Photoinduced interdiffusion in nanolayered Se∕As2S3 films: Optical and x-ray photoelectron spectroscopic studies

Abstract

Photoinduced interdiffusion was observed with above band gap light in nanolayered Se/ As 2 S 3 films. It is discussed in terms of the optical parameters such as band gap, Urbach edge ͑E e ͒ ͓F. Urbach, Phys. Rev. 92, 1324 ͑1953͔͒, and B 1/2 ͑Tauc's parameter͒ ͓J. Tauc et al., Phys. Status Solidi 15, 627 ͑1966͔͒. Experimental data of B 1/2 and E e for as-prepared samples do not show clear correlation implied by the Mott-Davis model ͓N. F. Mott and E. A. Davis, Electronic Process in Non-crystalline Materials ͑Clarendon, Oxford 1979͒, p. 287͔. It is also shown that the optical parameters can be changed with a change in the Se sublayer thickness. Variations of these optical parameters as a function of modulation period and photoinduced interdiffusion were discussed in terms of the quantum confinement effect and changes in the valence and conduction bands. We proposed a model to explain the mechanism of Se diffusion in As 2 S 3 , which suggests that diffusion takes place through the wrong bonds. X-ray photoelectron spectroscopy ͑XPS͒ is used to investigate the chemical alternations in the bonding. The proposed model was supported by the XPS data.

Kinetics and chemical analysis of photoinduced interdiffusion in nanolayered Se/As2S3 films

“…The diffusional intermixing affects several parameters of the multilayer structure, such as optical absorption edge, layer thickness, photoluminescence intensity, electrical conductivity, and photoconductivity. 2,3,6 The experiments indicate that the light-and thermostimulated diffusion effects are rather similar. The changes in these properties are explained based on the formation of ternary solid solution due to interdiffusion.…”

“…Earlier studies on Se/ As 2 S 3 multilayered samples ͓where the Se, As 2 S 3 sublayer thickness ratio was very small ͑0.3 or less͔͒ reported the value of p = 2. 2,3 We assume that the main reason for the quadratic energy dependence of ␣h in those samples was the quantum confinement effect, which is different in the present samples in two ways. First, the Se sublayer thickness ͑less than 5 nm͒ was lower compared to the 9 nm sublayer thickness of the present sample.…”

“…[1][2][3] These amorphous nanolayered chalcogenide ͑ANC͒ structures are similar to the crystalline superlattices yet distinct from ideal crystalline superlattices produced by molecular beam epitaxy. The ANCs can be considered as well-correlated layers with good periodicity and smooth interface.…”

“…Equation ͑1͒ is valid for a number of amorphous materials in the spectral region of large ␣ ͑10 4 Ͻ ␣ Ͻ 10 5 ͒, i.e., Tauc region. 2,3,13 In the special case where both the conduction and valence band edges have a parabolic shape, the value of p is 2. This quadratic dependence was observed in many chalcogenide glasses like As 2 S 3 , As 2 Se 3 , As 2 Te 3 , etc., 14 but not all chalcogenide glasses.…”

“…Our previous studies revealed that the bandgap and Tauc parameter increase after photoinduced diffusion. 2,3 It was assumed that diffusion takes place mainly in strained sites where wrong bonds are close to Se 2 0 sites ͑the subscripts and superscripts denote the coordination number and charge state, respectively͒. 3 Diffusion takes place through the reaction of Se 2 0 with the wrong S-S ͑S 2 0 ͒ and As-As bonds.…”