Design and Fabrication of Nanoscale Metal Interconnections for Transparent Deformable Electronic Devices

김동욱 포항공과대학교 일반대학원 2021 국내박사

As transparent displays and touch screens begin to be introduced to the public, the technology for transparent deformable electronic devices is attracting enormous attention as a next-generation electronic technology. Deformable optoelectronic devices, such as displays, solar cells, touch screens, and smart windows that maintain their functions under mechanical deformations have been developed, and various approaches to transparent deformable electrodes have been studied intensively. Despite these interests, transparent deformable nano- and micro-scale integrated interconnections that are easy to be patterned and positioned are receiving somewhat less attention. Since the most successful and feasible concept leading to deformable devices is linking rigid islands of active device components (transistor, light-emitting devices, photovoltaics, etc.) with deformable interconnections, developing transparent interconnections that can retain good electrical performance under high mechanical strain is highly required. In this work, I designed and fabricated three different types of transparent deformable nanoscale metal interconnections; (i) One-dimensional (1D) metal nanolines which were deposited on the flexible substrates by simple and reliable nanofiber (NF) photolithography, (ii) 1D wavy stretchable single metal NF which were individually positioned by the electrohydrodynamic (EHD) printing and metallized through room-temperature electroless plating, and (iii) Two-dimensional (2D) foldable and stretchable gold (Au) film hybrid electrodes which were composed of the anisotropic conductive ultrathin films (ACUFs) and the ultrathin Au film electrodes. These fabricated metal interconnections were not only electrically deformable but also optically transparent due to their nanoscale dimensions, and were able to be individually positioned and patterned in desired positions, shapes, and alignments. Also, all three deformable interconnections are fabricated by low-temperature processes and can easily expanded to large-scale production. The fabricated nanoscale metal interconnections were used as the interconnecting electrodes in the transparent and deformable field-effect transistors (FETs) array and the transparent electrodes of the deformable light-emitting devices.

Investigation on Thermal Stability of Metal-Interlayer-Semiconductor Contacts Using Carbon Implantation

이동훈 포항공과대학교 일반대학원 2022 국내석사

Effects of carbon implantation (C-imp.) on the thermal stability of MIS (Metal-Interlayer-Semiconductor) contact were investigated. The experiment was conducted on both Si and Ge substrates. To improve the thermal stability in MIS contact, C-imp. into MIS structures was applied. The current density (J) - voltage (V) characteristics showed that C-imp. changed the rectifying behavior to the ohmic-like behavior. The Schottky barrier height (SBH) was also reduced by the C-imp. These improvements can be beneficial to reduce the lower contact resistivity (ρc) with the rapid thermal annealing (RTA) temperatures ranging from 450 to 600 ℃. From the transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) mapping, the MIS contact with C-imp. showed the suppression of oxygen diffusion into Ti layer. From the secondary ion mass spectrometry (SIMS) analysis, the segregation of P dopant at the interface was more facilitated with C-imp.. Thus, the C-imp. is promising to improve the thermal stability and to realize low contact resistivity of MIS contact.

The design of all-dielectric metasurface for selectively blocking Near-Infrared region of solar spectrum

김은종 포항공과대학교 일반대학원 2020 국내석사

Blocking near-infrared region (NIR) is indispensable for saving energy consumed to maintain an interior temperature in buildings. The methods of blocking NIR are divided into two types. One is blocking NIR without visible light for applying to windows. The other is blocking the overall solar spectrum including visible light because the visible light accounts for 45% in the solar spectrum. Thus, we designed the metasurface of two types, respectively. In the transparent metasurface blocking NIR, simultaneously enhancing transmission in visible light and blocking in NIR remains challenging. Here, we demonstrate the trans-parent all-dielectric metasurface selectively blocking the NIR by using TiO2 nanocylinder and ITO layer. The ITO layer is implemented as a back reflector because the ITO is trans-parent in visible light whereas the ITO becomes reflective materials in the long-wavelength region (λ > 1500 nm). The designed metasurface exhibits a high average transmittance of 70% in visible light and high solar energy rejection (SER) of 90% in NIR. Furthermore, the performance of the designed metasurface is maintained over a wide range of an incident angle of light. Therefore, the metasurface gives an advanced guide-line for design energy-saving applications. In the opaque metasurface blocking NIR, the previous all-dielectric metasurfaces have difficulty in reflecting overall NIR because the reflection region is too narrow. Here, we demonstrate the all-dielectric metasurface blocking almost overall NIR by using amor-phous Si (a-Si) and SiO2. Since amorphous Si has a high refractive index (~3.3) and high extinction coefficient in NIR, the designed metasurface exhibits high reflection (1050 nm ≤ λ ≤ 2320 nm) as well as high absorption (λ < 1040 nm), thereby leading to high solar energy rejection (SER) of 94% in NIR. The performance of the designed metasurface is independent over a wide range of incident light. Furthermore, a-Si substrate constituting the metasurface can be readily deposited on other materials such as glass and plastic film, so the proposed metasurface has high applicability for a large-area fabrication rather than crystalline Si and GaAs.

