  We report on the origin of high Si flux observed during the use of Si as a
doping source in plasma assisted MBE growth of \b{eta}-Ga2O3. We show on the
basis of secondary ion mass spectroscopy (SIMS) analysis that Si flux is not
limited by the vapor pressure of Si but by the formation of volatile SiO. The
low sublimation energy of SiO leads to weak dependence of the SiO flux of Si
cell temperature and a strong dependence on the background oxygen pressure.
Extended exposure to activated oxygen results in reduction of SiO flux due to
the formation of SiO2 on the Si surface. The work reported provides key
understanding for incorporating Si into future oxide-based semiconductor
heterostructure and device MBE growth.
