非晶態金屬氧化物薄膜電晶體之環境敏感性與可靠度

Abstract

近年來，隨著液晶顯示器的尺寸逐漸增大，用來使液晶旋轉的薄膜電晶體所需要的電子遷移率也越來越高。但是，傳統的非晶矽薄膜電晶體其電子遷移率太低(< 1 cm2 / Vs)，因此擁有高電子遷移率(10~100 cm2 / Vs)的非晶態金屬氧化物薄膜電晶體對於應用在未來的顯示器上是非常有潛力的。因此，非晶態金屬氧化物薄膜電晶體將是我們要研究的課題之一。雖然非晶態金屬氧化物薄膜電晶體有非常高的電子遷移率，但卻常常受到環境、照光以及長時間操作偏壓等影響，而產生臨界電壓的偏移，使元件特性面臨一個不穩定的狀態。 本論文首先將探討從非晶態金屬氧化物薄膜電晶體的製程條件來改善元件的穩定性。在此部分，我們探討溶膠凝膠態之非晶態氧化銦鋅薄膜電晶體沉積後退火的必要元素，以調整薄膜的化學成分。相較於未經退火的非晶態氧化銦鋅薄膜電晶體，真空退火後的薄膜電晶體其電性傾向導體態；而經過氧退火的薄膜電晶體則具有開關特性，且退火時間延長時有更佳的電性。因此，我們得到以下的結論：單獨只有熱能時，並沒有辦法使非晶態氧化銦鋅薄膜電晶體達到退火的目的，還必須有氧分子的參與才行。 接著，則改變周遭環境之氛圍、光線以及溫度，來測試元件的穩定度。在環境氛圍的方面，隨著環境相對溼度的增加，溶膠凝膠態之非晶態氧化銦鎵鋅薄膜電晶體中之電子遷移率提高而臨界電壓降低，但是次臨界擺幅卻劣化，且有一個類似汲極誘導能障下降的現象產生。所以，我們提出了水偶極的模型來解釋這些不尋常的劣化現象。在環境光線的部分，溶膠凝膠態之非晶態氧化銦鎵鋅薄膜電晶體的導電度，會因為吸附氧分子的脫附而隨著照光時間的延長而增加，而若是將光線移除，氧的再吸附行為就可使元件特性恢復。 此外，我們也比較溶膠凝膠態之非晶態氧化銦鎵鋅薄膜電晶體與濺鍍式氧化銦鎵鋅薄膜電晶體對於溫度的敏感度。我們發現，在不含氧的環境下，熱激發主宰且提升元件特性。在含氧的環境下，氧吸附的形式改變以及氧與氧空缺的再結合主導了元件特性。接著，我們研究在不同溫度下，氧在濺鍍式非晶態氧化銦鎵鋅薄膜電晶體背通道吸附的能力；較高的環境溫度可幫助氧更易吸附在氧化銦鎵鋅薄膜上，且進一步地修復部分的缺陷，使態位密度減少。 除了周遭環境因素的改變之外，也加入偏壓，來監控在實際操作的狀況下，元件穩定度與劣化機制。對於應用在主動式陣列有機發光元件上，我們評估了濺鍍式非晶態氧化銦鎵鋅薄膜電晶體，在熱載子偏壓下的電容─電壓曲線的劣化。由真空下，我們發現閘極對汲極電容曲線與閘極對源極電容曲線都往正方向偏移，劣化機制主要是由閘極介電質中電子的捕獲所造成的。而在氧環境下，因為汲極端電場誘導的氧吸附，閘極對汲極電容曲線有一個很明顯的正方向偏移；而閘極對源極電容曲線的劣化，不只有正方向的偏移發生，還有奇特的兩階段開關行為。此兩階段的開關行為，主要是由於閘極介電質中的電子捕獲與電場誘導氧吸附在通道上所造成，尤其是在汲極端的氧吸附。閘極介電質中，電子的捕獲增加了源極端的能障，此外電場誘導的氧吸附更進一步抬升汲極端的能帶，因此而在閘極對源極電容曲線造成兩階段開關的行為。 最後，我們提出一些建議來期望得到高穩定度與特性好的非晶態金屬氧化物薄膜電晶體，且更一步期望實現全透明的顯示器的理想。

