Modulating the Schottky barrier of MXenes/2D SiC contacts via functional groups and biaxial strain: a first-principles study

Two-dimensional (2D) graphene-like SiC has attracted intense interest recently due to its unique electrical and physical properties. In implementing 2D semiconductors in device applications, one of the main challenges so far has been the formation of a high-quality Schottky barrier owing to the strong Fermi level pinning (FLP) at the interface of traditional metal-2D semiconductor contacts. In this paper, the 2D MXenes Ti₃C₂T₂ (T = F, O, OH) are proposed to serve as electrodes for 2D SiC. The structural and barrier properties of the Ti₃C₂T₂/SiC contacts were systematically investigated based on first-principles calculations combined with the GGA-PBE and HSE06 functionals. It is found that Ti₃C₂T₂ can be bonded with 2D SiC by van der Waals (vdW) interactions. Weak FLP is exhibited at Ti₃C₂T₂/SiC vdW contacts. The type of contact can be tuned by changing the functional T group of Ti₃C₂T₂. Ti₃C₂F₂/SiC and Ti₃C₂O₂/SiC contacts exhibit a p-type Schottky contact and p-type Ohmic contact, respectively, whereas an n-type Ohmic contact occurs in the Ti₃C₂(OH)₂/SiC contact. In addition, the calculated tunneling possibility (T_B) is ∼20% between Ti₃C₂T₂ and SiC, indicating weak bonding at the Ti₃C₂T₂/SiC vdW junctions. Furthermore, the Schottky barrier height and T_B of the Ti₃C₂(OH)₂/SiC contacts can be modulated via the biaxial strain. The controllable contact type and barrier in Ti₃C₂T₂/SiC contacts provide guidelines for developing high-performance 2D SiC optoelectronic and electronic devices.

Intriguing electronic, optical and photocatalytic performance of BSe, M2CO2 monolayers and BSe-M2CO2 (M = Ti, Zr, Hf) van der Waals heterostructures

Using density functional (DFT) theory calculations, we have investigated the electronic band structure, optical and photocatalytic response of BSe, M₂CO₂ (M = Ti, Zr, Hf) monolayers and their corresponding BSe–M₂CO₂ (M = Ti, Zr, Hf) van der Waals (vdW) heterostructures. Optimized lattice constant, bond length, band structure and bandgap values, effective mass of electrons and holes, work function and conduction and valence band edge potentials of BSe and M₂CO₂ (M = Ti, Zr, Hf) monolayers are in agreement with previously available data. Binding energies, interlayer distance and Ab initio molecular dynamic simulations (AIMD) calculations show that BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures are stable with specific stacking and demonstrate that these heterostructures might be synthesized in the laboratory. The electronic band structure shows that all the studied vdW heterostructures have indirect bandgap nature – with the CBM and VBM at the Γ–K and Γ-point of BZ for BSe–Ti₂CO₂, respectively; while for BSe–Zr₂CO₂ and BSe–Hf₂CO₂ vdW heterostructures the CBM and VBM lie at the K-point and Γ-point of BZ, respectively. Type-II band alignment in BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures prevent the recombination of electron–hole pairs, and hence are crucial for light harvesting and detection. Absorption spectra are investigated to understand the optical behavior of BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures, where the lowest energy transitions are dominated by excitons. Furthermore, BSe–M₂CO₂ (M = Ti, Zr, Hf) vdW heterostructures are found to be potential photocatalysts for water splitting at pH = 0, and exhibit enhanced optical properties in the visible light zones.

Using density functional theory calculations, we have investigated the electronic band structure, optical and photocatalytic response of BSe, M₂CO₂ (M = Ti, Zr, Hf) monolayers and their corresponding BSe–M₂CO₂ (M = Ti, Zr, Hf) van der Waals heterostructures.

Van der Waals heterostructure of graphene and germanane: tuning the ohmic contact by electrostatic gating and mechanical strain

Recent exciting developments in synthesis and properties study of the Germanane (GeH) monolayer have inspired us to investigate the structural and electronic properties of the van der Waals GeH/Graphene (Gr) heterostructure by the first-principle approach. The stability of the GeH/Gr heterostructure is verified by calculating the phonon dispersion curves as well as by thermodynamic binding energy calculations. According to the band structure calculation, the GeH/Gr interface is n-type Ohmic. The effects of different interlayer distances and strains between the layers and the applied electric field on the interface have been investigated to gain insight into the van der Waals heterostructure modifications. An interlayer distance of 2.11 Å and compressive strain of 6% alter the contact from Ohmic to Schottky status, while the electric field can tune the GeH/Gr contact as p- or n-type, Ohmic, or Schottky. The average electrostatic potential of GeH/Gr and the Bader charge analysis have been used to explain the results obtained. Our theoretical study could provide a promising approach for improving the electronic performance of GeH/Gr-based nano-rectifiers.

Non-invasively improving the Schottky barrier of MoS2/metal contacts by inserting a SiC layer

The applications of two-dimensional (2D) materials in electronics, optoelectronics, and spintronics are limited by the high contact resistance at the metal/semiconductor interface owing to the strong Fermi-level pinning. In this study, an interlayer insertion strategy is proposed to solve this problem, and first principles calculations are done to study the influences of inserting a SiC layer on the Schottky barrier and electronic properties of MoS2/metals (Mg, Al, In, Cu, Ag, Au, Pd, Ti, and Sc). The average charge value substantially increased (≥0.060 e) at the interface between SiC and MoS2 layers, and then no tunneling barrier appeared except for the MoS2/Au contact by inserting the SiC layer. Moreover, ΦSB,N almost decreases for the MoS2/metal contacts by inserting the SiC layer. When Ti, Cu, Au, and Pd are used as electrodes, the n-type Schottky barrier is formed with the ΦSB,N values of 0.479 eV, -0.073 eV, 0.498 eV, and 0.225 eV, respectively. However, if Al, In, Mg, and Ag are used as electrodes, the systems are transformed into Ohmic contact. These findings provide a practical guideline for depinning the Fermi level at contact interfaces and designing the high performance TMD-based nanoelectronic devices.