| 摘要: | ｢鐵磁/半導體｣以及｢鐵磁/半導體/鐵磁｣異質接面中自旋注入的特性已經成為目前很重要的課題,主要是因其可應用在自旋電子元件中.最近已經發現透過外加一層具有Schottky barrier的半導體或是一層絕緣體,可以有效的改善其自旋注入效率.在本計畫中,具有寬能隙的極性和非極性氮化鋁薄膜層被選為中間層的半導體材料.為了瞭解由氮化鋁的表面極性所導致的介面電荷堆積對於在｢鐵/氮化鋁/鐵｣這種異質接面中其自旋傳輸特性的影響,我們會先建立自旋極化的第一原理計算方法來得到兩種情況下的自旋極化電流和磁阻效應.另外,我們也會利用tight-binding (TB)的方法加上非平衡格林函數的形式所推導出的通用方程式來預測spin-transfer torque(T||) 在兩種情況下的行為.但是由於在｢鐵/極性氮化鋁/鐵｣的異質接面中有接面電荷堆積的存在,我們可以將這種結構視為在理想的磁性穿隧接面(MTJ)上加入兩層分布在接面上的雜質層,其雜質層中多種混合的雜質電位則可以由第一原理自洽所計算出的接面能階分布來決定. 因此,我們可以利用非平衡格林函數的方法來修正原先的通用方程式,並將其用來預測在極性氮化鋁異質面中T||的行為.我們相信本計畫中所得到的計算結果,不但將會對於基本的自旋傳輸理論給予很多物理上的解釋,同時也能對於未來以半導體為主的自旋電子元件之發展提供一些有用的想法 The spin injection into ferromagnet(FM)/semiconductor(SC) and FM/SC/FM heterojunctions are rapidly growing fields, due to their potential applications in spintronics devices. Recently, the spin injection efficiency has been successfully improved via inserting a tunneling slab, which can be either Schottky barrier SC or insulators (I). In this project, we choose the wide band gap AlN as the central SC layer with polar (pAlN) and non-polar (npAlN) surfaces. In order to understand the effect of AlN surface polarity-induced interfacial charge accumulation on the spin transport properties, we’ll first generalize the first-principles spin-polarized pseudofunction (PSFS) method with the integrated piecewise thermal equilibrium approach, so the spin-polarized current densities and TMR can be obtained in both Fe/pAlN/Fe and Fe/npAlN/Fe junctions. Moreover, we’ll apply the tight-binding model with the non-equilibrium Keldysh Green’s formulism to predict the non-collinear spin-transfer torque, T||, effect in both cases. For Fe/npAlN/Fe junction, which can be treated as an ideal MTJ, my newly derived general expression shows that non-collinear T|| can be estimated directly from the spin current densities solely in parallel and anti-parallel configurations. While for Fe/pAlN/Fe junction, with opposite interfacial charge accumulations at Fe/Al and Fe/N interfaces, this case can be treated as a disordered MTJ with multi impurity potentials at two interfaces, corresponding to the interfacial energy states calculated from the first-principles PSFS method. Then, the non-equilibrium Keldysh Green’s function method with Dyson equation can be applied to derive a new general expression of T|| for pAlN case. We believe our theoretical results will give more physical insights of fundamental spin transport mechanisms and also will provide some guidelines for future SC-based sprintronics applications. 研究期間：10008 ~ 10107 |
