Impact of O sub(2) flow rate on the growth rate of ZnO(0001) and ZnO(000[Formulaomitted]) on GaN by plasma-assisted molecular beam epitaxy

We studied the effects of a varying [Formulaomitted] flow rate on the growth of ZnO(0001) and ZnO(000[Formulaomitted]) layers on [Formulaomitted] templates by plasma-assisted molecular beam epitaxy. The [Formulaomitted] flow rate through the O-plasma source was varied between 0.25 and 4.5standard cubic centimeters per minute (sccm) corresponding to a growth chamber pressure between 3.0[Formulaomitted] and 5.0[Formulaomitted] Torr. We found that the change of the [Formulaomitted] flow rate had a profound effect on the ZnO layer growth rate. A maximum growth rate was reached for an [Formulaomitted] flow rate of 1.0-2.0sccm. The same growth rate dependence on the [Formulaomitted] 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 [Formulaomitted] flow rate between 1.0 and 2.0sccm. Essentially all emission lines exhibited a maximum intensity for an [Formulaomitted] flow rate between 1.0 and 2.0sccm thus coinciding with the flow rate yielding the maximum growth rate.