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We have studied the effects of plasma exposure on GaAs with Raman scattering, Hall mobility, capacitance-voltage (C-V), and Secondary ion mass spectroscopy (SIMS) measurements. The Raman studies show that the dependence of carrier removal on plasma species and substrate temperatures is consistent with the thermal migration of the plasma species. The free-carrier removal due to a plasma exposure is apparently dependent on the impurities present in the material. After a hydrogen plasma exposure, a decrease in carrier concentration and mobility occurs for the S-doped samples, while an increase in mobility and a decrease in carrier concentration occurs for the Si-doped samples. The production of: surface roughness due to various surface treatments was also observed by means of Raman spectroscopy and ellipsometry. The observed narrowing of the longitudinal optical (LO) phonon line and the plasmon-phonon (L+) line may be correlated with the degree of annealing. The degradation at the interface between Si-N films and GaAs substrates was observed to depend on the means of film deposition such as plasma-enhanced deposition (PED)and chemical-vapor deposition (CVD).