Observation of ballistic avalanche phenomena in nanoscale vertical InSe/BP heterostructures

Impact ionization, which supports carrier multiplication, is promising for applications in single photon detection(1) and sharp threshold swing field effect devices(2). However, initiating the impact ionization of avalanche breakdown requires a high applied electric field in a long active region, which hampers carrier multiplication with a high gain, low bias and superior noise performance(3,4). Here we report the observation of ballistic avalanche phenomena in sub-mean free path (MFP) scaled vertical InSe/black phosphorus (BP)(5-9) heterostructures(10). We use these heterojunctions to fabricate avalanche photo-detectors (APDs) with a sensitive mid-infrared light detection (4 mu m wavelength) and impact ionization transistors with a steep subthreshold swing (<0.25 mV dec(-1)). The devices show a low avalanche threshold (<1 V), low noise figure and distinctive density spectral shape. Our transport measurements suggest that the breakdown originates from a ballistic avalanche phenomenon, where the sub-MFP BP channel support the lattice impact ionization by electrons and holes and the abrupt current amplification without scattering from the obstacles in a deterministic nature. Our results provide new strategies for the development of advanced photodetectors(1,11,12) via efficient carrier manipulation at the nanoscale.