Structural and electronic properties of a van der Waals heterostructure based on silicene and gallium selenide: effect of strain and electric field

P. T. T. Le ab, Nguyen N. Hieu *c, Le M. Bui d, Huynh V. Phuc e, Bui D. Hoi f, B. Amin g and Chuong V. Nguyen *h
aTheoretical Physics Research Group, Advanced Institute of Materials Science, Ton Duc Thang University, Ho Chi Minh City, Viet Nam. E-mail: lethithuphuong@tdtu.edu.vn
bFaculty of Applied Sciences, Ton Duc Thang University, Ho Chi Minh City, Viet Nam
cInstitute of Research and Development, Duy Tan University, Da Nang 550000, Viet Nam. E-mail: hieunn@duytan.edu.vn
dNTT Hi-tech Institute, Nguyen Tat Thanh University, Ho Chi Minh City, Viet Nam
eDivision of Theoretical Physics, Dong Thap University, Dong Thap, Viet Nam
fDepartment of Physics, University of Education, Hue University, Hue, Viet Nam
gDepartment of Physics, Hazara University, Mansehra 21300, Pakistan
hDepartment of Materials Science and Engineering, Le Quy Don Technical University, Ha Noi 100000, Viet Nam. E-mail: chuongnguyen11@gmail.com

Received 3rd September 2018 , Accepted 26th October 2018

First published on 30th October 2018


Combining van der Waals heterostructures by stacking different two-dimensional materials on top of each other layer-by-layer can enhance their desired properties and greatly extend the applications of the parent materials. In this work, by means of first principles calculations, we investigate systematically the structural and electronic properties of six different stacking configurations of a Si/GaSe heterostructure. The effect of biaxial strain and electric field on the electronic properties of the most energetically stable configuration of the Si/GaSe heterostructure has also been discussed. At the equilibrium state, the electronic properties of the Si/GaSe heterostructure in all its stacking configurations are well kept as compared with that of single layers owing to their weak van der Waals interactions. Interestingly, we find that a sizable band gap is opened at the Dirac K point of silicene in the Si/GaSe heterostructure, which could be further controlled by biaxial strain or electric field. These findings open up a possibility for designing silicene-based electronic devices, which exhibit a controllable band gap. Furthermore, the Si/GaSe heterostructure forms an n-type Schottky contact with a small Schottky barrier height of 0.23 eV. A transformation from the n-type Schottky contact to a p-type one, or from the Schottky contact to an ohmic contact may occur in the Si/GaSe heterostructure when strain or an electric field is applied.


1 Introduction

Two-dimensional (2D) materials such as graphene,1–3 transition metal dichalcogenides (TMDs),4–6 hexagonal boron nitride,7 and phosphorene8–11 have received widespread attention owing to their extraordinary structural, electronic, and optical properties. Among these, graphene has become the most interesting 2D material over the past decade because of its unique properties such as high carrier mobility, quantum Hall effect, and massless Dirac fermions.2 However, the lack of a valuable band gap in graphene has limited its application in novel high-performance nanoelectronic devices, for instance, field effect transistors (FETs). Therefore, opening a sizable band gap in graphene is still challenging for the research community. To achieve this point, in parallel with the efforts on changing graphene properties, other research fields have been intensively gaining in importance in the past five years. This relates to seeking new types of 2D materials, which are similar to graphene but distinct in their electronic properties.

Recently, silicene, a silicon analogue of graphene, which is a 2D crystal with buckled honeycomb structure, has been synthesized experimentally on various metallic substrates.12,13 Also, a silicene FET has been fabricated for the first time,14 showing its promising applications for designing high-speed switching devices. Unlike graphene, silicene has a buckling honeycomb lattice with the sp3-hybridization of Si atoms. This structure makes silicene more flexible than graphene. The electronic band structure of silicene is very similar to that of graphene with a linear dispersion at the Dirac K point of the Brillouin zone. The linear dispersion curves of silicene cross the Fermi level at the Dirac K point, resulting in its zero band gap. To date, Si is the basis of the electronic industry, thus, technologies based on silicene may be more easily integrated into existing circuitry. To design novel high-performance electronic devices based on silicene, it is required to open its sizable band gap at the Dirac point around the Fermi level, but keep its high carrier mobility. Currently, a useful approach, which can be modulated effectively to open a band gap in silicene, is van der Waals (vdW) heterostructures, created by stacking silicene on top of other 2D materials.15–19 For instance, Ding and his group15 have investigated the electronic properties of silicene/GaS heterobilayers from first-principles calculations. They showed that the combination of silicene and GaS monolayers results in a band gap of 166 meV, opened at the Dirac point. The value of an opened band gap in silicene can also be modulated by the bias voltage and strains. Fan and his co-workers16 have investigated the structural and electronic properties of the silicene/InSe vdW heterostructure by means of density functional theory (DFT). They showed that such heterostructure has a band gap of about 149 meV, which is opened at the Dirac point of silicene. Moreover, this band gap can also be tuned by applying an electric field or strains. These findings suggest that the vdW heterostructures created by stacking silicene on top of other 2D materials are still challenging.

