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Synthesis and Characterization of CdS Nanocrystals in a Novel Phosphate Glass
Donghwan Kim,Chawon Hwang,Donggun Gwoo,Taehee Kim,Youngseok Kim,Namjin Kim,류봉기 대한금속·재료학회 2011 ELECTRONIC MATERIALS LETTERS Vol.7 No.4
Zinc-borophosphate glasses doped with various concentrations (≤ 5 wt. %) of cadmium sulfide (CdS) were synthesized. Nanocrystals of CdS were developed in the glass matrices at different concentrations, with emphasis on the quantum confinement effect. The effects of CdS content on the optical and various other properties were investigated. The optical characterization of the glasses was carried out using UV-Visible spectrophotometry. There was a red shift in the optical cut-off with increasing CdS content in the glass. The band gap of the CdS nanocrystals doped in the glass matrix ranged from 3.5 eV to 4.2 eV. The structural characterization,which was carried out using X-ray diffraction, shows the CdS to have hexagonal crystal structure. The average size and shape of the nanocrystals doped in the glass matrix were determined using transmission electron microscopy (TEM). The particle sizes of the doped CdS crystals were in the range of 3 nm to 6 nm for 2 wt. % CdS, and 15 nm to 20 nm for 5 wt. % CdS.
Sung Min Kim,Eun Jung Yoon,Min Sang Kim,Ming Li,Chang Woo Oh,Sung Young Lee,Kyoung Hwan Yeo,Sung Hwan Kim,Dong Uk Choe,Sung Dae Suk,Dong-Won Kim,Donggun Park 대한전자공학회 2006 Journal of semiconductor technology and science Vol.6 No.1
We demonstrate highly manufacturable Multi-channel Field Effect Transistor (McFET) on bulk Si wafer. McFET shows excellent transistor characteristics, such as 5~6 times higher drive current than planar MOSFET, ideal subthreshold swing, low drain induced barrier lowering (DIBL) without pocket implantation and negligible body bias dependency, maintaining the same source/drain resistance as that of a planar transistor due to the unique feature of McFET. And suitable threshold voltage (VT) for SRAM operation and high static noise margin (SNM) are achieved by using TiN metal gate electrode.
Filter-and-Forward Transparent Relay Design for OFDM Systems
Donggun Kim,Junyeong Seo,Youngchul Sung IEEE 2013 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY Vol.62 No.9
<P>In this paper, the filter-and-forward (FF) relay design for orthogonal frequency-division multiplexing (OFDM) transmission systems is considered for improving system performance over simple amplify-and-forward (AF) relaying. Unlike conventional OFDM relays performing OFDM demodulation and remodulation, to reduce processing complexity, the proposed FF relay directly filters the incoming signal in the time domain with a finite impulse response (FIR) and forwards the filtered signal to the destination. Three design criteria are considered for optimizing the relay filter. The first criterion is the minimization of the relay transmit power subject to per-subcarrier signal-to-noise ratio (SNR) constraints, the second criterion is the maximization of the worst subcarrier channel SNR subject to source and relay transmit power constraints, and the third criterion is the maximization of the data rate subject to source and relay transmit power constraints. It is shown that the first problem reduces to a semi-definite programming (SDP) problem by semi-definite relaxation (SDR), and the solution to the relaxed SDP problem has rank one under a mild condition. For the latter two problems, the problem of joint source power allocation and relay filter design is considered, and an efficient algorithm is proposed for each problem based on alternating optimization and the projected gradient method (PGM). Numerical results show that the proposed FF relay significantly outperforms simple AF relays with an insignificant increase in complexity. Thus, the proposed FF relay provides a practical alternative to the AF relaying scheme for OFDM transmission.</P>
Filter-And-Forward Relay Design for MIMO-OFDM Systems
Kim, Donggun,Sung, Youngchul,Chung, Jihoon IEEE 2014 IEEE TRANSACTIONS ON COMMUNICATIONS Vol.62 No.7
<P>In this paper, the filter-and-forward (FF) relay design for multiple-input–multiple-output (MIMO) orthogonal frequency-division multiplexing (OFDM) systems is investigated. Due to the considered MIMO structure, the FF relay design is investigated in the framework of joint design together with the linear MIMO transceiver. As the design criterion, first, the minimization of weighted sum mean square error (MSE) is considered. The joint design in this case is approached based on alternating optimization that iterates between the optimal design of the FF relay for given MIMO precoding and decoding matrices and the optimal design of MIMO precoding and decoding matrices for a given FF relay filter. Second, a more advanced problem of joint design for rate maximization is considered. The second problem is approached based on the obtained result regarding the first problem of weighted sum MSE minimization and the existing result regarding the relationship between weighted MSE minimization and rate maximization. Numerical results show the effectiveness of the proposed FF relay design method and significant performance improvement by the proposed FF relay over widely considered simple AF relays for MIMO-OFDM systems.</P>
Two-Stage Beamformer Design for Massive MIMO Downlink By Trace Quotient Formulation
Donggun Kim,Gilwon Lee,Youngchul Sung Institute of Electrical and Electronics Engineers 2015 IEEE Transactions on Communications Vol. No.
