Strain distribution in GaN/AlN superlattices grown on AlN/sapphire templates: comparison of X-ray diffraction and photoluminescence studies

A series of GaN/AlN superlattices (SLs) with various periods and the same thicknesses of GaN quantum wells and AlN barriers has been investigated. X-ray diffraction, photoluminescence (PL) and transmission electron microscopy (TEM) techniques are used to study the influence of thickness of AlN and GaN sublayers on strain distribution in GaN/AlN SL structures. Detailed X-ray diffraction measurements demonstrate that the strain occurring in SLs generally decreases with an increase of well/barrier thickness. Fitting of X-ray diffraction curves allowed the real thicknesses of the GaN wells and AlN barriers to be determined. Since blurring of the interfaces causes deviation of calculated data from experimental results, the quality of the interfaces has been evaluated as well and compared with results of TEM measurements. For the samples with thinner wells/barriers, the presence of pinholes and threading dislocations has been observed in TEM measurements. The best quality of interfaces has been found for the sample with a well/barrier thickness of 3 nm. Finally, PL spectra show that due to the quantum-confined Stark effect the PL peak energies of the SLs decreased with increasing the width of the GaN quantum wells and AlN barriers. The effect is well modeled by ab initio calculations based on density functional theory applied for tetragonally strained structures of the same geometry using a full tensorial representation of the strain in the SLs.