Energy Distribution Measurement of Nonequilibrium Carriers Using a Quantum Dot

　

Toshiyuki Kobayashi1,4, Shoei Tsuruta1,3,4, Satoshi Sasaki1, Toshimasa Fujisawa1, Yasuhiro Tokura2, and Tatsushi Akazaki1,4
1Physical Science Laboratory, 2Optical Science Laboratory, 3Tokyo University of Science, 4JST-CREST

　　A semiconductor quantum dot (QD) confines electrons in a nanometer-scale region. This results in discrete quantized energy levels of electrons in a QD. These quantized energy levels can be easily tuned by controlling the gate voltage, thus enabling us to use a QD as a high-resolution energy analyzer (or spectrometer) for the electrons near the QD.
　　We used this feature of QDs to measure the energy distribution of ballistic nonequilibrium electrons and holes emitted from a quantum wire (Fig. 1). Nonequilibrium carriers were emitted by applying a bias voltage (Vpc) to a quantum point contact. The emitted current was again focused by applying a perpendicular magnetic field (B⊥) and analyzed with a QD (Fig. 2). When the energy of the nonequilibrium carriers coincides with the quantized energy levels, those carriers can resonantly tunnel through the QD and can be detected as an electric current. Therefore, when the quasi-chemical potential of the carriers is aligned at the energy levels of the QD, the differential conductance of the QD exhibits a peak and appears as a Coulomb diamonds (Fig. 3).
　　We found that when the bias energy is small, the energy distribution does not broaden. However, at a high bias (~1 meV), the distribution broadened owing to enhanced electron-electron scattering, which causes carrier energy relaxation [1]. This result is important for future experiments related to quantum information transfer between quantum bits.

[1] T. Kobayashi, et al., Phys. Stat. Sol. (c) 5 (2008) 162.

Fig. 1. Scanning electron micrograph of device and measurement setup.
Fig. 2. (a,b) Electron accumulation and (c,d) hole accumulation near quantum dot.
Fig. 3. (a) Coulomb diamond realized by ballistic hot carrier injection. Arrows indicate peaks caused by hot carriers tunneling through QD levels. (b) Normal Coulomb diamond by Vsd.
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