Ryuji Oshima

Strain-compensated InAs/GaNAs quantum dots for use in high-efficiency solar cells

We have investigated GaAs-based p-i-n quantum dot solar cells (QDSCs) with 10 up to 20 stacked layers of self-assembled InAs quantum dots (QDs) grown by atomic hydrogen-assisted molecular beam epitaxy. The net average lattice strain was minimized by using the strain-compensation technique, in which GaNAs dilute nitrides were used as spacer layers. The filtered short-circuit current density beyond GaAs bandedge was 2.47 mA/cm2 for strain-compensated QDSC with 20 stacks of InAs QD layers, which was four times higher than that for strained QDSC with identical cell structure.

Increase in photocurrent by optical transitions via intermediate quantum states in direct-doped InAs/GaNAs strain-compensated quantum dot solar cell

We have developed a technique to fabricate quantum dot (QD) solar cells with direct doping of Si into InAs QDs in GaNAs strain-compensating matrix in order to control the quasi-Fermi level of intermediate QD states. The Si atoms were evenly incorporated into QDs during the assembling stage of growth such that a uniform array of partially filled QDs has been obtained. Nonradiative recombination losses were also reduced by Si doping and a photocurrent increase due to two-step photon absorption was clearly measured at room temperature detected under filtered air-mass 1.5 solar spectrum.

Characteristics of InAs/GaNAs strain-compensated quantum dot solar cell

We have fabricated and compared the performance of GaAs-based p-i-n quantum dot solar cells with ten multilayer stacked structures of self-assembled InAs quantum dots embedded with GaNxAs1−x strain-compensating spacer layers. Reducing the thickness of the spacer layer, and hence increasing the nitrogen composition in GaNxAs1−x, from 40 nm (x=0.5%) to 15 nm (x=1.5%) thereby fulfilling the net strain-balanced condition, resulted in a steady increase in the short-circuit density, while a decreasing trend for the open-circuit voltage was observed. The observed results can be interpreted in terms of the difference in the quantum confinement structure.

Ultra-high stacks of InGaAs/GaAs quantum dots for high efficiency solar cells

We report ultra-high stacked InGaAs/GaAs quantum dot (QD) solar cells fabricated by the intermittent deposition of In0.4Ga0.6As under an As2 source using molecular beam epitaxy. We obtain a 400-stack In0.4Ga0.6As QD structure without using a strain balancing technique, in which the total number of QDs reaches 2 × 1013 cm−2. Photoluminescence and cross-sectional scanning transmission electron microscope measurements indicate that the In0.4Ga0.6As QD structure exhibits no degradation in crystal quality, no dislocations and no crystal defects even after the stacking of 400 QD layers. The external quantum efficiency and the short-circuit current density of multi-stacked In0.4Ga0.6As QD solar cells increase as the number of stacked layers is increased to 150. Such ultra-high stacks and good cell performance have not been reported for QD solar cells using other material systems. The performance of the ultra-high stacked QD solar cells indicates that InGaAs QDs are suitable for use in high efficiency solar cells requiring thick QD layers for sufficient light absorption.

Multiple stacking of self-assembled InAs quantum dots embedded by GaNAs strain compensating layers

We have investigated a growth technique to realize high-quality multiple stacking of self-assembled InAs quantum dots (QDs) on GaAs (001) substrates, in which GaNxAs1−x dilute nitride material was used as a strain compensation layer (SCL). The growth was achieved by atomic hydrogen-assisted rf molecular beam epitaxy, and the effect of strain compensation was systematically investigated by using high-resolution x-ray diffraction measurements. By controlling the net average lattice strain to a minimum by covering each QD layer with a 40-nm-thick GaN0.005As0.995 SCL, we obtained a superior QD structure with no degradation in size homogeneity. Further, no dislocations were generated even after 30 layers of stacking, and the area density of QDs amounted to as high as 3×1012cm−2. The photoluminescence peak linewidth was improved by about 22% for QDs embedded in GaNAs SCLs as the accumulation of lattice strain with increasing growth of QD layers was avoided, which would otherwise commonly lead to degradation of ...

