• 한국진공학회:학술대회논문집
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• 한국진공학회 2013년도 제44회 동계 정기학술대회 초록집
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• pp.187-187
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• 2013

양자점(Quuantum dot, QD)은 0차원 특성을 가지는 구조로 양자 구속 효과로 인하여 bulk와 는 다른 구조적, 광학적, 전기적 특성을 가지고 있다. InAs QD는 size와 barrier의 bandgap 조절을 이용하여 쉽게 bandgap을 바꿀 수 있는 장점이 있어 solar cell, semiconductor laser diode, infrared photodetector 등으로 많은 연구가 이루어지고 있다. 일반적으로 Stranski-Krastanov (SK) mode로 성장한 InAs QD는 보통 GaAs epilayer와의 lattice mismatch (7%)를 이용하여 성장을 하고 이로 인하여 strain을 가지고 있고 QD의 density와 stack이 높을수록 strain이 커진다. 하지만 sub-monolayer (SML) QD 같은 경우 wetting layer가 생기는 지점인 1.7 ML이하에서 성장되는 성장 방식으로 SK-QD보다는 작은 strain을 가지게 된다. 또 QD의 size가 작아 SK-QD보다 큰 bandgap을 가지고 있다. 본 연구에서는 분자선 에피택시(molecular beam epitaxy, MBE)를 이용하여 semi-insulating GaAs substrate 위에 InAs QD를 0.5/1/1.5/1.7/2/2.5 monolayer로 성장을 하였다. GaAs과 InAs의 성장온도와 성장속도는 각각 $590^{\circ}C$, 0.8 ML/s와 $480^{\circ}C$, 0.2 ML/s로 성장을 하였으며 적층사이의 interruption 시간은 10초로 고정하였고 10주기를 성장하였다. Photoluminescence (PL)측정 결과 SML-QD는 size에 따라서 energy가 1.328에서 1.314 eV로 약간 red shift를 하였고 SK-QD의 경우 1.2 eV의 energy정도로 0.1 eV이상 red shift 하였다. 이는 QD size에 의하여 energy shift가 있다고 사료된다. 또 wetting layer의 경우 1.41 eV의 energy를 가지는 것으로 확인 하였다. SML-QD는 SK-QD 보다 반치폭(full width at half maximum, FWHM)이 작은 것은 확인을 하였고 strain field의 감소로 해석된다. 하지만 SML-QD의 경우 SK-QD보다 상대적으로 작은 PL intensity를 가지고 있었다. 이를 개선하기 위해서는 보다 높은 QD density를 요구하게 되는데 growth temperature, V/III ratio, growth rate 등을 변화주어서 연구할 계획이다.

• Nuclear Engineering and Technology
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• 제5권1호
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• pp.55-64
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• 1973

압축변형시킨 순천(0.007% 탄소포함)을 8$0^{\circ}C$이하에서 저온열처리(ageing)한 후, 열처리시간에 따라 증가하는 저항복점(lower yield point)의 변화를 압축시험으로 측정하여 그 strain ageing 효과를 조사했다. 본 실험에서 압축변형된 순철의 strain ageing 속도는 저항복응력의 증가가 그치고 중가의 60% 정도에 이를 때까지 열처리시간의 2/3승에 비례했으나, 이미 알려진 인장변형의 경우에서보다 느렸다. 이 60%의 증가는 순철을 6$0^{\circ}C$에서 약 5시간 열처리함으로써 얻었다. 압축변형된 순철의 strain ageing을 위한 활성화에너지는 열처리의 초기단계(6$0^{\circ}C$에서 약 30분)에서 21,500 cal/mole이었는데, 이것은 인장변형의 경우에서 알려진 것보다 대략 10%가 큰 값이다. 이 증가는 잔유응력에 의해 결정내에 형성되는 strain field로 설명되었다. 열처리공정의 둘깨단계(6$0^{\circ}C$에서 약 5시간까지 계속되는)에서는 그 활성화에너지가 다소 감소되는 경향이 있었다.

