Rutherford backscattering and channeling have been used to characterize the structure of an epitaxial GaN layer grown by metal-organic chemical vapor deposition on a Si(111) substrate. Using the channeling angular scan around an off-normal <1<(2)over bar>13> axis in the {10 (1) over bar0} plane of the GaN layer, the tetragonal distortion e(T), which is caused by the elastic strain in the epilayer, was determined. Moreover, the depth dependence of the e(T) was obtained by this technique. Our results show that the e(T) of the GaN epilayer is drastically reduced from 0.41% to 0.22% by inserting a 20 nm AlN interlayer. A fully relaxed (e(T)=0) GaN layer with a thickness of <1.5 mu m is expected. (C) 2014 Elsevier B.V. All rights reserved.