氮化鎵材料結構與光學特性研究

Abstract

在本論文中,我們利用共焦顯微鏡與近場光學顯微鏡研究兩種氮化鎵材料結構的光學特性。一種為利用氫化物氣相磊晶法(HVPE)成長之氮化鎵薄膜,另一種為利用有機金屬氣相沈積法(MOCVD)成長之氮化銦鎵/氮化鎵多重量子井結構。在氮化鎵薄膜研究方面,我們針對利用HVPE法成長氮化鎵薄膜上所發現的V型缺陷結構,觀察其光學影像與螢光影像。並利用氦鎘雷射(波長為325nm)作為激發光源,來量測V型缺陷在不同位置所發出的螢光光譜,我們發現其螢光光譜隨著不同位置而有所改變,當從表面沿著六角形倒立角錐面至角錐中心時,其光譜中心波長由373.5nm紅移至379.1nm。此紅移的現象可能為V型缺陷結構應力的釋放所造成。並在V型缺陷的中心位置量測到另外一個波峰,中心波長為386.7nm,與SEM縱向剖面影像對照,此波峰可能起源於形成缺陷起點之穿透位錯所造成。 在研究利用MOCVD法成長之氮化銦鎵/氮化鎵多重量子井結構光學特性方面,我們使用氦鎘雷射來作為單光子激發光源,在單光子共焦螢光影像上,發光的區域約佔全部面積的90%,比較其螢光影像上不同亮暗區域發光情形,發現其亮暗區域橫截面光強信號大小相差五倍。而在利用鈦藍寶石雷射作為雙光子激發光源時,固定雷射波長為900nm,從其共焦螢光影像上發現,發光的區域只佔全部面積的30%,其亮暗區域橫截面光強信號大小卻相差十倍。因此在雙光子共焦螢光影像上亮暗區域對比較單光子共焦螢光影像更為顯著。由雙光子與單光子激發的結果,可以知道在雙光子激發下,更容易從其共焦螢光影像判定氮化銦鎵/氮化鎵多重量子井發光的均勻性以及效率,藉以鑑別多重量子井(MQW)品質的優劣。

In this thesis, we study the structural and optical properties of two types of GaN material structures by confocal microscope and scanning near-field optical microscope. One is the undoped GaN films grown on (0001) sapphire substrate by hybrid vapor phase epitaxy (HVPE) and another is the InGaN/GaN MQW structure grown by metalorganic chemical vapor deposition (MOCVD). We study the room-temperature micro-luminescence images from V-shaped inverted pyramids in undoped GaN films. As the excitation laser spot at 325nm was translated from the surface toward the center of the invert pyramid along its slope, the center wavelength of the PL peak shows a trend of monotonic red-shift of from 373.5nm to 379.1nm. This could be attributed to the 3-dimensional release of stress and associated decrease of build-in piezoelectric field in the V-defects. A distinct and strong emission at 386.7nm was observed at the apex of the V-defect. This could be originated from the threading dislocation at the onset of the defect. We also study the InGaN/GaN MQW structure grown by MOCVD under He-Cd laser and Ti:Sapphire laser pumping. In the single photon confocal image by He-Cd laser pumping, the bright region is 90% to the measure of area. But in the two photon confocal image by Ti:Sapphire laser pumping, the bright region is only 30%. By comparing single photon and two photon confocal images, we found the contrast of the cross section of light intensity between bright region and dark region under two photon excitation is much stronger than the single photon. From the results, the use of the two photon excitation technique combined with confocal microscope provides a novel tool for characterization the quality of InGaN/GaN MQW structures.