Quantum Interference and Electron Transport in Synthesized GaN Nanowires

科技的進步，不僅引領了產業的腳步，帶動經濟的發展，甚至影響政治的局勢。而應用在電子產業裡的奈米技術，則為當前科技主要發展項目的其中之一。其進展速度，明顯地反應在電子產品快速更新的腳步上。目前元件尺寸縮小的程度，已經逐漸使得量子效應，顯著地表現出來。經由研究這些量子效應，在設計新概念的元件上，可以提供很多的可能性。氮化鎵是目前熱門半導體材料的其中之一。本篇論文主要研究氮化鎵奈米線的成長、分析、元件製備、以及電子的傳輸特性，尤其是量子干涉現象，例如普適性電導震盪以及弱侷域效應。在第一章，先介紹基本的概念，首先提到氮化鎵奈米線的結構，電學特性與潛在應用。接著介紹當尺寸縮小時，量子效應所產生的一些特殊現象。最後介紹電子在物質中的干涉效應。第二章是關於氮化鎵奈米線的成長，一開始先提到奈米線成長的機制和方法。接著介紹此實驗所用的氮化鎵奈米線的製備。其中包括了成長設備的建構，反應物和催化劑等相關訊息。詳細地描述成長的方法，最後並提出成長的一些心得和討論。奈米線的特性分析則放在第三章，包括了分析的一些方法，例如利用掃描式電子顯微鏡以及穿隧式電子顯微鏡，觀察外觀和細部結構。並利電子束撞擊樣本時所產生的X光頻譜，分析奈米線的成份。奈米線晶體結構的判定，則是利用電子束繞射，以及X光繞射的資料來分析。在加上光致發光的方法瞭解能隙的結構。綜合這些特性分析的結果，我們可以驗證成長出來的奈米線的確是氮化鎵。第四章詳述把氮化鎵奈米線製作成元件的方法和經過。主要手續包括微影和金屬沈積等製程，微影的部份包括電子束微影以及光學微影，而金屬沈積也佔了很重要的因素。寬能隙半導體和金屬的接面必須小心處理，才能使後續的量測順利的進行。電子干涉效應必須在低溫下才容易觀察到，因此我們使用了混合液氦的低溫系統，這個低溫系統以及電性量測的建構也會在本章介紹。第五章介紹了電性的量測結果和分析。首先藉由分析電場效應來得知奈米線中的載子密度，以及電遷移率。經由此電性分析，我們得知摻雜量和成長參數的關係。重摻雜和輕摻雜的奈米線都被量測到，我們篩選電導度最高的幾個樣本，在低溫下進行磁導率量測。我們觀察到普適性電導震盪和弱侷域效應，並分析之，最後得知了在氮化鎵奈米線中，電子相位的同調長度。

關鍵字

氮化鎵；奈米線；量子干涉；弱侷域效應；普適性電導震盪

並列摘要

The advance of technology not only leads the trend of industry but also promotes the development of economy. Sometimes, it even affects the political situations. Nanotechnology applied in electronic industry is one of the representatives. The progress of advancement can be sensed from the pace of the renew of electronic products. Quantum mechanical phenomena has been emerging as a result of progressive downsize of the electronic devices. Studying these phenomena opens up a route to create devices with new concepts. Gallium nitride is one of the most popular semiconductor materials for electronic devices. In this thesis, we focus on the study of gallium nitride nanowires, including synthesis, characterization, and transport measurements especially for electron interference phenomena such as universal conductance fluctuations and weak localization. In chapter one, we will present an introductory overview to gallium nitride nanowires, including crystal structure, electronic properties, and its potential applications. In addition, the foundation transport theory in constrained matters and quantum interference effects are introduced. Chapter two introduces the synthesis of gallium nitride nanowire. Nanowire growth mechanism is explained first. Then we show the nanowire synthesis method and setup of the growth facilities. Those facilities we used and parameters we applied are also presented and discussed. Characterization of synthesized nanowires is shown in the chapter three. We used scanning electron microscopy and transmission electron microscopy to study the morphology of the nanowires. By analyzing the energy dispersive x-ray spectra, we identified the elemental constitution of the nanowires. Moreover, the crystal structure of the nanowires is determined by both electron and x-ray diffractions. We also measured the optical gap from photoluminescence spectra. We summed up those characteristics and confirmed that the nanowires were indeed gallium nitride nanowires. In order to perform transport measurements, gallium nitride nanowire devices were made. Chapter four shows these nanowire device fabrication processes. The main process includes lithography and metal deposition. Both are important for device fabrication and measurements. The contact issues, which should be carefully addressed, for the wide energy gap semiconductor and metal are also discussed. Furthermore, electron interference phenomena in nanowires are expected to be observed at low temperature. Thus we applied cryogenic system in our measurements. Principle of the cryogenic system and setup of our measurement are also introduced therein. Chapter five presents the results of measurements. First, field effect behaviors are analyzed. We calculate the carrier densities and mobilities for nanowires from those data. From those electronic properties, we know how to adjust the growth parameters that are related to electronic properties of the synthesized GaN nanowires. These devices show the best performance in terms of electrical conductance and were chosen for magnetoconductance measurement at low temperature. Universal conductance fluctuations and weak localization were measured. The phase coherence length of electrons in GaN nanowires thus estimated by analyzing these behaviors.

並列關鍵字

GaN nanowire ； quantum interference ； weak localization ； universal conductance fluctuations

參考文獻

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[2] Kuykendall, T. et al. Crystallographic alignment of high-density gallium nitride nanowire arrays. Nature 3, 524 (2004).

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[4] Chen, G. D., Smith, M., Lin, J. Y. & Jiang, H. X. Fundamental optical transitions in GaN. Appl. Phys. Lett. 68, 2784 (1996).

[5] Reshchikov, M. A. & Morkoc, H. Luminescence properties of defects in GaN. J. Appl. Phys. 97, 061301 (2005).

國際替代計量

Quantum Interference and Electron Transport in Synthesized GaN Nanowires

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