Very recently, gallium selenide (GaSe), one of the 2D metal monochalcogenide materials has been successfully synthesized by various experimental approaches, such as vapor phase deposition20 and epitaxy methods,21 making posssible its potential applications for designing nanoelectronic and optoelectronic devices.22 Theoretical calculations based on the DFT method have pointed out that monolayer GaSe is a semiconductor with an indirect band gap.23–25 An indirect band gap of monolayer GaSe can also be tuned by strain,25 and the number of layers.26 These findings indicate that monolayer GaSe seems to be a promising candidate as a suitable substrate for silicene, which is yet to be investigated thoroughly. Previously, we found that the band gap near the Fermi level at the Dirac point of graphene can be opened when it is adsorbed on the GaSe monolayer and bilayer.27,28 In addition, the electronic properties of the graphene/GaSe vdW heterostructure could also be controlled by applying an electric field, or by changing biaxial and vertical strains. Chen et al. investigated the atomic and electronic properties of silicene/ZnS vdW heterostructures.29 They demonstrated that the ZnS monolayer is a promising material which can be used as a substrate for silicene. Moreover, they showed that the opened band gap and the electronic properties of the silicene/ZnS vdW heterostructure could be tuned by applying an electric field or biaxial strain. On the other hand, it should be noted that the main characteristics in 2D vdW heterostructures are charge transfer and band alignment.30–37 Aziza et al.32 demonstrated that in the graphene/GaSe heterostructure produced via a vdW epitaxy, the GaSe layer tuned the charge density of the graphene layer by shifting the Dirac point toward lower binding energies, indicating that electrons are transferred from graphene to the GaSe layer. Pierucci et al.36 demonstrated that the band alignment in the graphene/MoS2 heterostructure allows a charge transfer process from MoS2 to graphene under illumination. Also, Chiu et al.31 found that type-II band alignment was formed in the MoS2/WS2 herostructure with a valence band offset of 0.83 eV and a conduction band offset of 0.76 eV. Thus, the investigation of 2D vdW heterostructures plays an important role for further research and possible applications of 2D materials.

Therefore, in this work, we design a 2D van der Waals heterostructure based on silicene and GaSe monolayers. The structural and electronic properties of the Si/GaSe heterostructure with different stacking configuations were investigated through first-principles calculations based on density functional theory. The effect of biaxial strain and electric field on the electronic properties of the most energetically stable configuration of the Si/GaSe heterostructure has also been discussed.

2 Computational details

In this work, all calculations are performed using the DFT method, which is implemented in the simulation package Quantum Espresso38 within the projected augmented wave (PAW) method.39 Perdew, Burke, and Ernzerhof (PBE)40 parametric generalized gradient approximation (GGA) is chosen to describe the exchange–correction functional. For the vdW interaction correction in the vdW heterostructure, we opt for Grimme's DFT-D2 method, which has been applied to describe the long-range weak vdW interaction.41 In addition, the Heyd–Scuseria–Ernzerhof (HSE) hybrid function42 is also used to obtain the correct band gap and Schottky barrier height of the Si/GaSe heterostructure. It also should be noted that for layered silicene and monolayer GaSe, spin–orbit coupling (SOC) plays an important role and can lead to a splitting in the valence band maximum (VBM) and conduction band minimum (CBM), resulting in a decrease in their band gaps.43,44 For instance, Aziza et al.43 demonstrated that in the presence of SOC effects, in the VBM of monolayer GaSe a split by 120 meV occurs along the ΓK direction. However, SOC effects hardly change the shape of the bands of the heterostructure. Therefore, the SOC effects are not considered in the following calculations. The energy cut-off is set to be 500 eV, and a 9 × 9 × 1 k-point grid is selected in the Brillouin zone (BZ) integration. All the atoms are fully relaxed until the total energy and the residue forces on each atom are converged to 106 eV and 0.001 eV Å−1, respectively. A vacuum layer of 20 Å in the z direction is used to avoid the interactions between adjacent slabs. In addition, a dipole correction is employed to cancel the errors of electrostatic potential, atomic forces, and total energy under periodic boundary conditions.

