• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.11a
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• pp.112-112
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• 2009

In this study, the effect of a long post-deposition thermal annealing(600 and 1000 $^{\circ}C$) on the surface acoustic wave (SAW) properties of polycrystalline (poly) aluminum-nitride (AlN) thin films grown on a 3C-SiC buffer layer was investigates. The poly-AlN thin films with a (0002) preferred orientation were deposited on the substrates by using a pulsed reactive magnetron sputtering system. Experimental results show that the texture degree of AlN thin film was reduced along the increase in annealing temperature, which caused the decrease in the electromechanical coupling coefficient ($k^2$). The SAW velocity also was decreased slightly by the increase in root mean square (RMS) roughness over annealing temperature. However, the residual stress in films almost was not affect by thermal annealing process due to small lattice mismatch different and similar coefficient temperature expansion (CTE) between AlN and 3C-SiC. After the AlN film annealed at 1000 $^{\circ}C$, the insertion loss of an $IDT/AlN/3C-SiC/SiO_2/Si$ structure (-16.44 dB) was reduced by 8.79 dB in comparison with that of the as-deposited film (-25.23 dB). The improvement in the insertion loss of the film was fined according to the decrease in the grain size. The characteristics of AlN thin films were also evaluated using Fourier transform-infrared spectroscopy (FT-IR) spectra and X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM) images.

• Journal of the Korean Crystal Growth and Crystal Technology
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• v.9 no.6
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• pp.574-579
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• 1999

A counter-flow type horizontal reactor of metal organic chemical vapor deposition was designed with the Reynolds and the Rayleigh numbers of Re = 4.5 and Ra = 215.8, respectively. The GaN thin films were grown and characterized by Hall measurement, double crystal X-ray diffraction analysis and photoluminescence measurement. The Si and Mg were also used for doping of GaN films. The dislocation density of $2.6{\times}10^8/\textrm {cm}^2$ was included in GaN films representing the geometrical lattice mismatch between sapphire substrates and GaN films. The Si doped n-GaN films provide the electron carrier density and mobility in the regions of $10^{17}~10^{18}/\textrm{cm}^3$ and 200~400 $\textrm{cm}^2$/V .sec, respectively. Mg doped p-GaN films were post-annealed and activated with the hole carrier density of $8{\times}10^{17}/{\textrm}{cm}^3$.

• Proceedings of the Korean Vacuum Society Conference
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• 2015.08a
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• pp.257.1-257.1
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• 2015

Cu(In,Ga)Se2 (CIGS) 박막 태양 전지에서 buffer layer는 CIGS 흡수층과 TCO 사이의 밴드갭 차이에 대한 문제점과 lattice mismatch를 해결하기 위해 필수적이다. 흔히 buffer layer 물질로는 CdS가 가장 많이 사용되고 있으나 Cd의 독성에 관한 문제가 야기되고 있다. 따라서 ZnS(O, OH) buffer layer가 친환경 물질로 기존의 CdS 버퍼 층의 대체 물질로 각광 받고 있으며, 단파장 범위에서 높은 투과율로 인해 wide band gap의 Chalcopyrite 태양 전지에 응용되는 buffer layer로 많은 연구가 이루어지고 있다. 또한 buffer layer를 최적화 하여 carrier lifetime과 양자 효율이 증가시킬 수 있는 특성을 가지고 있다. 이 연구에서는 Cu(In,Ga)Se2 (CIGS) 박막에 화학습식공정 (CBD) 방법을 이용하여 최적화된 ZnS(O, OH)의 증착 조건을 찾고, 고품질의 buffer layer를 제조하기 위한 실험에 초점을 맞췄다. 또한, buffer layer의 막질을 개선하고 균일한 막을 제조하기 위해 processing parameters인 시약의 농도, 제조 시간 및 온도 등의 다양한 변화를 통해 실험을 진행하였다. 그 후 최적화된 ZnS(O, OH) buffer layer의 특성 분석을 위해 X-ray diffraction(XRD), photoluminescence (PL), scanning electron microscope (SEM) and GD-OES을 이용하였고, 이를 통해 제조된 CIGS 박막 태양전지는 light induced current-voltage (LIV) and external quantum efficiency (EQE)를 통해 특성 분석을 실시 하였다. 결과적으로, 제조된 ZnS(O, OH) buffer layer의 $ZnSO4{\cdot}7H2O$의 농도는 0.16 M, Thiourea는 0.5 M, NH4OH는 7.5 M, 그리고 반응 온도는 77.5 oC의 조건 하에 CIGS 기판 위에 균일하고 균열이 없는 ZnS(O, OH) 박막을 제조하였으며 이때 제조된 태양전지의 소자 특성은 Voc = 0.478 V, Jsc = 35.79 mA/cm2, FF = 47.77%, ${\eta}=8,18 %$이다.

