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Germanium telluride is a high performing thermoelectric material that additionally serves as a base for alloys such as GeTe–AgSbTe 2 and GeTe–PbTe. Such performance motivates exploration of other GeTe alloys in order understand the impact of site substitution on electron and phonon transport. In this work, we consider the root causes of the high thermoelectric performance material Ge 1− x Mn x Te. Along this alloy line, the crystal structure, electronic band structure, and electron and phonon scattering all depend heavily on the Mn content. Structural analysis of special quasirandom alloy structures indicate the thermodynamic stability of the rock salt phase over the rhombohedral phase with increased Mn incorporation. Effective band structure calculations indicate band convergence, the emergence of new valence band maxima, and strong smearing at the band edge with increased Mn content in both phases. High temperature measurements on bulk polycrystalline samples show a reduction in hole mobility and a dramatic increase in effective mass with respect to increasing Mn content. In contrast, synthesis as a function of tellurium chemical potential does not significantly impact electronic properties. Thermal conductivity shows a minimum near the rhombohedral to cubic phase transition, while the Mn Ge point defect scattering is weak as indicated by the low K L dependence on the Ge–Mn fraction (Fig. 10). From this work, alloys near this phase transition show optimal performance due to low thermal conductivity, moderate effective mass, and low scattering rates compared to Mn-rich compositions.
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Hopping charge transport in hydrogenated amorphous silicon–germanium alloy thin films
Measurements of the dark conductivity and thermoelectric power in hydrogenated amorphous silicon–germanium alloys (a-Si 1- x Ge x :H) reveal that charge transport is not well described by an Arrhenius expression. For alloys with concentrations of Ge below 20%, anomalous hopping conductivity is observed with a power-law exponent of 3/4, while the temperature dependence of the conductivity of alloys with higher Ge concentrations is best fit by a combination of anomalous hopping and a power-law temperature dependence. The latter has been attributed to charge transport via multi-phonon hopping. Corresponding measurements of the Seebeck coefficient reveal that the thermopower is n-type for the purely a-Si:H and a-Ge:H samples but that it exhibits a transition from negative to positive values as a function of the Ge content and temperature. These findings are interpreted in terms of conduction via hopping through either exponential band tail states or dangling bond defects, suggesting that the concept of a mobility edge, accepted for over five decades, may not be necessary to account for charge transport in amorphous semiconductors.
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- Award ID(s):
- 2011401
- PAR ID:
- 10411318
- Date Published:
- Journal Name:
- Journal of Applied Physics
- Volume:
- 131
- Issue:
- 22
- ISSN:
- 0021-8979
- Page Range / eLocation ID:
- 225110
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1063/5.0077441
