載子效應對摻氮二氧化鈦薄膜室溫鐵磁性的影響

Abstract

室溫鐵磁性在自旋電子學裡一直都被視為是很重要的一環，在近期的研究當中，寬能隙的半導體材料逐漸受到了重視，在摻雜入少量磁性的原子後可以觀察到室溫鐵磁性，而逐漸受到重視。但是更令人驚訝的是，在摻雜一些非磁性的元素，或甚至未摻雜任何元素，竟然也同樣地擁有室溫鐵磁性。本研究，利用脈衝雷射沉積的方法製備TiN_(2-x) O_x 固溶結構，發現該固溶結構在適當的氮摻雜比例時，可具有室溫鐵磁性。為了探討缺陷與其衍生之載子濃度變化對失溫鐵磁性的影響，我們亦將該固溶結構在氧氣下進行不同時間的退火處理。研究結果顯示在退火的過程中，由X-光繞射實驗(XRD)可以看到固溶體慢慢地朝向Anatase(004)的結構靠近，而由光致螢光光譜(photoluminescence PL)可以發現氮摻雜在退火的時候，部分的氮確實被氧氣所取代，最後由(X-ray photon spectroscopy XPS)可證明氧空缺也在氧退火中慢慢地減少。進一步以XPS解析靠近費米能階的電子結構時，發現到由氮與氧造成p-軌域耦合所造成Stoner splitting的部分並未隨著氧退火而被破壞。本實驗的結果也顯示此系統之室溫鐵磁性，符合束縛磁極化子Bound magnetic polarons， (BMPs)模型所揭示之特徵。

Obtaining room-temperature ferromagnetism (RTFM) in semiconductors has been considered as one of the key ingredients in realizing much anticipated spintronics. Recently, there is an increasing interest in searching for RTFM in several wide bandgap semiconductors by doping with non-magnetic elements. In this study, we have made a series of TiN_(2-x) O_x thin films, which is believed to play an important role in inducing carrier doping necessary for obtaining RTFM with various oxygen annealing treatments. It is expected that, after proper annealing, the process may lead to different degrees of N-doping in the TiN_(2-x) O_x matrix, and hence RTFM. Preliminary x-ray diffraction results have evidently indicated that the primary diffraction peak of oxygen annealed TiN_(2-x) O_x appears to shift more drastically toward the Anatase TiO_2 (004). Furthermore, by utilizing photoluminescence (PL) and X-ray photoelectron spectroscopy, N impurity and oxygen vacancy were proved to decrease with prolong oxygen annealing. However, from our study, the density of states near Fermi energy was found to remain exactly the same regardless the duration of annealing treatment. This result implies that the p-p coupling interaction between N and O are not affected by oxygen annealing, and hence, the features of Stoner splitting that gives rise o local magnetic moments. The annealing-induced weakening of RTFM thus is probably caused by the reduction of itinerant carriers associated with N-doping and oxygen vacancies. Our results are in line with the general features predicted by the bound magnetic polaron (BMP) model for the occurrence of RTFM in this system.