Fabrication of lateral graphene/h-BN Moiré uperlattices for ultrafast terahertz spectroscopy
Electronic Semiconductors, Components and Circuits, Integrated Systems, Sensor Technology, Theoretical Electrical Engineering
Final Report Abstract
The aim of this small project was the establishment of a cooperation with the group of Dr. Juan Delgado Notario of the University of Salamanca, with regard to the investigation of THz field-effect transistors based on h-BN-encapsulated graphene monolayers (h-BN: hexagonal Boron Nitride). The Spanish group has perfected the fabrication technology for such GFET devices, and this project enabled us to send a student to Salamanca to jointly fabricate first devices for characterization, and thus lay the foundation for an ongoing extended cooperation. A monolayer (ML) of crystalline graphene placed on top of multilayer crystalline h-BN will form a large-scale hexagonal moiré pattern with a lattice constant of approximately 14 nm, if the in-plane crystal axes have the same orientation. The moiré pattern formation is due to a strain distribution forcing the C-C bonds of the graphene to be stretched in order to commensurately compensate the mismatch in lattice periodicity of graphene relative to h-BN. The electrons in graphene experience a periodic hexagonal potential, and hence a modified band structure. In this band structure, novel fundamental phenomena can be demonstrated and investigated. Our intention was and is to observe Bloch oscillations (BOs) upon excitation of electron wavepackets with ultrashort THz pulses. This goal has not been achieved yet. An additional topic of this project, and one which does not require the existence of a moiré lattice, could be successfully completed: The first direct observation of plasma waves traversing the entire channel of a GFET. For these purposes, the student learnt to master the whole process from exfoliation over dry-transfer of the individual layers to e-beam lithography and dry-etching for the patterning of the stacks of van-der-Waals materials. He fabricated two types of GFETs which included bow-tie THz antenna structures to couple THz radiation to the graphene, the devices being optimized for the observation of plasma waves, respectively for the detection of BOs. With the first device, we could directly observe – with s-SNOM measurements conducted at the NanoGune Research Center in San Sebastian – plasma waves traversing the entire, more than 2-µm-long channel of the GFET. A detailed study at 2.52 THz revealed even the formation of a standing wave at the end of the channel proving that the plasma wave was reflected with a reflection coefficient close to one at the end of the channel. With the second sample, we looked for BOs in THz-pulse transmission measurements at cryogenic sample temperatures. Preliminary DC currentvoltage measurements unfortunately had not revealed the signature expected for a moiré superlattice. We nevertheless performed THz measurements to test a new LiNbO3 source and to optimize the THz radiation coupling to the graphene channel for follow-up studies on BOs with new samples in the near future.
