Abstract

A series of samples of single layers and twenty alternating thin layers of Si₃N₄ and a-Ge were prepared on quartz substrates by ion beam deposition. The optical transmission and reflection spectra were measured in the 300–2400-nm wavelength range. The analysis used a computer search routine to find a suitable set of values for the parameters used for describing the complex index of refraction n(λ) – ik(λ) and thickness for each layer in the multilayer stack. First, the single films were analyzed and the index n(λ), the extinction coefficient k(λ), and therefore the absorption coefficient a = [4πk(λ)]/λ for each of these materials was obtained. Second, the stack was described by ten pairs of Si₃N₄/a-Ge films. The thickness of the Si₃N₄ and a-Ge were allowed to vary, the index of refraction of the Si₃N₄ and a-Ge were varied, and the absorption coefficient of the a-Ge was varied. Only the absorption coefficient of the Si₃N₄ was held fixed at the values given by the single film study. The values of the band gap of the a-Ge were seen to change from 0.88 eV for a single layer of a-Ge (170 nm thick) to 0.97 eV for ten thin layers of a-Ge (4.35 nm) interspersed with Si₃N₄ films (3.68 nm). Methods of measuring the band gap that are based on incoherent light and coherent light are compared.

© 1986 Optical Society of America