The authors fabricated GaN-based light-emitting diodes (LEDs) on two different GaN templates with the same LED structure. One on thin GaN template (∼2μm) with high dislocation density [low (109cm−2)] grown by metal-organic vapor-phase epitaxy (sample A) and the other on thick GaN template (∼20μm) with comparatively low dislocation density [high (108cm−2)] by hydride vapor-phase epitaxy (sample B). In order to understand the mechanism of leakage current in LEDs, the correlation between current-voltage characteristics and etch pit density of LEDs was studied.

GaN epitaxial films are grown on c‐plane sapphire substrates with CrN buffer. The GaN layers show high crystalline quality and smooth surface morphology without being cracked. Selective etching of CrN buffer is performed by wet etching using conventional Cr metal etchant, which results in success ful lift‐off of GaN thick layers. We confirm that the crystalline quality of GaN does not change through the etching process. These results indicate that the chemical lift‐off process using CrN buffer is promising for production of both freestanding GaN substrates and vertical‐structure devices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

A grazing incidence x-ray diffraction with a synchrotron radiation and a cross-sectional high-resolution transmission electron microscopy were performed on the sapphire surface nitrided at 1080°C for 30min. The thickness of the nitrided layer was about 2nm. It was found out that the wurtzite, zinc-blende, and 30° rotated zinc-blende aluminum nitrides were formed on the sapphire surface. The 30° rotated zb-AlN formed the incoherent interface and has higher activation energy of formation, while the nonrotated zb-AlN formed the coherent interface.

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Free-standing GaN layers were successfully prepared by self lift-off process. Single crystalline ZnO buffer layer and GaN layer were successively grown on sapphire substrate by plasma assisted molecular beam epitaxy. Thick GaN film was grown on this template substrate for the realization of stress-free freestanding substrate by hydride vapor phase epitaxy. The a-axis and c-axis lattice constants of free-standing GaN were 3.189Å and 5.185Å, respectively. Peak positions of photoluminescence spectrum were D 0 X of 3.4715 eV and FX A of 3.4791 eV. These results suggest that the stress-free GaN layers were successfully prepared by self lift-off process.

In order to understand the origin of leakage current, light emitting devices were grown on two different templates with apparently different dislocation density: one on thin GaN template (∼2 µm) with higher dislocation density (low × 10 9 cm -2 ) prepared by metal-organic vapor-phase epitaxy (sample A), and the other on thick GaN template (∼20 µm) with comparatively low dislocation density (high × 10 8 cm -2) by hydride vapor-phase epitaxy (sample B). Especially, the template B showed very low value of the dislocation density for a screw component, 2.2 × 10 7 cm -2 evaluated by transmission electron microscope and 2.3 × 10 7 cm -2 approximated by the Williamson-Hall plot which was evaluated by high resolution X-ray diffraction, respectively. On the other hand, sample A showed one order higher, low × 10 8 cm -2 , than that of sample B for a screw component. Sample A showed the larger leakage current (more than two orders of magnitude) than sample B in a forward-biased region and a reverse-biased region also. It is expected that the screw dislocation were strongly contributed to the leakage current of forward and reverse I-V regions in LEDs.

Thick GaN films were grown by HVPE (Hydride Vapor Phase Epitaxy) on c-sapphire substrates with GaN nano-rod buffer layers. Lateral epitaxial growth mode was adapted to grow GaN thick films on nanostructure buffer. Thick GaN films were self-separated during cooling down by thermal stress caused by the difference of thermal expansion coefficient (TEC) between GaN and sapphire. Since the nano-rod buffer consists of nano-rods and voids, it is mechanically weaker than planar GaN layers and contributes to the self-separation of GaN thick films. 200 µm-thick free-standing GaN substrates show smooth surface morphology without any microcracks. The full width at half maximum (FWHM) of (0002) X-ray rocking curve is 619 [arcsec]. Donor-bound exciton emission is observed at 3.4718 eV in low-temperature photoluminescence spectra, which is nearly the same peak position as bulk GaN.1 Introduction Free-standing GaN (FS-GaN) substrates are strongly desired to fabricate high performance and reliable GaN based devices. But, there are many problems to realize high quality FS-GaN substrates due to the large difference of material properties between GaN films and dissimilar substrate. Especially, residual strain causes cracks and bending in free-standing GaN substrates. Several attempts to reduce residual stress in hetero-epitaxially grown GaN layers have been reported [1,2]. Nakamura et al. prepared a free-standing GaN substrate by polishing away c-sapphire after HVPE growth of thick GaN layer, and fabricated laser diodes with improved performance and life time [3]. Kelly et al. first demonstrated a laser lift-off (LLO) technique to separate a HVPE grown GaN layer from sapphire [4]. However, polishing of sapphire substrates after HVPE growth of GaN is required prior to laser lift-off because laser beams are scatted by GaN grown at a back surface and edge. This polishing process induces big mechanical stress in the GaN/sapphire system. In addition to mechanical stress, the GaN/sapphire system suffers from thermal stress through LLO process.Recently, there have been a few reports on self-separation of GaN substrates: void-assisted separation (VAS) technique [5], self-separation using low-temperature buffer [6], and self-separation using nanocolumns structure [7]. But, in the VAS technique, additional processes like Ti metal deposition on MOCVD (Metal Organic Chemical Vapor Deposition) GaN templates are required. Self-separation techniques using LT buffer or nano-structures have such problems that FS GaN substrates have limited by size, inferior crystal quality, and poor reproducibility.In this study, we propose a new buffer layer using a GaN nano-rod structure to obtain stress free FSGaN substrates. GaN thick films are self-separated during cooling down after HVPE growth due to thermal stress and mechanical weakness of GaN nano-rod buffer.