Preparation And Thermoelectric Properties Of Mg-Sn-Si Thin Films

Thermoelectric materials can directly convert electric energy and heat energy into each other based on the thermoelectric effect.They possess a wide application prospect in the field of temperature difference power generation and electronic refrigeration.Ternary solid solution Mg₂（Sn,Si）thermoelectric material are paid more attentions because Si and Sn are the same main group,possessing larger atomic mass difference,and the band gap can be regulated in a small range by changing the Sn/Si ratio.Thus,the scattering process of carriers can be controlled and the thermoelectric properties are improved.In this paper,a series of Mg₂Sn_1-xSi_x（x=0.2,0.4,0.5,0.6,0.7）films with different Mg contents and Mg₂Sn_1-xSi_x（x=0.4,0.5,0.6）films with different Bi contents were deposited by magnetron sputtering.The influence of Mg,Si and Bi content on phase composition,surface and cross-section morphology,room temperature carrier concentration and mobility and thermoelectric properties of the deposited films were analyzed.The main studied results are as follow:For Mg-rich Mg₂（Sn,Si）solid solution films,with the increasing metal Mg phase,the room temperature carrier concentration increases and the mobility decreases due to more interface between metal Mg phase and Mg₂（Sn,Si）phase.With the increasing metal Mg phase content,the room temperature conductivity increases and the Seebeck coefficient decreases.It is attributed to the results of competition between the increase of carrier concentration and the decrease of mobility in the deposited films.With the increasing metal Mg phase content,the power factor of deposited Mg₂（Sn,Si）films gradually decreases,suggesting that in the range of metal Mg phase content studied.It is not conducive to improve the power factor that metal Mg phase exists in the Mg₂（Sn,Si）films.For Mg₂（Sn,Si）solid solution films with different Si contents,with the increasing Si content,the carrier concentration shows a trend of decrease while the mobility shows a trend of increase.This is due to substitution of Sn ion by Si ions in Mg₂Sn lattice,which will cause certain lattice shrinkage distortion,reducing the occurrence of Mg vacancy in Mg₂Sn lattice.As a result,the hole concentration decreases with the increase of Si content in the deposited Mg₂（Sn,Si）films,that is,the carrier concentration decreases.Moreover,the mobility increases due to the weakened scattering process produced by the decrease of Mg vacancy in lattice.With the increase of Si content,the conductivity decreases and the Seebeck coefficient increases.The films with Si content of 10.8 at.%possess a maximum power factor of 2.76 m W·m^-1·K^-2at 303°C due to the moderate Seebeck coefficient and conductivity.Therefore,in Mg₂（Sn,Si）solid solution films,the optimal power factor can be obtained by controlling Si content of 10.8 at.%.For Mg₂（Sn,Si）thin films with different Bi doped contents,with the increasing Bi content,the carrier concentration increase and the mobility decrease.Similar to Si ion occupying Sn position,Bi ion occupying Sn or Si position in Mg₂（Sn,Si）lattice will cause large lattice expansion distortion,promoting the production of Mg vacancy in Mg₂（Sn,Si,Bi）lattice.The more Bi content,the more Mg vacancy in Mg₂（Sn,Si,Bi）lattice.As a results,the holes concentration in the deposited film increase with increasing Bi content.The mobility decreases due to increasing scattering process produced by the increase of Mg vacancy.With the increasing Bi content,the conductivity first decreases and then increases and the Seebeck coefficient first increased and then decreased.The Mg₂（Sn,Si）solid solution films with Si of10.2 at.%and Bi of 1.3 at.%possess the largest power factor of 2.93 m W·m^-1·K^-2at 160°C due to the higher Seebeck coefficient and moderate conductivity.Therefore,it is beneficial to improve the power factor of the Mg₂（Sn,Si）films by doping 1.2 at.%-1.6 at.%of Bi element.