• 대한전자공학회논문지SD
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• 제49권5호
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• pp.10-15
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• 2012

본 논문에서는 고효율 고출력 DUV(Deep Ultra Violet)-LED(Light Emitting Diodes)의 구현을 위하여 n-side에서 p-side로 well 두께를 다르게 형성하는 비대칭 MQW 에피구조를 제안하였다. 제안된 구조의 물리적 해석을 위해 상용화된 3차원 시뮬레이터 SimuLEDTM을 이용하여 소자의 전기적/광학적 특성을 비교 분석 하였다. 시뮬레이션 결과, 본 연구에서 제안한 B구조(n-side에서 p-side 방향으로 well 두께를 2 nm, 3 nm, 4 nm 로 제작)의 에피층을 가지는 UV-LED는 기본 MQW 에피구조를 가지는 UV-LED와 동작전압은 8.9 V로 동일한 값을 가졌지만 광출력은 기본구조의 10.6 mW 에 비해 약 1.17배 향상된 12.4 mW의 값을 가지는 것을 확인하였다.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제45회 하계 정기학술대회 초록집
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• pp.99-99
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• 2013

In this talk, I will present two research works in progress, which are: i) mapping of piezoelectric polarization and associated charge density distribution in the heteroepitaxial InGaN/GaN multi-quantum well (MQW) structure of a light emitting diode (LED) by using inline electron holography and ii) in-situ observation of the polarization switching process of an ferroelectric Pb(Zr1-x,Tix)O3 (PZT) thin film capacitor under an applied electric field in transmission electron microscope (TEM). In the first part, I will show that strain as well as total charge density distributions can be mapped quantitatively across all the functional layers constituting a LED, including n-type GaN, InGaN/GaN MQWs, and p-type GaN with sub-nm spatial resolution (~0.8 nm) by using inline electron holography. The experimentally obtained strain maps were verified by comparison with finite element method simulations and confirmed that not only InGaN QWs (2.5 nm in thickness) but also GaN QBs (10 nm in thickness) in the MQW structure are strained complementary to accommodate the lattice misfit strain. Because of this complementary strain of GaN QBs, the strain gradient and also (piezoelectric) polarization gradient across the MQW changes more steeply than expected, resulting in more polarization charge density at the MQW interfaces than the typically expected value from the spontaneous polarization mismatch alone. By quantitative and comparative analysis of the total charge density map with the polarization charge map, we can clarify what extent of the polarization charges are compensated by the electrons supplied from the n-doped GaN QBs. Comparison with the simulated energy band diagrams with various screening parameters show that only 60% of the net polarization charges are compensated by the electrons from the GaN QBs, which results in the internal field of ~2.0 MV cm-1 across each pair of GaN/InGaN of the MQW structure. In the second part of my talk, I will present in-situ observations of the polarization switching process of a planar Ni/PZT/SrRuO3 capacitor using TEM. We observed the preferential, but asymmetric, nucleation and forward growth of switched c-domains at the PZT/electrode interfaces arising from the built-in electric field beneath each interface. The subsequent sideways growth was inhibited by the depolarization field due to the imperfect charge compensation at the counter electrode and preexisting a-domain walls, leading to asymmetric switching. It was found that the preexisting a-domains split into fine a- and c-domains constituting a $90^{\circ}$ stripe domain pattern during the $180^{\circ}$ polarization switching process, revealing that these domains also actively participated in the out-of-plane polarization switching. The real-time observations uncovered the origin of the switching asymmetry and further clarified the importance of charged domain walls and the interfaces with electrodes in the ferroelectric switching processes.

• 한국항행학회논문지
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• 제8권2호
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• pp.91-97
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• 2004

본 논문은 수직 구조와 고성능 특성을 가진 InGaAsP 다중양자우물(MQW; Multiple Quantum Well) 비대칭 페브리페롯 변조기(AFPM; Asymmetric Fabry-Perot Modulators)에 관한 연구결과로써 광대역 동작특성과 저가격이 요구되는 피코셀과 같은 차세대 광대역 무선통신 시스템에 응용 가능성을 제안하고자 한다. 이 AFPM은 <-2V 동작전압과 광섬유와 간단히 결합되어 결합손실이 3dB 내외인 장점과 -3dB 주파수응답 특성이 10GHz인 특성을 보이므로 광대역 및 다중 무선서비스가 요구되는 시스템에 적용 가능할 것으로 기대된다. 이를 위한 간단한 링크실험 결과 92dB/Hz의 SFDR(Spurious Free Dynamic Range)과 약 40dB의 IMD(Inter-Modulation Distortion)의 결과를 보였다.