Abstract
The measurement of the Seebeck coefficient of thin film (100 nm) amorphous germanium telluride containing~31% oxygen under dark conditions and when exposed to monochromatic light in the 400 nm to 1800 nm wavelength region is reported. Exposure of the films to light is found to reduce the absolute value of the Seebeck coefficient compared to that measured in the dark. Furthermore, the magnitude of this reduction displays a distinctive spectral dependence over the wavelength range covered. The observed behaviour suggests that these measurements provide a method determining the optical bandgap of thin amorphous chalcogenide films. Further analysis of the data, along with that of X-ray photoelectron spectroscopy and photoconductivity studies, is used to determine the presence of sub-bandgap defect states and their role in determining the optical response of the Seebeck coefficient.