Sputtering Preparation And Thermoelectric Properties Of Doped Mg₃Bi₂ Thin Films

Thermoelectric materials are functional materials that can transform waste heat and electricity into each other and have a wide range of applications in the field of waste heat temperature difference power generation and electronic refrigeration.Mg₃Bi₂ is a semi-metallic material with negative band gap,high electrical conductivity and low lattice thermal conductivity.Therefore Mg₃Bi₂ is considered as a promising thermoelectric material for low temperature region.It is of great significance to further improve the thermoelectric properties of Mg₃Bi₂ materials by means of low dimensionalization,doping and nanocomposites.In this study,Mg₃Bi₂ thin films with different Mg and Bi contents and Mg₃Bi₂-based thin film materials doped with Sn and Sb elements were obtained by alternately sputtering using high-purity Mg targets and Mg-Bi alloy targets.The chemical composition,phase composition,microstructure and thermoelectric properties of deposited films were examined and studied.The main study results are as follows:The Mg-rich deposited films consist of amorphous Mg₃Bi₂ phase and their room temperature conductivity increases with the increase of the residual Mg content in the deposited films.The room temperature Seebeck coefficient and power factor（PF）show a trend of first increasing and then decreasing with increasing the residual Mg content in deposited films.Unfortunately,the Mg-rich deposited films possess a lower power factor and poor thermoelectric properties.The Bi-rich deposited films consist of crystalline Mg₃Bi₂ phase and their room temperature carrier concentration gradually increases but the mobility keeps decreasing with the increase of the residual Bi content in the deposited films.The power factor of the Bi-rich Mg₃Bi₂ films is improved due to the results of competition between decreases of the conductivity and increase of Seebeck coefficient,produced by the metallic Bi phase in the deposited films compared with the amorphous Mg₃Bi₂ films.However,its optimal power factor is still not high,only 0.40 m W·m^-1·K^-2.The doping of a small amount Sn can also induce the crystallization of Mg₃Bi₂ phase due to the reaction of Sn with Mg to form Mg₂Sn,which promotes the crystallization and growth of Mg₃Bi₂ phase by the action of Mg₂Sn crystal template.At a small Sn content,the deposited film contains Mg₃Bi₂ and Mg₂Sn phases,constituting Mg₃Bi₂/Mg₂Sn nanocomposite films.With the increase of Mg₂Sn content in the films,the carrier concentration shows a trend of frist increasing and then decreasing and the mobility varys in the opposite law.The phase interface of the nanocomposite film can modulate the carrier transport and increase the Seebeck coefficient of the film,thus effectively improving the power factor of the material.With further increasing the amount of Sn doping,the Mg₂Sn becomes the main phase and Bi in the film enters the Mg₂Sn lattice,forming Mg₂（Sn,Bi）solid solution.The power factor of the deposited Mg₂（Sn,Bi）films decreases significantly compared with that of the Mg₃Bi₂/Mg₂Sn nanocomposite film.The doping of Sb in deposited Mg₃Bi₂ films forms Mg₃（Bi,Sb）₂ solid solution films.The conductivity reduces,the Seebeck coefficient substantially increases and the power factor shows a trend of first increasing and then decreasing with the increase of Sb doping.The film with Sb doping of 0.4 at.%has the largest power factor due to the results of competition between conductivity and Seebeck coefficient.Both Mg₃Bi₂/Mg₂Sn nanocomposite and the Sb element doping were beneficial to increase the power factor of the Mg₃Bi₂ films.The Mg₃Bi₂/Mg₂Sn nanocomposite films with 10.3 at.%Sn doping possess the highest PF value of 2.85 m W·m^-1·K^-2 in the medium temperature region and the Mg₂（Sn,Bi）solid solution films with 0.4 at.%Sb doping possess the highest PF value of 1.47 m W·m^-1·K^-2 in the room temperature region.