Band edge optical absorption in intrinsic silicon: Assessment of the indirect transition and disorder models

Abstract

Two competing theories of the optical absorption edge of intrinsic crystalline silicon, the indirect transition theory, and the recent disorder (Urbach edge) theory, are assessed critically. The assessment of the indirect transition theory includes an attempt to predict, by the use of the luminescence spectrum and the principle of detailed balance, the optical absorption in the wavelength range where it is mediated by the simultaneous absorption of up to three phonons. The inability of this method to accurately reproduce the experimental data is used to illustrate limitations of the indirect transition theory: its neglect of band degeneracy and its failure to explain the different absorption replica shapes for different phonons. In its published form, the disorder theory does not explicitly address the question of the role of indirect processes or the nature of the strong transitions which trigger the disorder processes. Some of the evidence presented in favor of the theory is shown to be in doubt. The possible synthesis of the two theories is proposed.