Band structure modulation of ZnSe/ZnTe nanowires under strain

First-principles calculations have been used to investigate the structural and electronic properties of unpassivated ZnSe, ZnTe and ZnX/ZnY [$\text{X}=\text{Se}\left(\text{Te}\right),\text{Y}=\text{Te}\left(\text{Se}\right)$] core/shell nanowires oriented along the [111] direction with hexagonal cross sections. The effects of quantum confinement and strain on the electronic properties of the nanowires have been explored for different diameters and core/shell thicknesses. We observe that strong band structure modulation is achievable through uniaxial strain. While for ZnTe nanowires, compression induces a direct-to-indirect band transition for diameters larger than 1.4 nm, there is no sign for a similar transition either for single component ZnSe or core/shell nanowires. The wave function analysis reveals a strong preference for localizing the electron states inside ZnSe rich regions.

Figure 1

(a) The zinc-blend conventional unit cells of bulk ZnSe and ZnTe with lattice constants obtained from our DFT calculations. (b) The largest atomic structure of ZnTe NW along the [111] direction. ZnSe NWs have the same initial geometries with Te atoms replaced by Se atoms. Blue, yellow, and red spheres represent Zn, Se, and Te atoms, respectively.

Figure 2

(a) The binding energies and (b) band gap energies of calculated ZnSe and ZnTe bulks and NWs as a function of diameter. (c) Density of state (DOS) of C-ZnSe NW. The gap between the surface states and the CBM ($E_s$) occurs because of the unpassivated dangling bonds at the surface.

Figure 3

Energy band structures of nonpassivated unstrained and strained (a) A-ZnSe, (b) B-ZnSe, (c) C-ZnSe, (d) D-ZnSe, (e) A-ZnTe, (f) B-ZnTe, (g) C-ZnTe, and (h) D-ZnTe NWs. The Fermi level is set to zero and is shown by the red dotted line. The insets show the optimized geometries of unstrained nanowires. The red arrows in (f)–(h) are used to show the indirect band gaps of compressed ZnTe NWs.

Figure 4

(a) The initial geometry of B-ZnSe NW oriented along [111] direction. The optimized B-type (b) ZnSe(1)/ZnTe(2), (c) ZnSe(2)/ZnTe(1) core/shell NWs where Se atoms are replaced by Te atoms in the related rings, and Zn, Se, and Te atoms are represented by blue, orange, and red spheres, respectively.

Figure 5

The optimized geometry of C-ZnSe(2)/ZnTe(2) NW (a) in ball representation to show the outer and inner positions of Te and Zn atoms, respectively, (b) in the $yz$ direction showing the bond length (in Å) extension, and (c) the deviation of bonding angles from tetrahedral bonding. Zn, Se, and Te atoms are represented by blue, orange, and red spheres, respectively.

Figure 6

The calculated stress vs lattice parameter along the wire axis for (a) ZnSe/ZnTe and (b) ZnTe/ZnSe core/shell NWs.

Figure 7

Energy band structures of nonpassivated C-type ZnSe/ZnTe and ZnTe/ZnSe core/shell NWs grown along the [111] direction. The Fermi level is set to zero and is shown by the dotted red line. The insets show the optimized geometries of the nanowires.

Figure 8

Calculated PDOS of NWs with type-C (a) ZnSe, (b) ZnTe, (c) ZnSe(2)/ZnTe(2), and (d) ZnTe(2)/ZnSe(2). A slight Gaussian broadening of 0.05 eV has been applied. The Fermi energy is set to zero.

Figure 9

Valence (blue) and conduction (white) wave functions for type-C (a) ZnTe, (b) ZnSe(1)/ZnTe(3), (c) ZnSe(2)/ZnTe(2), (d) ZnSe(3)/ZnTe(1), (e) ZnSe, (f) ZnTe(1)/ZnSe(3), (g) ZnTe(2)/ZnSe(2), and (h) ZnTe(3)/ZnSe(1) NWs are shown. Blue, yellow, and red spheres represent Zn, Se, and Te atoms, respectively. Isosurfaces are drawn at 25$\%$ of maximum.