利用磁力探針顯微鏡測量氧化鋅摻雜鈷奈米線的居禮溫度

Abstract

ZnO為一室溫下具有寬能階3.37 V的半導體，是個適合在短波長及紫外光波段的光電子學應用上的材料。近年來已有物理學家根據理論以及實驗的預測結果顯示以ZnO或GaN為主的稀磁性半導體相當的被看好具有室溫鐵磁性，因此本實驗即利用磁力探針顯微鏡觀察ZnO參雜鈷的奈米線的磁力性質。 實驗上，利用高溫爐管成長出ZnO奈米線後，再利用離子佈植法將不同濃度的鈷打入樣品得到Zn0.89Co0.11O、 Zn0.92Co0.08O、Zn0.95Co0.05O 、Zn0.98Co0.02O，不同參雜濃度的奈米線材料，選取佈植能量為40 kev以及鈷濃度為11 %的氧化鋅奈米線為此實驗的量測樣品，因為Co離子佈值後會破壞奈米線的外型，隨著佈植劑量增高破壞越明顯，因此將奈米線放置在高溫爐中以氣壓8x10 torr， 600 C的溫度下做熱退火6小時的處理，以修復大部份的缺陷並增加氧的空缺提高奈米線的鐵磁性。 由磁力探針顯微鏡我們量測到氧化鋅參雜鈷奈米線具有室溫鐵磁性，且觀測到磁區的翻轉，也發現許多奈米線有多個磁區的分布，掃描得到的磁力影像顯示奈米線具有亮暗的帶狀磁區分別代表反平行或平行於探針的磁化方向。藉由量測不同溫度下奈米線的磁力影像可得到一個磁力和溫度的函數，並預測出奈米線的居禮溫度大約在450 K左右，高於其他稀磁性半導體的居禮溫度，適合用來發展室溫中應用的電子元件。

ZnO is a semiconductor of wide bandgap 3.37 V in the room temperature, and it is suit for optoelectronics application in a short wave length and ultraviolate waveband. In recent year, physicist have predict that dilute magnetic semiconductor such as ZnO and GaN have the room temperature ferromagnetism according to theory and the anticipation of experiment result. Therefore, in this experiment, we use Magnetic Force Microscopy to observe magnetic property of Zn1-xCoxO nanowires. In our experiment, we use high temperature furnace to grow up ZnO nanowires, and implant different concentration of cobalt into ZnO nanowires by ion implanting. We can get the Zn1-xCoxO nanowire for different concentration of cobalt as Zn0.89Co0.11O, Zn0.92Co0.08O, Zn0.95Co0.05O , Zn0.98Co0.02O nanowires. We select Zn0.89Co0.11O nanowires for our sample which has implanted by 40 kev. After cobalt ion implanting, it will destroy the surface of the nanowires, and more apparent by increasing dose capacity. In order to increase the ferromagnetism of nanowires, we annealed nanowires at 873 k in vacuum 8×10-6 torr for 6h.It can increase numbers of oxygen vacancy to increase the ferromagnetism of Zn0.89Co0.11O nanowires. We have measured the room-temperature ferromagnetism of individual Zn1-xCoxO nanowires by using Magnetic Force Microscopy and observed the reverse of domain. We found that nanowires have multi domains and the phase image revealed both dark and bright domains along the edges of the nanowires indicate parallel or antiparallel to the magnetization direction of the tip.

By measuring phase images of nanowires at different heating temperature, we can get a function of phase difference and temperature. We predict the Curie Temperature is about 450 K, and the transition temperature is considerably higher than the Curie temperature of most DMS. More importantly,it is high enough for the purpose of device applications at room temperature.