金屬氧化物薄膜電晶體之特性研究與光電元件開發

Abstract

本論文旨在探討金屬氧化物薄膜電晶體 (IGZO TFT) 之基本光電特性，並以其開發出一些衍生的光電元件。高效能 IGZO TFT 雖十分容易被製作出來，然而其易變性以及不穩定性拖延了其成為實質量產元件的時程。IGZO TFT之不穩定性通常表現在載子trapping的行為上，並且無論是電子或電洞都可能在電場下被trap在界電層表面而導致臨界電壓的不確定性。在本論文中，我們著力於瞭解其不穩定行為的機制並澄清一些改善方式，如熱退火及鈍化層，的作用機制。我們發現，其不穩定性與 IGZO 薄膜之氧含量可說息息相關。而且其在兩種不同極性的閘極偏壓下呈現出十分對稱的穩定性行為；也就是高氧含量 IGZO 對應到較好的負偏壓容忍度及較差的正偏壓忍受度，而低氧含量元件則恰巧完全相反。由實驗得知，在低溫成長的 IGZO 薄膜，以其所製成的 TFT 將只能在單一個極性的閘極偏壓下表現出高穩定度，而在另一個極性上則一定是差的。如果在實際電路的操作上可以減少對負偏壓的強度需求，我們將可以在較低氧含量的條件下製作出低溫可用的元件。但如果對兩種極性的強度需求都很高的話，那唯有再施予熱退火來提高在兩種極性偏壓下的整體穩定度；但此時低溫元件的期望將不存在。 IGZO 的易變性則反映在載子濃度上，鍍膜條件、退火條件、光照、環境氣氛都可能造成其摻雜濃度的變異。雖然後續熱退火可提升元件穩定度，但載子濃度將可能依爐管氣氛而上升或下降，此將限制實際可用的退火條件；也就是說退火氣氛的選擇是需要被謹慎考慮的。強紫外光亦可能造成氧空缺而提高載子濃度，故強紫外光照射將可用以輕微調整IGZO薄膜之氧含量（有可能因此而提升元件穩定度）或甚至使之導通化(可將導電度提升 4 個 order 之多)。以強紫外光處理之 IGZO 將可直接作為源/汲極 contact之 用，其效果與金屬 contact 相較不遠。 本論文提出一較不同於一般說法的論述，即照光可視為一種提高載子trapping反應的機制而不是照光本身造成臨界電壓的下降(往負壓方向移動)。照光可以同時加強電子及電洞的 trapping；在光照下量測轉移特性曲線容易發現曲線往負壓移動，是因為量測本身對元件同時施與了正壓及負壓，只不過負壓所造成的電洞trapping 較明顯而已。雖然 IGZO 屬透明性材料，事實上其對光有強烈的反應，並且範圍由紫外一路延伸到紅外光。照光將造成施子摻雜(氧空缺)及電洞或電子在界電層表面的 trapping. 雖然 IGZO 對可見光有所反應，在元件 turn on 的狀態下會形成電子trapping而觀測到即時 (real-time) 的負光電流。本論文亦嘗試以引入另一光吸收層的方式來提高IGZO TFT 對可見光的感測能力。結構上非常簡單，只是單純的在標準的IGZO TFT上以懸塗的方式覆蓋一層有機的低能隙材料 (P3HT, Eg = 2.1 eV) 即形成具高度可見光感測能力之光電晶體。其作動機制為：P3HT/IGZO 接面為一 p-n diode 並具一內建電壓(build in voltage)，P3HT的低能隙使得光激發激子在P3HT內部產生並在內建電場的驅動下將電子注入 IGZO 而形成背通道。抵消內建電壓需要大量的電子電洞跨立在 P3HT/IGZO 介面兩側，也就是說此時將有大量的電子存在於遷移率(mobility)很高的 IGZO 而產生明顯的光電流。此元件對光的反應為呈現正的光電流，這與標準元件的負光電流不同。 IGZO TFT 目前在應用上的弱點為穩定度的問題。然而，其缺點在另一個角度上看來確有可能是個優點。一個對正閘極偏壓穩定度很差的元件(電子容易被trapping) 可以被輕易製作出來而不需要熱退火處理；也就是說它可以是完全低溫的元件。容易電子trapping 代表其極適合作為一記憶體 (memory)，而且不需要任何刻意附加的電子trapping 機構。正偏壓可作為一寫入 (writing, 20s, VG = 20 V) 動作將臨界電壓往正壓方向移動；而搭配光照的負偏壓則可作為一抹除 (erasing, 20s, VG = - 20 V, light) 動作將臨界電壓往負壓方向移動。此記憶體可以具有可觀的臨界電壓window (30 V) 及開/關比 (on/off ratio = 105)。此外，此記憶體是高度可複寫性的元件。另一個應用是即時的紫外光偵測器，適度的施加正偏壓可快速恢復受到UV照射而往負壓移動的臨界電壓；也就是說它可以具有即時的光偵測能力，這是一般光電晶體所達不到的。此論文所發展的光偵測器，具有即時且可觀的 light/dark ratio，可達104 ~ 105。為目前金屬氧化物光偵測器的最高值。 應用上另一個重要的元件即為相反器 (inverter)。若要以金屬氧化物製作出一邏輯電路的標準單元，inverter，勢必要面臨一個根本的問題，就是CMOS結構無法使用，因為金屬氧化物基本上缺乏 p 型材料。所以臨界電壓的調控成為唯一可行方式。調控臨界電壓可以以傳統雙閘極結構來達成。本論文則提出另一新式的雙閘極結構，就是直接在底閘極電晶體的背通道上鍍上一金屬層作為浮閘。稱為浮閘是因為其在元件操作時是完全不接電源的。浮閘對 IGZO 所施予的電壓為功函數差異所造成的內建電壓，因為浮閘與 IGZO 間只有原生氧化層，也就是幾乎沒有界電層，所以浮閘對元件的超控力非常強；些微的內建電壓即可大幅移動 IGZO TFT 的臨界電壓。無論是增強型 (enhancement mode) 或著是空乏型 (depletion mode) TFT 皆可被製作出來，而且還維持非常好的元件特性。此元件的優勢為：高性能、不需額外電源及不需製作上介電層。一性能良好的相反器在本論文中藉著此雙閘結構被成功的開發出來。

In this study, the fundamental electro-optical characteristics of metal oxide thin film transistor (IGZO TFT) were investigated, and some important derivative devices were also developed. In fact, a high performance IGZO TFT is fabricated easily. However, its variability and instability retard the progress of commercialization. For its instability, it is presented by the behavior of carrier trapping. Either of electron and hole could be trapped at the dielectric surface under an electric field to result the undermined position of threshold voltage. In this study, we attempt to clarify the