Light-induced reversible optical properties of hydrogenated amorphous silicon: a promising optically programmable photonic material

Reversible optical properties (refractive index, n, and extinction coefficient, k) upon light soaking and annealing are predicted to originate from the metastable properties of hydrogenated amorphous silicon (a-Si:H). Optically programmable photonic devices can be demonstrated when a-Si:H is established as a potential programmable photonic material. Therefore, the effects of prolonged high intensity light soaking and annealing are investigated. A set of a-Si:H films is deposited by inductively coupled plasma-enhanced chemical vapor deposition (ICP-PECVD) near the amorphous/nano-crystalline phase transition that have significantly different hydrogen content and microstructures. Spectroscopic ellipsometry shows that the imaginary part of the pseudo dielectric function <ε₂> values near the peak clearly decreases after light soaking and this decrease can be reversed after annealing. No loss of Si-bonded hydrogen is observed after repeated cycles of light soaking and annealing and an inverse correlation between this reversibility and the hydrogen content is found. A change in the bulk optical properties is likely the main contributor to the observed metastable effect, suggesting a reversible refractive index change. Repeated cycles of reversible <ε₂> behavior are demonstrated which together with the magnitude of reversibility (as high as 3.7% at 3.6eV) illustrates the potential of a-Si:H for future programmable photonic devices.