AVS 68 Session 2D+EM-MoM: 2D Materials: Electronic, Mechanical, and Optical Properties

Atomically thin, single-crystalline 2D electronic materials have recently emerged, offering a remarkably wide range of building blocks of nanostructures, ranging from metals (e.g. graphene), large gap insulators (BN), to semiconductors (transition metal dichalcogenides and black phosphorous). One key advantage of these van der Waals materials lies in the flexibility of stacking different types of materials to form heterostructures, providing a design platform for achieving novel device functionality.With the ability to probe electronic structures with atomic resolution, scanning tunneling microscopy/spectroscopy (STM/S) has played a crucial role in advancing our understanding of the electronic structures of 2D electronic materials and their heterostructures.

In this talk, I will present a comprehensive combination of different modes of scanning tunneling spectroscopy beyond the conventional constant height tunneling spectroscopy.I will use a few examples to show how such comprehensive investigations provide an in-depth understanding of the electronic structure evolutions from monolayer to complex heterostructures (including moire superlattices and beyond).

Next-generation technology nodes will reach the sub-nanometer range via the introduction of ultra-thin and short-channel devices. Traditional semiconductors such as silicon (Si) suffer from device-degrading effects such as roughness-induced scattering or short-channel effects at this size scale. Two-dimensional transition metal dichalcogenides (2D TMDs) do not suffer from these effects. Therefore, they could be candidate materials for the next-generation nodes. Single-layer molybdenum disulfide (SL-MoS₂) devices exhibit carrier mobilities exceeding that of Si-based devices with similar thicknesses, but suffer from high contact resistance, reducing their utility in device technologies. Our previous results show that the physics at the interface between SL-MoS₂ and gold (Au) is complex and that thermal annealing (TA) under certain conditions could lead to lowered contact resistance by direct chemical hybridization and/or doping.

We performed detailed in situ Raman spectroscopy of exfoliated SL-MoS₂ on Au during and after annealing at temperatures ranging from room temperature (RT) to 300 °C in Ar. This revealed doping and mechanical strain induced in the SL-MoS₂ by the underlying Au layer. Using peak deconvolution, we identify the presence of two sub-peaks comprising each of the well-known MoS₂ fingerprint peaks (A_1g and E_2g), consistent with previous studies. The ratio of intensities and relative shifting of the sub-peaks indicates a strong interaction between the MoS₂ and the Au layer, which could be due to hybridization between the MoS₂/Au. The well-known Si peak at 520 cm^-1 diminished as the temperature was increased. As such, we conclude that the MoS₂ delaminated from the Au layer at elevated temperatures. The coupling between the Au and MoS₂ was reestablished upon cooling to RT after annealing as indicated by the reappearance of the Si peak. Further, as the MoS₂ was annealed at progressively higher temperatures, it became increasingly electron-doped by the Au with the strain remaining constant at 1% in tension.

Our data shows that TA and cooling of the interface between SL-MoS₂ and Au produces stronger coupling, suggesting an increased level of hybridization between the MoS₂/Au. The hybridization results in a nearly charge-neutral MoS₂-Au heterostructure, indicating that TA could be a path toward achieving intrinsic contacts for device applications. Finally, the strain between these surfaces remains unchanged upon annealing, implying that annealing promotes stronger coupling between SL-MoS₂/Au, without increasing strain. Thus, annealing presents an ideal route towards generating charge-neutral MoS₂/Au contacts with minimal interfacial strain.

2D layered materials are emerging as potential candidates for energy-efficient conventional devices, as well as new memristive and neuromorphic device architectures. The focus to date has been on new semiconductor materials, particularly transition metal dichalcogenides (TMDs). Yet, feasible technology, especially in the optoelectronics field, relies on clean interfacing, particularly to a suitable dielectric. HfS₂ is one of the few materials that offer a native oxide conversion pathway, previously demonstrated in various experimental set-ups. Here we explore the oxide formation kinetics on HfS₂ layers and the achievable interface quality in HfS₂/HfO_x devices.

To accomplish this, we employ several HfS₂ oxidation methods, from thermal to plasma and pulsed water vapour exposure, and compare the electrical switching behaviour of MIM and MIS device structures while characterizing their dielectric properties via spectroscopic ellipsometry. The oxidative behaviour of HfS₂ and the buried HfS₂/HfO_x interface are observed optically using a combination of reflectance spectra and ellipsometric modelling. In order to determine the viability of such an approach for device fabrication, the natively grown HfOx is benchmarked against ALD grown hafnia.

Thus, our study is a step towards finer control of interface quality in HfS₂/HfO₂ devices through calibration of the native oxidation pathway.

