| Summary: | Two-dimensional van der Waals (vdW) ferroelectrics are of great technological interest for high-density electronics, such as non-volatile memories and field-effect transistors, due to their intrinsic ferroelectric polarisation. Their integration with graphene offers opportunities to create ferroelectric/graphene hybrid systems for novel applications, such as ferroelectric field effect transistors. Also in these systems, graphene can provide a sensitive probe of the ferroelectric polarisation, which is difficult to measure in thin ferroelectrics. Within the family of vdW ferroelectrics, CuInP2S6 (CIPS) has attracted widespread attention due to its relatively high Curie temperature (Tc = 315 K), out-of-plane ferroelectricity, ionic conductivity and large optical band gap at room temperature (Eg = 2.7 eV). This thesis focuses on the electrical and optical properties of CIPS in bulk and thin layers. In particular, it explores CIPS/graphene heterostructures and the unique phenomena that emerge from the charge transfer at the CIPS/graphene interface.
A series of planar and vertical devices based on CIPS/graphene heterostructures, including field effect transistors (FETs) and tunnel junctions (TJs), are investigated to study the charge transfer across the CIPS/graphene interface. Resistive switching and memristive effects are observed in the transport characteristics of the devices. These are attributed to ferroelectric polarisation and charge trapping in the CIPS layer. In the FET, electrostatic gating is used to control the charge transfer at the CIPS/graphene interface. The gate-induced time-dependent charge transfer is slow (τ > 100 s) and depends on the sweep rate and range of applied gate voltage. The hysteresis and memristive effects are also sensitive to temperature and light illumination. A significant increase in hysteresis at temperatures T > 200 K suggests charge transfer from/to deep localised states in the CIPS layer. Also, light illumination leads to photoionisation of these states and a light activated slow redistribution of charges.
Finally, this thesis reports on the quantum Hall effect in graphene encapsulated by CIPS. Electrostatic gating of the graphene channel enables the Fermi energy to be tuned so that electrons in the localized states of CIPS are in equilibrium with the current-carrying, delocalized states of graphene. Due to the presence of strongly bound states in this hybrid system, a quantum Hall plateau is achieved at room temperature in relatively modest magnetic fields. This phenomenon offers the prospect for the controlled manipulation of the quantum Hall effect at room temperature.
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