• 대한용접접합학회:학술대회논문집
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• 대한용접접합학회 2002년도 Proceedings of the International Welding/Joining Conference-Korea
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• pp.250-254
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• 2002

Intergranular corrosion of austenitic stainless steels is a conventional and momentous problem during welding and high temperature use. One of the major reasons for such intergranular corrosion is so-called sensitization, i.e., chromium depletion due to chromium carbide precipitation at grain boundaries. Conventional methods for preventing sensitization of austenitic stainless steels include reduction of carbon content in the material, stabilization of carbon atoms as non-chromium carbides by the addition of titanium, niobium or zirconium, local solution-heat-treatment by laser beam, etc. These methods, however, are not without drawbacks. Recent grain boundary structure studies have demonstrated that grain boundary phenomena strongly depend on the crystallographic nature and atomic structure of the grain boundary, and that grain boundaries with coincidence site lattices are immune to intergranular corrosion. The concept of "grain boundary design and control", which involves a desirable grain boundary character distribution, has been developed as grain boundary engineering. The feasibility of grain boundary engineering has been demonstrated mainly by thermomechanical treatments. In the present study, a thermomechanical treatment was tried to improve the resistance to the sensitization by grain boundary engineering. A type 304 austenitic stainless steel was pre-strained and heat-treated, and then sensitized, varying the parameters (pre-strain, temperature, time, etc.) during the thermomechanical treatment. The grain boundary character distribution was examined by orientation imaging microscopy. The intergranular corrosion resistance was evaluated by electrochemical potentiokinetic reactivation and ferric sulfate-sulfuric acid tests. The sensitivity to intergranular corrosion was reduced by the thermomechanical treatment and indicated a minimum at a small roll-reduction. The frequency of coincidence-site-lattice boundaries indicated a maximum at a small strain. The ferric sulfate-sulfuric acid test showed much smaller corrosion rate in the thermomechanically-treated specimen than in the base material. An excellent intergranular corrosion resistance was obtained by a small strain annealing at a relatively low temperature for long time. The optimum parameters created a uniform distribution of a high frequency of coincidence site lattice boundaries in the specimen where corrosive random boundaries were isolated. The results suggest that the thermomechanical treatment can introduce low energy segments in the grain boundary network by annealing twins and can arrest the percolation of intergranular corrosion from the surface.

• 한국진공학회:학술대회논문집
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• 한국진공학회 1999년도 제17회 학술발표회 논문개요집
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• pp.29-29
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• 1999

The synthesis of layered structures on the nanometer scale has become essential for continued improvements in the operation of various electronic and magnetic devices. Abrupt metal-metal interfaces are desired for applications ranging from metallization in semiconductor devices to fabrication of magnetoresistive tunnel junctions for read heads on magnetic disk drives. In particular, characterizing the interface structure between various transition metals (TM) and aluminum is desirable. We have used the techniques of MeV ion backscattering and channeling (HEIS), x-ray photoemission (ZPS), x-ray photoelectron diffraction(XPD), low-energy ion scattering (LEIS), and low-energy electron diffraction(LEED), together with computer simulations using embedded atom potentials, to study solid-solid interface structure for thin films of Ni, Fe, Co, Pd, Ti, and Ag on Al(001), Al(110) and Al(111) surfaces. Considerations of lattice matching, surface energies, or compound formation energies alone do not adequately predict our result, We find that those metals with metallic radii smaller than Al(e.g. Ni, Fe, Co, Pd) tend to form alloys at the TM-Al interface, while those atoms with larger atomic radii(e.g. Ti, Ag) form epitaxial overlayers. Thus we are led to consider models in which the strain energy associated with alloy formation becomes a kinetic barrier to alloying. Furthermore, we observe the formation of metastable fcc Ti up to a critical thickness of 5 monolayers on Al(001) and Al(110). For Ag films we observe arbitrarily thick epitaxial growth exceeding 30 monolayers with some Al alloying at the interface, possible driven by interface strain relief. Typical examples of these interface structures will be discussed.

• 한국진공학회지
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• 제6권3호
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• pp.263-266
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• 1997

$In_{0.1}Ga_{0.9}As$/GaAs의 계면 특성을 상온에서 photoreflectance(PR) 측정을 통하여 연구 하였다. 에피층의 두께가 증가함에 따라 PR 신호에서 Franz-Keldysh oscillation(FKO)의 주 기가 감소하였고, 계면 전기장은 감소되었다. 이것은 InGaAs와 GaAs의 이종접합계면 부근 에서 격자부정합에 의한 결함이 증가되었기 때문으로 생각된다. 에피층의 두께가 300$\AA$보다 적은 경우 두께가 얇아짐에 따라 InGaAs층이 임계 두께에 가까워져 strain의 영향으로 밴 드갭 에너지가 크게 이동하였다.

