No abstract

2 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 impact ionization of avalanche breakdown requires a high applied electric field in a long active region, hampering carrier-multiplication with 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 5 /black phosphorus (BP) 6-9 heterostructures 10 . We use these heterojunctions to fabricate avalanche photodetectors (APD) and impact ionization transistors with sensitive mid-IR light detection (4 μm wavelength) and steep subthreshold swing (SS) (<0.25 mV/dec). The devices show a low avalanche threshold (<1 volt), 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.The upper panel of Fig. 1a shows the schematic of our heterostructure device with electrical connections. It consists of a thin γ-rhombohedral InSe/BP heterostructure connected to bottom and top metal electrodes on substrate. The detailed fabrication processes are presented in the Methods Section. During our measurements, we define the biased electrode connected to BP (~10 nm) as the drain and the grounded electrode connected to InSe (also ~10 nm) as the source. Considering that the lateral resistance of unstacked InSe is much smaller than that of the junction (Supplementary Section 1a), carriers also mainly transport vertically along the nanoscale InSe/BP channel. As a result, n-type InSe and p-type BP 13-15 form a vertical vdW heterojunction. The bottom panel of Fig. 1a 3 schematically shows the lattice structure at the junction interface. Notably, we assembled the InSe/BP junction in a glove box, resulting in a nearly perfect interface. In Fig. 1b, the high-resolution transmission electron microscope image verifies that the atomic stack is clean without the presence of any contamination or amorphous oxide even after all the device fabrication processes.We first characterized the transport properties of our heterostructure devices. With proper gate voltage 16 (10 V here, to ensure a necessary low doping level of BP and InSe, see below for details), the vertical vdW junction presents a standard rectification behaviour as a regular pn diode under moderate bias. In contrast, the reverse-biased current abruptly increases approximately 5 orders above a certain threshold voltage (~-4.8 V here), as shown in Fig. 1c. This "hard-knee" rapid change in current signals a typical avalanche breakdown 16, 17 resulting from impact ...