Correlation between transport, optical and structural properties in AlGaN/GaN heterostructures

Unintentionally doped AlGaN/GaN heterostructures were grown by plasma-assisted molecular beam epitaxy on GaN/Al2O3 templates. An optical, structural, and electrical characterization has been performed to determine and correlate the main factors that limit the low temperature mobility of the 2DEG. Hall effect measurements reveal a high electron density as well as differences in the Hall mobility between samples. Cross-section transmission electron microscopy images exhibit V-shaped pits in the AlGaN top layer of all samples, linked to threading dislocations. Results from cathodoluminescence suggest that the V-shaped pits and the regions close around may have different Al composition than the regions between pits. The AlGaN low-temperature photoluminescence spectra is dominated by nothing donor–acceptor pairs emission in samples having the lowest mobility. The enhanced electric field by the 2DEG charge in samples with high donor–acceptor densities, together with the alloy disorder and surface roughness arising from the pit regions, are most likely the dominant factors that limit the mobility to very low values.