Time-resolved Charge Detection in Transition Metal Dichalcogenide Quantum Dots

Abstract

We investigate electronic transport through gate-defined quantum dots in molybdenum disulfide MoS$_2$ using an integrated charge detector. We observe a crossover from two weakly coupled single dots to a strongly coupled double quantum dot. In the regime of extremely weak dot-lead coupling, where the direct transport current is below the detection limit, we measure the dot occupation via charge detection and access the few-electron regime. Due to the large band gap of MoS$_2$, tunneling rates can be sufficiently suppressed to resolve individual tunneling events. These results establish a platform for single-shot spin- and valley-to-charge conversion and highlight the potential of transition-metal dichalcogenide quantum dots for quantum information applications.

Figures (4)

• Figure 1: (a) False-color AFM image of the entire device. The MoS2 flake is purple and stretches horizontally over the whole device. The scale bar is 5µm. (b) Cross-section of the stack with the four layers of MoS2 encapsulated by two flakes of hBN and partially in contact with two distinct monolayer graphene sheets forming the electrical contacts to the MoS2. (c) Zoom-in image of the gate structure close to the dots. The center barrier connected to $\mathrm{V_{CB}}$ divides the device in two halves. On the top half, the detector dot side, $\mathrm{V_{Det}}$ controls the detector dot. On the bottom half, the signal dot side, the five gates connected to $\mathrm{V_{LB}}$, $\mathrm{V_{PG,L}}$, $\mathrm{V_{MB}}$, $\mathrm{V_{PG,R}}$, $\mathrm{V_{RB}}$ control dot L and dot R. The scale bar is 100nm. (d) Coulomb diamond measurement of the dot sitting between $\mathrm{V_{MB}}$, $\mathrm{V_{PG,R}}$ and $\mathrm{V_{RB}}$. (e) Current through the signal dot and (f) through the detector dot, measured simultaneously for a source drain bias $V_\mathrm{sd}=100µV$. The steps in the detector signal always occur at the same gate voltage as the resonances of the signal dot, but can even be observed when the resonances on the signal side are suppressed.
• Figure 2: (a) Measurement of the detector current while sweeping the two barriers $\mathrm{V_{MB}}$ and $\mathrm{V_{LB}}$ and keeping $\mathrm{V_{PG,L}}$ constant. Orange arrows point to a set of parallel resonances, with the last one of them marked by the red arrow. The yellow star marks the barrier configuration of the measurement in (b). (b) Detector current when sweeping $\mathrm{V_{PG,L}}$ at fixed barrier voltages. The dotted line indicates the plunger gate voltage at which the measurement in (a) was taken.
• Figure 3: (a) Time trace of $I_\mathrm{det}$ showing different current levels for the electron being in and out of the dot. (b) Histogram of tunneling in and tunneling out events at a certain $V_\mathrm{{PG,R}}$ (marked by the arrow in (c) and fits that show the exponential distribution). (c) Tunneling in ($\mathrm{\Gamma_{in}}$) and tunneling out rates ($\mathrm{\Gamma_{out}}$) as a function of the plunger gate voltage $V_\mathrm{{PG,R}}$. The arrow indicates the rates extracted from (b).
• Figure 4: (a) Resonances of a double dot controlled by the plunger gates $V_\mathrm{PG,L}$ and $V_\mathrm{PG,R}$ with constant barrier voltages. (b) Double dot measurement with $V_\mathrm{MB} =$-5V, showing exclusively capacitively coupled dots. We indicate the number of electrons for the dot under $V_\mathrm{PG,R}$ by n and for $V_\mathrm{PG,L}$ by m. (c) The same measurement with $V_\mathrm{MB} =$-4.5V, thus increasing the tunneling coupling, rounding the resonances at their crossings. The color scale is the same for (a), (b) and (c).