The Fabrication And Thermoelectric Properties Of C-axis Oriented BiCuSeO Thin Films Deposited By Pulsed Laser Deposition

BiCuSeO-based chalcogenide is a p-type oxide semiconductor with layered tetragonal crystal structure,which shows great potential in medium/high temperature thermoelectric applications due to the intrinsic low thermal conductivity.The polycrystalline bulks of BiCuSeO have been studied intensively,however,very little has been done to understand the thermoelectric properties and their dependence on film growth condition.Compared to the bulks,two-dimensional themoelectric thin films have attracted intensive research due to the advantage of application in the microthermoelectric power generation and refrigeration,which are important for the potential application in the themoelectric devices.In addition,the thermoelectric properties can be significantly improved due to the anisotropy of BiCuSeO thin films as the c-axis oriented growth.In this work,we have prepared high quality c-axis oriented BiCuSeO thin films on commercial wafers by pulsed laser deposition?PLD?.In order to understand and control of thermoelectric properties,the deposition conditions and chemical element doping have been investigated in detail.All the contents are show as the following:1.The BiCuSeO films have been synthesis on three different single crystal substrates by PLD The effects of substrates and temperature on the crystal structure,micro morphology and the transport properties of the films have been investigated.The single phase c-axis BiCuSeO films which coherent with the SrTiO₃ substrate can be obtained under the optimal deposition temperature of 330°C.The BiCuSeO film exhibits a metallic behavior over the temperature range from20³50 K.At room temperature,the resistivity of the film is about 12.5 m?·cm,which is much lower than that reported for bulk polycrystalline samples.The calculated room temperature power factor is about 3.3?Wcm^-1K^-22 which is much highter than the corresponding polycrystalline ceramics,suggesting that c-axis oriented BiCuSeO epitaxial thin films might promote the use of thermoelectric thin film in thermoelectric devices.2.Thesingle-crystal-likeBi_1-xPb_xCuSeO?x=0.04,006,0.08?andBi_1-xCa_xCuSeO?x=0.025,0.05,0.075?thin films have been successfully fabricated on the SrTiO₃?001?substrates with c-axis preferred orientation.The microstructure and high temperature thermoelectric properties as the function of the content of the Pb and Ca-doping have been investigated.The result showed that the Pb-doping could effectively enhanced the electrical conductivity and the power factor due to the enhanced hole carrier concentration.Compared with Ca-doping,Pb-doping is more favorable to the thermoelectric properties of intrinsic BiCuSeO films.3.Highly c-axis-textured Bi CuSeO thin films were grown directly on amorphous glass substrate by PLD.The resistivity?²7.1 m?cm?of the films at room temperature was found to be much lower than those reported for polycrystalline bulks.The electrical transport properties of the films can be fitted by the three dimensional variable-range-hopping?3D VRH?conduction formula at the low temperature range.In addition,detailed microstructural analysis revealed the existence of amount of amorphous grain boundaries throughout the films,leading to a significant decrease of the thermal conductivity.Under this circumstance,the thermoelectric performance of the present BiCuSeO thin films is expected to be greatly enhanced as compared to that of bulks,demonstrating the potential application in thin film thermoelectric devices.