一維奈米電子學及自旋電子學之物理與元件(I)

Abstract

近年來，奈米碳管與奈米線等由下而上自我組裝，而非由大至小蝕刻製造的奈米材料，已被廣泛的應用並展示其在製作奈米尺度電子與光電等元件之高度可行性。由於半導體製程不斷縮小，發展奈米電子學已成為掌握未來科技之鑰。結合電子之自旋與傳輸雙重特性的自旋電子學，早期多以發展金屬自旋電子元件為主，如利用巨磁阻、穿隧磁阻與自旋閥效應等，製造磁電阻式隨機存取記憶體與硬碟讀取頭元件。近年來，由於半導體自旋電子材料（如掺錳砷化鎵）的蓬勃發展，與鐵磁-半導體接面技術的突破，自旋電子學的研發已漸漸轉移到自旋電流引入注射與自旋電子傳輸性質的探討，以及自旋場效電晶體、自旋發光二極體、電驅動磁疇翻轉等元件的開發與應用。本團隊在前期奈米國家型科技計畫裡（2004/08–2007/12），研究單根氧化鋅奈米線與氧化鋅掺鈷奈米線的電性與磁性，成功製作出高真空退火後的室溫鐵磁性氧化鋅掺鈷奈米線，除仔細運用各種材料分析儀器確定鈷元素均勻分布外，我們使用SQUID驗證其本徵稀磁半導體特性，並使用磁力顯微鏡偵測出單根奈米線中磁疇的變化。我們更測量出磁疇強度隨溫度上升而減弱，描繪出居禮定律的溫度變化曲線，又確認高於400 K的居禮溫度。有此室溫鐵磁性稀磁半導體奈米線材料，結合我們對單根氧化鋅奈米線傳輸性質的測量技術，本期計畫擬製作可在室溫操作的巨磁阻元件、穿隧磁阻元件、自旋閥元件、自旋場效電晶體、電驅動磁疇翻轉元件等；並擬結合自旋電子理論研究，設計開發新型自旋電子元件。自1998年，Ohno研究砷化鎵掺錳材料為科技新知帶來了深遠的影響，本團隊藉由氧化鋅掺鈷的奈米線新材料，結合自旋電子與奈米電子的應用科技，將帶給全世界學界與產業界更大的震撼。 由於本團隊在電子束微影技術的精進，目前已經成功將半導體與金屬奈米顆粒置入兩奈米電極間之30-50奈米寬的間隙內，並成功測得奈米顆粒的電流-電壓特性曲線與電阻的溫度變化。本期計畫將進一步結合分子電子學理論專家，設計與開發分子電晶體、與分子磁鐵自旋電晶體。近十年來，諸多領域的科學家們嘗試以分子製作新型式的電晶體以及各類電子元件，分子電子學已成為繼傳統半導體電子學而起的一個嶄新研發領域。 在微米尺度下，電子元件的電子傳輸性質可以用自由電子氣來近似描述，並以半古典理論方式解釋。但是在奈米電子元件裡，原子分子的特性和電子的波動性質愈來愈明顯，分子電子學電子傳輸性質的研究，顯然必須及早正視這些關鍵因素。 本期計畫擬以磁性半導體-半導體-磁性半導體系統的巨磁阻量測與理論研究為基礎，拓廣到分子系統，即以saturated molecules 取代兩磁性電極中間的半導體。由於此系統的尺度微小，所以電子自旋容易保存，加上電子隧穿特性的影響，我們預期此系統的穿隧磁阻效應將很顯著，從而有利於應用在奈米尺寸磁自旋電子元件（如spin filter）的設計與製作。在理論方面，我們將以密度泛涵理論計算方式，開發分子自旋電子學的理論與程式，研究電子自旋的傳輸性質與量子多體效應，並且探討自旋分子元件運作的物理機制與材料特性。在實驗方面，我們將結合理論計算結果，開發可行的分子自旋電子系統，探討分子自旋電子元件設計的可行性，並將量測與探究此新穎系統的物理現象。奈米接面系統的電子自旋傳輸性質，是一個非常新穎而重要的研究課題。這些開創性的研究工作，需要開發新的實驗技術以及新的理論與程式，本計畫勢將為分子自旋電子學的未來發展開啟一扇大門。

In this project, we propose to study in depth both the fundamental science and industrial applications of nanoelectronic and spintronic devices. We will concentrate on one-dimensional nanodevices built from metal oxide nanowires, and also study molecular junctions and devices. We propose to investigate conducting metal oxide nanowires (such as RuO2 and In2-xSnxO3 or ITO nanowires) which are potentially useful as interconnects in nanoelectronic circuitries. In addition, we will study ZnO and other potentially important semiconductor nanowires. The ion-implantation technique will be applied to make diluted magnetic semiconductor (DMS) Co-doped ZnO (Zn1-xCoxO) nanowires, where room-temperature ferromagnetic (RTFM) ordering with a Curie temperature of as high as 450 K has recently been established by this research team. Focused electron beam (from an SEM) will be applied to modulate the magnetization of single Zn1-xCoxO nanowires. The modulation can induce ferromagnetic (FM) to paramagnetic transition in selected sections in a given individual Zn1-xCoxO nanowire, making alternative FM and paramagnetic sections. The FM and paramagnetic sections would be of several submicrons to several microns long. Therefore, highly-integrated series-connected nanowire field-effect transistors (FETs) may be constructed in a single DMS nanowire. Such functional nanodevices could trigger an industrial revolution in nanoscience and nanotechnology. In order to realize industrial applications in nanoelectronics and spintronics, we propose to investigate the electrical and magnetic properties of certain metal and semiconductor nanowires, and molecular junctions. Electron-beam lithographic technique will be employed to facilitate electrical-transport measurements on individual nanowires over a wide range of temperature (from 300 K down to sub-Kelvin temperatures) and in magnetic fields, as well as in the presence of a gate voltage where appropriate. Quantum electron and phonon transport phenomena in individual nanowires and nanodevices will be investigated. In particular, we will elaborate the electron spin-dependent transport as a sensitive probe for studying the physical properties of DMS nanowires. SQUID magnetometry and magnetic force microscopy (MFM) will be employed to investigate the magnetic properties of DMS Zn1-xCoxO nanowires. While the SQUID magnetometry method probes the average magnetizations of an ensemble of a huge amount of nanowires, the MFM method probes the magnetic dynamics (e.g., the domain wall motion and switching) in individual DMS nanowires. In addition to research on the fundamental science in nanostructures, this project will be devoted to the development of practicable nanoelectronic and spintronic devices. We aim not only to make significant impacts on the nanoelectronics industries but also to discover new science in the nanoscale systems.