摘要:本計畫的目的在於研究與發展由寬能隙金屬氧化物半導體所構成之透明pn接面,並進一步探討與提升其紫外光頻段之光伏特性。在這為期二年的計畫中,第一年我們以開發p型透明金屬氧化物半導體(以氧化銅鋁類別的材料為主)為目標,針對其結構、電性(電洞傳導特性)、光學性質與製程條件間的相關性進行分析,並進行基礎能帶研究,涵蓋能帶以及能帶間隙中的缺陷分佈等;第二年則將此p型透明金屬氧化物半導體與n型透明金屬氧化物半導體(以氧化鋅類別的材料為主)結合,進行透明pn接面的製作與研究,並針對能帶間隙中的缺陷分佈與pn接面光伏特性的相關性進行系統性的研究。最後將此pn接面製程轉移至塑膠基板,完成可撓性透明pn接面。在薄膜的製作上將採用低溫磁控濺鍍法搭配熱處理,在薄膜結構檢測部分將採用拉曼光譜(Raman Spectroscopy)以及X光繞射(X Ray Diffraction)進行分析;電性與载子傳導特性檢測則利用Van der Pauw與光激電流(Photocurrent)等實驗方法,光學性質上則是進行穿透/反射/吸收光譜的檢定。至於在能帶研究上,將採用光電子譜(Photoemission and Inverse Photoemission Measurements)搭配光或電激發螢光光譜(Photo- Electro- Luminacence)、與熱激發電流量測(Thermally Stimulated Current Measurement)等方法來獲取透明金屬氧化物半導體能帶與能帶間隙中缺陷分佈。在pn接面完成後將透過外部量子效率(External Quantum Efficiency)以及紫外光頻譜波段下電流-電壓特性曲線的量測來獲取其光伏特性,並希望藉此計畫能系統性的分析出pn金屬氧化物半導體基礎特性與其光伏效應的相關性,進而建立一模型準則來協助此類元件之開發與設計。
Abstract: Transparent metal oxide semiconductor is a unique group of materials, which have optical transparency as well as relatively good electrical conductivity when there is intentional incorporation of additional doping elements or there are native stoichiometric point defects which acts like donors or acceptors. Their major applications include transparent electrodes for flat panel displays, touch screens, photovoltaic, heating window, electromagnetic shielding and etc. Recently, by suppressing the donor-like intrinsic defects such as oxygen vacancy, several transparent metal oxide semiconductors have been successfully integrated into thin film transistors (TFTs) as the channel layers to demonstrate their filed-effect functionalities. However, to become good candidate materials for “versatile invisible electronics” applications, metal oxide semiconductors with features of p-channel operation and/or bipolar operation capability are also desirable. Because of the large electronegativity of oxygen, the valence band edge of most of the oxide semiconductors is strongly localized on oxygen ion, which causes the migration of positive holes becomes difficult even under an applied electric field. Until a decade ago, CuMO2 (M: Al, Ga, In) was reported as a p-type transparent conducting oxide based on the modification of band structure by using Cu as the cation to reduce the localization of the valence band edge. Today the development of high performance p-type transparent oxide semiconductors and transparent p-n junction is still a challenge topic.
The goal of this project is to develop and to have a fundamental understanding of transparent pn junction based on metal oxide semiconductors and then extend their application to photovoltaic device in ultraviolet regime. In this two-year research project, first we will focus on developing p-type transparent post-transition metal oxide semiconductors (particularly focus on CuAlO2-based materials) and investigating their structural, electrical and optical properties, followed by study of their electronic bandgap structures (including band-to-band and subbandgap structure). The thin film will be prepared by low-temperature RF reactive magnetron sputtering. The structural analysis will be primarily investigated by Raman spectroscopy and X-ray diffraction. Van der Pauw measurement, seekbeck measurement, dark electrical conductivity measurement, and photocurrent measurement will be performed for electrical analysis. The electronic band structure will be characterized by transmission/reflection/absorption spectra, photoemission and inverse photoemission spectroscopy, electro- and photo-luminescence and thermal stimulated current measurement.
In the second year, we will move onto to integrate the p-type metal oxide semiconductor with n-type oxide semiconductor (mainly ZnO-based materials) to form a transparent p-n junction. After the fabrication of the p-n junction, current-voltage characteristics will be evaluated and rectifying response will be checked. Then photovoltaic performances, such as the conversion efficiency and photovoltage, will be characterized by external quantum efficiency measurement and I-V characteristics under UV luminance. The results will further be correlated with the structural, optical, electronic transport properties and the electronic band structure of the junction layers, and feedback to optimize the junction layers. Once the on-glass p-n junction is optimized, the entire process will be moved onto plastic substrates to realize flexible transparent p-n junction. Through this project, we expect a guideline for identifying device-quality transparent metal oxide semiconductor will be established.