| Many of the recent researches have witnessed that the approaches of nanostructured and low-dimensionality improve the thermoelectric figure of merit. The many internal interfaces in nanostructures would effectively scatter phonons and reduce the lattice thermal conductivity significantly. Additionally, the nanostructures could increase the power factor due to low energy filtering of the carriers at the interfaces. Meanwhile, quantum confinement of the nanostructures could enhance the thermopower by modifying the band structures. Lead telluride (PbTe) is the premiere thermoelectric material for intermediate temperature (500-900K) applications. The highest reported ZT=3 at 550K for PbTe based material was achieved in PbTe quantum dot superlattices (QDSL). As a result, a substantial researches focus on such low dimensional structures.In this paper we grew the PbTe films and PbTe/CdTe multiple quantum wells (MQW) by off-axis magnetron co sputtering. The thermoelectric properties of PbTe film and MQW were investigated. We designed and fabricated an experimental setup for the measurement of the Seebeck coefficient and electrical conductivity of thin films. The hierarchical structures of PbTe films could be controlled by changing the co sputter Telluride (Te) flux (0.06A/s,0.13A/s,0.4A/s) and substrate temperature (Ts=220℃,25a0℃,285℃,320℃). The evolution of the nanostructure with the growth thicknesses was observed. High-quality PbTe/CdTe MQWs were obtained and the MQWs thermoelectric properties were measured as a function of well width (a=16,26 and36nm), barrier width (b=4,14, and28nm),number of periods (N=1,5,15). The main contents are as follows:1:The pyramids on PbTe/BaF2 thin film surface were favor to form under low co-sputter Te flux(<0.6A/s). The pyramids have the sizes ranged from 150-200 nm with the height to base aspect ratio about 1:2. The morphology evolution with the thickness indicated the columnar growth was related to the triangular pyramid formation under the off-axis co-sputtering and low Te flux condition. The islands and nanorods with different crystallographic planes were observed at the thickness of 27nm.The competitive growth occurred as the thickness increased. The columnar growth were highly textured along the (111) orientation to eventually form a triangular pyramid on the surface.2:The shape transitions from triangular pyramid to truncated octahedrons and triangular shape plates were gradually taking place with the Te flux increased from 0.13A/s to 0.16A/s. The flower-shaped structure with the diameter about 1μm was observed on the surface of the sample grown at 220℃ with Te flux rate of 0.13A/s The smooth surface and micro truncated octahedrons structure was obtained at 250℃. Truncated octahedrons and triangular plate were formed on the surface at 285℃. The higher Te flux of 0.4 A/s resulted in the formation of the triangular pits and triangular plate on the surface. The evolution of the surface was mainly attributed to the change in the ratio of growth rate between the{111} and{100} plane. The Te flux rate inhabits the aggressive growth between the{111} and{100} plane.3:The measurements of PbTe/CdTe quantum well with various structures were performed in the range of 300-600K. The electric conductivity increased and the Seebeck coefficient decreased as the temperature rose. The effects of the well width, barrier width and number of periods on the thermoelectric properties were investigated. The function of the conductivity and Seebeck coefficient as the temperature is explained by the activation energy of the grain. The optimized sample reached the highest Seebeck coefficient value of 497μV/K at room temperature. The transition of the Seebeck coefficient from p-type to n-type was occurred at 530K in PbTe/CdTe MQW samples, The immiscible of the PbTe/CdTe interface in the MQWs and the large loss of Te at highr temperature would contribute to the p-n transition, which were confirmed by the thermal cycle. |
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