Enhanced Shot Noise in Tunneling through a Stack of Coupled Quantum Dots

We have investigated the noise properties of the tunneling current through vertically coupled self-assembled InAs quantum dots. We observe super-Poissonian shot noise at low temperatures. For increased temperature this effect is suppressed. The super-Poissonian noise is explained by capacitive coupling between different stacks of quantum dots.

Figure 1

(a) Growth scheme of the active part of the device. (b) Conduction band structure $E_{\mathrm{CB}}$ and the Fermi energy $E_F$ of a stack of two QDs. If a positive bias voltage is applied the electrons tunnel in positive $z$ direction through the device.

Figure 2

(a) Current-voltage characteristics of the sample at $T\approx 1.4\mathrm{K}$. The arrow marks the peak which is discussed further. Inset: Schematic band structure of the coupled quantum dots where resonant tunneling is observed. (b) Typical noise spectra of the sample for different bias voltages. The solid line demonstrates the results of fitting the function $A/f+S_0$ to the spectrum at $V_{\mathrm{SD}}=-186.75\mathrm{mV}$. The horizontal line shows $S_0$.

Figure 3

Blow up of the peak marked in Fig. 2 in the current-voltage characteristic measured at $T\approx 1.4\mathrm{K}$ (dashed line, left axis) and shot noise amplitude $S_0$ as given by fitting the function $A/f+S_0$ to the noise spectra shown in Fig. 2 (dots, right axis). The squares correspond to the spectra shown in Fig. 2b. The scale of the right axis was chosen such that the dashed line corresponds on this axis to full shot noise $S_{\mathrm{full}}=2eI$ as expected for a single tunneling barrier structure (Fano factor $\alpha =1$).

Figure 4

(a) Differential conductance for a peak in the $I\mathrm{\text{−}}V$ characteristic at $T\approx 1.4\mathrm{K}$ and $T\approx 2.7\mathrm{K}$. (b) The filled symbols show the Fano Factor $\alpha$ at $T\approx 1.4\mathrm{K}$ and the open circles show $\alpha$ at $T\approx 2.7\mathrm{K}$.