The dislocation distribution of high-quality single-crystal gallium nitride (GaN) films grown by the hybrid vapor phase epitaxy was analyzed. This study examined the domain structure of GaN from the dislocation distribution on the macroscale by optical microscopy. The surface structure of GaN consisted of domains with microcolumns as the substructure. The inner domains contained a lower density of dislocations but a large number of these dislocations were observed along the domain boundaries. The existence of a domain boundary structure doubly increased the total dislocation density.

Polarity issues affecting III-V nitride semiconductors are reviewed with respect to their determination and control. A set of conditions crucial to the polarity control of GaN is provided for each of the following growth techniques; molecular beam epitaxy (MBE), pulsed laser deposition (PLD) and hydride vapor phase epitaxy (HVPE). Although GaN films might have been deposited by identical growth methods using the same buffer layer technologies, there is often a conflict between the resulting polarities achieved by different research groups. In this paper, we present the implications of the conditions used in each of the processes used for two-step metalorganic chemical vapor deposition (MOCVD), demonstrating systematic control of the polarity of GaN films on sapphire substrates. The potential for confusion in polarity control will be explained, taking into account the implications clarified in our studies. The correlation between the polarity and the growth conditions will be discussed in order to provide a mechanism for the determination and control of the crystal polarity during the growth of GaN films.

We have investigated the dislocation of GaN films grown by hydride vapor phase epitaxy (HVPE) on c-plane sapphire substrates using transmission electron microscopy (TEM), etch pit density (EPD) characteristics, and micro photoluminescence (PL). Micro PL mapping is a nondestructive method for observing defect sites as dark spots which reveal the dislocations causing non-radiative recombination centers in the GaN film surface. The dark spots reveal a decrease in threading dislocation sites with increasing the thickness of GaN films. In order to illustrate the correlation between the thickness and the dislocation density of GaN, the micro PL analysis method was used to observe the dislocation densities of a GaN film with a low dislocation density.

We have studied epitaxial GaN layers grown by hydride vapour phase epitaxy (HVPE) on porous GaN sublayers formed on SiC substrates. It was shown that these layers can be grown with good surface morphology and high crystalline quality. X-ray, Raman and photoluminescent (PL) measurements showed that the stress in the layers grown on porous GaN was reduced to 0.1-0.2 GPa, while the stress in the layers grown directly on 6H-SiC substrates remains at its usual level of about 1 GPa. Thus, we have shown that growth on porous GaN sublayer is a promising method for fabrication of high quality epitaxial layers of GaN with low strain values.

We report ongoing experiments on the growth of GaN by hydride vapor phase epitaxy (HVPE), using a newly designed Aixtron horizontal reactor. Growth was carried out on c-plane Al2O3 substrates on which a thin GaN layer had been predeposited by MOVPE and patterned using a dielectric mask. The mask pattern was designed to give information on the growth rate and morphology along different directions, and contained both a star-shaped pattern and arrays of parallel stripes of various widths and orientations. All growths were performed at atmospheric pressure and ~1050°C deposition temperature. For the range of experimental conditions investigated the maximum ratios of lateral to vertical growth velocities of around 2 and coalescence of the layer after approximately 10 μm of growth were observed for stripes oriented along the <100> direction.

Free standing GaN platelets were fabricated by hydride vapor phase epitaxy (HVPE). The platelets having a current maximum size of 7×6×0.1 mm3 were obtained by HVPE growth of ~100 μm thick GaN layers on SiC substrates and subsequent removal of the substrates by reactive ion etching (RIE). Surface of the GaN platelets was characterized by reflectance high energy election diffraction (RHEED), and Auger electron spectroscopy (AES). Crystal structure and optical properties of the platelets were studied by x-ray diffraction and photoluminescence (PL), respectively. Raman spectroscopy was also applied for material characterization. Residual strain was detected in the crystals. The stress was eliminated by high temperature anneal.