MBE grown GaN/AlGaN lateral Schottky barrier diodes for high frequency applications

The authors report the results of studies of lateral and vertical Schottky barrier diodes (SBD) based on molecular beam epitaxy (MBE) grown GaN/AlGaN heterostructures with two dimensional electron gas (2DEG) present at the heterostructure interface. The epilayers were grown by plasma assisted molecular beam epitaxy under metal-rich conditions on commercially available freestanding (0001) GaN substrates. Here, the authors present detailed results for two structures with electron sheet density (N_2DEG) of 4.6 × 10¹² and 1 × 10¹³cm⁻² and room temperature mobility of μ_2DEG = 1925 cm²/V s and μ_2DEG = 1760 cm²/V s, respectively. The processing of lateral and vertical Schottky barrier diodes was performed by laser writer using shallow mesas and Ni/Au metallization for Schottky barriers. The direct current electrical tests performed on lateral and vertical Schottky barrier diodes yielded a turn-on voltage of 0.6 and 1 V, respectively. For both cases, the reverse current registered in the experiment was very low and did not exceed 1 μA/mm for reverse voltage V_R = −5 V while the breakdown voltage for both structures was approximately V_B = −90 V, which is a good result for nonpassivated surface and anode/cathode distance of the order of a few micrometers. The comparison of current–voltage characteristics for lateral devices with different electron sheet densities shows significant disparity in differential resistances at on-state, which is attributed to the increase of ohmic contact resistance with aluminum mole fraction increase in the AlGaN epilayer. Lateral Schottky diodes demonstrated the low frequency noise amplitude comparable with that reported for regular GaN-based Schottky diodes of much larger area. Low frequency noise characterization on our SDBs is presented. Additionally, the authors present as an example our first result of 140 GHz radiation detection/rectification experiment done on the SBD grown by MBE on (0001) GaN/sapphire. The similar experiments on the SBD freestanding samples will be the subject of forthcoming studies.