First-principles Study Of Ag-doped ZnO

Over the past decades, with the development of photoelectric informationtechnology, the study of device of ultraviolet (UV) luminescence in the short wavehas become hot. ZnO, a wide-band semiconductor material, the large exciton bindingenergy (60meV) of which can ensure efficient excitonic emission and ultraviolet(UV) luminescence at room temperature, become one of the major focuses in theresearch field of semiconductor short-wavelength devices. As a new kind of novelsemiconductor of theⅡ-Ⅵgroup, it has a lot of unique photoelectric properties. But,ZnO thin film is n-type material in general, so that it is hard to achieve p-type doping.Thereby, p-type doping is a key technology of optic electronic applications, which isa hotspot of this study.The researchers were mainly attracted in nitrogen doped ZnO in an early phase.However, sufficient high quality p-type ZnO with low resistivity and high mobility isyet to be achieved. Moreover, compared with the other dopants, the Ag doped ZnO,was less studied especially in the theory. Based on that background, a method usingfirst principles and pseudopotentials according to density functional theory is appliedto calculate the geometric structure, the formation energy of impurities and theelectronic structure of ZnO doped with Ag. We outline the main contents and resultsin the study as follow:Firstly, we calculated the lattice parameters, the band structure, the bondstructure and the density of states (DOS) of ZnO with wurtzite structure. It waspredicated a direct band semiconductor material in theory. The valence-bandmaximum (VBM) and conduction-band minimum (CBM) both exist the center ofBrillouin Zone. And the calculated band gap is 0.916eV, which is smaller than theexperimental result for the well-known artifact of the traditional density functionaltheory (DFT).Secondly, the native point defects are calculated by the Vienna Ab initioSimulation Package (VASP). Compared with the other native point defects', theformation of Zn_i is the smallest. At the same time, Zn_i behaves as donor. As a result, the existence of Zn_i in as-grown ZnO may be the main reason that as-grown ZnO isnearly always n-type.Thirdly, the first-principles with pseudopotentials method based on the densityfunctional theory was applied to calculate the geometric structure, the formationenergy of impurities and the electronic structure of ZnO doped with Ag. Thecalculated conclusions show that the O-rich condition is better for p-type dopingthan the Zn-rich condition. The Ag dopant prefers to occupy the substitutional Znsite, which results in a deep acceptor. Furthermore, self-compensation via Ag_i,doping by Ag is almost impossible. As a result, the deep acceptor level is the mainreason for the p-type doping difficulty in ZnO.At last, in the Ag-doped ZnO, the complex would possibly behave as acceptor ifit exists. Simultaneously, we studied the feasibility of codoping Ag and H in theZnO.