氮化鎵和砷化鎵二維電子系統之磁傳輸

Magnetotransport in GaN and GaAs two-dimensional electron systems

陳光耀(Kuang Yao Chen)

指導教授 ： 梁啟德

國立臺灣大學/理學院/物理研究所/博士(2010年)

https://doi.org/10.6342/NTU.2010.01459

摘要

摘要
我們執行了在氮化鋁鎵/氮化鎵和砷化鋁鎵/砷化鎵異質結構之二維電子系統下的低溫磁傳輸量測，這篇論文是主要分成下面四個主題。

一、 成長在p型矽基板的氮化鋁鎵/氮化鎵高電子遷移率之電晶體(HEMT)

我們報告了成長在p型矽基板之氮化鋁鎵/氮化鎵高電子遷移率結構的磁傳輸特性。藉著使用氮化矽的處理技術，二維電子系統的霍爾遷移率是被提升大概三倍並且Shubnikov-de Haas (SdH)振盪是被觀察到，反之這些振盪並無法在未經氮化矽處理的HEMT結構被發現。此外，我們也調查量子干涉效應在經氮化矽處理和未經處理的HEMT結構，弱局部化(weak localization)和電子-電子交互作用力(electron-electron interaction)效應對於經氮化矽處理結構之導電性的修正項是大幅的減少。因此我們的氮化矽處理技術對於改善成長在氮化鋁鎵/氮化鎵HEMT結構的效率是有用的，這是可以整合CMOS的技術。

二、 探查在砷化鎵二維電子系統下藍道(Landau)量子化伴隨絕緣態到量子霍爾態相變的存在

我們研究在砷化鋁鎵/砷化鎵異質結構之弱無序二維電子系統下的直接絕緣態到量子霍爾態的相變，藉著增加垂直於二維電子系統的磁場， 由於藍道量子化所造成的磁阻振盪的開始是可以被觀察在直接絕緣態到量子霍爾態的相變的附近。然而，我們研究顯示這樣的相變並不發生在μB=1，即使是當藍道量子化出現在這個相變點的附近。不同電子遷移率對於直接絕緣態到量子霍爾態的相變和藍道量子化是應該要被考慮。

三、 藉由電流加熱去探查絕緣態到量子霍爾態

我們進行了對於二維砷化鎵電子系統之磁傳輸的量測，這直接絕緣態到量子霍爾導體態的相變是被觀察藉由增加垂直於二維電子系統的磁場並且在低溫下，相似的結果可由改變電流並固定晶格溫度模式被觀察到，並且從電流加熱的模式，電子有效溫度和電流之間的關係 是可以被獲得，a定義為冪次常數。我們發現a的值在相變點的兩邊是不同，這間接指出了不同的非彈性散射機制是分別存在低磁場之絕緣態和量子霍爾導體態。

四、 量子霍爾效應之半整數結構的實驗證據

我們在砷化鋁鎵/砷化鎵異質結構之高遷移率二維電子系統下提出了磁傳輸的實驗調查，從我們的資料顯示，當我們增加電流，異常非變化點是在量子霍爾電阻被發現。然而，在高磁場下，電流分布不均勻性和邊緣狀態暗指了在高電流引發的半整數量子霍爾平台可能是由於在二維電子系統的斜對角之局部電場所產生。

關鍵字

二維電子系統 ； 磁阻率 ； 量子霍爾效應 ； 相變 ； 電子有效溫度

並列摘要

Abstract
We have performed low-temperature magnetotransport measurements on two-dimensional electron systems (2DESs) in an AlGaN/GaN heterojunction and an AlGaAs/GaAs heterostructure. This thesis is mainly separated into the following four topics.

1. Al0.15Ga0.85N/GaN high electron mobility transistor structure grown on p-type Si substrates

We report magnetotransport characteristics of AlGaN/GaN high electron mobility transistor (HEMT) structures grown on p-type Si (111) substrates. By using a SiN treatment technique during the crystal growth, the Hall mobility of the 2DES can be enhanced (>3 times) and Shubnikov-de Haas (SdH) oscillations can be observed whereas these oscillations are not found in the untreated HEMT structures. Moreover, we also investigate quantum correction effects in the SiN treated and untreated HEMT structures. Both weak localization (WL) and electron-electron interaction (EEI) correction terms to the conductivity of the SiN treatment are greatly reduced. Therefore, our SiN treatment is useful in improving the performance of AlGaN/GaN HEMT structure grown on Si, which is integrated with the CMOS technology.

2. Probing Landau quantization with the presence of insulator-quantum Hall transition in a GaAs two-dimensional electron system

We investigate the direct insulator-quantum Hall (I-QH) transition in disordered 2DES in an AlGaAs/GaAs heterostructure. By increasing magnetic field perpendicular to 2DES, the onset of magneto-resistivity oscillations due to Landau quantization can be observed near the direct I-QH transition. However, our study shows that such a transition does not occur at μB=1 even when Landau quantization appears near the transition point. Different mobilities should be considered for the I-QH transition and Landau quantization.

3. Probing insulator-quantum Hall transitions by current heating

Magnetotransport measurements have been performed in a two-dimensional GaAs electron system. The direct insulator to quantum Hall conductor transition is observed at low temperatures by increasing the magnetic field B perpendicular to the 2DES. Similar result is found by changing current I and from current heating model, a relation between electron effective temperature Te and current can be obtained. Here a denotes the exponent of the power law. We find that the value of the exponent a is different on the both sides of the transition point, which indicates that the different inelastic scattering mechanisms are present in the low-field insulator and in the quantum Hall conductor, respectively.

4. Experimental evidence of half integer structures in the quantum Hall effect

We present an experimental investigation of magnetotransport in high-mobility two-dimensional electron systems in an AlGaAs/GaAs heterostructure. According to our data, anomalous invariant points are found in quantum Hall resistivity when we increase the current I. However, at high magnetic fields the inhomogeneity of current distribution and edge state imply that the presence of half-integer quantum Hall plateaus induced by higher current levels are possibly due to the local electric field at the diagonally opposite corners of 2DES.

並列關鍵字

two-dimensional electron system ； magnetoresistivity ； Shubnikov-de Haas ； quantum Hall effect ； phase transition ； electron effective temperature

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