First Principles Study On The Electronic Structure And Stablity Of Zno With Different Doping Configurations

ZnO is a wide band-gap (3.37eV) II-VI commpound semiconductor with a large exciton binding energy (60meV), which makes it to have potential applications in the short-wavelength optoelectronic devices, and has been an international investigation hotspot presently. Unintentional ZnO is the m-type conductive semiconductor due to its residual defects. The preparation of a stable and reproducible p-type ZnO is the bottle-neck in the present study. To fully achieve the application of ZnO in the optoelectronic-field, it is necessary and significant to understand the electronic band structure of the doping ZnO. On the other hand, the band-gap engineering requires adjusting the band gap of a semiconductor, which can be realized by adding the doped element into ZnO. However, alloying will result in the decline in the thermal stability and the film quality. It is helpful to study the strucutre stability for the ZnO alloying and the film growth.With the first-principles methods on the basis of density functional theory, the formation enthalpies, lattice parameters and electronic structures of (ZnM)O (M=Mg, Cd, Be, Cu) are calculated and the statbility of alloys is discussed. Especially, the influence of doping configurations is considered and discussed in the present study. In addition, we calculate the formation energy and acceptor ionization energy of N substituting for O site (No) in ZnMgO and ZnBeO alloys. On the basis of calculations, the p-type conductivity of Mg-N and Be-N co-doped ZnO is analyzed and discussed. On the other hand, the optical properties of ZnO under high pressure and N doped CU2O are investigated in detail. The main conclusions are as follows:(1) Cd, Mg, and Be are the effective elements to adjust the band gap of ZnO, the dependence of wurtzite ZnMO (M=Cd, Mg, Be) alloys on the dopant content x is Eg=3.28-5.04x+4.60x2(RT optical band gap), Eg=3.43+2.24x+0.68x2(OK band gap), and Eg=3.43+2.94x+1.77x"(OK band gap), respectively. By the comparison with the photoemission spectra in experiment, doping configurations might be one of the reasons to cuase the PL broadening. The calculations of formation energy suggest both of Cd and Mg doping will results in the instability of alloys with wurtzite structure. For ZnCdO alloy, the wurtzite ZnCdO alloy will transit into rocksalt phase when the Cd content is larger than80%. If the influence of doping configurations is encounted, the wurtzite and the rocksalt phases are suggested to co-exist when the Cd content is larger than75%while the wurtzite and the zinc-blende phases are suggested to co-exist in the range of Cd contents from25%to75%. For ZnMgO alloy, wurtzite ZnMgO alloy is stable when the Mg content is lower than37.5%and will transit into rocksalt phase when the Mg content exceeds37.5%. In the all range of Mg contents, zinc-blende ZnMgO alloy is always in the metastable phase.(2) The calculations of the formation energy of No in ZnMgOand ZnBeO alloys suggest nitrogen atoms are preferred to substitute the oxygen sites without the nearest neighbour doping atoms. The formation energy of No will increase about0.5or0.2eV for per nearest neighbour Mg or Be atom, respectively. The ionization energy of No acceptor is also related to the nearest neighbour doping atoms. For ZnMgO alloys, the No acceptors with4nearest neighbour Mg atoms have the lowest ionization energy, about0.24eV, suggesting the No acceptors still are the deep acceptors. For ZnBeO alloys, the No acceptors with2nearest neighbour Be atoms have the lowest ionization energy, about0.1eV, indicating the No acceptors can be regarded as the low acceptors and the corresponding Be content is about11%. With the results of formation energy and ionization energy of No in ZnMgOand ZnBeO alloys, Mg-N co-doping is not recommended to realize p-type conductivity for ZnO because No in ZnMgO is not an effective acceptor. At the given Be content (11%), Be-N co-doping could realize the p-type conductivity, but the carrier concentration is not expected to be high enough.(3) With increasing Cu content in Zn1-xCuxO alloy, band gap of Zn1-xCuxO alloy is reduced and is attributed to the upward shift of valenece band maximum and down shift of the conduction band bottom. The calculations of formation energy suggest the wurtzite Zn1-xCuxO alloy begins to transit into tenorite phase when x is larger than40%. At the given Cu content, the alloys with different doping configurations have the formation enthalpies in difference largely; hence resulting in the wurtzite and tenorite ZnCuO alloys could co-exist at a low Cu content. In addtion, Cu is found to be favorable to substitute Zn with the sites in a same plane.(4) A new high-pressure tetragonal phase (B10) of ZnO is investigated with an ah initio calculation based on density-functional-theory and is compared with the cubic rocksalt structure and CsCl structure phases at high pressure. It is found that B10phase has a more covalence nature than B2phase. The band gap energies of B1,B2, and B10phases are determined to be3.73,3.15, and3.27eV, respectively. Consider the underestimate of the band gap energy, all the three high-pressure phases should be regarded as insulators. The B10phase has a similar optical response to B2phase, but different from B1phase. The similarity of dielectric function between B10and B2phases are the result of the similar profiles of electronic density of state.(5) N-doped Cu2O films are deposited at different temperatures by sputtering a CuO target in the mixture of Ar and Ni. By analysis of transmission spectra, it is found that the N-doped Cu2O films are changed to be a direct allowed band-gap semiconductor and the optical band gap energy is enlarged to be2.52±0.03eV for the films deposited at different temperatures. The first-principles calculations indicate that the energy band gap increase by25%, being in good agreement with the experimental results. The change, from a direct forbidden band-gap transition to a direct allowed band-gap transition, can be attributed to the occupation of2p electrons of N at the top of valence band in the N-doped CU2O films.