Proximity effects of beryllium-doped GaN buffer layers on the electronic properties of epitaxial AlGaN/GaN heterostructures

AlGaN/GaN/Be:GaN heterostructures have been grown by rf-plasma molecular beam epitaxy on free-standing semi-insulating GaN substrates, employing unintentionally-doped (UID) GaN buffer layers with thicknesses, dUID, varying between 50 nm and 500 nm. We have found that the heterostructures with UID buffers thicker than 200 nm exhibit much improved Hall properties and inter-device isolation current compared to heterostructures with dUID < 200 nm. The output conductance of devices fabricated on these heterostructures increases as dUID decreases below 200 nm, and devices with gate lengths of 240 nm and 1 μm exhibited no significant difference in output conductance. Evidence of buffer trapping is observed in devices for which dUID ⩽ 100 nm. The observed effects are tentatively explained by the presence of parallel conduction paths in samples with non-optimized UID buffer thickness.