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.
Highlights
Ultra-thin hydrogenated amorphous silicon (a-Si:H) films grown on crystalline silicon (c-Si) substrates are very interesting structures for photovoltaic applications and their importance has increased within recent years
The study was conducted in 1993, yet quantum confinement effects on the optical bandgap continue to be reported in literature for ultra-thin films of amorphous silicon and the Tauc-Lorentz bandgap measured by optical techniques such as spectroscopic ellipsometry and transmission spectrophotometry are commonly used to detect the presence of nano-clusters smaller than exciton wavelengths
Another a-Si:H film was deposited on c-Si in the same procedure described above and was used to provide a confirmation between the a-Si:H layer thickness obtained from spectroscopic ellipsometry (SE) and the thickness measured by atomic force microscopy
Summary
They have shown that the Tauc-Lorentz optical model yielded a much better fit to experimental data for amorphous silicon than the Forouhi-Bloomer model that was developed earlier and was at that time widely used to describe optical absorption of amorphous semiconductors In their extensive study on the transition of silicon from amorphous to micro-crystalline phase, Collins et al.[8] have relied primarily on real-time spectroscopic ellipsometry performed in-situ, to reveal the nature and the evolution of amorphous silicon during PECVD growth. The study was conducted in 1993, yet quantum confinement effects on the optical bandgap continue to be reported in literature for ultra-thin films of amorphous silicon and the Tauc-Lorentz bandgap measured by optical techniques such as spectroscopic ellipsometry and transmission spectrophotometry are commonly used to detect the presence of nano-clusters smaller than exciton wavelengths. Our method aims at testing the validity of the Tauc-Lorentz model for ultra-thin a-Si:H films (
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