Silicon oxynitride films with varying oxygen/nitrogen ratio were grown from SiH4, N2O, and NH3 by means of a plasma-enchanced chemical vapor deposition process. The elemental composition of the deposited films was measured by a variety of high-energy ion beam techniques. To determine the chemical structure we used Fourier transform infrared absorption spectroscopy and electron-spin resonance. Ellipsometric data and values for mechanical stress are also reported. We show that the entire range of compositions from silicon oxide to silicon nitride can be covered by applying two different processes and by adjusting the N2O/NH3 gas flow ratio of the respective processes. It is suggested that the N2O/SiH4 gas flow ratio is the major deposition characterization parameter, which also controls the chemical structure as far as the hydrogen bonding configuration is concerned. We found that the films contain significant amounts of excess silicon and that the mechanical stress in the oxynitrides is lower than in plasma nitride. The electron-spin density is low (∼1017/cm3) in all samples.

The anneal behavior of plasma-enhanced chemical vapor deposited silicon oxynitride films has been studied using Fourier transform infrared absorption spectroscopy, nuclear reaction analysis, and electron-spin resonance. The anneal temperature range was 500–1000 °C. It is observed that the oxynitrides which contain only N–H bonds are thermally stable in the temperature range under study. The layers which also contain Si–H bonds are considerably less thermally stable. Abundant hydrogen effusion from these layers is observed at temperatures as low as 600 °C, accompanied by cracking and shrinkage of the films. It is suggested that the coexistence of both Si–H and N–H bonds offers the possibility for cross linking and that consequently the decomposition temperature of both types of bonds is lowered. Evidence for the occurrence of cross linking is found in the infrared difference spectra. Consistently, the silicon unpaired electron density does not increase upon annealing. The Si–H and N–H bands effectively shift towards higher wave numbers upon annealing at higher temperatures. This is ascribed to the inhomogeneity in bond strength, which in turn is related to a variation in electronegativity of the surrounding groups.

Polycrystalline silicon wafers have been subjected to annealing (700 °C, 1 h) and to a hydrogen plasma (350 °C, 30 min) during the processing of solar cells. The annealing treatment enhances the bulk minority-carrier recombination lifetime by 19%, presumably by impurity gettering. The plasma treatment improves the lifetime by 26%; hydrogen passivation accounts for at least 2/3 of this improvement. Gettering and passivation are found to be complementary: application of both treatments results in a 43% increase in lifetime compared to standard.

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Plasma-enhanced chemical vapor deposition (PECVD) is used for the deposition of a silicon nitride anti-reflection coating (ARC) onto 10 x 10 cm2 Wacker Silso polycrystalline silicon solar cells. It is found that the short-circuit current I,, is improved by 7 to 10% in comparison to reference cells with a standard screenprinted TazO5 coating. Part of the increase in Iscis because of a smaller reflectivity of the silicon nitride ARC. The other part of the improvement comes from an enhanced average minority-carrier diffusion length (Lmi"). The increase in Lmin results from hydrogen passivation, and is attributed to the generation of hydrogen ions during PECVD of Si3N4. Furthermore it is shown that the passivation effect by PECVD of Si3N4 is comparable to that obtained with a 1/2 hour hydrogen plasma treatment, and that it is stable during a 1 hour anneal at 700 "C. We did not observe a significant influence of the substrate temperature during Si3N4 deposition in the range of 350 to 450°C.