Recently, higher electron mobility is needed for thin film transistors (TFTs) to twist the liquid crystal in large-size displays. However, the electron mobility of conventional amorphous-silicon (α-Si:H) TFTs is very low (< 1 cm2 / Vs). As a result, amorphous metal-oxide TFTs with high mobility (10~100 cm2 / Vs) is very promising for the application of future displays. Accordingly, the exploration of amorphous metal-oxide TFTs becomes an important topic. Despite higher electron mobility of amorphous metal-oxide TFTs, the device performance is always affected by the ambience, illumination, and long-term bias stress, resulting in the threshold voltage shift and a metastable state. This dissertation investigated the improvement of device stability by changing fabricating process conditions firstly. In this part, we explored the essential elements of post-deposition annealing to adjust the film stoichiometry for sol-gel derived amorphous indium-zinc-oxide thin film transistors (α-IZO TFTs). Compared with the as-deposited α-IZO TFTs, the electrical characteristics of vacuum-annealed one tends to the conducting properties, while the oxygen-annealed one appears switch characteristics and exhibits better performance as the annealing duration extends. Hence, only the heating energy alone cannot achieve the purpose of annealing, the oxygen-containing environment is also required. Then, the surrounding ambience, light, and temperature will be altered to monitor the device stability. With increasing relative humidity, the electron mobility and the threshold voltage of sol-gel derived amorphous indium-gallium-zinc-oxide (α-IGZO) TFTs enhance, while the subthreshold swing degrades and the drain-induced-barrier-lowering appears. Hence, a water dipole model is proposed to explain these anomalous deteriorations. Under the light-illumination, the conductivity of sol-gel derived α-IGZO TFTs will increase, due to the oxygen desorption. On the other hand, the oxygen re-adsorption will cause a recovery behavior once the light source is removed. Furthermore, we compare the temperature sensitivity between sol-gel derived and sputtered α-IGZO TFTs. In the ambience without oxygen, thermal activation dominates and enhances the device performance. In oxygen-containing environment, the varying form of adsorbed oxygen and the combination of oxygen and vacancies dominate the device performance. Then, the adsorbing capability of oxygen on the back-channel in sputtered α-IGZO TFTs were examined at various ambient temperatures. Results imply that higher ambient temperature can assist the oxygen to adsorb on α-IGZO film easily and can further passivate part of the traps in α-IGZO film, leading to a decrease in the density-of-states. The experiment of bias stress is also performed to simulate the device operation and to find the degradation mechanism. For application in active-matrix organic light-emitting diodes, the anomalous capacitance-voltage degradation of sputtered α-IGZO TFTs under hot carrier stress is explored. In vacuum, both the gate-to-drain capacitance (CGD) and the gate-to-source capacitance (CGS) curves exhibit positive shifts due to the electron trapping in the gate insulator. While in an oxygen-rich environment, the CGD-VG curve shows a significantly positive shift due to the electric-field-induced oxygen adsorption. The degradation in the CGS-VG curve is not only the positive shift, but also the anomalous two-step turn-on behavior. This phenomenon can be ascribed to the electron trapping in the gate insulator and electric-field-induced oxygen adsorption on the channel layer, especially in the area adjacent to the drain terminal. The electron trapping increases the source energy-barrier, with the electric-field-induced oxygen adsorption further raising the energy band near the drain, resulting in a two-step turn-on behavior in the CGS-VG curve. Lastly, we address some suggestions for future research topic to obtain the amorphous metal-oxide TFTs with high stability and good performance. Finally, we hope that the fully transparent large area electronics for future display will be realized.