유한요소 모델을 이용한 풍력발전 블레이드 설계

박선호 포항공과대학교 대학원 2010 국내박사

The objective of this work was to design a 750kW wind turbine blade (IEC type class IA) using the finite element method (FEM). Only 3D finite element blade model was used during the whole detailed design procedure. The blade model is 24.1m long and consists of glass-epoxy laminate. The blade structure comprises multi-axial sandwich shells with two main UD glass tape spar caps and a UD reinforced trailing edge as well as a tri-axial (3AX) glass fabric in the blade root to transfer loads into the blade connection. 11,160 4-node shell elements and over 5,000 element groups were generated to model the composite blade of various stacking sequence. I-DEAS, commercial FEM code was applied in drawing and meshing, and ABAQUS in analysis. Four extreme cases which cover all design load cases specified in the GL guideline were defined and applied for structural analyses. The static and fatigue load carrying capacity, tip deflections and modal characteristics were evaluated to satisfy the strength and stability requirements. From calculations, it was verified that the designed rotor blade achieved the sufficient static and fatigue safety margins (S.F. or R.S.F > 1.0) in spite of increased natural frequencies. According to the final results of structural design, the prototype of the rotor blade was manufactured by the resin injection molding (RIM) method and passed all full-scale proof tests for certification, natural frequency test and static test. During the tests, any fracture, wrinkle or buckling of the blade did not occur and measurement results were in agreement with FE calculations within deviations of GL guideline. As a result, the design process of wind turbine blade using FEM was confirmed and established as reliable one.

오염 토양 세척 공정에서의 계면활성제 재사용을 위한 연구

안치규 포항공과대학교 대학원 2010 국내박사

Remediation of soil contaminated with hazardous hydrophobic organic compounds (HOCs) such as polycyclic aromatic hydrocarbons (PAHs) is a major environmental concern due to the toxic and carcinogenic properties of these compounds. PAHs are released into the environment as a result of combustion of fossil fuels or by accidental discharge. Due to their hydrophobicity, PAHs have low water solubility and are strongly sorbed to soil and sediment. Therefore, biodegradation of PAHs is very slow, resulting in their persistence in environments for long periods of time. A potential technology for rapid removal of PAHs sorbed to soils is soil washing with a surfactant solution. While the use of surfactants significantly enhances the performance of soil washing, operation costs are significantly increased as surfactant dosages are increased. Even though many approaches to reduce surfactant dosage have been introduced such as physical separation or chemical degradation of contaminants from surfactant solution, some of these have many drawbacks due to high-energy requirements, incomplete separation, or formation of potentially hazardous intermediates. Therefore, it is necessary to develop a surfactant recovery technology that is more simple, economic, and effective. In this study, selective adsorption of a HOC using activated carbon as a means of recovering surfactant after a soil washing process was investigated. In order to evaluate feasibility of selective adsorption process, we examined in various experimental conditions. And also we developed mathematical partitioning model for soil washing and selective adsorption to elucidate each system. As a model system, phenanthrene (PHE) was selected as a representative HOC and five different of nonionic surfactants (Triton X-100 (TX100), Tween 40 (TW40), Tween 80 (TW80), Brij 30 (B30), Brij 35 (B35)) and one of anionic surfactant (sodium dodecyl sulfate: SDS) were used. Four types of activated carbons that differed in size (Darco 100 (D100), Darco 20-40 (D20), 12-20 (D12) and 4-12 (D4) mesh sizes) were used in adsorption experiments. The efficiency of the selective adsorption process was represented as selectivity parameter. A selectivity value larger than 1 indicates that more contaminants relative to surfactant are adsorbed onto the activated carbon and that surfactant recovery is theoretically possible. In various surfactant solutions, the selectivity was much higher than 1, but surfactant recovery and soil washing efficiency were significantly differed by surfactant types. This indicated that to select proper surfactant, we should consider the soil washing efficiencies, surfactant losses, PHE removal and surfactant recovery efficiencies. In the surfactant mixtures as nonionic-anionic surfactant (TX100-SDS system), the sorbed amount of TX100 onto activated carbon was reduced by addition of SDS. Moreover the selectivity was also much higher than 1, thus activated carbon adsorption can be a good method for surfactant recovery in nonionic-anionic surfactant mixtures as well as in pure nonionic surfactant solutions. The model system (PHE/TX100) for soil washing and selective adsorption was analyzed using a mathematical partitioning model and compared to intrinsic sorption of PHE without the effect of sorbed surfactant. PHE was sorbed onto activated carbons or soils in a greater amount than an estimated value by intrinsic sorption, even though surfactant molecules covered most of activated carbon and soil surfaces. The enhanced sorption of PHE was attributed to the function of sorbed surfactant as an effective sorbent in soil washing and surfactant recovery process both. As a result, the molar solubilization ratio for sorbed surfactant (MSRs) was higher than that for micellar surfactant in bulk solution (MSR). In various soil washing scenarios, activated carbon adsorption process was evaluated using mathematical model. The simulation results suggest that only a small amount of activated carbon is necessary to effectively remove contaminants from surfactant solutions. And selective adsorption process using highly adsorptive activated carbon (D100) was more efficient than without selective adsorption process or using lower adsorptive activated carbon (D20). The adsorption technology using activated carbon is simple, fast, and inexpensive, and thus may be a reasonable alternative to recovery surfactant in soil washing process.