• 한국자기학회:학술대회 개요집
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• 한국자기학회 2013년도 임시총회 및 하계학술연구발표회
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• pp.19-20
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• 2013

Thermoelectic properties of the typical thermoelectric host materials, the tellurides and selenides, are known to be noticeably changed by their volume change due to the strain [1]. In the bismuth telluride ($Bi_2Te_3$) crystal, a substitution of rare-earth element by replacing one of the Bi atoms may cause the change of the lattice parameters while remaining the rhombohedral structure of the host material. Using the first-principles approach by the precise full potential linearized augmented plane wave (FLAPW) method [2], we investigated the Ce substitution effect on the thermoelectric transport coefficients for the bismuth telluride, employing Boltzmann's equation in a constant relaxation-time approach fed with the FLAPW wave-functions within the rigid band approximation. Depending on the real process of re-arrangement of atoms in the cell to reach the equilibrium state, $CeBiTe_3$ was found to manifest a metal or a narrow bandgap semiconductor. This feature along with the strong correlation effect originated by the 4f states of Ce affect significantly on the thermoelectric properties. We showed that the position of the strongly localized f-states in energy scale (Fig. 1, f-states are shaded) was found to alter critically the transport properties in this material suggesting an opportunity to improve the thermoelectric efficiency by tuning the external strain which may changing the location of the f-sates.

• 광물과 암석
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• 제35권3호
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• pp.259-271
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• 2022

광물의 격자구조가 특정한 방향성을 보이는 격자선호방향은 광물의 변형 조건에 따라 다르기 때문에, 해당 광물과 이를 포함한 암석의 변형 조건을 연구하는데 있어 유용하다. 이번 연구에서는 경기육괴의 남서부지역에 위치한 유구읍 추계리 금계산 일대의 각섬암류를 채취하여 암석내부 각섬석과 장석의 격자선호방향을 후방산란전자회절 기기를 사용하여 분석하였다. 분석결과 유구지역의 각섬석에서는 type IV와 type I 두가지 격자선호방향이 관찰되었다. 유구지역의 각섬암류 내 각섬석은 격자선호방향에 관계없이 강체회전에 의해 변형을 받은것으로 보이며, 암석의 변형정도가 결정입도와 격자선호방향에 영향을 준 것으로 생각된다. 각섬석의 결정입도가 커서 변형을 가장 작게 받은것으로 생각되는 시료에서는 각섬석이 강한 type I 격자선호방향을 보여주었다. 이에 반해, 각섬석의 결정입도가 작아 고변형을 받은 것으로 생각되는 시료들에서는 각섬석이 약한 type IV 격자선호방향을 보여 주었다. 유구지역에서 관찰되는 다양한 암석의 변형정도는 각섬암류와 인접해있는 페리도타이트에서도 관찰된 바 있어, 유구지역이 다양한 수준의 변형을 받았음을 지시한다.

• 한국전기전자재료학회:학술대회논문집
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• 한국전기전자재료학회 2010년도 하계학술대회 논문집
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• pp.31-31
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• 2010

