An improved analysis for band edge optical absorption spectra in hydrogenated amorphous silicon from optical and photoconductivity measurements

Abstract

The uncertainties inherent in the normalization of subgap photoconductivity spectra to the optical absorption spectra α(hv) in a-Si:H based films have been addressed. An analysis is presented which is based on optical transitions of constant dipole matrix element between parabolic distributions of extended states and exponential distributions of localized tail states. This analysis has been used to normalize the two sets of results accurately, as verified by photothermal deflection spectroscopy measurements, and is shown to be useful in the commonly encountered cases, in which the two spectra do not overlap over an extended region. Improved quantitative fits of α(hv), for photon energy from ∼1.5 to 2.4 eV, obtained on different a-Si:H based films indicate that the localized exponential band tail regions extend ∼60–70 meV above the optical gap.

Analysis of a-Si:H subgap absorption spectra obtained from absolute cavity ringdown absorption spectroscopy using an empirical DOS model

“…To test this hypothesis we have deconvoluted the absorption spectra but now using different matrix elements for the various optical transitions instead of a constant R 2 ðEÞ. Only optical transition (1)(2)(3)(4)(5) that completely dominates the spectrum above 1.7 eV, is still taken 10 A 2 because there is already good agreement between model and measured data. Fig.…”

“…2 and the model described above, we start the modeling procedure by fitting the data at energies between 1.5 and 3 eV to fix some optical parameters that will be needed for accurate fitting of the subgap region. The dominant types of optical transitions in this energy range are (1)(2)(3)(4)(5), (1)(2)(3)(4) and (2)(3)(4)(5). Because these transitions determine the Urbach slope in the absorption spectrum, we use both the T-R and tf-CRDS data to fit this region.…”

“…Due to the ultra-high sensitivity of tf-CRDS, the subgap absorption is measured with high precision and a detailed analysis of the subgap absorption with a densityof-states (DOS) model will give valuable information on the defect states and optical properties of the film. An empirical model is used, based upon earlier work from Curtins and Favre [3] and recently used by others [4,5], to deconvolute the absorption spectra of the 1031 nm thick a-Si:H film, with a main focus on the subgap absorption region. With this model we explore the possibilities of the technique to study a 4 nm thick aSi:H film, which is expected to be completely dominated by surface and interface defects.…”

Deviations from square-root distributions of electronic states in hydrogenated amorphous silicon and their impact upon the resultant optical properties

“…6, we plot ffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 ð hxÞ p as a function of hx corresponding to the experimental 2 ð hxÞ results of Jackson et al [15]. Following Jiao et al [20], we determine the Tauc optical gap by linearly extrapolating these experimental results. This linear extrapolation is determined using a least mean squares fit, and only corresponds to values of ffiffiffiffiffiffiffiffiffiffiffiffiffi ffi 2 ð hxÞ p between 0.2 and 2.5.…”

03/02418 Low-mobility solar cells: a device physics primer with application to amorphous silicon

“…We are neglecting reflection either at the front or the back of the a-Si:H layer. Additionally, the absorption properties of a-Si:H do vary somewhat with its preparation conditions; the values we have chosen are fairly typical [24]. Note that, for a film that is 1000 nm thick, the total photogeneration is about 8.8 Â 10 16 cm À2 /s.…”