(Invited) X-Ray Metrology of AlN Single Crystal Substrates

Abstract

As development of GaN and SiC material systems for applications in power switching progresses, the ultrawide-bandgap (UWBG) materials, with bandgaps greater than GaN, are gaining increased attention, in pursuit of improved power device performance. Substrate technology is critical to improve this field. High-quality native substrates, possessing low lattice and thermal expansion mismatches to epitaxial active layers, enable epitaxial growth with low densities of extended defects, leading to improved device performance and lifetime. Lateral and vertical power switching devices have been demonstrated in the emerging AlGaN alloy system [1]. Aluminum nitride (AlN) possesses a close lattice match to high Al mole fraction AlGaN alloys and a high thermal conductivity, which make it an excellent substrate for UWBG AlGaN-based devices. Physical vapor transport (PVT) growth has emerged as the most promising technique for the production of large, high-quality AlN single crystal substrates. Seeded growth technology developed by HexaTech enables iterative expansion of single crystal size, while suppressing low angle grain boundary (LAGB) formation, a primary source of grown-in dislocations. X-ray diffraction (XRD) is well suited to accurately and non-destructively probe the extended defect structure, composition, and strain of single crystals and epitaxial films. A variety of x-ray techniques were used throughout AlN substrate processing, from evaluation of as-grown crystals, through substrate polishing, to analysis of epitaxial films grown on AlN. High-resolution x-ray rocking curves (HRXRRC) and x-ray topography (XRT) were used to determine the primary dislocation types in AlN bulk crystals and their likely formation mechanisms [2, 3]. Synchrotron-based XRT images demonstrated that the expanded diameter of freestanding AlN boules contained low densities of threading dislocations (<104 cm-3), and no basal plane dislocations or LAGBs [4]. Furthermore, XRT images acquired on a commercial diffractometer were used for quality control of AlN seed crystals. The advent of a new generation of x-ray area detectors, capable of a wide dynamic range and rapid data acquisition, opened the possibility for using XRT in production. HRXRRC and surface-sensitive reciprocal space maps were used to monitor sub-surface damage removal during substrate polishing [5]. These results, together with atomic force microscopy and defect selective etching images, demonstrated that chemo-mechanically polished AlN surfaces were free of plastic damage [4]. Polished AlN surfaces were also studied by x-ray reflectivity, yielding long-range data about surface roughness and density. Finally, epitaxial AlGaN films and heterostructures deposited on AlN substrates were studied by a combination of these x-ray techniques, providing essential information on composition, thickness, relaxation, and heterostructure design. XRD has become a valuable metrology tool in the development of AlN technology. This talk will review the implementation of complementary x-ray based measurements in the processing of AlN, from boule growth to AlGaN-based devices.