Reversible above band gap absorption properties of Hydrogenated amorphous silicon (a-Si:H)

The vast majority of Staebler-Wronski Effect(SWE) studies have been focused on understanding the nature of metastable defects in hydrogenated amorphous silicon(a-Si:H), i.e. the optoelectrical properties for photon energies below the bandgap. However, the effects of prolonged light exposure and annealing on the optoelectrical properties above the bandgap have not been thoroughly investigated. For photonic integrated circuits even refractive index changes as little as 1%, may have a significant impact. Therefore, this work aims to study possible changes in the absorption properties above the bandgap as enabled by high intensity visible light soaking and annealing. The parameter used in this work to study such changes in absorption is the imaginary part of the pseudo dielectric function <e2> derived by spectroscopic ellipsometric (SE) measurements. A set of a-Si:H films was deposited near the amorphous/nanocrystalline phase transition at different substrate temperatures having different microstructures by inductively coupled plasma enhanced chemical vapour deposition(ICP-PECVD). For an a-Si:H film the <e2> value exhibits considerable change over time, as the top surface undergoes oxidation during prolonged air exposure. Therefore, the light-induced and annealing effects on these samples are investigated with respect to the corresponding twin samples that are stored at room temperature in air to serve as a reference. The measured <e2> values around the electronic polarization resonance clearly decrease after light soaking and can be completely reversed after annealing (nearly 4%; see attached figures), indicating familiar metastable nature of defects in hydrogenated amorphous silicon. Since no loss of bonded hydrogen is detected after repeated light soaking and annealing cycles and considering the reversibility of the <e2> change, it is believed that a change in the bulk optical properties is the main contributor to the observed metastable effect suggesting reversible refractive index change. Optical simulations which take the sample surface oxidation into account suggest that the effect may be explained through metastable volumetric expansion.