Disorder and optical absorption in amorphous silicon and amorphous germanium

doi:10.1016/S0022-3093(96)00612-6

Journal of Non-Crystalline Solids

Volume 210, Issues 2–3, March 1997, Pages 249-253

https://doi.org/10.1016/S0022-3093(96)00612-6 Get rights and content

Abstract

The role that disorder plays in shaping the functional form of the optical absorption spectra of both amorphous silicon and amorphous germanium is investigated. Disorder leads to a redistribution of states, which both reduces the empirical optical energy gap and broadens the optical absorption tail. The relationship between the optical gap and the breadth of the absorption tail observed in amorphous semiconductors is thus explained.

Introduction

In a defect-free crystalline semiconductor, the optical absorption spectrum terminates abruptly at the energy gap. In contrast, in an amorphous semiconductor, a tail in the absorption spectrum extends into the gap region [1]. In both amorphous silicon and amorphous germanium it is observed that the optical gap decreases and the absorption tail broadens with increased levels of thermal or structural disorder 2, 3, 4, 5. Cody et al. 3, 4, 6 assert that there is a `universal' relationship between the optical gap and the breadth of the absorption tail in these group IV amorphous semiconductors. Grein and John [7] find that this relationship applies to a broad class of amorphous and disordered semiconductors.

The fundamental basis for this `universal' relationship has yet to be established. Cody [6] points out that the absence of a proper theory for optical absorption, which spans the transition from below to above the gap, has hindered attempts to study this relationship. Recently we have developed a semi-classical model suitable for the analysis of the optical absorption spectrum of these amorphous semiconductors, which extends from below to above the gap 8, 9. In the present work, we use our model to study how disorder shapes the form of the optical absorption spectrum, and demonstrate the origin of the relationship between the optical gap and the breadth of the absorption tail.

Section snippets

Discussion

In an amorphous semiconductor, the optical absorption coefficient, α(ℏω)=D²(ℏω)J(ℏω), where D²(ℏω) denotes the optical transition matrix element and J(ℏω) represents the joint density of states (JDOS) function. As D²(ℏω) exhibits a weak functional dependence compared with that of J(ℏω) [10], our semi-classical analysis focuses on J(ℏω) [9]. In this analysis, we determine the form of J(ℏω) by averaging the local JDOS function over a Gaussian distribution of local energy gaps. Two principal

Conclusions

Disorder in amorphous silicon and amorphous germanium leads to a redistribution of states, from band to tail, in a one-to-one manner, thus allowing for a greater number of possible band to tail and tail to tail transitions. As a result, both a decrease in the empirical optical gap and a broadening of the absorption tail occur with increasing levels of disorder. The variations in the absorption spectrum can be attributed to variations in the disorder parameter, σ, while the mean energy gap

Acknowledgements

The authors wish to thank Dr C. Aversa and Professor S. John for interesting and helpful discussions. Financial assistance from the Natural Sciences and Engineering Research Council of Canada, Ontario Hydro, and the University Research Incentive Fund is gratefully acknowledged.

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