賴昆佑

This dissertation describes the growth methods of metal-organic chemical vapor deposition (MOCVD) to enhance crystal qualities of the key materials in deep ultraviolet light-emitting diodes (DUV LEDs). The methods include pulsed flow for high-quality AlN buffer and reduced V/III ratio for BN window layer. Specifically, H2 flow is essential for the growth of AlN and BN, as it not only suppresses the gas-phase pre-reaction between metal-organic source and NH3 but also etches the point defects on the epitaxial surface. To shorten the growth time of DUV LEDs without sacrificing crystal qualities, pulsed-flow of precursors is one of the most effective way, particularly at the substrate temperature below 1200℃. In this regard, a 1.5-μm AlN buffer containing two pulsed-NH3-flow AlN layers was attained at a single substrate temperature of 1180 °C. The AlN buffer exhibits atomically flat surface and the x-ray diffraction (XRD) (102) peak width of 427 arcsec. To further shorten the AlN-buffer growth, a double-pulsed-flow of NH3 and trimethylaluminum (TMA) and pulsed-H2 etching process were used to control the grain size of nucleation islands. The 1.5-μm-thick AlN epilayer exhibits a root-mean-square surface roughness of 0.25 nm (scanned area: 5x5 μm2). The pulsed etching technique facilitates the crystal transformation of AlN from three-dimensional (3D) nucleation to two-dimensional (2D) layer-by-layer growth. The last part of this work aims to clarify the growth mechanism of BN on AlN. The BN layer is to replace the resistive p-type AlGaN for DUV LEDs. The lattice transformation of BN from cubic (cBN) to rhombohedral (rBN) structure was confirmed with the results of XRD, transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). According to the cross-sectional TEM images, the BN growth favors low V/III ratios so that H2 gas can effectively suppress the gas phase pre-reaction and catalyze the crystal transformation from small cBN nano-islands to rBN monolayer. The growth of BN is summarized as follows: H2 flow suppressing gas-phase pre-reaction → cBN nucleation layers → crystal transformation → rBN continuous growth.