Relaxation processes of AlGaN/GaN heterostructures grown onto single crystal GaN(0001) substrates

We study the growth and relaxation mechanisms of AlxGa1-xN/GaN heterostruct ures deposited by molecular beam epitaxy (MBE) onto GaN single crystals. We use a combination of transmission electron microscopy and finite element c alculations (3D FE) to analyze the structure, the defects and the strained state. We compare heterostructures with 10%, 20% and 30% of Al ( chi(Al) = 0.1, 0.2, 0.3) corresponding to low misfits of 0.25%, 0.5% and 0.75%, respe ctively This provides us with specimens that are dislocation free or disloc ated above a certain critical thickness, to study the interplay between ela stic (relaxation in laterally limited structures such as sinusoidal undulat ions and islands) and plastic strain relaxation. The heterostructures with Al contents chi(Al) = 0.2 and 0.3 become unstable against undulation format ion. The undulation amplitude A increases with increasing layer thickness b y conserving the wavelength lambda of roughly 200 nm. At an aspect ratio A/ lambda approximate to 1/20 plastic relaxation by dislocation formation at t he undulation valleys sets in. 3D FE calculations yield a maximum elastic s train relaxation of approximate to 15%. The inhomogeneous strain distributi on in the undulated AlGaN layer induces an inhomogeneous strain distributio n in the thin GaN interlayer also. Detailed strain analysis yields a rather unusual result for the GaN interlayers:the valleys and the ridges exhibit almost zero strain whereas the slopes of the undulation are in compression. This result can possibly be utilized in novel quantum structures.