原子層沉積法成長單晶氧化鋅薄膜之光學與晶體結構特性研究

Abstract

我們成功地利用原子層沉積法成長纖維鋅礦結構單晶氧化鋅薄膜於c-plane與m-plane藍寶石□板，並且利用X-ray繞射與穿遂式電子顯微鏡研究其晶體結構特性。 經過高溫退火處理之後的氧化鋅單晶薄膜，其晶體品質顯示明顯的改進，並發現基面堆疊缺陷(basal stacking fault)為其主要的晶體結構缺陷。我們在c-plane藍寶石□板上成長出單晶氧化鋅薄膜，其磊晶面關係為(0001){10-10}ZnO||(0001){10-10}Al2O3 ；而在m-plane 藍寶石□板上之磊晶面關係為 (10-10)<0001>ZnO||(10-10)<-12-10>Al2O3 。以光激螢光光譜觀察到以3.325 eV為中心的主要發光波段，可能來自於基面堆疊缺陷。由於基面堆疊缺陷的原子結構可視為一層非常薄的閃鋅礦晶體內嵌在纖維鋅礦結構之中，形成的基面堆疊缺陷量子井結構。我們也研究了熱退火處理對單晶氧化鋅薄膜的晶體結構特性和發光特性所造成的影響。另外，我們也透過時間解析光激螢光實驗來測定此基面堆疊缺陷量子井結構的發光機制與近能帶間隙發光的特性。我們觀察到光激載子被基面堆疊缺陷量子井結構局限後，形成量子局限激子。而此量子局限激子更持續地受到由許多局限能態所形成的局限效應所束縛。這些局限能態可能是由隨機分佈的基面堆疊缺陷之間的量子耦合效應所形成；量子耦合效應是由於局限在不同量子井中的電子之間發生波函數交疊的關係，此現象亦形成了局限能態。由於測得的氧化鋅薄膜的低施子濃度以及近能帶能隙發光並無激子遷移的現象，所以我們排除了靠近基面堆疊缺陷的施子以及離子成份濃度擾動這兩個因素形成局限能態的可能性。

We have successfully grown mono-crystalline ZnO epitaxial films on c-plane and m-plane sapphire substrates by using the atomic layer deposition. X-ray diffraction and transmission electron microscopy were employed to verify the structural properties of the ZnO thin films. The structure of the ZnO epi-films exhibits significantly improvement upon thermal annealing and intrinsic types of basal plane stacking faults (BSFs) are the predominant structural defects in the ZnO films after thermal treatment. The ZnO epi-films grown on the c-plane and m-plane sapphires have the epitaxial relationships of (0001){10-10}ZnO||(0001){10-10}Al2O3 and (10-10)<0001>ZnO||(10-10)<-12-10>Al2O3, respectively. The BSF is found to contribute to the emission at 3.325 eV in the photoluminescence (PL) spectra of the annealed ZnO films. This is attributed to quantum-well (QW) structure formed by the BSF, which has the thin layer of zinc blend structure embedded in the wurtzite structure ZnO layer. The influence of thermal annealing to the structural and optical properties of the ZnO epi-films was also investigated. Through the time-resolved PL, we determined the decay times of the BSF related emission and the near-band-edge (NBE) emission. The QWs formed by the BSFs are found to trap the carriers to form BSF-bound excitons. The PL measurements revel that the BSF-bound excitons are influenced by the localization effect, which consists of localization states, and these bound excitons migrate among these localization states. Such localization states are attributed to the quantum coupling effect among the random distributed BSFs; the quantum coupling effect results from the wave function overlapping of the electrons bound in QWs and leads to the localization states. Because of the obtained low donor concentration and near band emission without the phenomenon of exciton migration, we exclude the donors in the vicinity of BSFs and the alloy density fluctuation from the origins of these localization states.