Fabrication and Characterization of GaN/AlN Resonant Tunneling Diodes

This chapter reviews our recent efforts on growth, fabrication, and characterization of GaN/AlN resonant tunneling diodes (RTDs). Working GaN/AlN RTDs were successfully demonstrated, and they could function well under the flux of very high current densities (e.g., ∼431 kA/cm2) without thermal breakdown. The high-speed nature of these devices was confirmed through switching experiments, achieving a 10–90% switching time of ≈55 ps. A fmax calculation shows a small-signal oscillation with frequency up to 164 GHz is possible. Unlike InGaAs/AlAs RTDs, the peak-to-valley current ratios (PVCRs) of GaN/AlN RTDs remain ∼1.5. Through computer modeling, temperature measurements, and material diagnosis, we reveal that there could be stronger inelastic scattering processes contributing to the valley current other than the coherent tunneling in the GaN/AlN RTDs. The possible inelastic mechanisms include optical phonons, interface roughness, and dislocations. Thus, the growth of high-quality GaN/AlN heterostructures and the evolution of bulk GaN substrates are critical for getting better performance devices. Finally, unipolar electroluminescence, without the presence of p-type doping, was observed in GaN/AlN RTDs. The interband tunneling process, which generates holes for the optical recombination, is likely due to the strong electric fields originating from the polarization effects native to wurtzite heterostructures.