Jinseo Kim

Tuning the electronic structure of tin sulfides grown by atomic layer deposition.

In this study, tin sulfide thin films were obtained by atomic layer deposition (ALD) using Tetrakis(dimethylamino)tin (TDMASn, [(CH3)2N]4Sn) and hydrogen sulfide (H2S). The growth rate of the tin sulfides (SnSx) was shown to be highly dependent on the deposition temperature, and reaction times of 1 second for the TDMASn and H2S were required to reach the saturation regime. Surface morphologies were smooth or rectangular with rounded corners as observed by a field emission scanning electron microscope (FE-SEM) and were dependent on temperature. X-ray diffraction results confirmed that the crystal structure of SnSx can be tuned by changing the ALD temperature. Below 120 °C, SnSx films appeared to be amorphous. In addition, SnSx films were SnS2 hexagonal at 140 and 150 °C and SnS orthorhombic above 160 °C. Similarly, the values of the optical band gap and binding energy showed significant differences between 150 and 160 °C. The electronic structures of SnSx were extracted by UPS and absorption spectroscopy, and the unsaturated Sn 3d molecular orbital (MO) states in the band edge were found to be responsible for the great improvement in electrical conductivity. This study shows that TDMASn-H2S ALD is an effective deposition method for SnSx films, offering a simple approach to tune the physical properties.

Multi-resistive Reduced Graphene Oxide Diode with Reversible Surface Electrochemical Reaction induced Carrier Control

The extended application of graphene-based electronic devices requires a bandgap opening in order to realize the targeted device functionality. Since the bandgap tuning of pristine graphene is limited to 360 meV, the chemical modification of graphene is considered essential to achieve a large bandgap opening at the expense of electrical properties degradation. Reduced graphene oxide (RGO) has attracted significant interest for fabricating graphene-based semiconductors since it has several advantages over other forms of chemically modified graphene; such as tunable bandgap opening, decent electrical properties, and easy synthesis. Because of the reduced bonding nature of RGO, the role of metastable oxygen in the RGO matrix is recently highlighted and it may offer emerging ionic devices. In this study, we show that multi-resistivity RGO/n-Si diodes can be obtained by controlling the RGO thickness at a nanometer scale. This is made possible by (1) a metastable lattice-oxygen drift within bulk RGO and (2) electrochemical ambient hydroxyl (OH) formation at the RGO surface. The effect demonstrated in a p-RGO/n-Si heterojunction diode is equivalent to electrochemically driven reversible electronic manipulation and therefore provides an important basis for the application of O bistability in RGO for chemical sensors and electrocatalysis.

Photochemical Hydrogen Doping Induced Embedded Two-Dimensional Metallic Channel Formation in InGaZnO at Room Temperature

The photochemical tunability of the charge-transport mechanism in metal-oxide semiconductors is of great interest since it may offer a facile but effective semiconductor-to-metal transition, which results from photochemically modified electronic structures for various oxide-based device applications. This might provide a feasible hydrogen (H)-radical doping to realize the effectively H-doped metal oxides, which has not been achieved by thermal and ion-implantation technique in a reliable and controllable way. In this study, we report a photochemical conversion of InGaZnO (IGZO) semiconductor to a transparent conductor via hydrogen doping to the local nanocrystallites formed at the IGZO/glass interface at room temperature. In contrast to thermal or ionic hydrogen doping, ultraviolet exposure of the IGZO surface promotes a photochemical reaction with H radical incorporation to surface metal-OH layer formation and bulk H-doping which acts as a tunable and stable highly doped n-type doping channel and turns IGZO to a transparent conductor. This results in the total conversion of carrier conduction property to the level of metallic conduction with sheet resistance of ∼16 Ω/□, room temperature Hall mobility of 11.8 cm(2) V(-1) sec(-1), the carrier concentration at ∼10(20) cm(-3) without any loss of optical transparency. We demonstrated successful applications of photochemically highly n-doped metal oxide via optical dose control to transparent conductor with excellent chemical and optical doping stability.

