Search

With this work we investigate several different ways to create marker layers for the ex situ characterisation of hydride vapour phase epitaxial (HVPE) gallium nitride (GaN) growth. Significant variations of the charge carrier concentration of GaN crystals become visible as marker lines in cross section investigations using optical microscopy and secondary electron microscopy (SEM). Pulsed intentional silicon and germanium doping leads to a good brightness contrast in cross section SEM micrographs and optical microscopy images. They appear as a black contrast in SEM. Pulsed manganese doping results in bright contrast marker layers in SEM, which is a result of the variation of the charge carrier concentration in the opposite direction, with respect to the background impurity level of the material. Marker layers are employed to determine the temperature transition point between 3D- and step flow GaN growth mode for the used set of growth parameters. A trend for the development of the growth rate of the c-facet as a function of the temperature is observed. [ABSTRACT FROM AUTHOR]

High-quality GaN layers were grown on sapphire (0001) substrates using laser molecular beam epitaxy (LMBE) by laser ablating a solid GaN target at different laser repetition rates (10–40 Hz) under a constant supply of r.f. nitrogen plasma. The effect of laser repetition rate on the structural and optical properties of GaN layers was systematically studied using high-resolution X-ray diffraction (HRXRD), field emission scanning electron microscopy, atomic force microscopy, Raman spectroscopy and photoluminescence (PL) spectroscopy. High-resolution X-ray rocking curve measurements revealed highly c-axis oriented GaN layers on sapphire grown at 30 Hz with a calculated screw dislocation density of ~1.42 × 107 cm−2, whereas the GaN layers grown at 10 or 40 Hz consisted the screw dislocation density in the range of 108–109 cm−2. Surface morphological analysis revealed a change in grain size as well as surface roughness as a function of laser repetition rate and is explained on the basis of growth kinetics. Vibrational Raman spectroscopy revealed that the GaN layer grown at 10 Hz shows an in-plane compressive stress of ~1 GPa, while the film grown at 30 Hz exhibits a minimum stress of ~0.3 GPa. The PL measurements show a highly luminescent band-to-band emission of GaN at 3.44 eV for the 10 Hz grown highly strained GaN layer and at 3.41 eV for the less strained film grown at 30 Hz along with a broad defect band emission centered around 2.28 eV. It is found that the GaN layers grown at 30 Hz have excellent structural and optical properties. We expect that the less strained thin and highly oriented GaN film grown by LMBE can further be utilized for developing prodigious low-temperature-grown nitride-based multilayer structures and devices. [ABSTRACT FROM AUTHOR]

In this paper, a MOCVD-GaN/Al2O3 (MGA) template was annealed under appropriate annealing conditions and N2 flow to form a porous template. The porous template was used for growing GaN by HVPE. The GaN crystal was easily separated from the porous template with the assistance of the microporous structure. The self-separated GaN crystal grown on the porous template showed a smaller full width at half maximum (FWHM) for (002) and (102) reflections in the HRXRD measurement than that grown on the MGA template. The PL results indicate that the optical quality of the GaN crystal on the porous template was improved and the dislocation density decreased. The Raman results showed that the stress in the GaN crystal grown on the porous template is much smaller than that on the MGA template. These results show that the crystalline quality of GaN crystals was improved by using the porous template. [ABSTRACT FROM AUTHOR]

A method to obtain high quality semipolar {1013} GaN grown on m-plane sapphire is presented. This method is similar to two-step nanoepitaxial lateral overgrowth (2S-NELOG) by combining a TiN interlayer and self-assembled SiO2 nanospheres. For the 2S-NELOG semi-GaN, the root-mean-square (RMS) roughness is 1.8 nm with a scan area of 5 x 5 μm². The reduction of the defect density is demonstrated using high resolution X-ray diffraction (HRXRD) and transmission electron microscopy (TEM). The full widths at half maximum (FWHMs) of the on-axis X-ray rocking curves (XRCs) are 381 and 524 arcsec, respectively rocking toward the [3032] direction and the [1210] direction. The anisotropy of the mosaic is lower compared to planar and TiN semi-GaN. In addition, Raman analyses also show the partial relaxation of the stress in the 2S-NELOG semi-GaN. [ABSTRACT FROM AUTHOR]

Synthesis of GaN nanoparticles (NPs) by a novel chemical co-precipitation method and the effect of nitridation temperature on the structural, optical and electrical properties have been reported. X-ray diffraction and high resolution transmission electron microscopy show the hexagonal wurtzite structure and highly crystalline nature of the GaN NPs. A strong blue luminescence was observed for all the GaN NPs at room temperature photoluminescence studies. Nevertheless, red and yellow luminescence were absent at the nitridation temperature of 1000 °C. The phonon frequency mode at the k-point of the Brillouin zone symmetry was observed at 271-273 cm-1 for GaN NPs by micro-Raman spectroscopy, which is normally absent for bulk GaN. The carrier concentration and mobility of GaN NPs synthesized at higher temperature were calculated to be 1.36 x 1017 cm-3 and 433 cm² V-1 s-1, respectively, by the Raman line shape analysis of the longitudinal-optical-phonon-plasmon coupled mode. [ABSTRACT FROM AUTHOR]