Origins and implications of intrinsic stress in hydrogenated amorphous silicon thin films

Abstract

Despite decades of research on hydrogenated amorphous silicon (a-Si:H), there remains much to be understood about the relationship between deposition conditions and the resulting structural, optical, and bulk properties of the material. In this work we investigate these correlations for a-Si:H films created using plasma enhanced chemical vapor deposition (PECVD), focusing on the creation of intrinsic stresses within the films. Through experimental examination of the deposition process pressure, we model the plasma ion momentum using a combination of theoretical models and empirical trends. We find that compressive stress is controlled by ion bombardment causing of peening the film, and leading to lattice distortion in the material. Conversely, tensile stress is created through bombarding ions collapsing nano-sized voids within the material, which are formed during the vapor-phase deposition. Combining our model of ion momentum with the theory of ion peening creating compressive stress, we are able to fit the process conditions to the observed the compressive regime of our films. Furthermore, by analyzing the hydrogen content in voids within our films, we are able to predict the film porosity, and thereby model the void collapse, yielding the tensile stresses. The balance between these compressive and tensile stress forces determines the final intrinsic stress state, and allows our refined model to fit the entire range of highly compressive to highly tensile film stresses. Finally, we present correlations between film structural properties and observed optical properties, real and imaginary refractive indices and optical band gap, factors important for the creation of a-Si:H based devices.