Relaxation Processes of AlGaN/GaN Heterostructures Grown onto Single Crystal GaN(0001) Substrates

Abstract

We study the growth and relaxation mechanisms of AlxGa1—xN/GaN heterostructures deposited by molecular beam epitaxy (MBE) onto GaN single crystals. We use a combination of transmission electron microscopy and finite element calculations (3D FE) to analyze the structure, the defects and the strained state. We compare heterostructures with 10%, 20% and 30% of Al (xAl = 0.1, 0.2, 0.3) corresponding to low misfits of 0.25%, 0.5% and 0.75%, respectively. This provides us with specimens that are dislocation free or dislocated above a certain critical thickness, to study the interplay between elastic (relaxation in laterally limited structures such as sinusoidal undulations and islands) and plastic strain relaxation. The heterostructures with Al contents xAl = 0.2 and 0.3 become unstable against undulation formation. The undulation amplitude A increases with increasing layer thickness by conserving the wavelength λ of roughly 200 nm. At an aspect ratio A/ λ ≈︂ 1/20 plastic relaxation by dislocation formation at the undulation valleys sets in. 3D FE calaculations yield a maximum elastic strain relaxation of ≈︂15%. The inhomogeneous strain distribution in the undulated AlGaN layer induces an inhomogeneous strain distribution 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.