow rate had a profound effect on the ZnO layer growth rate. A maximum growth rate was reached for an O2 flow rate of 1.0--2.0 standard cubic centimeters per minute. The same growth rate dependence on the O2 flow rate was observed for ZnO(0001) layers that were grown on GaN/4H-SiC buffer layers for verification. To assess the amount of active O contributing to the ZnO-growth, the spectral composition of the plasma was investigated with optical emission spectroscopy. The integrated optical emission line intensity reached a maximum for an O2 flow rate between 1.0--2.0 standard cubic centimeters per minute. Essentially all emission lines exhibited a maximum intensity for an O2 flow rate between 1.0--2.0 standard cubic centimeters per minute thus coinciding with the flow rate yielding the maximum growth rate.