First-principles study of the mechanical and optical properties of amorphous hydrogenated silicon and silicon-rich silicon oxide

We use first-principles density-functional theory (DFT) calculations to predict mechanical and optical property variation with composition for hydrogenated amorphous Si (a-Si:H) (at.% H=0, 5.9, 11.1, and 15.8) and a-SiOx (x=0, 0.5, 1.0, 1.5, and 2.0). A better understanding of the properties of a-Si:H and amorphous silicon oxide (a-SiO_x) is technologically important, particularly for photovoltaic and optoelectronic device applications, respectively. However, relatively little reliable property information is available for these amorphous materials except for the well-studied end-point cases of a-Si and a-SiO₂. Our DFT calculations within the generalized gradient approximation predict that addition of H to a-Si monotonically reduces the elastic modulus (Y) by 18% and bulk modulus (B) by 16% as H incorporation increases to 15.8at.% in a-Si:H. Similarly, addition of O to a-Si monotonically reduces Y by 35% and B by 38% as x increases to 2.0 in a-SiO_x. Our optical spectra for the complex dielectric function, ε (ω), exhibit intensity reduction in the E₂ transition peak of Im [ε (ω)] and reduction in the low-frequency dielectric constant { ε_o = lim_ω→0 Re [ε (ω)] } as either H or O are added to a-Si while the a-SiO_x spectra additionally resolve a vivid blueshift of both the fundamental absorption edge and E ₂ transition energy as O content increases. Considering the large variation in reported experimental measurements and the limited availability of previous computational results, our property predictions provide valuable insight into the mechanical and optical behavior of a-Si:H and a-SiOx materials.