Up- and Downconversion Luminescence in Ho3+,Yb3+-Co-Doped Y2O3 Transparent Ceramics Prepared by Spark Plasma Sintering

박철우,인준형,박재화,심광보,류정호 대한금속·재료학회 2019 대한금속·재료학회지 Vol.57 No.9

We employed spark plasma sintering (SPS) in the presence of sintering aids to fabricate highquality Ho3+,Yb3+-co-doped Y2O3 transparent ceramics suitable for use as a laser gain medium, and analyzed their microstructure and optical properties. Ho3+,Yb3+-co-doped Y2O3 transparent ceramics with different contents of Yb3+ were fabricated by SPS and exhibited high transparency in both near-infrared and visible regions. All specimens showed high transparency in both the near-infrared and visible-light regions, and sample microstructure was almost unaffected by the amount of doped Yb3+. The transmission spectra of 1 mol% Yb3+ doped specimens exhibited transmissivities of 73.4 and 81.0% at wavelengths of 700 and 1500 nm, respectively. Relatively high transmissivity was quickly achieved through the addition of a sintering aid (1 mol% La2O3) which accelerated mass transport during sintering. The effect of Yb3+ doping on the upconversion photoluminescence of Y2O3:Ho3+,Yb3+ was examined. Irradiation at 980 nm resulted in strong green photoluminescence (552 nm) and weak red photoluminescence (669 nm). Excitation at 447 and 980 nm resulted in strong green emission (5F4, 5S2 → 5I8) from Ho3+. The effects of Yb3+ content and laser input power on emission intensity and luminescence decay were investigated in detail. Under excitation at 980 nm, the presence of Yb3+ increased emission intensity over the entire range, while the opposite behavior was observed under excitation at 447 nm.

LAN을 이용한 VOD 시스템의 구현

김윤범,인준형,최윤식,이정수 한국방송∙미디어공학회 1996 방송공학회논문지 Vol.1 No.2

본 논문에서는 차세대 영상 서비스 기술인 VOD(Video-on-Demand)시스템을 LAN을 이용하여 구현하였다. LAN을 이용한 VOD 시스템은 네트워크 병목현상과 이에 따른 시간 지연 때문에 기존의 네트워크 구현으로는 원활한 서비스가 되지 못하므로, 그러한 문제점을 해결하기 위한 방안으로, 본 논문에서는 토팔러지의 변경과 그에 따른 네트워크 장비의 선정, 개선된 프로토콜의 제정 등이 제시되었다. 결론적으로 구현된 시스템은 세그먼트 당 최대 3∼4명의 클라이언트를 수용할 수 있었고, 전송하고자 하는 영상 데이터의 크기에 따라 미리 네트워크 버퍼의 크기를 최적화하는 것이 서비스의 품질을 높이는데 필수적이라는 것을 알 수 있었다. In this paper, a VOD(Video -on-Demand) system is implemented using LAN(Local Area Network). The use of LAN causes the time-delay problem due to its narrow channel limitation, as a result, the service using the conventional LAN used not to be popular. In this paper, to solve these problems, we develop the modified topology, the selection methodology of the network equipment, and the dedicated(improved) protocol. Consequently, we can serve at most 3 or 4 clients at the same time, and we conclude it is necessary to optimize the network buffer size depending on the size of video data.

전력반도체용 GaN 기판 산업동향

이희애,박재화,이주형,박철우,강효상,인준형,심광보,Lee, Hee Ae,Park, Jae Hwa,Lee, Joo Hyung,Park, Cheol Woo,Kang, Hyo Sang,In, Jun Hyeong,Shim, Kwang Bo 한국결정성장학회 2018 한국결정성장학회지 Vol.28 No.4

최근 전세계적으로 환경오염 및 에너지 고갈에 대한 대책 마련의 일환으로 에너지 재생 및 절약 소자인 고효율 친환경 전력반도체로서 GaN 전력소자에 대한 관심이 고조되어가고 있다. 이를 위해서, GaN 단결정 기판의 사용이 절실히 요구되는 바, Yole사 보고서(2013)와 국내 외 GaN 관련 산학연의 최신 발표(2017)를 바탕으로 최근 GaN 단결정 산업동향을 리뷰하였다. GaN 단결정 기판에 대한 연구개발은 저결함, 대구경을 목표로 지속적으로 진행되고 있으나, 아직 시장형성이 활성화되고 있지 못하다. The demand on use of GaN single crystal substrates for high efficient and environment - friended high power electronic semiconductor has be increased. The industrial business trend on GaN substrate along with its research activities has been reviewed through the recent scientific and technical in formations on the basic of Yole report (2013). The research on the GaN single crystal substrate has been performed continuously for the purpose of the high quality and larger diameter, but its market has not been activated yet.

