This review was based on a recent concerted effort to establish benchmarks as far as defects were concerned, and identify the basic issues involved. Samples were analyzed for extended defects by transmission electron microscopy and chemical etches, for polarity by electric force microscopy and convergent-beam electron diffraction, for point defects by deep level transient spectroscopy, for optical quality and deep defects by photoluminescence, for vacancies by positron annihilation, for donor- and acceptor-like states within the gap by ODMR and electron paramagnetic resonance, and for carrier transport targeted for defects and impurities by variable temperature and magnetic field-dependent Hall measurements. Hydride VPE samples of 1.5, 5.5 and 55μm thickness were investigated for defects by transmission electron microscopy, and their polarity was found to be Ga-polar, as expected, from convergent-beam electron diffraction combined with simulations. The density of misfit dislocations at the substrate/EPI interface was determined to be on the order of 1013/cm2 based on high-resolution electron microscopy images. The threading dislocation density decreased gradually with distance from the interface, reaching a value of about 108/cm2 at the surface of a 55μm film. A 200μm-thick laser separated and free-standing HVPE grown GaN template was also characterized similarly. The free surface and substrate sides were confirmed to be Ga- and N-polarity, respectively. The density of dislocations near to the N-face was determined to be, in order, 3 x 107 and 4 x 107 by cross-sectional transmission electron microscopy and plan-view transmission electron microscopy, respectively. Identical observations of the Ga-face revealed the defect concentration to be less than 107/cm2 by plan-view transmission electron microscopy and 5 x 105/cm2 by cross-sectional transmission electron microscopy.

Comprehensive Characterization of Hydride VPE Grown GaN Layers and Templates. H.Morkoç: Materials Science and Engineering R, 2001, 33[5-6], 135-207