응축상 극초고속 분자동역학의 펨토초 형광 및 흡수 분광학적 연구 : Femtosecond time-resolved fluorescence and absorption spectroscopies for ultrafast molecular processes in condensed phase

엄인태 포항공과대학교 대학원 2010 국내박사

A molecule promoted to an electronic excited state undergoes relaxation processes by the dynamic molecule/bath interaction, where the bath may be intramolecular, intermolecular, or radiative in origin. The relaxation processes include population relaxation, vibrational and vibronic relaxations/dephasings, and solvation. Inter and intramolecular chemical reactions may also occur in the electronic excited states. These processes are manifested in the absorption and emission spectra of the relevant chemical species, which can be observed by various spectroscopies. Since all these processes involve nuclear motions directly or indirectly, their basic time scales fall in the femtosecond range, and therefore, they can be examined most straightforwardly by the femtosecond time-resolved spectroscopies. Furthermore, when the excitation is made impulsively, i.e., the excitation occurs faster than the period of the related molecular vibration, nuclear wavepackets in both the electronic ground and excited states are created, whose dynamics can also be directly scrutinized through the oscillations of the spectra in time. Wavepacket motions provide wealth of information on not only the vibrational/vibronic relaxation processes but also other related relaxation processes and chemical reactions. In this work, femtosecond time-resolved fluorescence spectra measurement technique is developed and applied to the study of a number of molecular processes subsequent to the electronic excitation. Specifically, dipolar solvation and vibronic relaxation dynamics of a coumarin in liquid, an intramolecular photochromic reaction dynamics of diarylethene compounds, and wavepacket dynamics in single-walled carbon nanotubes are investigated. Through the measurement of the whole emission and absorption spectra with a time-resolution faster than certain molecular vibrations, it was demonstrated that accurate measurements of the spectral change in femtosecond time resolution provides detailed information on most of the dynamical processes of the molecule not easily accessible by other techniques. Nuclear wave packet motions of the carbon nanotube detected by the impulsive transient absorption also provides spectroscopic as well as dynamical information otherwise difficult to obtain from stationary spectroscopies such as Raman scattering. First, a femtosecond light source based on a multipass Ti:sapphire amplifier was fabricated utilizing chirped pulse amplification technique. A 20 fs pulse output of a home-made cavity dumped Ti:sapphire oscillator at 800 nm was stretched to 2 ps only by material dispersion without a conventional grating stretcher. By the multi-pass amplification process, pulse energy was increased from 40 nJ to 30 μJ with 12 ps pulse