Effect of metal intermixing on the Schottky barriers of Mo(100)/GaAs(100) interfaces

The electronic and structural properties of Mo(100)/GaAs(100) interfaces and Mo diffusion into GaAs are explored using first principle calculations. Our results show that the interface undergoes substantial atomic rearrangement with respect to the bulk structures and the bilayer of the GaAs adjacent to the interface becomes conducting. We study the n-type Schottky barrier height's dependence on Mo interdiffusion in the GaAs, with values ranging from ∼0.9eV to ∼1.39eV. This range is caused by the diffusants acting as additional n-type doping at the surface and their interaction with the metal-induced gap states.S.P.H. was supported by the EPSRC Grant No. EP/I009973/1, P.V.S. was supported by the Royal Society. Access to the HECToR high-performance computing facility was made available via our membership of the UK's HPC Materials Chemistry Consortium, which was funded by EPSRC (EP/F067496)

First-principles structure determination of interface materials: The NixInAs nickelides

This is the final version of the article. Available from American Physical Society via the DOI in this record.We present here a first-principles study of the ternary compounds formed by Ni, In, and As, a material of great importance for self-aligned metallic contacts in next-generation InAs-based MOS transistors. The approach we outline is general and can be applied to study the crystal structure and properties of a host of other new interface compounds. Using the ab initio random structure searching approach we find the previously unknown low-energy structures of NixInAs and assess their stability with respect to the known binary compounds of Ni, In, and As. Guided by experiments, we focus on Ni3InAs and find a rich energy landscape for this stoichiometry. We consider the five lowest-energy structures, with space groups Pmmn, Pbcm, P21/m, Cmcm, and R3¯. The five low-energy structures for Ni3InAs are all found to be metallic and nonmagnetic. By comparison to previously published TEM results we identify the crystal structure observed in experiments to be Cmcm Ni3InAs. We calculate the work function for Cmcm Ni3InAs and, according to the Schottky-Mott model, expect the material to form an Ohmic contact with InAs. We further explicitly consider the interface between Cmcm Ni3InAs and InAs and find it to be Ohmic with an n-type Schottky barrier height of -0.55eV.This work was supported in part by the EPSRC Grants No. EP/G007489/2, No. EP/J010863/1, and No. EP/I009973/1. All data supporting this study are provided as Supplemental Material accompanying this paper [25]. Computational resources from the University College London and London Centre for Nanotechnology Computing Services as well as HECToR and Archer as part of the UKCP consortium are gratefully acknowledged

First principles electronic and elastic properties of fresnoite Ba2TiSi2O8 (article)

This is the final version of the article. Available from IOP Publishing via the DOI in this record.The research materials supporting this publication can be accessed in ORE at http://hdl.handle.net/10871/30403Electronic, structural and elastic properties of fresnoite, Ba2TiSi2O8 (BTSO), are obtained via first principles calculations. The electronic properties having been comparatively analysed using both the generalised gradient approximation and the hybrid functional method. The indirect band gap of BTSO is found to change significantly through the choice of functional; it shows an increase from 3.79 eV to 5.72 eV. A small indirect gap of 0.33 eV is also present directly above the conduction band edge, which allows for small optical transitions similar to that of defect transitions. The titanium orbitals are dominant near the conduction band edge, with oxygen orbitals being the main contributor to the valence band edge. Dielectric and elastic properties of the material are also obtained, with the bulk modulus being 131.73 GPa and the elastic moduli along the [1 0 0] and [0 0 1] directions being 180.57 GPa and 102.56 GPa, respectively. Theoretical values for Raman frequencies are reported for BTSO. Finally, Bader charge analysis reveals the barium and titanium atoms in BTSO are comparable to their charges in BaTiO3. However, due to the presence of the Si–O bonds, oxygen exhibits a significant charge redistribution. Through the choice of functional, charge can become more localised on the oxygen atoms.Via our membership of the UK's HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work used the ARCHER UK National Supercomputing Service (www.archer.ac.uk). We acknowledge financial support from the Engineering and Physical Sciences Research Council (EPSRC) of the United Kingdom, via the EPSRC Centre for Doctoral Training in Metamaterials (Grant No. EP/L015331/1)

