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Electronic properties of MoS 2 /MoO x interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts

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ABSTRACT

In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations.

No MeSH data available.


(a) Side view of the atomic structure of the MoO3 unit cell. The interlayer metal to metal distance and the vdW gap are indicated by d(Mo-Mo) and d(O-O), respectively. (b) The MoO3 4 × 1 × 4 supercell used for the defects study, showing the layered structure along the [010] direction. Red and purple spheres represent O and Mo atoms, respectively. (c) Electronic band structure of bulk MoO3, showing the indirect band gap (indicated by an arrow) with the CBM at Г (0.0 0.0 0.0) and the VBM at U (0.5 0.0 0.5) high-symmetry k-points.
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f1: (a) Side view of the atomic structure of the MoO3 unit cell. The interlayer metal to metal distance and the vdW gap are indicated by d(Mo-Mo) and d(O-O), respectively. (b) The MoO3 4 × 1 × 4 supercell used for the defects study, showing the layered structure along the [010] direction. Red and purple spheres represent O and Mo atoms, respectively. (c) Electronic band structure of bulk MoO3, showing the indirect band gap (indicated by an arrow) with the CBM at Г (0.0 0.0 0.0) and the VBM at U (0.5 0.0 0.5) high-symmetry k-points.

Mentions: We first investigate the electronic properties of bulk and single layer Molybdenum-trioxide (MoO3) in detail. MoO3 shows two phases (α and β): α-MoO3 is stable in an orthorhombic crystal structure with space group Qh26 (Pbnm) (the unit cell lattice parameters are a = 3.962 Å, b = 13.855 Å, and c = 3.699 Å), and the β phase is observed only at high pressure and is metastable at ambient conditions4041. In the α-MoO3 phase (the one considered in this work), each unit cell contains four MoO3 formula units and has an easy cleavage (010) plane, as shown in Fig. 1(a,b). Each monolayer consists of two sublayers, with periodically arranged MoO6 octahedra. Thus, the crystal structure contains three distinct oxygen atoms due to their different coordination: asymmetrical bridging oxygen (unequal bond length with Mo), symmetric bridging oxygen (two Mo bonds with the same bond length and an elongated bond to the next sublayer), and terminal oxygen (single bond Mo-O). The terminal oxygen atom is preferentially deficient during an exfoliation process. The interlayer metal to metal distance d (Mo-Mo) is ~7.00 Å (the Mo-Mo distance within the same layer is 4.03 Å), and the effective vdW gap, d (O-O), is ~0.799 Å (See Fig. 1(a)). The electronic band structure for bulk MoO3 is shown in Fig. 1(c), which indicates that the band gap is of indirect type with the valance band maximum (VBM) at U (0.5 0.0 0.5) and the conduction band minimum (CBM) at Г (0.0 0.0 0.0) point. Our obtained band gaps (Eg) are 1.9 and 2.7 eV at the GGA and HSE levels of calculation, respectively. The HSE result is closer to the experimental values of 3.2 eV for bulk and 2.8 eV for polycrystalline MoO3, as obtained from absorption spectra measurements40. As can be seen in the DOS shown in Fig. 2(a), the MoO3 CBM is mainly contributed by Mo d states and the VBM by O p states.


Electronic properties of MoS 2 /MoO x interfaces: Implications in Tunnel Field Effect Transistors and Hole Contacts
(a) Side view of the atomic structure of the MoO3 unit cell. The interlayer metal to metal distance and the vdW gap are indicated by d(Mo-Mo) and d(O-O), respectively. (b) The MoO3 4 × 1 × 4 supercell used for the defects study, showing the layered structure along the [010] direction. Red and purple spheres represent O and Mo atoms, respectively. (c) Electronic band structure of bulk MoO3, showing the indirect band gap (indicated by an arrow) with the CBM at Г (0.0 0.0 0.0) and the VBM at U (0.5 0.0 0.5) high-symmetry k-points.
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Related In: Results  -  Collection

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f1: (a) Side view of the atomic structure of the MoO3 unit cell. The interlayer metal to metal distance and the vdW gap are indicated by d(Mo-Mo) and d(O-O), respectively. (b) The MoO3 4 × 1 × 4 supercell used for the defects study, showing the layered structure along the [010] direction. Red and purple spheres represent O and Mo atoms, respectively. (c) Electronic band structure of bulk MoO3, showing the indirect band gap (indicated by an arrow) with the CBM at Г (0.0 0.0 0.0) and the VBM at U (0.5 0.0 0.5) high-symmetry k-points.
Mentions: We first investigate the electronic properties of bulk and single layer Molybdenum-trioxide (MoO3) in detail. MoO3 shows two phases (α and β): α-MoO3 is stable in an orthorhombic crystal structure with space group Qh26 (Pbnm) (the unit cell lattice parameters are a = 3.962 Å, b = 13.855 Å, and c = 3.699 Å), and the β phase is observed only at high pressure and is metastable at ambient conditions4041. In the α-MoO3 phase (the one considered in this work), each unit cell contains four MoO3 formula units and has an easy cleavage (010) plane, as shown in Fig. 1(a,b). Each monolayer consists of two sublayers, with periodically arranged MoO6 octahedra. Thus, the crystal structure contains three distinct oxygen atoms due to their different coordination: asymmetrical bridging oxygen (unequal bond length with Mo), symmetric bridging oxygen (two Mo bonds with the same bond length and an elongated bond to the next sublayer), and terminal oxygen (single bond Mo-O). The terminal oxygen atom is preferentially deficient during an exfoliation process. The interlayer metal to metal distance d (Mo-Mo) is ~7.00 Å (the Mo-Mo distance within the same layer is 4.03 Å), and the effective vdW gap, d (O-O), is ~0.799 Å (See Fig. 1(a)). The electronic band structure for bulk MoO3 is shown in Fig. 1(c), which indicates that the band gap is of indirect type with the valance band maximum (VBM) at U (0.5 0.0 0.5) and the conduction band minimum (CBM) at Г (0.0 0.0 0.0) point. Our obtained band gaps (Eg) are 1.9 and 2.7 eV at the GGA and HSE levels of calculation, respectively. The HSE result is closer to the experimental values of 3.2 eV for bulk and 2.8 eV for polycrystalline MoO3, as obtained from absorption spectra measurements40. As can be seen in the DOS shown in Fig. 2(a), the MoO3 CBM is mainly contributed by Mo d states and the VBM by O p states.

View Article: PubMed Central - PubMed

ABSTRACT

In an electronic device based on two dimensional (2D) transitional metal dichalcogenides (TMDs), finding a low resistance metal contact is critical in order to achieve the desired performance. However, due to the unusual Fermi level pinning in metal/2D TMD interface, the performance is limited. Here, we investigate the electronic properties of TMDs and transition metal oxide (TMO) interfaces (MoS2/MoO3) using density functional theory (DFT). Our results demonstrate that, due to the large work function of MoO3 and the relative band alignment with MoS2, together with small energy gap, the MoS2/MoO3 interface is a good candidate for a tunnel field effect (TFET)-type device. Moreover, if the interface is not stoichiometric because of the presence of oxygen vacancies in MoO3, the heterostructure is more suitable for p-type (hole) contacts, exhibiting an Ohmic electrical behavior as experimentally demonstrated for different TMO/TMD interfaces. Our results reveal that the defect state induced by an oxygen vacancy in the MoO3 aligns with the valance band of MoS2, showing an insignificant impact on the band gap of the TMD. This result highlights the role of oxygen vacancies in oxides on facilitating appropriate contacts at the MoS2 and MoOx (x < 3) interface, which consistently explains the available experimental observations.

No MeSH data available.