Dmitry Zemlyanov

Integrating two-dimensional semiconductors, such as MoS2, with dielectric materials remains a central challenge for their use in future logic technologies. While seed layers are typically introduced to promote dielectric nucleation and adhesion, we show that they also critically govern charge transfer doping and, in turn, transistor performance. Back-gated monolayer MoS2 transistors passivated on their top surface with a Ta-seed/HfOx dielectric stack were fabricated and characterized electrically and physically using Raman, photoluminescence, and X-ray photoelectron spectroscopies. Threshold voltage and on-current varied strongly with Ta-seed thickness and deposition conditions, and these changes correlated with signatures observed across all spectroscopic probes. The results reveal that the seed layer both introduces disorder into the MoS2 channel and modifies the interfacial charge environment, controlling charge transfer between HfOx and MoS2. Optical spectroscopy shows that the on-current tracks seed-induced disorder, whereas X-ray photoelectron spectroscopy indicates that the threshold voltage correlates with shifts in the local electrostatic environment associated with interfacial charge transfer. Better performance was obtained with ultrathin 0.2 nm Ta-seed layers deposited under oxygen-poor conditions, which limit deposition-induced damage while facilitating charge transfer. These findings identify seed-layer engineering as a key strategy for controlling disorder and interfacial doping in MoS2 devices and establish multimodal spectroscopy as a practical approach during fabrication for process development and monitoring.

This work investigates the p-doping effect of Nitric Oxides (NO_X) annealing on transistors with a channel from a network of carbon nanotubes (CNTs), with a focus on extension doping of top-gated transistors and RF characterization. NO_X annealing at 100 °C for 1 h induces oxidation in CNTs, which introduces a doping band inside the nanotube band gap near the valence band. The benefits and drawbacks of NO_X annealing on the performance of back-gated (BG) CNT transistors are characterized. The doping band is beneficial in that it reduces the Schottky barrier for hole injection and increases the hole carrier concentration in the channel. However, we also note that NO_X annealing deteriorates the inverse subthreshold swing (SS) and drain-induced barrier lowering (DIBL) by introducing interface traps and worsens any existing short channel effects. Our capacitance analysis confirms that the increase in source-drain and interface-trap capacitances degrades both SS and short channel effects. To circumvent the drawbacks of NO_X annealing, we employ it to create extension doping regions in p-type, top-gated transistors with a scaled high-k gate oxide, leading to an increase in drain current from 5.09 to 13.95 μA at V_OV = -2 V and V_DS = -1 V. Because the effect of NO_X annealing is limited to the extension regions and has little impact on the channel region, negative effects on the SS and DIBL are mitigated. Finally, an RF transistor is fabricated, and NO_X annealing is used to enhance the cutoff frequency by approximately 10-fold.

The helium ion microscope (HIM) focused ion beam (FIB) has emerged as a powerful tool to directly pattern nanostructures below 10 nm due to its high‐resolution capabilities and the inert nature of the ion source. These attributes make HIM FIB particularly interesting for patterning 2D materials such as transition metal dichalcogenides (TMDs) to investigate transport phenomena at the nanoscale. Reported here is the fabrication of MoS ₂ nanoribbon devices using HIM FIB‐induced etching (FIBIE) with XeF ₂ , allowing for reduced ion dose compared to direct sputtering. While patterning is efficacious, the devices exhibit performance degradation with decreasing nanoribbon width due to damage up to 150 nm beyond the patterned edge. Incorporating an hBN encapsulation improves device performance by one order of magnitude, although the lateral extent of damage remains unchanged. The spatial distribution of damage is shown to be determined by the forward‐ and backscattered ions and electrons, while the hBN encapsulation layer substantially reduces damage from XeF ₂ interactions in unexposed regions. Raman and photoluminescence (PL) measurements corroborate these findings, while ion/solid interaction simulations further elucidate the resolution limits imposed by substrate interactions. This work provides critical insights and a practical pathway for utilizing HIM FIBIE in 2D TMD functional device patterning.

Transition metal dichalcogenides are attractive channel materials for an aggressive dimension scaling of double-gated field-effect transistors. Some challenges, however, remain nowadays, including finding a suitable top high-κ dielectric and a way to deposit it. Recently, an amorphous Ta2O5 interface layer grown on top of MoS2 has been shown to improve the device's electrical performance compared to a deposited amorphous Al2O3 film. In this work, we investigate the origin of such an improvement, combining experimental and first-principles approaches. The former suggests a comparable n-type doping of the 2D material for the Al2O3 and Ta2O5 interface layer based on x-ray photoelectron spectroscopy. Within the first-principles approach, we construct several amorphous Ta2O5 atomistic models and their interfaces with a MoS2 or WS2 monolayer. We demonstrate that a clean interface can in principle be achieved between the 2D material and Ta2O5 with a proper surface passivation, with a defect-free electronic bandgap a potential fluctuation within the 2D material under control. Little differences are found between MoS2 and WS2 in the corresponding interface at the theoretical level. The electronic properties are found to strongly depend on the coordination of the metal atoms at the oxide surface. Combining these analyses, we conclude that a higher interface quality for the Ta2O5 layer with MoS2 than for the Al2O3 one is a plausible explanation for the improved performances.

We demonstrate the growth of small-rotation-angle TaSe₂ moiré structures on a Au(111) substrate, providing important insights on the direct synthesis of small-angle twisted two-dimensional layers for future twistronics.

Most two-dimensional (2D) materials experimentally studied so far have hexagons as their building blocks. Only a few exceptions, such as PdSe₂, are lower in energy in pentagonal phases and exhibit pentagons as building blocks. Although theory has predicted a large number of pentagonal 2D materials, many of these are metastable and their experimental realization is difficult. Here we report the successful synthesis of a metastable pentagonal 2D material, monolayer pentagonal PdTe₂, by symmetry-driven epitaxy. Scanning tunnelling microscopy and complementary spectroscopy measurements are used to characterize this material, which demonstrates well-ordered low-symmetry atomic arrangements and is stabilized by lattice matching with the underlying Pd(100) substrate. Theoretical calculations, along with angle-resolved photoemission spectroscopy, reveal monolayer pentagonal PdTe₂ to be a semiconductor with an indirect bandgap of 1.05 eV. Our work opens an avenue for the synthesis of pentagon-based 2D materials and gives opportunities to explore their applications such as multifunctional nanoelectronics.

Most two-dimensional (2D) materials experimentally studied so far have hexagons as their building blocks. Only a few exceptions, such as PdSe2, are lower in energy in pentagonal phases and exhibit pentagons as building blocks. While theory has predicted a large number of pentagonal 2D materials, many of them are metastable and their experimental realization is difficult. Here we report the successful synthesis of a metastable pentagonal 2D material, the monolayer pentagonal PdTe2, by symmetry-driven epitaxy. Scanning tunneling microscopy and complementary spectroscopy measurements are used to characterize the monolayer pentagonal PdTe2, which demonstrates well-ordered low-symmetry atomic arrangements and is stabilized by lattice matching with the underlying Pd(100) substrate. Theoretical calculations, along with angle-resolved photoemission spectroscopy, reveal monolayer pentagonal PdTe2 is a semiconductor with an indirect bandgap of 1.05 eV. Our work opens an avenue for the synthesis of pentagon-based 2D materials and gives opportunities to explore their applications such as multifunctional nanoelectronics.