GaSe heterostructure

Journal Pre-proof Stacking and electric field effects on the band alignment and electronic properties of the GeC/GaSe heterostructure Dat D. Vo, Vo T.T. Vi, Tan Phat Dao, Tuan V. Vu, Huynh V. Phuc, Nguyen N. Hieu, Nguyen T.T. Binh, Chuong V. Nguyen

PII: DOI: Reference:

S1386-9477(19)31887-9 https://doi.org/10.1016/j.physe.2020.114050 PHYSE 114050

To appear in:

Physica E: Low-dimensional Systems and Nanostructures

Received date : 16 December 2019 Revised date : 10 February 2020 Accepted date : 21 February 2020 Please cite this article as: D.D. Vo, V.T.T. Vi, T.P. Dao et al., Stacking and electric field effects on the band alignment and electronic properties of the GeC/GaSe heterostructure, Physica E: Low-dimensional Systems and Nanostructures (2020), doi: https://doi.org/10.1016/j.physe.2020.114050. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

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Stacking and electric field effects on the band

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alignment and electronic properties of the GeC/GaSe heterostructure

Dat D. Vo a,b , Vo T. T. Vi c , Tan Phat Dao d , Tuan V. Vu a,b ,

Huynh V. Phuc e , Nguyen N. Hieu f , Nguyen T. T . Binh f,∗ , Chuong V. Nguyen g,∗

a Division

of Computational Physics, Institute for Computational Science, Ton Duc Thang University, Ho Chi Minh City, Vietnam

b Faculty

of Electrical & Electronics Engineering, Ton Duc Thang University, Ho Chi Minh City, Vietnam.

c Department d Center

of Physics, University of Education, Hue University, Hue, Vietnam

of Excellence for Green Energy and Environmental Nanomaterials, Nguyen Tat

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Thanh University, Ho Chi Minh City, Vietnam

e Division f Institute

of Theoretical Physics, Dong Thap University, Dong Thap, Viet Nam

of Research and Development, Duy Tan University, Da Nang, Viet Nam of Materials Science and Engineering, Le Quy Don Technical University,

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g Department

Ha Noi, Viet Nam

ABSTRACT

Combining different two-dimensional materials into layered van der Waals heterostruc-

tures are recently considered as an effective route to enhance the electronic properties of the

Preprint submitted to Elsevier

10 February 2020

Journal Pre-proof constituent materials and to extend the application range in next-generation nanodevices. Here, the GeC/GaSe heterostructure and its electronic properties controlled by electric field have been constructed and systematically investigated using first-principles calculations. Five different staking patterns of the GeC/GaSe heterostructure are constructed to consider

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the stacking effects on the electronic properties. We find that the GeC/GaSe heterostructure is mainly characterized by the weak van der Waals forces, dominating between GeC

and GaSe layers, preserving their intrinsic properties in GeC/GaSe heterostructure. The GeC/GaSe heterostructure exhibits the type-II band alignment, where the electron-hole pairs are separated, making it suitable for fabricating next-generation optoelectronic nan-

odevices. Moreover, the stacking configurations have little affect the structural and electronic properties of the GeC/GaSe heterostructures. The pattern-I stacking configuration

has the lowest binding energy and shortest interlayer distance as compared with other stacking patterns of the GeC/GaSe heterostructure. Furthermore, our results demonstrate

that the electric field is considered as an effective route to modulate the electronic properties of GeC/GaSe heterostructure from semiconductor to metal. This finding makes the GeC/GaSe heterostructure promising material for optoelectronic nanodevices.

Keywords: GeC monolayer; GaSe monolayer; DFT calculations; Electronic properties;

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van der Waals heterostructures.

Introduction

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The successful exfoliation of graphene [1], a member of two-dimensional (2D) materials family in 2004 has opened up new opportunities for designing nextgeneration high-performance nanoelectronic and optoelectronic devices, such as ∗ Corresponding authors. Email addresses: [email protected] (Dat D. Vo), [email protected] (Nguyen T. T . Binh), [email protected] (Chuong V. Nguyen).

