광소자 적용을 위한 RF-Magnetron Sputtering 및 원자층 증착방법으로 형성된 ZnS 박막 특성 분석

Abstract

증가하고 있는 필요 에너지원에 대한 수요로 인하여 화석연료를 비롯한 에너지 자원의 사용량은 날로 증가하고 있는 가운데, 과도한 자원 사용에 따른 에너지 자원의 고갈, 지구 온난화 및 환경오염의 문제가 우려되고 있는 현실이다. 이러한 문제점 들을 해결하기 위한 하나의 방편으로서 신재생 에너지원의 개발이 각광을 받고 있다. 이에 따라서, 유망한 신재생 에너지원의 하나로서 태양광 에너지를 전력으로 전환할 수 있는 태양전지 소자에 관한 관심은 최근 크게 증가하고 있다. 현재 Si 반도체를 기반으로 한 단결정, 다결정 bulk Si 태양전지는 20% 이상의 에너지 전환 효율을 보이며 에너지 문제를 해결할 수 있는 태양전지로서 주목받고 있다. 하지만 Si 반도체 물질을 사용함으로서 야기되는 불가피한 문제점들, 부각되는 단점으로 인해서 차세대 태양전지 개발에 대한 필요성 또한 증가하고 있다. 차세대 태양전지로서 Cu, Ga, In, Se(S) 4종 성분을 이용한 화합물 박막 태양전지는 bulk Si계 태양전지의 단점인 낮은 광흡수 계수, 비-직접 천이형 반도체 물질 사용으로 인한 에너지 손실 문제, 상대적으로 많은 량의 Si 물질을 사용함으로서 제한되는 재료비 절감 문제, 장기간 사용에 의한 효율 저하 문제 등을 해결할 수 있을 것으로 기대되고 있다. 한편으로 공정 단가 절감, 개인 휴대 가능한 수준의 취급 편의를 갖춘 태양전지 제작을 위해서 유연기판 상의 제작, roll to roll 공정의 확립과 같은 기술의 개발이 추진되어 왔다. 4성분계 박막 태양전지의 에너지 전환효율에 큰 영향을 주는 층 중에 하나는 N-type buffer 층으로서 주로 CdS 물질로 증착되며, 현재로서는 고효율 확보를 위한 최적의 buffer 층으로 평가 받고 있다. 단, 인체에 유해한 Cd 물질 사용으로 인한 환경 안전성의 문제, 300-500nm 파장 대역 빛의 광흡수에 의한 효율 저하 등의 문제가 있어, 대체 박막 층인 ZnS 박막을 증착함으로서 상대적으로 환경에 더욱 무해하며, 단파장 대역의 광흡수를 향상시킬 수 있는 박막 태양전지를 제작하려는 시도가 진행이 되고 있다. ZnS 물질은 약 ~3.7 eV의 직접 천이 형 밴드 갭(direct band gap) 에너지을 갖는 에너지 밴드구조를 갖고 있으며. 가시광선 영역에서 80% 이상의 높은 광투과율을 보인다. 투명 전도성 및 형광특성과 같은 다양한 광 특성을 가짐에 따라서 LED, bio-molecular sensor 등을 비롯한 광소자 제작에 사용되는 것으로 알려져 있다. 본 연구에서는 RF-magnetron sputtering 방식 및 atomic layer deposition 방법으로 ZnS 박막을 증착하여, 실험 조건에 따른 ZnS 박막의 특성변화를 분석하였으며, 박막태양전지 buffer 층으로의 제작 가능성을 검토하였다. ZnS 박막의 기본적인 물성 분석을 위하여 소다 석회 유리, silicon 기판 상에 증착을 시도하였으며, 유연기판에의 적용을 위해 polyethylene-terephtalate 폴리머 기판 상에 증착을 시도하여 특성을 분석, 비교하였다. Oxygen 비율 조절을 위한 annealing 과정을 oxygen pure, air ambient에서 각각 진행하였으며, 각 박막의 특성 변화를 관찰하였다. 또한 Atomic layer deposition 방식을 이용한 박막 증착을 실시하여 vacuum, sulfuric (H2S+N2), oxygen ambient에서 annealing 후의 박막 특성 변화를 관찰하였으며, ZnS 박막의 구조적, 광학적, 전기적, 화학적 특성을 확인하였다. 각 분석 방법으로서, X-ray 회절분석 (XRD, X-ray diffraction analysis)을 통해 ZnS 박막의 grain size와 phase 분석을 실시하였다. 제작된 ZnS 박막은 hexagonal wurtzite (002)/Zinc-blend (111) 방향의 preferred orientation을 보이는 박막 성장 양상을 보였으며, annealing 온도 및 ambient 조건에 따라서 hexagonal wurtzite ZnO (002) 방향의 preferred orientation을 갖는 회절 패턴을 보여 고온, oxygen 유무에 따른 상변화 현상을 보였다. 또한 공정 조건에 다른 grain size 변화가 비례적으로 나타나는 관계를 나타내어 공정 조건 조절에 따라서 ZnS 박막의 물성 조절이 가능함을 보여주었다. UV-visible spectrometer를 이용하여 박막의 광 투과도 측정 결과 450nm 이상의 장파장 대역에서 최대 90%의 광투과율을 갖는 파장대역이 발견되었으며, Tauc plot을 이용한 밴드 갭 에너지 계산 결과 박막 두께 증가에 따라서 밴드 갭 에너지가 선형적으로 감소하는 경향을 보였다. Scanning electron microscope(SEM)를 이용하여 박막표면의 미세구조 관찰을 실시하여 XRD 관찰을 통한 grain size 측정결과와 비교 분석을 하였으며, 상변화에 다른 표면 미세구조의 변화를 파악하였다. 박막의 Zn, S, O의 조성비율을 검토하기 위해 SEM 장비에 준비된 energy dispersive spectrometer(EDS) 분석장비를 이용해 각 박막 증착 공정 별 성분 변화를 관찰함으로서, 총체적으로 박막 태양전지, LED와 같은 광소자 적용을 위한 ZnS 박막의 물성의 제어가 sputtering, ALD, annealing 공정 조건을 조절함에 따라서 가능함을 확인할 수 있었다.| While the amount of energy usage and demand of energy sources was increaing, there were predictions about crisis of energy exhaustion and environment polution causing global warming. To resolve these problems, renewable energy for clean and sustainable energy source was received attentions. As one of the promising renewable energy source, photovoltaic energy collected by photovoltaic electronic device has attracted and showed positive potential. Silicon solar cell based on bulk size silicon semiconductor material was well focused as one of the best photovoltaic device showing high energy