氧化鋅於半極性氮化鎵模板成長之微結構研究

Abstract

氧化鋅(ZnO)的寬能隙及高激子結合能，使其具多元的半導體與光電應用。氧化鋅/氮化鎵(GaN)異質接合，因兩者同為六方晶之纖鋅礦(wurtzite)結構，有較小晶格失配比和相似的熱膨脹係數可用於磊晶結構之元件。非極性與半極性結構可避免或降低極化效應，提升發光效率。本論文主要研究非極性ZnO成長於半極性面GaN上的成長現象與特徵。 本研究係利用有機金屬化學氣相沉積法在r面藍寶石(sapphire)上成長出表面有刻面之a晶向GaN為模板，其表面具有非極性的a面及半極性的{10-11}和{11-22}等晶面。ZnO和摻雜鋁之ZnO(AZO)薄膜以化學氣相沉積法成長在GaN模板上。研究的重點分別針對沉積於各個非極性和半極性GaN面上的ZnO及AZO，以掃描式電子顯微鏡(SEM)、X光能譜分析儀(EDS)、聚焦離子束顯微鏡(FIB)、高解析X光繞射儀(HRXRD)以及穿透式電子顯微鏡(TEM)進行觀察與分析，探討表面形貌及微結構與成長之關係。 對GaN表面形貌的觀察與分析發現，於GaN半極性{10-11}面上有L條紋形貌，分別平行GaN的[1-210]和[10-1-2]兩個方向；在{11-22}面上的條紋則是沿著[11-2-3]GaN方向。沉積ZnO之後，在半極性{10-11}和{11-22}面上，ZnO呈現出島狀形貌和原本GaN不同。其中在{10-11}面上，島狀結構沿[1-210]GaN方向排列，形成條紋狀形貌；{11-22}面上之ZnO則是如同原本GaN的形貌緻密的排列。 GaN模板上的ZnO薄膜從垂直試片方向進行XRD分析，發現ZnO磊晶成長在GaN上，其晶向關係是[0001]ZnO //[0001]GaN和[-1100]ZnO//[-1100]GaN。然而，{11-22}半極性面GaN上的ZnO 經由TEM微結構分析，發現兩種結構：一是ZnO(11-2 2)//GaN(11-22)和[1-100]ZnO//[1-100]GaN，另一則是ZnO(-101-1)//GaN(0002)和[5-72-3]ZnO//[1-100]GaN。半極性(10-11)面GaN上的ZnO薄膜亦發現兩種微結構：一種是ZnO(10-11)//GaN(10-11)和[1-210]ZnO//[1-210]GaN，另一是ZnO(0002)//ZnO(10-11)和[-2110]ZnO//[1-10-1]ZnO。 對GaN模板上的AZO薄膜進行同樣分析，發現AZO在a面GaN和半極性(11-22)面GaN上有和ZnO相似的形貌，但是在半極性(10-11)面GaN上卻截然不同。從XRD垂直試片方向分析，發現AZO薄膜和GaN模板有和ZnO相同的磊晶關係；(10-11)面AZO薄膜的XRD圖譜，也跟ZnO薄膜的結果雷同；但AZO是以(0002)的面向磊晶成長於GaN(10-11)上，方向關係是[-2110]ZnO//[1-10-1]GaN。

ZnO/GaN heterojunction has been suggested as a strong candidate for many device applications due to their wurtzite structure with small lattice mismatch and thermal mismatch. Nonpolar and semipolar ZnO/GaN can avoid and reduce the polarization to raise the light emission efficiency. This thesis work focus on understanding of growth of semipolar ZnO films on GaN from microstructural characterization. ZnO and Al-doped ZnO (AZO) were epitaxially grown on faceted a-oriented GaN template on r-plane sapphire. The faceted GaN was grown by metal-organic chemical vapor deposition (MOCVD), while ZnO and AZO were grown by chemical vapor deposition. The MOCVD-grown GaN template consisted of facets of a-plane and semipolar planes ({10-11} and {11-22}) on which simultaneous growth of nonpolar- and semipolar-oriented ZnO and AZO were explored. The surface morphology and the relation between microstructure and growth were identified by using scanning electron microscopy (SEM) with energy dispersive spectrometer (EDS), focused ion beam (FIB), high resolution X-ray diffraction (HRXRD), and cross-sectional transmission electron microscopy (XTEM). On GaN {10-11} facets, the surface exhibits an L-shape or corner-like morphology which consists of connected stripes along [11-2-3]GaN and [1-210]GaN , whereas on GaN {11-22} facet surface, striation or stripe-like morphology can be seen along [11-2-3]GaN . The SEM observations show that the ZnO grown on semipolar GaN {10-11} facets exhibit the island growth mode with striation morphology along [1-210]GaN, and similar morphology on GaN{11-22} facets. The symmetric XRD pattern shows that ZnO grown on the faceted GaN exhibit (11-20) reflections only, the in-plane orientation relationship of [0001]ZnO//[0001]GaN and [-1100]ZnO//[-1100]GaN . On semipolar GaN (11-22), the epitaxial ZnO can form in two different semipolar orientations as evidenced by TEM and XRD. One orientation relationship is shown to be ZnO(11-22)// GaN(11-22) and [-1100]ZnO//[-1100]GaN, the other is ZnO(-101-1)// GaN (0002) and [5-72-3]ZnO// [-1100]GaN. On semipolar GaN(10-11) facet, it is also found that there exist two kinds of ZnO microstructure: one is ZnO (10-11) // GaN (10-11) and [1-210]ZnO//[1-210]GaN, and the other is ZnO (0002) // ZnO (10-11) and [-2110]ZnO//[1-10-1]ZnO. The morphologies of AZO films on nonpolar a-plane and semipolar GaN{11-22} facets exhibit similar to those of ZnO. However, it is smooth on GaN{10-11} different from ZnO striation morphology. The XRD analyses of AZO show the same result as the ZnO. Further XTEM observations, however, reveal that the AZO grown on semipolar GaN(10-11) facet is in the orientation relationship of AZO (0002) // GaN (10-11) and [-2110]ZnO//[1-101]GaN.