In this communication, results are presented of the application of etching in molten E+M etch (KOH-NaOH eutectic mixture with 10% MgO) for studying defects in GaN. The method was used to study defects on differently oriented cleavage and basal planes of GaN single crystals, MOCVD-, MBE- and HVPE-grown epitaxial layers and LD and LED structures. Dislocations, dislocation loops and stacking faults have been revealed on $(10\, \bar{\text{\scriptsize 1}}\, 0)$, $(1\,\bar{\text{\scriptsize 2}}\,10)$ and $\{0001\}$ Ga- and N-polar planes. Diversified etch pit morphology was observed depending on the crystallographic orientation of the etched samples and was correlated with the crystallographic symmetry of the GaN lattice. Etching results were calibrated using TEM analysis.

Two approaches to defect-selective etching used for revealing and analysis of defects in GaN and SiC are described and critically evaluated. These are: (i) orthodox etching which results in formation of pits on the defect sites and (ii) electroless etching, which yields protruding etch features. The mechanisms of surface reactions that are responsible for the distinct differences in the morphology of defect-related etch features are discussed. The most frequently used etching systems for GaN and SiC and the methods of verification of their reliability in revealing different types of defects are described.

Heteroepitaxial GaN layers grown on sapphire by metal organic vapour phase epitaxy (MOVPE) have been characterised by conventional transmission electron microscopy (TEM) on planar and cross-sectional samples, Large Angle Convergent Beam Electron Diffraction (LACBED) and by high-resolution transmission electron microscopy (HRTEM). Hollow tubes termed nanopipes were resolved on planar view and cross-sections of heteroepitaxial GaN. For advanced studies of the nature of nanopipes the LACBED method was employed. The recognition between perfect structure and screw distortion around nanopipes was performed with high accuracy using Zone Axis LACBED images.

Photochemical (PEC) etching and transmission electron microscopy (TEM) have been used to study the defects in hetero-epitaxial GaN layers. TEM proved that PEC etching reveals not only dislocations but also nanopipes in the form of protruding, whisker-like etch features. It is shown by diffraction contrast techniques that the nanopipes are screw coreless dislocations. An example is shown of the transformation of a normal full-core screw dislocation into a nanopipe. The PEC/TEM experiments indicate the presence of electrically active (recombinative) species in the vicinity of the nanopipes.

We used single crystals of GaN, obtained from high-pressure synthesis, as substrates for Metalorganics Vapor Phase Epitaxy growth of violet and UV laser diodes. The use of high-quality bulk GaN leads to the decrease of the dislocation density to the low level of 105 cm−2, i.e. two orders of magnitude better than typical for the Epitaxial Lateral Overgrowth laser structures fabricated on sapphire. The low density and homogeneous distribution of defects in our structures enables the realization of broad stripe laser diodes. We demonstrate that our laser diodes, having 15 μm wide stripes, are able to emit 1.3-1.9 W per facet (50% reflectivity) in 30 ns long pulses. This result, which is among the best ever reported for nitride lasers, opens the path for the development of a new generation of high power laser diodes.

Impurity atmospheres around dislocations have been studied in n-type Si-doped liquid encapsulated Czochralski (LEC) GaAs substrates by micro-Raman spectroscopy, diluted Sirtl-like etching with light (DSL) method, and electron-beam-induced current (EBIC). A complete morphological study of the recombinative atmospheres revealed by photoetching was achieved by phase stepping microscopy (PSM), which is an optical interferometry technique allowing to obtain the surface topography with a high vertical resolution (in the nanometer range). The minority carrier diffusion length was measured by EBIC at different points of the atmospheres. Structural distortion at the regions surrounding the dislocation core were observed by micro-Raman spectroscopy. The carrier depletion depth and the recombination of the photogenerated carriers were also studied by Raman spectroscopy, obtaining a good agreement with the EBIC data and the photoetching rates. Impurity gettering and diffusion and defect reactions involving As interstitials are assumed to play a major role in the formation of the recombinative atmospheres.

The recent progress in high pressure crystallization of GaN is reported. The results of the growth from the solutions of atomic nitrogen in pure Ga and in its alloys with Mg, Ca and Zn are discussed. It is shown that the growth mechanisms and the physical properties of the crystals depend on the type of dopant added into the solution. In particular, high resistivity (104-106Ωcm) GaN crystals of improved structural quality can be grown from solutions containing Mg. It was also observed that the addition of Mg, Ca and Zn suppress yellow photoluminescence commonly observed in GaN crystals grown without an intentional doping. The preparation of surfaces of GaN substrates for homoepitaxy by mechanical and mechanochemical polishing is discussed. It is shown that atomically flat, thermally stable surfaces are possible to obtain by the applied procedures.

Some most interesting results concerning homoepitaxial growth by MOCVD and MBE is shortly reviewed. In particular, it is shown that perfectly matched (strain free) GaN layers can be deposited on the highly resistive GaN:Mg substrates.

Quantitative energy-dependent EBIC measurements have been used to calibrate the photoetching rate in HF-CrO3 aqueous solutions (DSL method: Diluted Sirtl-like etching with the use of Light) as a function of dopant concentration in LEC grown n-type GaAs samples containing growth striations. The relative DSL etching rate depends on the width of the surface depletion region associated with the semiconductor-etching solution interface, i. e. greater etch rates correspond to smaller dopant concentrations. These results are in agreement with the electrochemical model of GaAs etching in the DSL etching system.

The paper presents a systematic study of grown-in and process-induced defects on LEC GaAs substrates. Defects have been revealed by photoetching the wafers with diluted Sirti-like solutions after various processing steps. MESFET arrays have been processed on the wafers and a systematic mapping of the I-V characteristics has been performed. A correlation between various defect configurations and the FET threshold voltage shifts has been established.