Bias-dependent layer-resolved Raman evidence for interfacial asymmetry in vertical graphene/MoSe2/graphene junctions on SiO2/Si substrates

Abstract

Vertical graphene/MoSe₂/graphene (Gr/MoSe₂/Gr) junctions are promising for tunable electronics as well as optoelectronics based on vertical transport. To identify the junction properties of a vertically stacked Gr/MoSe₂/Gr junction on SiO₂/Si, we combine in situ bias-dependent Raman spectroscopy with temperature-dependent transport measurements. At zero bias, the Raman response of the two graphene electrodes appears as a single overlapped feature. Under finite bias, this response separates into two distinct contributions, which allows layer-resolved extraction of the chemical potential shifts of the two graphene electrodes. The extracted shifts are strongly asymmetric, with Gr_bot showing a consistently smaller response than Gr_top under both bias polarities. This result indicates that the junction is better described as an asymmetric back-to-back Schottky diode with a less electrostatically tunable bottom interface. The device also exhibits a clear hysteresis loop in the cyclic current–voltage characteristics, indicating history-dependent hysteretic transport characteristics. At room temperature, the transport remains nearly symmetric, but the polarity asymmetry becomes stronger at higher temperature. A plausible origin is an asymmetric junction formed by different defect environments at the two interfaces. MoSe₂ can contain intrinsic Se vacancies. Upon additional exposure of the upper MoSe₂ surface, some of these vacancy sites can undergo O substitution, whereas the lower interface can retain a stronger Se vacancy character. These in-gap Se vacancy states may contribute to Fermi level pinning. A temperature-dependent change in this pinning condition may further change the effective barrier profile and thereby enhance the transport asymmetry. These results show that interface quality and transfer history play a central role in determining the electrostatic response of vertical Gr/MoSe₂/Gr heterostructures.