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.
Highlights
Amorphous semiconductors, and chalcogenides in particular, are interesting materials that display a broad range of physical phenomena resulting in wide-ranging studies of their fundamental properties and their application [1, 2]
Amorphous germanium telluride (a-GeTe) is of particular interest as it forms the basis of Ge–Sb–Te alloy systems that have been exploited in phase-change memory (PCM) applications [3, 4]. a-GeTe has a crystallisation temperature [ 150 °C and can be reversibly switched between the amorphous and crystalline state at high speed [5]
Modelling of a-GeTe using density functional theory (DFT) approaches has led to the prediction of mid-gap states upon melt quenching, though with a narrower bandgap (* 0.4 eV) [16]
Summary
Chalcogenides in particular, are interesting materials that display a broad range of physical phenomena resulting in wide-ranging studies of their fundamental properties and their application [1, 2]. Despite the significant research attention this and other applications have attracted, a full understanding the electronic and thermoelectric properties of these materials remains a challenge due to their non-crystalline structure. Modelling of a-GeTe using density functional theory (DFT) approaches has led to the prediction of mid-gap states upon melt quenching, though with a narrower bandgap (* 0.4 eV) [16]. Thermoelectric power studies of bulk GeTe have yielded a Seebeck coefficient (S) value of * 35 to 80 lV K-1 at 300 K [17,18,19,20,21]. The study of 500 nm annealed polycrystalline (rhombohedral) GeTe films yielded a value of S * 80 lV K-1 [25] similar to the bulk. In all of the above it is likely that some of the variation in reported properties between studies may be due to the effects of oxidation
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