The Microstructure And Thermoelectric Properties Of Cu₂Se-Based Films By Sputtering

Cu₂Se materials have excellent thermoelectric properties due to the structure of"phonon liquid-electron crystal"（PLEC）.In this thesis,the Cu₂Se films were deposited on Si O₂/Si substrates by magnetron sputtering using a powder sintered Cu₂Se compound target.The influences of Cu content,Ag doping,film thickness and nanomultilayer modulation period on the thermoelectric properties and carrier transport processes of Cu-Se films were also discussed.The main results are following:（1）The p-type Cu-Se films with different Cu contents were prepared using targets with different atomic ratio（[Cu]/[Se]）of Cu to Se.The deposited films took place phase transformation from p-Cu Se₂to p-Cu Se₂+γ-Cu Se,toγ-Cu Se,toγ-Cu Se+β-Cu_2-xSe,toβ-Cu_2-xSe,toα-Cu₂Se+unknown,toα-Cu₂Se+unknown+Cu phase with increasing[Cu]/[Se]in the deposited films from 0.49 to 5.74.When the[Cu]/[Se]in the films is in the range of2.65-2.91,all films are composed ofβ-Cu_2-xSe phase.The depositedβ-Cu₂Se phase films are rich in Cu.At room temperature（RT）,the carrier concentration decreases in the order of the Cu Se₂,Cu Se andβ-Cu_2-xSe phase;but the carrier mobility exhibits an opposite trend.In the studied temperature range,the Seebeck coefficient and the power factor decreases in the order ofβ-Cu_2-xSe,Cu Se₂and Cu Se phase.In onlyβ-Cu_2-xSe phase films,[Cu]/[Se]=2.65 films with moderate degree of（111）preferred orientation has the highest power factor in whole studied temperature range due to proper carrier concentration and mobility.（2）Theβ-Cu_2-xSe phase films can be obtained at RT usingα-Cu₂Se phase compound targets.The appropriate amount of Ag-doping can significantly increase the Seebeck coefficient,thereby obtaining a higher power factor.The Ag atom doped forms a nano-sized Cu Ag Se second phase in the films.The depositedβ-Cu₂Se phase films are rich in Cu.With the increase of Ag content,the carrier concentration of the depositedβ-Cu_2-xSe films increases first and then decreases,but the carrier mobility exhibits an opposite trend.The trend of the resistivity and Seebeck coefficient is decreasing and then increasing.The film with Ag content of 1.37at%has the highest power factor due to the significantly higher Seebeck coefficient.（3）The Cu-Se films with different thickness were prepared by changing the deposition time using a powder sintered Cu₂Se compound target.The deposited film is mainly composed ofβ-Cu_2-xSe phase and there is also a very small amount ofα-Cu₂Se phase for the thinner film.The depositedβ-Cu₂Se phase films are Cu-rich.With the increase ofβ-Cu_2-xSe film thickness,the carrier concentration,carrier mobility and columnar grain diameter increase;while the Seebeck coefficient and resistivity decrease.The films with a minimum thickness of 0.84μm have the highest power factor.Theβ-Cu_2-xSe films have significant size effect on the thermoelectric properties.（4）Theβ-Cu_2-xSe layers in bothβ-Cu_2-xSe/a-C andβ-Cu_2-xSe/SiC nano-multilayer films are Cu-rich and have dense nanoscale columnar grains.The carrier concentration and carrier mobility decrease and the resistivity and Seebeck coefficient increase with reducing the modulation period ofβ-Cu_2-xSe/a-C orβ-Cu_2-xSe/SiC.For theβ-Cu_2-xSe/a-C nano-multilayer film,The thermal conductivity also decrease with reducing the modulation periods.The multilayer film with modulation period 160 nm（the smallest modulation period）,the thermal conductivity and ZT value reach 0.41W·m^-1·K^-1and 0.42 at RT,respectively,with a high power factor of 1.13 m W·m^-1·K^-2at 300℃.For theβ-Cu_2-xSe/SiC nano-multilayer films,the multilayer film with the smallest modulation period（210 nm）has the highest power factor reaching 0.39 m W·m^-1·K^-2and 0.59 m W·m^-1·K^-2at RT and 325°C,respectively.Enhanced thermoelectric properties in the nano-multilayer structureare attributed to scattering effect at the layer interface and grain interface.（5）The layer interface scattering effect of theβ-Cu_2-xSe/a-C nanomultilayer film is significantly stronger than that of theβ-Cu_2-xSe/SiC nanomultilayer film due to the higher built-in electric field at the layer interface.These results suggest that the nano-multilayer structure is an effective way to improve thermoelectric properties for thin films.