mechanism of instable behavior of IGZO TFT and reveal how the improving treatments, thermal annealing and passivation, take effect. We have found that the instability is strongly dependent on the oxygen content in IGZO thin film. The instability of IGZO TFT under different polar gate bias stresses will exhibits a symmetric behavior. Higher oxygen content IGZO will correspond to well endurance against negative bias stress and poor endurance under positive bias stress; the TFT with lower oxygen content will exhibit opposite instability behavior. The IGZO TFT with a lower temperature deposited IGZO active layer only can endue one polar bias stress; the stability under another polar bias stress is always poor. The post treatment, thermal annealing, can improve the entire stability under both polar bias stresses. However, the expectation of low temperature fabrication will be destroyed. For its variability, it is presented by the variation in carrier concentration. Any of deposition condition, annealing condition, illumination and surrounding atmosphere could make a change in dopant concentration. Although post thermal annealing can improve the stability of IGZO TFT, the carrier concentration will also be increased or decreased, depends on the furnace atmosphere. Therefore, the annealing conditions like atmosphere should be chose carefully. Besides, strong ultraviolet illumination could make oxygen vacancy to increase the carrier concentration. Therefore, strong ultraviolet can be employed to slightly modulate the oxygen content (the stability under bias stress may be improved) or even make conductive (the conductivity of IGZO can be raised by four orders). The light treated IGZO domain can serve as the source/drain contact in IGZO TFT and its performance is comparable with the metal pad. In this thesis, we propose an assumption different from the common explanation on light effect. We consider that illumination can be regarded as a prompter enhancing the carrier trapping process. Illumination alone will not make the negative shift in threshold voltage. The negatively shifted transfer curve under illuminatin is caused by the applied negative gate bias, which results hole trapping, when scanning the gate voltage. Although IGZO is a transparent material, in fact, it exhibits intense response to light and the effective color ranges from ultraviolet to infrared. Illumination will make dopping process and electron/hole trapping at the dielectric surface. Although IGZO TFT is responsive to visible light and could exhibit a real-time negative photo current caused by electron trapping, we also introduce a light absorption layer in the IGZO TFT to improve the sensibility to visible light. The proposed photo transistor possesses a simple structure; an organic low bandgap material, P3HT (Eg = 2.1 eV), is spun coat on the standard bottom gate IGZO TFT. The P3HT/IGZO junction is a p-n diode and forms a build-in voltage drop across the junction. The excitions will be generated by visible light at the low band gap P3HT layer