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차가운 백색광 양자점 광발광 다이....

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글쓴이 최고관리자 등록일 14-10-22 20:34
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    미국의 미시간 대학(University of Michigan)과 독일의 OSRAM Opto Semiconductors GmbH의 연구진은 양자점 구조를 사용해서 차가운 백색광 광발광 다이오드를 개발하는데 성공했다. 

    이 장치는 질화 인듐 갈륨(InGaN) 양자점 능동 펌프(active pump)와 컨버터(convertor)로 구성되었다. 펌프 부분은 컨버터 영역 속의 더 긴 황색/오렌지 파장에서 변환된 청색의 스펙트럼 피크를 가진 광을 방출했다. 상업적으로, 백색 LED는 청색 또는 자외선에 의한 더 복잡한 형광체를 기반으로 주로 만들어진다. 그러나 이러한 장치들은 전력 손실이 발생하고, 가열로 인해서 형광체가 분해되는 경향이 있다. 

    양자 우물 펌프/컨버터 시스템은 광자 속의 전자-홀 재결합을 억제하는 경향을 가진 편광 유도된 전기장으로부터 발생되는 효율 제한 문제를 가진다. 양자점은 이런 분극 유도형 전기장을 감소시키는 변형 완화 프로세스(strain relaxation process)로 형성되고, 광자의 재결합 효율을 향상시킨다. 그러나 양자점 속의 캐리어 주입에 대한 배리어는 양자 우물보다 주로 더 높다. 

    양자점 구조들은 Veeco Gen 930 시스템을 사용해서 사파이어 템플릿 위의 c-평면 GaN 위에 플라즈마 보조 분자빔 에피택시(plasma-assisted molecular beam epitaxy)에 의해서 성장되었다. 양자점의 다중 층들은 컨버터와 펌프 영역 속의 GaN 배리어에 의해서 분리되었다. 장(長)-파장 컨버터 양자점 적층은 12nm의 배리어를 가진 541~550°C에서 성장되었다. 펌프 적층은 16 nm의 배리어를 가진 585~592°C에서 성장되었다. 

    n-GaN 300 nm 버퍼와 150 nm 접촉 층은 720°C에서 성장되었다. p-AlGaN 전자 차단층과 p-GaN 접촉층은 680℃에서 성장되었다. 이번 연구진은 백색광과 효율을 조절할 목적으로 양자점 형성 프로세스의 매개변수와 양자점 층의 개수를 최적화했다. 메사(mesa) LED는 600μmx600μm이었다. 장치는 p-GaN 접촉층 위에 반투명한 니켈/금 윈도우와 물질의 측면 위의 알루미늄 미러가 결합되었다. 턴온(turn-on) 전압은 20-24Ω의 면저항을 가진 5.5~6.0V이었다. 

    이 장치는 평면 양자점 LED보다 전류에 따라서 더 작은 청색-편이를 가졌다. 이번 연구진은 양자점 층과 비교했을 때 성장 프로세스 속의 변형 이완 때문에 양자점 층 속의 더 작은 편광에 기여한다. 외부 양자 효율(external quantum efficiency, EQE)은 37~45A/cm2의 주입 전류 밀도에서 최대치를 가진다. 100A/cm2에서 효율 저하는 14~17%이다. 

    작은 청색 이동은 각각 25A/cm2와 100A/cm2에서 4375K에서 4585K까지 상관 색온도(correlated color temperature)에서 210K의 작은 변화를 초래했다. 유사한 작은 변화들이 다른 장치에서도 발견되었다. 

    차가운 백색은 4500 K에 해당하고 뜨겁고 부드러운 백색은 3000 K에 해당한다. 이번 연구진은 “상관 색온도에서의 이런 수치는 모든 양자 우물 컨버터 LED에서 달성하기가 어렵다. 높은 방사성 효율을 가진 장파장(600 nm 이상) 방출이 이러한 InGaN/GaN 양자 우물로 달성하기가 어렵기 때문”이라고 말했다. 

    양자 우물 또는 형광체 기반의 색상 변환은 5500 K 이상의 더 높은 상관 색온도를 초래한다. InGaN/GaN 양자점 컨버터 LED가 차가운 백색 및 뜨거운 백색 광원의 구현에 중요할 것이라고 이번 연구진은 믿고 있다. 이 연구결과는 저널 Applied Physics Letters에 “Monolithic phosphor-free InGaN/GaN quantum dot wavelength converter white light emitting diodes” 라는 제목으로 게재되었다(http://dx.doi.org/10.1063/1.4896304). 

    그림. (a) InGaN/GaN 양자점 파장 변환 백색 LED 헤테로구조의 구조도. (b) 450 nm의 최대 방출을 가진 자기 조직된 In0.24Ga0.76N/GaN 양자점의 원자 힘 현미경 이미지. (c) 600 nm에서 최대 방출을 가진 In0.37Ga0.63N/GaN 양자점. (d) 청색 발광 펌프 점(pump dot)의 상온 광발광. (e) LED 헤테로구조의 상온 광발광. 

    Comments (2)

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