Thickness and doping effects observed in impure vacuum deposited a-Si films

Abstract

Films of a-Si have been prepared by vacuum deposition from crucibles of BeO and BN in a hydrogen ambient. The electrical conductivity of these impure films was modified by the addition of aluminum that was coevaporated during the film deposition. The changes observed are consistent with the assumption that the aluminum doping results in a p-type film for low conconcentrations (<2.3 at.%) whereas high concentrations introduce band structure changes as well. Thickness dependent sample resistivities are observed. This result is interpreted using a layer model to approximate two surface layers with resistivities ρs = 108 Ω∙cm and a bulk layer with ρB = 8 × 103 Ω∙cm. Each high-resistance layer, which is approximately 0.12 μm thick, seems to be caused by the increased hydrogen concentration at both surfaces, which is observed in the hydrogen profiling study. It is suggested that these high-resistance surface layers are due to a wider band gap material with the Fermi level near midgap. The bulk material has a smaller band gap with the Fermi level closer to one of the band edges.

Structure and optical properties of aSiAl and aSiAlHx magnetron sputtered thin films

“…Further increase of the Al concentration resulted in a further reduction of the optical band gap to 0.6 eV, as derived from UV-Vis NIR data. This is in agreement with El-Naggar and Bakry 10 and Talukder et al, 18 who reported a continuous decrease in optical band gap upon doping aSi with Al. They attributed this decrease of the measured optical band gap to an increase in disorder of the material, owing to doping.…”

“…Introduction of Al into aSi reduces the optical band gap from 1.4 eV for aSi to 0.7 eV for Si 0.82 Al 0. 18 . Further increase of the Al concentration resulted in a further reduction of the optical band gap to 0.6 eV, as derived from UV-Vis NIR data.…”

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

“…They concluded that the Tauc optical gap associated with a-Si: H increases as the thickness of the film diminishes. Similar results have been found by Talukder et al, 3 Demichelis et al, 4,5 Chacorn and Haneman, 6 Nguyen et al, 7 and El-Naggar. 8 This increase in the Tauc optical gap associated with a-Si: H corresponding to decreased film thickness has been attributed to a number of factors.…”

“…Cody et al 1 hypothesize that this observation occurs due to a fundamental curvature in the spectral dependence of ͱ ␣ប, ␣͑ប͒ denoting the optical-absorption spectrum, thicker films sampling the optical-absorption spectrum over a lower range of photon energies, and thus, taking this curvature into account, yielding lower Tauc optical gaps than their thinner counterparts; the determination of the optical gap using the most common form of the approach of Tauc et al 2 requires a linear extrapolation of the spectral dependence of ͱ ␣ប, a plot of the dependence of ͱ ␣ប on the photon energy, ប, often being referred to as a Tauc plot, the intercept of this linear extrapolation with the abscissa axis corresponding to the Tauc optical gap. 9 Talukder et al, 3 Demichelis et al, 4,5 and El-Naggar, 8 who all employed the approach of Tauc et al 2 in their determinations of the optical gap associated with a-Si: H, 10 instead argued that this increase in the Tauc optical gap associated with a-Si: H corresponding to diminished film thickness arises as a consequence of the increasingly important role that inhomogeneities play in thinner films. 11 Nguyen et al, 7 who measured the optical properties of extremely thin a-Si: H and crystalline silicon ͑c-Si͒ films, i.e., films as thin as 0.0005 m, suggested that the increases in the optical gap that they observed corresponding to diminished film thickness arise as a consequence of quantum confinement effects for the case of c-Si and possibly the increasingly important role of hydrogen for the case of a-Si: H; a link between quantum confinement and the presence of hydrogen within a-Si: H was suggested by Brodsky 12 and O'Leary et al 13,14 Clearly, each of these effects can contribute to this experimentally observed phenomenology.…”

Effect of thickness on the structural and optical properties of GaN films grown on Si(111)

“…This change might be due to the fact that the surface layers of the film can vary in a fundamental way from the bulk layer. This is owing to an unintentional grading of the constituents of the material as interpreted by Talukder [18]. The optical energy gap, E g , was determined from Tauc's expression [19] for direct band gap semiconductor by a linear extrapolation of the plot of (ahm) 2 versus hm to the energy axis.…”