矽基稀磁半導體材料開發及其特性研究

Si-based diluted-magnetic-semiconducting materials and the characterization

姚佳賢(Jia-Hsien Yao)

指導教授 ： 金重勳

國立清華大學/工學院/材料科學工程學系/博士(2010年)

摘要

稀磁半導體具有可在同一個材料中控制電荷和自旋自由度的特性，因此被認為是極有可能實現自旋電子元件的重要候選人之一，在各種稀磁半導體的系統中，四族的半導體材料(矽、鍺)具有可和現代電子工業快速整合的優勢，因此最受到矚目。在本篇論文的研究中，我們將著重於矽基稀磁半導體的新材料開發，以及其相關物理性質之研究，例如:微結構分析、磁性、以及電性量測等。本論文可以細分成三個主要的研究主題: 第一個研究主題著重於具有室溫鐵磁性之稀磁半導體材料的開發與研究，我們成功地利用磁控濺鍍法成長鉻添加氫化非晶矽薄膜，並觀察到高於室溫的鐵磁性，在經由X光繞射儀和電子顯微鏡鑑定其微結構特性後，發現所有的初鍍態試片皆呈現非晶結構，並且沒有任何的二次磁性相或鉻團簇產生。由異常霍爾效應的觀察，以及磁化量對溫度的關係中，我們發現在此材料中的磁性來源機制可以被磁性子的展透理論合理地解釋。我們利用延伸X光射線吸收微結構光譜分析鉻原子的局域環境，進一步確認了我們成功地將鉻添加於氫化非晶矽中，沒有發現任何的團簇物或是氧化物產生。除此之外，我們亦發現氫化處理會影響材料的磁性和電性表現，只有當薄膜中的斷鍵被氫填補之後，才能觀察到試片的鐵磁性表現。在第二個研究主題方面，延續在鉻添加氫化非晶矽的實驗，我們開發了具有室溫鐵磁性的錳添加氫化非晶矽薄膜，並利用一樣的分析方法去探究結構、磁性，以及電性性質。我們也再次觀察到氫化處理提升了試片的各項性質。然而值得注意的是，在磁化量對溫度的關係上，錳添加的試片具有和鉻添加試片不一樣的行為。並且異常霍爾效應在錳添加氫化非晶矽薄膜中可以在高達150 K的溫度下被觀察到。此外，我們也進一步利用居禮-偉斯和三維自旋波定理去適套磁化量對溫度的關係曲線。這些量測及分析結果顯示試片的鐵磁性是源自於巡迴式載子所誘發的。最後，在第三個研究主題方面，我們探討後退火處理對試片性質的影響。我們發現後退火處理可以提升試片的磁性及電性表現，例如飽和磁化量、導電度，以及載子濃度等。此外，我們利用電子彈回偵測法去量測氫濃度，發現在低溫退火的過程中，氫都沒有逸散。因此我們推測後退火處理能提升試片的各項性質，是因為熱能促使氫擴散，使氫能更均勻的分佈於試片中，降低缺陷密度。更重要的是，經退火處理的錳添加氫化非晶矽試片在室溫下可以明顯地觀察到異常霍爾效應。這是四族稀磁半導體中所被觀察到的最高溫度，同時這個量測結果也代表試片的磁性來源是載子所誘發的，且可被實際應用於自旋電子元件中。

關鍵字

氫化非晶矽 ； 磁性半導體 ； 異常霍爾效應 ； 錳攙雜 ； 鉻攙雜 ； 載子誘發鐵磁性

並列摘要

Diluted magnetic semiconductors (DMSs) is one of the most promising candidates to realize spintronics devices with the potentials to control both charge and spin degrees of freedom in one material. Among various systems, group IV-based (Si, Ge) DMSs specially attract much attention due to the compatibility with existing industry of modern electronics. In our works, we aimed at exploring to develop new Si-based DMSs materials, and the corresponding distinct physical properties. These included microstructure, ferromagnetism, and transport properties. Three main topics have been involved.
In the first topic, we have demonstrated room-temperature ferromagnetism in Cr-doped hydrogenated amorphous Si (a-Si1-xCrx:H) thin films grown by sputtering method. The microstructure of these samples was identified by XRD and HRTEM, in which all as-deposited films are amorphous without any clusters or second phases. Pronounced anomalous Hall effect (AHE) and temperature dependent magnetization (M-T) curve both suggest the origin of the ferromagnetism may arise from percolation of magnetic polarons. Furthermore, the local environments around Cr atom were analyzed by EXAFS, confirming that successful incorporation of an unusually high concentration of Cr magnetic ions in a-Si:H thin films nearly free of Cr clusters and oxides. In addition, the effect of hydrogenation shows precipitous influence on magnetic and electrical properties of the samples. Only when dangling bonds in the sample are largely hydrogenated, will robust ferromagnetic order be observed.
In the second topic, we have developed Mn-doped a-Si:H thin films, which also show room-temperature ferromagnetic order. The structural, magnetic and electrical properties were also studied by the analyses discussed in Cr-doped samples. Moreover, the enhancement of these properties by hydrogenation was observed again. However, it is noted that the behaviors in M-T and AHE measurements are quite different from those of Cr-doped ones. The M-T curve fits very well by combination of Curie-Weiss law and 3D spin-wave model and AHE was reproducibly obtained at 150 K. These results suggest that the origin of ferromagnetism may arise from the itinerant carrier mediated mechanism.
Finally, in the last topic, we have studied the effect of post-annealing treatment on the properties of a-Si1-xMnx:H thin films. Annealing manifestly enhances saturation magnetization, electrical conductivity, and carrier concentration. Moreover, the hydrogen concentration was determined by ERD methods. The results revealed that there is no hydrogen effusion during low temperature annealing. The reason of property enhancement by annealing arises from the thermal energy which promotes hydrogen diffusion, hence a more homogeneous distribution and lessens defect density. In particular, AHE was clearly and reproducibly observed at room temperature in annealed samples, which is the highest temperature ever observed in Si-based DMSs. These results indicate that the origin of ferromagnetism arises from hole-mediated mechanism, which is essential for the realization of practical Si-based spintronics devices.

並列關鍵字

hydrogenated amorphous silicon ； magnetic semiconductor ； anomalous Hall effect ； Mn-doping ； Cr-doping ； carrier-mediated ferromagnetism

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