A method for absolute evaluation of alloy composition and strain in AlGaN buffer layers in GaN/Si system is described, which can be applied to any III-nitride alloys hetero-epitaxially grown on Si (111) or sapphire (0001) substrate. Absolute measurements of reciprocal space maps of (10-14) nitride layers are performed with the help of Si (313) reciprocal lattice points. The alloy composition and strain-relaxation state in all layers are directly calculated from the absolute values of reciprocal lattice points referenced to the Si substrate. The proposed method of analysis using Si substrate is more accurate than currently favored method in which an epitaxial GaN layer is used as the reference. The knowledge of strain state and alloy composition with greater accuracy would be useful in designing and growing III-nitride devices.

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