Shot Noise Spectroscopy on a Semiconductor Quantum Dot
in the Elastic and Inelastic Cotunneling Regimes


Yuma Okazaki1, 2, Satoshi Sasaki1, 2, and Koji Muraki1
1Physical Science Laboratory, 2Tohoku University

   Shot noise is a time-dependent current fluctuation reflecting the discreteness of charge carriers. Measurements of shot noise in coherent conductors such as a quantum dot (QD) are expected to reveal dynamical mechanisms and underlying correlation in electron transport. Electron cotunneling is a higher-order tunneling process observed in the Coulomb blockade regime. Cotunneling is classified into elastic and inelastic processes; the former involves only the QD ground state, while the latter accompanies dynamical charge fluctuation between the ground and excited states. Shot noise measurements have been reported only for the inelastic cotunneling regime in a carbon nanotube QD [1]. For semiconductor QDs, on the other hand, shot noise measurements in the cotunneling regime are made more challenging by the even lower cotunneling current inherent to semiconductor QDs [2].
   Here we study the shot noise in a semiconductor QD in the cotunneling regime. We fabricate a small QD [Fig. 1(a)], in which the level spacing ƒ’E can be increased up to 1 meV. Owing to this largeƒ’E, we can obtain cotunneling current high enough as compared to the resolution of our noise measurement setup. Figure 1(b) shows the conductance G as a function of source drain bias Vsd. For a small bias range (|Vsd| < 1 mV), we find small but finite conductance due to elastic cotunneling. For a large bias range (|Vsd| > 1 mV), on the other hand, G is strongly enhanced due to the strong inelastic cotunneling. Figure 1(c) shows the Fano factor F, which is determined from the measured current noise Sout and current I [F = Sout / 2eI with e being the electron charge]. We observe the Poissonian Fano factor F ~ 1 in the elastic cotunneling regime. On the other hand, we observe an enhancement of the Fano factor up to F ~ 2.5 (super-Poissonian Fano factor) in the inelastic cotunneling regime. This large Fano factor results from the fact that the inelastic cotunneling is accompanied by an emission of excess electrons, which leads to electron bunching and enhances the shot noise [3]. The observed difference in the value of the Fano factor can be utilized as a new spectroscopic measurement, by which we can quantitatively distinguish the microscopic origins of charge transport.

[1] E. Onac et al., Phys. Rev. Lett. 96 (2006) 026803.
[2] S. Gustavsson et al., Phys. Rev. B 78 (2008) 155309.
[3] Y. Okazaki, S. Sasaki, and K. Muraki, Phys. Rev. B 87 (2013) 041302(R).

Fig. 1. (a) Device and the setup. The current noise is fed to the LC resonator, and is measured using cold-amplifier. (b) Conductance G and (c) Fano factor F as a function of Vsd.