and the electrons will be injected into the IGZO by the build in electric field to form a back channel. The injected electrons in IGZO possess high mobility to conduct abundant photo current. This proposed photo transistor response to illumination by a positive photo current which is different from the standard device with negative photo current. To date, the drawback of IGZO TFT, limits the realistic application, is the instable behavior. However, for other applications, this drawback may be an advantage. An unstable IGZO TFT, exhibits poor endurance to positive gate bias and have abundant electron trap states, can be fabricated easily at room temperature without post annealing treatment. Possessing abundant electron trap states represents that it is an excellent memory without any intended mechanism for carrier trapping. The writing operation is defined as a VG pulse (VG = 20 V, 20s) to move the transfer curve in the direction of positive gate voltage; the erasing operation is defined as an illumination accompanied VG pulse (VG = - 20 V, 20s, UV) to move the transfer curve in the direction of negative gate voltage. This proposed memory exhibits a significant threshold voltage window of 30 V and an on/off ratio of 105. Furthermore, this device performs well repeatability under writing/erasing cycles. Another feasible application of this device is the real-time photo detector. The UV made negative shift in threshold can be compensated by positive VG pulses promptly, which leads to a real-time photo transistor. In fact, generally photo transistors are unable to perform real-time sensing. The proposed real-time photo detector in this thesis has a significant light/dark ratio of 105. This is the best metal oxide based photo detector nowadays. Another important device for application is inverter. If we attempt to fabricate a basic logic unit, inverter, using metal oxide based transistors, we will face a problem of the unfeasibility of CMOS structure because the lack of p-type metal oxide material. Therefore the threshold voltage modulation seems the unique solution. Threshold voltage modulation can be achieved by a conventional dual gate structure. In this study, we propose a novel dual gate TFT using a floating metallic back gate; the gate is floating, implying additional power supply is not necessary during device operation. The bias provided by floating gate to IGZO body comes from the work function difference. Since there is almost no dielectric layer located between floating gate and IGZO, the controllability of floating gate to IGZO body is much high; a little build in voltage can bring a noticeable threshold voltage shift. No matter depletion mode and enhance mode TFTs with excellent performance can be fabricated using this dual gate structure. The advantages of this dual gate device are high performance, unnecessary of additional power supply and unnecessary of additional dielectric layer. Finally, a high performance inverter comprised of the proposed dual gate TFTs is demonstrated in this study.