射頻磁控濺鍍摻氮氧化鋅薄膜之微結構與物性研究

Studies of microstructure and physical properties of the RF-sputtered N-doped ZnO films

王清鍾(Ching-Chung Wang)

指導教授 ： 傅昭銘

國立臺灣大學/理學院/物理研究所/博士(2014年)

https://doi.org/10.6342/NTU.2014.10313

摘要

本論文研究使用射頻磁控濺鍍法製備摻氮氧化鋅稀磁性半導體薄膜，探討樣品製備過程中，氮氣對氧化鋅薄膜的晶體結構、表面型態、光學性質、室溫鐵磁性與高頻電傳特性等物理性質的影響，以對稀磁性半導體薄膜的磁性來源、機制及氧化鋅薄膜的高頻電傳特性有所了解。
首先我們探討在氬氣及氮氣（N2）流量為1:1的條件下，於c面藍寶石基板上製備三種不同厚度之摻氮氧化鋅薄膜的反常拉曼峰、室溫鐵磁性與薄膜本質缺陷之間的關係。X射線繞射結果顯示所有薄膜皆具有高度c軸指向的多晶結構。光致螢光光譜數據顯示在3.2 eV附近有一高強度之寬帶，而在2.0 eV附近則有一弱帶存在。藉由多峰擬合法，推斷鋅空缺（VZn）、鋅間隙（Zni）及氮置換氧（NO）缺陷的發射為高強度寬帶形成之主因，然而弱帶則是氧空缺（VO）及氧間隙（Oi）缺陷的發射所形成的。這些缺陷也與拉曼光譜數據中位於277、511、584、及644 cm−1這四個反常拉曼峰有關。此外，這三個不同厚度的摻氮氧化鋅薄膜皆觀測到具有室溫鐵磁性。隨著薄膜厚度的增加，飽和磁化強度先迅速變弱，而後再增強。綜合分析結果發現薄膜的磁性與VZn、Oi及NO缺陷的濃度變化有密切的關連，同時薄膜的飽和磁化強度與c軸晶格應變有正比關係。
另外我們則是探討藉著改變氮氣的流量（氬氣的流量固定），於c面藍寶石基板上製備未摻及不同摻氮濃度之氧化鋅薄膜的微結構、本質缺陷與高頻電傳特性之間的關係。實驗發現薄膜的表面形貌與高頻電傳特性因氮氣流量的改變而有強烈的變化。於濺鍍過程中，隨著氮氣的引入，薄膜晶粒變小且表面有類似六角形結構的奈米柱形成，在未摻雜的氧化鋅薄膜中並未發現此種結構。與未摻雜的氧化鋅薄膜相對比，拉曼光譜顯示所有摻氮氧化鋅薄膜具有四個反常拉曼峰，分別位於276、510、586、及644 cm−1，這些反常拉曼峰與Zni、Oi、VO、NO缺陷及氮的相關複合缺陷有關，且其峰值與氮氣流量成正比。在阻抗頻譜的分析中，藉由磚層（brick layer）模型和等效電路模型的引入，進一步分析不同氮氣流量下，薄膜晶粒和晶界等微觀結構的變化與對應於阻抗頻率響應之貢獻。分析量測結果顯示，隨著氮氣流量的增加，薄膜晶粒和晶界的弛豫頻率逐漸變小，此與晶粒和晶界中的缺陷濃度增加有關。此外，薄膜的交流電導率也隨著氮氣流量的增加而減小，這是由於更多的NO受體缺陷與氧化鋅薄膜的本質施體缺陷（VO及Zni）補償所致。
本研究分析摻氮氧化鋅稀磁性半導體的磁性及頻率相關之電傳特性，相信可對氧化鋅材料未來實際自旋及高頻元件應用，奠定良好之基石。

關鍵字

氧化鋅 ； 氮 ； 本質缺陷 ； 室溫鐵磁性 ； 阻抗分析 ； 稀磁性半導體

並列摘要

The goal of this study is to understand the influence of nitrogen doping on the crystal structure, surface morphology, room temperature ferromagnetism, optical and high-frequency magneto-electrical properties of zinc oxide (ZnO) thin films deposited by radio-frequency magnetron sputtering.
First, we investigated the intrinsic defects responsible for the anomalous Raman peaks and the room temperature ferromagnetism in the nitrogen-doped ZnO (ZnO:N) thin films deposited on c-plane (0001) sapphire (Al2O3) substrates. The X-ray diffraction results indicate that all of the films are grown with a highly c-axis-oriented poly-crystalline structure. All photoluminescence spectra exhibit an intensive broad band centered at approximately 3.2 eV as well as a weak band at approximately 2.2 eV. By using a multi-peak fitting method, we deduce that zinc vacancies (VZn), zinc interstitials (Zni), and substituted nitrogen atoms at the oxygen sites (NO) are responsible for the emissions embedded in the intensive broad band, while oxygen vacancies (VO) and ox-ygen interstitials (Oi) are responsible for the weak band. These defects in the ZnO:N films may be associated with the four anomalous Raman peaks at approximately 277, 511, 584 and 644 cm−1. Room temperature ferromagnetism has been observed for all of the ZnO:N films. With increasing the film thickness, the saturation magnetization first decreases rapidly and then increases. The analyzed results reveal that the magnetism of these films has a close correlation with the variation of VZn, Oi, and NO defects. In the meantime, the saturation magnetization of the ZnO:N films has a direct proportional dependence to the change in c-axis lattice strain.
Moreover, we investigated the microstructure, intrinsic defects, and high-frequency magneto-electrical properties of undoped and N-doped ZnO thin films, which were prepared with different N2 gas flow rates on c-plane (0001) sapphire (Al2O3) substrates. The structure and high-frequency magneto-electrical properties of the ZnO:N films vary drastically with the variation of N2 flow rate. With the introduction of N2 gas during deposition, the grain size of the film decreases and short hexagonal-like nanorods grown at grain surface are observed. In comparison with the undoped ZnO film, Raman spectra of the ZnO:N films reveal four anomalous peaks at 276, 510, 586 and cm−1, which are attributed to Zni, Oi, VO, NO defects and nitrogen-related defect complexes. Complex impedance spectra of all the films were analyzed by an equivalent circuit, employing two sets of parallel resistance and capacitance components in series to represent the ox-ide grain and grain boundary contributions, respectively. The analyzed results imply that the grain and grain boundary relaxation frequency become smaller with increasing the N2 flow rate. The reduction of relaxation frequencies is due to an increase of defect concentration in grains and grain boundaries. In addition, as the N2 flow rate is raised, the calculated ac conductivity decreases gradually. The reason for the decrease in con-ductivity could be the compensation of more NO acceptors for the intrinsic donors (VO and Zni) of ZnO films.

並列關鍵字

zinc oxide (ZnO) ； nitrogen (N) ； intrinsic defect ； room temperature ferromagnetism ； impedance spectroscopy ； diluted magnetic semiconductor (DMS)

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