氫化物氣相磊晶成長氮化鎵薄膜之微結構與光學特性研究

Abstract

在本論文中，我們利用原子力顯微鏡、X射線繞射、冷激發光光譜、光激螢光顯微鏡、穿透式及掃描式電子顯微鏡來研究以氫化物氣相磊晶(HVPE)法成長在氧化鋁(Al2O3)基板上之氮化鎵薄膜(膜厚由5.58 μm至 22.9 μm)的微結構與光學特性。由原子力顯微鏡影像分析發現，表面缺陷密度會隨磊晶膜厚的增加而由2.5x109 cm-2降至5.7x104 cm-2；X射線繞射的量測結果顯示當膜厚增加時，氮化鎵轉動曲線高斯分佈訊號之半高寬值由587 arcsec縮減至309 arcsec；由冷激發光(photoluminescence)的量測結果發現，隨著磊晶當膜厚增加時，近能帶邊緣放射(near band edge emission)的強度與黃冷光(yellow luminescence)的強度比值也逐漸增加；根據以上的結果，我們得出以下的結論：以HVPE磊晶成長之氮化鎵薄膜，其晶體品質會隨著膜厚的增加而變好，尤其以膜厚最厚的樣品(22.9 μm)品質最好，其線差排(threading dislocation)密度約可降低達2.14x106 cm-2。 除了對氮化鎵薄膜品質的研究外，針對在膜厚為22.9 μm的樣品中所發現的V型缺陷(V-defect)，亦進一步進行觀察與分析。由掃瞄式電子顯微鏡所拍得之影像可清楚觀察到此缺陷源於線差排且在表面形成一倒立六角錐；此外，在光學微結構發光特性的量測分析發現，當以紫外光激發樣品表面時，V型缺陷內所發出的泠光相較於其他無缺陷區域所發出的泠光會顯得特別強，表示樣品表面受紫外激發光後之冷光強度分佈與V型缺陷結構有關。

We studied the micro-structural and optical properties of undoped GaN films grown on sapphire substrates by HVPE with film thickness ranging from 5.58 μm to 22.9 μm by atomic force microscope (AFM), X-ray diffraction (XRD), photoluminescence (PL), micro-fluorescence microscope, transmission electron microscope (TEM) and scanning electron microscope (SEM). The AFM images indicate that the estimated defect density decreases from 2.5x109 cm-2 to 5.7x104 cm-2 with increasing film thickness. The full width at half maximum of X-ray rocking curve for GaN peak narrows from 587 to 309 arcsec as the film thickness increases from 5.58 μm to 22.9 μm. The PL spectra show that both the intensity of the near band edge emission peak and the intensity ratio of the near band edge emission peak to the yellow luminescence increase substantially as the film thickness increases. Based on the above results, the thicker HVPE grown GaN films showed better crystal quality, especially the thickest one, 22.9 μm-thick GaN film. The estimated density of threading dislocation (TD) of the 22.9 μm-thick GaN film is approximately 2.14x106 cm-2. Besides the crystal quality study of the GaN films, we also investigated a specific defect structure, V-defect, found in the 22.9 μm-thick GaN film. The SEM top-view and cross-section images show that the V-defect initiates at TD with and terminate near the surface with a hexagonal inverted pyramid. The influence of surface morphology on luminescent property of GaN films has also been studied by micro-fluorescence microscopy. The enhanced luminescence at the V-defect site indicates a correlation of luminescent morphology with the structure of V-defect.