Thick GaN films were grown on two different low-temperature GaN (LT-GaN)-buffer layers, i.e., one- and two-step LT-GaN buffer, by hydride vapor-phase epitaxy (HVPE). NH4Cl layer was included in two-step LT-GaN, which was evaluated by X-ray diffraction patterns. Many voids were observed at the interface for both samples. We will discuss the mechanism of void formation for both samples. The full-width at half-maximum (FWHM) values of (0 0 0 2) omega-rocking curves were 390 and 440 arcsec for 200-mu m-thick GaN on one-and two-step LT-GaN, respectively. The 200-mu m-thick high-temperature GaN (HT-GaN) film on two-step LT-GaN was self-separated without any cracks after cooling down. NH4Cl layer included into two-step LT-GaN buffer was effectively contributed to the voids formation and self-separation in realizing a stress-free free-standing GaN (FS-GaN) substrate. (C) 2007 Elsevier B.V. All rights reserved.