We developed and tested three MBE-compatible processes for the deposition of high-quality low-temperature silicon oxides and oxynitrides in the ultra high vacuum at substrate temperatures between room temperature and 500 degrees C, gas enhanced evaporation (GEE), plasma enhanced evaporation (PEE) and plasma enhanced oxidation (PEO). The deposited layers were thoroughly investigated and compared with respect to their electrical, optical and stoichiometrical properties by means of ellipsometry, mechanical profilometry, Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), Rutherford backscattering (RBS), Fourier transform infrared ((FTIR) spectroscopy, and by electrical measurements (I-V, C-V) on MOS structures. A model of the growth mechanism for each of the processes is suggested.