Negative differential resistance in dislocation-free GaN∕AlGaN double-barrier diodes grown on bulk GaN

Double-barrier GaN resonant tunneling diodes with AlGaN barriers were fabricated on bulk (0001) single-crystal GaN substrates. Layers were grown using molecular-beam epitaxy with a rf plasma nitrogen source. Single diodes of $6\mu \mathrm{m}$ diameter were prepared by inductively coupled plasma reactive ion etching. For many diodes clear negative differential resistance is observed around $2\mathrm{V}$ with peak currents around $10\mathrm{kA}∕{\mathrm{cm}}^2$ and a peak-to-valley ratio of about 2 at room temperature. Its observation does not depend on specific conditions of measurement; however, it slowly decays after each measurement. The mechanism behind this decay is investigated since it is obviously prohibiting the usage of GaN resonant tunneling diodes so far. It is shown not to be caused by catastrophic breakdown of the devices.