Polarity Control of Top Gated Black Phosphorous FETs by Workfunction Engineering of Pre-Patterned Au and Ag Embedded Electrodes

We propose and experimentally demonstrate top-gated complementary n- and p-type black phosphorous field effect devices (FETs) by engineering the workfunction of pre-patterned electrodes embedded in a SiO
₂
bottom layer. Pre-patterned electrodes offer the advantages of reducing the exposure time of exfoliated flakes to oxidant agents with respect to top-contacted devices and maximizing the accessible area for sensing applications. The presented devices are realized by mechanical exfoliation of multilayer black phosphorous flakes on top of pre-patterned embedded source and drain contacts. A capping layer consisting of 15-nm thick Al
₂
O
₃
is deposited to prevent flakes degradation and serves as top gate dielectric. The silicon substrate can be exploited as back gate to program the FETs threshold voltage. We deposited both Au and Ag embedded contacts to investigate the impact of electrodes workfunction on BP FETs polarity. Au contacted devices show p-type conduction with ON/OFF current ratio 140 and holes mobility up to 40 cm
²
V
^-1
s
^-1
. Devices with Ag contacts exhibit prevalent n-type conduction with ON/OFF ratio 1700 and electron mobility 2 cm
²
V
^-1
s
^-1
. The reported results represent a substantial improvement with respect to reported alternative implementations of black phosphorous FETs with pre-patterned, non-embedded electrodes. Moreover, we demonstrate that Ag is a promising metal for electron injection in black phosphorous FETs.