Nearly stress-free substrates for GaN homoepitaxy

High-quality 300 mu m thick GaN crack-free layers grown by hydride vapor phase epitaxy (HVPE) on c-plane sapphire without buffer layers and separated from the substrate by laser lift-off were investigated by high resolution X-ray diffraction (XRD), low-temperature photoluminescence and cathodoluminescence. All these characterization techniques confirm the high structural quality of the resulting material. Lateral X-ray mapping of the free-standing bulk-like GaN shows a homogeneous compressive stress of less than 40 MPa and a heterogeneous stress of about 80 MPa. The formation of twin grains (domains) were observed both in the reciprocal space mapping of the (20.5) reflection and in rocking curve measurements. The latter ones revealed an estimated lateral coherence length of about 1.2 mu m. The crystallite size along the c-axis is estimated to be larger than 20 mu m. An upper limit of the density of dislocations with a component of the Burgers vector along the c-axis (screw and mixed type) of 1.3 x 10(7) cm(-2) was extracted from the XRD data, while transmission electron microscopy measurements revealed a dislocation density of 1.7 x 10(7) cm(-2). Thus, these layers are suitable as lattice-parameter and thermal-expansion matched substrates for strain-free homoepitaxy of GaN-based device heterostructures. (c) 2006 Elsevier B.V. All rights reserved.