Preparation And Thermoelectric Properties Of Thin Films Of Nano CrSi₂ And Mg₂Si

Utilizing high vacuum magnetron sputtering equipment, nano-structured CrSi₂ film with different Ti doping content and nano-structured Mg₂Si film with different Al doping content were prepared by alternately depositing Cr/Ti and Si layer or Mg/Al and Si layer on the Si（100） substrate. The field emission-scanning electron microscope（FESEM）, X-ray diffraction spectra（XRD）, Hall coefficient tester and Seebeck coefficient resistance measuring system were used to study the properties and parameters of deposited films,including surface morphology, crystal structure, carrier concentration, room temperature conductivity, Seebeck coefficient, electric resistivity and power factor with temperature in different deposition technique. Meanwhile, the rule and mechanism of these properties and parameters were discussed. It was shown that deposition process greatly influenced these parameters and properties of the deposited films.Only single phase nano-structured CrSi₂ films with Ti-doping were obtained by alternately deposition of Cr/Ti and Si layer and subsequent vacuum anneal at 500°C for 6h. Deposited films possess relatively homogeneous grains with size of 7⁸nm and relatively smooth dense surface. The grain size of deposited film slightly increases with the increases of Ti content in films. With the increases of Ti content in deposited films, room temperature Hall coefficient and hole carrier mobility monotonically decreases but hole concentration increases. Thus the conductivity of deposited film first increases to reach a maximum and then decreases with increasing Ti content in films. Seebeck coefficient of Ti-doped CrSi₂ films monotonically decreases from positive to negative value as the temperature increases. This suggests that the deposited films evolve from P-type semiconductor to N-type semiconductor. The Seebeck coefficient and power factor of Ti-doped CrSi₂ films reaches-108μV/K and 0.45μW.m-1.K-2 at 618°C, respectively.Al-doped single phase Mg₂Si nano-structured films were obtained by alternately deposition of Mg/Al and Si layer by using high substrate bias. The grain size of films is 5⁶nm and evenly distributed. When the sputtering power of Mg target is 30 W, 35 W and 40 W, the most Mg₂Si phase is obtained at 35 W. When substrate bias varies in the range of 150 V to 300 V, the higher the bias is, the more the amount of produced Mg₂Si phase is. The values of Seebeck coefficient are negative and their absolute values slowly increase with increasing the temperature from room temperature to 400°C. The absolute values of Seebeck coefficient sharply increase and reach a maximum value at certain temperature when temperature is over 400°C. The absolute values of Seebeck coefficient then decrease with further increases in temperature. For the films with Al doping contents of 3.30at%, 8.71at%, 11.95at% and 21.49at%, the maximum absolute value of Mg₂Si film`s Seebeck coefficient was approximately 240μV/K, 380μV/K, 375μV/K and 280μV/K, respectively.The electric resistivity of Al-doped Mg₂Si nano-structured films slowly increase when temperature increases from room temperature to 400°C. Its variation rule is similar to Seebeck coefficient. A sharply increase in the absolute values of the Seebeck coefficient and the electric resistivity occur when temperature is over 400°C. The absolute values of the Seebeck coefficient and electric resistivity then decreases with further increases in temperature.