氧化鋅與氧化鎂鋅特性研究及其於 透明薄膜電晶體之應用

Abstract

本篇論文主要在研究氧化鋅(ZnO)和氧化鎂鋅(MgZnO)薄膜的機械、光學、電子等性質及其在薄膜電晶體之應用。 我們以射頻磁控濺鍍(radio frequency magnetron sputtering)法在玻璃、矽晶圓(p-type)和塑膠(PI、PET)基板上沈積氧化鋅或氧化鎂鋅薄膜，並透過拉伸實驗、XRD、SEM、穿透頻譜了解其機械、光學、電子等特性。結果發現玻璃和PET上的氧化鋅膜在可見光區的穿透率均可達60%以上而氧化鎂鋅膜在可見光區的穿透率均可達80%以上。當的鎂含量增加時紫外光吸收邊界會往短波長移動，能帶寬增加，在可見光區的穿透率均可達80%以上。 利用金屬-絕緣層-半導體(Metal-Insulator-Semiconductor,MIS)實驗尋找最佳的薄膜電晶體絕緣層。因為Mg0.1Zn0.9O、Mg0.2Zn0.8O、Mg0.3Zn0.7O的漏電流很大，所以相對介電常數無法計算出。Mg0.4Zn0.6O(O2:Ar=1:6)、Mg0.5Zn0.5O(O2:Ar=0、1:9、1:6、1:3， 5、10mtorr)和MgO(O2:Ar=1:4)的相對介電常數均大於7以上。當氧化鋅摻雜鎂含量越多時，晶格結構會趨向立方體結構，並能有效降低漏電流。在10mtorr、O2:Ar=1:9下Mg0.5Zn0.5O薄膜之C-V曲線中遲滯迴路較小，代表正游離電荷較少。至於Mg0.5Zn0.5O我們發現氧通量對其漏電流密度影響不大。 在薄膜電晶體實驗中，我們測試的主動層有MgxZn1-XO(x=0、0.01、0.05、0.1、0.2)而介電層有氮化矽(SiNx)加二氧化矽(SiO2)、Mg0.5Zn0.5O、氧化鎂(MgO)。實驗結果發現以SiO2加SiNx作為絕緣層搭配氧化鋅或氧化鎂鋅作為主動層之薄膜電晶體的漏電流很大，而且輸出特性曲線在飽和區有〝overshoot〞現象。此外由於SiNx與氧化鎂鋅界面缺陷密度高使薄膜電晶體的特性不佳，其臨限電壓很大(~20V)、載子遷移率很低(~10-3(cm2V-1s-1))、開關比很小(~103)。而以Mg0.5Zn0.5O為介電層的薄膜電晶體，由於Mg0.5Zn0.5O的絕緣性不佳，導致閘極漏電流過大。至於以MgO為介電層的薄膜電晶體則有不錯的電性，其臨限電壓＜10V、載子遷移率~1(cm2V-1s-1)、開關比~105。從實驗的結果可以發現，當主動層沉積時沒有加氧通量或微量鎂時，因為氧化鋅的本質載子濃度過高，所以在負偏壓時電晶體無法關閉。當通入氧或增加微量鎂時，因氧或鎂降低氧化鋅的本質載子濃度，所以薄膜有電晶體的特性。不過，當氧通量或鎂含量過高時，則又無電晶體特性。

The thesis focused on the study of mechanical, optical and electrical properties of Zinc Oxide (ZnO) & Magnesium Zinc Oxide (MgZnO) thin films, and their applications in thin film transistors (TFTs). The thin films were deposited by radio frequency magnetron sputtering on various substrates, such as glasses, p-type Si wafers, polyimide (PI) and polyethylene terephthalate (PET). We then investigated film properties by tensile test, XRD, SEM, and optical transmittance measurement. The optical transmittances of >60% and >80% were obtained in the ~μm-thick ZnO films and ~ Å -thick MgZnO films in the visible light wavelength range as they are deposited on both glass and PET substrates. The ultraviolet absorption edge moved to a shorter wavelength, corresponding to a wider band gap, with the increase of magnesium contents. We investigated the electrical properties of the dielectric layers through metal-oxide-semiconductor (MIS) experiments. The dielectric constants of ~2000 Å -thick Mg0.1Zn0.9O, Mg0.2Zn0.8O and Mg0.3Zn0.7O thin films were unable to be determined because of large leakage currents. Relative dielectric constants of 7 or above were obtained for Mg0.4Zn0.6O(O2:Ar=1:6, 5mtorr), Mg0.5Zn0.5O(O2:Ar=0、1:9、1:6、1:3, 5、10 mtorr) and MgO(O2:Ar=1:4, 3mtorr). As Mg content increased, the leakage current of MgZnO thin films reduced significantly. Among all the thin films tested, Mg0.5Zn0.5O deposited at 10 mtorr from gas composition of O2:Ar=1:9 showed smallest hysteresis in the C-V measurement, which indicated the amount of positive mobile charges was smallest. In addition, we observed that the leakage current density of Mg0.5Zn0.5O thin films was not correlated to the oxygen partial pressures during deposition. We fabricated TFTs with various active layers, such as MgxZn1-XO (x=0、0.01、0.05、0.1、0.2), and various gate dielectric layers, such as film stack of silicon nitride (SiNx) and silicon dioxides (SiO2), Mg0.5Zn0.5O and MgO. We observed 'overshoot' at the onset of the saturation region in the output characteristics of TFTs with film stack of SiNx and SiO2 as gate dielectric layer. Because of the interface defects between SiNx and ZnO or MgZnO, the TFTs exhibited large threshold voltages of ~ 20V, low mobilities of ~10-3 cm2V-1s-1 and on/off ratios of ~ 10-3. TFTs with Mg0.5Zn0.5O gate dielectric exhibited large gate leakage currents, which might result from the poor insulating of the Mg0.5Zn0.5O. From experimental results, we found that the off currents of TFTs with active layers deposited at small amount of oxygen flow or deposited from targets containing small amount Mg were reduced significantly. However, when excess oxygen flow or Mg content was applied, no TFT characteristics could be obtained.