Jongseob Kim - Academia.edu (original) (raw)
Papers by Jongseob Kim
ACS Applied Materials & Interfaces, Feb 21, 2012
We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxi... more We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxide (ZnO) nanowires by molecular adsorption. We find that spontaneous dissociative adsorption of fluorine molecules introduces halfemptying of otherwise fully filled oxygen-derived surface states. The resulting surface Fermi level is so close to the valence band maximum of the ZnO nanowire that the nanowire undergoes significant p-type charge transfer doping. Those half-filled surface states are fully spin-polarized and lead to surface ferromagnetism that is stable at room temperature. We also analyze the kinetic control regime of the surface transfer doping and find that it may result in nonequilibrium steady states. The present results suggest that postgrowth engineering of surface states has high potential in manipulating ZnO nanostructures useful for both electronics and spintronics.
Nano Letters, 2008
One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the e... more One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.
Electronic Materials Letters, 2014
ABSTRACT Normally-off AlGaN/GaN HEMTs have been fabricated by employing a recessed-gate structure... more ABSTRACT Normally-off AlGaN/GaN HEMTs have been fabricated by employing a recessed-gate structure and oxygen plasma treatment and outstanding improvement of V th variation is observed. The origin of the observed positive V th shift and reduced variation window induced by oxygen plasma treatment is investigated by computational methods. Formation energy calculations for oxygen inclusions in III-N reveal that a negatively charged V Al - O N complex in the AlGaN passivation layer can be a major source of V th variation in AlGaN/GaN hetero-structured devices. Calculated trap energy levels are used as the parameters of a device simulation, which indicated that significant V th variation can be induced by a small fluctuation in the AlGaN layer thickness and defect densities. Our theoretical investigation shows that normally-off AlGaN/GaN HEMTs having reliable V th variation can be produced by oxygen inclusions accompanying a recessed-gate structure.
2006 8th International Conference on Solid-State and Integrated Circuit Technology Proceedings, 2006
Abstract The device characteristics of Si-nanowire FET (Si-NWFET) are investigated with non-equil... more Abstract The device characteristics of Si-nanowire FET (Si-NWFET) are investigated with non-equilibrium Green's function (NEGF) method. In this study, we characterize the effect of channel modulation by engineering dopant profiles, oxide thickness, and corner rounding ...
MRS Proceedings, 2005
ABSTRACT The interests of low-k dielectric materials to reduce capacitance in multilevel metal in... more ABSTRACT The interests of low-k dielectric materials to reduce capacitance in multilevel metal interconnects of integrated circuits are well known in the semiconductor industry. Mechanical properties of low-k film are currently the main issues. Improved hardness and modulus are desirable because, when building a multilayered stack and doing sequential processing, films go through chemical mechanical planarization. In this proceeding, we reports the Young's moduli of the typical low k materials, and the effects of various factors for Young's moduli of materials, such as, structures of precursors, density, and porosity. Using atomistic molecular dynamics simulation with experimental measurements, the Young's moduli of films of amorphous silicon oxide in which 25% of Si-O-Si chains were replaced by Si-(CH3 H3C)-Si, Si-CH2-Si, Si-(CH2)2-Si, Si-(CH2)3-Si, Si-(CH2)4-Si, Si-(CH2)6-Si, were measured and analyzed. The predicted trends of Young's moduli of films formed by above precursors are in good consistent with those observed from experiments. The Young's moduli of materials are largely dependent on the densities of materials. Young's modulus of material increases as the density of the material increases. The chemical properties, chain length, and connectivity of material take effects on the Young's modulus of material. Given the same densities of material the smaller number of cavities per unit volume the material has, the lower Young's modulus it shows. Based on the results, the method of predict mechanical properties of materials by the conjunction of basic experimental measurements and atomistic simulation will be discussed.
2006 8th International Conference on Solid-State and Integrated Circuit Technology Proceedings, 2006
The reliability issues, including 100k cycle's endurance and 2 hours high temperature storag... more The reliability issues, including 100k cycle's endurance and 2 hours high temperature storage (HTS: 150oC, 200oC and 250oC) of sub-90nm NAND Flash cells, are studied. Furthermore, the trap generation models in endurance and interface trap recovery model in HTS are ...
