Negative Differential Conductance and Electron Interference Effects in GaN/AlN Resonant Tunneling Diodes with Metallic Collector

Resonant tunneling transport in semiconductor heterostructures that exhibit broken inversion symmetry due to spontaneous and piezoelectric polarization fields has been under scrutiny over the last few years [PRX 7, 041017 (2017)]. Engineering this quantum transport regime in polar III-Nitride semiconductors is of high interest for manufacturing ultra-fast electronic oscillators and intersubband lasers. Towards this goal, we have recently elucidated the implications of the broken inversion symmetry on the quantum transport characteristics of GaN/AlN resonant tunneling diodes (RTDs) [PRApplied 11, 034032 (2019)]. This new understanding on broken-symmetry effects has prompted us to study the possibility of recovering symmetric resonant tunneling injection using a metallic collector to screen polarization charges [PRApplied 13, 034048 (2020)]. Here, we report the transport characteristics of such RTD structure in which the n-type GaN collector layer is replaced with the Ti/Au/Ni metallic stack. Thanks to the exponentially enhanced resonant tunneling transmission of carriers, this novel heterostructure design enables the injection of electrons directly from the metallic collector into the wide-bandgap semiconductor. Repeatable room-temperature negative differential conductance is measured in multiple devices, attesting to the high degree of electronic coherence. Temperaturature-dependent resonant tunneling spectroscopy of the polar heterostructure is also reported between 4.2K and 300K. Moreover, the robustness of this quantum interference phenomenon is verified by the generation of self-sustained microwave oscillations with a fundamental frequency of 0.22 GHz. These results raise hopes for the development of fully-epitaxial resonant tunneling structures capable of seamlessly interfacing III-Nitride semiconductor devices with strongly correlated materials, including superconductors and ferromagnets, thereby enhancing Andreev reflection and spin injection, respectively.