Tetsuo Kodera

Experimental Observation of Enhanced Electron–Phonon Interaction in Suspended Si Double Quantum Dots

Silicon-based suspended double quantum dots (SDQDs) were fabricated to study and control the strength of the electron–phonon interaction. A distinctive and large inelastic tunneling was observed in single-electron transport measurement and well explained by the emission of phonons that interact strongly with electrons owing to the phonon modulation in the suspended film. The first time observation of the enhancement of the electron–phonon interaction in Si SDQDs as well as the good agreement between the experimental results and the theoretical simulations are encouraging preliminary results that allow us to envision the observation of the tailoring of the electron–phonon interaction in SDQDs.

Fine and Large Coulomb Diamonds in a Silicon Quantum Dot

We experimentally study the transport properties of silicon quantum dots (QDs) fabricated from a highly doped n-type silicon-on-insulator wafer. Low noise electrical measurements using a low temperature complementary metal-oxide-semiconductor (LTCMOS) amplifier are performed at 4.2 K in liquid helium. Two series of Coulomb peaks are observed: long-period oscillations and fine structures, and both of them show clear source drain voltage dependence. We also observe two series of Coulomb diamonds having different periodicity. The obtained experimental results are well reproduced by a master equation analysis using a model of double QDs coupled in parallel.

Suspended Quantum Dot Fabrication on a Heavily Doped Silicon Nanowire by Suppressing Unintentional Quantum Dot Formation

We aim at embedding a quantum dot on a suspended nanowire by solving the problem of unintentional quantum dot formation, which exacerbates in a suspended nanowire. The origin of this worsening is the higher potential barrier presumably owing to the enhancement of random-dopant-induced potential fluctuation and/or higher degree of surface roughness and surface trapped charges on suspended nanowires. The higher barrier was successfully decreased by adopting a higher doping concentration as well as wider constriction patterns. Consequently, we can control the quantum dot formation in the suspended nanowire and successfully defined a single-quantum dot by patterning the double constrictions on the heavily doped suspended nanowire.

Realization of Lithographically‐Defined Silicon Quantum Dots without Unintentional Localized Potentials

We have fabricated lithographically‐defined Si quantum dots (QDs) within a metal‐oxide‐semiconductor field‐effect transistor (MOSFET) structure. In this architecture, the top gate is used to tune the carrier density whereas side gates control the potentials of the QDs and tunneling barriers. These lithographically‐defined and electrically‐tunable Si QDs were successfully realized without unintentional localized potentials.

Direct observation of subband structures in (110) PMOSFETs under high magnetic field: Impact of energy split between bands and effective masses on hole mobility

The band structures and carrier transport in (110) pFETs are thoroughly studied over a wide temperature range under high magnetic fields. In (110) pFETs, the degenerate hole bands in bulk Si are separated into the higher energy band (H band) and the lower energy band (L band). The energy difference between these bands is experimentally evaluated. The effective masses of each band are directly obtained from the Shubnikov-de Haas (SdH) oscillation analysis. It is demonstrated that mobility in the higher energy band is worse than that in the lower energy band, resulting in sharp mobility drop at higher surface carrier concentrations (Ns) and a clear hump in Id-Vg characteristics at low temperatures of less than 20 K. In order to further enhance mobility in (110) pFETs, the increase in the energy split between H and L bands is important.

Pauli-spin blockade in a vertical double quantum dot holding two to five electrons

We use a vertical double quantum dot (QD) to study spin blockade (SB) for the two-to five-electron states. SB observed for the two- and four-electron states is both assigned to Pauli exclusion with formation of a spin triplet state, and lifted by singlet-triplet admixing due to fluctuating nuclear field. SB observed for the five-electron state is caused by combined Pauli effect and Hunds rule. We observe a hysteretic behavior of the SB leakage current for up and down sweep of magnetic field, and argue that SB and its lifting by hyperfine interaction are subtle with the spin configuration and modified depending on the inter-dot detuning and number of electrons.

Spin-related tunneling in lithographically-defined silicon quantum dots

We realized lithographically-defined electrically-tunable silicon quantum dots (Si QDs) without unintentional localized potentials by improving device structures and fabrication techniques. Carrier density was tuned with a top gate and QD-potentials were controlled with the side gates. We succeeded in observing spin-related tunneling phenomena using the double QD device.