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Journal Pre-proof graphene-based field-effect transistors (FETs) [2,3], photodetectors [4], ultracapacitors [5]. One can observe that there have been a large number of the scientific research on graphene, not only applications [6,7] but also fundamentals [8,9]. In par-

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allel with the graphene’s research, other 2D materials, including silicene [10,11], germanene [12,13], phosphorene [14–17], transition metals dichalcogenides [18,19] and monochalcogenides [20,21] have attracted tremendous considerable because of their extraordinary physical and optical properties, which are desirable for opto-

electronics and nanoelectronics. Among these diverse 2D materials, GaSe monolayer– a member of 2D monochalcogenides, has successfully been synthesized in experiment using a micromechanical cleavage technique [20] or a vapor phase deposition approach [22]. Unlike the gap-less graphene, GaSe monolayer is semiconductor with a band gap of about 2 eV. The intriguing properties of GaSe monolayer, such as thermal stability, high photoresponsivity of 1.7 A/W at zero gate voltage [22], anisotropic Hall-mobility of aboult 215 cm2 /V×s [23], making it promising candidate for future nanoelectronic devices, such as FETs [20], photodetectors [22].

More recently, a new type of group–IV compounds, graphene-like GeC has

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considered as a potential material for future optoelectronic devices because it shows the excellent stability and exhibits the semiconducting behavior with a large band gap of about 2.1 eV [24]. It is interesting that the electronic properties of GeC monolayer can be engineered by strain [25,26], electric field [27], surface functionalization [28,29]. For instance, Lu et al. [25] claimed that the indirect–direct

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band gap transition can be realized in GeC monolayer through strain engineering. Li et al. [29] found that the non-magnetic semiconductor can be converted into ferromagnetic/antiferromagnetic semiconductor when Ge atoms/C atoms are fully hydrogenated to GeC monolayer, suggesting its great potential application for magnetic nanoelectronics. More interestingly, one can find that the electronic and optical properties of GeC monolayer can be controlled by stacking it on top of another 3

Journal Pre-proof 2D materials to form the vertical layered heterostructures. It should be noted here that the formation of the vertical heterostructures, which consists of two or more different 2D materials has recently been considered as an effective route to modify

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and enhance the electronic and optical properties of the constituent monolayers. Currently, a large number of vertical heterostructures between 2D materials

has been experimentally synthesized and theoretically proposed, such as SnSe2 /MoS2 [30,31], graphene/MoS2 [32–34], phosphorene/TMDs [35–37] etc. Especially, to date, the

combination between GeC and other 2D materials, including GeC/MoS2 [38], GeC/MSSe (M = Mo, W) [39], GeC/ZnO [40,41], blue phosphorus/GeC [42], GeC/WS2 [43]

has also been received great interest owing to the preservation of the intriguing electronic properties and the enhancement of the optical properties. For instance,

Din et al. [39] have claimed that the type-II band alignment and larger Rashba spin splitting in the GeC/MSSe (M = W, Mo) heterostructures make them promising materials for designing electronic, spintronic, and future renewable energy devices.

Further, Gao et al. [40] have demonstrated that the GeC/ZnO heterostructure shows an outstanding absorption coefficient in the visible light region, which is as twice

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as that of components, predicting its suitable candidate for photocatalyst devices.

However, up to date, there is still no work that focused on the combination between GeC and GaSe monolayers and their electronic and optical properties, as well as the stacking and electric field effects. Therefore, in this work, we construct the GeC/GaSe heterostructures with different stacking configurations and investi-

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gate their electronic and optical properties at the ground state using first principles calculations. The stacking and electric field effects on the electronic properties of GeC/GaSe heterostructures were also considered.

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Computational methods All the calculations are performed within density functional theory (DFT) and

simulated via the Quantum Espresso package [44,45]. The generalized gradient

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approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) [46] approach and

projector augmented wave (PAW) [47] are adopted for describing the exchange correlation energy and the ion-electron interactions, respectively. The traditional DFT methods always underestimate the long-range forces, which are mainly contributed

in layered structures, thus we employ the DFT-D3 method to resolve this issue. A cut-off energy of 500 eV and a Monkhorst-Pack k-point mesh of 9 × 9 × 1 are

employed in all structural and electronic properties calculations. All geometric op˚ and timization are calculated with the forces and energy convergence of 0.01 eV/A 10−6 eV, respectively. The spurious interactions, existing between the adjacent cells

along the z direction of heterostructure were separated by a large vacuum thickness ˚ of 30 A.

