Preparation And Performance Optimization Of Bi-Te Based Thermoelectric Thin Film

Thermoelectric（TE）materials can realize the direct conversion between thermal energy and electric energy.Thermoelectric conversion technology based on TE materials is a green and clean energy conversion technology,which plays an important role in thermoelectric power generation,device heat dissipation,waste heat recovery and so on.TE materials have more advantages than other energy conversion systems in milliwatt to microwatt power level for electronic device.With many advantages compared with bulk TEs,such as higher integration density,heat pumping capability,faster response time,more compact size,thin film TE materials are more suitable for the development trend of modern integrated circuit（IC）industry and have broad application prospects in the field of refrigeration and thermal management of high-power microelectronic devices.However,limited strategies can be utilized to tailor the performance of thin film through controlling the microstructure,as compared to the bulk TE materials.As the Bi₂Te₃ thermoelectric material with the highest thermoelectric conversion efficiency in the low temperature region,how to further improve the thermoelectric properties of the Bi₂Te₃ film has become a key issue to promote its development.This project starts with structure optimization,component optimization and process optimization to systematically study the preparation,characterization and performance optimization of Bi-Te-based thermoelectric films,in order to obtain Bi₂Te₃thermoelectric film materials with excellent performance.The growth process of thermoelectric thin films largely determines their performance.In this dissertation,the films are grown by magnetron sputtering deposition process.From the perspective of the growth process of Bi₂Te₃ based thermoelectric thin films,the effects of growth process factors on their thermoelectric properties are systematically studied.By preparing the sample films with sputtering power,sputtering pressure and sputtering temperature as a single variable,combined with the thermoelectric performance analysis results,the best growth process parameters of the films are finally obtained.Bi₂Te₃ based thermoelectric films have the best thermoelectric performance when the sputtering power is 55 W,the sputtering pressure is 0.4 Pa and the sputtering temperature is 573.15 K,and the power factor reaches the optimal value of 7.28μW·cm^-1·K^-2.On the basis of the best growth process,the effects of original doping and film thickness on the thermoelectric properties of Bi₂Te₃ films are studied by using the method of component optimization,that is,doping Bi or te elements in Bi₂Te₃ alloy by co sputtering,adjusting the content of Bi and Te in Bi₂Te₃ films,and changing the deposition time to obtain films with different thickness.By analyzing the thermoelectric properties of the sample films,it is found that although Bi doping fails to improve the thermoelectric properties of Bi₂Te₃ films,Bi₂Te₃ films doped with an appropriate amount of Bi form acceptor doping after Bi occupies te,which makes the films change from n-type semiconductor materials to p-type semiconductor materials,realizing the change of material types,it provides a research idea for p-type optimization of n-type thermoelectric thin films.With the increase of Te content,the film growth mechanism gradually changed from layered growth mechanism to island growth mechanism;The grain growth mechanism changes from lamellar growth mechanism to spiral growth mechanism with excessive Te,which reduces the compactness and increases the roughness of the film.The recombination of excess Te atoms and V_Te defects leads to the decrease of carrier concentration,the decrease of conductivity,the increase of Seebeck coefficient,and the change trend of power factor first decreases and then increases.When the Te content of Bi₂Te₃ film is 67.62%,the power factor reaches the optimal value of 12.75μW·cm^-1·K^-2.With the increase of film thickness,due to the growth time of the film,the phenomenon of lattice stacking faults in the film increases,resulting in more defects,aggravating the scattering of carriers,and gradually reducing the carrier mobility.At the same time,the growth time of the film increases.Due to the volatile characteristics of Te at high temperature,the Te content in the film gradually decreases,resulting in the increase of carrier concentration.Under the joint influence of mobility and carrier concentration,the conductivity increases,the Seebeck coefficient decreases,and the power factor increases first and then decreases.When the thickness of Bi₂Te₃ film is913 nm,its power factor reaches the optimal value of 14.99μW·cm^-1·K^-2.Finally,the thermoelectric performance of Bi-Te-based Thermoelectric Films is significantly improved through the combination of structural optimization,component optimization and process optimization.After optimization,the power factor of the films without any optimization treatment is reduced from the initial 2.01μW·cm^-1·K^-2increased to 14.99μW·cm^-1·K^-2,and the Seebeck coefficient and conductivity were maintained at higher levels of-110.40μV/K and 1230.05 S/cm respectively.