First Principles Calculations Of P-type Doping Of Bi₂se₃ And Studies On Bi₂se₃ Single Crystal Growth

Bi₂Se₃, as conventional thermoelectric material, has been discovered to be topological insulator which has the largest band gap in topological insulators, about 0.3 eV. The as-grown Bi₂Se₃ is n-type, therefore, its p-type doping is very important for its application as topological insulator as well as thermoelectric material. In this graduate paper, we investigated p-type doping of Bi₂Se₃ with first principle calculation.With the analysis on the electronic orbit in all-electron calculation, the pseudopotential of Mg, Se, Bi was generated with QUANTUM-ESPRESSO package. Their reference configurations are [Ne]3S², [Ar]3d¹⁰4s²4p⁴, [Xe]4f¹⁴5d¹⁰6s²6p³ respectively and PBE-type generalized gradient approximation(GGA) or PW91-type local dengsity approximation is applied. The cell parameters and band structure of bulk material of Mg, Se, Bi and those of their compounds are used to test the quality of these pseudopotentials, which indicates high quality of the generated pseudopotential of Mg and Se and also suggests high impacts of Bi 5d electrons.The formation energy of intrinsic defects of Bi₂Se₃, Se vacance VSe,and antisites defects Bi_Se, Se_Bi, were calculated , which suggests the impossibility of Bi_Se. The formation energy of Se_Bi is negative, and that of VSe is only 0.057Ry. Therefore, the n-type characteristics of Bi₂Se₃ is induced by both Se_Bi and VSe. Due to higher Bi content in it , there must exist planar defects of Bi in as-grown as-grown Bi₂Se₃.The formation energy of defects X_Bi^q,X_Se^q(q=-1,-2) and X between quintuple layer of Bi₂Se₃, X_int, (X:Ca,Mg) were calculated The doping of Bi₂Se₃ with Ca and Mg atoms, is studied. The formation energy of X_Bi^q(X:Ca,Mg; q=0,-1,-2) is megative , while that of other defcts are positive, which suggests that Ca and Mg can dope Bi₂Se₃ into p-type. Comparing the results of Ca with those of Mg , we suppose Ca is a better p-type dopant than Mg.The Mg-doped Bi₂Se₃, Bi_2-xMg_xSe₃(x=0,0.05), were grown via a process of two-step melting, starting with high-purity elements ( Bi, 99.999%; Se, 99.99%; Mg, optical purity). We have found that the content of Se in as-grown Bi_2-xMg_xSe₃ can be enhanced by the temperature gradient along the axis of quartz ampoules in solidification methods. Some slices with layered structure were separated from the grown sample. By the crystal-direction determination device of X-Ray, these slices are not single crystal.