Light irradiation induced brittle-to-ductile and ductile-to-brittle transition in inorganic semiconductors

The intrinsic brittleness of inorganic semiconductors prevents them from extended engineering applications under extreme conditions of high temperature and pressure, making it essential to improve their ductility. Here, we applied the constrained density functional theory to examine the relationship between plastic deformation and photonic excitation in sphalerite ZnS and related II-IV semiconductors. We find that ZnS transforms from a dislocation dominated deformation mode in the ground state to a twin dominated deformation mode with bandgap electronic excitations, leading to brittle failure under light illumination. This agrees very well with recent mechanical experiments on single crystal ZnS. More interesting, we predict that the ZnTe and CdTe display the opposite mechanical behavior compared to ZnS, exhibiting ductility close to metallic level with bandgap illumination, but typical brittle failure in the dark state. Our results provide a general approach to design more shapeable and tougher semiconductor devices by controlling exposure to electronic excitation.

Figure 1

(a) A schematic illustration showing the structure of sphalerite ZnS with the stacking sequence AaBbCc along the $\langle 111\rangle$ direction and the top view of the {111} plane of ZnS based on the two normal vectors of $\langle 11\overline{2}\rangle$ and $\langle 1\overline{1}0\rangle$. A, B, C and a, b, c indicate Zn layers and S layers, respectively. The black rectangle denotes the normal Burgers vectors $1/2\langle 11\overline{2}\rangle a_0$ and $1/2\langle 1\overline{1}0\rangle a_0$. The dotted lines denote the two possible slip paths within the {111} plane of ZnS. Path I and path II indicate two plausible plastic deformation paths occurring between two widely spaced layers like Cc and two closely spaced layers such as Ac. (b) Three-dimensional GSF energy map of path I for the {111} plane of ZnS obtained by DFT calculation. P, F, and T represent perfect stacking, unstable stacking fault, and unstable twinning fault, respectively. (c), (d) GSF energy of path I (c) and path II (d) as a function of the displacement $u/|\vec{b}|$ along the $\langle 11\overline{2}\rangle$ direction, where $\vec{b}$ is the length of the Burgers vector along the specific slip direction, and $u$ denotes the magnitude of displacement. Here $0{\mathrm{h}}^{+}0{\mathrm{e}}^{-}, 1{\mathrm{h}}^{+}1{\mathrm{e}}^{-}$ and $2{\mathrm{h}}^{+}2{\mathrm{e}}^{-}$ represent the results for the ground state, one electron-hole exited state, and two electron-hole excited state, respectively. ${\gamma }_{\mathrm{us}}, {\gamma }_{\mathrm{isf}}$, and ${\gamma }_{\mathrm{ut}}$ correspond to unstable stacking fault energy, intrinsic stacking fault energy, and unstable twin fault energy, respectively. (e), (f) GSF energy of path I (e) and path II (f) as a function of the displacement $\mathrm{u}/|\vec{b}|$ along the $\langle 1\overline{1}0\rangle$ direction. Schematic view of (g), (i) unstable stacking-fault structure, and (h), (j) unstable twinning-fault structure for ZnS. The blue arrows indicate the relative shift of top layer to bottom layer toward the $\langle 11\overline{2}\rangle$ direction.

Figure 2

(a) Electron localization function maps in the $\left\{111\right\}$ plane of the ground state. (b), (c) Charge density difference between the ground state and one electron-hole excited state for the unstable stacking-fault structure of (b) path I and (c) path II. The blue and red color isosurfaces denote losing and capturing electrons. (d), (e) Planar average charge density difference [(d) path I, (e) path II] between the ground state and one electron-hole excited state for the unstable stacking-fault structure along the $\langle 111\rangle$ direction.

Figure 3

GSF energies for both path I and path II associated with the displacement along the $\langle 11\overline{2}\rangle$ direction for (a) ZnSe, (b) ZnTe, and (c) CdTe.