含錳三五族砷化物半導體材料之鐵磁性質研究

Ferromagnetism in Mn-doped III–V semiconducting materials

黃柏瑋(Po-Wei Huang)

指導教授 ： 黃金花

國立清華大學/工學院/材料科學工程學系/博士(2011年)

https://doi.org/10.6843/NTHU.2011.00091

摘要

在這篇論文裡，我們首先探討於(001)及(311)A 兩種不同晶向砷化鎵基板上所成長之鎵錳砷薄膜的平面磁異向性。由磁化量對溫度的關係顯示，(311)A鎵錳砷其退火後居禮溫度的提升量較(001)鎵錳砷為小。而在磁滯曲線的研究中，(311)A鎵錳砷展現了單純的平面磁異向性，且具有沿著晶向的磁易軸；相反地，退火前後的(001)鎵錳砷其磁易軸皆偏離晶向。我們分析了各晶向的靜磁能並提出計算單軸及雙軸磁異向性能量及決定磁易軸方向的算式。
其次，我們探討(001)鎵錳砷薄膜與氧化錳薄膜之間的交換場行為。我們觀察到覆蓋一極薄層(6 nm)氧化錳的鎵錳砷不論有無經過磁場退火，其磁滯曲線皆同時具有交換偏移場及磁化量縱向偏移的現象，其中交換偏移場先隨退火時間的增加而減少但在30分鐘後則重新提升，而磁化量縱向偏移對退火時間的變化則不明顯。藉由X射線能譜儀的分析，我們發現氧化錳薄膜同時呈現一氧化錳(MnO)及四氧化三錳(Mn4O3)兩種氧化態，且隨退火時間的增加前者含量減少但後者含量相反地反而增加。有鑑於此，我們提出了一個簡單的載子自旋組態模型來說明在退火過程中，存在於氧化錳及四氧化三錳均勻介面的 “被固定住”未補償自旋造就了所觀察到接近恆定的磁化量縱向偏移。
最後，我們探討在Stranski–Krastano模式下成長的奈米尺度銦錳砷，其結構與磁性質和錳含量、生長溫度、及生長時間的關係。由於錳元素在不同成長溫度下的不同表面活性，銦錳砷奈米點及銦錳砷奈米(柱)線可分別在320450°C間及高於450°C的成長溫度下成長出。我們特別探討在380°C下所成長的奈米點其磁性質和錳含量及生長時間的關係。微結構分析顯示低於44%的錳濃度下能成長出閃鋅礦結構的鐵磁銦錳砷奈米點；而錳濃度在44%(含)以上的銦錳砷奈米點，能在零場冷及場冷的磁化量對溫度曲線的分歧處定義阻隔溫度，經由記憶效應的測試，發現奈米點的自旋呈現長程有序的超鐵磁態。我們針對不同成長時間的奈米點的磁化量與溫度關係之分析，提出了一個簡單的模型來解釋銦錳砷奈米點之載子自旋組態的隨奈米點成長之轉變型式。最後，我們發現高溫( 450°C)及高錳濃度(33%)為成長有序銦錳砷奈米(柱)線的必要條件。

關鍵字

三五族砷化物 ； 稀磁半導體 ； 磁異向性 ； 交換自旋偶合 ； 奈米點 ； 奈米線

並列摘要

In this dissertation, we first investigate the in-plane magnetic anisotropy of Ga0.93Mn0.07As thin films grown on (001) and (311)A GaAs substrates. Temperature dependent magnetization (MT) measurements reveal a less enhancement in Curie temperature (TC) of the (311)A Ga0.93Mn0.07As upon annealing compared to the (001) counterpart. However, in the studies of magnetic hysteresis loops, the (311)A Ga0.93Mn0.07As exhibits a simple in-plane magnetic anisotropy with easy-axis consistently set along the crystallographic orientations, while deviated easy-axis was observed in both the as-grown and annealed (001) Ga0.93Mn0.07As. We analyzed the in-plane azimuthal magnetostatic energy and proposed an equation for calculating the angular dependent uniaxial and biaxial anisotropic energy and locating the easy-axis exactly.
Next, we present a different observation on the exchange biasing of Ga0.95Mn0.05As by ultra thin (6 ) MnOx. We have simultaneously observed exchange bias (HE) and vertical magnetization (M) shift in as-grown and field-annealed MnOx /Ga0.95Mn0.05As bilayers. HE initially decreases with increasing annealing time ta and then increases when ta > 30 min, while M shift remains almost unchanged with ta. X-ray photoelectron spectroscopy (XPS) analysis reveals that MnOx is composed of MnO and Mn3O4, and the volume amount ratio of Mn3O4 to MnO increases with increasing ta. A simple model of spin configurations based on a uniform MnOMn3O4 interface with constant “pinned” uncompensated interfacial spins during field annealing is proposed to account for the observed exchange-biased phenomena.
The last of this study is to explore the influences of the Mn concentration, growth temperature, and growth time on the structure and magnetic properties of (In, Mn)As nanostructures under StranskiKrastanov (S–K) growth mode. The (In, Mn)As nanodots and elongated dots (or nanowires), all have room-temperature ferromagnetism, can be grown at 320 oC ≦ Tg < 450 oC and Tg ≧ 450 oC, respectively. We particularly investigate the effects of the growth time and Mn concentration on the magnetic properties of the (In, Mn)As nanodots grown at 380 oC. Microstructure analysis reveals that the In1-xMnxAs nanodots with x < 0.44 present zinc-blende structure. For the In1-xMnxAs nanodots with x ≧ 0.44, a blocking temperature (TB) slight lower than TC can be located at the bifurcation between the zero-field-cooled and field cooled MT curves. For these nanodot samples with ultrahigh Mn concentration, the spin of dots exhibits a long-range order state at T < TB and the magnetic spin configuration is confirmed as superferromagnetic ordering by memory effect test. By analyzing the MT relations of the In0.56Mn0.44As nanodot samples with different growth times, a simple model is proposed to account for the spin configuration transition with quantum dots growth evolution. At last, it is found that high Mn concentration and high growth temperature are both necessary for the growth of well-ordered nanowires.

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