Nano letters, Jan 14, 2015
The dependency of dopant-distributions on channel diameters in realistically sized, highly phosph... more The dependency of dopant-distributions on channel diameters in realistically sized, highly phosphorus-doped silicon nanowires is investigated with an atomistic tight-binding approach coupled to self-consistent Schrödinger-Poisson simulations. By overcoming the limit in channel sizes and doping densities of previous studies, this work examines electronic structures and electrostatics of free-standing circular silicon nanowires that are phosphorus-doped with a high density of ∼2 × 10(19) cm(-3) and have 12 nm-28 nm cross-sections. Results of analysis on the channel energy indicate that the uniformly distributed dopant profile would be hardly obtained when the nanowire cross-section is smaller than 20 nm. Insufficient room to screen donor ions and shallower impurity bands are the primary reasons of the nonuniform dopant-distributions in smaller nanowires. Being firmly connected to the recent experimental study (Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 15254-15258), this work establishe...
physica status solidi (RRL) - Rapid Research Letters, 2010
Nano Letters, 2010
We investigate peculiar dopant deactivation behaviors of Si nanostrucures with first principle ca... more We investigate peculiar dopant deactivation behaviors of Si nanostrucures with first principle calculations and reveal that surface dangling bonds (SDBs) on Si nanostructures could be fundamental obstacles in nanoscale doping. In contrast to bulk Si, as the size of Si becomes smaller, SDBs on Si nanostructures prefer to be charged and asymmetrically deactivate n- and p-type doping. The asymmetric dopant deactivation in Si nanostructures is ascribed to the preference for negatively charged SDBs as a result of a larger quantum confinement effect on the conduction band. On the basis of our results, we show that the control of the growth direction of silicon nanowire as well as surface passivation is very important in preventing dopant deactivation.
The Journal of Physical Chemistry Letters, 2014
One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photov... more One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photovoltaic cell is its long carrier lifetime. We investigate the role of the intrinsic defects of CH3NH3PbI3 on its outstanding photovoltaic properties using density-functional studies. Two types of defects are of interest, i.e., Schottky defects and Frenkel defects. Schottky defects, such as PbI2 and CH3NH3I vacancy, do not make a trap state, which can reduce carrier lifetime. Elemental defects like Pb, I, and CH3NH3 vacancies derived from Frenkel defects act as dopants, which explains the unintentional doping of methylammonium lead halides (MALHs). The absence of gap states from intrinsic defects of MALHs can be ascribed to the ionic bonding from organic-inorganic hybridization. These results explain why the perovskite MALHs can be an efficient semiconductor, even when grown using simple solution processes. It also suggests that the n-/p-type can be efficiently manipulated by controlling growth processes.
![Research paper thumbnail of Ab initio investigations on the HOSO[sub 2]+O[sub 2]→SO[sub 3]+HO[sub 2] reaction](https://attachments.academia-assets.com/43771545/thumbnails/1.jpg)
](The Journal of Chemical Physics, 2000
HOSO 2 radical is the key intermediate for the oxidation SO 2 to SO 3 by OH radical in the atmosp... more HOSO 2 radical is the key intermediate for the oxidation SO 2 to SO 3 by OH radical in the atmosphere. The structural aspects and the energetics of the reaction HOSO 2 ϩO 2 →SO 3 ϩHO 2 have been studied using Mo "ller-Plesset ͑MP2͒ and density functional ͑DFT͒ techniques with 6-31G** and triple-, quadruple-, and quintuple-quality basis sets including diffuse basis functions. The detailed theoretical analyses have further revealed that this reaction could proceed through the formation of intermediate complexes and an intramolecular proton transfer like transition state. The energetics of these intermediate reactions has been studied in detail. The use of MP2 methods to study such radical mechanisms had some characteristic symmetry-breaking problem with larger basis sets. This unphysical situation with larger basis set MP2 calculations in this hypervalent system has been explained through the interpretation of the relevant energy surface.
ACS Applied Materials & Interfaces, 2012
We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxi... more We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxide (ZnO) nanowires by molecular adsorption. We find that spontaneous dissociative adsorption of fluorine molecules introduces half-emptying of otherwise fully filled oxygen-derived surface states. The resulting surface Fermi level is so close to the valence band maximum of the ZnO nanowire that the nanowire undergoes significant p-type charge transfer doping. Those half-filled surface states are fully spin-polarized and lead to surface ferromagnetism that is stable at room temperature. We also analyze the kinetic control regime of the surface transfer doping and find that it may result in nonequilibrium steady states. The present results suggest that postgrowth engineering of surface states has high potential in manipulating ZnO nanostructures useful for both electronics and spintronics.
We investigate using first-principles calculations the atomic structure of the orthorhombic phase... more We investigate using first-principles calculations the atomic structure of the orthorhombic phase of Ta 2 O 5 . 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 high-symmetry structural model, where all Ta and O atoms have the correct formal oxidation states of þ5 and À2, 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 Ta 2 O 3 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 Ta 2 O 5 observed in experiments is attributed to the intrinsic structural variations, and oxygen vacancies that drive the collective relaxation of the O sublattice.
