Unlocking High-Performance, Ultra-Low Power van der Waals Photo-Transistors: Toward Back-End-of-Line in-Sensor Machine Vision Applications.

Recent reports on machine learning and machine vision (MV) devices have demonstrated the potential of two-dimensional (2D) materials and devices. Yet, scalable 2D devices are being challenged by contact resistance and Fermi level pinning (FLP), power consumption, and low-cost CMOS compatible lithography processes. To enable CMOS + 2D, it is essential to find a proper lithography strategy that can fulfill these requirements. Here, we explored a modified van der Waals (vdW) deposition lithography and demonstrated a relatively new class of van der Waals field effect transistors (vdW-FETs) based on 2D materials. This lithography strategy enabled us to unlock high-performance devices evident by high current on-off ratio (Ion/Ioff), high turn-on current density (Ion), and weak FLP. We utilized this approach to demonstrate a gate-tunable near-ideal diode using a MoS2/WSe2 heterojunction with an ideality factor of ∼1.65 and current rectification of 102. We finally demonstrated a highly sensitive, scalable, and ultralow power phototransistor using a MoS2/WSe2 vdW-FET for back-end-of-line integration. Our phototransistor exhibited the highest gate-tunable photoresponsivity achieved to date for white light detection with ultralow power dissipation, enabling ultrasensitive optoelectronic applications such as in-sensor MV. Our approach showed the great potential of modified vdW deposition lithography for back-end-of-line CMOS + 2D applications.

Lattice Transformation from 2D to Quasi 1D and Phonon Properties of Exfoliated ZrS2 and ZrSe2.

Recent reports on thermal and thermoelectric properties of emerging 2D materials have shown promising results. Among these materials are Zirconium-based chalcogenides such as zirconium disulfide (ZrS2 ), zirconium diselenide (ZrSe2 ), zirconium trisulfide (ZrS3 ), and zirconium triselenide (ZrSe3 ). Here, the thermal properties of these materials are investigated using confocal Raman spectroscopy. Two different and distinctive Raman signatures of exfoliated ZrX2 (where X = S or Se) are observed. For 2D-ZrX2 , Raman modes are in alignment with those reported in literature. However, for quasi 1D-ZrX2 , Raman modes are identical to exfoliated ZrX3 nanosheets, indicating a major lattice transformation from 2D to quasi-1D. Raman temperature dependence for ZrX2 are also measured. Most Raman modes exhibit a linear downshift dependence with increasing temperature. However, for 2D-ZrS2 , a blueshift for A1g mode is detected with increasing temperature. Finally, phonon dynamics under optical heating for ZrX2 are measured. Based on these measurements, the calculated thermal conductivity and the interfacial thermal conductance indicate lower interfacial thermal conductance for quasi 1D-ZrX2 compared to 2D-ZrX2 , which can be attributed to the phonon confinement in 1D. The results demonstrate exceptional thermal properties for Zirconium-based materials, making them ideal for thermoelectric device applications and future thermal management strategies.