Byungki Ryu

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.

Defects Responsible for the Hole Gas in Ge/Si Core-Shell Nanowires

The origin of the ballistic hole gas recently observed in Ge/Si core-shell nanowires has not been clearly resolved yet, although it is thought to be the result of the band offset at the radial interface. Here we perform spin-polarized density-functional calculations to investigate the defect levels of surface dangling bonds and Au impurities in the Si shell. Without any doping strategy, we find that Si dangling bond and substitutional Au defects behave as charge traps, generating hole carriers in the Ge core, while their defect levels are very deep in one-component Si nanowires. The defect levels lie to within 10 meV from or below the valence band edge for nanowires with diameters larger than 33 A and the Ge fractions above 30%. As carriers are spatially separated from charge traps, scattering is greatly suppressed, leading to the ballistic conduction, in good agreement with experiments.

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.

Substantial enhancement in intrinsic coercivity on M-type strontium hexaferrite through the increase in magneto-crystalline anisotropy by co-doping of group-V and alkali elements

The effect of d1 impurity doping in Sr-hexaferrite (SrM) on the magnetic anisotropy is investigated. First-principles calculations revealed that group-V elements (V, Nb) are stabilized with co-doping of alkali elements. Na1+/K1+ doping at Sr2+-site is found to be critical to form the d1 impurities at Fe-site. Experimentally, Na–V doped SrM shows the intrinsic coercivity of ∼5.4 kOe, which is ∼300% enhancement compared to undoped SrM and comparable value to La–Co co-doped SrM. Finally, the spin-orbit coupling from non-vanishing angular momentum of d1 impurity in SrM should be a main factor for such a substantial improvement of intrinsic coercivity.

Defects responsible for the Fermi level pinning in n+ poly-Si/HfO2 gate stacks

Based on density functional calculations, we propose a defect model that can explain flat band voltage shifts, especially in n+ poly-Si/HfO2 gate stacks. For two interface structures, with Si electrodes on top of crystalline and amorphous HfO2, we find the formation of O-vacancies at the interface, which exhibit weak Si–Si dimer bonds and low formation energies, very different from those in the oxide. Due to weak dimer bonds, charge trap levels lie near the Si conduction band edge, leading to the Fermi level pinning and flat band voltage shifts in n+ poly-Si gate electrodes.

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.

First-principles study of the electronic structure of aluminate nanotubes

We report the results of first-principles theoretical calculations for the electronic structure of aluminate nanotubes. A tubular structure in the form of AlO2 is energetically stable and exhibits metallic conduction. Due to weak interactions between Li atoms and nanotubes, Li doping does not alter the stability of AlO2 nanotubes and only increases the Fermi level. On the other hand, stable AlO nanotubes can be obtained by hole doping with Be and Mg impurities.

Enhancing Thermoelectric Performances of Bismuth Antimony Telluride via Synergistic Combination of Multiscale Structuring and Band Alignment by FeTe2 Incorporation

It has been a difficulty to form well-distributed nano- and mesosized inclusions in a Bi2Te3-based matrix and thereby realizing no degradation of carrier mobility at interfaces between matrix and inclusions for high thermoelectric performances. Herein, we successfully synthesize multistructured thermoelectric Bi0.4Sb1.6Te3 materials with Fe-rich nanoprecipitates and sub-micron FeTe2 inclusions by a conventional solid-state reaction followed by melt-spinning and spark plasma sintering that could be a facile preparation method for scale-up production. This study presents a bismuth antimony telluride based thermoelectric material with a multiscale structure whose lattice thermal conductivity is drastically reduced with minimal degradation on its carrier mobility. This is possible because a carefully chosen FeTe2 incorporated in the matrix allows its interfacial valence band with the matrix to be aligned, leading to a significantly improved p-type thermoelectric power factor. Consequently, an impressively high thermoelectric figure of merit ZT of 1.52 is achieved at 396 K for p-type Bi0.4Sb1.6Te3-8 mol % FeTe2, which is a 43% enhancement in ZT compared to the pristine Bi0.4Sb1.6Te3. This work demonstrates not only the effectiveness of multiscale structuring for lowering lattice thermal conductivities, but also the importance of interfacial band alignment between matrix and inclusions for maintaining high carrier mobilities when designing high-performance thermoelectric materials.

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.