duration at high 10 kHz repetition rate. The pulse compressor consisted of a grating and a prism pairs compensated the large amounts of second and third order dispersions in the amplified pulses to give 70 fs pulses with the pulse energies 14 μJ. These intense femtosecond pulses are used to generate frequency tunable pulses in visible range by the noncollinear optical parametric amplification (NOPA) process. The second harmonic of the fundamental at 400 nm amplified visible frequency components in white light continuum, and generated 30 fs visible pulses tunable from 460 nm to 660 nm. Solvation dynamics is one of the most important relaxation processes in liquid, as it is directly related to all the spectroscopic and chemical reaction properties in liquids. To experimentally investigate the fastest solvation process, polar solvation dynamics of coumarin 153 are investigated by the dynamic Stokes shift function, S(t). In small protic solvents, it is known that an initial ultrafast component below 50 fs constitutes more than half of the total solvation dynamics. We used fluorescence up-conversion technique via two photon absorption process, which can provide 40 fs time resolution for the whole emission wavelength range. Moreover, time-resolved fluorescence spectra were recorded directly without the conventional spectral reconstruction method. A temporal oscillation in frequency of the emission spectrum in the solvation curve is clearly observed. By virtue of the ultrahigh time resolution, the intramolecular contribution and the solvation contribution on the fluorescence decay can be separated. In the obtained S(t), initial solvation time scale is 120 fs, invariant to the solvents used in this experiment, although its amplitude varies in different solvents. The capability to measure the whole time-resolved fluorescence spectra gives an opportunity to explore the dynamics along the excited state potential surface involving a chemical reaction. Ring closure reaction dynamics of 1,2-bis(2-methylbenzo[b]thiophene-3-yl)hexafluorocyclopentene (BTF6) was investigated by means of the time resolved fluorescence up-conversion technique. Unlike a single wavelength data, direct spectra measurement gives an unambiguous picture on the excited state. By the 2D correlation analysis for the time-resolved fluorescence spectra, two different emission bands in the fluorescence of the open ring isomer were resolved. Instantaneous formation of the closed ring isomer was observed by the transient absorption spectra measurement using white light continuum probe. The global analysis of time-resolved fluorescence and TA spectra gave the ultrafast ring closure reaction time scale less than 1 ps. Moreover, the ring closure reaction of BTF6 entirely took place only on the ultrafast time scale, so that there was no solvent viscosity dependence on long decay time constants. Nuclear wavepackets generated by the impulsive excitation can be applied to the coherent vibrational spectroscopy. The coherent wavepacket oscillations of the radial breathing modes (RBM) in micelle-suspended single-walled carbon nanotubes (SWNT) were investigated by pump/probe technique. SWNTs are resonantly excited by the lowest E 여기 상태 (excited state)로 전이된 분자는 내적•외적 상호작용에 의해 입자 수 이완, 위상 이완, 진동 이완, 용매화 등의 경로로 다시 바닥 상태 (ground state)로 돌아가거나 분자 내/ 분자 간 화학반응을 겪기도 한다. 