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

Abstract: Initiating impact ionization of avalanche breakdown essentially requires applying a high electric field in a long active region, hampering carrier-multiplication with high gain, low bias and superior noise performance. Here we report the observation of ballistic avalanche phenomena in sub-MFP scaled vertical indium selenide (InSe)/black phosphorus (BP) heterostructures. The heterojunction is engineered to avalanche photodetectors (APD) and impact ionization transistors, demonstrating ultra-sensitive mid-IR light detection (4 {\mu}m wavelength) and ultra-steep subthreshold swing, respectively. These devices show an extremely low avalanche threshold (<1 volt), excellent low noise figures and distinctive density spectral shape. Further transport measurement evidences the breakdown originals from a ballistic avalanche phenomenon, where the sub-MFP BP channel enables both electrons and holes to impact-ionize the lattice and abruptly amplify the current without scattering from the obstacles in a deterministic nature. Our results shed light on the development of advanced photodetectors and efficiently facilitating carriers on the nanoscale.

Observation of Ballistic Avalanche Phenomena in Nanoscale Vertical InSe/BP Heterostructures

The paper under consideration reports a notable investigation into the ballistic avalanche phenomena within nanoscale vertical indium selenide/black phosphorus (InSe/BP) heterostructures. The research team successfully observed and characterized avalanche breakdown in these vertical van der Waals (vdW) heterojunctions, distinctively contributing to the development of advanced avalanche photodetectors (APDs) and impact ionization MOSFETs (IMOS).

Key Findings

The study documents several salient features associated with the ballistic avalanche breakdown:

1. Sub-MFP Scalability with Low Threshold Voltage: The novel heterostructures exhibit avalanche phenomena at voltages lower than 1 V, which is significantly reduced compared to the typical requirement in conventional avalanche devices. This is attributed to the sub-mean free path (MFP) channel length, which facilitates ballistic transport.
2. Low Noise Figures and Distinctive Noise Spectra: The InSe/BP devices demonstrated reduced noise levels, characterized by a unique 1/f noise spectrum deviating from the white noise typically associated with conventional APDs. This suggests a deterministic nature of the ballistic avalanche process.
3. Robustness and Hysteresis-Free Operation: The heterostructures maintain operational stability and repeatability over multiple cycles, exhibiting negligible hysteresis (<0.03 V). This property is particularly advantageous for applications in photonics and electronics requiring precise control.
4. Ballistic Avalanche Mechanism: The underlying ballistic avalanche mechanism is theorized to result from a deterministic sequence of carrier impact ionization facilitated by the exceedingly small channel dimensions. The symmetric band structure of BP enhances the equal ionization likelihood of electrons and holes, a pivotal factor in this deterministic process.

Practical Implications

The mechanical robustness and intrinsic noise properties of the studied heterostructures offer them as promising platforms for next-generation APDs, particularly in mid-infrared (mid-IR) light detection applications. The observed high multiplication factors and low noise profiles imply potential use in quantum information systems, where signal integrity is critical.

IMOS devices crafted from these heterostructures show unprecedented subthreshold swing (SS) values (<0.25 mV/dec), outperforming the traditional limitation of 60 mV/dec at room temperature. This indicates their applicability in energy-efficient transistors for future electronics.

Theoretical Significance and Future Directions

The clarification of a ballistic avalanche mode contributes fundamentally to the theoretical understanding of nanoscale impact ionization and charge transport dynamics. This mode, underpinned by ballistic transport regimes, draws a stark differentiation from conventionally understood random scattering-induced avalanche processes.

Future research should focus on detailed quantum transport analysis to deepen our understanding of the 3D ballistic transport behavior and its implications. Additionally, exploring heterostructures with alternative 2D material combinations could further diversify the applications and efficiency profiles of such devices.

The paper provides a significant advancement in the field of quantum electronics, reinforcing the importance of understanding carrier dynamics at the nanoscale. By leveraging the ballistic avalanche effect, it opens new avenues for device innovation in photodetector and transistor technology.