Free standing GaN layers with GaN nanorod buffer layer

Abstract

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.

The hydride vapor phase epitaxy of GaN on silicon covered by nanostructures

“…Hydride vapor phase epitaxy (HVPE) has already proven to be well suited for the fabrication of FS-GaN with threading dislocation densities below 10 4 cm −2 [1]. Commonly, sapphire wafers are used as substrates for the HVPE process to grow FS-GaN [1][2][3].…”