First-principles investigation on electronic properties and band alignment of group III monochalcogenides

Using first-principles calculations, we investigated the electronic properties and band alignment of monolayered group III monochalcogenides. First, we calculated the structural and electronic properties of six group III monochalcogenides (GaS, GaSe, GaTe, InS, InSe, and InTe). We then investigated their band alignment and analysed the possibilities of forming type-I and type-II heterostructures by combining these compounds with recently developed two-dimensional (2D) semiconducting materials, as well as forming Schottky contacts by combining the compounds with 2D Dirac materials. We aim to provide solid theoretical support for the future application of group III monochalcogenides in nanoelectronics, photocatalysis, and photovoltaics.

sample the first Brillouin zone. The tetrahedron methodology with Blöchl corrections 28 was used for all the calculations, only except the Gaussian smearing methodology 29 with a smearing of 0.01 eV was employed for band structure calculations. To avoid interaction between adjacent images, a relatively large vacuum space of 20 Å was inserted in the normal direction. After the fully relaxation, the force on each atom was less than 0.01 V/Å. All the calculations were performed in a spin-restricted manner.

Results and Discussions
The crystal structure of M III X is shown in Fig. 1. A M III X monolayer is formed by four covalently bonded atomic planes in an X-M III -M III -X sequence. The relaxed lattice constants of GaS, GaSe, GaTe, InS, InSe, and InTe are 3.62, 3 We observed an interesting feature in the band structures of these M III Xs -band convergence. For all of the M III X monolayers, in addition to the VBM located at the Γ-M high-symmetry line, there is another valley in the valence band along the K-Γ high-symmetry line. The difference between the energies of these two valleys is only 4, 25, 34, 18, 18, and 3 meV for the GaS, GaSe, GaTe, InS, InSe, and InTe monolayers, respectively, which are all much lower than 52 meV (2k B T 300K , i.e. twice the thermal energy at room temperature). This means that band convergence may occur in the valence band of GaS, GaSe, GaTe, InS, InSe, and InTe. When both valleys contribute to the total electrical conductivity (σ), the power factor (P = S 2 σ, where S presents the Seebeck coefficient) is considerably increased. Since the Seebeck coefficient is one of the main factors of a material's ability to efficiently produce thermoelectric power, the band convergence in the GaS, GaSe, GaTe, InS, InSe, and InTe monolayers may allow them to be used as thermoelectric materials, as previously shown for MoS 2 30-32 and phosphorene 33 . Indeed, Tung et al. 34 found that the maximum P of a p-type (and n-type) InSe monolayer can reach 0.049 (and 0.043) W/K 2 m at 300 K in the armchair direction.
The band alignment of M III Xs is shown in Fig. 3. The energy levels of CBM and VBM were calculated with reference to the vacuum level, while the vacuum level was determined through the calculation of the planar averaged electrostatic potential. The VBM values of GaS, GaSe, GaTe, InS, InSe, and InTe monolayers are −6.88, −6.40, −5.78, −7.02, −6.56, and −5.91 eV, respectively. Meanwhile, the CBM values of GaS, GaSe, GaTe, InS, InSe, and InTe monolayers are −3.59, −3.63, −3.65, −4.38, −4.26, and −3.84 eV, respectively. As reported in a previous investigation 35 , the reduction potential ( + E H /H 2 ) and oxidation potential ( ) of water are −4.44 and −5.67 eV, respectively. These values lie just within the bandgaps of the GaS, GaSe, GaTe, InS, InSe, and InTe monolayers, suggesting that these 2D materials can potentially serve as photocatalysts for water splitting, as previously reported by Zhuang et al. 2 .
Recent studies show that 2D-material-based Schottky contacts have great potential in nanoelectronic devices [36][37][38] and sensors 39 . To explore the opportunities of forming a Schottky contact between each M III X monolayer and each 2D Dirac material, we calculated and obtained values of 4.23 and 4.60 eV as the the work functions of graphene and silicene, respectively. Figure 3 shows that graphene can form n-type Schottky contacts  www.nature.com/scientificreports www.nature.com/scientificreports/ with GaS, GaSe, GaTe, and InTe, and n-type Ohmic contacts with InS and InSe. These predictions are in good agreement with results of recent studies 40,41 . Meanwhile, silicene can form n-type Schottky contacts with GaS, GaSe, GaTe, InS, InSe, and InTe. Overall, our findings are expected to be useful to the design of Schottky devices with dedicated Schottky barrier height.
In addition, p-n junctions are fimportant for building nanoelectronic devices. Control of the carrier type in 2D semiconducting materials is a fundamental requirement for the application of nanoelectronic devices in various fields. Peng et al. 42 developed a method for direct inject the carrier into a nanochannel by using a metal electrode with proper work function. That is a p-type (or n-type) device channel can be obtained by chosing a metal electrode with higher (or lower) work function than that in the channel. Figure 3 shows the band alignment of GaS, GaSe, GaTe, InS, InSe, and InTe. The work functions of widely used metals for electrode like Y, Al, Cu, Ag, Au, and Pt are also shown for comparison 43 . Obviously, when Al and Ag are used as the electrode material, both the Al/InS and Ag/InS interfaces may form an Ohmic contact. Furthermore, when Y is used as the electrode material, all of the Y/M III X interfaces are highly probably form an Ohmic contacts, inducing effective carrier injection as well as enhancement of contact performance.
Using first-principles calculations, we systematically investigated the electronic properties and band alignment of a family of 2D semiconducting materials-group III monochalcogenides (GaS, GaSe, GaTe, InS, InSe, and InTe). We found that all six M III X materials are indirect-bandgap semiconducting materials (the bandgaps of GaS, GaSe, GaTe, InS, InSe, and InTe are 3.29, 2.77, 2.13, 2.63, 2.30, and 2.07 eV, respectively). Interestingly, we discovered band convergence in all of the M III X materials, indicating their potential for thermoelectric applications. The calculated results for band alignment of the M III Xs indicate that all of the M III X monolayers are potential photocatalysts for water splitting. Moreover, the M III X monolayers can form type-II heterostructures with other popular 2D semiconducting materials, which is a critical requirement for photocatalyst and photovoltaic applications. We also found that most M III X monolayers can form n-type Schottky contacts with graphene and silicene. In addition, when elemental Y is used as the electrode material, all of the Y/M III X interfaces may form Ohmic contacts. We believe our findings can help to extend the application of group III monochalcogenides in thermoelectrics, photocatalysis, photovoltaics, and nanoelectronics.  Table of heterostructures formed between M III Xs (GaS, GaSe, GaTe, InS, InSe, and InTe) and various 2D semiconducting materials (MoS 2 , MoSe 2 , WS 2 , WSe 2 , BlackP, BlueP, arsenene, h-BN, g-GaN, and germanane). Type-I and type-II heterostructures are shown in yellow and red, respectively.