氧化鋅鉿薄膜性質及薄膜電晶體之研究

Abstract

本論文中首先探討磁控濺鍍法及溶液凝膠法製備的氧化鋅鉿薄膜材料性質．並應用氧化鋅鉿於下閘極薄膜電晶體． 首先，利用磁控濺鍍法在室溫下成長氧化鋅鉿之薄膜於玻璃基板，濺鍍靶材的鉿之成分為0、2.5、5、7.5及10 at%，沉積的薄膜在空氣氣氛下分別進行30分鐘的200、300 、400、500 及600 °C的後退火處理。在鉿(Hf)摻雜入ZnO會導致材料呈現非晶化的現象。而薄膜之內建應力沉積完時處於壓應力的狀況，隨著退火溫度上升，其應力開始釋放甚至最終變成張應力。HfxZn1-xO薄膜在可見光區段具有80%以上的穿透度。其光學能隙會隨鉿含量提升而增高，但隨著退火溫度上升而下降。此情況歸咎於薄膜內應力狀況的變化及晶粒的成長。當鉿的含量提高時，材料結構的破壞及較高的能隙會產生較多的載子阱及在導帶有較少的熱激發載子，而促使氧化鋅鉿之電阻率提高。此外吾人亦針對氧化鋅鉿�氧化鋅的異質結構之電性進行探討。大量的介面載子會於氧化鋅鉿及經歷過退火溫度高於500度之試片出現形成，高導電介面會使整體結構電阻大幅下降。而電阻的下降量亦隨著氧化鋅鉿的鉿含量及膜厚增加而上升。 另一方面，本研究同時利用溶液凝膠法在室溫下成長氧化鋅鉿之薄膜於玻璃基板，前驅液之鉿成分為0、2.5、5及10 at%，沉積後的薄膜於空氣氣氛下分別進行30分鐘300 、400、500 及600 度的後退火處理。對於未經處理之薄膜，鉿摻雜會增進薄膜之結晶性。薄膜光學能隙會隨著鉿含量提升而增高，但隨著退火溫度上升而下降。此情況歸咎於薄膜內應力狀況的變化、晶粒的成長及氧化鉿在退火後析出所造成。氧化鋅之電阻率會隨著退火溫度上升而下降，是由於薄膜之結晶性增加及缺陷量下降所致。相反的，氧化鋅鉿之電阻率隨退火溫度上升而上升則是由於氧化鉿析出及材料內部氧空缺含量下降造成。鉿的摻雜會增進鋅及氧元素之間的鍵結，同時退火處理也會改善金屬元素及氧元素之間的鍵結情況。 最後，透過磁控濺鍍法來成長下閘極氧化鋅鉿薄膜電晶體及上閘極氧化鋅鉿/氧化鋅異質接面薄膜電晶體。對下閘極氧化鋅鉿薄膜電晶體而言，氧化鋅鉿通道層其鉿成分為0、0.5、1及2.5 at%，電晶體的介電層為電子束蒸鍍成長之氧化鎂。比較之下，當鉿含量為0.5 at%時，元件有較佳表現。臨界電壓、次臨界擺幅、載子遷移率及電流開關比分別為4.1 V、1.66 V/dec 、9.0 cm2/Vs 及 >107。對上閘極薄膜電晶體，氧化鋅鉿/氧化鋅異質接面能大幅改善元件電性，元件之臨界電壓、次臨界擺幅及載子遷移率分別為0.38 V、1.79 V/dec 及158 cm2/Vs。

This thesis reports the properties of the HfxZn1-xO thin films deposited by rf-sputtering and sol-gel method. The sputter-deposited HfxZn1-xO thin films were then used as the active layers of thin film transistors (TFTs) on the glass substrates. The sputtering HfxZn1-xO thin films with various Hf contents are sputter-deposited on glass substrates from HfxZn1-xO (x = 0, 2.5, 5, 7.5 and 10 at%) targets at room temperature. The incorporation of Hf in the ZnO film leads to the amorphization of the materials. As the annealing temperature increases, the built-in stresses in the thin films are relaxed. The optical bandgap increases with the Hf content, yet it decreases with the annealing temperature. This can be attributed to the alteration of strain (stress) status in the films and the slight grain growth. Hf doping increases the resistivity of ZnO owing to the disorder of the material structure and the higher bandgap. The electrical properties of the rf-sputtered HfxZn1-xO/ZnO heterostructures were also investigated. A highly conductive interface is formed at the interface between HfxZn1-xO and ZnO thin films as the ZnO annealing temperature exceeds 500°C, leading to the apparent decrease of the electrical resistance. The resistance decreases with an increase of either thickness or Hf content of the HfxZn1-xO capping layer. The second part of thesis reports the characterization of sol-gel derived HfxZn1-xO thin films deposited on glass substrates. The incorporation of Hf in the films increases the crystallinity of the as-deposited films. The bandgap increases with the Hf content but reduces after thermal annealing because of the relaxation of built-in stress, atomic rearrangement, and precipitation of HfO2. The resistivity of ZnO decreases as the annealing temperature increases owing to the improvement of crystallinity and reduction of defect densities. On the contrary, the resistivity of HfxZn1-xO thin films increases with the annealing temperature owing to the precipitation of HfO2 and reduction of oxygen vacancies. The electrical properties of sol-gel derived Hf0.1Zn0.9O/ZnO and ZnO/HfxZn1-xO heterostructures were also investigated. The amount of resistance reduction increases as the bottom layer annealing temperature increases. According to the XPS result, the incorporation of Hf in the films improves the Zn-O bonding state, and thermal annealing enhances metal-oxygen bonding. Sputtered HfxZn1-xO (x=0.005, 0.01 and 0.025) is used as the channel layer of the bottom-gate TFT with MgO dielectric layer deposited by e-beam evaporation. The Hf0.005Zn0.995O shows the better device performance. The Vth, S.S., μlin and on-off ratio values for the Hf0.005Zn0.995O TFT were 4.1 V, 1.66 V/dec, 9.0 cm2/Vs and >107. In the top gate TFTs, Hf0.1Zn0.9O/ZnO hetertstructure is used as the channel of TFTs. The multilayer enhances the electrical properties of TFTs. The Vth, S.S., and μlin, values for the Hf0.1Zn0.9O/ZnO TFT were 0.38 V, 1.79 V/dec, and 158 cm2/Vs.