Multi-scale Simulations of Metal-Semiconductor Nanoscale Contacts

PublishedAn electron transport simulations via a metal-semiconductor interface is carried out using multi-scale approach by coupling ab-initio calculations with 3D finite element ensemble Monte Carlo technique. The density functional theory calculations of the Mo/GaAs (001) interface show electronic properties of semiconductor dramatically change close to the interface having a strong impact on the transport. Tunnelling barrier lowers and widens due to a band gap narrowing near the interface reducing resistivity by more than one order of magnitude: from 2.1 × 10-8Ω.cm2 to 4.7 × 10-10Ω.cm2. The dependence of electron effective mass from the distance to the interface also plays a role bringing resistivity to 7.9 × 10-10Ω.cm2.This work was supported by the EPSRC grants EP/I010084/1, EP/I009973/1, and HECToR facility computer resource EPSRC grant EP/F067496. PVS was supported by the Royal Society

Hypersonic modes in nanophononic semiconductors

Frequency gaps and negative group velocities of hypersonic phonon modes in periodically arranged composite semiconductors are presented. Trends and criteria for phononic gaps are discussed using a variety of atomic-level theoretical approaches. From our calculations, the possibility of achieving semiconductor-based one-dimensional phononic structures is established. We present results of the location and size of gaps, as well as negative group velocities of phonon modes in such structures. In addition to reproducing the results of recent measurements of the locations of the band gaps in the nanosized Si/Si0.4Ge0.6 superlattice, we show that such a system is a true one-dimensional hypersonic phononic crystal

Effect of charge transfer on band alignment in 2D|3D heterostructures: A study of HfS 2 | HfO 2 interfaces (article)

This is the final version. Available from the American Physical Society via the DOI in this record. The dataset associated with this article is in ORE at https://doi.org/10.24378/exe.4644HfS 2 | HfO 2 interfaces present a uniquely interesting study in band alignment. The band alignment between materials determines the viability of many electronic devices. We have modeled a variety of two-dimensional|three-dimensional (2D|3D) interfaces HfS 2 | HfO 2 interfaces, showing that the band alignment can change substantially depending on the geometric alignment. Our results have shown that there exists almost no electronic reconstruction when layers of HfS 2 are placed on a HfO 2 substrate. Conversely, when the in-plane connection between HfS 2 and HfO 2 is made (lateral), there is a more significant interface reconstruction present. In the latter case, all examples considered yielded a type I alignment, whereas in the case that layers were parallel to the HfO 2 substrate (stacked) we found that the alignment was either type I or type II with a very small difference between the valance band offsets of the two constituents. We show that the range in the barrier heights between these two systems can vary by up to 2.46 eV. This variation is driven by the amount of charge transfer across the interface and indicate that 2D|3D interfaces have considerably more tunability in their band alignment than 2D|2D or 3D|3D interfaces.Leverhulme TrustEngineering and Physical Sciences Research Counci

Computational study of the enhancement of graphene electrodes for use in Li-ion batteries via forming superlattices with transition metal dichalcogenides

This is the final version. Available on open access from the American Chemical Society via the DOI in this record. In our study, we examined nine transition metal dichalcogenide (TMDC)-graphene superlattices as potential Li-ion intercalation electrodes. We determined their voltages, with ScS2-graphene in T- and R-phases showing the highest at around 3 V, while the others ranged from 0 to 1.5 V. Most superlattices exhibited minimal volumetric expansion (5 to 10%), similar to NMC (8%), except for SnS2-T and NiS2-T, which expanded up to nearly 20%. We evaluated their capacities using a stability metric, EIS, and found that ScS2-T, ScS2-R, and TiS2-T could be intercalated up to two Li ions per MX2 unit without decomposing to Li2S, yielding capacities of 306.77 mA h/g for both ScS2 phases and 310.84 mA h/g for TiS2-T, roughly equivalent to LiC2. MoS2-T could accept Li up to a limit of a = 15/16 in LiaMoS2Cb, corresponding to a capacity of 121.29 mA h/g (equivalent to LiC4). Examining the influence of graphene layers on MoS2-T, we observed a voltage decrease and an initial EIS decrease before effectively flat lining, which is due to charge donation to the middle graphene layer, reducing the electron concentration near the TMDC layer. As graphene layers increased, overall volume expansion decreased with Li intercalation, which is attributed to the in-plane expansion changing. Our results underscore the potential of TMDC-graphene superlattices as Li-ion intercalation electrodes, offering low volumetric expansions, high capacities, and a wide voltage range. These superlattices all show an increase in the capacity of the graphene.Engineering and Physical Sciences Research Council (EPSRC

