Choong Ki Kim

Korea University, Sŏul, Seoul, South Korea

Are you Choong Ki Kim?

Claim your profile

Publications (11)29.67 Total impact

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Background and purpose: Periodic limb movements (PLM) during sleep (PLMS) are associated with cortical and cardiovascular activation. Changes in cerebral hemodynamics caused by cortical activity can be measured using near-infrared spectroscopy (NIRS). We investigated oscillatory components of cerebral hemodynamics during PLM and different sleep stages in restless legs syndrome (RLS) patients with PLMS. Methods: Four female RLS patients with PLMS, and four age- and sex-matched normal controls were included. PLM and sleep stages were scored using polysomnography, while the spontaneous cerebral hemodynamics was measured by NIRS. The phase and amplitude of the cerebral oxyhemoglobin concentration [HbO] and the deoxyhemoglobin concentration [Hb] low-frequency oscillations (LFOs) were evaluated during each sleep stage [waking, light sleep (LS; stages N1 and N2), slow-wave sleep (stage N3), and rapid eye movement (REM) sleep]. In RLS patients with PLMS, the cerebral hemodynamics during LS was divided into LS with and without PLM. Results: The cerebral hemodynamics activity varied among the different sleep stages. There were changes in phase differences between [HbO] and [Hb] LFOs during the different sleep stages in the normal controls but not in the RLS patients with PLMS. The [HbO] and [Hb] LFO amplitudes were higher in the patient group than in controls during both LS with PLM and REM sleep. Conclusions: The present study has demonstrated the presence of cerebral hemodynamics disturbances in RLS patients with PLMS, which may contribute to an increased risk of cerebrovascular events.
    Preview · Article · Jan 2016 · Journal of Clinical Neurology
  • [Show abstract] [Hide abstract]
    ABSTRACT: Objectives: We developed a multimodal optical imaging system for intraoperative visualization of sentinel lymph nodes (SLNs). This study is to validate our system by showing SLNs in the lung through combined optical color and fluorescent image with indocyanine green (ICG) and ICG with human serum albumin (HSA). Methods: Identical ICG concentrations of ICG only or ICG:HSA was injected into the rat footpad and porcine lung. Absolute amounts of the fluorescents were scaled on the basis of animal weights. The entire procedures were recorded using color and near-infrared (NIR) charge-coupled device (CCD) cameras simultaneously, and the 2 images were merged by real-time image processing software. All fluorescence intensity signals to background ratio (SBR) and retention rates at SLN for both fluorescents were estimated and compared. Results: This newly developed intraoperative color and fluorescence optical imaging system successfully visualized the SLNs in animal experiments. The SLNs were identified 100% for both rat and pig under in vivo conditions. Real-time image processing software overcame the low signal of NIR fluorescence images. ICG and ICG:HSA provided no significantly different SBR in the SLN images for both rat thigh and pig lung. Conclusions: The intraoperative optical imaging system enabled real-time image-guided surgery during SLN mapping in lung in an animal model. The ICG retention rate was similar to ICG:HSA. ICG alone can be useful for SLN imaging during lung cancer surgery.
    No preview · Article · Mar 2013 · The Journal of thoracic and cardiovascular surgery
  • Source
    Soo Ho Bae · Sang Jun Lee · Young Ho Kim · Hee Chul Lee · Choong Ki Kim
    [Show abstract] [Hide abstract]
    ABSTRACT: We study the 1/f noise currents and dark currents in LWIR HgCdTe photodiodes. The measured dark currents of the diodes processed by post implantation annealing with different annealing times are analyzed using current model fitting methods. The different dark current components, such as diffusion current, generation-recombination current, band-to-band tunneling current, and trap assisted tunneling current, at various bias voltages can be separated from the measured dark currents. By the fitting analysis, some physical parameters are extracted and different annealing effects can be explained by the parameters. The improvements in diode characteristics by post implantation annealing can be explained by the changes of trap density, donor concentration, minority carrier lifetime, and generation lifetime. The 1/f noise currents are measured over a wide range of reverse bias voltages, and correlated with the extracted dark currents by superposition of the noise generated by the different dark current mechanisms. It turns out that the band-to-band tunneling has a smaller correlation with the 1/f noise than other current components, and the trap center seems to be responsible for the 1/f noise characteristics of the LWIR HgCdTe photodiodes.
