Strain and stress are dominant features of nearly all epitaxial layers
and heterostructures made out of group-III nitrides. This is mainly for
the following three reasons: (1) the lattice mismatch between GaN, AlN
and InN, (2) the mismatch in the coefficients of thermal expansion
between the layers and the substrate, and (3) the large and
inhomogeneous density of extended defects in nitride layers. A
comprehensive picture of the strains in nitride heterostructures is
essential for the fabrication and optimization of most devices. In this
paper, we will discuss the three main sources of strain mentioned above
by means of individual examples. First, the strain variation in GaN
layers grown on SiC substrates is discussed with respect to the lattice
mismatch between the GaN and the Al(Ga)N buffer layer, the morphological
evolution during growth and the occurence of a phase separation in the
buffer layer. Second, the inhomogeneous stress and crystallographic
orientation in GaN layers grown by either pendeo-epitaxy on SiC or
cantilever-epitaxy on Si substrates are analyzed in view of the
thermally induced load after growth. Third, the application of SiN
interlayers to reduce the dislocation density in GaN layers and the
impact of these layers on the strain in GaN is presented.

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1. Characterization of Growth Defects in GaN Layers withX-ray Microbeam
2. Characterization of chrystallographic properties and defects via X-ray microdiffraction in GaN(0001) layers