Metastable hydrogenated amorphous silicon (a-Si:H) as reversible programmable photonic material

Hydrogenated amorphous silicon (a-Si:H) is known to exhibit light-induced metastable properties that are reversible upon annealing. Although these metastable properties suggest the existence of reversible optical properties of a-Si:H as well, very little is known about this effect. If indeed properly identified and characterized, such reversible optical properties may find application in the reversible programmable photonic integrated circuits (PICs) that can enable multiple functionalities on the same chip, similar to field-programmable gate arrays (FPGAs). However, the required reversible effective refractive index change due to light soaking and annealing has not been reported yet nor has it been thoroughly investigated. Therefore, the effects of prolonged high intensity light soaking and annealing on a-Si:H on the near infrared (NIR) optical properties are studied in this work. A thin-film interferometric technique was developed to detect minute changes probed using a NIR laser source (1465-1575 nm). Using this approach, an increase in refractive index resulted in a red shift of the sharp reflection minimum and a blue shift for the decrease in refractive index. To detect the changes in optical properties more precisely, double-layered thin films were used: a-Si:H was deposited by inductively coupled plasma-enhanced chemical vapour deposition (ICP-PECVD) on SiO2, which was in turn deposited by PECVD on a crystalline silicon substrate. The a-Si:H deposition temperature was set to 80 °C and 300 °C, such that significantly different structural properties, e.g. hydrogen content and density, could be achieved. An irreversible blue shift was observed during the first cycle of annealing and light soaking after the deposition. However, from the second cycle onwards, a red shift of the spectrum due to light soaking, i.e. reversal of the annealed state was observed. It appeared that the initial irreversible changes are inevitable and only after these changes reversibility is observable. The reversibility was sustained after further cycles of annealing and light soaking. The reversibility appears for both a-Si:H deposited at 80 °C and 300 °C. However, the magnitude of the reversibility for a-Si:H deposited at 80 °C is significantly larger when compared to a-Si:H deposited at 300 °C. This suggests a correlation of the metastable properties of a-Si:H on the hydrogen content and density of the material, i.e. porous films (deposited at 80 °C) are more susceptible to light-induced change than dense films (deposited at 300 °C). The magnitude of the reversibility in refractive index for a-Si:H deposited at 80 °C is estimated to be around 0.03%. Although small, this metastable change should be sufficient for an application in reversible programmable optical switch. These results therefore indicate that a-Si:H has potential in enabling reversible programmable PICs and work to implement this material in a photonic device is currently ongoing.