Periodic composition modulations in InGaN epitaxial layers

Abstract

InGaN epitaxial layers grown by metalorganic chemical vapor deposition were investigated in order to understand the occurrence of composition modulations in the GaN–InN system. The In contents of the samples were determined to be x=0.21 and 0.31. Transmission electron microscopy was performed on [0001], [101̄0], and [112̄0] zone-axis specimens. Plan-view images display a domain structure, representing regions in which the directions of the modulations differ. Intersections between domains occur in 〈101̄0〉, and 〈112̄0〉, and other directions. Satellite spots appear in selected-area diffraction patterns. These observations can be explained by diffraction effects resulting from periodic composition modulations. An equation was derived relating the spacing between the satellites and the reflections to the wavelength of the modulations in the wurtzite structure. The sample with x=0.21 had a wavelength of λ=3.1±1.3 nm and the one with x=0.31 had λ=3.2±1.3 nm. Since Young’s modulus is isotropic in the (0001) plane, no particular direction is favored for the modulations based on strain energy considerations. This result is consistent with the observation of the variously oriented domains and satellites.