氮化鋁鎵/氮化鎵二維電子系統之自旋特性研究

Spin-dependent properties of two-dimensional AlGaN/GaN electron systems

邱凱信(Kai-Sin Cho)

指導教授 ： 陳永芳

國立臺灣大學/理學院/物理研究所/博士(2006年)

https://doi.org/10.6342/NTU.2006.03098

摘要

摘要
我們研究了二維氮化鎵電子系統在旋光以及磁場下的電特性測量◦本論文包含下列幾部分 ◦ 一開始藉由波長325 nm的雷射激發，圓偏振所引導出來的自旋光電流在氮化鋁鎵/氮化鎵超晶格結構(Al0.15Ga0.85N/GaN superlattices)中第一次被觀察到 ◦ 藉由改變旋光的相位(例如:左旋光到右旋光)，可以改變光電流的方向◦ 其中，325nm引導出來的光電流大小比遠紅外光所引導出來的光電流多兩個數量級◦ 我們的結果證明了spin splitting 確實存在AlGaN/GaN superlattices的樣品中◦ 然而，為了探測造成 spin splitting的物理機制，我們測量了低溫下的二維電子在氮化鋁鎵/氮化鎵異質結構中的磁性傳輸◦藉由在Shubnikov–de Haas震盪的拍頻效應，我們可以量測零場的能階分裂◦ 我們的量測結果證明了在氮化鋁鎵/氮化鎵異質結構下，結構轉置不對偁是造成能階分裂的主因◦ 此外，我們在氮化鋁鎵/氮化鎵異質結構中亦做了微波調制的Shubnikov-de Hass磁性量測並且得到相同的結論 ◦
然而，為了得到更完美的物理圖像，我們在同一塊樣品上做了旋光以及磁場下的測量◦ 經過定量的分析，我們可以將光學以及電學的實驗結果連結並做出合理的解釋◦ 再一次的我們證明了結構轉置不對偁是造成能階分裂的主因◦ 最後，我們探測了高場下電子的自旋行為，藉由Shubnikov-de Haas震盪的峰值分裂，我們可以估計不同藍道能階的g-係數大小 ◦ 實驗結果發現我們的測量值比一般塊材(bulk)的值來的大◦其原因來自於多體粒子之間交換能的增加所導致◦另一方面，g-係數會隨著藍道係數的減少而增大 ◦ 其原因在於比較少的藍道係數會對應到比較多的電子電子交互作用所導致 ◦

關鍵字

氮化鋁鎵/氮化鎵 ； 異質結構 ； 旋光 ； 電子自旋 ； 能階分裂

並列摘要

Abstract
We have investigated spin properties using optical (circular photogavinic effect) and transport measurement (Shubnikov-de Haas oscillations) in two-dimensional GaN electron systems. This dissertation contains the following parts. At the beginning, the circular photogalvanic effects (CPGE), induced by ultraviolet (325 nm) radiation, have been observed in the (0001)-oriented Al0.15Ga0.85N/GaN superlattices. The CPGE current changes sign upon reversing the radiation helicity, and it is up to two orders of magnitude larger than that obtained by far-infrared radiation. This result suggests the existence of a sizeable spin splitting in AlGaN/GaN superlattices. In order to detect the underlying mechanism behind spin splitting, low-temperature magnetotransport measurements were performed on AlGaN/GaN two-dimensional electron systems. By studying the beating pattern in the Shubnikov–de Haas oscillations in a perpendicular magnetic field, we are able to measure the zero-field spin-splitting energies in our systems. Our experimental results demonstrate that the Rashba term due to structural inversion asymmetry is the dominant mechanism which gives rise to the measured zero-field spin splitting in our wurzite AlGaN/GaN structures. In addition, to confirm our results, we also report zero-field spin splitting using microwave-modulated Shubnikov-de Hass (SdH) oscillations in an Al0.25Ga0.75N/GaN heterostructure and get the same conclusion.
In order to obtain a unified physical picture, we compare CPGE and SdH measurements in the same sample. Both experiments are tested in (0001)-oriented Al0.25Ga0.75N/GaN heterostructures. Though a quantitative analysis, we establish the link between the beating of SdH oscillations and the CPGE. This shows that the underlying mechanism responsible for both phenomena is the Rashba effect. At last, we investigate spin in high magnetic regime. By measuring the positions of a pair of spin-split SdH maxima, we are able to estimate the g-factors at different Landau level (LL) indices. We find the g-factor is enhanced over its bulk value in GaN (~2) due to many-body exchange interactions. Moreover, the measured g-factor increases with decreasing LL index, indicating that many-body electron–electron interactions become stronger as the number of occupied LLs decreases.

並列關鍵字

AlGaN/GaN ； circular photogavinic effect ； Shubnikov-de Haas oscillations ； spin splitting ； heterostructures

參考文獻

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