Modulation of van der Waals and classical epitaxy induced by strain at the Si step edges in GeSbTe alloys

Thick Does the Trick: Genesis of Ferroelectricity in 2D GeTe-Rich (GeTe)m (Sb2 Te3 )n Lamellae

The possibility to engineer (GeTe)m (Sb2 Te3 )n phase-change materials to co-host ferroelectricity is extremely attractive. The combination of these functionalities holds great technological impact, potentially enabling the design of novel multifunctional devices. Here an experimental and theoretical study of epitaxial (GeTe)m (Sb2 Te3 )n with GeTe-rich composition is presented. These layered films feature a tunable distribution of (GeTe)m (Sb2 Te3 )1 blocks of different sizes. Breakthrough evidence of ferroelectric displacement in thick (GeTe)m (Sb2 Te3 )1 lamellae is provided. The density functional theory calculations suggest the formation of a tilted (GeTe)m slab sandwiched in GeTe-rich blocks. That is, the net ferroelectric polarization is confined almost in-plane, representing an unprecedented case between 2D and bulk ferroelectric materials. The ferroelectric behavior is confirmed by piezoresponse force microscopy and electroresistive measurements. The resilience of the quasi van der Waals character of the films, regardless of their composition, is also demonstrated. Hence, the material developed hereby gathers in a unique 2D platform the phase-change and ferroelectric switching properties, paving the way for the conception of innovative device architectures.

On the van der Waals Epitaxy of Homo-/Heterostructures of Transition Metal Dichalcogenides

Layered materials held together by weak van der Waals (vdW) interactions are a promising class of materials in the field of nanotechnology. Besides the potential for single layers, stacking of various vdW layers becomes even more promising since unique properties can hence be precisely engineered. The synthesis of stacked vdW layers, however, remains to date, hardly understood. Therefore, in this work, the vdW epitaxy of transition metal dichalcogenides (TMDs) on single-crystalline TMD templates is investigated in depth. It is demonstrated that the role of lattice mismatch is insignificant. More importantly is the role of surface energy, calculated using density functional theory, which plays an essential role in the activation energy for adatom diffusion, hence nucleation density. This in turn correlates with defect density since the stacking sequence in vdW epitaxy is generally poorly controlled. Moreover, the vapor pressure of the transition metal is also found to correlate with adatom diffusion. Consequently, the proposed study enables important and new insight in the vdW epitaxy of multilayer 2D homo-/heterostructures.

Peculiar alignment and strain of 2D WSe2 grown by van der Waals epitaxy on reconstructed sapphire surfaces

The increasing scientific and industry interest in 2D MX₂ materials within the field of nanotechnology has made the single crystalline integration of large area van der Waals (vdW) layers on commercial substrates an important topic. The c-plane oriented (3D crystal) sapphire surface is believed to be an interesting substrate candidate for this challenging 2D/3D integration. Despite the many attempts that have been made, the yet incomplete understanding of vdW epitaxy still results in synthetic material that shows a crystallinity far too low compared to natural crystals that can be exfoliated onto commercial substrates. Thanks to its atomic control and in situ analysis possibilities, molecular beam epitaxy (MBE) offers a potential solution and an appropriate method to enable a more in-depth understanding of this peculiar 2D/3D hetero-epitaxy. Here, we report on how various sapphire surface reconstructions, that are obtained by thermal annealing of the as-received substrates, influence the vdW epitaxy of the MBE-grown WSe₂ monolayers (MLs). The surface chemistry and the interatomic arrangement of the reconstructed sapphire surfaces are shown to control the preferential in-plane epitaxial alignment of the stoichiometric WSe₂ crystals. In addition, it is demonstrated that the reconstructions also affect the in-plane lattice parameter and thus the in-plane strain of the 2D vdW-bonded MLs. Hence, the results obtained in this work shine more light on the peculiar concept of vdW epitaxy, especially relevant for 2D materials integration on large-scale 3D crystal commercial substrates.

Phase change thin films for non-volatile memory applications

The rapid development of Internet of Things devices requires real time processing of a huge amount of digital data, creating a new demand for computing technology. Phase change memory technology based on chalcogenide phase change materials meets many requirements of the emerging memory applications since it is fast, scalable and non-volatile. In addition, phase change memory offers multilevel data storage and can be applied both in neuro-inspired and all-photonic in-memory computing. Furthermore, phase change alloys represent an outstanding class of functional materials having a tremendous variety of industrially relevant characteristics and exceptional material properties. Many efforts have been devoted to understanding these properties with the particular aim to design universal memory. This paper reviews materials science aspects of chalcogenide-based phase change thin films relevant for non-volatile memory applications. Particular emphasis is put on local structure, control of disorder and its impact on material properties, order-disorder transitions and interfacial transformations.

Strain engineering of van der Waals heterostructures

Modifying the strain state of solids allows control over a plethora of functional properties. The weak interlayer bonding in van der Waals (vdWaals) materials such as graphene, hBN, MoS₂, and Bi₂Te₃ might seem to exclude strain engineering, since strain would immediately relax at the vdWaals interfaces. Here we present direct observations of the contrary by showing growth of vdWaals heterostructures with persistent in-plane strains up to 5% and we show that strain relaxation follows a not yet reported process distinctly different from strain relaxation in three-dimensionally bonded (3D) materials. For this, 2D bonded Bi₂Te₃-Sb₂Te₃ and 2D/3D bonded Bi₂Te₃-GeTe multilayered films are grown using Pulsed Laser Deposition (PLD) and their structure is monitored in situ using Reflective High Energy Electron Diffraction (RHEED) and post situ analysis is performed using Transmission Electron Microscopy (TEM). Strain relaxation is modeled and found to solely depend on the layer being grown and its initial strain. This insight demonstrates that strain engineering of 2D bonded heterostructures obeys different rules than hold for epitaxial 3D materials and opens the door to precise tuning of the strain state of the individual layers to optimize functional performance of vdWaals heterostructures.