• Proceedings of the Korean Institute of Electrical and Electronic Material Engineers Conference
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• 2009.06a
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• pp.235-235
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• 2009

This paper describes the influence of a polycrystalline (poly) 3C-SiC buffer layer on the surface acoustic wave (SAW) properties of poly aluminum nitride (AlN) thin films by comparing the center frequency, insertion loss, the electromechanical coupling coefficient ($k^2$), andthetemperaturecoefficientoffrequency(TCF) of an IDT/AlN/3C-SiC structure with those of an IDT/AlN/Si structure, The poly-AlN thin films with an (0002)-preferred orientation were deposited on a silicon (Si) substrate using a pulsed reactive magnetron sputtering system. Results show that the insertion loss (21.92 dB) and TCF (-18 ppm/$^{\circ}C$) of the IDT/AlN/3C-SiC structure were improved by a closely matched coefficient of thermal expansion (CTE) and small lattice mismatch (1 %) between the AlN and 3C-SiC. However, a drawback is that the $k^2(0.79%)$ and SAW velocity(5020m/s) of the AlN/3C-SiC SAW device were reduced by appearing in some non-(0002)AlN planes such as the (10 $\bar{1}$ 2) and (10 $\bar{1}$ 3) AlN planes in the AlN/SiC film. Although disadvantages were shown to exist, the use of the AlN/3C-SiC structure for SAW applications at high temperatures is possible. The characteristics of the AlN thin films were also evaluated using FT-IR spectra, XRD, and AFM images.

• Korean Journal of Materials Research
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• v.24 no.12
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• pp.645-651
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• 2014

GaN is most commonly used to make LED elements. But, due to differences of the thermal expansion coefficient and lattice mismatch with sapphire, dislocations have occurred at about $109{\sim}1010/cm^2$. Generally, a low temperature GaN buffer layer is used between the GaN layer and the sapphire substrate in order to reduce the dislocation density and improve the characteristics of the thin film, and thus to increase the efficiency of the LED. Further, patterned sapphire substrate (PSS) are applied to improve the light extraction efficiency. In this experiment, using an AlN buffer layer on PSS in place of the GaN buffer layer that is used mainly to improve the properties of the GaN film, light extraction efficiency and overall properties of the thin film are improved at the same time. The AlN buffer layer was deposited by using a sputter and the AlN buffer layer thickness was determined to be 25 nm through XRD analysis after growing the GaN film at $1070^{\circ}C$ on the AlN buffer CPSS (C-plane Patterned Sapphire Substrate, AlN buffer 25 nm, 100 nm, 200 nm, 300 nm). The GaN film layer formed by applying a 2 step epitaxial lateral overgrowth (ELOG) process, and by changing temperatures ($1020{\sim}1070^{\circ}C$) and pressures (85~300 Torr). To confirm the surface morphology, we used SEM, AFM, and optical microscopy. To analyze the properties (dislocation density and crystallinity) of a thin film, we used HR-XRD and Cathodoluminescence.

• Proceedings of the Korean Vacuum Society Conference
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• 1995.02a
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• pp.36-36
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• 1995

The cubic (zinc blende) CdSe eppilayers were grown on GaAs(100) substrates by electron beam (e-beam) evapporation technique. X-ray scans with copper $K\alpha$ radiation indicate that the CdSe eppilayers are zinc blende. The lattice pparameter obtained from the (400) reflection is 6.077$\AA$, which is in excellent agreement with the value repported in the literature for zinc blende CdSe. The orientation of as-grown CdSe eppilayer is determined by electron channeling ppatterns(ECpp). The crystallinity of heteroeppitaxial CdSe layers were investigated based on the double crystal x-ray rocking curve(DCRC). The deppendence of the rocking curve width on layer thickness was studied. The FWHM(full width at half maximum) of CdSe eppilayers grown on GaAs(100) substrates is decreasing with increasing eppilayer thickness. The carrier concentration and mobility of the as-grown eppilayers deduced Hall data by van der ppauw method, are about 7$\times$1017 cm-3 and 2$\times$102 $\textrm{cm}^2$ / sec at room tempperature, resppectively. The energy gapp was determinded from the pphotocurrent sppectrum. In pphotocurrent sppectrum of a 1-${\mu}{\textrm}{m}$-thick CdSe eppilayer at 30K, the ppeak at 1.746 eV is due to the free exciton of cubic CdSe. In summary, We have shown that eppilayers of zinc blende CdSe can be grown on GaAs(100) substrates by e-beam, desppite the large mismatch between eppilayer and substrate, as well as the natural ppreference for CdSe to form in the wurtzite structure.

