E.-A. Choi

O-vacancy as the origin of negative bias illumination stress instability in amorphous In–Ga–Zn–O thin film transistors

We find that O-vacancy (VO) acts as a hole trap and plays a role in negative bias illumination stress instability in amorphous In–Ga–Zn–O thin film transistors. Photoexcited holes drift toward the channel/dielectric interface due to small potential barriers and can be captured by VO in the dielectrics. While some of VO+2 defects are very stable at room temperature, their original deep states are recovered via electron capture upon annealing. We also find that VO+2 can diffuse in amorphous phase, inducing hole accumulation near the interface under negative gate bias.

Structural and electronic properties of crystalline InGaO3 (ZnO)(m)

Based on theoretical calculations, we find that the crystal structure of InGaO3(ZnO)m consists of an alternating stack of a wurtzite (Ga∕Zn)–O block and an In–O octahedral layer. In the (Ga∕Zn)–O block, the Ga atoms favor a modulated boundary structure against a flat boundary structure. The band spectrum shows that hole carriers are spatially confined whereas electrons move more freely through the whole crystal. The characteristics of a superlattice structure appears especially in the flat boundary structure. The band gap decreases with m due to the reduction in the quantum confinement effect.

Enhanced electron-mediated ferromagnetism in Co-doped ZnO nanowires

We perform density functional calculations to investigate the magnetic coupling properties of Co-doped ZnO nanowires (NWs). The ferromagnetism of NWs is strongly affected by the position of the minority Co ta levels and their population that is controlled by additional electron doping. While the antiferromagnetic state is energetically more favorable than the ferromagnetic state in carrier-free NWs, electron doping greatly enhances the stability of ferromagnetism. Compared with bulk ZnO, the minority ta levels relative to the conduction band edge have a tendency to decrease with decreasing of the wire diameter, indicating that electron concentrations to achieve the ferromagnetism are much reduced. The short-range nature of the magnetic coupling between two Co ions suggests that sufficiently high doping levels of the Co ions are needed to yield ferromagnetic NWs.

The effect of impurities on hydrogen bonding site and local vibrational frequency in ZnO

For isovalent impurities such as Be, Mg, Ca, Sr, and Cd and group-I element such as Na in ZnO, first-principles local-density-functional calculations show that the interstitial position of H depends on the type of impurities, either occupying a bond center (BC) site or an antibonding (AB) site adjacent to the impurity atom. The AB site is more favorable in the vicinity of Na, Ca, Sr, and Cd, while the stable position is the BC site in the case of Be. We find that both electronegativity and atomic size play a role in switching the H interstitial position between the BC and AB sites. Previous studies have suggested that two infrared lines observed at 3611 and 3326 cm−1 result from hydrogen atoms positioned at BC and AB sites, respectively. The results for the H bonding sites and defect concentrations suggest that Ca is the most probable impurity as the origin of the infrared line at 3326 cm−1. However, for impurities around which H is positioned at the AB site, the calculated local vibrational frequencies a...

Local bonding effect on the defect states of oxygen vacancy in amorphous HfSiO4

We perform first-principles calculations to investigate the defect properties of O vacancies in amorphous HfSiO4. For atomic models generated from molecular dynamics simulations, we find that O vacancies, which have only Hf atoms or a mixture of Hf and Si in the neighborhood, behave as charge trap centers, similar to those in HfO2. On the other hand, O vacancies surrounded by only Si atoms are energetically most favorable and have very high trap energies for both electron and hole carriers. Thus, these defects are suggested to be responsible for the reduction of threshold voltage instability.

The Electronic Structure of Oxygen Vacancy in Amorphous HfSiO4

We perform first‐principles density functional calculations to investigate the electronic structure of O‐vacancy in amorphous HfSiO4. We find that O‐vacancies surrounded only by Si atoms are energetically most favorable and exhibit the defect levels similar to those of α‐quartz SiO2. When O‐vacancies have a mixture of Si and Hf atoms or only Hf atoms in the neighborhood, they behave as charge traps, similar to those in HfO2. We suggest that Si‐surrounded O‐vacancy defects play a role in improving the threshold voltage instability in HfSiO4‐based metal‐oxide‐semiconductor devices.

The effect of Mn ions on the growth of cubic GaN

We perform first‐principles pseudopotential calculations to investigate the effect of Mn delta‐doping on the stability of the zinc‐blende and wurtzite structures in GaN. In bulk GaN, it is possible for the zinc‐blende structure to become a stable phase againt the wurtzite one with increasing the concentration of the delta‐doped Mn ions. On GaN surfaces, it costs very high energies to form inversion domains with a Mn exposure because the Mn–N bond is not strong enough to stabilize the inverted structures. In the initial growth of GaN, we also examine the formation of stacking faults by the Mn delta‐doping, which generate cubic bonds, and find that the stacking faults easily occur on the Ga‐polar surface rather than on the N‐polar one. On the Ga‐polar surface, the Mn ions tend to remain on the surface than in the bulk region, suggesting that these ions act as a surfactant.