Transport properties of heavily Si doped high Al mole fraction Al_xGa_1−xN grown by MBE on single-crystal AlN substrates

Controlled n-type doping in ultrawide bandgap AlGaN alloys is critical for power electronics, deep UV photonics applications. While the bandgap can be increased by increasing Al, a sharp increase in resistivity at above ∼80% Al has been reported in Si doped AlGaN, though the exact mechanism remains unclear. In this study, Si-doped AlGaN films with a wide range of doping concentrations and Al% are grown by molecular beam epitaxy on low dislocation density, single crystal AlN substrates. Electronic transport of these films was studied to investigate the upper limits of both Al% and Si doping. Degenerate doping and high electron density were achieved in films with up to 85% Al. Above this, the dopant thermal activation energy sharply increased; electron thermal activation energies were measured in samples with Al% up to and including binary AlN, which exhibited activation energy of 200 meV. Several mechanisms to explain this sharp increase in dopant activation energy, including the stabilization of deep DX-centers; decreased permittivity from decreased polar optical phonon screening, are identified. Additionally, the self-compensation phenomenon at very high donor densities was investigated. By minimizing the impacts of foreign impurities and native defect complexes, carrier densities of 7.6 x 10¹⁹ cm⁻³ several times higher than maximum reported in similar MBE, MOCVD grown films are achieved. Furthermore, the degenerate doping is pushed to higher Al of up to 85%. This work demonstrates that AlGaN layers with >5.5eV bandgaps can be made highly conductive for electron transport in optoelectronic devices.