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DOI: 10.1039/D4NA00482E
(Paper)
Nanoscale Adv., 2024, Advance Article

Son T. Nguyen^{a},
Chuong V. Nguyen*^{b},
Huynh V. Phuc^{c},
Nguyen N. Hieu^{de} and
Cuong Q. Nguyen*^{de}
^{a}Faculty of Electrical Engineering, Hanoi University of Industry, Hanoi 100000, Vietnam. E-mail: nguyensontung@haui.edu.vn
^{b}Department of Materials Science and Engineering, Le Quy Don Technical University, Hanoi 100000, Vietnam. E-mail: chuong.vnguyen@lqdtu.edu.vn
^{c}Division of Physics, School of Education, Dong Thap University, Cao Lanh 870000, Vietnam
^{d}Institute of Research and Development, Duy Tan University, Da Nang 550000, Vietnam. E-mail: nguyenquangcuong3@duytan.edu.vn
^{e}Faculty of Natural Sciences, Duy Tan University, Da Nang 550000, Vietnam

Received
11th June 2024
, Accepted 26th July 2024

First published on 26th July 2024

Minimizing the contact barriers at the interface, forming between two different two-dimensional metals and semiconductors, is essential for designing high-performance optoelectronic devices. In this work, we design different types of metal–semiconductor heterostructures by combining 2D metallic MX_{2} (M = Nb, Hf; X = S, Se) and 2D semiconductor SiH and investigate systematically their electronic properties and contact characteristics using first principles calculations. We find that all the MX_{2}/SiH (M = Nb, Ta; X = S, Se) heterostructures are energetically stable, suggesting that they could potentially be synthesized in the future. Furthermore, the generation of the MX_{2}/SiH metal–semiconductor heterostructures leads to the formation of the Schottky contact with ultra-low Schottky barriers of a few tens of meV. This finding suggests that all the 2D MX_{2} (M = Nb, Ta; X = S, Se) metals act as effective electrical contact 2D materials to contact with the SiH semiconductor, enabling electronic devices with high charge injection efficiency. Furthermore, the tunneling resistivity of all the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs is low, confirming that they exhibit high electron injection efficiency. Our findings underscore fundamental insights for the design of high-performance multifunctional Schottky devices based on the metal–semiconductor MX_{2}/SiH heterostructures with ultra-low contact barriers and high electron injection efficiency.

More recently, silicane (SiH), a novel 2D material has been predicted by full hydrogenation of monolayer silicene on both sides.^{13,14} It should be noted that silicane can also be synthesized by mechanical exfoliation, a strategy that has been used to obtain germanane (GeH).^{15} Additionally, hydrogenation opens a band gap, stabilizes the structure and eliminates conductivity in the silicene monolayer.^{16} Moreover, the electronic, thermoelectric and optical properties of the SiH monolayer are sensitive to external conditions, including strains,^{17} electric fields^{18} and constructing heterostructures.^{19–22} Among those strategies, construction of heterostructures has been proven to be one of the most effective strategies to enhance the physical properties of the SiH monolayer. For instance, we previously investigated the electronic properties and carrier mobility of the BP/SiH heterostructure as well as the effect of an electric field. The combination of SiH and BP monolayers gives rise to an enhancement in the optical absorption and carrier mobility compared to the constituent monolayers. Sheng et al.^{23} combined the InSe/SiH heterostructure and demonstrated that such combination leads to an enhancement of photocatalytic efficiency. Furthermore, the combination between SiH and PtSe_{2} (ref. 24) or AlAs^{25} also leads to an enhancement in the absorption coefficient and photocatalytic properties. It is found that the physical properties of the SiH material can be tuned when it is combined with other 2D semiconductors. However, to date, the combination between SiH and other 2D metals has not yet been extensively investigated.

In this work, we design MX_{2}/SiH MSHs (M = Nb, Ta; X = S, Se) by stacking the 2D MX_{2} (M = Nb, Ta; X = S, Se) metals above on top of the SiH semiconductor using first-principles calculations. It is demonstrated that all the 2D MX_{2}/SiH (M = Nb, Ta; X = S, Se) metal–semiconductor heterostructures form the Schottky contact with ultra-low contact barriers of a few tens of meV. Our findings could provide fundamental insights and open an avenue for the design of high-performance multifunctional Schottky devices based on the metal–semiconductor MX_{2}/SiH heterostructures with ultra-low contact barriers and high electron injection efficiency.

