基於IGZO 薄膜電晶體基體效應所發展之可見光感測與臨界電壓調控技術

Abstract

非晶氧化銦鎵鋅(a-IGZO)為具有潛力的光電材料，尤其是應用於顯示器上，故了解其對於可見光的基本反應有其必要性。在本研究中，我們對a-IGZO 薄膜電晶體在不同波長光照下進行了光反應之研究，發現其行為與光波長及元件之操作模式有明顯的相依性，此可作為將來在系統設計上的參考。為了改製透明的a-IGZO 薄膜電晶體成為對可見光區有高度感測能力的光感測器，引入以窄能隙高分子半導體P3HT，覆蓋於底閘極a-IGZO TFT 主動層上來形成光電晶體。加入P3HT 覆蓋層的元件其光感測靈敏度被大幅提升，其具有大量光電流的主要原因來自於光照造成臨界電壓的改變。經過一系列不同操作模式下的照光實驗，我們合理推測照光所引起臨界電壓改變的機制為照光過程中由P3HT 層所激發出的激子(電子電洞對)在P3HT/IGZO 介面被內建電場拆解並順著電場方向將電子引入IGZO 層並在基體內(背通道)累積或被捕捉而造成臨界電壓值的改變。 此外，在本研究中也發現藉由覆蓋層結構的引入會造成臨界電壓值改變，推測是覆蓋層與IGZO 主動層之費米能階不同而導致介面形成電偶極。我們發現a-IGZO 薄膜電晶體的臨界電壓位置可經由一系列不同費米能階的金屬覆蓋層來有效的調變，由於IGZO 主動層後通道與不同費米能階之覆蓋層間形成不同程度與極性的電偶極，此會感應出不同的基體電壓(基體效應)來改變元件臨界電壓值。因此，我們提出一個加入金屬覆蓋層的結構來提升元件效能與調變其臨界電壓值而不會造成元件效能的折損與漏電。於此更發現元件遷移率(mobility)可經由覆蓋層的引入而大幅提升，此法將可作為一簡單而有效的製程。

a-IGZO is the high-potential material for optoelectronic application, display specially. Therefore, the photo-response to visible light of a-IGZO transistor must be understood. In this study, we discuss the photo-response of a-IGZO TFT under illumination with various wavelengths and find out that a-IGZO TFT is strongly wavelength and operation mode dependent. This study could provide a useful direction for future system design. In order to reform the transparent a-IGZO thin film transistor to become a visible light photo-sensor with adequate sensitivity, we introduce a narrow bandgap polymer semiconductor, P3HT, capping onto the active layer of bottom-gate a-IGZO TFT to form a photo-transistor. The large photocurrent of P3HT-capped a-IGZO photo-transistor may be caused by the light-induced threshold voltage shift. By a series of experiments made in different operation modes under illumination, the reasonable mechanism of light-induced threshold voltage shift is proposed. The excitons are generated in P3HT by illumination and then are dissociated by the build-in electric field at P3HT/IGZO junction. The electrons dissociated from excitons drift into IGZO and then be trapped or accumulate at the back channel in IGZO TFT during illumination. Furthermore, in this study, it was found that the threshold voltage position is changed by introducing the capping layer. It is speculated that electric dipoles are formed during the process of fermi-level equilibration while the junction between capping layer and IGZO form. We demonstrate that the threshold voltage position of a-IGZO TFT could be effectively adjusted by capping layer with various fermi-levels. By the electric dipoles with various magnitudes and different polarities forming between IGZO back channel and capping layer with various fermi-levels, the body voltage could be adjusted (body effect) and then affect the device threshold voltage. In this study, we propose a novel structure with capping metal layer onto the active layer of bottom-gate a-IGZO TFT to provide a powerful solution of enhancement of device performance and threshold voltage modulation that would not cause current leakage and performance degradation. In addition, the device mobility increases significantly after introducing the metal capping layer. In summary, capping metal layer seems a simple and effective approach to fabricate a feasible metal oxide transistor.