3 Results and discussion

Before investigating the structural and electronic properties of the vdW heterostructure based on single-layered silicene and gallium selenide (GaSe), we first check their atomic structures and band structures, as illustrated in Fig. 1. After optimization, the obtained lattice constants for single-layered Si and GaSe are 3.85 Å and 3.783 Å, respectively. These values are in good agreement with reported theoretical and experimental values.15,26,32,45 Moreover, the lattice mismatch between silicene and GaSe is only 1.8%, indicating that the vdW heterostructure based on these two materials can be formed easily with precise stacking in 2D directions. We find that at the equilibrium state, the single-layered silicene is a semimetal with zero band gap, as shown in Fig. 1(c). In contrast, the GaSe is a semiconductor with an indirect band gap of 1.60 eV, forming between the VBM along the ΓM path and the CBM at the Γ point, as shown in Fig. 1(d).
image file: c8cp05588b-f1.tif
Fig. 1 Left panel shows (a) the atomic structure and (b) the band structure of single-layered silicene at the equilibrium state. Right panel shows (c) the atomic structure and (d) the band structure of a single-layered GaSe monolayer at the equilibrium state. The blue, violet, and yellow balls stand for Si, Ga, and Se atoms, respectively. The Fermi level is set to be zero.

Fig. 2 displays the Si/GaSe heterostructure with six different stacking configurations, namely, AA-I, AA-II, AB-I, AB-II, AC-I, and AC-II. For AA-I, and AA-II stacking configurations, Si atoms of two sublattices of silicene locate on top of both Ga and Se atoms in the heterostructure, as shown in Fig. 2(a) and (b). On the other hand, for the AB-I and AB-II stacking configurations, one Si sublattice atom is located directly on top of the Se atom and the other one is on top of the Ga–Se hexagonal center, as shown in Fig. 2(c) and (d). For the AC-I and AC-II types of the Si/GaSe heterostructure, one Se atom is positioned directly on the center of two Si atoms, as illustrated in Fig. 2(e) and (f). The structural parameters of the Si/GaSe heterostructure for different stacking configurations are listed in Table 1. Our calculations show that the AB-I stacking configuration of the Si/GaSe heterostructure is the most stable one with the lowest total energy. In addition, after geometric optimization, the interlayer distance, denoted by d as shown in Fig. 2, between the silicene layer and topmost Se layer of the GaSe in the Si/GaSe heterostructure for six different stacking configurations is listed in Table 1. One can see that the average interlayer distance d in all six stacking configurations of the heterostructure is about 3.4 Å, which has the same magnitude as that in other vdW heterostructures based on 2D materials, such as graphene/WSe2,46,47 graphene/phosphorene,48,49 graphene/GaN,50,51 graphene/SiC,52 and TMD/ZnO.53 This indicates that the interaction between silicene and GaSe in all six stacking configurations is related to the weak vdW interaction.


image file: c8cp05588b-f2.tif
Fig. 2 Top view and side view of the six considered different stacking configurations of the Si/GaSe heterostructure. The blue, violet, and yellow balls stand for Si, Ga and Se atoms in the heterostructure, respectively.
Table 1 Band gap opening Eg (meV), interlayer distance d (Å), silicene buckling height ΔSi–Si (Å), and GaSe buckling height ΔGa–Se (Å) of the Si/GaSe heterostructure for six different stacking configurations
Configurations E g (meV) d (Å) Δ Si–Si (Å) Δ Ga–Se (Å)
AA-I 116.1 3.378 0.481 1.151
AA-II 100.7 3.487 0.486 1.149
AB-I 100.5 3.516 0.484 1.150
AB-II 140.3 3.391 0.485 1.151
AC-I 189.8 3.438 0.485 1.148
AC-II 197.2 3.420 0.488 1.148