일련의 동역학적 과정들은 분자의 흡수/형광 스펙트럼에 반영되므로 다양한 분광학적 방법에 의해 관측될 수 있는데, 이들은 모두 분자의 진동 모드에 직•간접적으로 연관되어 있기 때문에 기본적으로 펨토초 (femtosecond, 10-15 초) 영역에서 일어난다. 따라서 이 과정들을 가장 직접적으로 연구할 수 있는 방법은 바로 펨토초 펄스를 이용한 시간 분해 분광학이다. 분자의 진동 주기보다 짧은 펄스에 의한 전이는 바닥 상태와 여기 상태에 핵파속 (nuclear wavepacket)을 형성하며, 이는 분광학적으로 스펙트럼의 시간적인 진동 (oscillation)으로 관측된다. 핵파속의 생성과 이완 과정은 분자의 진동 이완 및 반응 경로 등에 관한 많은 정보를 내포하고 있다. 본 연구에서는 펨토초 시간 분해 형광 및 흡수 스펙트럼 측정 기술을 Coumarin분자의 극성 용매화 반응, Diarylethene 분자의 광변색 반응, 단일벽 탄소나노튜브의 결맞는 진동 동역학 (coherent vibrational dynamics)과 같은 응축상의 화학적 시스템에 적용/분석 하였다. 특히, 분자의 특정 진동 주기보다 빠른 시간 분해능의 스펙트럼 측정 기술을 구현함으로써 일반적으로는 접근이 어려운 극초단 시간 영역의 상세한 동역학 정보를 얻을 수 있었다. 2장에는 증폭된 펨토초 펄스 광원을 기반으로 주파수 변환이 가능한 가시광 영역의 광 매개 증폭기 (optical parametric amplifier)제작 과정을 기술하였다. 자체 제작한 공진기 덤핑 방식의 티타늄:사파이어 레이저에서 10 kHz의 반복 속도로 출력된 중심파장 800 nm의 20 fs, 40 nJ 펄스를 매질의 분산만 이용하여 2 ps 길이로 늘리고, 또 다른 티타늄:사파이어 이득 매질에서의 다중 통과 방식으로 30 μJ, 12 ps의 펄스로 증폭하였다. 회절발과 프리즘쌍의 조합으로 펄스 압축단을 구성하여 증폭 직후의 펄스가 가지는 큰 주파수 분산을 상쇄하여14 μJ 에너지의 70 fs 펄스를 얻을 수 있었다. 증폭된 펄스는 이차 조화파 (400 nm)와 백색광 (continuum) 생성 과정을 거쳐 광매개 증폭기의 광원으로 사용 되었고 가시광 영역에서 파장 변환이 자유로운 30 fs, 100 nJ 펄스를 생성할 수 있었다. 3장에는 용액상에서 가장 중요한 이완과정 중 하나인 용매화 동역학에 관한 실험적 결과들을 기술하였다. 용액상에서 분자의 분광학적, 화학적 성질은 용매화 동역학에 직접적으로 연관되어있다. 시간적으로 가장 빠른 용매화 동역학을 실험적으로 관측하기 위해 Coumarin 153 분자의 동적 스톡스 이동 함수를 측정하여 극성 용매화 과정을 관찰하였다. 분자량이 작은 알코올 용매에서는 전체 용매화 과정의 50% 이상이 50 fs 이하의 매우 빠른 속도로 일어난다고 알려져 있다. 이러한 빠른 시간 영역에서의 정확한 측정을 위하여 이광자 흡수를 이용한 형광 업-컨버전 기술을 통해 40 fs의 시간 분해능을 구현하였다. 또한, 일반적으로 사용되는 시간 윤곽으로부터의 재구성법이 아닌 스펙트럼의 직접 스캔 방식을 통해 가장 정확한 시간 분해 스펙트럼을 측정할 수 있었다. 그 결과 형광 스펙트럼 전체가 시간적으로 진동하는 모습까지도 상세하게 관측할 수 있었고 그로부터 Coumarin 탐침 분자의 진동 모드가 형광 신호에 미치는 영향을 분리하여 순수한 용매화 동역학만을 관측할 수 있었다. 측정된 용매화 함수로부터 120 fs의 초기 용매화 시간 상수를 얻을 수 있었고 실험에서 사용된 몇 가지 용매들은 초기 용매화의 속도에는 영향을 미치지 못한다는 결론을 얻었다. 4장에는 1,2-bis(2-methylbenzo[b]thiophene-3-yl)hexafluorocyclo pentene (BTF6) 의 고리 닫힘 반응 동역학을 형광 스펙트럼의 직접 스캔 방식을 이용하여 분석한 결과를 기술하였다. 단일 파장에서 측정된 형광의 시간 윤곽의 한정된 정보와는 달리, 시간 분해 스펙트럼의 세밀한 측정을 통해 고리 열린 형태의 두 구조 이성질체를 분광학적으로 구분하고, 반응 동역학에 관한 명확한 모델을 제시할 수 있었다. 특히 시간 분해 스펙트럼에 대한 2차원 상관 (correlation)분석과 전역적 (global) 분석 방법을 병행하여 BTF6의 고리 열린 형태 형광은 2개의 서로 다른 밴드 (펨토초 수준으로 감쇠하는 높은 에너지의 밴드와 150 ps 수준으로 천천히 감쇠하는 낮은 에너지의 밴드)로 구성되어 있다는 결론을 얻었다. 특히 다양한 용매를 이용한 실험 결과들로부터 반응이 용매의 점성과는 무관하며 오직 극초단 시간 영역에서만 진행되고 150 ps의 형광 감쇠 성분은 반응을 하지 않는 분자의 형광 수명에 해당한다는 사실을 알 수 있었다. 이와 더불어 고리 닫힌 형태의 생성을 순간 흡수 스펙트럼을 통해 측정함으로써 BTF6의 고리 닫힘 반응은 1 ps보다 훨씬 빠른 시간에서만 일어남을 밝혔다. 5장에는 짧은 펄스 광원을 통한 결맞는 진동 동역학 연구의 예로써 마이셀 분산된 단일벽 탄소 나노튜브의 방사형 대칭 진동 (radial breathing mode, RBM) 생성과 이완을 순간 흡수 분광법을 이용하여 측정하고 그로부터 나노 튜브 집합