Results and discussion

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Before constructing the GeC/GaSe heterosutructure, we first check the lattice constants of both GeC and GaSe monolayers at the ground state, along with their electronic and thermodynamic properties. These calculations are depicted in

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Fig. 1. Similar graphene, GeC monolayer shows a planar honeycomb structure, as illustrated in Fig. 1(i-a). Whereas, GaSe monolayer displays a layered structure, as illustrated in Fig. 1(ii-a). The lattice constants of GeC and GaSe monolayer are ˚ and 3.82 A, ˚ respectively, which are consistent with other calculated to be 3.27 A reports [48,49]. Both the GeC and GaSe monolayers exhibit semiconductor characteristics with large band gap values. The GeC monolayer has a direct band gap, 5

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Fig. 1. (a) Top and side views of the atomic structures, (b) PBE band structure, (c) HSE

band structure, and (d) phonon dispersion spectrum of (i) GeC monolayer and (ii) GaSe monolayer, respectively.

as depicted in Fig. 1(i-b,c), whereas an indirect band gap is observed in the GaSe

monolayer, as depicted in Fig. 2(ii-b,c). The band gaps of GeC and GaSe mono-

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layers at the ground state are calculated to be 2.14 eV and 1.90 eV, respectively. Furthermore, we find that the depth of the band inversion of monolayer GaSe at

the Γ point is calculated to be 96 meV, which is in good agreement with previous experiment [50] and theoretical reports [51]. In addition, both the GeC and GaSe monolayer are stable at the ground state, as illustrated in the phonon dispersions

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spectrums in Fig. 1(i-d) and Fig. 1(ii-d).

We now turn to construct the GeC/GaSe heterostructures by placing GeC

monolayer above on top of the GaSe surface. Due to difference in the lattice constants between GeC and GaSe monolayers, thus, to combine the GeC/GaSe heterostructure with the small lattice mismatch, we use a large supercell, containing a √ √ (2 × 2) supercell of GeC and ( 3 × 3) supercell of GaSe. The lattice mismatch 6

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Fig. 2. Top view and side view of the relaxed atomic structures of GeC/GaSe heterostructure for different stacking configurations of (a) pattern-I, (b) pattern-II, (c) pattern-III, (d) pattern-IV, (e) pattern-V, respectively. The cyan, purple, yellow and pink balls stand for

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Carbon, Germanium, Selenium and Gallium atoms, respectively.

in the GeC/GaSe heterostructure is small of 1.8 %, which will affect insignificantly the electronic properties of the constituent monolayers. Five representative stacking configurations of the GeC/GaSe heterostructure are depicted in Fig. 2. The obtained

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interlayer distance (D) and the binding energy in all five stacking configurations of the GeC/GaSe heterostructure are calculated and listed in Tab. 1. One can observe the interlayer distance D between GeC and the topmost Se layer of GaSe part is ˚ to 3.492 A. ˚ The pattern-I stacking configuration has the ranging from 3.462 A shortest interlayer distance and the lowest binding energy, whereas the pattern-II has the longest D and highest Eb . The nature causing such difference in the inter7

Journal Pre-proof Table 1 The interlayer distance (D), binding energy (Eb ), band gap (Eg ) and band alignment of the GeC/GaSe heterostructure for different stacking configurations.

Pattern-I Pattern-II Pattern-III Pattern-IV Pattern-V

˚ D, A

˚ −2 Eb , meV/A

Eg , eV

Band alignment

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Stacking patterns

3.462

-14.50

1.344

Type-II

3.498

-12.43

1.349

Type-II

3.492

-12.98

1.345

Type-II

3.477

-13.64

1.344

Type-II

3.481

-13.32

1.352

Type-II

layer distances and binding energies can be originated from the different interlayer interaction caused by the different stacking configurations of the GeC/GaSe het-

erostructures. It should be mentioned here that the interlayer distance and binding energy of the GeC/GaSe heterostructures are comparable with those in other typ-

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ical 2D van der Waals heterostructures, such as GeC/MoS2 [38], GeC/ZnO [40], MoS2 /Tellurene [52]. It indicates that the GeC layer is also bonded to GaSe layer via the weak van der Waals forces in all five patterns of the Gec/GaSe heterostructures.

