Substitutional Electron and Hole Doping of ${\mathrm{WSe}}_2$: Synthesis, Electrical Characterization, and Observation of Band-to-Band Tunneling

Transition-metal dichalcogenides (TMDs) such as ${\mathrm{MoS}}_2$, ${\mathrm{MoSe}}_2$, and ${\mathrm{WSe}}_2$ have emerged as promising two-dimensional semiconductors. Many anticipated applications of these materials require both $p$-type and $n$-type TMDs with long-term doping stability. Here, we report on the synthesis of substitutionally doped ${\mathrm{WSe}}_2$ crystals using Nb and Re as $p$- and $n$-type dopants, respectively. Hall coefficient and gate-dependent transport measurements reveal drastically different doping properties between nominally 0.5% Nb- and 0.5% Re-doped ${\mathrm{WSe}}_2$. While 0.5% Nb-doped ${\mathrm{WSe}}_2$ (${\mathrm{WSe}}_2:\mathrm{Nb}$) is degenerately hole doped with a nearly temperature-independent carrier density of approximately ${10}^{19}{\mathrm{cm}}^{-3}$, electrons in 0.5% Re-doped ${\mathrm{WSe}}_2$ (${\mathrm{WSe}}_2:\mathrm{Re}$) are largely trapped in localized states below the mobility edge and exhibit thermally activated behavior. Charge transport in both ${\mathrm{WSe}}_2:\mathrm{Nb}$ and ${\mathrm{WSe}}_2:\mathrm{Re}$ is found to be limited by Coulomb scattering from ionized impurities. Furthermore, we fabricate vertical van der Waals–junction diodes consisting of multilayers of ${\mathrm{WSe}}_2:\mathrm{Nb}$ and ${\mathrm{WSe}}_2:\mathrm{Re}$. Finally, we demonstrate reverse rectifying behavior as a direct proof of band-to-band tunneling in our ${\mathrm{WSe}}_2:\mathrm{Nb}/{\mathrm{WSe}}_2:\mathrm{Re}$ diodes.

Figure 1

(a) Resistivity as a function of temperature for ${\mathrm{WSe}}_2:\mathrm{Nb}$ (the red squares) and ${\mathrm{WSe}}_2:\mathrm{Re}$ (the blue circles). (b) Linear behavior of Hall resistivity as a function of the magnetic field at 300 K is shown. The positive slope for ${\mathrm{WSe}}_2:\mathrm{Nb}$ and the negative slope for ${\mathrm{WSe}}_2:\mathrm{Re}$ confirms $p$-type and $n$-type doping, respectively. (c) Inferred carrier density vs temperature for Nb- and Re-doped ${\mathrm{WSe}}_2$. Freezing of charge carriers is not observed in ${\mathrm{WSe}}_2:\mathrm{Nb}$, whereas the carrier concentration decreases with temperature for ${\mathrm{WSe}}_2:\mathrm{Re}$. (d) Hall mobility vs temperature for Nb- and Re-doped ${\mathrm{WSe}}_2$. A weak dependence on temperature is clearly seen for ${\mathrm{WSe}}_2:\mathrm{Nb}$, whereas the mobility decreases by roughly a factor of 3 for ${\mathrm{WSe}}_2:\mathrm{Re}$ as the sample is cooled.

Figure 2

Linear behavior of drain current as a function of the drain-source voltage for different gate voltages are shown for (a) Nb-doped ${\mathrm{WSe}}_2$ and (b) Re-doped ${\mathrm{WSe}}_2$. [Inset in (a)] An AFM image of ${\mathrm{WSe}}_2:\mathrm{Nb}$ FET. (c) The dependence of conductivity on the back-gate voltage for hole-doped ${\mathrm{WSe}}_2:\mathrm{Nb}$ when the drain voltage is $-1\mathrm{V}$. (d) The dependence of conductivity for electron-doped ${\mathrm{WSe}}_2:\mathrm{Re}$ when the drain voltage is $-0.1\mathrm{V}$.

Figure 3

(a) Current as a function of the applied bias $V_b$ on Si while ${\mathrm{WSe}}_2:\mathrm{Re}$ is grounded. The $p\text{−}n$ junction characteristics are shown in both forward- and reverse-bias conditions. (Upper inset) Schematic diagram of the heterojunction device. (Lower inset) The order of rectification from forward to reverse bias of 2 V. (b) Qualitative band diagrams at forward and reverse biases. BTBT tunneling is shown in reverse bias and the hole-electron recombination process is shown in forward bias.