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.

Using free standing layer of InGaAsN we have succeeded in measuring the optical absorption in very broad spectral range (0.8-2.5eV). This gave us an insight into the conduction band density of states for the energies higher than the energy gap of this compound. By combining Hall effect measurements with determination of plasma edge frequency in infrared reflectivity for differently doped samples we were able to deduce the density of states, conduction band electrons effective mass and dispersion relation. In particular it turned out that both i) experimentally measured dispersion relation of the conduction band shows extremely high degree of nonparabolicity and consistently ii) the effective mass of electrons is few times larger than that corresponding to GaAs of the same electron concentration. So far the obtained experimental results are in line with recently proposed band anticrossing model of the electronic structure of III-N-V alloys.

Compacts of composites SiC-diamond were made by infiltration of Si into nanocrystalline diamond powders in a toroid-type press under the pressure of 7.7 GPa at 1300 °C. In-situ high pressure diffraction studies of these processes were performed in MAX80 cubic anvil press at a pressure of 8.5 GPa in temperatures up to 1800°C in HASYLAB at DESY, Hamburg, Germany. Sintering was performed for (i) pure nanocrystalline diamond powders, (ii) a mixture of nanocrystalline powders of diamond and nanocrystalline SiC, (iii) a mixture of nanocrystalline diamond with microcrystalline Si powders and (iv) compacts of nanocrystalline diamond infiltrated by Si. The SiC-diamond composites obtained by infiltration of Si have best physical properties: hardness similar to conventional diamond compacts (approximately 50 GPa), highest density 3.35 g cm-3 and uniform nanocrystalline microstructure.