Sungjin Kim

Hidden structural order in orthorhombic Ta2O5.

We investigate using first-principles calculations the atomic structure of the orthorhombic phase of Ta2O5. Although this structure has been studied for decades, the correct structural model is controversial owing to the complication of structural disorder. We identify a new low-energy highsymmetry structural model where all Ta and O atoms have correct formal oxidation states of +5 and −2, respectively, and the experimentally reported triangular lattice symmetry of the Ta sublattice appears dynamically at finite temperatures. To understand the complex atomic structure of the Ta2O3 plane, a triangular graph-paper representation is devised and used alongside oxidation state analysis to reveal infinite variations of the low-energy structural model. The structural disorder of Ta2O5 observed in experiments is attributed to the intrinsic structural variations, and oxygen vacancies that drive collective relaxation of the O sublattice.

Influence of V-pits on the efficiency droop in InGaN/GaN quantum wells

We discuss the influence of V-pits and their energy barrier, originating from its facets of (101¯1) planes, on the luminescence efficiency of InGaN LEDs. Experimental analysis using cathodoluminescence (CL) exhibits that thin facets of V-pits of InGaN quantum wells (QWs) appear to be effective in improving the emission intensity, preventing the injected carriers from recombining non-radiatively with threading dislocations (TDs). Our theoretical calculation based on the self-consistent approach with adopting k⋅p method reveals that higher V-pit energy barrier heights in InGaN QWs more efficiently suppress the non-radiative recombination at TDs, thus enhancing the internal quantum efficiency (IQE).

Highly area efficient and cost effective double stacked S/sup 3/ (stacked single-crystal Si) peripheral CMOS SSTFT and SRAM cell technology for 512M bit density SRAM

For the first time, the highest density SRAM, such as 512M bit SRAM, is developed by implementing the smallest 25F/sup 2/S/sup 3/ SRAM cell technology, whose cell size is 0.16/spl mu/m/sup 2/, and area saving peripheral SSTFT (stacked single-crystal thin film transistor) technology. The SSTFT are used as the peripheral CMOS transistors as well as the cell transistors to save area to make the SRAM products comparative to the DRAM cell based products in the density and the cost. In the S/sup 3/ SRAM cell, the load PMOS and pass NMOS transistors are stacked over the planar pull-down NMOS transistors to drastically reduce the cell size. Also, in a periphery, the core logic transistors are stacked on the ILD to save the layout area for maximizing cell efficiency for the products.

Ferroelectric Coupling Effect on the Energy‐Band Structure of Hybrid Heterojunctions with Self‐Organized P(VDF‐TrFE) Nanomatrices

Ferroelectric coupling effects on the energy-band structure of hybrid heterojunctions are investigated using hybrid photovoltaic devices with poly(3-hexylthiophene-2,5-diyl) (P3HT)/ZnO and poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)). The self-organized P(VDF-TrFE):P3HT photoactive layer forms a novel architecture consisting of P3HT domains in a P(VDF-TrFE) matrix. The energy-band structure at the interface of the p-n heterojunction is tuned by artificial control of the ferroelectric polarization of the P(VDF-TrFE) material, consequently modulating the photovoltaic performance of the hybrid photovoltaic devices.

ZnO nanotips and nanorods on carbon nanotube/Si substrates: anomalous p-type like optical properties of undoped ZnO nanotips

ZnO nanotips and nanorods were grown on screen-printed multi-walled carbon nanotube (MWCNT) films via thermal chemical vapor deposition at relative low growth temperatures of 400 and 500u2009°C. Uniform formation of ZnO nanotips and nanorods occurred on MWCNT-printed Si substrates, but were rarely observed on bare Si substrates at the same growth temperatures. In photoluminescence (PL) measurements, it was found that ZnO nanorods exhibit typical intrinsic optical properties, while ZnO nanotips revealed p-type like luminescence behavior. Acceptor-related emission bands originating from neutral acceptor-bound exciton, free-to-acceptor and donor-acceptor pair transitions are clearly observed in temperature-dependent PL spectra of ZnO nanotips.