Fig. 1 (a) Top and side views of the atomic structures and (b) band structures of monolayers SiH and MX_{2} (M = Nb, Ta; X = S, Se). The Fermi level is set to be zero. |

We further design the MX_{2}/SiH MSHs by vertically stacking the 2D MX_{2} metals above on top of the 2D SiH semiconductor. The atomic structures of the MX_{2}/SiH MSHs are depicted in Fig. 2 and S1 of the ESI.† We investigated four different stacking patterns (SP) of the MX_{2}/SiH MSHs. The most energetically favorable SP is depicted in Fig. 2. To minimize the effects of strain caused by the lattice mismatch between the two different layers, the MX_{2}/SiH MSHs are designed by using (2 × 2) and supercells of MX_{2} and SiH layers, respectively. The overall lattice mismatch for the MX_{2}/SiH MSHs is less than 2%. This value is still small and affects insignificantly the electronic properties of these 2D materials. After geometric optimization, the interlayer distances between the MX_{2} and SiH layers are obtained and listed in Table 1. The interlayer distances vary from 2.31 Å for the TaS_{2}/SiH to 2.33 Å for the NbS_{2}/SiH and to 2.37 Å for the TaSe_{2}/SiH and to 2.39 Å for the NbSe_{2}/SiH MSHs. This finding indicates that the interlayer distances in Nb(Ta)S_{2}/SiH are shorter than those in the Nb(Ta)Se_{2}/SiH MSH. In addition, we can find that these values of the interlayer distances in MX_{2}/SiH heterostructures are comparable to those in other SiH-based heterostructures, such as AlAs/SiH,^{25} SiH/CdI_{2} (ref. 31) and InSe/SiH.^{23} Furthermore, we check the stability of all the MX_{2}/SiH MSHs by calculating the binding energy as follows:

(1) |

Fig. 2 (a) Top view and (b) side views of the atomic structures of the MX_{2}/SiH MS-vdWHs (M = Nb, Ta; X = S, Se). |

a, Å | d, Å | E_{b}, meV Å^{−2} |
Φ_{p}, eV |
Φ_{n}, eV |
Contact types | |
---|---|---|---|---|---|---|

NbS_{2}/SiH |
6.62 | 2.33 | −14.50 | 0.010 | 2.15 | p-ShC |

NbSe_{2}/SiH |
6.80 | 2.39 | −12.61 | 0.024 | 2.15 | p-ShC |

TaS_{2}/SiH |
6.63 | 2.32 | −14.17 | 0.014 | 2.14 | p-ShC |

TaSe_{2}/SiH |
6.81 | 2.37 | −12.72 | 0.067 | 2.12 | p-ShC |

The projected band structures of the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs are depicted in Fig. 3(a). One can find that the band structures of the MX_{2}/SiH MSHs appear to be a sum of the band structures of the constituent MX_{2} metal and SiH semiconductor. More interestingly, the combination between MX_{2} metals and the SiH semiconductor leads to generation of metal/semiconductor heterostructures. Depending on the position of the band edges of the semiconductor relative to the Fermi level of metal, the combined heterostructure can form either the Schottky contact (ShC) or ohmic contact (OhC), as illustrated in Fig. 3(b). By analyzing the projected band structures, we observe that all the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs lead to generation of the Schottky contact. In the band structures of the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs, the Fermi level of the MX_{2} metal lies between the band edges of the SiH semiconductor. In addition, we find that the VBM of the SiH semiconductor is closer to the Fermi level than its CBM, indicating that all the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs exhibit the p-type Schottky contact. The Schottky contact barriers for the p-type and n-type Schottky contact (ShC) can be obtained as: Φ_{p} = E_{F} – E_{VBM} and Φ_{n} = E_{CBM} – E_{F}, where E_{VBM} and E_{CBM}, respectively, are the energy of the VBM and CBM of the semiconductor SiH. E_{F} is the Fermi level of the MX_{2}/SiH heterostructure. The contact barriers for the MX_{2}/SiH MSHs are listed in Table 1. It is interesting that the p-type Schottky barriers in all the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs are ultra-low. The NbS_{2}/SiH MSH shows the smallest p-type Schottky barrier of 10 meV, while the largest Schottky barrier is observed in the TaSe_{2}/SiH MSH, which is only 67 meV. The observed ultra-low contact barrier in the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs indicates that the charge injection efficiency in these heterostructures is highly advantageous.^{35,36} Thereby, the 2D MX_{2} metals act as effective electrical contact 2D materials to contact with the SiH semiconductor, enabling electronic devices with high charge injection efficiency.

