(a) Illustration of the baseline device growth stack (Sample A). (b) HAADF-STEM image of similarly grown test structure. (c) Isometric drawing of the device prior to oxide and pad metal depositions. (d) Top–down image of the Ti/Au pad contact. (e) Photograph of 7 × 10 μm2 GaN RTD structure showing the three DC-coupled electrodes and the RTD mesa device under test (DUT). Strong violet light was observed emitting from the RTD structures under bias, but this was identified as the long-wavelength tail of a much stronger near-UV emission at approximately 360 nm.