| AlxGa1-xN is a direct band-gap material with broad bandwidth, stable chemical property, thermo-stability and anti-radiation capacity. It's wavelength can very continuouslly from 200 to 365 nm, making it to have potential applications in lots of areas, such as dissociation of pollutant materials, water and air purification, solid-state lighting, etc. Device performance is highly influenced by the crystal quality of epitaxial films. However, the difficulties in the growth of AlGaN-based material with high Al content have been the key factors for limiting the development of the ultraviolet light-emitting diode (UV-LED). Therefore, the study on the growth and characterization of AlGaN epitaxial film is of great importance. This paper is mainly on the development of metalorganic chemical vapor deposition (MOCVD) crystal growth technology for good crystal quality and crack free AlxGai-xN epilayer with high Al content on sapphire substrate through a series of growth and characterization experiments. The physical properties of AlInGaN quaternary material were investigated experimentally and theoretically too. Through the optimization of the MOCVD growth process, high-quality AlGaN/GaN quantum wells were prepared, and the interface quality was studied in-depth. In addition, the epitaxial growth of GaN-and AlGaN-based ?-nitrides on non-polar and semi-polar sapphire substrates were studied systemmatically. High resolution X-ray diffraction (HR-XRD), scanning electron microscope (SEM), photoluminescence (PL) spectroscopy, X-ray photoelectron spectroscopy (XPS), synchrotron radiation X-ray spectroscopy, and other characterization technologies we used to study the crystal quality, surface morphology, and optical properties of AlGaN epitaxial films. The major research contents and the results achieved in this study are listed as follows:1. High temperature A1N (HT-A1N) layer was grown as an interlayer under the optimized pulse mode, then high-quality AlxGa1-xN films were grown on the HT-A1N layer, with Al component varied from 0 to 87%. The effects of HT-A1N interlayer on reducing crack density and improving the crystal quality were investigated systematically. The results show that the crystal quality of AlGaN film can be improved greatly when the thickness of HT-A1N was about 200 nm. It was found that as the Al composition x increases, the c/a ratio decreases almost linearly, and moves away further from the ideal value of 1.633, while the internal parameter u in wurtzite AlxGa1-xN increases with the increase of x. This is attributed to the fact that the Al-N bonding character becomes more ionic with the increase of Al composition.2. Raman spectroscopy was used to study the strain variation in the AlxGa1-xN films. The results showed that Ai(LO) phonon mode frequency shift is more obvious, and the Raman peak monotonously shifted to higher frequency with increasing Al composition. The broadening of the Ai(LO) phonon mode was mainly resulted from long-range order of AlxGa1-xN films. The PL spectra of AlxGa-xN film was investigated with special attention on the origin of yellow band luminescence. The 317 nm band-edge emission peak, which was caused by free carriers combination, showed "blue shift" behavior as the temperature decreased from 300 to 10 K for the Al0.26Ga0.74N film.3. Extended X-ray absorption fine spectroscopy (EXAFS) spectra were measured to study the local electronic structures of AlxGa1-xN materials at the Ga K-edge. The EXAFS measurement results for the AlxGa1-xN epi-layers demonstrate that the first shell Ga-N bond length is nearly Al composition-independent. However, the second shell Ga-Ga bond length is strongly dependent on Al composition and is significantly longer than that of Ga-Al bond. On the other hand, the Ga-Al bond length was essentially composition independent. By analyzing the Ga 3d, Al 2p and N Is core-level XPS spectra of AlxGa1-xN epi-layers, it was verified that the surface of AlxGa1-xN is a composite of oxides and nitrides of gallium and aluminum. Furthermore, it was identified that the Ga-0 components were converted to Al-O components when the AlxGa1-xN sample was exposed to air.4. AlInGaN quaternary epitaxial layers were grown on GaN/sapphire templates by MOCVD at an optimal growth temperature 890 ?. The optical and structural properties were characterized by various kinds of technical tools. The full width at half maximum (FWHM) of the X-ray diffraction rocking curves was found to decrease as the Al/In mole ratio was increased. The density and size of V-shaped defect pits observed on the surface of AlInGaN epitaxial layers with energy-dispersive X-ray spectroscopy could be reduced by lowering the growth pressure. It was discovered with the variable temperature PL spectra that the emission peak energy for the AlInGaN epitaxial layers demonstrated an S-shaped shift behavior with increasing the measurement temperature. This phenomenon can be explained by taking into account the localized excitonic states influenced by the size fluctuation and the inhomogeneous distribution of In-rich clusters within the AlInGaN epitaxial layers.5. The impact of the purge time and other key growth process paramters on the interface quality of the AlGaN/GaN quantum well was studied. The optimized value of purge time after growth of AlGaN barrier layer and GaN well layer was determined to be 4 min and 2 min, respectively. The mechanism of defect formation in quantum wells was investigated. Through the analysis of microstructure of MQW samples by SEM, high quality AlGaN/GaN MQWs with different thickness of barriers were grown successfully. The effect of thickness of quantum barrier on the luminescence properties of MQWs was studied. The PL spectra of the MQW samples showed that the quantum-confined Starker effect caused by band bending, was responsible for the red shift of emission peak. The "blue shift" of the peak was caused by the quantum confinement effect when the quantum well width was decreased.6. The GaN and AlGaN epitaxial films were grown on R-plane, A-plane and M-plane sapphire substrates successfully. The SEM and XRD results showed that better quality of the polar [0001] oriented GaN could be achieved, when the nucleation layer was grown with a high ?/? ratio. The GaN epi-layer with a crystal orientation of [1122] was obtained on the M-plane sapphire substrate with good crystal quality. It was found that the C content was decreased by Si-doping for the Si-doped AlGaN sample grown on A-plane sapphire substrate. However, the C content was identified to be increased for Si-doped AlGaN grown on M-plane sapphire substrate. A much better crystal quality was obtained for the sample grown on A-plane substrate than on M-plane substrate. For all the AlGaN samples grown on A- and M-plane sapphire substrates, low intensity of blue band luminescence were observed in PL measurement, indicating good crystal quality. The successful growth of non-polar and semi-polar GaN and AlGaN epi-layer is very meaningful for making high brightness AlGaN-based UV-LED in the future.7. The electronic and optical performances of the GaN-based LEDs with various kinds of electron blocking layers (EBLs) have been numerically evaluated by using the commercial (APSYS) simulation software. The simulation results show that when the p-In0.018Al0.089Ga0.893N/GaN superlattice (SL) EBL, which is lattice-matched to the GaN is used, the hole injection efficiency into the active region from the p-GaN layer can be greatly increased, and the electron leakage can be significantly reduced. Moreover, it was found that the efficiency droop at high injection current density could be remarkably improved for the GaN-based LEDs with the p-In0.018Al0.089Ga0.893N/GaN SL EBL. |
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