Overview

Gallium nitride films of increasing thickness have been grown on either AlN or AlGaN substrates. The state of stress of these biaxially stressed layers gradually changed from compression to tension with regard to both their average strain and their local strain along the [0001] growth direction. The components of both the compressive and tensile stresses are caused by the mismatch in lattice parameters between the GaN and the buffer layer and the mismatch in the coefficients of thermal expansion between GaN and SiC, respectively. The compressive stress is partially relieved within the first 20 nm in the GaN film grown on the AlN buffer layer. The relief of the remaining stress follows an exponential dependence on the thickness of the GaN layer with values for the characteristic decay length of 0.24 m and 0.64 m for the AlN and AlGaN buffer layer, respectively. The relaxation mechanism is discussed in terms of the formation of misfit dislocations via surface undulations.

Keywords: Strain; GaN; buffer layers.

1. Introduction

Silicon carbide has several advantages as a substrate for the growth of III-Nitride films and heterostructures, including moderately close lattice matching to AlN and GaN, higher thermal conductivity and relatively easy cleavage along [11 20].