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High-quality GaN with smooth surface morphology has been grown on vicinal a-plane sapphire substrates by metalorganic chemical vapor deposition. The misorientation angles of vicinal a-plane sapphire substrates were changed systematically and the results were compared with the growth on exact a- and c-plane sapphire substrates. Surface morphology and crystalline qualities are found to be very sensitive to misorientation angles of a-plane sapphire substrates and the misorientation angle was optimized to be 0.25°. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) spectra were measured for sub-nanometer-thick GaN quantum wells (QWs) with Al[sub 0.8]Ga[sub 0.2]N barriers, which were grown by atmospheric-pressure metal-organic chemical-vapor deposition. The thickness of the GaN QW layers was systematically varied from 1 to 4 ML. We clearly observed a PL peak at room temperature at a wavelength as short as 247 nm (5.03 eV) from 1-ML-thick QWs. The effective confinement energy, or difference between this recombination energy and the band gap of bulk GaN, is as large as 1.63 eV. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Quarter-wave reflectors consisting of sets of GaN and Al[sub 0.34]Ga[sub 0.66]N layers have been grown on sapphire substrates by atmospheric-pressure metal organic chemical vapor deposition. A periodic structure with flat interfaces was observed by high-resolution scanning electron microscopy. X-ray diffraction measurements were performed to characterize the structures, from which the Al content x in the Al[sub x]Ga[sub 1-x]N layers was determined to be 0.34. No cracks could be seen on the surface of the reflectors by optical microscopy. The measured peak reflectivity at 390 nm increases with the number of pairs and reaches as high as 96±2% in the 35-pair reflector. © 1998 American Institute of Physics. [ABSTRACT FROM AUTHOR]

In continuation to the steady state detection of below-gap states by combining a below-gap excitation (BGE) light with an above-gap excitation (AGE) light, we have studied the time-response of both donor-acceptor pair luminescence (DAP) and yellow luminescence (YL) in undoped GaN after switching off the BGE light. An extremely slow recovery of YL with a time constant of up to 28 s was observed, which is interpreted as a relaxation process of electrons from a below-gap state chosen by a BGE energy of 1.17 eV. Trap parameters were quantitatively determined by studying the intensity change and subsequent recovery time of DAP as a function of AGE and BGE power densities. The BGE energy dependence of the intensity change and subsequent recovery time of DAP and YL implies an energy distribution of traps in the band-gap of GaN. The combination of time-resolved PL measurement under two-wavelength excitation scheme was shown to be powerful for understanding the carrier recombination dynamics including YL among below-gap states. [Copyright &y& Elsevier]