Design of memristor materials from ordered-vacancy zincblende semiconductors

Memristors with promising applications in nonvolatile memory and unconventional computing have attracted much interest for both materials study and device development. Memristors are not commonly realized in zincblende-like semiconductors that could have optimum lattice matching with Si or GaAs substrates in semiconductor technologies, whereas often based on metal oxides with movable oxygen vacancies. Here, we propose the ordered-vacancy zincblende (OVZ) semiconductors as a type of memristor materials. Based on first-principles calculations on the ${\mathrm{Al}}_2\text{−}X\text{−}{Y}_4$ group of semiconductors, we select ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$ as the best candidate that is lattice matched to Si, with medium energy barriers of ∼1 eV for vacancy/ion diffusion, comparable to the metal-oxide memristor materials, suggesting that ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$ could be segregated into ion-rich versus vacancy-rich structures via ion drift under electric operation. We find from defect calculations that both ${\mathrm{V}}_{\mathrm{Cd}}$ and ${\mathrm{Cd}}_i$ are shallow defects, suggesting a bipolar conduction with electron transport dominated. We further find that the electron-rich ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$ structure can be both electrically conductive and optically transparent, showing potential applications as transparent memristors. Our study therefore opens the way of designing OVZ memristor materials with good compatibility with semiconductor technologies, as well as potentially optimum properties for memristor devices.

Figure 1

(a) Crystal structure and symmetry group of ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$. (b) The HSE06 band structure of ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$, the blue dots represent the position of VBM and CBM. (The VBM is set to be 0.) (c) The calculated electron localization function of ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$. (d) The table presents the HSE06 band gaps of the ${\mathrm{Al}}_2\text{−}X\text{−}{Y}_4$ ($X=\mathrm{Zn}$, Cd; $Y=\mathrm{S}$, Se, Te) compounds based on the structure of (a).

Figure 2

(a) Stable chemical potential region for ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$; the potential competing phases are shown in the other regions. (b) Calculated formation energies for various defects in ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$, under both the Cd-rich and Cd-poor conditions. (c) Schematic diagram of the ${\mathrm{Cd}}_i/{\mathrm{V}}_{\mathrm{cd}}$ defects formation.

Figure 3

(a) Representation of the minimum energy path of Cd atom diffusing to the Cd vacancy in the pristine $\left(2\times 2\times 1\right)$ supercell of ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$ with a Cd vacancy. (b) Representation of the MEP of charged defect $V_{Cd}^{2-}$ ion diffusing to the center of the adjacent S tetrahedron in the Cd-deficient $\left(2\times 2\times 1\right)$ supercell of ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$. The blue dots in the insets describing the positions of the diffusing ions. (c) Representation of the MEP of the $C{d}_i^{2+}$ ion diffusing to the center of the adjacent S tetrahedron in the Cd-rich $\left(2\times 2\times 1\right)$ supercell of ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$ (the corresponding CI-NEB images were presented in Fig. S4).

Figure 4

(a) The transmission spectra $\left(T_{xx}\right)$ of a freestanding 0.2-μm-thick ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$ slab with various carrier densities $\left(n_{\mathrm{e}}\right)$. (b) The absorption coefficient (${\alpha }^{xx}={\alpha }^{yy},{\alpha }^{zz}$) of ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$ with $n_{\mathrm{e}}=6.15\times {10}^{19}\mathrm{c}{\mathrm{m}}^{-3}$, and plasma frequency (${\omega }_p^{xx}={\omega }_p^{yy}=0.451\mathrm{eV}/\hslash , {\omega }_p^{zz}=0.452\mathrm{eV}/\hslash$) are given in the inset of (b). (c) The transmission spectra $\left(T_{xx}=T_{yy},T_{zz}\right)$ of a freestanding 0.2-μm-thick ${\mathrm{Al}}_2\mathrm{Cd}{\mathrm{S}}_4$ slab with optically smooth surfaces $\left(n_{\mathrm{e}}=6.15\times {10}^{19}\mathrm{c}{\mathrm{m}}^{–3}\right)$. (d) Representation of the real-space electron density distribution (green spheres) of the above doped system. The electrical conductivity evaluated from the plasma frequency is ${\sigma }^{xx}={\sigma }^{yy}=0.136\times {10}^3\mathrm{S}/\mathrm{cm}$; ${\sigma }^{zz}=0.136\times {10}^3\mathrm{S}/\mathrm{cm}$. Note that, the blue spheres represent the internal sections of the green spheres. See Supplemental Material Sec. 1 for the evaluation methods for optical property and conductivity.