The growth of the high-quality GaN epilayers is of significant technological importance because of their commercializedoptoelectronic applications as high-brightness light-emitting diodes (LEDs) and laser diodes (LDs) in the visible and ultraviolet spectral range. The GaN-based heterostructural epilayers have the polar c-axis of the hexagonal structure perpendicular to the interfaces of the active layers. The Ga and N atoms in the c-GaN are alternatively stacked along the polar [0001] crystallographic direction, which leads to spontaneous polarization. In addition, in the InGaN/GaN MQWs, the stress applied along the same axis contributes topiezoelectric polarization, and thus the total polarization is determined as the sum of spontaneous and piezoelectric polarizations. The total polarization in the c-GaN heterolayers, which can generate internal fields and spatial separation of the electron and hole wave functions and consequently a decrease of efficiency and peak shift. One of the possible solutions to eliminate these undesirable effects is to grow GaN-based epilayers in nonpolar orientations. The polarization effects in the GaN are eliminated by growing the films along the nonpolar [$11\bar{2}0$] ($\alpha$-GaN) or [$1\bar{1}00$] (m-GaN) orientation. Although the use of the nonpolar epilayers in wurtzite structure clearly removes the polarization matters, however, it induces another problem related to the formation of a high density of planar defects. The large lattice mismatch between sapphiresubstrates and GaN layers leads to a high density of defects (dislocations and stacking faults). The dominant defects observed in the GaN epilayers with wurtzite structure are one-dimensional (1D) dislocations and two-dimensional (2D) stacking faults. In particular, the 1D threading dislocations in the c-GaN are generated from the film/substrate interface due to their large lattice and thermal coefficient mismatch. However, because the c-GaN epilayers were grown along the normal direction to the basal slip planes, the generation of basal stacking faults (BSFs) is localized on the c-plane and the generated BSFs did not propagate into the surface during the growth. Thus, the primary defects in the c-GaN epilayers are 1D threading dislocations. Occasionally, the particular planar defects such as prismatic stacking faults (PSFs) and inversion domain boundaries are observed. However, since the basal slip planes in the $\alpha$-GaN are parallel to the growth direction unlike c-GaN, the BSFs with lower formation energy can be easily formed along the growth direction, where the BSFs propagate straightly into the surface. Consequently, the lattice mismatch between film and substrate in $\alpha$-GaN epilayers is mainly relaxed through the formation of BSFs. These 2D planar defects are placed along only one direction in the cross-sectional view. Thus, the nonpolar $\alpha$-GaN films have different atomic arrangements along the two orthogonal directions ($[0001]_{GaN}$ and $[\bar{1}100]_{GaN}$ axes) on the $\alpha$-plane, which are expected to induce anisotropic biaxial strain. In this study, the anisotropic strain relaxation behaviors in the nonpolar $\alpha$-GaN epilayers grown on ($1\bar{1}02$) r-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVO) were investigated, and the formation mechanism of the abnormal zigzag shape PSFs was discussed using high-resolution transmission electron microscope (HRTEM).

• 한국광학회지
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• 제21권6호
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• pp.230-234
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• 2010

큰 복굴절 값을 갖는 광자결정광섬유는 실리카 단일 물질로 구성되어 온도 민감성이 매우 낮다. 이를 이용하여 온도변화에 영향을 받지 않는 스트레인 및 구부림 센서를 구현하였다. 이때, 스트레인 변화에 따른 민감도는 $1.41\;pm/{\mu}{\varepsilon}$ 이고, 구부림에 대한 민감도는 타원코어 장축 방향과 단축 방향으로의 구부렸을 때 각각 $0.93\;nm/m^{-1}$, $-1.6\;nm/m^{-1}$ 로 차이가 있었다.

• 한국재료학회지
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• 제23권7호
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• pp.373-378
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• 2013

In order to clarify the effect of C/Ti atom ratios(${\chi}$) on the deformation behavior of $TiC_{\chi}$ at high temperature, single crystals having a wide range of ${\chi}$, from 0.56 to 0.96, were deformed by compression test in a temperature range of 1183~2273 K and in a strain rate range of $1.9{\times}10^{-4}{\sim}5.9{\times}10^{-3}s^{-1}$. Before testing, $TiC_{\chi}$ single crystals were grown by the FZ method in a He atmosphere of 0.3MPa. The concentrations of combined carbon were determined by chemical analysis and the lattice parameters by the X-ray powder diffraction technique. It was found that the high temperature deformation behavior observed is the ${\chi}$-less dependent type, including the work softening phenomenon, the critical resolved shear stress, the transition temperature where the deformation mechanism changes, the stress exponent of strain rate and activation energy for deformation. The shape of stress-strain curves of $TiC_{0.96}$, $TiC_{0.85}$ and $TiC_{0.56}$ is seen to be less dependent on ${\chi}$, the work hardening rate after the softening is slightly higher in $TiC_{0.96}$ than in $TiC_{0.85}$ and $TiC_{0.56}$. As ${\chi}$ decreases the work softening becomes less evident and the transition temperature where the work softening disappears, shifts to a lower temperature. The ${\tau}_c$ decreases monotonously with decreasing ${\chi}$ in a range of ${\chi}$ from 0.86 to 0.96. The transition temperature where the deformation mechanism changes shifts to a lower temperature as ${\chi}$ decreases. The activation energy for deformation in the low temperature region also decreased monotonously as ${\chi}$ decreased. The deformation in this temperature region is thought to be governed by the Peierls mechanism.