Optical bandgap of ultra-thin amorphous silicon films deposited on crystalline silicon by PECVD

Abstract

An optical study based on spectroscopic ellipsometry, performed on ultrathin hydrogenated amorphous silicon (a-Si:H) layers, is presented in this work. Ultrathin layers of intrinsic amorphous silicon have been deposited on n-type mono-crystalline silicon (c-Si) wafers by plasma enhanced chemical vapor deposition (PECVD). The layer thicknesses along with their optical properties –including their refractive index and optical loss- were characterized by spectroscopic ellipsometry (SE) in a wavelength range from 250 nm to 850 nm. The data was fitted to a Tauc-Lorentz optical model and the fitting parameters were extracted and used to compute the refractive index, extinction coefficient and optical bandgap. Furthermore, the a-Si:H film grown on silicon was etched at a controlled rate using a TMAH solution prepared at room temperature. The optical properties along with the Tauc-Lorentz fitting parameters were extracted from the model as the film thickness was reduced. The etch rate for ultrathin a-Si:H layers in TMAH at room temperature was found to slow down drastically as the c-Si interface is approached. From the Tauc-Lorentz parameters obtained from SE, it was found that the a-Si film exhibited properties that evolved with thickness suggesting that the deposited film is non-homogeneous across its depth. It was also found that the degree of crystallinity and optical (Tauc) bandgap increased as the layers were reduced in thickness and coming closer to the c-Si substrate interface, suggesting the presence of nano-structured clusters mixed into the amorphous phase for the region close to the crystalline silicon substrate. Further results from Atomic Force Microscopy and Transmission Electron Microscopy confirmed the presence of an interfacial transitional layer between the amorphous film and the underlying substrate showing silicon nano-crystalline enclosures that can lead to quantum confinement effects. Quantum confinement is suggested to be the cause of the observed increase in the optical bandgap of a-Si:H films close to the a-Si:H/cSi interface.

Passivating electron‐selective contacts for silicon solar cells based on an a‐Si:H/TiOx stack and a low work function metal

“…The features of the ATOM contact structures with TiO x were characterized by TEM and EDX and are shown in Figure . The rough c‐Si/a‐Si:H interface is because of a phase transition region of a few nanometers from crystalline to fully amorphous . The TiO x layer, of which the thickness is around 2 nm, was deposited uniformly on the i‐a‐Si:H surface, showing no isolated islands.…”

Broadband anti-reflection coating using dielectric Si3N4 nanostructures. Application to amorphous-Si-H solar cells

“…Actually, its thickness is limited to about 300 nm by material diffusion length. In this case, light trapping mechanisms are important to improve cell performance [3,4]. There exist a strong activity and interest in improving efficiency and related figures of merit.…”

Theoretical investigation on the passivation layer with linearly graded bandgap for the amorphous/crystalline silicon heterojunction solar cell

“…Experimentally, a‐Si:H bandgap can be narrowed by increasing the crystallinity or reducing the hydrogen content . When a‐Si:H is deposited on c‐Si, nano‐structured clusters can be mixed into the amorphous phase for the region close to c‐Si surface . Rough epitaxial growth can also occur at the a‐Si:H/c‐Si interface .…”