以穿透式電子顯微鏡研究氮化鎵薄膜之結構和缺陷

Abstract

在本論文中，我們使用穿透式電子顯微鏡研究成長於(0001)方向的氧 化鋁單晶上之氮化鎵薄膜的晶體結構及缺陷。利用低壓有機金屬氣相磊晶 系統 (LP-MOCVD) 在(0001)方向的氧化鋁單晶上先成長低溫(525 C)的氮 化鎵緩衝層，再成長高溫(1025 C)的氮化鎵薄膜。在成長過程中，使用兩 種不同的有機金屬，即triethylgallium (TEGa) 和 trimethylgallium (TMGa)，成長氮化鎵薄膜。我們以穿透式電子顯微鏡比較兩種有機金屬成 長的薄膜之緩衝層的結構，繞射圖形及影像 ，以得到薄膜的晶格常數、 與氧化鋁單晶間的晶格差距、及薄膜上的差排。由結果得知，有機金屬 TMGa和TEGa所形成的氮化鎵薄膜之緩衝層，有不同的結 構，TMGa所形成 的緩衝層是柱狀晶結構，而TEGa所形成的緩衝層是碟狀的晶粒堆疊 而成 。氮化鎵及氧化鋁單晶之晶格差距，發生在(1 0 -1 0)GaN // (2 -1 -1 0)Sapphire 和 (2 -1 -1 0)GaN // (3 -3 0 0)Sapphire，此差距 約16.1%。氮化鎵薄膜晶格常數，在最初成長的氮化鎵薄膜晶格常數小於 最後成長的薄膜。經穿透式電 子顯微鏡影像得知，TEGa成長的薄膜比 TMGa成長的薄膜有較低的差排密度。若氮化 鎵薄膜成長較慢會得到高品 質的氮化鎵薄膜。 In this thesis, we have studied the structure and defect of GaN films grown on (0001)sapphire by transmission electron microscope (TEM). We have grown low temperature (525 C) GaN buffer layer on (0001) sapphire substrate then grown GaN film at high temperature (1025 C) by low-pressure metalorganic chemical vapor deposition (LP-MOCVD). In our LP-MOCVD system, we used different MO source, triethylgallium (TEGa) and trimethylgallium (TMGa) to grow GaN films. We compare the diffraction patterns and TEM images of GaN films grown by two MO source to indicate the buffer layer structure,the lattice parameters of GaN films, lattice misfit between GaN and sapphire,and dislocations of GaN films. Two types structure of LT-GaN buffer layers are formed by two types MO source during GaN epitaxial growth. One is columnar structure from TMGa source and the other is stacked like disc stacked together from TEGa source. Lattice misfits of GaN/ sapphire interface based on (1 0 -1 0)GaN // (2 -1 -1 0)Sapphire and (2 -1 -1 0)GaN // (3 -3 0 0)Sapphire are about 16.1%.The lattice parameters a & c of GaN films of initial layers are smaller than those of layers close to the surface (final stage). From TEM images, the dislocation density of GaN films by TEGa source is less than that by TMGa source. The growth rate of GaN is slower which results in better crystallization and less defect density.