ECR-plasma deposited oxygen-free SiN(x) films for low- and high-reflectivity coatings for GaAs based devices

We have developed optical coatings with oxygen-free amorphous silicon nitride (a-SiN(x)) and amorphous silicon (a-Si) thin films. The a-Si and a-SiN, films were deposited from the mixture of Ar and SiH4, and from that of Ar, SiH4, and N2, respectively, on GaAs at 90 deg C using an electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) technique. No oxygen was found in the films within the detection limit of Rutherford Backscattering Spectroscopy (RBS). The Raman measurements showed almost no surface darmage after the film deposition on GaAs. The RMS surface roughness of 140 nm thick film on GaAs, determined by the atomic force microscopy (AFM) was better than 0.8 nm whereas the original surface roughness of the substrate was 0.25 nm. Using a single layer of a-SiN(x), film as an anti reflection (AR) coating the reflectivity of the GaAs wafer could be reduced down to 1.7x10(exp -4) at the wavelength lambda=1000 nm. In case of high-reflectivity (HR) coatings fa bricated with a stack of alternating layers of a-SiN(x), and a-Si a reflectivity over 92 per cent at lambda=1000 nm could be achieved with only 4 alternating layers.