Air‑Stable MoTe₂ Phototransistor with Asymmetric Contacts Yields Strong Photovoltaic Response and Sub‑Millisecond Response

Background

Two‑dimensional (2D) materials such as graphene and transition metal dichalcogenides (TMDs) have transformed nanoscience by offering atomically thin semiconductors with sizable bandgaps and exceptional mechanical flexibility [1–9]. Among them, 2H‑MoTe₂ is notable for its indirect bandgap of 0.83 eV in bulk and a direct bandgap of 1.1 eV when thinned to a monolayer [22,23]. This material has been explored for spintronics, field‑effect transistors (FETs), photodetectors, and solar cells [24–33]. Electrical performance of MoTe₂ devices hinges on the quality of metal contacts. Appropriate electrode materials can induce p‑ or n‑type doping and achieve low‑barrier Ohmic contact [34–40]. While many studies compare contact metals, few have systematically examined how asymmetric contact geometries influence device physics, particularly in photovoltaic photodetectors.

Here we fabricate an air‑stable, p‑type, multi‑layer MoTe₂ phototransistor with asymmetric source/drain contact cross‑sections. Using scanning photocurrent microscopy (SPCM), we map the spatial potential profile and demonstrate that asymmetric contacts give rise to non‑zero net photocurrent and photovoltaic behaviour, even in the off‑state. The device delivers sub‑millisecond photoresponse and operates up to 1550 nm, making it suitable for communication applications.

Results and Discussion

Two back‑gated multi‑layer MoTe₂ devices (D1 and D2) were fabricated on 300 nm SiO₂/p⁺-Si substrates. Optical microscopy, AFM, and Raman spectroscopy confirmed the quality and thickness of the MoTe₂ channel. Device D1, shown in Figure 1, has a channel length of 10 µm and a 23 nm thick MoTe₂ film. The source and drain contact cross‑sections are 6.5 µm and 4.8 µm, respectively, providing a pronounced asymmetry (see Additional file 1: Figure S1).

Electrical characterization (Figure 1c) shows clear p‑type behaviour with an on/off ratio of 6.8 × 10³ at 1 V source‑drain bias. The field‑effect mobility is 14.8 cm²/Vs. Even at reduced bias (100 mV), the on/off ratio remains above 6.0 × 10³ (Additional file 1: Figure S3). The device exhibits minimal hysteresis and retains stability in air.

Under 637 nm illumination (Figure 2), the source‑drain current increases with laser power. When the gate voltage is set to 5 V, the transistor enters the off‑state yet still displays a measurable photocurrent, evidencing photovoltaic action. The open‑circuit voltage (V_OC) saturates at 50 mV, while the short‑circuit current (I_SC) rises from 0 to 1.6 nA as the illumination power increases from 0 to 4.2 mW. Importantly, V_OC and I_SC remain unchanged when the polarity of the applied bias is reversed, confirming that the photocurrent originates from an intrinsic photovoltaic effect rather than thermoelectric or photothermoelectric artefacts (Figure 2d).

SPCM mapping (Figure 3) at 1200 nm wavelength reveals that strong photocurrent peaks emerge near the MoTe₂/electrode interfaces under short‑circuit conditions. The peaks shift towards the channel centre as the back‑gate voltage is increased from –5 V to +5 V, indicating that the potential step at the contacts dominates carrier separation at low bias. When the source‑drain bias is increased (Figure 4), the photocurrent peak progressively moves to the channel centre, confirming that the applied bias eventually supersedes the contact‑induced potential step.

These observations demonstrate that asymmetric contact geometry creates a net photocurrent even when the device is electrically off. The photovoltaic response is robust across devices with varying degrees of asymmetry (D2 and additional samples in Additional file 1: Figure S8) and persists across a wide spectral range.

Time‑resolved measurements (Figure 5a) show rise times of 20 µs and fall times of 127 µs under zero source‑drain bias, while the times increase to 210 µs and 302 µs, respectively, at 1 V bias. This behaviour reflects the transition from contact‑dominated to channel‑dominated photocurrent generation. Spectral response measurements (Figure 5b) confirm that the device is sensitive from 1200 nm to 1550 nm, covering the standard optical communication window.

Conclusions

We have fabricated an air‑stable, p‑type MoTe₂ phototransistor with asymmetric source/drain contacts that exhibits a pronounced photovoltaic response in the off‑state. Scanning photocurrent microscopy has revealed that metal‑induced potential steps at the MoTe₂/electrode interfaces are the key driver of carrier separation under low bias. The device delivers sub‑millisecond photoresponse and operates across the visible and near‑infrared spectrum up to 1550 nm, underscoring its potential for low‑power photodetectors and optical communication applications.

Methods/Experimental

Source, drain, and gate electrodes were patterned on 300‑nm SiO₂/p⁺-Si substrates using standard UV photolithography, followed by selective etching of the SiO₂ beneath the gate and e‑beam evaporation of 5 nm/100 nm Cr/Au. Multi‑layer MoTe₂ crystals were mechanically exfoliated from 2H‑MoTe₂ single crystals grown by chemical vapor transport (TeCl₄ transport agent, 750 °C → 700 °C, 3 days). The MoTe₂ flakes were transferred onto the patterned electrodes using polyvinyl alcohol (PVA) as a transfer medium, which was subsequently dissolved in water and rinsed with isopropyl alcohol. AFM (SPA‑300HV) and Raman spectroscopy (514‑nm laser, 100× objective) characterized thickness and crystal quality.

Electrical and photoresponse measurements were performed with an Agilent B1500A semiconductor analyzer and Lakeshore probe station under 637 nm laser illumination (spot > 200 µm). Time‑resolved photocurrent was recorded with a DL1211 preamplifier and Keysight MSOX3024T oscilloscope. Spatially resolved photocurrent used a home‑made SPCM setup with a SuperK EXTREME white‑light laser, tunable via a SuperK SELECT filter, focused by a 20× objective. Photocurrent was extracted with a DL1211 preamplifier and SR830 lock‑in amplifier at a 1 kHz chopping frequency.

Abbreviations

2D

Two‑dimensional

2H‑MoTe₂

2H‑type molybdenum ditelluride

AFM

Atomic force microscopy

FET

Field‑effect transistor

I_PC

Photocurrent

I_SC

Short‑circuit current

I_sd

Source‑drain current

PVA

Polyvinyl alcohol

TMDs

Transition metal dichalcogenides

V_bg

Back‑gate voltage

V_OC

Open‑circuit voltage

V_sd

Source‑drain voltage

τ_fall

Fall time

τ_rise

Rise time