ACS Applied Materials & Interfaces, Feb 21, 2012
We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxi... more We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxide (ZnO) nanowires by molecular adsorption. We find that spontaneous dissociative adsorption of fluorine molecules introduces halfemptying of otherwise fully filled oxygen-derived surface states. The resulting surface Fermi level is so close to the valence band maximum of the ZnO nanowire that the nanowire undergoes significant p-type charge transfer doping. Those half-filled surface states are fully spin-polarized and lead to surface ferromagnetism that is stable at room temperature. We also analyze the kinetic control regime of the surface transfer doping and find that it may result in nonequilibrium steady states. The present results suggest that postgrowth engineering of surface states has high potential in manipulating ZnO nanostructures useful for both electronics and spintronics.
Nano Letters, 2008
One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the e... more One of the major challenges toward Si nanowire (SiNW) based photonic devices is controlling the electronic band structure of the Si nanowire to obtain a direct band gap. Here, we present a new strategy for controlling the electronic band structure of Si nanowires. Our method is attributed to the band structure modulation driven by uniaxial strain. We show that the band structure modulation with lattice strain is strongly dependent on the crystal orientation and diameter of SiNWs. In the case of [100] and [111] SiNWs, tensile strain enhances the direct band gap characteristic, whereas compressive strain attenuates it. [110] SiNWs have a different strain dependence in that both compressive and tensile strain make SiNWs exhibit an indirect band gap. We discuss the origin of this strain dependence based on the band features of bulk silicon and the wave functions of SiNWs. These results could be helpful for band structure engineering and analysis of SiNWs in nanoscale devices.
Electronic Materials Letters, 2014
ABSTRACT Normally-off AlGaN/GaN HEMTs have been fabricated by employing a recessed-gate structure... more ABSTRACT Normally-off AlGaN/GaN HEMTs have been fabricated by employing a recessed-gate structure and oxygen plasma treatment and outstanding improvement of V th variation is observed. The origin of the observed positive V th shift and reduced variation window induced by oxygen plasma treatment is investigated by computational methods. Formation energy calculations for oxygen inclusions in III-N reveal that a negatively charged V Al - O N complex in the AlGaN passivation layer can be a major source of V th variation in AlGaN/GaN hetero-structured devices. Calculated trap energy levels are used as the parameters of a device simulation, which indicated that significant V th variation can be induced by a small fluctuation in the AlGaN layer thickness and defect densities. Our theoretical investigation shows that normally-off AlGaN/GaN HEMTs having reliable V th variation can be produced by oxygen inclusions accompanying a recessed-gate structure.
2006 8th International Conference on Solid-State and Integrated Circuit Technology Proceedings, 2006
Abstract The device characteristics of Si-nanowire FET (Si-NWFET) are investigated with non-equil... more Abstract The device characteristics of Si-nanowire FET (Si-NWFET) are investigated with non-equilibrium Green's function (NEGF) method. In this study, we characterize the effect of channel modulation by engineering dopant profiles, oxide thickness, and corner rounding ...
MRS Proceedings, 2005
ABSTRACT The interests of low-k dielectric materials to reduce capacitance in multilevel metal in... more ABSTRACT The interests of low-k dielectric materials to reduce capacitance in multilevel metal interconnects of integrated circuits are well known in the semiconductor industry. Mechanical properties of low-k film are currently the main issues. Improved hardness and modulus are desirable because, when building a multilayered stack and doing sequential processing, films go through chemical mechanical planarization. In this proceeding, we reports the Young's moduli of the typical low k materials, and the effects of various factors for Young's moduli of materials, such as, structures of precursors, density, and porosity. Using atomistic molecular dynamics simulation with experimental measurements, the Young's moduli of films of amorphous silicon oxide in which 25% of Si-O-Si chains were replaced by Si-(CH3 H3C)-Si, Si-CH2-Si, Si-(CH2)2-Si, Si-(CH2)3-Si, Si-(CH2)4-Si, Si-(CH2)6-Si, were measured and analyzed. The predicted trends of Young's moduli of films formed by above precursors are in good consistent with those observed from experiments. The Young's moduli of materials are largely dependent on the densities of materials. Young's modulus of material increases as the density of the material increases. The chemical properties, chain length, and connectivity of material take effects on the Young's modulus of material. Given the same densities of material the smaller number of cavities per unit volume the material has, the lower Young's modulus it shows. Based on the results, the method of predict mechanical properties of materials by the conjunction of basic experimental measurements and atomistic simulation will be discussed.
2006 8th International Conference on Solid-State and Integrated Circuit Technology Proceedings, 2006
The reliability issues, including 100k cycle's endurance and 2 hours high temperature storag... more The reliability issues, including 100k cycle's endurance and 2 hours high temperature storage (HTS: 150oC, 200oC and 250oC) of sub-90nm NAND Flash cells, are studied. Furthermore, the trap generation models in endurance and interface trap recovery model in HTS are ...
