Ultrafast Photocurrent Response and High Detectivity in Two-Dimensional MoSe2-based Heterojunctions.

Two-dimensional (2D) transition metal dichalcogenide (TMDC) materials have garnered great attention on account of their novel properties and potential to advance modern technology. Recent studies have demonstrated that TMDCs can be utilized to create high-performing heterostructures with combined functionality of the individual layers and new phenomena at these interfaces. Here, we report an ultrafast photoresponse within MoSe2-based heterostructures in which heavily p-doped WSe2 and MoS2 flakes share an undoped MoSe2 channel, allowing us to directly compare the optoelectronic properties of MoSe2-based heterojunctions with different 2D materials. Strong photocurrent signals have been observed in both MoSe2-WSe2 and MoSe2-MoS2 heterojunctions with a photoresponse time constant of ∼16 µs, surmounting previous MoSe2-based devices by three orders of magnitude. Further studies have shown that the fast response is independent of the integrated 2D materials (WSe2 or MoS2) but is likely attributed to the high carrier mobility of 260 cm2 V-1 s-1 in the undoped MoSe2 channel as well as the greatly reduced Schottky barriers and near absence of interface states at MoSe2-WSe2/MoS2 heterojunctions, which lead to reduced carrier transit time and thus short photocurrent response time. Lastly, a high detectivity on the order of ∼1014 Jones has been achieved in MoSe2-based heterojunctions, which supersedes current industry standards. These fundamental studies not only shed light on photocurrent generation mechanisms in MoSe2-based heterojunctions but also open up new avenues for engineering future high-performance 2D optoelectronic devices.

Improved Contacts and Device Performance in MoS2 Transistors Using a 2D Semiconductor Interlayer.

We report a contact engineering method to minimize the Schottky barrier height (SBH) and contact resistivity of MoS2 field-effect transistors (FETs) by using ultrathin 2D semiconductors as contact interlayers. We demonstrate that the addition of a few-layer MoSe2 between the MoS2 channel and Ti electrodes effectively reduces the SBH at the contacts from ∼100 to ∼25 meV, contact resistivity from ∼6 × 10-5 to ∼1 × 10-6 Ω cm2, and current transfer length from ∼425 to ∼60 nm. The drastic reduction of SBH can be attributed to the synergy of Fermi-level pinning close to the conduction band edge of the MoSe2 interlayer and favorable conduction-band offset between the MoSe2 interlayer and MoS2 channel. As a result of the improved contacts, MoS2 FETs with Ti/MoSe2 contacts also demonstrate higher two-terminal mobility.

Near-infrared optical transitions in PdSe2 phototransistors.

We investigate electronic and optoelectronic properties of few-layer palladium diselenide (PdSe2) phototransistors through spatially-resolved photocurrent measurements. A strong photocurrent resonance peak is observed at 1060 nm (1.17 eV), likely attributed to indirect optical transitions in few-layer PdSe2. More interestingly, when the thickness of PdSe2 flakes increases, more and more photocurrent resonance peaks appear in the near-infrared region, suggesting strong interlayer interactions in few-layer PdSe2 help open up more optical transitions between the conduction and valence bands of PdSe2. Moreover, gate-dependent measurements indicate that remarkable photocurrent responses at the junctions between PdSe2 and metal electrodes primarily result from the photovoltaic effect when a PdSe2 phototransistor is in the off-state and are partially attributed to the photothermoelectric effect when the device turns on. We also demonstrate PdSe2 devices with a Seebeck coefficient as high as 74 µV K-1 at room temperature, which is comparable with recent theoretical predications. Additionally, we find that the rise and decay time constants of PdSe2 phototransistors are ∼156 µs and ∼163 µs, respectively, which are more than three orders of magnitude faster than previous PdSe2 work and two orders of magnitude over other noble metal dichalcogenide phototransistors, offering new avenues for engineering future optoelectronics.

Reversible photo-induced doping in WSe2 field effect transistors.

We report a reversible photo-induced doping effect in two-dimensional (2D) tungsten diselenide (WSe2) field effect transistors on hexagonal boron nitride (h-BN) substrates under low-intensity visible light illumination (∼10 nW µm-2). Our experimental results have shown that this reversible doping process is mainly attributed to two types of defects in h-BN substrates. Moreover, the photo-doped WSe2 transistors can be stable for more than one week in a dark environment and maintain the high on/off ratio (108) and carrier mobility, since there are no additional impurities involved during the photo-induced doping process to increase the columbic scattering in the conducting channel. These fundamental studies not only provide an accessible strategy to control the charge doping level and then to achieve a writing/erasing process in 2D transistors, but also shed light on the defect states and interfaces in 2D materials.

High-Performance WSe2 Phototransistors with 2D/2D Ohmic Contacts.

We report high-performance WSe2 phototransistors with two-dimensional (2D) contacts formed between degenerately p-doped WSe2 and undoped WSe2 channel. A photoresponsivity of ∼600 mA/W with a high external quantum efficiency up to 100% and a fast response time (both rise and decay times) shorter than 8 µs have been achieved concurrently. More importantly, our WSe2 phototransistor exhibits a high specific detectivity (∼1013 Jones) in vacuum, comparable or higher than commercial Si- and InGaAs-based photodetectors. Further studies have shown that the high photoresponsivity and short response time of our WSe2 phototransistor are mainly attributed to the lack of Schottky-barriers between degenerately p-doped WSe2 source/drain contacts and undoped WSe2 channel, which can reduce the RC time constant and carrier transit time of a photodetector. Our experimental results provide an accessible strategy to achieve high-performance WSe2 phototransistor architectures by improving their electrical transport and photocurrent generation simultaneously, opening up new avenues for engineering future 2D optoelectronic devices.