Metal oxide thin-film transistor (TFT) using metal oxide semiconductor as a channel layer attracts great attention. Metal oxide TFT offers many advantages compared to amorphous Si, which is currently used in industry, such as high mobility and good electrical stability. The metal oxide has large band gap, usually over 3.3 eV, and rarely interact with visible light. It means that the fabricated thin-film is transparent, which have a potential to act as a channel layer in TFT device. Moreover, the metal oxide TFT exhibits good electri-cal property even in amorphous phase. The amorphous film can be prepared by relatively easy and quick process while crystalline film usually requires slow and special process. Since metal oxide TFT offers many advantages, it is expected that the TFT could be applied to many practical applications: backplane of high resolution AMLCD, large are AMOLED, flexible display, and transparent display.
The technical trends in AMLCD TV are large area, high resolution, and high frame rate. In the case of large are, for example, user experiences better reality when the size of the display is enlarged to 100 inches from 70 inches since the field of view in increased to 49 degree from 35 degree. As the size of the display increases the physical size of the pixel increases too. Thus, the resolution of the display should be improved for comfortable watching. More specifically, it is expected that the resolution would be increased to UD level (4000 x 2000) and the physical time that can be assigned for formatting each image gets shorter due to the RC delay in the circuit. The image blurring has been a problem of the LCD due to the slow physical response time of the liquid crystal. The possible way to solve the image blurring problem is improving frame rate. However the restricted mobility of amorphous Si, less than 0.5 cm2/V.s, cannot satisfy the technical requirements for driving backplane. Thus metal oxide is considered as a potential candidate for the back plane of next generation display, currently.
The initial research on metal oxide TFT is leaded by Wager group in Oregon State University, USA, and Hosono group in TIT, Japan. They used vacuum based deposition technique, such as ion beam assisted deposition, for the preparation of channel layer and reported the unique benefits of metal oxide TFT and its great potential for next generation active materials. Since a few results were reported about the flexible elec-tronics using oxide TFT, the research is accelerated both in academic and industrial fields. The research trends in oxide TFT can be categorized into 3 parts. They are 1) exploring novel materials for better performance and stability, 2) studying device physics to improve the performance and stability of the device, and 3) preparing oxide TFTs using solution based process. The solution based process offers many advantages such as direct patterning, uniform film in large area, and low damaging process with reasonable process price. Since the solution process can be applied to ink-jet or roll-to-roll process which could prepare the film where it is required, expensive photolithography in no longer required. In addition, it is expected that the electronic circuits can be fabricated with simple and normal condition using roll-to-roll printer like printing a paper or book, soon.
In this study, solution processed metal oxide semiconductors are thoroughly investigated to improve the electrical performance and stability. We have developed novel metal oxide semiconductor aluminum indium oxide (AIO), which exhibits high performance and enables low temperature process. The structure of AIO is based on indium oxide. Some part of indium atoms is substituted to aluminum to transform the transparent conductive oxide, indium oxide, to semiconductor, AIO. In the first part of the research, we have demonstrated the transformation of electrical behavior, confirmation of structural modification, and optimization of the AIO TFTs. In the second part, we have developed the post annealing process which could decrease the process temperature. The post annealing under high oxidation atmosphere facilitates the conversion of hydroxide related species to oxide and improves the electrical performance of the AIO TFT with 250 ℃ annealing.
In order to decrease the oxide formation temperature, we suggest the aqueous route. The unique indi-um complex structure, [In(H2O)6]3+, allows the formation of In2O3 at low temperature. The In2O3 TFT were activated at low temperature even with 175 ℃ annealing. The optimized 200 ℃ annealed In2O3 TFT exhib-ited good electrical performance with low standard deviation, in terms of mobility and sub-threshold slope. The vacuum post annealing facilitates the condensation reaction and activates the In2O3 TFT even even with 100 ℃ annealing. The transparent In2O3 TFT was fabricated on a flexible PEN substrate with 200 ℃ an-nealing process. The resulting device was transparent and flexible with good electrical performance and sta-bility against external positive and negative gate bias stress. The aqueous route also enables the high perfor-mance application. The optimized 350 ℃ annealed oxide TFT exhibited mobility over 12 cm2V-1s-1 with good bias stability. The study of Al, Ga, and Zn doping effects on the In2O3 TFT indicates that the cation, has higher biding energy with oxygen, effectively decreases the electron trap sites and improves the negative bias stability.
금속 산화물 반도체는 약 3 eV 이상의 넓은 밴드갭은 가지고 있어 광학적으로 투명하며, 비정질 상태에서도 우수한 n-type 반도체 특성을 갖는다. 이와 같은 장점으로 대면적 균일도와 저온 공정이 가능할 것으로 여겨지는 차세대 디스플레이 구동 반도체 재료로 AMLCD나 AMOLED와 같은 디스플레이분야의 응용뿐만 아니라 RFID나 transparent tag와 같은 전자 회로에도 응용 가능한 큰 잠재력을 지니고 있다. 특히, 진공 공정에 비해 간단하고 경제적인 방법으로 소자를 제작할 수 있는 용액 공정은 금속 산화물 반도체의 실용화 가능성을 높일 수 있다. 또한 비교적 낮은 온도에서 뛰어난 전기적 특성을 보이는 금속 산화물 반도체의 특성을 잘 활용하면 저렴한 유리 기판이나 유연한 플라스틱 기판에도 용액 공정을 통해서 TFT를 구현할 수 있는 가능성이 있다.
본 연구에서는 솔-젤 공정을 활용하여 저온 공정 가능한 산화물 반도체를 제작하고자 하였다. 솔-젤 공정은 저온에서 유리 및 산화물을 제작할 수 있는 대표적인 용액 공정으로, 저렴한 가격으로 균일한 박막을 형성할 수 있다는 장점이 있다. 이를 달성하기 위하여 새로운 조성의 반도체 재료를 개발하고자 하였다. 그리고 추가 열처리를 통하여 저온 공정을 구현하고 그 특성을 향상시키고자 하였다. 또 기존의 알코올 기반의 접근 방식에서 탈피하여 전구체 수용액을 제안하고 이를 유연 기판위에 제작하고자 하였다. 그리고 최종적으로 우수한 전기적 특성과 신뢰성을 을 갖는 산화물 반도체를 제작하고자 하였다.