利用原子層沉積技術成長氧化鋅薄膜與氧化鎂緩衝層之研究

Abstract

本論文乃研究利用原子層沉積技術 (Atomic Layer Deposition, ALD) 成長之氧化鋅薄膜的光電特性。論文可分為三個部分： 第一部分，為了改善氧化鋅薄膜氧空缺過多的問題，本論文採用低溫ALD製程製備氧化鋅薄膜，使其內電子濃度降低，並改善其結晶品質和電子遷移率 (electron mobility)。實驗結果顯示，電子濃度可降至1016 cm^(-3)，載子遷移率為4.66 〖cm〗^2/Vs。 第二部分，同樣是為了降低氧化鋅薄膜的電子濃度，我們使用臭氧(O3)作為precursor之ALD製程製作氧化鋅薄膜。藉由調控不同的製程參數，發現其電子濃度可有效降至1014 cm^(-3)，載子遷移率為37 〖cm〗^2/Vs。 此外，以X光繞射儀檢驗，發現此氧化鋅薄膜幾乎沒有結晶，且載子遷移率高，因此可應用於薄膜電晶體的製作。 最後，我們於藍寶石基板及矽基板上成長氧化鋅薄膜，並在基板與氧化鋅層中間插入一層以ALD成長之氧化鎂緩衝層，並比較有無緩衝層的差異。實驗結果發現，有氧化鎂緩衝層之氧化鋅薄膜，PL強度顯著上升，且隨著氧化鎂緩衝層厚度增加而增加。此外，成長於矽基板上的氧化鋅，在有加上氧化鎂緩衝層時，其PL峰值隨氧化鎂緩衝層厚度上升有藍移 (blue shift) 的現象，推測其應為氧化鎂緩衝層施加給氧化鋅層的壓應力 (compressive strain) 釋放了原本基板所給予氧化鋅薄膜之張應力 (tensile stress) 所導致。氧化鎂緩衝層所帶來的發光性質之改善，可進一步應用於製作氧化鋅發光二極體。

This thesis presents the optical, electronic and crystalline characterization of ZnO thin films prepared by atomic layer deposition (ALD). It can be divided into three main sections： In the first section, we presented the ZnO thin films grown at low temperatures on sapphire substrates by ALD in order to conquer the problems resulting from the numerous oxygen vacancies in the ZnO thin films grown at higher temperature. Various schemes and experimental conditions were also applied to improve the crystal quality and carrier mobility of the ZnO thin films. The results show that lowering the deposition temperature can significantly suppress the electron concentration in the ZnO thin films. The electron concentrations on the ZnO thin films prepared at low temperatures can be suppressed to 1016 〖cm〗^(-3) with the mobility of 4.66 〖cm〗^2/Vs. In the second part, in order to lower the electron concentration introduced by the oxygen vacancies, ozone gas was used as the precursors in the ALD process of ZnO thin films. Experimental results indicate that the electron concentration as low as 1014 〖cm〗^(-3) and the carrier mobility as high as 49 〖cm〗^2/Vs can be achieved. In addition, X-ray diffraction characterization shows amorphous phase of the ZnO thin films on corning glass. Along with the high carrier mobility, the resulting ZnO thin films exhibit good potential for the application on thin film transistors (TFTs). In the last section, ZnO thin films were grown on (0002) sapphire and (111) Si substrates with and without an MgO buffer layer inserted in between. We found that the MgO buffer layer enhances the intensity of the near band edge (NBE) room-temperature photoluminescence (PL) from the ZnO thin films, and the PL intensity increases with the thickness of the MgO buffer layer. In addition, we observed remarkable blue-shifts of the NBE emissions of ZnO thin films with the insertion of MgO buffer layers of different thicknesses for the case on (111) Si substrate. This phenomenon could be attributed to the relaxation of the tensile stress due to the lattice mismatch between ZnO and (111) Si substrate when an MgO buffer layer is introduced. The improvement of the ZnO optical properties with the insertion of MgO buffer layer can be applied to ZnO-based photonic devices, such as ZnO-based light-emitting diodes (LED) and laser diode.