Liu, Xiao, Thomas H. Metcalf, Qi Wang, and Douglas M. Photiadis. "Elastic properties of several silicon nitride films." MRS Online Proceedings Library (OPL) 989 (2007).

Slightly varying deposition parameters at fixed ion beam speed / Energy or later by at fix deposition parameters and vary ion beam speed... Characterize the film at each run... Finally you will arrive at required film property at optimized parameters.. You can refer my articles.

Stoichiometric Si3N4 forms at high temperature, and you have a restriction with temp by using GaAs substrate.

Check the thermal expansion mismatch btw si3N4 and GaAs.

In the case e-beam which is done at very low vacuum level, there are always chances of building up high residual stresses in the deposited film, which will tend to peel with increasing thickness.

I feel you can rather sputter a silicon wafer in pure nitrogen sputtering gas, at a hgih sputtering pressure 30 to 40 mT, and get some kind of a Silicon nitride layer, thought it will nitrogen deficient, but you can probably get an insulating layer if this is your requirement

In my lab we could even deposit SiO2 films by sputtering a silicon wafer in pure oxygen.

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