Daria Belotcerkovtceva

Encapsulation of two-dimensional (2D) materials with hexagonal boron nitride (hBN) is widely used to fabricate ultraclean van der Waals (vdW) heterostructures for optoelectronic and quantum transport studies. These devices are often operated under strong out-of-plane displacement fields that tune the electronic properties of the active 2D material, while the surrounding hBN is typically assumed to remain electronically inert. This assumption, however, neglects defect states that are present even in high-quality hBN crystals. Here, we probe their response to the applied displacement field using monolayer graphene encapsulated between hBN crystals as a sensitive charge sensor. Increasing the displacement field induces notable charge transfer between graphene and hBN, producing additional electron doping of ≈10–20 × 109 cm–2 even at the moderate displacement field (∼0.5 V nm–1), while simultaneously increasing carrier-density inhomogeneity of the graphene layer. Both effects are consistent with ionization of defect states within the hBN bandgap, with the bulk density ∼1–7 × 10–5 nm–3 varying across different hBN crystals. Our results show that displacement-field-induced defect ionization in hBN can modify the electronic response of vdW heterostructures, with direct implications for device design and interpretation of experiments.

Leakage current is a physical phenomenon that critically affects the operation and reliability of mainstream electronic devices and heterostructures for diverse applications. Two-dimensional materials are being integrated into the structure of ultrascaled electronic devices, but the leakage current across them is still not well understood. Here we analyse the leakage current across hexagonal boron nitride (hBN), molybdenum disulfide and tungsten disulfide of different thicknesses, and compare it with industrial-quality SiO₂/n⁺⁺Si samples. The samples are analysed at the nanoscale and at the device level, and the experimental data are complemented with computational modelling assisted by technology computer-aided design and density functional theory. First, we demonstrate that the surface roughness of the bottom electrode dramatically alters the leakage current when an electric field is applied. Second, we show that in multilayer two-dimensional materials, the energy bandgap and density of atomic defects are key factors that determine the leakage current; however, in monolayer two-dimensional materials, the leakage current is mainly determined by sample thickness, understood as the electrode-to-electrode distance. Consequently, leakage current across monolayer hBN is higher than that across monolayer molybdenum disulfide and tungsten disulfide despite hBN having a bandgap nearly three times larger, due to its approximately 50% lower thickness. Third, we establish an equivalence (in terms of leakage current) between hBN and SiO₂ films of different thicknesses, which can be used to predict the performance and reliability of two-dimensional-material-based nano-electronic devices, such as transistors and memristors.

High current-treated AlO_x passivated-graphene (CTPG) with enhanced carrier mobility offers promise for stability and scalability in graphene nanoelectronic and spintronic devices.

Emergent technologies are required in the field of nanoelectronics for improved contacts and interconnects at nano and micro-scale. In this work, we report a highly-efficient nanolithography process for the growth of cobalt nanostructures requiring an ultra-low charge dose (15 μC cm⁻², unprecedented in single-step charge-based nanopatterning). This resist-free process consists in the condensation of a ∼28 nm-thick Co₂(CO)₈ layer on a substrate held at −100 °C, its irradiation with a Ga⁺ focused ion beam, and substrate heating up to room temperature. The resulting cobalt-based deposits exhibit sub-100 nm lateral resolution, display metallic behaviour (room-temperature resistivity of 200 μΩ cm), present ferromagnetic properties (magnetization at room temperature of 400 emu cm⁻³) and can be grown in large areas. To put these results in perspective, similar properties can be achieved by room-temperature focused ion beam induced deposition and the same precursor only if a 2 × 10³ times higher charge dose is used. We demonstrate the application of such an ultra-fast growth process to directly create electrical contacts onto graphene ribbons, opening the route for a broad application of this technology to any 2D material. In addition, the application of these cryo-deposits for hard masking is demonstrated, confirming its structural functionality.