타원편광 분석을 통한 인듐틴옥사이드 박막의 광학적 특성 및 반사방지막 응용

Abstract

본 논문에서는 TCO (transparent conducting oxide)의 일종인 인듐틴옥사이드 (ITO; indium tin oxide)를 사용하여 실리콘 태양전지에 응용 가능한 무반사 코팅을 목표로 연구를 수행하였다. 박막은 전자빔 증착법을 이용하여 제작되었고, 최초에 제작된 박막을 타원편광분석기 (Ellipsometer)를 사용하여, 증착 조건에 대한 박막의 유전 함수를 정의하였다. 분산관계식인 로렌츠 진동자 모델 (Lorentz oscillator model)를 통해서 4.6 eV에 공명진동을 하는 진동자와, 드루드 모델 (Drude model)을 채택하여 자유전자에 대한 플라즈마 진동수 (plasma frequency)를 가정하여 일반적으로 축퇴된 n형의 와이드 밴드갭 반도체라고 생각되는 ITO의 유전함수를 결정할 수 있었다. ITO 박막에 나노 스케일의 빈 공간 (Void fraction)이 존재하는 경우를 가정하여, ITO의 유전함수와의 유효매질근사 (effective medium approximation)를 통한 다층 박막 구조의 반사방지막을 시뮬레이션 하였다. 컴퓨터를 통하여 최적의 해를 구한 뒤, 이를 통해서 시뮬레이션 된 반사방지막 제작을 수행하였다. 저온에서 열처리로 ITO의 질을 향상시킬 수 있는 조건을 찾기 위해, 각각 150℃, 175℃ 200℃의 온도에서 0 sccm 부터 80 sccm 사이의 산소 분위기 내에서 1분 동안 RTA를 통해 열처리를 수행하여, ITO가 150℃, 산소 60 sccm에서 TCO로서 가장 좋은 성능을 낸다는 사실을 확인하였다. 박막 내부에 조작 가능한 나노 스케일의 빈 공간을 만드는 작업은 경사입사증착법 (glancing angle deposition)을 사용하여 이루어 졌으며, 기존의 경사입사증착법은 박막에 복굴절성 (birefringence)을 유발할 가능성이 있어, 경사입사증착법을 행할 때 기판을 회전시켰다. 제작된 박막은 주사전자현미경 (scanning electron microscope) 통하여, 박막이 기둥형 구조로 성장되어 나노 스케일의 빈공간이 생성된 것을 확인하였고, 이것은 타원편광분석을 통해서 교차 확인 되었다. 초기에는 박막에 존재하는 빈 공간의 함량을 계산할 때, 로렌츠 진동자 모델과 유효매질근사를 거쳐서 얻어지는 지극히 간접적인 방식을 사용하였으나, 차후에 경사입사증착법의 사영효과 (shadowing effect) 제거하는 이론적 접근을 통해서, 주사전자현미경으로부터 직접 측정된 박막의 두께와 증착각을 통한 연산으로 직접적이면서 간단한 방식으로 박막의 빈 공간 함량을 계산할 수 있었다. 완성된 반사방지막은 2층 무반사 코팅으로, 1층의 두께가 26 nm이고 2층의 두께가 58 nm이다. 이 박막은 400 nm와 600 nm 사이에서 6.57%의 평균 반사율을 나타내었고, 기존 실리콘 기판에 비해서 반사도가 크게 줄어든 것을 확인할 수 있었다. 본 연구를 통해서 이루어진 ITO를 이용한 다층 반사방지막 설계는 그 동안 TCO로서 그저 투명할 뿐인 도체로 인식되던 ITO에 새로운 전기를 가져다 줄 것으로 기대된다. |Indium tin oxide (ITO) thin films with function of anti-reflection, which is well known material for transparent conducting oxide(TCO), was studied in terms of silicon solar cells. The films were fabricated by using electron beam evaporation. At first, dielectric functions of indium tin oxide was determined by utilizing ellipsometry with a couple of dispersion relations. With Lorentz oscillator model, one of the dispersion relations, the resonance frequency corresponding to the photon energy of 4.6 eV was considered, and with Drude model which is also one of the dispersion relations, free electron with plasma frequency was considered as well, because ITO is thought of as degenerated n-type wide band-gap semiconductor. Supposing void fraction in the ITO thin films, computer simulation of multi-layer antireflection coatings was carried out using effective medium approximation between varied void fraction and dielectric function of ITO. From the simulation, optimal condition for antireflection using multi-layer ITO thin films was found and the fabrication of anti-reflection coatings of the optimal condition was performed. To find condition for annealing in the low temperature, the ITO films was annealed for 1 min at 150℃, 175℃, and 200℃ of temperature from 0 sccm to 80 sccm of oxygen flow rate, respectively. The greatest properties of ITO as TCO were found at 150℃ of temperature and 60 sccm of oxygen flow rate during processing of finding annealing condition. The void fraction of ITO thin films was made using glancing angle deposition (GLAD) technique. Because the conventional GLAD technique could cause birefringence in the ITO films, the sample holder of vacuum chamber was rotated by 5 rpm during glancing angle deposition. The fabricated thin films were examined by using scanning electron microscope (SEM) which found the nano columnar structure and formation of void between the columns. These void fractions were cross-checked by ellipsometer. During calculation of the void fraction of