Photopyroelectric spectroscopy of a-Si:H thin semiconducting films on quartz.

Abstract

Photopyroelectric Spectroscopy has proven to be a sensitive qualitative [1] and quantitative [2] technique for thin film spectroscopic applications. An important feature of this back-surface detection technique, not shared with the more conventional front-surface photothermal detection methods (Photothermal Deflection Spectroscopy, PDS; and Photoacoustic Spectroscopy, PAS) is its ability to measure directly and separately two independent spectrally-varying parameters: the optical absorption coefficient [3] and the nonradiative quantum efficiency. PDS of thin semiconducting films of amorphous hydrogenated Si [4] readily yields information about the product of the optical absorption coefficient, α(λ), and the nonradiative quantum efficiency, η(λ). The standard assumption is, however, that η(λ) is not a sensitive function of the exciting photon energy. This assumption is generally wrong, and nonradiative quantum efficiencies have been found photoacoustically to vary by one order of magnitude [5] across the optical gap in Ge doped As 2 Se 3 chalcogenide glasses. PAS yields amorphous thin film spectra similar to PDS [6]. The working assumption has been that PA spectra are essentially accurate above the optical gap, as η(λ) is expected to be independent of photon energy. Kitamura et al. [5] were able to derive extended η(λ) spectra of (As 2 Se 3 )100-x Ge x glasses upon combining PA spectra with optical absorption coefficient information obtained in an independent spectrophotometric experiment using ordinary polished bulk samples. These authors, however, were not able to guarantee that the glasses and the bulk samples had the same (or even nearly similar) α(λ) spectra.