Photochemical tuning of ultrathin TiO2/p-Si p-n junction properties via UV-induced H doping

We report a modified TiO2/p-Si electronic structure that uses ultraviolet exposure for the incorporation of H. This structure was characterized using various photoelectron spectroscopic techniques. The ultraviolet (UV) exposure of the TiO2 surface allowed the Fermi energy level to be tuned by the insertion of H radicals, which induced changes in the heterojunction TiO2/p-Si diode properties. The UV exposure of the TiO2 surface was performed in air. On UVexposure, a photochemical reaction involving the incorporation of UV-induced H radicals led to the creation of a surface Ti-O-OH group and caused interstitial H doping (Ti-H-O) in the bulk, which modified the electronic structures in different ways, depending on the location of the H. On the basis of the band alignment determined using a combined spectroscopic analysis, it is suggested that the UV-induced H incorporation into the TiO2 could be utilized for the systematic tuning of the heterojunction property for solar cells, photocatalytic applications, and capacitors.

Creation of a Short-Range Ordered Two-Dimensional Electron Gas Channel in Al2O3/In2O3 Interfaces

The tuning of electrical properties in oxides via surface and interfacial two-dimensional electron gas (2DEG) channels is of great interest, as they reveal the extraordinary transition from insulating or semiconducting characteristics to metallic conduction or superconductivity enabled by the ballistic transport of spatially confined electrons. However, realizing the practical aspects of this exotic phenomenon toward short-range ordered and air-stable 2DEG channels remains a great challenge. At the heterointerface formed after deposition of an Al2O3 layer on a nanocrystalline In2O3 layer, a dramatic improvement in carrier conduction equivalent to metallic conduction is obtained. A conductivity increase by a factor of 1013 times that in raw In2O3, a sheet resistance of 850 Ω/cm2, and a room temperature Hall mobility of 20.5 cm2 V-1 s-1 are obtained, which are impossible to achieve by tuning each layer individually. The physicochemical origin of metallic conduction is mainly ascribed to the 2D interfacially confined O-vacancies and semimetallic nanocrystalline InOx (x < 2) phases by the clustered self-doping effect caused by O-extraction from In2O3 to the Al2O3 phase during ALD. Unlike other submetallic oxides, this 2D channel is air-stable by complete Al2O3 passivation and thereby promises applicability for implementation in devices.

Evolution of Defect-Associated Subband Energy States in Nanocrystalline TiO2 Films on Si and Ge Substrates

We identified the electronic states in the conduction- and valence-band edges associated with intrinsic defects in nanocrystalline TiO2 layers on Si and Ge substrates. This was accomplished through spectroscopic study with soft X-ray photoemission spectroscopy and visible-ultraviolet spectroscopic ellipsometry. The interpretation of the spectra based on molecular orbital (MO) theory well explains the origin of empty and occupied states of band edge in TiO2 with the correct assignment of MO states. The evolution of these band-edge states under thermal nanocrystalline growth and interfacial chemical mixing as a function of type of substrate was investigated, taking into consideration the asymmetric local bonding distortion and fast atomic diffusion at the grain boundary. The engineering solution to utilize TiO2 as a gate dielectric on Ge substrate is demonstrated by implanting SiON/Si interfacial layer. This study suggests that the control of electronically active defect density and energy level in nanoscale TiO2 thin films is strongly affected by thermal grain expansion and interfacial chemistry, depending on the semiconductor substrates used.

Comparison on the Physical & Chemical Characteristics in Surface of Polished Wafer and Epi-Layer Wafer

【Physical and chemical changes in a polished wafer and in

Design of a C/SiC functionally graded coating for the oxidation protection of C/C composites

2.5{\mu}m

Improving p-to-n transition and detection range of bimodal hydrogen-sensitive nanohybrids of hole-doped rGO and chemochromic Pd-decorated-MoO3 nanoflakes

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Complementary Schottky diode formation with carbon buffer and p-doped single layer graphene on intrinsic SiC via fluorine intercalation

4{\mu}m