Papers by Author: Wook Bahng

Abstract: We investigated the effect of the substrate temperature on the electrical and the optical properties of ZnO/4H-SiC structures. The n-type ZnO layer was grown on p-type 4H-SiC substrate by pulsed laser deposition to form p-n hetero-junction diode structure. The n-type ZnO thin films were deposited by pulsed laser deposition at different temperatures of 200, 400, and 600 °C, respectively. It was shown from transmission line method (TLM) and auger electron spectroscopy (AES) data that the sheet resistance of ZnO on SiC was increases from ~760 Ω/square to ~4000 Ω/square as the deposition temperature increases and the oxygen outdiffusion decreases. The I-V characteristics with and without illumination has also been studied.

Abstract: We investigated the effect of the metal work-function and doping concentration on the barrier height of Ni-contacts with embedded nano-particles (NPs) on 4H-SiC surfaces. Both n-type epitaxial layers with ND=1×1016 cm-3, and layers doped by phosphorous implantation to a doping concentration of ~1×1019 cm-3 are used. The barrier height is reduced with increasing doping concentration and the silver (Ag) nano-particles (R~18.5 nm) further enhances the local electric field of the electrical contacts to 4H-SiC in comparison to gold (Au) nano-particles (R~20.2 nm). In the case of ion-implanted samples, the barrier height of the fabricated SiC diode structures with embedded Ag-NPs was significantly reduced by ~0.09 eV and ~0.25 eV compared to the samples with Au-NPs and the sample without NPs, respectively.

Abstract: The characteristics of Ga-doped zinc oxide (GaZnO) thin films deposited at different substrate temperatures (TS~250 to 550oC) on 4H-SiC have been investigated. Structural and electrical properties of GaZnO thin film on n-type 4H-SiC (100)were investigated by using x-ray diffraction, atomic force microscopy (AFM), Hall effect measurement, and Auger electron spectroscopy (AES). Hall mobility is found to increase as the substrate temperature increase from 250 to 550 oC, whereas the lowest resistivity (~3.3 x 10-4 Ωcm) and highest carrier concentration (~1.33x1021cm-3) values are observed for the GaZnO films deposited at 400 oC. It has been found that the c-axis oriented crystalline quality as well as the relative amount of activated Ga3+ Introduction ions may affect the electrical properties of GaZnO films on SiC.

Abstract: The 50W Quasi-resonant mode SMPS which adopted a normally-on-type SiC JFET as a switch has been designed and characterized. A simple decision circuit and an auxiliary power supply was utilized to safely protect the JFET from an in-rush current at initial operation stage and to provide sufficient negative voltage for a complete JFET drive. Even without a refine engineering, the SMPS showed 96% efficiency at a full load state.

Abstract: We have investigated the electrical and physical properties of the oxidized-SiN with or without post oxidation annealing (POA) in N2 gas. A significant reduction in interface-trap density (Dit) has been observed in the oxidized-SiN with N2 POA for 60 min if compared with other oxides. The reason for this has been explained in this paper.

Abstract: The effects of post annealing etch process on electrical performances of a 4H-SiC Schottky diodes without any edge termination were investigated. The post etch was carried out using various dry the dry etch techniques such as Inductively Coupled Plasma (ICP) and Neutral Beam Etch (NBE) in order to eliminate suspicious surface damages occurring during a high temperature ion activation process. The leakage current of diodes treated by NBE measured at -100V was about one order lower than that of diode without post etch and a half times lower than that of diode treated by ICP without a significant degradation of forward electrical characteristics. Based on the above results, the post annealing process was adapted to a junction barrier Schottky diode with a field limiting ring. The blocking voltages of diode without post annealing etch and diodes treated by ICP and NBE were -1038V, -1125V, and -1595V, respectively.

Abstract: The effect of the doping concentration and space of both p-grid and FLR on the electrical performances of 4H-SiC JBS diode has been investigated. A 4H-SiC JBS diode with the p-grid space of 3um, the FLR space of 3um, and the doping concentration of 5E18cm-3 showed the highest blocking voltage of 1500V.

Abstract: We have investigated the electrical and physical properties of high-temperature (1300, 1400 oC) grown dry oxide with or without post oxidation annealing (POA) in nitric oxide (NO) gas. A significant reduction in interface-trap density (Dit) has been observed in 1300 oC-grown dry oxide with or without NO POA if compared with the Dit of 1400 oC-grown dry oxide. The reason for this has been explained in this paper.

Abstract: This paper demonstrates the breakdown voltage characteristics of different edge termination structures including aluminum (Al)-deposited guard ring and Al-deposited guard ringassisted field limiting ring (FLR) for a 4H silicon carbide (SiC) Schottky barrier diode (SBD). In order to investigate the application feasibility of the Al-deposited junction termination to a high breakdown voltage SiC-SBD, two types of SiC-SBDs are fabricated using conventional photolithography, electron beam evaporation, and thermal treatment techniques without ion implantation and thermal oxidation procedures. The breakdown voltage characteristics of the SiCSBDs are significantly dependent on the Al-deposited edge termination. The SiC-SBD without the Al-deposited edge termination shows less than 250 V breakdown voltage, while the Al-deposited guard ring and Al-deposited guard ring-assisted FLR structures show roughly 700 V and 1200 V breakdown voltages, respectively. The prominent improvement in the breakdown voltage characteristics is attributed to the electric field lowering at the Schottky contact edge by the Al deposition edge termination.

Abstract: A double delta-doped channel 4H-SiC MESFET is proposed to kick out degradation of the DC and RF performances caused by the surface traps, by forming a quantum-well-like potential well and separating an effective channel from the surface. To obtain an optimum device structure, the DC and RF performances of double delta-doped channel MESFETs having various delta-doping concentrations but the same pinch-off voltage with that of conventional MESFET were also investigated. The SilvacoTM simulation results show that the double delta-doped channel MESFET achieved more improvement of the drain current, the cut-off frequency, and the maximum oscillation frequency for higher delta-doping concentration near the gate. In all cases, DC and RF performances for double delta-doped channel MESFETs are much improved than those of the conventional MESFET.