REN Dahua, QIAN Kai, LI Qiang, ZHANG Yuan, ZHANG Teng

(1.School of Information Engineering, Hubei Minzu University, Enshi 44500, China; 2.School of Advanced Materials and Mechatronic Engineering, Hubei Minzu University, Enshi 44500, China)

Abstract: The electronic structure and optical property of stacked GaN-WS2 heterostructure are explored with HSE06 calculation based on density functional theory. The direct band gap of GaN-WS2 heterostructure is 1.993 eV, which is obviously a type-II band alignment semiconductor. Furthermore, the optical property of GaN-WS2 heterostructure such as absorption coefficient is analyzed. These new findings enable GaN-WS2 heterostructure to be promising candidates for photovoltaic cells and electronic devices in visible light.

Key words: GaN-WS2 heterostructure; band structure; optical property; calculation

1 Introduction

Van der Waals (vdW) heterostructures (HS) are used to design unusual electronic devices due to their excellent physical properties and outstanding performances[1]. Moreover, HSs have been the focus as their band gaps and atomic layer thicknesses are tunable[2].Vertical stacking HS, such as BiFeO3-Nb:SrTiO3[3],n-VO2-p-GaN[4], WTe2-MoTe2[5], MoS2-WS2[6,7], and Germanane-Antimonene[8]HS can be put into innovative applications in nanoelectronic devices, for example ultrathin photodetectors[9], solar cells[10], memory devices[11], flexible sensors and transistors[12,13]and so on.As is known, structural properties are the key factors to determine applications, and the electronic structures are very sensitive to thicknesses[6-7]. HS can offer the chance to explain the interaction between the different monolayers.

In these years, lots of researchers have payed growing attractions to 2D GaN. On one side, 2D GaN has been directly experimentally synthesized via graphene encapsulation[14], which is desirably applied in optoelectronic fields. On another side, 2D GaN has theoretically been proved to be stable[15-17]. Moreover,Sanderset alhave calculated the electronic and optical properties of 2D GaN based on density functional theory (DFT) and many-body perturbation theory (MBPT)[18]. Particularly, applications of 2D GaN can be expanded to energy-efficient display, nonlinear optics and water purification[18]. As is revealed that the strong polarization field in 2D GaN resulted in the quantum confined Stark effect. Sunet alhas systematically investigated the electronic properties of graphene-g-GaN HS based onab initiocalculations and discovered that a decreasing interlayer distance leaded to a transition from n-type to p-type Schottky contact[19]. In addition,monolayer tungsten disulfide (WS2) is a typically direct band gap (2.38 eV) semiconductor[6,7]. Recently, Heet alreported that the band gap (1.48 eV) of the MoSe2-WSe2HS is lower than the band gaps of individual WSe2(1.60 eV) and MoSe2(1.50 eV) layers[20]. Interestingly, 2D GaN and 2D WS2can be stably stacked to form HS, which could have prominently physical properties.

Generally, the superior structures and excellent properties of GaN-WS2vdW HS is significantly valuable to explore. Furthermore, the electronic and optical properties of GaN-WS2HS have not systematically been investigated. It is very necessary to study the structural, electronic and optical properties of GaNWS2vdW HS, which could potentially provide a platform for applications in photovoltaic cells and electronic devices.

2 Computational methods and theories

Ab initiocalculations are implemented in the Viennaab initiosimulations package(VASP)[21-24].Firstly, the generalized gradient approximation (GGA)of the Perdew-Burke-Ernzerhof(PBE) functional[25,26]and HSE06 calculation[27]were used. Secondly, the Kohn-Sham equations of DFT are solved with the projector augmented wave (PAW) method[28]. Thirdly, the energy cutoff was 600 eV and Brillouin zone sampling of K-mesh was 13×13×1 for relaxation and density of states (DOS) calculations. Accurately, K-mesh points for energy band structures calculations were set as 15×15×1 and the energy criteria was set as 10-5eV until the residual force was smaller than -10-3eV/Å. Especially, the vacuum layer thickness was set more than 25.0 Å to avoid spurious interactions in the neighboring images. Significantly, the long ranged vdW interaction is important to hold the 2D HS together[29]. Therefore the vdW-DF2 functional, the Langreth-Lundqvist formalism, was used to describe long-range electron correlation effects[30].

