氮化硼(boron nitride, BN)近年來成為深紫外光發光二極體所需的理想P型材料，因此具備sp2鍵結的六方氮化硼(h-BN)受到很大關注，h-BN具備高能隙(~ 6.0 eV)與低電洞活化能(~ 30 meV)，能使深紫外光不被氮化硼吸收，還能大幅提升電洞濃度，增加元件的外部量子轉換效率。 然而同為sp2鍵結的菱形(rhombohedral)氮化硼(r-BN)，其晶格結構與六方氮化硼相似，在常用的X射線繞射儀(x-ray diffraction, XRD)量測中，六方氮化硼與菱形氮化硼的訊號位置幾乎重疊，兩者僅差不到0.1°，無法明確分辨六方氮化硼與菱形氮化硼的結構。因此，我們藉由高解析度穿透式電子顯微鏡(high resolution transmission electron microscopy, HRTEM)的影像，觀察h-BN與r-BN在晶格結構上的差異。 為了得到高品質的BN的磊晶層，本研究利用有機金屬氣相沉積法(metal-organic chemical vapor deposition, MOCVD)成長BN，先在藍寶石基板上成長單晶氮化鋁(AlN)，再接著成長BN。我們透過五三比的調變來改善BN的晶格品質，並藉由XRD及HRTEM分析BN的晶格結構。根據三組五三比(2454、663、237)的分析結果，我們發現h-BN與r-BN會共存於磊晶層，且五三比為663的BN磊晶層有最佳的晶格品質。 ;Hexagonal Boron Nitride (h-BN) with sp2 bonding is an attractive p-type material for the application in deep-ultraviolet light emitting diodes (DUV LEDs). This is due to the wide bandgap (~ 6.0 eV) and low activation energy of acceptors (~30 meV) of h-BN, rendering high transmission of DUV photons and high p-type conductivity for DUV LEDs. However, rhombohedral BN (r-BN), also with sp2 bonding, is of a similar lattice structure with h-BN, which yields almost identical X-ray diffraction (XRD) angles. With the diffraction-peak difference less than 0.1°, h-BN and r-BN can not be differentiated by XRD patterns alone, but requires high-resolution transmission electron microscope (HRTEM) to reveal the minor difference in lattice between h-BN and r-BN. In this study, we perform epitaxial growth of BN by metal-organic chemical vapor deposition (MOCVD) on single-crystalline aluminum nitride (AlN), which is previously grown on the sapphire substrate. In order to improve the crystal qualities of BN, V/III ratios (i.e. the molar-flow ratio of NH3 to triethylborane) was varied from 237 to 2454. According to the characterization results with XRD and HRTEM, the MOCVD-grown BN wafer contains hexagonal and rhombohedral lattices at the BN/AlN interface, and the wafer grown with the V/III ratio of 663 exhibit superior qualities to those attained with the ratios of 237 and 2454.
