• 한국진공학회:학술대회논문집
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• 한국진공학회 2012년도 제42회 동계 정기 학술대회 초록집
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• pp.358-359
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• 2012

최근 분극 특성이 상이한 무분극 GaN 에피성장에 관한 심도 있는 연구와 함께 전자-전공 캐리어의 주입 및 캐리어의 거동, 방출되는 편광 특성 및 다양한 물리적 특성들에 대해 보고되고 있으며, 광학적 특성 및 물리적 특성의 확보를 위한 많은 연구가 활발히 진행 중이다 [1]. GaN의 ohmic 접촉(ohmic contact)의 형성은 발광 다이오드(light emitting diode), 레이저 다이오드(Laser), 태양전지(solar cell)와 같은 고신뢰도, 고효율 광전자 소자를 제조하기 위해서는 매우 중요하다 [2]. 그러나 이와 함께 병행 되어야 할 무분극 p-GaN 의 ohmic contact에 관한 연구는 많이 이루어지고 있지 않는 실정이다. 따라서 본 논문에서는 r-plane 사파이어 기판 상에 성장된 p-GaN에서의 ohmic 접촉 형성 연구를 위하여 Ni/Au ohmic 전극의 접촉저항 특성을 연구하였다. 본 실험에서는 성장된 a-plane GaN의 Hole농도가 $3.09{\times}1017cm3$ 인 시편을 사용하였다. E-beam evaporation 장비를 이용하여 Ni/Au를 각각 20 nm 그리고80 nm 증착 하였으며 비접촉저항을 측정하기 위해 Circle-Transfer Length Method (C-TLM) 패턴을 사용하였다. 샘플은 RTA (Rapid Thermal Annealing)를 사용하여 $300^{\circ}C$에서 $700^{\circ}C$까지 온도를 변화시키며 전기적 특성을 비교하여 그림 1(a) 나타내었다. 그림에서 알 수 있듯이 $400^{\circ}C$에서 가장 낮은 비접촉저항 값인 $6.95{\times}10-3{\Omega}cm2$를 얻을 수 있음을 발견하였다. 이 때의 I-V curve 도 그림1(b)에 나타낸 바와 같이 열처리에 의해 크게 향상됨을 알 수 있다. 그러나, $500^{\circ}C$ 이상 온도를 증가시키면 다시 비접촉 저항이 증가하는 것을 관찰하였다. XRD (x-Ray Diffraction) 분석을 통하여 $400^{\circ}C$ 이상열처리 온도가 증가하면 금속 표면에 $NiO_2$가 형성되며, 이에 따라 오믹특성이 저하 된다고 사료된다. 또한 $Ni_3N$의 존재를 확인 하였으며 이는 nonpolar surface의 특성으로 인해 nitrogen out diffusion 현상이 동시에 발생하여 계면에는 dopant로 작용하는 질소 공공을 남기고 표면에 $Ni_3N$을 형성하여 ohmic contact의 특성이 저하되기 때문인 것으로 사료된다.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2010년도 제39회 하계학술대회 초록집
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• pp.207-207
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• 2010

Both ZnO and GaN have excellent physical properties in optoelectronic devices such as blue light emitting diode (LED), blue laser diode (LD), and ultra-violet (UV) detector. The ZnO/GaN heterostructure, which has a potential to achieve the cost efficient LED technology, has been fabricated by using radio frequency (RF) sputtering, pyrolysis, metal organic chemical vapor deposition (MOCVD), direct current (DC) arc plasmatron, and pulsed laser deposition (PLD) methods. Among them, the PLD system has a benefit to control the composition ratio of the grown film from the mixture target. A 500-nm-thick ZnO film was grown by PLD technique on c-plane GaN/sapphire substrates. The post annealing process was executed at some varied temperature between from $300^{\circ}C$ to $900^{\circ}C$. The morphology and crystal structural properties obtained by using atomic force microscope (AFM) and x-ray diffraction (XRD) showed that the crystal quality of ZnO thin films can be improved as increasing the annealing temperature. We will discuss the post-treatment effect on film quality (uniformity and reliability) of ZnO/GaN heterostructures.

• 한국결정성장학회지
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• 제26권2호
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• pp.58-61
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• 2016

Basic 암모노써멀 방법을 사용하여 벌크 GaN 단결정을 성장시켰다. 종자 결정은 Hydride vapor phase epitaxy법으로 성장한 c면의 GaN 템플릿을 사용하고 광화제는 sodium metal, amide, azide를 사용하였다. 결정 성장 온도는 $500{\sim}600^{\circ}C$, 성장 압력은 2~3 kabr에서 실시하였다. c 축의 결정 성장 속도는 운용 압력이 증가함에 따라 선형적으로 증가하였다. Cathodoluminescence로 측정한 평균 전위 밀도는 $1{\times}10^5/cm^2$였다. Double Crystals X-ray Diffraction로 측정한 (002)면의 반치폭은 Ga 면에 대해서는 약 270 arcsec, N 면에 대해서는 약 80 arcsec이었다.