Nano letters, Jan 14, 2015
The dependency of dopant-distributions on channel diameters in realistically sized, highly phosph... more The dependency of dopant-distributions on channel diameters in realistically sized, highly phosphorus-doped silicon nanowires is investigated with an atomistic tight-binding approach coupled to self-consistent Schrödinger-Poisson simulations. By overcoming the limit in channel sizes and doping densities of previous studies, this work examines electronic structures and electrostatics of free-standing circular silicon nanowires that are phosphorus-doped with a high density of ∼2 × 10(19) cm(-3) and have 12 nm-28 nm cross-sections. Results of analysis on the channel energy indicate that the uniformly distributed dopant profile would be hardly obtained when the nanowire cross-section is smaller than 20 nm. Insufficient room to screen donor ions and shallower impurity bands are the primary reasons of the nonuniform dopant-distributions in smaller nanowires. Being firmly connected to the recent experimental study (Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 15254-15258), this work establishe...
physica status solidi (RRL) - Rapid Research Letters, 2010
Nano Letters, 2010
We investigate peculiar dopant deactivation behaviors of Si nanostrucures with first principle ca... more We investigate peculiar dopant deactivation behaviors of Si nanostrucures with first principle calculations and reveal that surface dangling bonds (SDBs) on Si nanostructures could be fundamental obstacles in nanoscale doping. In contrast to bulk Si, as the size of Si becomes smaller, SDBs on Si nanostructures prefer to be charged and asymmetrically deactivate n- and p-type doping. The asymmetric dopant deactivation in Si nanostructures is ascribed to the preference for negatively charged SDBs as a result of a larger quantum confinement effect on the conduction band. On the basis of our results, we show that the control of the growth direction of silicon nanowire as well as surface passivation is very important in preventing dopant deactivation.
The Journal of Physical Chemistry Letters, 2014
One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photov... more One of the major merits of CH3NH3PbI3 perovskite as an efficient absorber material for the photovoltaic cell is its long carrier lifetime. We investigate the role of the intrinsic defects of CH3NH3PbI3 on its outstanding photovoltaic properties using density-functional studies. Two types of defects are of interest, i.e., Schottky defects and Frenkel defects. Schottky defects, such as PbI2 and CH3NH3I vacancy, do not make a trap state, which can reduce carrier lifetime. Elemental defects like Pb, I, and CH3NH3 vacancies derived from Frenkel defects act as dopants, which explains the unintentional doping of methylammonium lead halides (MALHs). The absence of gap states from intrinsic defects of MALHs can be ascribed to the ionic bonding from organic-inorganic hybridization. These results explain why the perovskite MALHs can be an efficient semiconductor, even when grown using simple solution processes. It also suggests that the n-/p-type can be efficiently manipulated by controlling growth processes.
The Journal of Chemical Physics, 2000
HOSO 2 radical is the key intermediate for the oxidation SO 2 to SO 3 by OH radical in the atmosp... more HOSO 2 radical is the key intermediate for the oxidation SO 2 to SO 3 by OH radical in the atmosphere. The structural aspects and the energetics of the reaction HOSO 2 ϩO 2 →SO 3 ϩHO 2 have been studied using Mo "ller-Plesset ͑MP2͒ and density functional ͑DFT͒ techniques with 6-31G** and triple-, quadruple-, and quintuple-quality basis sets including diffuse basis functions. The detailed theoretical analyses have further revealed that this reaction could proceed through the formation of intermediate complexes and an intramolecular proton transfer like transition state. The energetics of these intermediate reactions has been studied in detail. The use of MP2 methods to study such radical mechanisms had some characteristic symmetry-breaking problem with larger basis sets. This unphysical situation with larger basis set MP2 calculations in this hypervalent system has been explained through the interpretation of the relevant energy surface.
ACS Applied Materials & Interfaces, 2012
We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxi... more We report first-principles theoretical investigation of p-type charge transfer doping of zinc oxide (ZnO) nanowires by molecular adsorption. We find that spontaneous dissociative adsorption of fluorine molecules introduces half-emptying of otherwise fully filled oxygen-derived surface states. The resulting surface Fermi level is so close to the valence band maximum of the ZnO nanowire that the nanowire undergoes significant p-type charge transfer doping. Those half-filled surface states are fully spin-polarized and lead to surface ferromagnetism that is stable at room temperature. We also analyze the kinetic control regime of the surface transfer doping and find that it may result in nonequilibrium steady states. The present results suggest that postgrowth engineering of surface states has high potential in manipulating ZnO nanostructures useful for both electronics and spintronics.
We investigate using first-principles calculations the atomic structure of the orthorhombic phase... more We investigate using first-principles calculations the atomic structure of the orthorhombic phase of Ta 2 O 5 . 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 high-symmetry structural model, where all Ta and O atoms have the correct formal oxidation states of þ5 and À2, 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 Ta 2 O 3 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 Ta 2 O 5 observed in experiments is attributed to the intrinsic structural variations, and oxygen vacancies that drive the collective relaxation of the O sublattice.