저주파 잡음 측정을 이용한 High-k 절연막 MOSFET 소자의 신뢰성에 관한 연구 : A Reliability Study on MOSFETs with High-k dielectrics using Low-Frequency Noise Measurements

최현식 포항공과대학교 대학원 2010 국내박사

This thesis describes the reliability study of MOSFETs with high-k dielectrics using low-frequency noise measurements. For La-incorporated high-k dielectrics, the oxide breakdown characteristics are verified using low-frequency noise measurements. For SiGe channel p-MOSFETs with high-k dielectrics, the oxide breakdown, the negative bias temperature instability (NBTI), and hot carrier instability (HCI) are observed using low-frequency noise and charge pumping measurements. The n-MOSFET with La-incorporated high-k dielectrics is measured before and after channel oxide breakdown. Low frequency noise (LFN) after channel soft oxide breakdown (SBD) of n-MOSFETs with a HfLaSiO gate dielectric and TaN metal gate shows a Lorentzian-like spectrum, which is not observed in HfSiO gate dielectric devices after channel SBD. This is related to the spatial location of the SBD spot. Because the La weakens atomic bonding in the interface layer, the SBD spot is generated close to the Si/SiO2 interface. This is verified using the time domain analysis. The reliability of SiGe channel p-MOSFETs with high-k dielectrics is studied. Si0.75Ge0.25 p-MOSFETs with HfSiO gate dielectrics show the higher breakdown voltage than the Si channel p-MOSFETs in TZDB measurements. At the first instant after TZDB, the increase of hole current is larger than the increase of electron current in Si0.75Ge0.25 p-MOSFETs, which is opposite to the Si channel p-MOSFETs. This supports that the breakdown in Si0.75Ge0.25 p-MOSFETs are occurred at interfacial layer (IL) and that in Si p-MOSFETs are occurred at bulk layer. These characteristics are also observed in BD measurements. Low-frequency noise (LFN) after BD of Si0.75Ge0.25 p-MOSFETs shows a small Lorentzian hump and that of Si p-MOSFETs shows an only increase for all measured frequency. This is evidence that the IL of Si0.75Ge0.25 p-MOSFETs is more fragile than that of Si p-MOSFETs because of the Ge induced trap generation. Finally, the NBTI and HCI of SiGe channel p-MOSFETs are studied. The short gate-length devices are more degraded by NBTI in respect of the threshold voltage shift (ΔVth) than the long gate-length devices in Si0.75Ge0.25 channel p-MOSFETs. This supports that the short gate-length devices contain more fragile bond than the long gate-length devices do. Under channel hot carrier (CHC) stress, the drain currents are severely degraded and the kink effect in subthreshold region is observed in Si0.75Ge0.25 channel p-MOSFETs.

전광 효과를 이용한 전자 빔의 다발 길이 측정 실험

노상순 포항공과대학교 대학원 2010 국내석사

An electro-optic sampling (EOS) experiment has been carried out at 60MeV electron linac of the Pohang Light Source. Coherent transition radiation (CTR) produced by relativistic electron bunches passed by an optically active ZnTe crystal. The induced birefringence was sampled with ultrashort titanium sapphire (Ti:Sa) laser pulses. The technique of EOS provides the possibility to measure the longitudinal charge distribution with very high resolution, determined by the width of the optical laser pulse. A 150 fs Ti:Sa laser is used to sample the birefringence which is induced in a nonlinear optical crystal by the co-moving electric field of a relativistic electron bunch, or, alternatively by the coherent transition radiation generated by the bunch. The initial linear polarization of the laser pulse is converted into a slightly elliptical polarization which is then converted into an intensity modulation. By shifting the timing of the laser pulse relative to the bunch in sub-picosecond steps, the time profile is obtained by sampling over many bunched.