<P>In this paper, the problem of outer beamformer design based only on channel statistic information is considered for two-stage beamforming for multi-user massive MIMO downlink, and the problem is approached based on signal-to-leakage-plus-noise ratio (SLNR). To eliminate the dependence on the instantaneous channel state information, a lower bound on the average SLNR is derived by assuming zero-forcing (ZF) inner beamforming, and an outer beamformer design method that maximizes the lower bound on the average SLNR is proposed. It is shown that the proposed SLNR-based outer beamformer design problem reduces to a trace quotient problem (TQP), which is often encountered in the field of machine learning. An iterative algorithm is presented to obtain an optimal solution to the proposed TQP. The proposed method has the capability of optimally controlling the weighting factor between the signal power to the desired user and the interference leakage power to undesired users according to different channel statistics. Numerical results show that the proposed outer beamformer design method yields significant performance gain over existing methods.</P>
Kim, Hong-Sik,Kumar, Melvin David,Kim, Joondong,Lim, Donggun Elsevier 2018 Sensors and actuators. A, Physical Vol.269 No.-
<P><B>Abstract</B></P> <P>The 2D layers of MoS<SUB>2</SUB> were deposited at RT, 200 °C and 400 °C over p-Si substrates using RF sputtering method. When the deposition temperature is increased beyond 200 °C, the MoS<SUB>2</SUB> layers were grown in vertical direction. XRD studies revealed that the prominent peak is shifted from 2θ = 24° to 13.5° and become narrow while increasing the temperature to 400 °C. The average absorption of 53.68% was exhibited by the sample deposited at 400 °C whereas it was reduced to 28.47% for the sample prepared at 200 °C. When the MoS<SUB>2</SUB>/p-Si photodetector was exposed to the red color wavelength, the high photocurrent of 130 μA was produced even at zero bias. The response and recovery times were measured as 38.78 μs and 43.07 μs respectively for λ ≈ 455 nm. The detectivity was found to be in the range of 109–1010 Jones. The proposed MoS<SUB>2</SUB> based photodetector is more beneficial in terms of quick transport of photogenerated carriers and fast response. The growth control and large-scale production of 2D materials would induce the enhanced photoelectric performances and practical device applications.</P> <P><B>Highlights</B></P> <P> <UL> <LI> 2D material was unformly grown onto 3 inch p-Si wafer by sputtering method. </LI> <LI> Temperature modulation controled the vertical growth direction of MoS<SUB>2</SUB>. </LI> <LI> The veritcal grown MoS<SUB>2</SUB> provided the singificantly enhanced photoresponses. </LI> </UL> </P>
Transparent Cu<sub>4</sub>O<sub>3</sub>/ZnO heterojunction photoelectric devices
Kim, Hong-Sik,Yadav, Pankaj,Patel, Malkeshkumar,Kim, Joondong,Pandey, Kavita,Lim, Donggun,Jeong, Chaehwan Academic Press 2017 Superlattices and microstructures Vol.112 No.-
<P><B>Abstract</B></P> <P>The present article reports the development of flexible, self-biased, broadband, high speed and transparent heterojunction photodiode, which is essentially important for the next generation electronic devices. We grow semitransparent p-type Cu<SUB>4</SUB>O<SUB>3</SUB> using the reactive sputtering method at room temperature. The structural and optical properties of the Cu<SUB>4</SUB>O<SUB>3</SUB> film were investigated by using the X-ray diffraction and UV–visible spectroscopy, respectively. The p-Cu<SUB>4</SUB>O<SUB>3</SUB>/n-ZnO heterojunction diode under dark condition yields rectification behavior with an extremely low saturation current value of 1.8 × 10<SUP>−10</SUP> A and a zero bias photocurrent under illumination condition. The transparent p-Cu<SUB>4</SUB>O<SUB>3</SUB>/n-ZnO heterojunction photodetector can be operated without an external bias, due to the light-induced voltage production. The metal oxide heterojunction based on Cu<SUB>4</SUB>O<SUB>3</SUB>/ZnO would provide a route for the transparent and flexible photoelectric devices, including photodetectors and photovoltaics.</P> <P><B>Highlights</B></P> <P> <UL> <LI> All metal oxide device is proposed for advanced transparent photoelectric devices. </LI> <LI> Transparent metal oxide (p-Cu<SUB>4</SUB>O<SUB>3</SUB>/n-ZnO) device is fabricated room temperature. </LI> <LI> Flexible p-n heterojunction photodetector is operating without a bias. </LI> </UL> </P>