Polarity effects on the growth of Si-doped AlGaN epilayers on AlN

We investigate the polarity dependent properties of Si-doped n-type AlGaN epilayers grown via metal organic chemical vapor deposition on Al- and N-polar AlN bulk substrates. While X-ray diffraction confirmed comparable crystalline quality for both polarities, photoluminescence spectroscopy revealed distinct optical signatures, suggesting varying levels of defect complexes involving aluminum vacancies and oxygen impurities. Electrical characterization via Hall-effect measurements showed a sharp contrast in transport properties. The N-polar epilayer exhibited a slight reduction in free electron concentration, but a significant decrease in electron mobility by over an order of magnitude compared to the Al-polar epilayer. Microstructural analysis through scanning transmission electron microscopy and energy-dispersive spectroscopy identified polarity-specific features at the AlGaN/AlN interface including increased lattice distortion and the presence of localized oxygen impurities in the N-polar samples, despite uniform Al and Ga distribution. These findings highlight substrate polarity as a key factor influencing the overall material quality and the performance of AlGaN-based devices, offering useful insights for optimizing n-type conductivity in polarity-engineered applications such as AlN and AlGaN-based vertical p-n diodes, vertical photoconductive semiconductor switch devices, and high electron mobility transistors.