전구체 용액을 이용한 비진공 방식의 CIS계 태양전지용 박막 제조 및 특성 평가

이동욱 포항공과대학교 대학원 2010 국내석사

Carbon-based energy such as coal, oil and natural gas faces difficulties with various aspects caused by increase in energy demands, production cost and decrease in its resources. Furthermore, such energy sources make huge amounts of carbon oxides, sulfur oxides and nitrogen oxides into atmosphere when they burn, which lead to greenhouse effect. According to these situations, a lot of researches for developing renewable energy have been studied until now. Thin film solar cells have the great potential to convert sunlight into electricity at low cost, because of high material use efficiency, high optical absorption property, tunable band-gap energy and so on. Recently, polycrystalline Cu(In,Ga)Se2 (CIGS) absorber layers are used for the most efficient thin film photovoltaic cells. However, previous reports were based on high vacuum deposition systems and high temperature heat treatment process such as thermal evaporation and sputtering technique. During several decades, so, production cost has not reduced under $1/W since CIGS solar cells developed first time, where renewable energy could be expected to substitute for fossil fuels. For achieving ‘grid parity’, non-vacuum and room-temperature deposition methods should be studied. In this thesis, CIS chalcopyrite absorber layers were synthesized on Mo-sputtered soda-lime glass substrate by non-vacuum coating using precursor solution and thermal annealing methods. Precursor solution was prepared by mixture of metal chloride precursors, polypropylenecarbonate(PPC) as a thermal-decomposable organic binder, and non-toxic acetone as a solvent. A low-cost, non-toxic precursor solution was deposited on substrate by simple doctorblade-coating method which can be applied to continuous roll-to-roll production. And then, precursors were converted into CIS crystalline phase in quart tube furnace under a low vacuum environment. The morphology, composition and crystallinity of the synthesized thin films were characterized by SEM, EDX, and XRD analysis. By XRD, TGA measurements, in addition, growth mechanism of CIS grains was analyzed for heat-treated samples at varied temperature. Chloride phase precursors decomposed and perfectly disappeared over 100˚C, but then they formed metal chalcogenide phase such as Cu2-xSe and In2Se3. Between 300˚C and 400˚C, CIS chalcopyrite structure started to evolve, finally completed its crystallization to chalcopyrite structure near 500˚C. Also, composition and concentration of precursors were major factors to forming thin film structure. In most cases, stoichiometric precursor paste could made 500nm thickness of continuous CIS film by 500˚C annealing in a vacuum environment for 30mins. By optical absorption spectrum, furthermore, CIS thin film synthesized on bare substrate showed great light absorbance behavior at visible and near-IR light region. And band-gap energy was calculated to be about 1.15eV by Tauc relation. However, SEM, EDX, XRD, AES results showed phase separation between CIS grains and amorphous Se-rich film. Although that film had very low contact resistance property, amorphous layer can act as electrophilic recombination centers. This phase separation phenomena should be solved for device fabrication and its high performance.

연 X-선 공명산란을 이용한 다층 전이금속산화물의 전자구조와 자성구조 연구 : Resonant Soft X-ray Scattering Study on the Electronic and Magnetic Structures of Oxide Superlattices

