PVA-assisted metal transfer for vertical WSe₂ photodiode with asymmetric van der Waals contacts

The vertical electronic and optoelectronic devices based on 2D materials have shown great advantages over lateral devices, such as higher current density, faster switch speed, and superior short-channel control. However, it is difficult to fabricate vertical device with conventional metal deposition methods due to the aggressive process usually results in damage to the contact region. Here, we develop a simple and effective metal transfer technique and fabricate p-type and n-type WSe₂ transistors by using metals with different work functions and subsequently create a vertical WSe₂ transistors with a 18-nm-thick channel, which retain good gate coupling effect. Furthermore, a vertical WSe₂ photodiode is constructed with graphene and Pt as asymmetric van der Waals (vdW) contacts. The work-function difference between graphene and Pt generates a built-in electric filed, leading to a high current rectification over 10⁵. Under 405 nm laser illumination, the device exhibits excellent self-powered photodetection properties, including a high responsivity of 0.28 A W^-1, fast response speed of 24 μs, and large light on/off ratio exceeding 10⁵ at zero bias, which surpass most of the vdW photodiodes. This work demonstrates that the metal transfer technique is a promising strategy for the construction of high-performance vertical optoelectronic devices.