Thermoelectric properties of doped and undoped mixed phase hydrogenated amorphous/nanocrystalline silicon thin films
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Thermoelectric properties of doped and amorphous/nanocrystalline silicon thin films
undoped
mixed
1245-A17-10
phase
hydrogenated
Y. Adjallah, C. Blackwell, and J. Kakalios School of Physics and Astronomy, University of Minnesota, University of Minnesota, Minneapolis, MN, 55455 ABSTRACT The Seebeck coefficient and dark conductivity for undoped, and n-type doped thin film hydrogenated amorphous silicon (a-Si:H), and mixed-phase films with silicon nanocrystalline inclusions (a/nc-Si:H) are reported. For both undoped a-Si:H and undoped a/nc-Si:H films, the dark conductivity is enhanced by the addition of silicon nanocrystals. The thermopower of the undoped a/nc-Si:H has a lower Seebeck coefficient, and similar temperature dependence, to that observed for undoped a-Si:H. In contrast, the addition of nanoparticles in doped a/nc-Si:H thin films leads to a negative Seebeck coefficient (consistent with n-type doping) with a positive temperature dependence, that is, the Seebeck coefficient becomes larger at higher temperatures. The temperature dependence of the thermopower of the doped a/nc-Si:H is similar to that observed in unhydrogenated a-Si grown by sputtering or following high-temperature annealing of a-Si:H, suggesting that charge transport may occur via hopping in these materials. INTRODUCTION Thermoelectric (TE) devices enable the direct conversion of thermal energy into electrical power using only solid-state materials with no moving parts. Temperature measurements, power generation and electronic refrigeration [1] in semiconductors may be performed through the Seebeck effect and the Peltier effect. The Seebeck coefficient S, also called the thermoelectric power or thermopower, is related to an intrinsic property of the material, and represents the voltage generated across two points on a material by a temperature gradient. The Seebeck coefficient is very small for metals, on the order of a few µV K and for semiconductors is on the order of few mV/K. We have investigated the thermoelectric properties of undoped and doped hydrogenated amorphous silicon (a-Si:H) films in which are embedded silicon nanocrystallite inclusions (a/ncSi:H) [2,3]. These materials are synthesized in a dual chamber co-deposition system where the silicon nanocrystals are generated in one plasma reactor and then injected into a second plasma chamber in which the surrounding a-Si:H matrix is deposited. Hydrogenated amorphous silicon is an attractive material for photovoltaic applications due to the ease of large area thin film deposition on a wide variety of substrates, which would also be advantageous for certain thermoelectric applications. In this report, we describe the dependence of the dark conductivity and thermopower on the nanocrystallite volume fraction in undoped and phosphorus doped n-type a-Si:H and a/nc-Si:H films. MATERIAL PREPARATION The mixed phase amorphous/nanocrystalline hydrogenated silicon thin films (a/nc-Si:H) are synthesized in a dual chamber co-deposition system described previously in detail [2,3]. Briefly,
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