Paper OX-TuP2
van der Waals Heterostructures of Graphene and β-Ga₂O₃ Nanoflake for Enhancement Mode MESFETs and Logic Applications

Tuesday, October 22, 2019, 6:30 pm, Room Union Station B

β-gallium oxide (β-Ga₂O₃) is a promising material for next-generation power electronics due to its wide band gap of ∼4.9 eV and excellent productivity. Interestingly, a single crystalline β-Ga₂O₃ with a monoclinic structure can be exfoliated into ultra-thin flakes along the (100) plane due to its strong in-plane force and weak out-of-plane force. The exfoliated β-Ga₂O₃ flakes can be easily integrated with 2D materials (h-BN, TMDCs) to form van der Waals heterostructures for a down-scaled novel (opto)electronic devices as well. Most of the fabricated β-Ga₂O₃ transistors exhibit n-type characteristics with a negative threshold voltage (V_th) due to oxygen vacancy or donor like impurities in β-Ga₂O₃. The negative threshold voltage of n-type β-Ga₂O₃ transistors allows only a depletion mode (D-mode) operation, which limits their implementation in the circuit design. However, an enhancement mode (E-mode) operation, allowing simple circuit designs and fail-safe operation under high voltage conditions, is preferred for power transistors.

We have demonstrated a method to control V_th of β-Ga₂O₃ Metal-Semiconductor Field Effect Transistor (MESFET) by using various morphology of van der Waals heterostructure of β-Ga₂O₃ and graphene to achieve an E-mode operation. The junction of β-Ga₂O₃ and graphene forms a Schottky barrier due to the difference of their work functions. In the same β-Ga₂O₃ nanoflake, the β-Ga₂O₃ MESFET with a double gate of the sandwich structure of graphene/β-Ga₂O₃/graphene showed positive V_th (E-mode operation) while the bottom gate only MESFET showed negative V_th (D-mode operation). Furthermore, a β-Ga₂O₃/graphene van der Waals heterostructure based monolithic Direct Coupled FET Logic (DCFL) inverter was demonstrated by integrating E-mode and D-mode MESFETs on single β-Ga₂O₃ nanoflake and exhibited good inverter characteristics. These results show a great potential of van der Waals heterostructure of β-Ga₂O₃/2D materials on future nanoscale smart power integrated circuit (IC) applications. The details of our results and discussions will be presented .