HVPE로 성장시킨 bulk GaN의 두께에 따른 광학적 특성 변화

이희애,박재화,이정훈,이주형,박철우,강효상,강석현,인준형,심광보 한국결정성장학회 2018 한국결정성장학회지 Vol.28 No.1

In this work, we investigated the variation of optical characteristics with the thickness of bulk GaN grown by hydride vapor phase epitaxy(HVPE) to evaluate applicability as GaN substrates in fabrication of high-brightness optical devices and high-power devices. We fabricated 2-inch GaN substrates by using HVPE method of various thickness (0.4, 0.9, 1.5 mm) and characterized the optical property with the variation of defect density and the residual stress using chemical wet etching, Raman spectroscopy and photoluminescence. As a result, we confirmed the correlation of optical properties with GaN crystal thickness and applicability of high performance optical devices via fabrication of homoepitaxial substrate. 본 연구에서는 고휘도 · 고출력 광학소자 제조에 GaN 기판으로서의 적용가능 여부를 평가하고자 HVPE 법으로성장된 bulk GaN 결정의 두께 증가에 따른 광학적 특성 변화를 분석하였다. HVPE를 이용하여 다양한 두께(0.4, 0.9, 1.5 mm 이상)의 2인치 GaN 기판을 제작한 뒤, 화학 습식 에칭, Raman, PL 등을 이용하여 기판의 결함밀도와 잔류응력 변화에따른 광학적 특성을 분석하였다. 이를 통해 제작된 GaN 기판의 결정 두께와 광학적 특성과의 상관관계를 확인하였으며, 동종기판의 제작을 통한 고성능 광학소자로의 응용가능성을 확인하였다.

화학적 기계적 연마 공정을 통한 bulk AlN 단결정의 표면 가공

이정훈,박철우,박재화,강효상,강석현,이희애,이주형,인준형,강승민,심광보 한국결정성장학회 2018 한국결정성장학회지 Vol.28 No.1

PVT법으로 성장된 AlN 단결정의 표면 평탄화 최적화 하기 위하여 기계적 연마 후 SiO2 slurry를 이용한 CMP 공정을 진행하였고 이에 따른 표면 형상, slurry 변화에 따른 가공 특성을 분석하였다. Slurry의 pH가 표면 연마 과정에 미치는 영향을 알아보기 위해 SiO2 slurry의 pH를 조절하였으며, 제타전위측정기를 통해 각각의 pH에 따른 zeta potential의영향과 MRR(material removal rate) 결과를 비교하였으며, 최종적으로 원자간력 현미경(atomic force microscope)을 이용한표면 거칠기 RMS(0.2 nm)를 얻을 수 있었다.

Ultra-fast densification of highly transparent Y2O3 ceramic with La2O3 as sintering aid by spark plasma sintering

박철우,박재화,강효상,이희애,이주형,인준형,심광보 한양대학교 세라믹연구소 2018 Journal of Ceramic Processing Research Vol.19 No.5

Highly transparent Y2O3 ceramics were produced using spark plasma sintering (SPS) at 1600 °C and 30 MPa for 5 min. Whenthe SPS process was applied with various amounts of La2O3 as dopant. The specimen doped with 3 mol% La2O3 showed thehighest density, and rapid particle growth and pore growth occurred, exhibiting that the relative density and average grainsize are 99.2% and 17.2 μm, respectively. The specimen showed excellent transmittance of 79.44% in the visible light region(600 nm), resulting that La2O3 would be a useful dopant for improving the transmittance and mechanical properties oftransparent Y2O3 ceramics produced with SPS.

Controlling of the heterogeniously growing GaN polycrystals using a quartz ring in the edge during the HVPE-GaN bulk growth

박재화,이희애,박철우,강효상,이주형,인준형,이승국,심광보 한양대학교 세라믹연구소 2018 Journal of Ceramic Processing Research Vol.19 No.5

The outstanding characteristics of high quality GaN single crystal substrates make it possible to apply the manufacture of highbrightness light emitting diodes and power devices. However, it is very difficult to obtain high quality GaN substrate becausethe process conditions are hard to control. In order to effectively control the formation of GaN polycrystals during the bulkGaN single crystal growth by the HVPE (hydride vapor phase epitaxy) method, a quartz ring was introduced in the edge ofsubstrate. A variety of evaluating method such as high resolution X-ray diffraction, Raman spectroscopy andphotoluminescence was used in order to measure the effectiveness of the quartz ring. A secondary ion mass spectroscopy wasalso used for evaluating the variations of impurity concentration in the resulting GaN single crystal. Through the detailedinvestigations, we could confirm that the introduction of a quartz ring during the GaN single crystal growth process usingHVPE is a very effective strategy to obtain a high quality GaN single crystal.

Effects of high-energy ball milling of commercial Y2O3 powders on their densification using spark plasma sintering

박철우,이정훈,강석현,박재화,김현미,강효상,이희애,이주형,인준형,심광보 한양대학교 세라믹연구소 2018 Journal of Ceramic Processing Research Vol.19 No.2

Commercial Y2O3 powders were treated by high-energy ball milling (HEBM) and then subjected to spark plasma sintering(SPS) in a furnace at 1600 oC under 30 MPa for 5 minutes to produce pure Y2O3 transparent ceramics. The sintered Y2O3bodies show a relative density of about 99.52% and a grain size of about 3.2 μm, which is approximately six times smaller thanthat of the specimen obtained using commercial powders. This result shows that HEBM is highly effective in improving thesinterability of Y2O3 with the controlled grain growth. The transmittance of all sintered specimen was similar in the infraredregion (2000 nm), but the specimen that underwent HEBM showed excellent transmittance in the visible light region (700 nm). In particular, the hardness and fracture strengths of the sintered bodies that underwent HEBM were 8.48 GPa and0.91MPa·m1/2, respectively, demonstrating excellent mechanical characteristics.