the ITO thin films, fairly indirect methods (dispersion relations and effective medium approximation) had been used in early days. Afterwards, with theoretical approach removing shadowing effect of GLAD along with real thickness of films measuring by SEM, void fraction of the films was more directly calculated. The fabricated anti-reflection coating was 2-layer anti-reflection coating. The thickness of the first layer was 26 nm, and the second was 58 nm. This ITO thin film on the silicon substrate shows the average reflectance of 6.57% between 400 nm and 600 nm. The reflectance was greatly lowered compared to the reflection of bare silicon substrate. The modeling of multi-layer anti-reflection coatings using ITO which was considered just as a TCO is expected to give new phase to ITO.; Indium tin oxide (ITO) thin films with function of anti-reflection, which is well known material for transparent conducting oxide(TCO), was studied in terms of silicon solar cells. The films were fabricated by using electron beam evaporation. At first, dielectric functions of indium tin oxide was determined by utilizing ellipsometry with a couple of dispersion relations. With Lorentz oscillator model, one of the dispersion relations, the resonance frequency corresponding to the photon energy of 4.6 eV was considered, and with Drude model which is also one of the dispersion relations, free electron with plasma frequency was considered as well, because ITO is thought of as degenerated n-type wide band-gap semiconductor. Supposing void fraction in the ITO thin films, computer simulation of multi-layer antireflection coatings was carried out using effective medium approximation between varied void fraction and dielectric function of ITO. From the simulation, optimal condition for antireflection using multi-layer ITO thin films was found and the fabrication of anti-reflection coatings of the optimal condition was performed. To find condition for annealing in the low temperature, the ITO films was annealed for 1 min at 150℃, 175℃, and 200℃ of temperature from 0 sccm to 80 sccm of oxygen flow rate, respectively. The greatest properties of ITO as TCO were found at 150℃ of temperature and 60 sccm of oxygen flow rate during processing of finding annealing condition. The void fraction of ITO thin films was made using glancing angle deposition (GLAD) technique. Because the conventional GLAD technique could cause birefringence in the ITO films, the sample holder of vacuum chamber was rotated by 5 rpm during glancing angle deposition. The fabricated thin films were examined by using scanning electron microscope (SEM) which found the nano columnar structure and formation of void between the columns. These void fractions were cross-checked by ellipsometer. During calculation of the void fraction of the ITO thin films, fairly indirect methods (dispersion relations and effective medium approximation) had been used in early days. Afterwards, with theoretical approach removing shadowing effect of GLAD along with real thickness of films measuring by SEM, void fraction of the films was more directly calculated. The fabricated anti-reflection coating was 2-layer anti-reflection coating. The thickness of the first layer was 26 nm, and the second was 58 nm. This ITO thin film on the silicon substrate shows the average reflectance of 6.57% between 400 nm and 600 nm. The reflectance was greatly lowered compared to the reflection of bare silicon substrate. The modeling of multi-layer anti-reflection coatings using ITO which was considered just as a TCO is expected to give new phase to ITO.