Physics and application of charge transfer in van der Waals heterostructures

Abstract

Since the discovery of graphene, the field of 2D materials has garnered massive interest from a materials science, basic physics and device application point of view. This results from the diverse range of electronic and transport properties observed in these systems. For example, graphene, which has a gapless Dirac fermionic band structure with extremely high carrier mobility, shows low photoresponse due to the absence of a band gap. However, layered transition metal dichalcogenides (TMDCs) such as MoS2, which possess a semiconducting band structure with disorder dominated hopping like transport mechanism and low carrier mobility, demonstrates high photoresponse due to the presence of a band gap. One of the major benefits of 2D materials is the possibility of stacking together isolated atomic planes of different materials in a layer by layer manner forming an atomic Lego or a van der Waals heterostructure. Proximity induced interaction between two or more 2D crystals with varied crystal structure and electronic properties leads to a plethora of possibilities for the emergence of new physics and/or device functionality. Consequently, van der Waals heterostructures have been utilized to design devices for a wide variety of applications such as electronic, piezoelectric, thermoelectric, optoelectronic and non-volatile information storage to name a few. For optoelectronic and memory-based applications, charge transfer between the constituent layers of the van der Waals heterostructure has proven to be of immense importance. There are reports of excellent photodetectors based on MoS2 graphene heterostructures where a transfer of photogenerated carriers from the MoS2 to graphene layer leads to high responsivity figures of ∼ 5 × 108 AW−1 at room temperature. In this thesis, we study the effect of vertical charge transfer in TMDC based van der Waals heterostructures aimed at non-volatile memory, memristor and bio-inspired synaptic applications. For this purpose, we use a trilayer stack of MoS2, hBN and graphene. Here hBN acts as a tunnel barrier separating the MoS2 channel from the graphene floating gate (FG). This design is motivated by our investigations into the ON/OFF switching mechanism in back gated TMDC FETs where we observed clear signatures of percolative switching in a disordered channel with low subthreshold slopes. An improvement in the subthreshold slope is brought about by capacitance engineering via extension of the FG, leading tohigh quality MoS2 FETs with near-ideal subthreshold slope (≈ 80 mV/decade) maintained for almost four decades of change in conductance. The device also demonstrates a large anti-hysteresis in the transfer characteristics due to the transfer of charges from the channel to the FG. This, coupled with a low OFF state current makes the MoS2 FG device ideal for energy efficient memory applications. The charge transfer process also leads to a hysteresis in the output characteristics which is indicative of a memristor like behaviour. Furthermore, the quanta of charge transferred can be controlled using short time period pulses at the gate and drain terminal. This leads to a multi-state memory device with repeated increase and decrease of the channel conductance resulting from the accumulation or depletion of electronic charges on the graphene FG. Pulsed charge transfer mediated changes in device conductance is analogous to the pulsed potentiation and depression of a biological synapse which is mediated via controlled release of neurotransmitters into the synaptic cleft. In addition to pulsed potentiation and depression, the device successfully replicates other synaptic properties such as paired pulse facilitation (PPF) and spike time dependent plasticity (STDP) while maintaining a low power dissipation (∼5 fJ per pulse), making it ideal for future neuromorphic applications.