The band structures of all five stacking patterns of the GeC/GaSe heterostruc-

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ture are depicted in Fig. 3 along with those of the GaSe and GeC monolayer. First, we can see that the electronic band structures of five stacking patterns of the GeC/GaSe heterostructures seem to be a combination between those of the constituent GeC and GaSe monolayers owing to the weak vdW forces between them, as above-mentioned. In addition, five stacking patterns of the GeC/GaSe heterostructures exhibit the semiconducting characteristics with the indirect band gap 8

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Fig. 3. Calculated band structures of the isolated (a) GaSe and GeC monolayers, and their GeC/GaSe heterostructures for different stacking patterns of (c) pattern-I, (d) pattern-II, (e) pattern-III, (f) pattern-IV, (g) pattern-V, respectively.

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of about 1.35 eV, as listed in Tab. 1. Moreover, we find that the formation of the

GeC/GaSe heterostructure tends to decrease in the depth of the band inversion at the Γ to 82 meV. Interestingly, we can see that the conduction band minimum (CBM) of such patterns is mainly contributed by the GaSe layer, whereas the valence band maximum (VBM) comes from the GeC layer. It indicates that the type-II band

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alignment is formed in all five stacking patterns of the GeC/GaSe heterostructure, making them suitable for fabricating next-generation optoelectronic nanodevices, where the electron-hole pairs are separated. As above-mentioned, we can conclude that the stacking configurations have little affect the structural and electronic properties of the GeC/GaSe heterostructures. The pattern-I stacking configuration has the lowest binding energy and shortest interlayer distance as compared with other 9

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Fig. 4. Electrostatic potential of the pattern-I stacking configuration of the GeC/GaSe het-

erostructure. The inset shows the charge density difference in the GeC/GaSe heterostructure. Red and green areas represent for the charge accumulation and depletion, respectively.

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stacking patterns of the GeC/GaSe heterostructure.

Fig. 4 depicts the electrostatic potential and the charge density difference of the pattern-I at the equilibrium state. One can see that the GeC has a deeper potential than the GaSe layer. The potential drop between GaSe and GeC layers is calculated to be a large of 8.92 eV, which will cause an electrostatic potential at the

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interface. In addition, it should be noted that the work functions play an important role in semiconductor-based heterostructures. Thus, we further calculate the work functions of isolated GeC and GaSe monolayers, which are calculated to be 4.91 eV and 5.58 eV. It indicates that when the GaSe contacts with GeC layer, the electrons will flow from GeC to GaSe layer due to the lower work function of GeC. Furthermore, the potential difference between the GaSe and GeC layer, as depicted in 10

Journal Pre-proof Fig. 4 causes a charge transfer from GeC to GaSe layer, where electrons flow from GaSe to the GeC layer, and the holes flow is opposite. Thus the GeC layer interface accumulates negative charge, while as the GaSe interface accumulates positive

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charges, as shown in the inset of Fig. 4, forming a built-in electric field, which can reduce the recombination of photogenerated electrons and holes.

Interestingly, when the GeC/GaSe heterostructure is used as a component of nanodevices, it may subject to the electric field. Therefore, it is possible to con-

sider the impact of the electric field on the electronic properties of the GeC/GaSe heterostructure. The schematic model of the electric field, applying perpendicular

along the z direction of the GeC/GaSe heterostructure is illustrated in the inset of Fig. 5. The electric field, pointing from the GaSe layer to GeC layer is defined as a positive direction. The variation of the band gap as a function of electric field is depicted in Fig. 5. We find that both negative and positive electric fields turn

to decrease in the band gap of the GeC/GaSe heterostructure. When the positive

˚ or the negative electric field of -0.3 V/A ˚ is applied, the electric field of +0.3 V/A semiconducting feature of the GeC/GaSe heterostructure is converted to the metal-

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lic one. It indicates that the semiconductor to metal transition can be achieved in

the GeC/GaSe heterostructure when it subjected to the electric field. This finding makes the GeC/GaSe heterostructure promising material for optoelectronic nanodevices.