Fig. 3 shows the band structures of all the considered stacking configurations of the Si/GaSe heterostructure. First, one can observe that the band structures of the Si/GaSe heterostructure for the six different stacking configurations seem to be a combination of the band structure of freestanding silicene and GaSe. This is due to the above-mentioned weak vdW interaction between the silicene layer and GaSe layer in the considered heterostructure. The linear dispersion forming between the occupied π state and unoccupied π* state is still located at the high symmetry K Dirac point. For freestanding silicene, as shown in Fig. 1(c), both the π and π* states cross the Fermi level, resulting in a gapless semiconductor. However, it can be seen from Fig. 3 that a small band gap has opened in all the stacking configurations of the Si/GaSe heterostructure around the Dirac K point. This means that silicene is converted from a semimetal to a semiconductor with a valuable band gap. The band gap of the Si/GaSe heterostructure for the six considered stacking configurations is 116.1 meV, 100.7 meV, 100.5 eV, 140.3 meV, 189.8 meV, and 197.2 meV, respectively, for AA-I, AA-II, AB-I, AB-II, AC-I, and AC-II stacking configurations. These values are much larger than that of the graphene/GaSe heterostructure.27 The reason for this difference is due to the overlap of the Si 3pz orbitals, which is weaker than that of C 2pz orbitals in graphene. Whereas, the origin of the band gap opening in the Si/GaSe heterostructure is the onsite energy difference between two sublattices, resulting in the sublattice symmetry breaking of Si atoms. This result was also observed in our previous vdW graphene-based heterostructures such as graphene/MoS2,54 graphene/phosphorene,55 and graphene/GaSe.27 Also, to understand in more detail the physical mechanism of the band gap opening in the considered Si/GaSe heterostructure, the tight-binding (TB) method is adopted in this work. According to the π-electron TB approximation, the dispersion of silicene near the Fermi level at the Dirac K point can be described as: image file: c8cp05588b-t1.tif, where Δ is the on-site energy difference between two sublattices of silicene, νF is the Fermi velocity, and k is the wave vector relative to the Dirac K point of silicene.


image file: c8cp05588b-f3.tif
Fig. 3 Band structures of the corresponding six stacking configurations of the Si/GaSe heterostructure. The Fermi level is set to be zero. The red and blue colors define the CBM and the VBM of the heterostructure.

We now focus on the most energetically favorable configuration of the Si/GaSe heterostructure. In Fig. 4(a) and (b) we display the band structures of the most stable stacking configuration of the Si/GaSe heterostructure using PBE and HSE calculations, respectively. We can see that for the Si/GaSe heterostructure, the obtained band gap, opening at the K Dirac point by PBE and HSE methods is 100.5 meV and 452 meV, respectively. In addition, it is clear that the Fermi level of silicene is closer to the CBM of the monolayer GaSe, which shows that an n-type semiconductor is obtained. Although PBE underestimates the obtained band gap, opening at the Dirac K point of the Si/GaSe heterostructure, the PBE can give reasonable trends and physical mechanisms, which possesses guiding function for future experimental studies. On the other hand, we can see that a gap of 0.61 eV appeared below the Fermi level in the energy ranging from −2.2 eV to −1.2 eV due to the orbital hybridization and overlap between the Si 3p orbital and Se 4p orbital. This appearance was also observed experimentally in other vdW heterostructures, such as graphene/MoS2 heterostructures.30,56,57 For instance, Diaz et al.57 demonstrated that the π-band of graphene can be opened when it is supported on the MoS2 substrate due to hybridization with states from the MoS2 substrate. To confirm the structural stability of the Si/GaSe heterostructure, we further calculate its phonon dispersion, as illustrated in Fig. 4(c). One can observe from Fig. 4(c) that there is no imaginary frequency in the phonon dispersion curves of the Si/GaSe heterostructure. It indicates that the heterostructure is stable at the equilibrium state. In Fig. 4(d) and (e) we show the charge density difference and the electrostatic potential of the most stable stacking configuration of the Si/GaSe heterostructure. It should be noted that the charge density difference can visualize the charge transfer in the heterostructure, and can be calculated as follows: Δρ = ρSi/GaSeρSiρGaSe, where ρSi/GaSe, ρSi, and ρGaSe are the charge densities of the Si/GaSe heterostructure, the freestanding silicene, and the isolated GaSe monolayer, respectively. As shown in Fig. 4, we find that electrons are mainly accumulated in the interspace between the silicene and GaSe layers. In addition, a Bader charge analysis shows that a charge of 0.041 e is transferred from the silicene layer to the topmost Se layer of the GaSe monolayer, resulting in a built-in electric field. Fig. 4(e) shows the electrostatic potential of the most stable stacking configuration of the Si/GaSe heterostructure at the equilibrium state. It can be seen that in the heterostructure, the electrostatic potential of silicene is quite a bit larger than that of the GaSe monolayer, resulting in a large potential drop across the heterostructure.


