Surface and subsurface structures of porous GaN prepared by anodizing epitaxial GaN layers grown on SiC substrates are investigated by atomic-force microscopy. Comparison of the images of the porous GaN surfaces with those taken on planes cleft perpendicular to the surface shows that the pores are formed along the boundaries of columnar structures of the original GaN films. X-ray investigations show that the porous GaN has less residual stresses than the initial GaN epitaxial layers. Use of porous GaN as a buffer layer for growth of low-stress GaN is proposed.

We have studied epitaxial GaN layers grown by hydride vapour phase epitaxy (HVPE) on porous GaN sublayers formed on SiC substrates. It was shown that these layers can be grown with good surface morphology and high crystalline quality. X-ray, Raman and photoluminescent (PL) measurements showed that the stress in the layers grown on porous GaN was reduced to 0.1-0.2 GPa, while the stress in the layers grown directly on 6H-SiC substrates remains at its usual level of about 1 GPa. Thus, we have shown that growth on porous GaN sublayer is a promising method for fabrication of high quality epitaxial layers of GaN with low strain values.

Epitaxial 4H–SiC layers were grown by chemical vapor deposition (CVD) on porous silicon carbide. Porous SiC substrates were fabricated by the formation of a 2 to 15 μm thick porous SiC layer on commercial off-axis 4H–SiC substrates. The thickness of CVD grown layers was about 2.5 μm. The concentration ${\mathrm{N}}_{\mathrm{d}}–{\mathrm{N}}_{\mathrm{a}}$ in the layers was about ${\text{7×10}}^{15} {\mathrm{cm}}^{\mathrm{-3}}.$ The layers were investigated for their surface roughness, crystal structure, deep level concentration, and minority carrier diffusion length. It was found that the characteristics of SiC epitaxial layers grown on porous SiC substrates were significantly improved compared to those of SiC layers grown on standard SiC substrates.

Raman scattering spectroscopy, scanning electron microscopy, and scanning acoustic microscopy were studied on porous $\mathrm{SiC}$ layers prepared by different technological routes and subjected to reactive ion treatment. The Raman spectra revealed a number of features specific for nanocrystallite materials, which can be used for characterization and diagnostics of porous $\mathrm{SiC}$ layers for technological applications.