Growth And Electrical Transport Properties Of Tin Selenide Crystals

The study of the Sn-Se system,?-? group semiconductor,can be traced back to1950s.Recently,because of extremely large thermoelectric properties observed in SnSe crystals,SnSe has been attracted much attention in thermoelectric community recently.Until now,many experimental and theoretical works have revealed the physical origins of the extremely large thermoelectric properties.These works not only elaborate the physical properties of layered semiconductor SnSe,but also advance thermoelectric theory.According to literatures survey,SnSe samples,grown by different groups,have quite different electrical and thermal properties.The understanding for defect types or formation mechanisms is also quite diverse.And most of above-mentioned works have been done at high temperature range?>300 K?,however there is nearly no electrical property characterization at low temperature.Besides,compared with research on p-type SnSe,the study of n-type SnSe is in the initial stage.Based on above-mentioned backgrounds,We adopted Bridgeman method,flux method,and physical vapor transport?PVT?method to grow a series of SnSe crystals in this thesis,and explored the relationship between electrical properties of SnSe crystals and their growth methods based on low-temperature electrical transport measurement.As to p-type SnSe degenerate semiconductor,those grown by PVT and Sn as solvent show electrical property of thermally activated semiconductor.However,the SnSe crystals grown by Bridgeman method and Na Cl as solvent show electrical behavior of metals.SnSe crystals prepared by Bridgeman method have lower holes concentration(?10¹⁷-10¹⁸cm^-3)and we observed a metal-insulator transition at 50 K-70 K,which is attributed to Anderson localization according to magnetoresistance analysis.Electrical transport measurement suggests that the SnSe crystals using Na Cl as solvent have higher holes concentration(?10¹⁹ cm^-3)compared with SnSe prepared by Bridgeman method.High carrier concentrations SnSe crystals,grown by flux method using Na Cl as solvent,showed the metallic electric property and it can be described by Landau Fermi liquid theory.Consequently,the SnSe crystal using Na Cl as solvent possesses higher power factor(31.5?W·cm^-1·K^-1at room temperature).As to n-type SnSe,CeBr₃-doped crystal shows metal-like conductivity with an electron concentration of 8×10¹⁸cm^-3,and its electrical performance is similar to SnSe prepared by flux method using Na Cl as solvent.The other single element doping n-type SnSe crystals,as well as SnSe crystals using Ce Cl₃ as solvent,exhibit non-degenerate semiconductor electrical property.At low temperature,we observed a metal-insulator transition in these crystals,implying probably temperature-dependent topology change in conduction band of SnSe.In order to research how microstructure influences thermoelectric performance in SnSe system,we growed SnSe-SnSe₂ eutectic crystals and conducted their thermal and electrical properties.Controlling the growth rate,we prepare different eutectics and find higher growth rate contributes to thinner layers of the two components,improving the proportion of interfaces.Increased interface enhances phonon scattering and reduces thermal conductivity.On the contrary,lower growth rate leads to thicker layers and increases the thermal conductivity beyond SnSe crystal.In conclusions,in this thesis,I have successfully grown n-or p-type SnSe crystals and SnSe-SnSe₂ eutectic compound by different growth methods,and characterized their electrical transport property.Based on these characterizations,I elucidate the relationship between electrical properties of SnSe and different growth methods.This work can further advance the optimization of thermoelectric properties of SnSe.