Modulation-doped Al x Ga 1- x N/GaN heterostructures with various thickness of Si-doped n-Al 0.22 Ga 0.78 N barrier (n-AlGaN) were deposited on (0001)-oriented sapphire (α-Al 2O 3) by MOCVD. The reciprocal space mappings (RSMs) of symmetric reflection (0002) and asymmetric reflection (1014) were measured with the methods of high resolution X-ray diffraction (HRXRD). The results indicate that the microstructure and strain status of barrier correlate to those of the i-GaN layer. The strained barrier starts to be relaxed while its thickness is larger than 750 and the critical thickness t c is larger than 500. The barrier holds an "abnormal" relaxation status
which probably results from the internal defects of n-AlGaN and the strain relaxation status at the i-GaN/α-Al 2O 3 interfaces.