Towards controllable Si-doping in oxide molecular beam epitaxy using a solid SiO source: Application to ${\hat{I}}^2$-Ga2O3

Abstract

The oxidation-related issues in controlling Si doping from the Si source material in oxide molecular beam epitaxy (MBE) is addressed by using solid SiO as an alternative source material in a conventional effusion cell. Line-of-sight quadrupole mass spectrometry of the direct SiO-flux ($\hat{I}{¦}_{SiO}$) from the source at different temperatures ($T_{SiO}$) confirmed SiO molecules to sublime with an activation energy of 3.3eV. The $T_{SiO}$-dependent $\hat{I}{¦}_{SiO}$ was measured in vacuum before and after subjecting the source material to an O${}_2$-background of $10^{-5}$ mbar (typical oxide MBE regime). The absence of a significant $\hat{I}{¦}_{SiO}$ difference indicates negligible source oxidation in molecular O${}_2$. Mounted in an oxygen plasma-assisted MBE, Si-doped ${\hat{I}}^2$-Ga2O3 layers were grown using this source. The $\hat{I}{¦}_{SiO}$ at the substrate was evaluated [from 2.9x10${}^9$ cm${}^{-2}$s${}^{-1}$ ($T_{SiO}$=700°C) to 5.5x10${}^{13}$ cm${}^{-2}$s${}^{-1}$ (T${}_{SiO}$=1000°C)] and Si-concentration in the ${\hat{I}}^2$-Ga2O3 layers measured by secondary ion mass spectrometry highlighting unprecedented control of continuous Si-doping for oxide MBE, i.e., $N_{Si}$ from 4x10${}^{17}$ cm${}^{-3}$ ($T_{SiO}$=700°C) up to 1.7x10${}^{20}$ cm${}^{-3}$ ($T_{SiO}$=900°C). For a homoepitaxial ${\hat{I}}^2$-Ga2O3 layer an Hall charge carrier concentration of 3x10${}^{19}$ cm${}^{-3}$ in line with the provided $\hat{I}{¦}_{SiO}$ ($T_{SiO}$=800°C) is demonstrated. No SiO-incorporation difference was found between ${\hat{I}}^2$-Ga2O3(010) layers homoepitaxially grown at 750°C and ${\hat{I}}^2$-Ga2O3(-201) layers heteroepitaxially grown at 550°C. The presence of activated oxygen (plasma) resulted in partial source oxidation and related decrease of doping concentration (particularly at $T_{SiO}$<800°C) which has been tentatively explained with a simple model. Degassing the source at 1100°C reverted the oxidation.