First-principles Study On The Thermoelectric Properties Of Element-doped Ge₂Sb₂Te₅

The chalcogenide layered pseudo-binary alloy Ge₂Sb₂Te₅is currently the most excellent phase change memory material.Its high electrical conductivity,low lattice thermal conductivity,low band gap and high carrier effective mass indicate its usefulness as a thermoelectric material.However,the large amount of holes brought by the high concentration of intrinsic Ge vacancies leads to higher carrier thermal conductivity and lower Seebeck coefficient,which limits the improvement of its thermoelectric efficiency.In order to conduct in-depth research and improvement on the thermoelectric application potential of GST materials,this thesis studied the crystal structure,electronic structure and thermoelectric properties of pristine Ge₂Sb₂Te₅and Ge₂Sb₂Te₅doped with Ag,Bi,and Sn elements through first-principles calculations,and the effect of Ge vacancy on them was discussed.The main conclusions are as follows:(1)Ge₂Sb₂Te₅in the K-H configuration has dynamic stability.The acoustic branch and optical branch of the phonon spectrum are obviously coupled,and the optical branch has a greater contribution to the thermal conductivity of the lattice.The band gap value of Ge₂Sb₂Te₅calculated by hybrid functionals is 256me V.The valence band is mainly contributed by Te atoms,and its band structure is not conducive to the improvement of thermoelectric performance;the conduction band is jointly contributed by three elements,and exists numerous carrier pockets with degeneracy of multiple bands,the effective carrier mass is as high as 2.44265 m₀,resulting in the maximum value of n-type doping PF/?which is about 2.16 times that of p-type doping.At a suitable carrier concentration,the peak value of ZT of n-type doping can reach above 3.0,but the high concentration of intrinsic Ge vacancies is not conducive to the improvement of the thermoelectric performance of Ge₂Sb₂Te₅.(2)Due to the limitation of symmetry,the thermoelectric performance of Ge₂Sb₂Te₅exhibits isotropy in the xy direction.Because of the higher S and lower?_e/?in the z direction,the PF/?of 3.52×10¹²Wm^-1K^-2s^-1can be obtained in the z direction,which is 1.45times of that in the xy direction;the lattice thermal conductivity in the z direction is only0.227 W/(m*K)at room temperature,which is less than half of the xy direction.Taken together,the thermoelectric performance of Ge₂Sb₂Te₅is anisotropic,and the thermoelectric performance in the z direction is significantly better than that in the xy direction.(3)In replacement doping,Ag,Bi,and Sn all tend to replace Ge,and the formation energies are all negative,indicating that replacement doping can proceed spontaneously.Ag doping will cause the Fermi-level to penetrate into the valence band,increase the band gap,and reduce the degeneracy of the energy band.High-concentration Ag doping has a negative impact on the thermoelectric performance.Sn doping does not significantly affect the energy band structure except for the small splitting of the energy band.Since the electronegativity of Sn is lower than that of Ge,the electron delocalization increases and the band gap decreases;Sn doping has a small overall effect on the thermoelectric properties of the electrons.Bi doping makes the Fermi level gradually penetrate into the conduction band,the energy band is split,and the band gap is reduced.Low-concentration Bi doping is expected to obtain better thermoelectric performance.(4)Ge vacancy formation energy is about-2?-3e V,indicating that Ge₂Sb₂Te₅has a significant tendency to spontaneously form Ge vacancies.The introduction of Ge vacancies causes the Fermi level to enter the valence band,and the energy band is significantly split and broadened.Due to band splitting,both?/?and?_e/?are greatly reduced,which makes it difficult to improve the thermoelectric performance of actual GST materials.The superposition of Ge vacancies and holes introduced by Ag doping causes the Fermi level to penetrate into the valence band,which is not conducive to obtaining excellent thermoelectric performance.Sn has a limited effect on Ge vacancies.The extra electrons provided by Bi are not enough to fill the large number of holes brought by Ge vacancies,so the Fermi level is still in the valence band;compared with Ag and Sn,Bi doping has less influence on?/?and?_e/?,A higher PF/?can be obtained,so methods such as doping and straining should be tried to eliminate the intrinsic Ge vacancies in the GST material.