We have grown GaN on porous SiC substrates and studied the effect of
substrate porosity on the overgrown film quality in terms of defect structure and density
and film strain. The growth was performed by plasma-assisted molecular beam epitaxy
(PAMBE). The GaN films were characterized by x-ray, transmission electron microscopy
(TEM) and wafer curvature measurements by surface profilometry. TEM images show
that the GaN film grown on porous substrates contains open tubes and a low dislocation
density in regions between tubes. We discuss various growth mechanisms that can lead to
these defect features in the GaN film. However, we do not find any overall improvement
in the x-ray rocking curve FWHM of the GaN films grown on porous substrates
compared to those on nonporous substrates. It was found that the GaN films grown on
porous SiC were significantly more strain relaxed compared to those grown on nonporous
substrate. We propose various mechanisms that can lead to the reduction in strain in GaN
films grown on porous substrates a