Pronounced Photovoltaic Response from Multi-layered MoTe2 Phototransistor with Asymmetric Contact Form

Electric Tuning of Vortex Ratchet Effect in NbSe2

Inversion symmetry breaking has played an important role in recent discoveries of nonreciprocal charge transport. Niobium diselenide, for example, lacks an inversion center in the monolayer form and can host prominent nonreciprocal transport property. Here, however, we observe a nonreciprocal transport signal in the second-harmonic channel of bulk-like NbSe2, in which inversion symmetry of the lattice seems preserved. The second-harmonic signal occurs along different in-plane current orientations and appears not only in the vortex-liquid regime but also even in the superconducting fluctuation regime without an applied magnetic field. By adding a direct current (DC) bias, we quantify the symmetry breaking effect in the vortex-liquid regime. The DC bias also suggests that the rectification effect at the contacts may account for the seemingly nonreciprocal transport at zero magnetic field. Our results demonstrate that DC biasing is a useful knob for addressing nonreciprocal charge transport in a wide range of materials.

Escalated Photocurrent with Excitation Energy in Dual-Gated MoTe2

Although van der Waals-layered transition metal dichalcogenides from transient absorption spectroscopy have successfully demonstrated an ideal carrier multiplication (CM) performance with an onset of nearly 2E_g, interpretation of the CM effect from the optical approach remains unresolved owing to the complexity of many-body electron-hole pairs. We demonstrate the escalated photocurrent with excitation photon energy by fabricating the dual-gate p-n junction of a MoTe₂ film on a transparent substrate. Electrons and holes were efficiently extracted by eliminating the Schottky barriers in the metal contact and minimizing multiple reflections. The photocurrent was elevated proportionately to the excitation photon energy. The boosted quantum efficiency confirms the multiple electron-hole pair generation of >2E_g, consistent with CM results from an optical approach, pushing the solar cell efficiency beyond the Shockley-Queisser limit.

Breaking symmetry in device design for self-driven 2D material based photodetectors

The advent of graphene and other two-dimensional (2D) materials offers great potential for optoelectronic applications. Various device structures and novel mechanisms have been proposed to realize photodetectors with unique detecting properties. In this minireview, we focus on a self-driven photodetector that has great potential for low-power or even powerless operation required in the internet of things and wearable electronics. To address the general principle of self-driven properties, we propose and elaborate the concept of symmetry breaking in 2D material based self-driven photodetectors. We discuss various mechanisms of breaking symmetry for self-driven photodetectors, including asymmetrical contact engineering, field-induced asymmetry, PN homojunctions, and PN heterostructures. Typical device examples based on these mechanisms are reviewed and compared. The performance of current self-driven photodetectors is critically assessed and future directions are discussed towards the target application fields.

MoTe2 van der Waals homojunction p-n diode with low resistance metal contacts

Although many studies have focused on transition metal dichalcogenide heterojunction p-n diodes, homojunction p-n diodes still require more extensive study. We present a van der Waals p-MoTe2/n-MoTe2 homojunction p-n diode with low resistance metal contacts. Such two-dimensional homojunction devices with low contact resistance can be used in various applications in the electronics industry. The device structure consists of stacked nanoflakes of p-MoTe2 and n-MoTe2. In this investigation, we implement a deep ultraviolet light-driven doping technique in a N2 gas environment to modulate the carrier concentration in a multilayered p-MoTe2 flake, which is consequently inverted to n-MoTe2. The deep ultraviolet light-driven doping provides environmental stability in the treated devices. We use ohmic metal contacts for the homojunction p-n diode and achieve excellent gate-dependent rectifying behavior with a rectification ratio of up to 104. Contrary to heterojunctions, the ideality factor is found to be 1.05 for the zero gate bias, indicative of good interface quality at the p-MoTe2/n-MoTe2 junction, owing to low charge trapping sites at the homojunction interface. In addition, low-temperature measurements are performed to determine the barrier height for different gate biases. This study contributes to research on van der Waals homojunction p-n diodes, which show much potential for nanoelectronic and optoelectronic devices.