image file: c8cp05588b-f4.tif
Fig. 4 Band structures of the most energetically favorable configuration of the Si/GaSe heterostructure given by PBE (a) and HSE (b) calculations. The Fermi level is set to be zero. (c) Phonon dispersion curves of the Si/GaSe heterostructure. (d) The top and side views of the charge density difference of the most energetically favorable configuration of the Si/GaSe heterostructure along the z-direction. The red and blue colors represent positive and negative values, respectively. (e) The electrostatic potential of the most stable stacking configuration of the Si/GaSe heterostructure along the z-direction.

More interestingly, the Si/GaSe heterostructure represents a metal–semiconductor contact, forming two different contact types of the Schottky and ohmic contact. The difference in the work functions of the silicene and GaSe monolayer leads to the charge transfer between them. Based on the Schottky–Mott model in the metal-semiconductor Si/GaSe heterostructure, an n-type Schottky barrier height, ΦB,n, is defined by the energy difference between the Fermi level, EF, and the CBM of the semiconducting GaSe, ECBM, that is ΦB,n = ECBMEF. On the other hand, a p-type Schottky barrier height, ΦB,p, is defined by the energy difference between the EF and the valence band maximum (VBM) of the semiconducting GaSe, that is ΦB,p = EFEVBM. When the ΦB,n or ΦB,p is a negative value, the Schottky contact will transform to an ohmic contact. We find that the obtained ΦB,n and ΦB,p of the Si/GaSe heterostructure given from the PBE/HSE method are 0.23 eV/0.56 eV and 1.2 eV/1.62 eV, respectively. Both PBE and HSE calculations show that the most energetically stable configuration of the Si/GaSe heterostructure forms an n-type Schottky contact. Moreover, it indicates that PBE methods are good at predicting correct trends and physical mechanisms of the Si/GaSe heterostructure.

We further investigate the effect of the biaxial strain on the electronic properties of the Si/GaSe heterostructure, as shown in Fig. 5. Biaxial strain is applied to the heterostructure by changing its lattice constant as ε = (aa0)/a, where a and a0 are the lattice constants of the heterostructure at the equilibrium state and under strain, respectively. It is obvious that the band gap opening in the Si/GaSe heterostructure can be controlled by applying biaxial strain. For instance, with increasing strain from −8% to +8%, the band gap, opening of silicene in the heterostructure increases from 90 meV to 180 meV, respectively. The variation of the band gap, opening in silicene versus the strain is illustrated in Fig. 5(b). It can be seen that the band gap opening in silicene depends linearly on the strain. In Fig. 5(c) we display the band structures of the heterostructure at different strains. One can observe that with increasing strain from −8% to +8%, the Fermi energy level of silicence shifts upwards from the valence band to the conduction band of the semiconducting GaSe monolayer. When the tensile strain is larger than +4%, the CBM of silicene crosses the Fermi level, leading to a transition from semiconductor to metal of the GaSe monolayer in the Si/GaSe heterostructure. This indicates that when the biaxial strain is larger than +4%, the n-type Schottky barrier height becomes a negative value, resulting in a transformation from Schottky contact to ohmic contact type, as shown in Fig. 5(d). From Fig. 5(a), we also find that the n-type Schottky barrier height of the Si/GaSe heterostructure decreases with increasing biaxial strain from −8% to +8%. When the strain is smaller than −4%, the n-type Schottky barrier height becomes larger than the p-type Schottky barrier height. The Si/GaSe heterostructure in this case of biaxial strain forms a p-type Schottky contact. This implies that by applying a compressive strain the transition from the n-type to the p-type Schottky contact occurs in the heterostructure.


image file: c8cp05588b-f5.tif
Fig. 5 (a) Schematic of the biaxial applied strain to the Si/GaSe heterostructure. (b) The dependence of the band gap of the Si/GaSe heterostructure on the biaxial strain. (c) (From left to right) Band structures of the Si/GaSe heterostructure under different biaxial strains of −8%, −4%, 0%, +4%, and +8%, respectively. The Fermi level is set to be zero. (d) The evolution of the n-type and p-type Schottky barrier height as a function of the biaxial strain.

