Light‐Induced Reversible Optical Properties of Hydrogenated Amorphous Silicon: A Promising Optically Programmable Photonic Material

Abstract

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 <e 2 > 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 <e 2 > behavior are demonstrated which together with the magnitude of reversibility (as high as 3.7% at 3.6 eV) illustrates the potential of a-Si:H for future programmable photonic devices.The ORCID identification number(s) for the author(s) of this article can be found under https://doi.

Metastable Refractive Index Manipulation in Hydrogenated Amorphous Silicon for Reconfigurable Photonics

“…It can be concluded that the surface of the a‐Si:H oxidized and saturated over time considering the reference sample (Figure S4a–d,b–e, Supporting Information). The sample that went through annealing and light soaking showed a higher oxidized state that can be related to high‐intensity light‐induced oxidation when compared to the reference sample . In addition, the reference sample going through surface oxidation over time also corroborates the observation made in Figure a, with the measured spontaneous blue‐shift over the aging period after first annealing step when the AR coating was left in ambient conditions (Figure S4d,e, Supporting Information).…”

“…The sample that went through annealing and light soaking showed a higher oxidized state that can be related to high-intensity light-induced oxidation when compared to the reference sample. [29] In addition, the reference sample going through surface oxidation over time also corroborates the observation made in Figure 2a, with the measured spontaneous blue-shift over the aging period after first annealing step when the AR coating was left in ambient conditions ( Figure S4d,e, Supporting Information). Therefore, it can be concluded that the Si-O related FTIR and XPS measurements corroborate the surface oxidation.…”

“…Loss of hydrogen in the a‐Si:H film was reported to cause a reduction in the effective refractive index . Also, surface oxidation can reduce the effective refractive index . The initial slow blue‐shift can be related to oxidation of the pristine surface of the resonant device that causes a decrease of the effective refractive index of a‐Si:H. The annealing‐induced initial blue‐shift can be related to the reduced refractive index of the a‐Si:H film.…”

“…The measurements were done in transmission mode between 340 and 7000 cm −1 . The hydrogen content of the as‐deposited a‐Si:H film was determined from an FTIR measurement to be 20.5% and the microstructure parameter, R *, was 0.21 . The crystallinity in the film was investigated using Raman spectroscopy using a Renishaw inVia Raman microscope.…”

“…[46,47] Also, surface oxidation can reduce the effective refractive index. [13,29,48] The initial slow blue-shift can be related to oxidation of the Adv. Optical Mater.…”