Yusuke Hoshi

Investigation of the open-circuit voltage in solar cells doped with quantum dots

Quantum dots (QDs) have attracted much attention for use in photovoltaic applications because of their potential for overcoming the limits of conventional single-junction devices. One problem associated with solar cells using QDs is that the open-circuit voltage (Voc) always decreases with the addition of QDs with respect to the reference cell without QDs. Here, we report the investigation of current–voltage characteristics in Ge/Si QD solar cells in the temperature range from 100 to 300 K. We show that even though Voc decreases with increasing temperature, it depends on the nominal Ge thickness, indicating that Voc reduction is primarily caused by a decrease in the bandgap energy of the cell. From photoluminescence decay measurements, we found that rapid carrier extraction from QDs occurred in the solar cells; this process eliminates the quasi-Fermi energy splitting between the QDs and the host semiconductor and causes Voc reduction in QD solar cells.

Spin accumulation created electrically in an n-type germanium channel using Schottky tunnel contacts

Using high-quality Fe3Si/n+-Ge Schottky-tunnel-barrier contacts, we study spin accumulation in an n-type germanium (n-Ge) channel. In the three- or two-terminal voltage measurements with low bias current conditions at 50 K, Hanle-effect signals are clearly detected only at a forward-biased contact. These are reliable evidence for electrical detection of the spin accumulation created in the n-Ge channel. The estimated spin lifetime in n-Ge at 50 K is one order of magnitude shorter than those in n-Si reported recently. The magnitude of the spin signals cannot be explained by the commonly used spin diffusion model. We discuss a possible origin of the difference between experimental data and theoretical values.

Cubic Rashba spin-orbit interaction of a two-dimensional hole gas in a strained-Ge/SiGe quantum well.

The spin-orbit interaction (SOI) of a two-dimensional hole gas in the inversion symmetric semiconductor Ge is studied in a strained-Ge/SiGe quantum well structure. We observe weak antilocalization (WAL) in the magnetoconductivity measurement, revealing that the WAL feature can be fully described by the k-cubic Rashba SOI theory. Furthermore, we demonstrate electric field control of the Rashba SOI. Our findings reveal that the heavy hole (HH) in strained Ge is a purely cubic Rashba system, which is consistent with the spin angular momentum m(j) = ± 3/2 nature of the HH wave function.

Enhanced carrier extraction from Ge quantum dots in Si solar cells under strong photoexcitation

We report studies of the carrier extraction mechanism in Si solar cells with Ge quantum dots (QDs), which enable the optical absorption of photons with energies below the band gap of the host. Photocurrent measurements revealed that the photocurrent in the QD solar cells increased superlinearly with increasing excitation intensity under strong photoexcitation, which differed greatly from the behavior of Si solar cells without Ge QDs. This nonlinear photocurrent generation indicates that the carrier extraction efficiency from QDs is enhanced under strong photoexcitation by nonlinear carrier extraction processes, such as two-step photon absorption and hot carrier generation via Auger recombination.We report studies of the carrier extraction mechanism in Si solar cells with Ge quantum dots (QDs), which enable the optical absorption of photons with energies below the band gap of the host. Photocurrent measurements revealed that the photocurrent in the QD solar cells increased superlinearly with increasing excitation intensity under strong photoexcitation, which differed greatly from the behavior of Si solar cells without Ge QDs. This nonlinear photocurrent generation indicates that the carrier extraction efficiency from QDs is enhanced under strong photoexcitation by nonlinear carrier extraction processes, such as two-step photon absorption and hot carrier generation via Auger recombination.

Electric-field control of spin accumulation signals in silicon at room temperature

We demonstrate spin accumulation signals controlled by the gate voltage in a metal-oxide-semiconductor field effect transistor structure with a Si channel and a CoFe/n+-Si contact at room temperature. Under the application of a back-gate voltage, we clearly observe the three-terminal Hanle-effect curves, i.e., spin accumulation signals. The magnitude of spin accumulation signals can be reduced with increasing the gate voltage. We consider that the gate controlled spin signals are attributed to the change in the carrier density in the Si channel beneath the CoFe/n+-Si contact. This study is not only a technological jump for Si-based spintronic applications with gate structures but also reliable evidence for the spin injection into the semiconducting Si channel at room temperature.

Bias current dependence of spin accumulation signals in a silicon channel detected by a Schottky tunnel contact

We study the electrical detection of spin accumulation at a ferromagnet-silicon interface, which can be verified by measuring a Hanle effect in three-terminal lateral devices. The device structures used consist of a semiconducting Si channel and a Schottky tunnel contact. In a low current-bias region, the Hanle-effect curves are observed only under forward bias conditions. This can be considered that the electrical detectability at the forward-biased contact is higher than that at the reverse-biased contact. This is possible evidence for the detection of spin-polarized electrons created in a Si channel.

Simultaneous enhanced photon capture and carrier generation in Si solar cells using Ge quantum dot photonic nanocrystals

We propose a novel solar cell structure with photonic nanocrystals coupled to quantum dots (QDs) for advanced management of photons and carriers. The photonic nanocrystals at the surface create an extra interaction between the photons and the QDs, which promotes light trapping. Photo-generated carriers can be efficiently transported by preparing vertically aligned QDs with electronic coupling. Implementation of the proposed structure was realized in crystalline Si solar cells with Ge QDs by development of a simple and practical formation method based on a wet chemical process without any lithography techniques. The wet process utilizes a periodically modulated etching rate induced by self-organized Ge QDs. The effectiveness of the proposed solar cell was demonstrated by the marked increase of the absolute conversion efficiency when compared with the control crystalline Si solar cells. It is found that light trapping by the photonic nanocrystals has a larger contribution to the efficiency improvement than the contributions from the carrier transport of the vertically aligned QDs.

A quantum-dot spin qubit with coherence limited by charge noise and fidelity higher than 99.9%

1 RIKEN Center for Emergent Matter Science, RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan 2 Department of Applied Physics, University of Tokyo, Tokyo 113-8656, Japan 3 JST, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama, 332-0012, Japan 4 Department of Physical Electronics, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8552, Japan 5 Institute of Industrial Science, University of Tokyo, Meguro-ku, Tokyo 153-8505, Japan 6 Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan 7 Department of Applied Physics and Physico-Informatics, Keio University, Hiyoshi, Yokohama 223-8522,

Temperature evolution of spin accumulation detected electrically in a nondegenerated silicon channel

We study temperature evolution of spin accumulation signals obtained by the three-terminal Hanle effect measurements in a nondegenerated silicon channel with a Schottky-tunnel-barrier contact. We find the clear difference in the temperature-dependent spin signals between spin-extraction and spin-injection conditions. In a spin-injection condition with a low bias current, the magnitude of spin signals can be enhanced despite the rise of temperature. For the interpretation of the temperature-dependent spin signals, it is important to consider the sensitivity of the spin detection at the Schottky-tunnel-barrier contact in addition to the spin diffusion in Si.

Ultrashallow Ohmic contacts for n-type Ge by Sb δ-doping

We demonstrate ultrashallow Ohmic contacts for n-Ge by the Sb δ-doping and low-temperature Ge homoepitaxy. We find that the segregation effect of Sb on Ge(111) is lower than that on Ge(100) for growth temperatures below 400 °C. Consequently, we achieve the δ-doping for Ge(111), having very high concentrations above 1020 cm−3 and abrupt profiles within nanometer-scale widths. By introducing the δ-doping to atomically controlled metal/Ge Schottky contacts, completely symmetric current-voltage characteristics, that is, low-resistivity Ohmic contacts are obtained owing to the effective tunneling conduction through the Schottky barrier.