박지회 포항공과대학교 대학원 2010 국내박사

다층 전이금속산화물의 계면에서 일어나는 전자구조의 재구조화를 연 X-선 공명산란을 이용해서 연구했다. 우리는 연 X-선 공명산란은 X-선 흡수분광법과 X-선 반사도의 장점을 결합한 것으로 전자구조를 깊이에 대한 함수로 구할 수 있으며 선평광과 원평광 X-선을 이용하면 스핀과 궤도함수의 구조도 얻을 수 있다. 또한 원자마다 다른 흡수끝머리 값을 이용하여 전자구조를 각각의 원자에 대해 개별적으로 구할 수 있다. 연 X-선 공명산란 결과로부터 LaAlO3/SrTiO3(LAO/STO)와 La2/3Ba1/3MnO3/LaNiO3(LBMO/LNO)의 계면에서 전자구조의 재구조화가 일어난다는 분명한 증거를 발견했으며 그 구조의 특성들도 구했다. 다층 전이금속산화물의 물리적인 성질은 계면의 전자구조에 크게 영향을 받으므로 구한 전자구조가 이 다층산화물의 전체적인 특성을 이해하는데 중요한 실마리를 제공할 것이다. LAO/STO 계면은 전도성, 초전도성, 자기저항과 같은 흥미로운 특성을 보이나 계면에서의 전자구에 대한 실험의 아래쪽 덩어리 부분으로부터 계면의 구조를 분리하는 것이 어려워 거의 이루어지지 않았다. 연 X-선 공명산란 실험을 통해 LAO/STO 다층박막 계면에서의 전자구조의 특성을 얻었다. (003) 다층박막 반사의 경우 대칭구조에 의해서 거의 금지반사가 되나 Ti L2,3와 O K 흡수끝머리 값 근처에서 산란된 X-선의 세기가 크게 증가하는 것을 볼 수 있으므로 계면에서의 전자구조가 변했다는 것을 알 수 있다. 그리고 σ 편광에 대한 π 편광일 때 산란된 X-선의 세기가 구면대칭인 경우에 기대되는 값보다 9배 정도 큰 값을 가진다. 이것은 Ti d 궤도함수의 겹칩이 없어짐으로 인해 생긴 Templeton 산란 때문이다. O K 흡수끝머리 근처에서의 연 X-선 공명산란 결과에서는 Ti t2g 궤도함수와 결합하고 있는 O p 궤도함수와 일치하는 봉우리가 둘로 갈라지과 각각의 봉우리의 높이가 편광에 따라 다른 것을 볼 수 있는데 이로부터 dxy 궤도함수가 다른 t2g 궤도함수들도다 약 1 eV 정도 낮은 에너지를 가진다는 것을 알 수 있다. 정방 찌그러짐의 영향을 확인하기 위해 STO와 NdGaO3 두 종류의 기판에서 키운 시료를 이용했는데 연 X-선 공명산란 측정결과 두 시료가 비슷한 모양을 가지는 것을 볼 수 있다. LBMO는 강자성 도체이고 LNO는 상자성 도체이므로 LBMO/LNO 다층박막은 전형적인 거대자기저항 물질들과 같은 구조를 같고 있으나 측정된 자기저항 값은 매우 작다. 연 X-선 공명자기산란 결과를 정량적으로 분석해서 전하과 자기 산란진폭을 구했다. 전하 (자기) 산란진폭의 허수 부분은 흡수분광 (자기원평광이색성) 스펙트럼과 같은 에너지의존성을 가지므로 이로부터 전자구조와 자기구조를 구할 수 있다. 실험으로 구한 흡수분광 스펙트럼과 비교를 통해서 Mn 당 약 0.5개의 전자가 Ni로 계면을 통해서 이동했다는 것을 알 수 있었다. 그리고 계면에서 Mn의 자기모멘트는 줄어들고 Ni에는 자기모멘트가 유도된다는 결과를 얻었다. 이와 같은 전하의 이동으로 인해 LNO는 상자성 도체에서 반강자성 부도체로 상전이가 일어나고 이것으로 일반적이지 않은 자기저항 값을 설명할 수 있다. 연 X-선 공명자기산란을 이용해 LBMO/LNO에서 일어나는 상호층 교환 결합에 대해서도 연구했다. 측정된 연 X-선 공명자기산란 히스테리시스 결과의 정량적인 분석으로부터 각각의 LBMO층에서의 자기히스테리시스 값을 구했으며 외부자기장의 없을 때 인접한 층의 자기모멘트느 거의 반평형 관계에 있다는 것을 확인할 수 있었다. 각층에서의 자기히스테리시스와 전체 시료의 자기히스테리시스 결과로부터 상호층 교환 결합 상수 값을 구했다. The depth-resolved electronic and magnetic structures of transition metal oxide superlattices were studied using resonant soft x-ray scattering. In order to obtain information on spin and orbital states, circularly and linearly polarized x-rays were used, respectively. Element-specificity was also achieved by exploiting the resonant enhancement in the vicinity of absorption edges. LaAlO3/SrTiO3 (LAO/STO) superlattices were studied to characterize their interface reconstructions. Near Ti L2,3 edges comparable intensities between σ- and π-polarizations observed at a (003) reflection, even though the angle between incident and scattered x-rays is around 70 degrees, are attributed to Templeton scattering caused by degeneracy-lifting of Ti $3d$ orbitals at the interfaces. From the results near O K edge it was found that the dxy orbital has lower energy by about 1 eV than other t2g orbitals. We used two LAO/STO superlattices grown on STO and GdGaO3 substrates in order to check the effect of tetragonal distortion on the interface electronic reconstruction. Experimental results for the both samples present the similar features. We studied reconstructed interface structures of a superlattice consisting a half metallic ferromagnet La2/3Ba1/3MnO3 (LBMO) and a metallic paramagnet LaNiO3 (LNO). In this superlattice, adjacent LBMO layers are coupled antiferromagnetically so that this superlattice has same structure as multilayers exhibiting giant magnetoresistance. However, its magnetoresistance is very small. The electronic and magnetic structures at the interfaces are resolved for those in the bulk by using quantitative analysis of soft x-ray resonant magnetic scattering (SXRMS) data. It was found that about 0.5 electron per Mn ion is transferred to the Ni ions through the interface, the magnetizations of the Mn ions are suppressed at the interface, and the LNO spacer layer becomes antiferromagnetic. As a result of charge transfer, the LNO layers changes from paramagnetic conductor to a antiferromagntic insulator, and the unusual magnetoresistance behavior is attributed to it. Finally, the interlayer exchange coupling in the LBMO/LNO superlattice was investigated. The magnetic hysteresis loop of each LBMO layer was extracted from measured SXRMS data, and the obtained layer-resolved LBMO hysteresis loops were utilized to determine the bilinear and biquadratic interlayer exchange coupling constants. From these results, we confirmed the adjacent LBMO layers in this superlattice are antiferromagnetically coupled.