The optical properties of 2D GaN-WS2HS have been described by the complex dielectric function ε(ω)=ε1(ω)+iε2(ω), where ε1(ω) and ε2(ω) are real and imaginary parts, respectively. Simultaneously, the imaginary part is calculated by summing up all possible transitions from the occupied to unoccupied states,which is closely related to the band structure in the absorption behaviors[31,32]. The imaginary is given by the following relationship:

wheree,mandωare the charge, mass of free electrons,and the frequency of incident photons, respectively.Meanwhile,M,i,jandfipresent the dipole matrix, the initial states, final states, and the Fermi distribution function fori-th state with wave vectork, respectively.The real part ε1(ω) is strongly associated with the imaginary ε2(ω) by the Kramers-Kronig formula[31-33]:

wherePpresents the principal value. Other optical coefficients such as absorption coefficient are also discussed, which is expressed as the following equation[33]:

3 Results and discussion

The structure of GaN-WS2HS is shown in Fig.1,which is exhibited in top and side view. After optimization, the hexagonal unit parameter of 2D GaN-WS2HS is 3.194 Å, which agrees well with other theoretical and experimental values[6,7]. The supercell (4×4) is used to calculate other properties.

In order to describe the vdW interactions of the two layers between WS2and GaN, the distance from Ga atom in GaN monolayer and S atom in WS2monolayer and the binding energy ΔEwas calculated:

where ΔErefers to the binding energy of GaN-WS2HS.MeanwhileEHS,EGaNandEWS2present the total energies of GaN-WS2HS, GaN monolayer and WS2monolayer,respectively. As is calculated, the distance between the two layers is approximately 3.081 Å, which means no bonds between Ga and S atoms are formed. The binding energy of GaN-WS2is -0.245 eV, which demonstrates that the interaction between two layers is physically van der Waals and the negative binding energy suggests that the GaN-WS2HS has favorable energies during the formation.

In the following, the electronic structure of GaNWS2HS has been calculated, as depicted in Fig.2.GaN-WS2HS is a direct band gap semiconductor with a type-II band alignment, which can have advantages in photovoltaic cells and photoelectronic devices for hindering the recombination of electron-hole pairs and increasing the light utilization. The valence band maximum (VBM) of the GaN-WS2HS is contributed by GaN layer meanwhile the conduction band minimum (CBM) of the GaN-WS2HS is contributed by WS2layer. Both of VBM and CBM of the GaN-WS2HS stay at the K point. It is also shown that the band gap of GaN-WS2HS is 1.993 eV, which is smaller than both individual GaN (3.48 eV) layer[15]and individual WS2(2.362 eV) layer within the frame of HSE06 calculations, which agrees well with the experimental values of WS2(2.38 eV)[6,7]. Therefore, the band gap of the GaN-WS2HS could be significantly changed by vertically stacked heterostructure. It is interesting that the stacked heterostructure of 2D GaN and 2D WS2offers a beautiful strategy to tune the band gap of WS2semiconductor, which could provide a chance at band engineering and electronic device design.

Fig.3 The total density of states (TDOS) and partial density of states (PDOS) of GaN-WS2HS(color online)

As reflected in the calculated band structure, the band gap is the contribution of GaN-WS2HS from the upshift CBM of 2D WS2at the K point and the downshift VBM of 2D GaN at the K point.

To gain insight into the band structure of GaNWS2HS, the total density of states (TDOS) and partial density of states (PDOS) are calculated, as depicted in Fig. 3. Then, the black line stands for the TDOS of GaN-WS2HS, meanwhile the blue and red lines present the DOS of GaN and WS2in GaN-WS2HS,respectively, as is shown in Fig.3. The main contribution of VBM of GaN-WS2HS is GaN and that of CBM is WS2. Obviously, the VBM and CBM are confined in GaN and WS2, indicating that GaN-WS2is a type-II semiconductor, which is particularly interesting for the spatial separation of the electrons and holes after photoexcitation and a highly desirable applications in photovoltaic fields.

Because the VBM and CBM for GaN-WS2HS are located at GaN and WS2monolayers, respectively,It is necessary to take the excitonic effect into account for optical properties of HS[33]. Taking GaN monolayer and WS2monolayer into consideration, the excitions are limited in each monolayer. Therefore, the absorption of GaN-WS2HS is calculated. As is shown in Fig.4, it is recognized that the absorption intensity is very strong in the visible light, which could be used in solar cells and optoelectronic devices.

4 Conclusions

The electronic and optical properties of GaN-WS2HS have been performed fromab initiocalculations.As is found that GaN-WS2HS is stable. Additionally,the GaN-WS2HS is a direct band gap semiconductor as well as a type-II semiconductor. Then, the absorption behavior of GaN-WS2HS is analyzed, indicating that GaN-WS2HS is very efficiently able to absorb the visible light, which can expand the application to photovoltaic fields. These results will help design the applications in solar cells and photodetectors in visible light.