Abdirash Akilbekov

Hydrogen adsorption on the ZnO ( ) surface: ab initio hybrid density functional linear combination of atomic orbitals calculations

Hydrogen atoms unavoidably presented in ZnO samples or thin films during their synthesis considerably affect electrical conductivity. Results of first principles hybrid functional linear combination of atomic orbitals calculations are discussed for hydrogen atoms incorporated in bulk or adsorbed upon non-polar ZnO (1¯ 100) surfaces. The energy of H incorporation, atomic relaxation, electronic density redistribution and modification of the electronic structure are compared for both surface adsorption and bulk absorption. It is shown that hydrogen forms a strong bonding with the surface O ions (Eads = 2.7eV) whereas its incorporation into bulk is energetically quite unfavorable. Hydrogen adsorption reduces the surface energy. Surface hydrogen atoms are very shallow donors, thus contributing to the electronic conductivity and ZnO metallization.

Ab initio calculations of the F centers in MgF2 bulk and on the (001) surface

We present and discuss the results of atomic and electronic structure calculations of the F centers in MgF2 bulk and on the (001) surface. The calculations are based on the B3PW Hartree–Fock and density functional theory hybrid exchange-correlation functional. Most of the electronic density of a missing fluorine ion is localized in the bulk vacancy and a little bit less—in a surface vacancy. It is shown that the electronic F center is a deep donor. The lattice distortion and defect formation energy on the neutral (001) surface and in the bulk are also compared.

Ab initio calculations of the H centers in MgF 2 crystals

MgF2 with rutile structure is important wide-gap optical material with numerous applications. We present and discuss the results of calculations for basic hole defects - interstitial F atoms (called also the colour H centres). This study is based on the large scale ab initio DFT calculations using hybrid B3PW exchange-correlation functional as implemented into CRYSTAL computer code. The electronic structure, atomic geometry, charge density distribution are calculated and discussed.

Effects of Doped Oxygen on ZnWO4 Crystal Luminescence

The results of the study of the spectral characteristics and photo cathodoluminescence zinc tungstate crystals subjected to heat treatment in an oxygen atmosphere, or flows of high-energy irradiation of oxygen ions. It was observed that additional introduction of oxygen leads to decline in luminescence’s effectiveness. It was discovered, that introduction of oxygen by thermal processing leads to changes in excitation spectrum. Decline of excitation effectiveness proportional to increase of excitation quanta energy from 4.5 to 6.5 eV was observed; in the meantime, in the unprocessed crystals it declined by only 25-30%. The effect can be explained by destruction of complex defects with luminescence centers during introduction of additional oxygen. It was demonstrated, that characteristic oxygen’s depth of entry during thermal processing is 20 nm.

Atomic and electronic structure of hydrogen on ZnO (11̄00) surface: ab initio hybrid calculations

Hydrogen atoms unavoidably incorporated into ZnO during growth of bulk samples and thin films considerably affect their electrical conductivity. The results of first principles hybrid LCAO calculations are discussed for hydrogen atoms in the bulk and on the non-polar ZnO (100) surface. The incorporation energy, the atomic relaxation, the electronic density redistribution and the electronic structure modifications are compared for the surface adsorption and bulk interstitial H positions. It is shown that hydrogen has a strong binding with the surface O ions (2.7 eV) whereas its incorporation into bulk is energetically unfavorable. Surface hydrogen atoms are very shallow donors, thus, contributing to the electronic conductivity.

Kr and Xe ion induced aggregation processes in LiF crystals during irradiation and thermal annealing

The efficiency of F and Fn center creation in LiF crystals irradiated with 147 MeV Kr and 195 MeV ions was studied via ion fluence and flux. In the case of Kr ions with flux of 8.9×10 9 ions/(cm 2 ×s) saturation of F centers (~ 10 19 cm -3 ) was observed at a fluence of 10 12 ions/cm 2 . Further irradiation decrease the F center concentration. In LiF irradiated with both ions an enhancement of electron color centers (F and Fn) via flux was observed. In LiF crystals irradiated with Kr ions an enhancement of Fn centers was observed after thermal annealing up to 600 K. The mechanisms of the processes are discussed.