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GaN single crystals are used as substrates for homo-epitaxial growth by MOCVD. Prior to growth, the N-face, or (0 0 0 1 ) plane, of the substrate crystals is polished to obtain off-angle orientations of 0, 2, and 4° towards the [1 1 2 0] direction. The hillock density of the homo-epitaxial films grown on the misoriented substrates is decreased as compared with the layers grown on the exact N-face. However, in addition to the hillocks, triangular-shaped pits are formed on the films grown on the misoriented substrates. The formation of the triangular-shaped pits is described by the blocking of the anisotropic step-flow growth.

We report results of a photoluminescence (PL) study of homoepitaxial N-polar GaN films grown by metal-organic chemical vapour deposition on vicinal (0 0 0 1 ) GaN single crystal substrates. Off-angles of 2° and 4° towards the [1 1 2 0] direction as well as 4° in the [1 0 1 0] direction were investigated. Along with a remarkable improvement of the epilayer morphology, a significant reduction of the unintentional/intrinsic donor concentration is achieved for all considered misorientations. As a consequence, PL spectra with narrow bound and free excitonic lines were observed. The misorientation of 4° towards the [1 1 2 0] direction results in an N-polar epilayer of the best optical quality.

In this communication two defect-selective etching methods for GaN are evaluated and critically compared: (i) orthodox etching in molten bases (KOH}NaOH eutectic denoted E) and in hot H 2 SO 4 /H 3 PO 4 acids (denoted HH etching) and (ii) photoelectrochemical (PEC) etching in aqueous KOH solutions. The parameters of etching are given for di!erent type of materials, i.e. bulk crystals and epitaxial layers, for both Ga- and N-polar M 0001 Nsurfaces. It is shown that molten bases are e!ective in revealing nano-pipes, inversion domains (IDs) and some dislocations, but the optimal etching parameters depend on the type of material, type and density of defects and polarity. Both orthodox etchants that result in the formation of pits on dislocations are also suitable for revealing micro-defects in heavily Mg-doped GaN single crystals but instead of pits, protruding etch features are formed. The reliability and limitations of these etching methods in revealing defects are demonstrated by means of transmission electron microscopy (TEM) calibration and by the selective formation of etch pits on dislocations introduced by indentation. Comparison with PEC etching, newly developed for GaN, is brie#y discussed. ( 2000 Published by Elsevier Science B.V. All rights reserved. PACS: 61.72.Ff

It is shown that electron diffraction patterns taken from small areas (typically 4–20 nm) of GaN cross-section TEM specimens allow unequivocal determination of the local polarity. As experimental data we used a set of diffraction patterns from an area with the same polarity but with an increasing thickness due to the wedge-shaped morphology of the TEM specimen. The polarity is determined using a least-squares refinement procedure in which the local misorientation and specimen thickness is refined for the two possible polarities. The refinements show that one polarity has much better agreement between calculated and experimental intensities of the reflections for all diffraction patterns of the set, allowing straightforward identification of polarity.

GaN single crystals were used as substrates for MOCVD growth. The ( 0 0 0 1 ) plane of the substrate crystals was polished to obtain off-angle orientations of 0, 2, and 4° towards the [ 1 0 1 0 ] direction. The highest misorientation resulted in a reduction of the hexagonal hillock density by nearly two orders of magnitude as compared with homo-epitaxial films grown on the exact ( 0 0 0 1 ) surface. The features that are still found on the 4° off-angle sample after growth can be explained by a model involving the interaction of steps, introduced by the misorientation, and the hexagonal hillocks during the growth process. Following from this explanation it could be concluded that surface diffusion is found to be not important during growth on the N-side. The material quality of the N-side was examined by photoluminescence (PL) measurements. The PL spectrum measured at 5 K shows dominant donor bound excitons with a FWHM of 1.4 meV as well as free excitonic transitions.

MOCVD-grown GaN on the N-polar surface of GaN substrates has been found to exhibit gross hexagonal pyramidal features (typically 10}50 lm in size depending on layer thickness). The evolution of the pyramidal defects is dominated by the growth rate of an emergent core of inversion domain (typically 100 nm in size). The inversion domains nucleate at a thin band of oxygen containing amorphous material (2}5 nm in thickness), being remnant contamination from the mechano-chemical polishing technique used to prepare the substrates prior to growth. Apart from pyramidal hillocks, the #at-topped hillocks are also formed. The arguments are presented on the association between these features and the core dislocations, which constitute the source of the growth steps. Improvement in the substrate polishing procedures allowed the e!ective elimination of these surface hillocks. ( 1999 Elsevier Science B.V. All rights reserved.