Dr. Wonjae Kim: 2D material based field-effect devices: fabrication and characterization
Since its discovery in 2004, graphene has been considered as one of the most promising materials for applications in nanoelectronics as it is a real two-dimensional (2D) crystal. This atomic thick crystal enables electrons to have extremely high carrier mobility (up to 105 cm2/Vs) by suppressing lattice scattering. The atomic layer is absolutely elastic but at the same time the atoms involved in sp2-hybridized bonds in the crystal yield the most robust structure which has a physical strength that is about 100 times higher than steel. Exploiting such extraordinary properties, state-of-the-art graphene devices including the gigahertz-range transistors, sensitively functioning sensors, transparent-flexible devices, and wearable applications have been demonstrated. However, the development of graphene devices as logic components is undermined due to the nature of the zero bandgap in graphene. Although a bilayer or graphene nanoribbon can open a few-hundred-meV energy gap, its breakthrough technology is challenging to implement.
Over an effort of bandgap engineering, several device architectures to avoid the critical bandgap limit in device applications have been proposed as, for example, graphene three-terminal junctions (TTJs) exploiting nonlinear transport arising only in the 2D regime or graphene-semiconductor heterojunctions utilizing the tunable Schottky effect.
The nonlinearity arises due to the inhomogeneous carrier distribution along the graphene channel. As a consequence, the output signals at the center of the channel are exhibited as electrical rectification when two terminals are biased anti-symmetrically. Interestingly, the sign of rectification is strongly dependent on the type of charge carriers: the output is always positive in hole and negative in the electron region. In addition, as graphene acts excellently as an electrode and interconnection, the TTJ device can be also utilized as a logic inverter when a gate is employed.
Recently, new 2D materials based on transition metal on dichalcogenides, such as, MoS2 and WS2, or a similar compound like GaSe have been discovered. They are the layered substances having a certain bandgap in the range of 1-2 eV. Accordingly, 2D material-based heterojunctions can be realized when graphene is put into contact on 2D semiconductor. A Schottky barrier is formed at the interface of two materials owing to the difference in work function. If a gate focusing on graphene is constructed, then, tunable Schottky diodes can be realized: the barrier height is tuned by the modulation of Fermi energy for graphene through the gate voltage. This promotes an on/off ratio when operating as a field-effect transistor. The heterojunction is further applicable for a photodetector. By light exposure, a photocurrent is generated as a part of excess carriers (electron-hole pairs) created through the 2D semiconductor is separated at the Schottky junction.
In this talk, the TTJ devices performing as a rectifier and inverter is presented. The rectifying device which is controlled by the gate operates at 100 kHz with 18% of efficiency (= VOUT/VIN). The devices fabricated utilizing CVD graphene and having tens of micrometer scale are showed for discussion. In addition, highly tunable and photosensitive Schottky diodes formed in 2D heterojunctions, i.e. graphene-GaSe and graphene-MoS2, are finally presented. The on/off ratio higher than 103 and photoconductivity with ~100 A/W achieved based on the about 30-nm-thick 2D semiconductor flake are focused. Device fabrication and its related technology will be further discussed during the talk.
Date: 23rd October
Time: 10:30 am