Pulsed laser deposition 방법에 의한 바나듐 산화물 박막 증착 및 증착 조건에 따른 박막 특성 변화 연구

학위논문(석사) --서울대학교 대학원 :재료공학부,2008.8.Maste

Photovoltaic Studies on Ferroelectric Oxides with Scanning Optical Probes

DoctorThe ferroelectric photovoltaics has recently regained much interest due to the scientific importance as well as potential applications in optoelectronics and solar cell devices. The ferroelectric photovoltaic effect are mainlycharacterized by very large photovoltage output, which is observed to be linearly proportional to ferroelectric crystal length along the ferroelectric polarization, not limited by the band gap of ferroelectric materials. Despite the advantage of large photovoltage, the ferroelectric photovoltaics has never been thought as a viable alternative to conventional solar cell devices because of low power conversion efficiency, which is mainly due to poor electrical transportproperties of ferroelectric materials. Furthermore, most of ferroelectric materials have a wide band gap of 3~4eV, limiting their use as solar cell devices due to low absorption of solar radiation. On the contrary, ferroelectric BiFeO3 has a relatively small band gap of 2.7eV and large polarization of about 100μC/cm2, suggesting a possibility of new concept of solar cell devices.Since the early observation of photovoltaic effect in ferroelectric materials, many researchers tried to find a true origin for the ferroelectric photovoltaic effect, which includes asymmetry in carrier excitation from a defectlevel to the conduction band, the asymmetry of elementary processes such as photoexcitation and scattering in a noncentrosymmetric medium, DC field created by the optical-rectification effect and depolarization field. Recent theoretical calculations indicate that an electrostatic potential step is present across the nanometer-scale ferroelectric domain walls, such as 71°, 109°, 180° domain walls of BiFeO3 and 90° domain wall of PbTiO3, producing intense internal field at the domain walls. The photogenerated carriers are thought to be effectively separated at the domain walls due to the presence of internal electric field, as in the depletion region of conventional semiconductor p-n junction-basedphotovoltaic devices. The electrostatic potential at the domain walls is additive. As a result, open circuit voltage is observed to be linearly proportional to the number ofdomain walls between the electrodes, producing large open circuit voltage, much larger than the band gap of ferroelectric BiFeO3. Besides ferroelectric domain wall,the ferroelectric domain itself also contributes to the photovoltaic response of BiFeO3, as observed in monodomain BiFeO3 single crystal. That is, the ferroelectric photovoltaic response is thought to be closely related to the ferroelectric domain structure.In order to investigate the interaction of ferroelectric ordering with light at the microscopic level, the scanning photocurrent microscopy, which is able to map the local photocurrent responses at the sub-micrometer scale,was used. The photovoltaic current of epitaxial (001) BiFeO3 film deposited on (001) SrTiO3 substrate was reversibly changed upon the ferroelectric switching. The sign of local photovoltaic current was found to be in closerelation with the local polarization orientation and its magnitude was spatially nonuniform. Specifically, the local photovoltaic response of BiFeO3 was enhanced near charged domain walls, which were produced as a result ofdomain pinning during ferroelectric switching. Through the spectrally resolved photovoltaic current measurement, the sub-band gap excitation, besides band-to-band transition, was observed near the charged domain walls, suggesting the enhancement of photovoltaic current is related to defect levels inside the band gap, which are able to locally lower the effective band gap of BiFeO3 near charged domain walls. Ionized oxygen vacancies can respond to an electric field, resulting in electromigration of oxygen vacancies toward negative electrode. The electromigrated oxygen vacancies are assumed to be accumulated near charged domain walls, producing defect levels. In otherwords, the increased photo-generation rate due to locally reduced effective band gap results in the enhancement of photovoltaic current near charged domain walls.The single ferroelectric domain contribution to the ferroelectric photovoltaic effect was also experimentally demonstrated using monodomain (110) BiFeO3 film. The photovoltaic current of monodomain BiFeO3 was reversibly switched upon the ferroelectric domain reversal, as in the case of multidomain (001) BiFeO3 film. The understanding of the microscopic interaction of ferroelectric order with light will provide an opportunity of using ferroelectric materials as a new concept of solar cells and optoelectronic devices. Furthermore, multiferroic character of BiFeO3, where the ferroelectric, ferroelastic and antiferromagnetic order parameters all interact with each other, suggest a possibility of multifunctional devices, in which the photovoltaic effect can be controlled by the electric, magnetic and/or stress fields

비아나 원환면과 벽-통과 공식

Doctor본 논문에서는 비아나의 원환면(Vianna's torus)과 란다우-긴즈버그 씨앗(Landau-Ginzburg seeds) 관하여 연구하였다. 이 논문의 주요결과로서, 모든 비아나의 원환면의 상대적인 그로모프 용량의 하계와 란다우-긴즈버그 씨앗(Landau-Ginzburg seeds)의 형태를 구체적으로 구해냈다. 보다 자세하게 다음과 같은 여러가지 결과를 증명하였다: (1) 비아나의 원환면들의 상대적인 그로모프 용량들의 하한(infimum)을 구했고 $0$보다 큼을 보였다. (2) 개별 비아나의 원환면의 상대적인 그로모프 용량의 하계를 구하고 보다 큰 보편적인 하계(universal lower bound)를 얻었다. (3) 그리고 마르코프 삼중(Markov triple)의 두 변이(mutation)에 따라 달라지는, 이보다 더 큰 하계를 구했다. (4) 벽-통과 공식을 통해 작은 마르코프 삼중에 해당하는 란다우-긴즈버그 포텐셜을 구하여 임계점(critical point)의 존재성을 알아보았다. (5) 마지막으로 특별한 마르코프 삼중인 $(2,b,c)$와 $(a,1,c)$에 해당하는 란다우-긴즈버그 씨앗의 세 방향들(directions)과 변이 방향을 결정하였다.In this thesis, we study an asymptotic behavior of Vianna’s tori $T_{a,b,c}$ in \CP^2 as $a + b + c \to \infty$ and its associated LG seeds. We first give an infimum of lower bound for the relative Gromov capacity of the torus$T_{a,b,c}$ over triples $(a, b, c)$. Then we find a lower bound for the relative Gromov capacity of individual torus and its improvements for any triple $(a, b, c)$. We illustrate the existence of critical points of LG potential Wa,b,c associated to $(a, b, c)$ and then give a description of directions of LG seeds in triples of the forms $(2, b, c)$ and $(a, 1, c)$

The electrical and structural analysis of Al doped ZnO deposited at low temperature using a MOCVD system

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The electrical and structural analysis of ZnO deposited at low temperature using a MOCVD system

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Effects of precursor flow rates during LPCVD on the properties of transparent conducting Al doped ZnO thin films

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