Computational studies of conductivity in wide-band-gap semiconductors and oxides

First-principles investigation of positively charged and neutral oxygen vacancies in amorphous silica

The structural parameters, electron localization functions, electron paramagnetic resonance (EPR) parameters, formation energies, and thermodynamic transition levels of various oxygen vacancy defects in amorphous silica are comprehensively and integrally investigated by using density functional theory. The trends of changes in the oxygen vacancy defect structure and electron localization induced by the increase in distance between defective silicon atoms are clearly identified. It is shown that the dimer configuration may be the potential structure of the Eδ' center. For the back-projected unpuckered configuration and the puckered configuration, whose EPR parameters are more consistent with the experimental values of the Eγ' center, the unpaired electron localized on the sp3 hybridized silicon atom is a common feature. Due to the three-coordinated oxygen atom in the forward-oriented configuration, the EPR parameters are closest to those of the Eα' center. Transformations of oxygen vacancy defects under different charge states are studied by sequentially adding and removing electrons. The thermodynamic transition level analysis reveals that the dimer and forward configurations may behave as deep traps for electron accumulation. The back-projected puckered fourfold-coordinated and fivefold-coordinated configurations are comparatively stable and may be able to function as shallow traps for electron transport. The neutral double unpuckered, neutral back-projected puckered fourfold-coordinated, and neutral back-projected unpuckered configurations are more likely to lose electrons during hole trapping. As the bias voltage is repeatedly changed, the defect density of the puckered configuration may reduce, while that of the dimer and unpuckered configuration may take an opposite trend.

Ferromagnetic Behavior and Magneto-Optical Properties of Semiconducting Co-Doped ZnO

ZnO is a well-known semiconducting material showing a wide bandgap and an n-type intrinsic behavior of high interest in applications such as transparent electronics, piezoelectricity, optoelectronics, and photovoltaics. This semiconductor becomes even more attractive when doped with a few atomic percent of a transition metal. Indeed, e.g., the introduction of substitutional Co atoms in ZnO (ZCO) induces the appearance of room temperature ferromagnetism (RT-FM) and magneto-optical effects, making this material one of the most important representatives of so-called dilute magnetic semiconductors (DMSs). In the present review, we discuss the magnetic and magneto-optical properties of Co-doped ZnO thin films by considering also the significant improvements in the properties induced by post-growth irradiation with atomic hydrogen. We also show how all of these properties can be accounted for by a theoretical model based on the formation of Co-V_O (oxygen vacancy) complexes and the concurrent presence of shallow donor defects, thus giving a sound support to this model to explain the RT-FM in ZCO DMSs.

Electron and hole stability in GaN and ZnO

We assess the thermodynamic doping limits of GaN and ZnO on the basis of point defect calculations performed using the embedded cluster approach and employing a hybrid non-local density functional for the quantum mechanical region. Within this approach we have calculated a staggered (type-II) valence band alignment between the two materials, with the N 2p states contributing to the lower ionization potential of GaN. With respect to the stability of free electron and hole carriers, redox reactions resulting in charge compensation by ionic defects are found to be largely endothermic (unfavourable) for electrons and exothermic (favourable) for holes, which is consistent with the efficacy of electron conduction in these materials. Approaches for overcoming these fundamental thermodynamic limits are discussed.