β-In₂Se₃ is a two-dimensional semiconductor that has long been believed to have a centrosymmetric crystal structure. α-In₂Se_3­ is a closely related, two-dimensional, ferroelectric semiconductor [1,2] and has shown promise in low-power, neuromorphic electronic devices [3,4] and transistors [5]. Much work has been devoted to exploring the performance of α-In₂Se₃ in electronic devices due its unique combination of properties; however large-area thin-film synthesis of α-In₂Se₃ has not been established. Since 2018 there have been reports of polarization in a β’-In₂Se₃ phase arising from displacements of the center layer selenium atoms [6–8]; however, these studies have not included any fabrication and characterization of electronic devices. One study from 2018 claims deposition of large-area, ferroelectric In₂Se_{3­ ­}thin-films via MBE; however, it is difficult to distinguish the α from β phase by the Raman spectra and TEM micrographs provided. Earlier this year, ferroelectric behavior was reported in transistors fabricated from metal-organic chemical vapor deposition (MOCVD) β-In₂Se₃ [9]. If β-In₂Se₃ does truly exhibit ferroelectric ordering, it would be a very promising material for low-power transistors and neuromorphic electronic devices.

In this study, we demonstrate molecular beam epitaxy (MBE) synthesis of β-In₂Se₃ thin-films and electrical characterization of β-In₂Se₃ transistors. We link processing parameters such as synthesis substrate temperature, and Se/ In flux ratio to resulting β-In₂Se₃ electrical properties and device performance.

[1]M. Küpers, et. al., Inorg. Chem. 57, 11775 (2018).

[2]J. Xiao, et. al.,Phys. Rev. Lett. 120, 227601 (2018).

[3]B. Tang, et. al., ACS Appl. Mater. Interfaces 12, 24920 (2020).

[4]L. Wang, et. al., Adv. Funct. Mater. 29, 1 (2019).

[5]M. Si, et. al., Nat. Electron. 2, 580 (2019).

[6]S. Li, et. al., Sci. Adv. 4, eaar7720 (2018).

[7]F. Zhang, et. al., ACS Nano 13, 7, 8004–8011 (2019).

[8]Z. Zhang, et. al., Adv. Mater. 34, (2022).

[9]S. Lee, et. al., 2D Mater. 9, 025023 (2022).

[9]S. M. Poh, et. al., Nano Lett. 18, 6340 (2018).

[10]S. Lee, et. al., 2D Mater. 9, 025023 (2022).

The discovery of graphene has triggered a great interest in inorganic as well as molecular two-dimensional (2D) materials. Over the last years, a remarkable progress in the mechanical characterization of free-standing 2D materials was achieved [1]. In this contribution, we compare mechanical characterization of membranes by means of bulge tests and nanoindentation experiments. The bulge test was found to be a reliable method, which is suited for any kind of membrane. Nanoindentation was found to be suited for a number of membrane types while others entail a high risk of rupturing. With these two methods, a number of different pristine and pyrolysed carbon nanomembranes (CNMs) were examined. CNMs form by irradiation induced cross-linking of molecular layers e.g. self-assembled monolayers of terphenylthiol (TPT) molecules. Cross-linking between adjacent molecules was induced by low energy electron exposure. Pyrolysis of CNMs is known to yield nanocrystalline graphene, which is in agreement with the observed stiffening of the membranes. Part of this contribution will be devoted to a comparison of different approaches to analyse pressure-deflection measurement data of bulge tests. A sum of the limits approach is suggested as most precise method that employs an analytic pressure-deflection expression for determining the mechanical properties.

[1] X. Zhang and A. Beyer, Nanoscale 13, 1443 (2021).

In this talk, I will discuss unique aspects of two-dimensional crystals that show intertwined nature between electronic, magnetic, optical properties and structural distortions. First, when a transition metal dichalcogenide (TMD) overlay another TMD forming moire superlattices, the softer one deforms itself to conform the mismatched lattices between them so that the unique whirlpool shaped lattice distortions occur [1]. I will show our recent theoretical and computational study shown that the pair of torsional distortions with the opposite chirality introduce characteristic fuzziness in Raman spectroscopy and interesting excitonic signatures irrespective twist angles. agreeing with experiments very well [1]. Second, a newly synthesized 2D carbon allotrope by linking biphenylene molecules is shown to host interesting type II Weyl fermions together with a possible magnetic orderings [2]. With external mechanical perturbations, our first-principles calculations including self-consistent extended Hubbard interactions reveal the anomalous Lifshitz transition of pair annihilations with merging onto zone-center saddle point.

[1] J. Kim, E. Ko et al., Nat. Mater. (2022). https://doi.org/10.1038/s41563-022-01240-2

[2] Y.-W. Son et al., Nano Lett.22, 3112 (2022).