Ki-ha Hong

The Role of Intrinsic Defects in Methylammonium Lead Iodide Perovskite

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.

Gate-All-Around (GAA) Twin Silicon Nanowire MOSFET (TSNWFET) with 15 nm Length Gate and 4 nm Radius Nanowires

GAA TSNWFET with 15 nm gate length and 4 nm radius nanowires is demonstrated and shows excellent short channel immunity. p-TSNWFET shows high driving current of 1.94 mA/mum while n-TSNWFET shows on-current of 1.44 mA/mum. Merits of TSNWFET and performance enhancement of p-TSNWFET are explored using 3D and quantum simulation

High-performance amorphous gallium indium zinc oxide thin-film transistors through N2O plasma passivation

Amorphous-gallium-indium-zinc-oxide (a-GIZO) thin filmtransistors (TFTs) are fabricated without annealing, using processes and equipment for conventional a-Si:H TFTs. It has been very difficult to obtain sound TFT characteristics, because the a-GIZO active layer becomes conductive after dry etching the Mo source/drain electrode and depositing the a-SiO2 passivation layer. To prevent such damages, N2O plasma is applied to the back surface of the a-GIZO channel layer before a-SiO2 deposition. N2O plasma-treated a-GIZO TFTs exhibit excellent electrical properties: a field effect mobility of 37cm2∕Vs, a threshold voltage of 0.1V, a subthreshold swing of 0.25V/decade, and an Ion∕off ratio of 7.

Strain-Driven Electronic Band Structure Modulation of Si Nanowires

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.

Asymmetric Doping in Silicon Nanostructures: The Impact of Surface Dangling Bonds

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.

Gate-all-around Twin Silicon nanowire SONOS Memory

We have developed gate-all-around (GAA) SONOS with ultra thin twin silicon nanowires for the first time. By using channel hot electron injection (CHEI) and hot hole injection (HHI) mechanisms, program speed of 1 mus at V_d = 2 V, V_g = 6 V and erase speed of 1 ms at V_d = 4.5 V, V_g = -6 V are achieved with 2~3 nm nanowire and 30 nm gate. Nanowire size below 10 nm dependencies on V_th shift (DeltaV_th) and the program/erase (P/E) characteristics are investigated. As nanowire diameter (d_nw) decreases, faster program speed and larger DeltaV_th are observed.

A Pathway to Type-I Band Alignment in Ge/Si Core-Shell Nanowires.

We investigate the electronic band structures of Ge/Si core-shell nanowires (CSNWs) and devise a way to realize the electron quantum well at Ge core atoms with first-principles calculations. We reveal that the electronic band engineering by the quantum confinement and the lattice strain can induce the type-I/II band alignment transition, and the resulting type-I band alignment generates the electron quantum well in Ge/Si CSNWs. We also find that the type-I/II transition in Ge/Si CSNWs is highly related to the direct to indirect band gap transition through the analysis of charge density and band structures. In terms of the quantum confinement, for [100] and [111] directional Ge/Si CSNWs, the type-I/II transition can be obtained by decreasing the diameters, whereas a [110] directional CSNW preserves the type-II band alignment even at diameters as small as 1 nm. By applying a compressive strain on [110] CSNWs, the type-I band alignment can be formed. Our results suggest that Ge/Si CSNWs can have the type-I band alignment characteristics by the band structure engineering, which enables both n-type and p-type quantum-well transistors to be fabricated using Ge/Si CSNWs for high-speed logic applications.

Atomistic study on dopant-distributions in realistically sized, highly P-doped Si nanowires.

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 establishes the first theoretical framework for understanding dopant-distributions in over-10 nm highly doped silicon nanowires.

Surface ferromagnetic p-type ZnO nanowires through charge transfer doping.

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.

Prediction of Young's Moduli of Low Dielectric Constant Materials by Atomistic Molecular Dynamics Simulation

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 Youngs moduli of the typical low k materials, and the effects of various factors for Youngs moduli of materials, such as, structures of precursors, density, and porosity. Using atomistic molecular dynamics simulation with experimental measurements, the Youngs moduli of films of amorphous silicon oxide in which 25% of Si-O-Si chains were replaced by Si-(CH 3 H 3 C)-Si, Si-CH 2 -Si, Si-(CH 2 ) 2 -Si, Si-(CH 2 ) 3 -Si, Si-(CH 2 ) 4 -Si, Si-(CH 2 ) 6 -Si, were measured and analyzed. The predicted trends of Youngs moduli of films formed by above precursors are in good consistent with those observed from experiments. The Youngs moduli of materials are largely dependent on the densities of materials. Youngs modulus of material increases as the density of the material increases. The chemical properties, chain length, and connectivity of material take effects on the Youngs modulus of material. Given the same densities of material the smaller number of cavities per unit volume the material has, the lower Youngs 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.