Assessment of layer composition and thickness in AlGaN/GaN HEMT structures by spectroscopic ellipsometry

AlGaN/GaN heterostructures for power applications, grown on sapphire substrates by metal organic vapor phase epitaxy (MOVPE) in a multiwafer reactor were analyzed by spectroscopic ellipsometry (SE). The thickness of the individual layers as well as the AlGaN bandgap energy were extracted from pseudodi-electric function spectra (epsilon) for photon energies ranging from 2 to 5 eV. The experimental (epsilon) spectra were analyzed using a multilayer approach, describing the dielectric functions of the individual layers by a parametric oscillator model. The potential of the technique will be demonstrated using on-wafer homogeneity as well as wafer-to-wafer variation within the same growth run in the MOVPE reactor as examples. SE was found to be capable of detecting relative variations in the Al content of 1% and the AlGaN layer thickness of 10%.