• 한국진공학회:학술대회논문집
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• 한국진공학회 2009년도 제37회 하계학술대회 초록집
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• pp.142-142
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• 2009

• 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.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2002년도 하계학술대회 논문집
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• pp.13-17
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• 2002

Ga$_2$O$_3$ nanomaterials were synthesized from mechanically ground GaN powders with thermal annealing Ga$_2$O$_3$ nanobelts were farmed in a nitrogen atmosphere, while Ga$_2$O$_3$ nanoparticles were formed inan oxygen atmosphere. The structural properties of the Ga$_2$O$_3$ nanomaterials were investigated by X-ray diffractometer (XRD) and high-resolution transmission eleotron microscope (HRTEM). The study of field emission scanning electron microscopy (FESEM) on the microstructures of nanomaterials revealed that the nanobelts are with the range of about 10∼200nm width and 10∼50nm thickness, and that nanoparticles are with the range of about 20∼50nm radius. On the basis of XRD and HRTEM data, we determined that the nanobelts grow toward a direction perpendicular to the (010) lattice plane and that they are enclosed by facets of the (10T) and (101) lattice planes. The formation of the nanobelts may be described by the vapor-solid(VS) mechanism, and the supersaturation device of gaseous phase may play an important role in the formation of Ga$_2$O$_3$ nanomaterials.

• 한국광학회:학술대회논문집
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• 한국광학회 2007년도 하계학술발표회 논문집
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• pp.237-238
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• 2007

A new strain analysis model, so called the stress matched model, in an epitaxial multilayer system is proposed. The model makes it possible to know the strain, the stress, the elastic strain energy in each epitaxial layer. Analytical formulas of strain parameters in each epitaxial layer are derived under assumptions that the substrate thickness is finite and the in-plane lattice constant is the same for all epitaxial layers for dislocation free growth. As an example, the model is applied to a 405nm InGaN/InGaN multiple quantum well laser diode. Analysis result shows that AlxGa1-xN layer with Al mole fraction of 0.06 and the thickness of 6${\mu}m$ is one of good templates for a laser. In fact, this layer structure coincides with experimentally optimized one.

• 한국결정성장학회지
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• 제25권2호
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• pp.45-50
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• 2015

본 논문에서는 Al 조성이 점진적으로 변화된 AlGaN 에피층을 HVPE (hydride vapor phase epitaxy) 방법에 의하여 성장하였다. 소스영역의 온도는 $950^{\circ}C$, 성장 영역의 온도는 $1145^{\circ}C$에서 연속적으로 (0001) 사파이어 기판위에 성장되었고, AlGaN 에피층은 시간당 100 nm의 성장률을 보였다. FE-SEM 측정과 EDS 측정으로부터 성장층의 Al 변화를 확인하였으며, AFM 측정결과 2인치 기판위에 성장된 graded AlGaN 에피층의 거칠기는 수십 nm였다. Al 조성의 변화는 XRD 측정에 의하여 확인하였으며, Al 조성 74 %의 (002) AlGaN의 주피크 관측과 함께 연속적으로 (002) AlN 층의 피크가 확인되었다. 이는 하나의 층에 사파이어 기판으로부터 Al 조성이 점진적으로 변화하는 에피층을 HVPE 방법으로 얻었음을 증명하며, 이 결과로부터 다양한 광소자 및 전자소자의 응용이 기대된다.

• 전자공학회논문지A
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• 제30A권10호
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• pp.33-40
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• 1993

A reflection type 16x8 S-SEED array from LP(Low Pressure)-MODVD-grown GaAs/AlGaAs extremely shallow quantum well(ESQW) structures, with 4% Al fraction, has been fabricated. Its intrinsic region consists of 50 pairs of alternating 100.angs. GaAs and 100.angs. $Al_{0.04}$Ga$_{0.96}$As layers. A multilayer reflector stack of $Al_{0.04}$/Ga$_{0.96}$ As(599$\AA$)/AlAs(723$\AA$) was incorporated for the reflection plane below the p-i-n structures. The device processing after the MOCVD growth includes the mesa etching, isolation etching, insulator deposition, p & n metallization, and AR(Anti-Reflection) coating. For switching characteristics of the S-SEED in the form of p-i-n ESQW diode, the maximum optical negative resistance was observed at 856nm. Reflectance measurements showed a change from 15.6% to 43.3% for +0.9V to -6V bias. The maximum contrast ration of the S-SEED array was 2.0 and all the 128 devices showed optical bistability with contrast ratios over 2.4 at 5V reverse bias.

• ETRI Journal
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• 제28권6호
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• pp.787-789
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• 2006

In this letter, we report that vertically well-aligned ZnO nanowires were grown on GaN epilayers and c-plane sapphire via a vapor-liquid-solid process by introducing a 3 nm Au thin film as a catalyst. In our experiments, epitaxially grown ZnO nanowires on Au-coated GaN were vertically well-aligned, while nanowires normally tilted from the surface when grown on Au-coated c-$Al_2O_3$ substrates. However, pre-growth annealing of the Au thin layer on c-$Al_2O_3$ resulted in the growth of well-aligned nanowires in a normal surface direction. High-resolution transmission electron microscopy measurements showed that the grown nanowires have a hexagonal c-axis orientation with a single-crystalline structure.