ABSTRACT Thick low defect AlN and AlGaN layers grown on ultra violet (UV) transparent substrates are considered as promising substrate materials for the UV light emitters and detectors. Electrically insulating thick AlN layers may serve as the substrates for AlGaN/GaN-based high power high electron mobility transistors (HEMTs). In this paper we report on crack-free up to 20 µm thick AlN layers grown by stress control HVPE on 2-inch sapphire substrates. As-grown surface had a characteristic pyramidal morphology. Being t hick enough, AlN layers can be polished to improve surface roughness. The minimum full width at half maximum (FWHM) values of AlN ω-scan x-ray (00.2) and (10.2) rocking curves was about 500 and 1000 arcsec, respectively. The XRD analysis was applied for the threading dislocation density evaluation in grown AlN layer. Screw dislocation density was found to be (3-7)x10 8 cm -2 for the layers from 3 to 12 µm thick. INTRODUCTION Hydride vapour phase epitaxy (HVPE) is known to produce low defect GaN substrate materials for electronic and optoelectronic GaN-based devices. High quality HVPE grown GaN layers and free-standing material have been recently demonstrated by several research teams as the substrates for GaN-based power devices [1], high-frequency transistors [2], and blue lasers [3]. However, GaN is not optically transparent in a UV spectral region for wavelengths shorter than 360 nm and wider band gap material are needed for deep UV applications. Due to large band gap (6.2 eV), high thermal conductivity (3.3 W/cm-K) and close thermal and lattice match to AlGaN layers, AlN is promising substrate material for advanced high power UV emitting devices (LEDs and LDs), UV detectors, ultra high power high frequency electronic devices (AlGaN/GaN HEMTs). However, bulk AlN substrates of required size (2-inch diameter and larger) are not available. One way to get over this issue is to fabricate AlN template substrates comprising thick low defect AlN layer grown on a foreign substrate. The main technical challenge here is to minimize stresses in such template substrate and to avoid cracking of the AlN layer. Recently we have reported on stress control HVPE technology to grow thick (>50µm) crack-free AlN layers on SiC substrates [4]. In this paper we report on crack-free AlN layers up to 20 µm thick grown by HVPE on 2-inch sapphire substrates. Results of 3-15 µm thick layers characterization are discussed.