氧化物半導體薄膜電晶體之研究

Abstract

透明氧化物半導體在近幾年相當受到注目且發展迅速，這些氧化物半導體因為含有具特殊電子結構(n-1)d10ns0 (n>=4)之重金屬陽離子，使得它們擁有許多優越且有趣的特性。它們的電子結構提供了良好的載子傳輸途徑，且不太會受原子排列狀況的影響，不論是結晶或非晶材料均可具備高載子傳輸率。它們可以在低溫或室溫環境下即成長出高品質薄膜，因而可適用在各式基板上。另外它們多是寬能隙材料，在可見光範圍是透明的，可用來提高平面顯示器的開口率與解析度，甚至可用以製作透明電子元件，以及許多新穎的應用。 本論文針對氧化物半導體，研究其在實際元件如薄膜電晶體上的可實用性，以單一氧化物材料如氧化鋅、混合氧化物材料如氧化鋅銦鎵，來製作各種微小化高品質的氧化物薄膜電晶體。由於單一氧化物材料容易結晶、產生晶界而劣化元件特性，因此本論文首先以熱退火方式使晶界融合，來製作多晶氧化鋅透明薄膜電晶體，並且研究其因尺度的縮放產生的效應。緊接著本論文研究薄膜厚度對於氧化鋅型態的影響，並進而以全蝕刻製程製作出非晶氧化鋅透明薄膜電晶體，證實其元件特性較多晶氧化鋅透明薄膜電晶體為佳。 而在混合氧化物材料氧化鋅銦鎵的部份，本論文搭配不同成長條件與絕緣層選擇，製作出高載子移動率與高穩定性之透明薄膜電晶體，並探討不同條件對元件特性的影響。進一步地，本論文將氧化鋅銦鎵薄膜電晶體製作在軟性塑膠基板上，並且同時製作一些小型積體電路如反相器與環形震盪器等，這些元件與電路在軟性基板上均仍可正常操作。另外，本論文亦提出以元件模擬軟體搭配適合的氧化物半導體模型，包含有能隙間能態密度分布模型與載子移動率模型等，準確地模擬出與實驗結果吻合的氧化物半導體薄膜電晶體元件特性，並針對其物理機制進行探討。

Transparent oxide semiconductors composed of heavy metal cations with specific electronic configuration of (n-1)d10ns0 (n>=4) have gained widely attentions in recent years. The ns orbitals provide efficient transport path which is not sensitive to the film disorder. Thus, high mobility can be obtained in both crystalline and amorphous oxide semiconductors. Oxide semiconductors are also suitable to a large variety of substrates because they can be deposited at low temperature or even room temperature. In addition, they are usually wide-bandgap materials and thus are transparent in the visible range, which can benefit the resolution of displays by increasing the aperture ratio or be used for transparent electronics. This dissertation investigates the application of oxide semiconductors for thin film transistors (TFTs). Various high performance TFTs based on oxide semiconductors with heavy metal cations, e.g. single-component oxide semiconductors such as zinc oxide (ZnO) and multi-component oxide semiconductors such as indium gallium zinc oxide (IGZO), were successfully fabricated.

The miniaturized transparent thin-film transistors (TTFTs) having various channel widths and lengths are fabricated using polycrystalline ZnO, and their typical and scaling behaviors are studied. Generally single-component oxide semiconductors easily form polycrystalline phases, and their grain boundaries would deteriorate the device performances. Thus a post annealing is performed to re-fuse the grain boundaries. Furthermore, the thickness dependent morphologies of ZnO are also studied in this dissertation. By simply reducing the thickness of ZnO films, ZnO can be intentionally grown into the amorphous phase without grain boundaries. Both top-gate and bottom-gate amorphous ZnO TTFTs of micrometer scales are effectively implemented using fully lithographic and etching processes.

As for the multi-component oxide semiconductors, a representative material system, amorphous indium gallium zinc oxide (a-IGZO), is studied in this dissertation. High mobility and high stability a-IGZO TTFTs are fabricated using different gate insulators and different deposition conditions for the channel. The influences of gate insulator and oxygen partial pressure on device performance and stability are investigated. A process for fine fabrication of a-IGZO TFTs and integrated circuits such as inverters and ring oscillators on flexible and transparent plastic substrates is also successfully developed in this dissertation. In addition to the experimental works, a model of the carrier transport and the subgap density of states in a-IGZO is reported for device simulation of a-IGZO TFTs operated in both the depletion mode and the enhancement mode. It is found that a simple model using a constant mobility and two-step subgap density of states reproduced well the characteristics of the a-IGZO TFTs.