    Preview · Article · Apr 2012 · Journal of Electronic Materials
  • Young-Ho Kim · Soo-Ho Bae · Hee Chul Lee · Choong Ki Kim
    [Show abstract] [Hide abstract]
    ABSTRACT: Effects of fixed charge on R0A value of ZnS-passivated x=0.3 HgCdTe n-on-p diode are explained as a shunt resistance that affects current-voltage (I-V) and dynamic resistance-voltage (Rd-V) characteristics. The fixed charge of 1�1011/cm2 to 2 � 1011/cm2 which is usually obtained with ZnS passivation makes the surface weakly inverted and reduces HgCdTe diode R0A value owing to the short generation lifetime of HgCdTe substrate. The gate-controlled diode and specially fabricated diode are used to explain the surface leakage current in the weak inversion and charge sheet model is used to explain the characteristics. It is found that the surface leakage current by the inverted channel in the weak inversion can reduce R0A more than other currents such as the generation current and tunneling current which are usually used to explain the surface leakage current of HgCdTe diode.
    No preview · Article · Jun 2000 · Journal of Electronic Materials
  • [Show abstract] [Hide abstract]
    ABSTRACT: The hydrogenation effects on HgCdTe diode performance are presented and the mechanism of hydrogenation is revealed. By the hydrogenation, R0A is increased by 30 times and photo-response is also improved. It is supposed that these are explained by the increased minority carrier lifetime by the hydrogenation. However, it is found from LBIC measurements that the minority carrier lifetime doesn’t increase by the hydrogenation. An important clue that explains the hydrogenation effects is found from Hall measurements. It is found that, after the hydrogenation, the doping concentration of Hg-vacancy doped substrate decreases and the mobility increases. For the heavily hydrogenated bulk substrate, it is also found that the hydrogen passivates the whole Hg-vacancy and reveals the residual impurity and p-type doping concentration is exponentially graded. From these measurements, the diffusion current model of gradually doped diode is proposed. This model shows that the diffusion current of the graded junction diode is 2 orders of magnitude smaller than that of the abrupt junction diode, which clearly explains the R0A increase by the hydrogenation. Medicisimulation to investigate the change of LBIC signal by the doping grading also coincides with the measurements. From these measurements and model, the hydrogenation effects are attributed to the grading of Hg-vacancy doped p-type substrate by the diffused hydrogen.
    No preview · Article · Jun 2000 · Journal of Electronic Materials
  • Source
    Seong Hoon Lee · Soo Ho Bae · Hee Chul Lee · Choong Ki Kim
    [Show abstract] [Hide abstract]
    ABSTRACT: In this paper, we report the effects of bromine etch and HNO3 post-etch treatment on the C-V characteristics of MIS devices with ZnS on LPE-grown HgCdTe wafers. C-V characteristics of most devices at 1 MHz showed more increased surface doping concentrations than the original values of the wafers. These apparent doping concentrations were strongly dependent on the surface treatments. These are thought to result from the surface treatment effects on the interface trap density and the surface minority carrier response time. By longer bromine etch and by HNO3 post-etch treatment, the interface trap density decreased and the surface minority carrier response time increased.
    Preview · Article · May 1998 · Journal of Electronic Materials
  • [Show abstract] [Hide abstract]
    ABSTRACT: This paper presents a new simple method of HgCdTe surface treatment which consists of chemical oxidation of HgCdTe with nitric acid and removal of the oxide with ammonium hydroxide. The electrical properties of the electron-beam deposition CdTe passivation of Hg0.7Cd0.3Te are investigated with regard to the effects of HgCdTe surface etching, exposure to nitric acid, and the new surface treatment method. As the HgCdTe surface is progressively etched with bromine in methanol (Br-MeOH), the surface becomes rougher and a higher density of fixed charge is induced at the interface between CdTe and HgCdTe. Exposure to HNO3 results in a very high density of fixed charge and performance degradation in metal insulator semiconductor (MIS) capacitors, which is due to the chemical oxide grown by HNO3. The oxide growth rate is enhanced as the concentration of HNO3 increases or as more H2O is added. This oxide can be removed with NH4OH. After the new surface treatment, MIS capacitors of Hg0.7Cd0.3Te show substantial improvement in electrical properties, such as low density of fixed charge and reduced hysteresis width, regardless of previous surface etching.
