利用原子層沉積技術成長氧化鋅及其光電性質之研究

Abstract

摘要 本論文利用原子層沉積技術在不同基材上成長氧化鋅(ZnO)並研究其光學性質。本論文分為三個不同的主題。在第一部分為氧化鋅成長在碳化矽(SC)基板上之研究，結果顯示在高溫下成長的氧化鋅，其成長機制為柱狀成長模式(columnar growth mode)。光激發光頻譜(photoluminescence ,PL)的研究顯示對於成長在碳化矽(SC)基板上之氧化鋅，其發生受激輻射(stimulated emission)之臨界(threshold)強度比成長在藍寶石(sapphire)基板上之氧化鋅來的大，其主要的原因在於碳化矽基板之折射率大於氧化鋅，因此無法形成平面波導的結構。此外，為了要在較低的溫度下成長高品質的氧化鋅，我們先在高溫下成長一層氧化鋅作為緩衝層，而後再進行低溫成長，研究結果顯示藉由兩段式的成長方式可達高品質氧化鋅薄膜。 在第二部份，我們主要探討成長氧化鋅在二氧化矽基板上及其受激輻射之機制。結果顯示由於具有平面波導的結構，成長在二氧化矽基板上之氧化鋅薄膜發生受激輻射之臨界強度較低。在不同的薄膜厚度以及退火時間之下，因 electron-hole plasma所造成的受激輻射會發生藍位移的現象，此歸因於氧化鋅和二氧化矽在高溫退火時所造成的互相擴散，進而減少電子電洞在高濃度時的多體效應(many-body effects)所造成的能帶限縮(bandgap renormalization)的現象。袁 第三部份主要研究利用原子層沉積技術成長氧化鋅奈米結構的方法。在藍寶石基板上先成長非常薄的氧化鋅，藉由高溫退火形成島狀的奈米晶粒，最後在高溫下成長氧化鋅形成氧化鋅的奈米柱狀結構。氧化鋅奈米柱具有高度c軸取向並且在紫外光的範圍具有顯著的光激發光(PL)現象。除此之外，藉由原子層沉積技術具有高包覆性的特點，我們在二氧化矽奈米顆粒之間的縫隙成長氧化鋅，在退火後形成氧化鋅奈米點。因量子侷限效應，自發放光(spontaneous emission)有藍位移的效果。 最後我們製作了n-ZnO/SiO2/p-GaN異質接面發光二極體。在低電流密度時，主要的發光來自於氮化鎵之鎂能階；而在高電流密度時，鎂能階的發光逐漸飽和而氧化鋅的發光變的顯著。我們亦發現在反向崩潰電壓時，氮化鎵也會因為撞擊游離化(impact ionization)的現象而放出非常強烈的紫外光。

Abstract This thesis studied the optical properties of the ZnO deposited by atomic layer deposition (ALD) on different substrates. It can be divided into three topics. The first topic is the epitaxial growth of ZnO on the SiC substrate. The columnar growth mode of ZnO on SiC was activated at high deposition temperatures. The photoluminescence measurements showed a high pumping threshold of stimulated emission, which was attributed to the lack of optical confinement due to the lower the refractive index of ZnO than that of SiC.. Two-step approach was used to grow ZnO with high crystalline quality at a low deposition temperature in the “ALD window”. By introducing a buffer layer deposited at high temperatures, highly orientated ZnO thin films could be grown on the buffer layer at a lower deposition temperature. The second topic is the stimulated emission in the polycrystalline ZnO thin films deposited on SiO2 The low-threshold stimulated emission in ZnO thin films was achieved. By controlling the film thickness and the annealing condition, the stimulated emission resulted from the electron-hole plasma (EHP) shifted toward larger photon energy and the lasing threshold increased. It could be attributed to the inter-diffusion between ZnO and SiO2, which reducds the many-body effects and modified the bandgap renormalization in ZnO. The third topic is the deposition of ZnO nanostructures by ALD. The initial island seeds as well as the selective growth of ZnO on homo- and hetero-interfaces were used to prepare the ZnO nanorods on the sapphire substrate. The ZnO island seeds were prepared by the initial ALD cycles followed with high-temperature post-annealing. Subsequent ALD growth proceeded preferentially on the ZnO island seeds over the sapphire substrate, leading to the formation of ZnO nanorods. Highly orientated ZnO nanorods with large-area uniformality and strong UV emission at room temperature were achieved. On the other hand, ZnO quantum dots were deposited into the voids between the SiO2 nanoparticles by using the excellent conformality of ALD. The ZnO dots were distributed uniformly in the SiO2 matrix. Because of the quantum confinement effects, significant blue-shift in the spontaneous emission spectrum was observed. We also presented the electroluminescence from the n-ZnO/SiO2/p-GaN heterojunction diode. The Mg deep-level emission in p-GaN appeared at low injection current density. With increasing the current density, the Mg deep-level emission from p-GaN saturated and electroluminescence from ZnO gradually dominated. Strong UV emission at reverse breakdown region was observed, which might be attributed to the impact ionization in GaN.