| Recently, ZnO has attracted much attention due to its wide applications invarious devices such as piezoelectric transducers, varistors, optical waveguides, andsolar cells. ZnO has a direct band gap of3.3eV, which made this material that had agood application of luminescence, and a large exciton binding energy of60meV. ZnOhave wide sources, and low price. ZnO has also been considered as a promisingmaterial for short wavelength optoelectronic devices.But there are still very real challenges to applying in industry. It is very difficultto obtain p-type ZnO, although recently it was demonstrated that high densities ofholes could be obtained with N or As as the dopant using novel doping techniques. So,there are still hot issues to get high quality p-type ZnO and the wide-band-gap alloymaterial. Here we investigate the defects in ZnO and the stable structures ofCaxZn1-xO alloy under high pressure using a first-principles pseudo potential method.And the obtained results are as follows:1) We perform first-principles calculations to investigate the semiconductor properties of B1ZnO under pressure. The formation enthalpy of several native pointdefects is related to a fine interplay between the internal strains, charges on the defectsand applied external pressures. For negatively charged states, the formation enthalpiesdecrease and the defect equilibrium concentration increases with pressure, and thesituation for the positively charged state is on the contrary. For Zn-rich ZnO at highpressures, it’s easier to form p-type material, because the oxygen vacancies VO act asdonor defects with higher formation enthalpy. The defect electronic transition levelsalso strongly depend on the pressure, with positive pressure coefficients and nospecial dependence on the charge.2) We have studied the stability of N-doped ZnO under pressure viafirst-principles DFT calculations. Introducing high pressure results in a decrease in theformation enthalpy, and an increase in the NO equilibrium concentration. Thecalculated values of defect formation volumes are always negative, indicated theformation enthalpy would decrease with pressure. With the increasing pressure, theimpurities energy level located at Fermi energy are weakened. The Bader chargeanalysis shows that pressure also makes bond energy stronger. Overall, pressureprovides an efficient way to approach p-type ZnO.3) The composition and structure of the CaO-ZnO alloy has been investigated athigh pressure using USPEX codes. In a range of0-60GPa, four stable alloystructures with different Ca concentrations were found. With the increasing pressure,more alloys with higher Ca concentrations appeared. However, the structure only canbe stable when the Ca-concentration is no more than50%, even if pressure isincreased to60GPa. When the Ca concentration increases, the stable structures ofCaZn6O7, CaZn5O6, CaZn3O4and CaZnO2undergo a hexagonal to monoclinic transition, and then transforms back to a hexagonal phase. We find that ground-statestructures do not share the cubic structure of ZnO and CaO. Accordingly, we posit thatthe symmetry of component structures has little impact on the alloy structure.Through the analysis of band structure, we note an almost linear increase in band gapas a function of Ca-concentration at the selected pressure. When the concentration isbelow50%the band gap increases with pressure. For CaZnO2, the band gap firstincreases and then decreases with pressure. And we found that under0GPa the fouralloy structures are in the metastable state. So we think our predicted alloys can besynthesized at high pressure and reserved at ambient conditions. |
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