    No preview · Article · Jun 1997 · Journal of Electronic Materials
  • Jung‐Yeal Lee · Chul‐Hi Han · Choong‐Ki Kim · Bok‐Ki Kim
    [Show abstract] [Hide abstract]
    ABSTRACT: The optical and electrical properties of polycrystalline silicon (poly‐Si) film with electron cyclotron resonance (ECR) plasma oxidation have been investigated. Density of dangling bonds in the poly‐Si film was decreased significantly after oxidizing by ECR oxygen plasma. It was found that oxygen ions generated during the ECR plasma oxidation process diffuse into the poly‐Si film, and passivate the dangling bonds. Also, it was found that the poly‐Si film with ECR plasma oxidation has better thermal stability than hydrogen‐passivated poly‐Si film, which can be explained by the higher energy of Si–O bond than that of Si–H. © 1995 American Institute of Physics.
    No preview · Article · Sep 1995 · Applied Physics Letters
  • Hi‐Deok Lee · Ho‐Jun Lee · Choong‐Ki Kim · Chul‐Hi Han
    [Show abstract] [Hide abstract]
    ABSTRACT: A self‐aligning etch method for silicon substrates is presented. A hole pattern is defined on the front side of the silicon substrate while etching of the hole takes place from the back side through the application of a current from the front side of the silicon substrate to a cathode electrode in a solution of HF, HNO3, and H2O. As the etching proceeds, the etched pattern in the back side gradually becomes self‐aligned with the front side pattern of the silicon substrate. Twenty five circular holes arranged in a 5×5 array are formed in a boron‐doped (100) silicon substrate. The etch method provides control over hole dimensions which are 520 μm in diameter and spaced 520 μm apart. The self‐aligning property of the etch method enables the formation of different shaped holes. © 1995 American Institute of Physics.
    No preview · Article · Jun 1995 · Applied Physics Letters
  • Ho‐Jun Lee · Young‐Jin Jeon · Chul‐Hi Han · Choong‐Ki Kim
    [Show abstract] [Hide abstract]
    ABSTRACT: It has been found that misfit dislocations in heavily boron‐doped layers originate from wafer edges. Moreover, the propagation of the misfit dislocation into a region can be suppressed by placing a surrounding undoped region formed by a chemical‐vapor deposited (CVD) oxide ring pattern before boron doping. A heavily boron‐doped layer with no misfit dislocations can be obtained by using a CVD oxide ring up to a boron dose of 5.4×10<sup>16</sup> cm<sup>2</sup> for an area of 2 cm×2 cm and up to a dose of 1.48×10<sup>17</sup> cm<sup>2</sup> for an area of 400 μm×400 μm.  
    No preview · Article · Jun 1994 · Applied Physics Letters
  • Yong Tae Kim · Suk‐Ki Min · Jong Sung Hong · Choong Ki Kim
    [Show abstract] [Hide abstract]
    ABSTRACT: Controlling the wafer temperatures from 200 to 500 °C at H 2 /WF 6 flow ratio equal to 24, low‐resistive (about 11 μΩ cm) tungsten thin films are deposited by plasma‐enhanced chemical vapor deposition. The as‐deposited tungsten films have (110), (200), and (211) oriented bcc structures and Auger depth profile shows that fluorine and oxygen impurities are below the detection limit of Auger electron spectroscopy.
    No preview · Article · Mar 1991 · Applied Physics Letters
  • Nam‐Deog Kim · Choong‐Ki Kim · Jin Jang · Choochon Lee
    [Show abstract] [Hide abstract]
    ABSTRACT: Hydrogenated amorphous silicon (a‐Si:H) thin‐film transistors (TFTs) which utilize two layers of amorphous silicon nitride with different composition as a gate insulator have been studied. The field‐effect mobility, subthreshold slope, and stability of an a‐Si:H TFT are enhanced by inserting a thin silicon‐rich nitride layer between the a‐Si:H and the gate insulator. The improvement of these characteristics appears to be due to both the decrease of the interface state density between the a‐Si:H and the top silicon‐rich nitride layer, and the good dielectric quality of the bottom nitride layer.
    No preview · Article · Jun 1989 · Applied Physics Letters