• Progress in Superconductivity
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• v.7 no.1
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• pp.69-76
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• 2005

Complex single buffer composed of yttrium and samarium oxide was deposited on the metallic substrates by MOCVD (metal organic chemical vapor deposition) method using single liquid source. Two different types of the substrates with in-plane textures of about $8{\sim}10$ degree of Ni and $3at.\%W-Ni$ alloy were used. Y(tmhd: 2,2,6,6-tetramethyl-3,5-heptane dionate)$_3$:Sm(tmhd)$_3$ of liquid source was adjusted to 0.4:0.6 to minimize the lattice mismatch between the complex single buffer and the YBCO. The epitaxial growth of $(Y_{x}Sm_{1-x})_{2}O_3$ was achieved at the temperature higher than $500^{\circ}C$ in $O_2$ atmosphere. However, it was found that the formation of NiO accelerated with increasing deposition temperature. By supplying $H_{2}O$ vapor, this oxidation of the substrate could be suppressed throughout the deposition temperatures. We could get the epitaxial growth on pure Ni substrate without the formation of NiO. The competitive (222) and (400) growths were observed at the deposition temperatures of $650\~750^{\circ}C$, but the (400) growth became dominant above $800^{\circ}C$. The $(Y_{x}Sm_{1-x})_{2}O_3$-buffered metallic substrates can be used as the buffer for YBCO coated conductor.

• Journal of the Korean Vacuum Society
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• v.7 no.4
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• pp.300-305
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• 1998

Crystallinity and structual properties of the epitaxially grown Pt films on $Al_2O_3$(0001) substrate by rf magnetron sputtering at a substrate temperature of $600^{\circ}C$ were studied by using backscattering spectrometry (BS)/channeling and transmission electron microscopy (TEM) measurements. $MeV^4$He ion BS/channeling results showed that the channeling minimum yield of Pt film with a thickness of 3500$\AA$ was 4%. This indicates an excellent crystallinity of Pt film. When the thickness of Pt film was less than 200 $\AA$, the channeling minimum yield of Pt film increased sharply with the decrease in film thickness. The Pt layer on $Al_2O_3$(0001) substrate grew epitaxially to the direction of (111) with six-fold symmetry. Cross-sectional TEM images also showed that Pt film on $Al_2O_3$(0001) substrate consist of twinned domains to release the strain induced by the lattice mismatch and the surface roughness of the film increased at the twin boundaries where the strain was contcentrated.

• Proceedings of the Korean Vacuum Society Conference
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• 2014.02a
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• pp.426.2-426.2
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• 2014

Wavefront control은 렌즈, 거울 등을 포함한 많은 광학소자를 대체할 수 있는 기술이며, 이는 광 직접 소자 개발에 매우 유용하다. 기존의 Distributed bragg reflector (DBR) 구조의 경우 lattice mismatch, 낮은 효율, 작은 굴절률 차이의 물질만을 사용해야 하는 문제 등으로 광 직접 소자에의 적용에는 한계가 있다. 본 연구에서는 이러한 한계점을 극복하고, 더 나아가 광학소자 구조 내에서의 빛의 거동을 조절하기 위해서 High-index contrast grating (HCG), 즉, 큰 굴절률 차이가 나는 물질로 이루어진 격자 구조 내의 빛이 가지는 waveguide 특성에 대한 연구가 수행되었다. 굴절률 차이가 큰 물질을 sub-wavelength의 주기적인 혹은 비주기적인 격자 구조로 만듦으로써 투과된 빛의 투과도와 위상 등을 조절할 수 있고 이를 통해 빛의 초점 거리, 휘어짐을 조절 할 수 있다. HCG 구조 내의 빛의 거동을 Rigorous coupled wave analysis (RCWA) 및 Finite element method (FEM) 계산을 이용하여 시뮬레이션 하였다. RCWA 계산을 통해 주기 격자구조의 투과도 및 반사도, 빛의 위상을 계산하여 비주기를 갖는 전체적인 HCG 구조를 결정하였고, FEM 계산을 통하여 그 구조 내에서 빛의 거동을 시뮬레이션 하였다. 1,300 nm 파장의 빛이 광원으로 사용되었고 시뮬레이션을 위해 낮은 굴절률의 물질로 ITO, 높은 굴절률의 물질로는 Si이 사용되었다. $15{\mu}m$ 포커싱, $7.91^{\circ}$의 휘어짐을 시뮬레이션 하였고, 실제 소자 공정을 하여 제작한 후, 광 측정 결과 포커싱은 $15{\mu}m$, 휘어짐은 $4.5{\sim}6.5^{\circ}$를 확인하였다. 이러한 결과를 바탕으로 HCG구조체를 통하여 빛의 엔지니어링이 가능함을 알 수 있었다. HCG구조체는 빛이 투과하는 광학 소자의 전반에 적용이 가능하며 더 나아가 인위적인 빛의 엔지니어링이 가능함을 시사한다.

• Korean Journal of Materials Research
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• v.11 no.4
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• pp.319-323
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• 2001

In order to investigate the effect of thickness on the properties of ZnO thin films, a series of films having different thickness were deposited on (0001) sapphire by using pulsed laser deposition(PLD). SEM and XRD analyses showed that, as the film thickness increases, the grain size increased and the crystallinity improved. Room-temperature PL spectra also exhibited that the intensities of both ultraviolet and deep level emission Peaks increased as the film thickness increased. Hall measurements at room- temperature revealed that, as the film thickness changes from 400 to 4000 , the carrier concentration of the film showed sharp decrease, which that of thicker film gradually saturated. Therefore, it is concluded that the strain due to the lattice mismatch between substrate and film is fully relaxed around the thickness of 4000 .