To have a clear picture of the electric field effects on the electronic properties

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of the GeC/GaSe heterostructures, we further calculate its band structures under different strengths of the negative and positive electric fields, as depicted in Fig. 6. It is clear that the electric field affects significantly the positions of the VBM and CBM relative to the Fermi level. When the negative electric field is subjected, the CBM tends to downshift towards to the Fermi level, whereas the VBM moves upwards. It leads to a decrease in the band gap of the GeC/GaSe heterostructure. When 11

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Fig. 5. Band gap value of the GeC/GaSe heterostructure as a function of applied electric

field. The inset is schematic model of applied electric field along the z direction of the GeC/GaSe heterostructure.

˚ is subjected, both the CBM and VBM of the the positive electric field of -0.3 V/A GeC/GaSe heterostructure shift towards and cross the Fermi level, as depicted in Fig. 6(a), resulting in a transition from the semiconductor to metal. Similar to the

case of the positive electric field, the transformation from semiconductor to metal

was also occurred in the GeC/GaSe heterostructure when the positive electric field

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˚ is subjected, as illustrated in Fig. 6(g). These findings suggest that the of +0.3 V/A electric field is considered as an effective route to modulate the electronic properties of GeC/GaSe heterostructure from semiconductor to metal.

Conclusion

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In summary, we have systematically investigated the electronic properties of

GeC/GaSe heterostructure using first-principles calculations. We find that the GeC/GaSe heterostructure characterizes by the weak vdW forces and exhibits the type-II band alignment in all five stacking patterns with electrons flowing from the GeC to GaSe layer. The stacking configurations show a little effect on the electronic properties 12

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Fig. 6. Electronic band structures of the pattern-I stacking configuration of the GeC/GaSe

˚ (b) -0.2 V/A, ˚ (c) -0.1 V/A, ˚ (d) 0 V/A, ˚ (e) under different electric fields of (a) -0.3 V/A,

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˚ (f) +0.2 V/A, ˚ and (g) +0.3 V/A, ˚ respectively. The Fermi level is set to be zero. +0.1 V/A,

of the GeC/GaSe heterostructure, whereas the electric field can tune the semiconducting feature of the GeC/GaSe heterostructure into metallic one. Our finding results provide abundant opportunities for using GeC/GaSe heterostructure in next-

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generation optoelectronic and nanoelectronic devices.

Acknowledgements

This work was supported by the Domestic Master/PhD Scholarship Programme

of Vingroup Innovation Foundation. 13

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[50] Z. B. Aziza, V. Z´olyomi, H. Henck, D. Pierucci, M. G. Silly, J. Avila, S. J. Magorrian, J. Chaste, C. Chen, M. Yoon, et al., Phys. Rev. B 98 (11) (2018) 115405. [51] V. Zolyomi, N. Drummond, V. Fal’Ko, Phys. Rev. B 87 (19) (2013) 195403.

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[52] W. Zhang, D. Chang, Q. Gao, C. Niu, C. Li, F. Wang, X. Huang, C. Xia, Y. Jia, J. Mater. Chem. C 6 (38) (2018) 10256–10262.

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 GeC/GaSe heterostructure and its electronic properties controlled by electric field have been constructed and systematically investigated.  GeC/GaSe heterostructure is mainly characterized by the weak van der Waals forces, dominating between GeC and GaSe layers, preserving their intrinsic properties in GeC/GaSe heterostructure.  The GeC/GaSe heterostructure exhibits the type-II band alignment, where the electron-hole pairs are separated, making it suitable for fabricating next-generation optoelectronic nanodevices.  Stacking configurations have little affect the structural and electronic properties of the GeC/GaSe heterostructures.  Electric field is considered as an effective route to modulate the electronic properties of GeC/GaSe heterostructure from semiconductor to metal.

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Author contributions

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Dat D. Vo: Software, Investigation, Validation. Vo T. T. Vi: Methodology, Software, Investigation, Funding acquisition. Tan Phat Dao: Investigation, Validation. Tuan V. Vu: Investigation, Validation. Huynh V. Phuc: Methodology, Investigation, Validation. Nguyen N. Hieu: Methodology, Investigation, Validation. Nguyen T. T. Binh: Conceptualization, Software, Investigation, Validation, Writing - Original Draft. Chuong V. Nguyen: Conceptualization, Supervision, Writing - Original Draft, Writing - Review & Editing