Quantum efficiency affected by localized carrier distribution near the V-defect in GaN based quantum well

It is known that due to the formation of in-plane local energy barrier, V-defects can screen the carriers which non-radiatively recombine in threading dislocations (TDs) and hence, enhance the internal quantum efficiency in GaN based light-emitting diodes. By a theoretical modeling capable of describing the inhomogeneous carrier distribution near the V-defect in GaN based quantum wells, we show that the efficient suppression of non-radiative (NR) recombination via TD requires the local energy barrier height of V-defect larger than ∼80u2009meV. The NR process in TD combined with V-defect influences the quantum efficiency mainly in the low injection current density regime suitably described by the linear dependence of carrier density. We provide a simple phenomenological expression for the NR recombination rate based on the model result.

Effects of external surface charges on the enhanced piezoelectric potential of ZnO and AlN nanowires and nanotubes

We theoretically investigate external surface charge effects on piezoelectric potential of ZnO and AlN nanowires (NWs) and nanotubes (NTs) under uniform compression. The free carrier depletion caused by negative surface charges via surface functionalization on vertically compressed ZnO and AlN NWs/NTs is simulated using finite element calculation; this indicates the enhancement of piezoelectric potential is due to the free carriers (electrons) being fully depleted at the critical surface charge density. Numerical simulations reveal that full coverage of surface charges surrounding the NTs increases the piezoelectric output potential exponentially within a relatively smaller range of charge density compared to the case of NWs for a typical donor concentration (∼1017 cm−3). The model can be used to design functional high-power semiconducting piezoelectric nanogenerators.

Semimetal-antiferromagnetic insulator transition in graphene induced by biaxial strain

We report first-principles calculations on antiferromagnetic spin ordering in graphene under biaxial strain. Using hybrid functional calculations, we found that semimetallic graphene sheets undergo a transition to antiferromagnetic insulators at a biaxial strain of 7.7% and that the band gap rapidly increases after the onset of this transition before reaching 0.9 eV at a biaxial strain of 12%. We examined the competition of the antiferromagnetic spin ordering with two-dimensional Peierls distortions upon biaxial strain, and found that the preceding antiferromagnetic insulator phase impedes the Peierls insulator phase. The antiferromagnetic insulator phase is destabilized upon carrier filling but robust up to moderate carrier densities. This work indicates that biaxially strained graphene represents a noble system where the electron-electron and electron-lattice interactions compete with each other in a simple but nontrivial way.

Polarization characteristics of semipolar (112̄2) InGaN/GaN quantum well structures grown on relaxed InGaN buffer layers and comparison with experiment

Partial strain relaxation effects on polarization ratio of semipolar (112̄2) InxGa1−xN/GaN quantum well (QW) structures grown on relaxed InGaN buffers were investigated using the multiband effective-mass theory. The absolute value of the polarization ratio gradually decreases with increasing In composition in InGaN buffer layer when the strain relaxation ratio (ε0y′y′−εy′y′)/ε0y′y′ along y′-axis is assumed to be linearly proportional to the difference of lattice constants between the well and the buffer layer. Also, it changes its sign for the QW structure grown on InGaN buffer layer with a relatively larger In composition (x > 0.07). These results are in good agreement with the experiment. This can be explained by the fact that, with increasing In composition in the InGaN subsrate, the spontaneous emission rate for the y′-polarization gradually increases while that for x′-polarization decreases due to the decrease in a matrix element at the band-edge (k‖ = 0).

Core–shell Si1−xGex nanowires with controlled structural defects for phonon scattering enhancement

We demonstrate for the first time core–shell Si1−xGex alloy nanowires that can suppress the phonon propagation in nanowires without reducing the electrical conductivity. Non-uniformly distributed structural defects in the outer shells of the Si1−xGex nanowires enhance boundary scattering during phonon transport, while a defect-free core provides a current path for electrical carriers.