Predicting Phase Stability at Interfaces

This is the final version. Available on open access from the American Physical Society via the DOI in this recordData availability: The data that supports this Letter is openly available in ORE at https://doi.org/10.24378/exe.4966We present the RAFFLE methodology for structural prediction of the interface between two materials and demonstrate its effectiveness by applying it to MgO encapsulated by two layers of graphene. To address the challenge of interface structure prediction, our methodology combines physical insights derived from morphological features observed in related systems with an iterative machine learning technique. This employs physical-based methods, including void-filling and n-body distribution functions to predict interface structures. For the carbon-MgO encapsulated system, we have shown the rocksalt and hexagonal phases of MgO to be the two most energetically stable in the few-layer regime. We demonstrate that monolayer rocksalt is heavily stabilized by interfacing with graphene, becoming more energetically favorable than the graphenelike monolayer hexagonal MgO. The RAFFLE methodology provides valuable insights into interface behavior, and a route to finding new materials at interfaces.Engineering and Physical Sciences Research Council (EPSRC)Leverhulme Trus

Properties of layered TMDC superlattices for electrodes in Li-ion and Mg-ion batteries

This is the final version. Available on open access from the American Chemical Society via the DOI in this record. In this work, we present a first-principles investigation of the properties of superlattices made from transition metal dichalcogenides for use as electrodes in lithium-ion and magnesium-ion batteries. From a study of 50 pairings, we show that, in general, the volumetric expansion, intercalation voltages, and thermodynamic stability of vdW superlattice structures can be well approximated with the average value of the equivalent property for the component layers. We also found that the band gap can be reduced, improving the conductivity. Thus, we conclude that superlattice construction can be used to improve material properties through the tuning of intercalation voltages toward specific values and by increasing the stability of conversion-susceptible materials. For example, we demonstrate how pairing SnS2 with systems such as MoS2 can change it from a conversion to an intercalation material, thus opening it up for use in intercalation electrodes.Engineering and Physical Sciences Research Council (EPSRC

Dominance of interface chemistry over the bulk properties in determining the electronic structure of epitaxial metal/perovskite oxide heterojunctions

This is the author accepted manuscript. The final version is available from the American Chemical Society via the DOI in this record.We show that despite very similar crystallographic properties and work function values in bulk Fe and Cr, epitaxial films of these metals on Nb:SrTiO3(001) exhibit completely different heterojunction electronic properties. The Cr/SrTiO3 interface is ohmic, whereas Fe/SrTiO3 forms a Schottky barrier with a barrier height of 0.50 eV. This difference arises because of variations in interface chemistry. In contrast to Cr [Chambers, S. A., Adv. Mater. 2013, 25, 4001.], in-diffused Fe exhibits a +2 oxidation state and occupies Ti sites in the perovskite lattice, resulting in negligible charge transfer to Ti, upward band bending, and Schottky barrier formation. The differences between Cr and Fe are understood by performing first-principles calculations of the energetics of defect formation, which corroborate experimental results.This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award #10122. The work described was conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830. S.P.H. was supported by the EPSRC Grant No.EP/I009973/1. Access to the HECToR high-performance computing facility was made available via S.P.H. membership of the U.K.’s HPC Materials Chemistry Consortium, which was funded by EPSRC (EP/F067496)