氧化鋅電晶體之製作及建立其電路元件模型

Abstract

近年來，金屬氧化物被人們廣泛研究，其中又以氧化鋅材料為主軸，其具有相當的可見光穿透率，高電子遷移率，並適合低溫製程等優點。氧化鋅薄膜電晶體被視為可撓性電路以及平面顯示器的新希望。但由應用於可撓性電路之氧化鋅電晶體受到低溫製程的影響，使得其穩定性降低。另一方面，其材料特性不同於傳統以單晶矽製作的電晶體，使得元件的分析窒礙難行。 在本篇論文中，我們在室溫下製作出高效能之氧化鋅薄膜電晶體。其操作電流高達1毫安培以上。同時我們針對常溫製程所面臨的問題加以討論，並且根據氧化鋅電晶體內部的物理機制，建立一個足以完整描述其電特性的元件模型。 首先，藉由觀察電特性較差的氧化鋅電晶體，我們發現其電容-電壓(Capacitance-Voltage, CV)曲線以及電流-閘極電壓(I-VG)曲線出現遲滯現象。同時，在電流-源極電壓(I-VDS) 曲線中，在飽和區間開始時，出現了過衝電流的現象。 對於I-VG中出現的遲滯現象，我們透過不同的偏壓方向、以及積分時間的準靜態CV量測，探討其絕緣層中的移動性離子電荷(mobile ionic charges)的影響，以解釋其特性曲線中的遲滯現象。 而針對飽和區開始時的過衝電流，我們透過晶界(grain boundary)物理模型加以解釋，並且經由X光繞射 (X-ray Diffraction)以及原子力顯微鏡(Atomic Force Microscopy)驗證氧化鋅薄膜的奈米晶體(nanocrystalline)之特性，取得晶粒大小。接著我們藉由射頻CV量測，以高低頻電容法求出其中的缺陷濃度，將上述實驗所得之參數放入模型當中進行模擬。最後，我們考慮電極之接觸電阻，以建立室溫下製作之氧化鋅薄膜電晶體之電路元件模型。

Recently, metal-oxide materials have drawn a lot of attention in research field. ZnO is suitable for room-temperature process. It has lots of advantages such as high transparency in the visible light region, and high electron mobility. Thus ZnO is regarded as a potential material for flexible electronics and panel displays. However, due to the fabrication process at room temperature, ZnO-based devices have lower stability than Si-based transistors. Moreover, since the material property of ZnO is unique and different from Si, the analysis of ZnO-based devices is more difficult, compared with traditional transistors. In this work, we demonstrate high-performance ZnO-based thin film transistors fabricated at room temperature. The operating current is 1.4 mA at VGS = 6V and VDS = 20V. The Ion-Ioff ratio is higher than 1.0×106. We also make a discussion on the problems encountered in fabricating ZnO-based TFT at room temperature. And then based on the grain boundary theory, we build a model. With this model, we can completely describe the carriers transport mechanism in the channel layers of ZnO-based TFTs fabricated at room temperature. First of all, we analyze ZnO TFTs with poor electrical properties. Hysteresis can be observed from IDS-VG curves and C-V curves of these devices. We also find out overshoots in IDS-VDS curves. In the first part of this thesis, in order to explain the hysteresis observed from transfer curves, the C–V profiles are described by qausi-static capacitance-voltage (QSCV) measurement in deferent integration time and from different sweep directions. According to the measurement results, the hysteresis I-V curves and C-V curves is result from the mobile ionic charges in the insulating layer. In the second part of this thesis, we use a model based on the theory of grain boundary to explain overshoots observed in the I-VDS curves. We verify the nanocrystalline property of ZnO thin film by X-ray diffraction (XRD) pattern and atomic force microscopy (AFM) image. Meanwhile, we use the “high-low-frequency capacitance method” to calculate the trap density in the channel layer. Finally, using parameters from the experimental results, we demonstrate an analytical IDS-VDS model in this work.