Spatial/temporal photocurrent and electronic transport in monolayer molybdenum disulfide grown by chemical vapor deposition

Abstract

We systematically investigate the spatial/temporal photocurrent in photodetectors and electronic transport in transistors/Hall-bar devices based on monolayer MoS2 grown by chemical vapor deposition (CVD). We found that the maximum photocurrent occurs when the laser spot is close to the metal/MoS2 contact and is tunable by the applied drain voltage, which can be explained by the modulation of the local electric field at the Schottky barrier, consistent with predictions from our quantum transport simulation. We observed that the maximum photocurrent at drain contact is much larger than the one at the source contact, and the DC currents show rectifying behavior. These phenomena can be explained by the different Schottky barrier heights at the two contacts. By measuring Hall-bar structure at various temperatures from 100 K to 400 K, we extracted the barrier heights at the source and drain contacts, separately. We found that the barrier height at drain contact is about 50 mV larger than the one at the source contact, consistent with the photocurrent and DC current observations. We measured the photocurrent at various powers, and a photoresponsivity of 3.07 mA/W was extracted at low powers. When the power increases above 20 μW, the photocurrent starts to saturate. Temporal response of the photocurrent is also dependent on the laser power. At high laser powers, photocurrent overshoot was observed. The photocurrent saturation at high powers and the overshoot in temporal photocurrent are likely due to the same mechanism: an accumulation of electrons in the channel, flattening out the band structure, since the laser spot is located near the drain contact in these measurements. These studies of photocurrents and electronic transport in CVD MoS2 highlight the importance of the contacts in the electronic/optoelectronic devices and reveal the physical mechanism of the photocurrent/electronic transport in these devices.

Van der Waals Coupled Organic Molecules with Monolayer MoS2 for Fast Response Photodetectors with Gate-Tunable Responsivity

“…To elucidate the underlying mechanisms in the improved response dynamics in ZnPc-treated MoS 2 detectors, it is therefore important to clarify the inherent slow response behavior in the present bare MoS 2 devices. For the persistent photoconductance in typical MoS 2 detectors, previous studies have considered the minority carrier trapping to inherent defects in MoS 2 , , the interface states with substrate, ,− and also the surface-adsorbed O 2 and H 2 O from ambient . Since in a control study on a 30 nm Al 2 O 3 -passivated device (see Figure S5), we find no enhancement in the photoresponse speed, it is believed that the ambient effects are negligible in the measured slow photoresponse dynamics of the MoS 2 detector.…”

Unveiling defect-mediated carrier dynamics in monolayer semiconductors by spatiotemporal microwave imaging

“…While ultrafast photoresponse can be detected by pump-probe experiments (44)(45)(46)(47)(48), the peak intensity of pulsed laser is usually too high for practical photodetectors. In contrast, the response time of previously demonstrated TMD photodetectors is usually on the order of milliseconds (9,(49)(50)(51) or even seconds (22,52), which is clearly limited by the transport through source/drain contact electrodes. The sub-μs carrier lifetime due to trap states can be inferred from photocurrent measurements (49), which requires device modeling and indirect analysis of the data.…”

Charge Separation at Mixed-Dimensional Single and Multilayer MoS2/Silicon Nanowire Heterojunctions

“…The positive photocurrent at the MoS 2 /metal interface (Figure 3b, right) of the grounded source contact is consistent with previous reports 29,36,37 of carrier separation by the built-in electric field (photovoltaic effect) at this interface. 29,36,38 For completeness, we note that photothermal effects may also contribute to a positive photocurrent at the contacts, 37 but our focus here is on the response of the vdW heterojunction.…”