Optical constants and band gap of wurtzite Al1-xScxN/Al2O3 prepared by magnetron sputter epitaxy for scandium concentrations up to x=0.41

Wurtzite Al1-xScxN thin films with scandium Sc concentrations up to x=0.41 were prepared by reactive pulsed DC magnetron co-sputtering at heater temperatures between 300 degrees C and 400 degrees C on Al2O3 substrates. Spectroscopic ellipsometry was used to determine the dielectric functions of wurtzite Al1-xScxN by modeling the spectra with a three-layer model involving parametric oscillator functions for the Al1-xScxN layers. By combining ellipsometry with transmission spectroscopy, we determined the composition dependence of the optical band gap E-g. For x<0.25, the experimentally determined band gap follows the theoretical prediction with an offset, which arises from residual sub-band gap absorption. For higher Sc concentrations, for which the band gap is expected to become indirect, the experimental data deviate from the theoretical values for the direct band gap. No absorption that can be attributed to cubic ScN crystallites was observed up to x=0.41, indicating a high phase purity in line with X-ray diffraction data.