Kee-Joo Chang

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.

Even-Odd Behavior of Conductance in Monatomic Sodium Wires

With the aid of the Friedel sum rule, we perform first-principles calculations of conductances through monatomic Na wires, taking into account the sharp tip geometry and discrete atomic structure of electrodes. We find that conductances (G) depend on the number (L) of atoms in the wires; G is G(0)( = 2e(2)/h) for odd L, independent of the wire geometry, while G is generally smaller than G(0) and sensitive to the wire structure for even L. This even-odd behavior is attributed to the charge neutrality and resonant character due to the sharp tip structure. We suggest that similar even-odd behavior may appear in other monovalent atomic wires.

Essential role of impedance in the formation of acoustic band gaps

We investigate acoustic band gaps (ABGs) in a two-dimensional lattice of cylinders for the cases of constant impedance, Z, and constant velocity, v. ABGs become wider for the case of constant v (varying Z), and become smaller, eventually disappearing in the opposite case. As the volume fraction increases, the upper (bottom) edge of the stop band increases (decreases) and then decreases (increases) in composites with impedance variation only, so that the midgap frequency changes very little and a larger ABG can be created. The upper (bottom) edge of the stop band increases (decreases) when the impedance ratio increases, so that the midgap frequency decreases slightly and the size of the ABG increases.

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.

Structure-Induced Ferromagnetic Stabilization in Free-Standing Hexagonal Fe1.3Ge Nanowires

Single-crystalline free-standing hexagonal Fe(1.3)Ge nanowires (NWs) are synthesized for the first time using a chemical vapor transport process without using any catalyst. Interestingly, Fe(1.3)Ge NWs are found to be ferromagnetic at room temperature, while bulk Fe(1.3)Ge has the lower critical temperature of 200 K. We perform first-principles density functional calculations and suggest that the observed strong ferromagnetism is attributed to the reduced distances between Fe atoms, increased number of Fe-Fe bonds, and the enhanced Fe magnetic moments. Both experimental and theoretical studies show that the magnetic moments are enhanced in the NWs, as compared to bulk Fe(1.3)Ge. We also modulate the composition ratio of as-grown iron germanide NWs by adjusting experimental conditions. It is shown that uniaxial strain on the hexagonal plane also enhances the ferromagnetic stability.

Atomic structure of shallow acceptor- and donor-hydrogen complexes in GaAs

Abstract The microscopic structures of an interstitial hydrogen atom in p- and n-type GaAs are determined using an ab initio pseudopotential method. For Be impurities substituting Ga ions, hydrogen sitting at the bond-center site more strongly bonds to one of the neighboring As atoms than to the acceptor, while a direct donor-H bond takes place in Si-doped samples. This character of hydrogen bonding of passivated SiH complexes is similar to that of SiH bonds in n-type Si. For group VI donors such as S, hydrogen strongly interacts with the neighboring host Ga breaking a donor-Ga bond.

Electric‐field‐enhanced dissociation of the hydrogen‐Si donor complex in GaAs

The passivation and dissociation process of the hydrogen‐Si donor complex in plasma‐hydrogenated GaAs was presented. The temperature dependent values of dissociation frequencies νd which the first‐order kinetics permit, satisfy the relation νd=5.7×1013 exp(−1.79±0.05 eV/kT) s−1 for the no‐biased anneals. During electric‐field‐enhanced anneal experiments, we confirm that no in‐diffusion from the passivated region to the bulk is observed in the temperature ranges below 150 °C, and that there is a dissociation frequency region independent of the annealing temperature. Finally, from the electric field annealing experiment on the passivated donor in n‐type GaAs, it is suggested that the hydrogen atom in Si‐doped GaAs exposed to the plasma hydrogen is negatively charged with the gain of free electrons and passivates the Si donor, and also that the hydrogen atom or the electron of the hydrogen‐Si donor complex can be easily released by the electric field.

Stability and segregation of B and P dopants in Si/SiO2 core-shell nanowires.

Using molecular dynamics simulations, we generate realistic atomic models for oxidized Si nanowires which consist of a crystalline Si core and an amorphous SiO(2) shell. The amorphous characteristics of SiO(2) are well reproduced, as compared to those for bulk amorphous silica. Based on first-principles density functional calculations, we investigate the stability and segregation of B and P dopants near the radial interface between Si and SiO(2). Although substitutional B atoms are more stable in the core than in the oxide, B dopants can segregate to the oxide with the aid of Si self-interstitials which are generated during thermal oxidation. The segregation of B dopants occurs in the form of B interstitials in the oxide, leaving the self-interstitials in the Si core. In the case of P dopants, dopant segregation to the oxide is unfavorable even in the presence of self-interstitials. Instead, we find that P dopants tend to aggregate in the Si region near the interface and may form nearest-neighbor donor pairs, which are energetically more stable than isolated P dopants.

Electronic structure and transport properties of hydrogenated graphene and graphene nanoribbons

The band gap opening is one of the important issues in applications of graphene and graphene nanoribbons (GNRs) to nanoscale electronic devices. As hydrogen strongly interacts with graphene and creates short-range disorder, the electronic structure is significantly modified by hydrogenation. Based on first-principles and tight-binding calculations, we investigate the electronic and transport properties of hydrogenated graphene and GNRs. In disordered graphene with low doses of H adsorbates, the low-energy states near the neutrality point are localized, and the degree of localization extends to high-energy states with increasing adsorbate density. To characterize the localization of eigenstates, we examine the inverse participation ratio and find that the localization is greatly enhanced for the defect levels, which are accumulated around the neutrality point. Our calculations support the previous result that even with a low dose of H adsorbates, graphene undergoes a metal–insulator transition. In GNRs, relaxations of the edge C atoms play a role in determining the edge structure and the hydrocarbon conformation at low and high H concentrations, respectively. In disordered nanoribbons, we find that the energy states near the neutrality point are localized and conductances through low-energy channels decay exponentially with sample size, similar to disordered graphene. For a given channel energy, the localization length tends to decrease as the adsorbate density increases. Moreover, the energy range of localization exceeds the intrinsic band gap.