conversion efficiency over 20% and now is predicted to solve upcoming energy crisis. However, there are some problems to be solved and there are also need for research on advanced next generation photovoltaic device. CIGS thin film solar cell, which is so called next generation photovoltaic device, is fabricated with compound quaternary absorber layer composed of Cu, In, Ga, Se(S). Disadvantages of bulk silicon based solar cell, low absorption coefficient, nature of indirect band sturcture, limitation of diminishing thickness of cell, long-term stability are supposed to be solved by using CIGS solar cell. Cost reduction, device portability, device fabrication with flexibility are expected to be obtained. Important one in quaternary thin film solar cell is N-type buffer layer usually deposited as CdS material which is conventionally used in quaternary thin film solar cell with high efficiency yield. However, it is not acceptable using Cd material for its high toxicity and absorption lose from 300-500nm short wavelength range. For these reasons, ZnS thin film for its replacement was suggested because of its cleanness and less absorption loss. ZnS material has about ~3.7 eV of direct band gap energy and highly transparent within visible wavelenth spectrum. This transparent and conductive ZnS is usually used in LED, bio-molecular sensor etc. for its opto-electronic property. In this research, RF-magnetron sputtering and atomic layer deposition method was preapred for ZnS deposition and film properties were analyzed whether it is acceptable for application in solar cell or other opto-electronic devices. ZnS film was deposited on soda-lime glass, silicon, polyethylene-terephtalate substrate. For oxygen control, annealing process was advanced within air, oxygen ambient. When it was prepared by atomic layer deposition, annealing in vacuum, sulfuric (H2S+N2), oxygen ambient was proceeded and film properties were compared. X-ray diffraction analysis was done for grain size and phase confirmation. Deposited ZnS films showed hexagonal wurtzite (002)/Zinc-blend(111) preferred orientation and change of XRD pattern from various annealing conditions with phase transformation. By UV-visible spectrometer, optical transmittance was confirmed and it showed up to 90% for maximum transmittance over 450nm of wavelength. Band gap energy of ZnS films were confirmed by Tauc plotting with film thickness. In this study, we found that band gap energy is linearly decreased with increasing thickness. scanning electron microscope was used for surface analysis and morphologies of ZnS films were assured with grain size calculation from X-ray diffraction. For composition analysis, energy dispersive spectrometer was used to check Zn, S, O composition. In this research, possibility of adaptation for opto-electronic device like solar cell or LED devices were considered and we concluded that film properties can be controlled by controlling fabrication conditions of sputtering, atomic layer deposition, annealing process and it is acceptable to be used in device fabrication.; While the amount of energy usage and demand of energy sources was increaing, there were predictions about crisis of energy exhaustion and environment polution causing global warming. To resolve these problems, renewable energy for clean and sustainable energy source was received attentions. As