利用同步輻射光源研究極性與非極性氧化鋅薄膜之生長與物理特性

Abstract

本論文係研究極性與非極性氧化鋅磊晶薄膜之成長、結構與其它物理性質及其間之關聯性。本論文分成兩個部分，第一部份，主要探討利用磁控濺鍍方法成長於藍寶石基板之高品質非極性氧化鋅(ZnO)與氧化鎂鋅(MgZnO)磊晶薄膜的成長與特性。第二部分，研究利用雷射濺鍍方法成長於矽基板之極化氧化鋅磊晶薄膜之成長與特性。 非極性高品質氧化鋅薄膜的製作是近幾年氧化鋅光電元件研發的重點項目，主要目的是克服傳統極性(c面向)氧化鋅在多重量子井結構會出現之量子侷限史托克效應(Quantum Confined Stark Effect,QCSE)。量子侷限史托克效應會使量子井能帶產生歪斜，使得電子、電洞的複合機率降低，不僅讓發光效率減弱且會使發光波長紅移。非極性氧化鋅之所以可以克服量子侷限史托克效應,是因為其極性軸-c軸平躺在成長平面上，其內建電場垂直於多重量子井的介面，因此對量子井的能帶結構不具影響力。在本研究裡，我們使用磁控濺鍍系統成功的於m面向藍寶石基板上長出高品質m面向氧化鋅磊晶薄膜，並藉由一數奈米厚低溫成長的氧化鋅作為緩衝層，有效地消除了其他非m面向的氧化鋅晶域。 a面向的氧化鋅都是另一非極性氧化鋅，可以藉由成長於r面向的藍寶石基板而獲得。我們於是利用r面向和m面向的藍寶石基板來製作a面向和m面向的非極性氧化鋅，藉由XRD、TEM、原子力顯微鏡(AFM)和低溫極化光致螢光光譜(LT Polarized-PL)去探討和比較其結構品質、缺陷密度、表面形貌和光學特性。 氧化鋅多重量子井結構(MQWs)以氧化鋅則為位能井(well)，而氧化鎂鋅則是做為位能屏障(barrier layer)的理想材料，因此在氧化鋅多重量子井結構的研發中高品質氧化鎂鋅的製作是一關鍵的課題。氧化鎂是立方體結構，氧化鋅卻是六角結構；隨著鎂的濃度增加到一定程度，氧化鎂鋅的結構會由六角變為立方體結構或形成相分離，因此具高鎂濃度的六角結構氧化鎂鋅的製作仍是一具挑戰性的課題。在本實驗裡，我們成功的在r面向和m面向的藍寶石基板上成長出r面向和m面向的單一晶相非極性氧化鎂鋅磊晶層。經由X光光電子能譜(XPS)分析，鎂濃度高達約35%。此外，氧化鎂鋅磊晶薄膜表面的粗糙程度，對於多重量子井結構的功能影響甚鉅；我們成長的氧化鎂鋅的表面粗糙度可小於1奈米，這對多重量子井結構的成長是一大進展。 另一部分，雖然c面向的極性氧化鋅在成長多重量子井時會出現量子侷限史托克效應的問題，但有研究顯示當氧化鋅量子井的厚度大於3奈米時，量子侷限史托克效應才有重要影響，而c面向的極性氧化鋅缺陷密度遠低於非極性氧化鋅；再者，矽基板具有價格便宜、優異品質與面積大等特性，而且可與已建立的矽電子工業技術做整合，所以許多研究團隊致力於成長c面向的極性氧化鋅於矽基板上。在這部份實驗裡，我們使用雷射濺鍍系統，藉由一數奈米厚的Gd2O3(Ga2O3)緩衝層成功的長出高品質c面向氧化鋅磊晶薄膜於矽(111)基板。在氧化鋅磊晶層裡，經由TEM的對比圖像分析，發現主要的缺陷是貫穿式的刃差排(edge-type dislocation) 和本質基面疊差(intrinsic basal plane stacking faults)；這些氧化鋅磊晶層都承受拉伸應力但卻沒有裂隙(cracks)產生。

In this thesis, we investigated the correlations between structural characteristic and physical properties of the polar and non-polar ZnO. It includes two subjects: one is studying the growth of high-quality non-polar ZnO and MgZnO grown on sapphire by using radio-frequency magnetron sputtering deposition; and the other one is studying the growth of polar ZnO grown on Si by using pulsed laser deposition. Non-polar (a-plane or m-plane) ZnO has attracted much attention recently because it can overcome the quantum confined Stark effect (QCSE), which degrades the performance of multiple quantum wells (MQWs) grown along the c-axis of ZnO. The QCSE, caused by the internal electric field lying along the interface normal of the MQWs grown along c-axis, skews the band-gap in the quantum well, which not only reduces the optical properties efficiency but also makes the emission wavelength red-shifted. Non-polar ZnO, having its internal electric field lying on the interface, effectively avoid the QCSE. In this work, high quality m-plane orientated ZnO films have been successfully grown on m-plane sapphire by using radio-frequency magnetron sputtering deposition. The introduction of a nanometer thick low temperature grown ZnO buffer layer effectively eliminates the inclusions of other undesirable orientations. Both a-plane and m-plane ZnO are the non-polar ZnO, and both of them can solve the problem caused by QCSE. A-plane ZnO has also been grown on r-plane sapphire. The structural and optical properties of the ZnO film were characterized by X-ray diffraction, transmission electron microscopy, atomic force microscopy, and low-temperature polarized photoluminescence. A comparison on the crystal quality, defect density, surface morphology and optical properties between the two kinds of non-polar ZnO was made. ZnO-based MQWs are consisted of ZnO wells and barrier layers made of materials with larger band-gap and compatible crystalline structure. Ternary MgxZn1-xO compound is often adopted as a barrier material. MgO has a cubic structure in its stable phase, but ZnO has a hexagonal structure. Phase segregation occurs as Mg content exceeds a critical level, above which cubic phase MgZnO emerges from the hexagonal phase one. Therefore, the growth of single phase hexagonal MgZnO films is still a challenge issue. In this work, we demonstrated the successful growth of single phase non-polar a-plane and m-plane MgZnO on r-plane and m-plane sapphire, respectively. Derived from the X-ray photoelectron spectroscopy (XPS) results, the Mg content is higher than 35%. Moreover, the small surface roughness (< 1 nm) of the obtained MgZnO films is another advance of the fabrication of high performance MQWs. Despite the polar c-plane ZnO has the QCSE problem, some reports indicated that QCSE will counteract the quantum confinement effects for wells with width larger than 3 nm. Furthermore, low costs, excellent quality, and large-area availability of Si wafers, and the unique possibility of integrating well-established Si electronics with ZnO-based optoelectronic devices make Si a desirable substrate for ZnO growth. In this work, high-quality (0001)-oriented ZnO epitaxial films were grown by pulsed laser deposition on Si(111) buffered with a nanometer-thick Gd2O3(Ga2O3) layer. TEM contrast analysis reveals that the major defect structures in the ZnO films are edge-type threading dislocations and intrinsic basal plane stacking faults. All the ZnO epi-films studied are under a tensile biaxial strain but no cracks were found.