When being applied to practical nanoelectronic devices, the heterostructure might be subjected to external electric fields, which will effectively modify its electronic properties. We thus investigate the effect of an external electric field applied perpendicularly to the Si/GaSe heterostructure, on its electronic properties. The effect of the external electric field on the electronic properties of the heterostructure is displayed in Fig. 6. It can be seen that by applying a negative electric field, the band gap opening in the Si/GaSe heterostructure continuously decreases with increasing strength of the electric field. In contrast, by applying a positive electric field, the opened band gap increases linearly with increasing strength of the electric field. The linear dependence of the opened band gap in the Si/GaSe heterostructure on the strength of the electric field, as shown in Fig. 6(b), is closely related to the giant Stark effect.58 In Fig. 6(c) we show the band structures of the Si/GaSe heterostructure under different strengths of electric field. We find that the Fermi energy level shifts upwards from the valence band to the conduction band of the semiconducting GaSe monolayer in the Si/GaSe heterostructure. When a negative electric field of −2 V nm−1 is applied, the CBM shifts downwards and crosses the Fermi level, leading to a transition from semiconductor to metal of the semiconducting GaSe monolayer in the Si/GaSe heterostructure. It also shows that the Schottky contact, forming between the metallic silicene and the semiconducting GaSe monolayer is transformed to an ohmic contact when the negative electric field is larger than −2 V nm−1. By applying a positive electric field, one can observe that the Fermi level shifts downwards from the conduction band to the valence band of the semiconducting GaSe monolayer. By applying a positive electric field, there is no transition from semiconductor to metal of the GaSe monolayer. The GaSe monolayer in the heterostructure still keeps a semiconducting behavior. This means that by applying an external electric field, there is no transition between the n-type and p-type Schottky contacts in the Si/GaSe heterostructure. The n-type Schottky barrier height increases with increasing strength of the positive electric field and decreases with increasing strength of the negative electric field. When the strength of the negative electric field is larger than −2 V nm−1, the n-type Schottky barrier height becomes a negative value, resulting in a transformation from a Schottky contact to an ohmic contact.


image file: c8cp05588b-f6.tif
Fig. 6 (a) Schematic of an external electric field applied perpendicularly to the Si/GaSe heterostructure. (b) The dependence of the band gap opening in the heterostructure on the strength of the electric field. (c) (From left to right) Band structures of the Si/GaSe heterostructure under different strengths of electric field. (d) The evolution of the n-type and p-type Schottky barrier heights as a function of electric field.

4 Conclusion

In summary, we have investigated the structural and electronic properties of the Si/GaSe heterostructure, as well as the effect of biaxial strain and an external electric field using density functional theory. Six different stacking configurations of the Si/GaSe heterostructure are proposed and investigated in detail. Our results show that the electronic properties of the Si/GaSe heterostructure seem to be a combination of those in the single layered components owing to the weak vdW interactions occurring in the heterostructure. Interestingly, we find that the Si/GaSe heterostructure in all the stacking configurations shows semiconducting behavior with a band gap of about 100 meV opening in silicene at the Dirac K point around the Fermi level. This band gap opening in the Si/GaSe heterostructure could be modulated by strain or electric field. This finding suggests that the Si/GaSe heterostructure could be used for designing novel high-performance microelectronic devices. Furthermore, at the equilibrium state, the Si/GaSe heterostructure in its most energetically stable configuration forms an n-type Schottky contact with a small Schottky barrier height of 0.23 eV. Both the Schottky contact and Schottky barrier height of the heterostructure could be controlled by applying biaxial strain or by applying an external electric field. When the tensile biaxial strain is larger than +4%, a transition from a Schottky contact to an ohmic one occurs in the heterostructure, whereas when the compressive biaxial strain is smaller than −4%, a transition from an n-type to a p-type Schottky contact was observed. A transition from the Schottky to an ohmic contact was also observed in the heterostructure when the negative electric field is smaller than −2 V nm−1.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research is funded by Vietnam National Foundation for Science and Technology Development (NAFOSTED) under Grant Number 103.01-2017.309. B. Amin acknowledges support from the Higher Education Commission of Pakistan (HEC) under Project No. 5727/KPK/NRPU/R&D/HEC2016.

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