LAUSR

DOI: 10.1002/aelm.201800138 a great challenge for top-gate RF device fabrication since atomic layer deposition (ALD) typically needs oxygen or water as precursor.[13–15] Even though recent research progress of capping BP with ALD high-κ dielectrics shows effective suppression of the black phosphorus surface oxidation,[16–18] this process still degrades the electric performance of BP transistors compared with the back-gate devices with minimal exposure to precursors.[19] As shown in previous studies, electric performance of BP FETs can be dominated by the channel dielectric interface where ALD high-κ dielectrics performs better than conventional SiO2. For instance, backgate BP transistors on high-κ substrate exhibit improved device performance in comparison with BP FETs on conventional SiO2. Also, encapsulation by hexagonal boron nitride results in great enhancement of the hole mobility of BP.[6,21,22] However, this approach requires multiple dry transfer steps for both black phosphorus and hexagonal boron nitride flakes with precise alignment for a single device, and thus it has extremely low throughput and yield. In this paper, we report a new approach toward high-performance BP RF transistors using a Damascene-like planarization process to create an embedded gate stack with high-κ dielectrics, which enables high-quality interface while avoids the precursor exposure to the BP channel surface at the same time.[23,24] Side-by-side comparison with two conventional topgate structures shows at least twice improvement in the radio frequency performance of the embedded gate devices. Systematic studies of the radio frequency performance from room temperature down to 20 K are carried out for the first time. A record high extrinsic fmax of 17 and 31 GHz for the device with 400 nm gate length has been achieved at room temperature and 20 K, respectively. The ratio of fmax/fT has been improved to over two, a twice improvement over previous results, showing a significant advantage in power gains compared with graphene transistors.[23,24]