  Thanks to its high refractive index contrast, band gap and polarization
mismatch compared to GaN, In0.17Al0.83N layers lattice-matched to GaN are an
attractive solution for applications such as distributed Bragg reflectors,
ultraviolet light-emitting diodes, or high electron mobility transistors. In
order to study the structural degradation mechanism of InAlN layers with
increasing thickness, we performed metalorganic vapor phase epitaxy of InAlN
layers of thicknesses ranging from 2 to 500 nm, on free-standing (0001) GaN
substrates with a low density of threading dislocations, for In compositions of
13.5% (layers under tensile strain), and 19.7% (layers under compressive
strain). In both cases, a surface morphology with hillocks is initially
observed, followed by the appearance of V-defects. We propose that those
hillocks arise due to kinetic roughening, and that V-defects subsequently
appear beyond a critical hillock size. It is seen that the critical thickness
for the appearance of V-defects increases together with the surface diffusion
length either by increasing the temperature or the In flux because of a
surfactant effect. In thick InAlN layers, a better (worse) In incorporation
occurring on the concave (convex) shape surfaces of the V-defects is observed
leading to a top phase-separated InAlN layer lying on the initial homogeneous
InAlN layer after V-defects coalescence. It is suggested that similar
mechanisms could be responsible for the degradation of thick InGaN layers.
