The dependence of the Tauc and Cody optical gaps associated with hydrogenated amorphous silicon on the film thickness: αl Experimental limitations and the impact of curvature in the Tauc and Cody plots

Abstract

Using a model for the optical spectrum associated with hydrogenated amorphous silicon, explicitly taking into account fundamental experimental limitations encountered, we theoretically determine the dependence of the Tauc and Cody optical gaps associated with hydrogenated amorphous silicon on the thickness of the film. We compare these results with that obtained from experiment. We find that the curvature in the Tauc plot plays a significant role in influencing the determination of the Tauc optical gap associated with hydrogenated amorphous silicon, thus affirming an earlier hypothesis of Cody et al. We also find that the spectral dependence of the refractive index plays an important role in influencing the determination of the Cody optical gap. It is thus clear that care must be exercised when drawing conclusions from the dependence of the Tauc and Cody optical gaps associated with hydrogenated amorphous silicon on the thickness of the film.

Optical bandgap of semiconductor nanostructures: Methods for experimental data analysis

“…Indeed, the reference sample analyzed with Cody plot gives invariably E g lower than 0.8 eV (from 0.6 eV to 0.7 eV), which does not agree with theory and experimental results on a-Ge (E g is around 0.8 eV). 17,33 It should be noted that while Cody plot gave some reliable results for a-Si, 18,21 this is not always true for other semiconductors, especially in a confined system. a-Si film is characterized by large tails in the bandgap, probably weakening the validity of Tauc assumption.…”

“…17 Given the very thin film used, the rule of 3<al < 10 cannot be applied for the Cody plot. 21 We performed the linear fitting for both Tauc and Cody plots, in the same energy range, for all samples. In Figs.…”

“…18 On the choice among Tauc or Cody plots, many papers are present in the literature. [17][18][19][20][21] Most of them focus on the light absorption in a-Si and a-Si:H, debating on the effect and magnitude of tail states in the band gap and on the validity of Tauc 20 or Cody 21 model. Recently, the two models have been compared in sputtered Ge quantum well (QWs), 19 showing the presence of a double slope in the Tauc plot and claiming that the Cody plot is able to provide a more unambiguous determination of the optical bandgap compared to Tauc plot.…”

Detection of surface electronic defect states in low and high-k dielectrics using reflection electron energy loss spectroscopy

“…However, a number of empirical measures have been established for delineating extended gap/defect states and determining the effective band gap of amorphous materials. 163 This allows for a comparison of the band gap of amorphous and crystalline materials and the extension of the theoretical concepts developed for crystalline materials to their amorphous counterparts. In this study, the band gap for the a-SiOC:H thin films was determined from the raw REELS spectra by linearly extrapolating the leading edge of the loss spectrum to the elastic peak baseline similar to the method of Miyazaki.…”

Atomic layer deposition of nickel–cobalt spinel thin films

“…There exist different approaches for obtaining information on the band gap from absorption spectra and two very common are plotting (αE) 2 and (αE) 1/2 versus E in order to estimate direct and indirect bandgaps, respectively. 51 Here, α is the absorption coefficient, which is calculated as α = ln((1 − R)(1 − A Gl )/T )/d, where R is the reflectance, A Gl the absorption of the borosilicate-glass substrate, T the transmission and d the film thickness, and E is the photon energy. For Co 3 O 4 , direct bandgaps at 1.5 and 2.1 eV, and indirect band gaps at 1.2 and 1.3 eV can be identified for the two main absorption peaks.…”