Tuning the Band Alignment and Electronic Properties of $\mathrm{Ga}\mathrm{Se}/\mathrm{Sn}$X${}_2$ (X = $\mathrm{S}$, $\mathrm{Se}$) Two-Dimensional van der Waals Heterojunctions via an Electric Field

Two-dimensional (2D) material-based van der Waals heterostructures (vdWHs) have been identified as an excellent platform from which to expand various device applications for light-emitting, photovoltaic, and field-effect transistors because of their different band alignments, including type-I, type-II, and type-III. However, it is difficult to achieve transformations between types of band alignment in a single heterostructure for diverse applications. In this study, the effects of a vertical electric field on the band alignment transition and electronic properties of 2D $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{S}}_2$ (${\mathrm{Sn}\mathrm{Se}}_2$) vdWHs are investigated systematically using density functional theory calculations. The study shows that the electric field can modulate not only the band gap but also the band alignment to produce multifunctional device applications. A positive electric field can control the band alignment transformation from type-II to type-I for electric field values of approximately 0.39 V/Å (0.12 V/Å), while a negative electric field can transform the type-II to type-III band alignments for electric field values of about −0.2 V/Å (−0.16 V/Å) in $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{S}}_2$ (${\mathrm{Sn}\mathrm{Se}}_2$) vdWHs. We trace these surprising results to the conduction band and valence band edge position movements for the linear decrease of $\mathrm{Ga}\mathrm{Se}$, while the linear increase of ${\mathrm{Sn}\mathrm{S}}_2$ (${\mathrm{Sn}\mathrm{Se}}_2$) occurs with the applied electric field. The present work may provide a direction for tunable multiple-band alignments in 2D vdWHs and help achieve multifunctional device applications.

Figure 1

(a) Top view and (b) side view of the relaxed geometric structure of $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{S}}_2$ (${\mathrm{Sn}\mathrm{Se}}_2$) vdWHs. (c) The first Brillouin zone of the hexagonal monolayer with the vectors in the reciprocal lattice, high-symmetry points, and integral path. (d) Schematic model of the $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{S}}_2$ (${\mathrm{Sn}\mathrm{Se}}_2$) vdWHs in the electric field.

Figure 2

The electronic structures of (a) and (d) 1 × 1 $\mathrm{Ga}\mathrm{Se}$ monolayer, and (b) and (e) 1 × 1 ${\mathrm{Sn}\mathrm{S}}_2$ (${\mathrm{Sn}\mathrm{Se}}_2$) monolayer, and the calculated electronic band structure for (c) $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{S}}_2$ vdWHs and (f) $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{Se}}_2$ vdWHs. The solid (dashed) line indicates the PBE (HSE06) method, and the horizontal dashed line indicates the Fermi level. The Fermi level is set at zero.

Figure 3

The schematic diagram of the band alignment and the projected band structures of the (a) $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{S}}_2$ and (b) $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{Se}}_2$ vdWHs using the PBE method, using a vacuum level as a common energy reference. E_g and E_F represent the energy of the band gap and the Fermi level. The conduction band offset (CBO) and the valence band offset (VBO) represent the band steps at the bottom of the conduction band and the top of the valence band of the two substances, respectively.

Figure 4

Lateral views of the three-dimensional charge density difference diagrams and the planar average charge density difference Δρ(z) along the z direction of (a) $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{S}}_2$ and (b) $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{Se}}_2$ vdWHs. The area of Δρ > 0 with a red and Δρ < 0 with a blue isosurface represent the charge accumulation and depletion.

Figure 5

The (a) projected band structures and (b) corresponding band alignments in external electric fields of −0.4, −0.2, 0, 0.2, and 0.4 V/Å for $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{S}}_2$ vdWHs. The red and blue lines represent the $\mathrm{Ga}\mathrm{Se}$ and ${\mathrm{Sn}\mathrm{S}}_2$ layers, respectively. The Fermi level is set to zero.

Figure 6

The (a) projected band structures and (b) corresponding band alignments in external electric fields of −0.2, −0.1, 0, 0.1, and 0.2 V/Å for $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{Se}}_2$ vdWHs. The red and green lines represent the $\mathrm{Ga}\mathrm{Se}$ and ${\mathrm{Sn}\mathrm{Se}}_2$ layers, respectively. The Fermi level is set to zero.

Figure 7

The evolution of the band gap and the band edge as a function of the electric field in the (a) and (c) $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{S}}_2$ and (b) and (d) $\mathrm{Ga}\mathrm{Se}/{\mathrm{Sn}\mathrm{Se}}_2$ vdWHs. The black, red, blue, and green lines represent the CBM of the $\mathrm{Ga}\mathrm{Se}$, the VBM of the $\mathrm{Ga}\mathrm{Se}$, the CBM of the ${\mathrm{Sn}\mathrm{S}}_2$ (${\mathrm{Sn}\mathrm{Se}}_2$), and the VBM of the ${\mathrm{Sn}\mathrm{Se}}_2$ (${\mathrm{Sn}\mathrm{Se}}_2$), respectively.