Furthermore, to examine the charge injection efficiency of all the MX_{2}/SiH MSHs, we calculate the tunneling probability and contact tunneling resistivity as follows:^{37}

(2) |

The w_{TB} and Φ_{TB} represent the tunneling width and tunneling height, which can be obtained by analyzing the electrostatic potential of the considered heterostructures. The electrostatic potentials of all the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs are depicted in Fig. 4. The obtained tunneling probability and contact tunneling resistivity of all the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs are listed in Table 2 and illustrated in Fig. 5. One can find that a higher tunneling probability is correlated with lower tunneling resistivity^{38} and enhanced electron injection. The tunneling probability of the NbSe_{2}/SiH MSH is higher than that of the other heterostructures, suggesting that the NbSe_{2} 2D metal acts as a superior electrical contact 2D material to contact with the SiH semiconductor to achieve high charge injection efficiency. Additionally, we find that the tunneling resistivity of all the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs is as low as that of the low-contact-resistance Bi/MoS_{2}.^{39,40} A lower tunneling resistivity correlates with a higher electron injection efficiency. Therefore, the electron injection efficiency in all the MX_{2}/SiH MSHs is high. These findings suggest that the MX_{2} 2D metals can act as promising electrodes for sub-10 nm field-effect transistors.^{41} Additionally, the work functions of NbS_{2}, NbSe_{2}, TaS_{2} and TaSe_{2} 2D metals are calculated to be 6.12, 5.90, 5.51 and 5.36 eV. It is obvious that the work function of the MX_{2} monolayers is still higher than that for the common electrode graphene (4.6 eV).^{42} The large value of the work functions in the MX_{2} 2D metals leads to an alignment towards the VBM of the 2D SiH semiconductor, forming a p-type Schottky contact.^{43}

Fig. 4 Calculated electrostatic effective potentials of (a) NbS_{2}/SiH, (b) NbSe_{2}/SiH, (c) TaS_{2}/SiH and (d) TaSe_{2}/SiH MSHs. |

Fig. 5 Calculated tunneling probability and contact tunneling resistivity of all the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs. |

In addition, as illustrated in Fig. 4, we can see that the potential of the SiH layer is higher than that of the NbS(Se)_{2} layer in their corresponding MSHs, but it is deeper than the potential of the TaS(Se)_{2} layer. The charges flow from the layer with a deeper potential to the layer with a higher one. Therefore, electrons are transferred from the NbS(Se)_{2} layer to the SiH layer in the NbS(Se)_{2}/SiH MSH. On the other hand, charges will flow from SiH to the TaS(Se)_{2} layer in the TaS(Se)_{2}/SiH MSH.

We further calculated the charge density difference (CDD) in the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs to visualize the charge redistribution and charge transfer between the two constituent monolayers. The CDD in the heterostructure can be obtained from the difference in the charge densities of the heterostructure (ρ_{MSH}) and the constituent metal (ρ_{M}) and semiconductor (ρ_{S}) as follows:

Δρ = ρ_{MSH} − ρ_{M} − ρ_{S}
| (3) |

The amount of charge transfer at the interface of MX_{2}/SiH MSHs is obtained as:^{44,45}

(4) |

The CDD for all the MX_{2}/SiH (M = Nb, Ta; X = S, Se) MSHs is depicted in Fig. 6. Yellow and cyan regions indicate positive and negative charges, respectively. For the NbS_{2}/SiH heterostructure, the positive charges are mainly accumulated in the NbS_{2} layer, while the negative charges are depleted in the SiH layer, as depicted in Fig. 6(a) and its inset. This finding indicates that the electrons are transferred from the NbS_{2} layer to the SiH layer, i.e. the metallic NbS_{2} layer loses electrons, while the semiconducting SiH layer gains them. A similar trend of charge transfer is also observed in the NbSe_{2}/SiH MSH, as illustrated in Fig. 6(b). On the other hand, for the TaS(Se)_{2}/SiH MSH, the positive charges are mainly accumulated on the side of the SiH layer, while the negative charges are depleted in the TaS(Se)_{2} layer. The electrons are transferred from the SiH layer to the TaS(Se)_{2} layer. From eqn (5), the amount of charge transfer at the interfaces of the NbS_{2}/SiH, NbSe_{2}/SiH, TaS_{2}/SiH and TaSe_{2}/SiH heterostructures is calculated to be 0.045, 0.012, 0.024 and 0.015e, respectively. Such amount of charge transfer is small, indicating that weak interactions are dominant at the interface of heterostructures. Furthermore, it should be noted that owing to the ultra-low Schottky barriers, the contact barriers and contact types in the MX_{2}/SiH MSHs can be easily tuned by external conditions.^{46–51} For instance, applying a compressive strain of −3% includes a transition from Schottky to the ohmic contact in the NbS_{2}/SiH heterostructure, as illustrated in Fig. S2 of the ESI.† The versatility in the contact behavior of the MX_{2}/SiH MSHs under external conditions makes them promising candidates for next-generation multifunctional devices.

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## Footnote |

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4na00482e |

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