one of the promising renewable energy source, photovoltaic energy collected by photovoltaic electronic device has attracted and showed positive potential. Silicon solar cell based on bulk size silicon semiconductor material was well focused as one of the best photovoltaic device showing high energy conversion efficiency over 20% and now is predicted to solve upcoming energy crisis. However, there are some problems to be solved and there are also need for research on advanced next generation photovoltaic device. CIGS thin film solar cell, which is so called next generation photovoltaic device, is fabricated with compound quaternary absorber layer composed of Cu, In, Ga, Se(S). Disadvantages of bulk silicon based solar cell, low absorption coefficient, nature of indirect band sturcture, limitation of diminishing thickness of cell, long-term stability are supposed to be solved by using CIGS solar cell. Cost reduction, device portability, device fabrication with flexibility are expected to be obtained. Important one in quaternary thin film solar cell is N-type buffer layer usually deposited as CdS material which is conventionally used in quaternary thin film solar cell with high efficiency yield. However, it is not acceptable using Cd material for its high toxicity and absorption lose from 300-500nm short wavelength range. For these reasons, ZnS thin film for its replacement was suggested because of its cleanness and less absorption loss. ZnS material has about ~3.7 eV of direct band gap energy and highly transparent within visible wavelenth spectrum. This transparent and conductive ZnS is usually used in LED, bio-molecular sensor etc. for its opto-electronic property. In this research, RF-magnetron sputtering and atomic layer deposition method was preapred for ZnS deposition and film properties were analyzed whether it is acceptable for application in solar cell or other opto-electronic devices. ZnS film was deposited on soda-lime glass, silicon, polyethylene-terephtalate substrate. For oxygen control, annealing process was advanced within air, oxygen ambient. When it was prepared by atomic layer deposition, annealing in vacuum, sulfuric (H2S+N2), oxygen ambient was proceeded and film properties were compared. X-ray diffraction analysis was done for grain size and phase confirmation. Deposited ZnS films showed hexagonal wurtzite (002)/Zinc-blend(111) preferred orientation and change of XRD pattern from various annealing conditions with phase transformation. By UV-visible spectrometer, optical transmittance was confirmed and it showed up to 90% for maximum transmittance over 450nm of wavelength. Band gap energy of ZnS films were confirmed by Tauc plotting with film thickness. In this study, we found that band gap energy is linearly decreased with increasing thickness. scanning electron microscope was used for surface analysis and morphologies of ZnS films were assured with grain size calculation from X-ray diffraction. For composition analysis, energy dispersive spectrometer was used to check Zn, S, O composition. In this research, possibility of adaptation for opto-electronic device like solar cell or LED devices were considered and we concluded that film properties can be controlled by controlling fabrication conditions of sputtering, atomic layer deposition, annealing process and it is acceptable to be used in device fabrication.