Probing subpicosecond dynamics using pulsed laser combined scanning tunneling microscopy

Time-resolved tunneling current measurement in the subpicosecond range was realized by ultrashort-pulse laser combined scanning tunneling microscopy, using the shaken-pulse-pair method. A low-temperature-grown GaNxAs1−x(x=0.36%) sample exhibited two ultrafast transient processes in the time-resolved tunnel current signal, whose lifetimes were determined to be 0.653±0.025 and 55.1±5.0ps. These values are of the same order of magnitude as those measured in the conventional pump–probe reflectivity measurement.

Probing ultrafast spin dynamics with optical pump-probe scanning tunnelling microscopy.

Studies of spin dynamics in low-dimensional systems are important from both fundamental and practical points of view. Spin-polarized scanning tunnelling microscopy allows localized spin dynamics to be characterized and plays important roles in nanoscale science and technology. However, nanoscale analysis of the ultrafast dynamics of itinerant magnetism, as well as its localized characteristics, should be pursued to advance further the investigation of quantum dynamics in functional structures of small systems. Here, we demonstrate the optical pump-probe scanning tunnelling microscopy technique, which enables the nanoscale probing of spin dynamics with the temporal resolution corresponding, in principle, to the optical pulse width. Spins are optically oriented using circularly polarized light, and their dynamics are probed by scanning tunnelling microscopy based on the optical pump-probe method. Spin relaxation in a single quantum well with a width of 6 nm was observed with a spatial resolution of ∼ 1 nm. In addition to spin relaxation dynamics, spin precession, which provides an estimation of the Landé g factor, was observed successfully.

InGaP-based InGaAs quantum dot solar cells with GaAs spacer layer fabricated using solid-source molecular beam epitaxy

We report InGaP-based multistacked InGaAs quantum dot (QD) solar cells with GaAs spacer layers. We obtain a highly stacked and well-aligned InGaAs QD structure with GaAs spacer layers in an InGaP matrix grown by solid-source molecular beam epitaxy. The photoluminescence intensity of the InGaAs QDs in the InGaP matrix increases as the number of QD layers increases, which indicates the growth of a high-quality InGaP-based multistacked InGaAs QD structure. The short-circuit current density and the conversion efficiency of the InGaP-based QD solar cells increase as the number of InGaAs QD layers increases.

InGaAs quantum dot superlattice with vertically coupled states in InGaP matrix

We report the formation of vertically coupled states in a 20-stack InGaAs quantum dot (QD) superlattice with GaAs spacer layers in an InGaP matrix. The InGaAs QD superlattices in the InGaP matrix have good optical properties even though the interdot spacing is reduced to 4.5 nm. We confirmed the vertically coupled states from the excitation power dependence in photoluminescence (PL) measurements. The PL peak of a QD superlattice shifts to a shorter wavelength as the excitation power is increased. The blue-shifted energy of the PL peak is 10 meV for a QD superlattice with an interdot spacing of 4.5 nm, whereas the blue shift is not observed for a multistacked QD structure with an interdot spacing of 17 nm. The vertically coupled states induce a blue shift in the PL peak wavelength as the excitation power density is increased. The vertical energy transfer between InGaAs QDs in an InGaP matrix is very attractive for use in solar cell devices.

Nanoscale probing of transient carrier dynamics modulated in a GaAs–PIN junction by laser-combined scanning tunneling microscopy

The modulation of carrier dynamics in a GaAs-PIN junction after photoexcitation by an ultrashort-pulse laser was probed by shaken-pulse-pair-excited scanning tunneling microscopy (SPPX-STM), which enables nanoscale mapping of time-resolved STM images. The effect of the built-in potential on the carrier dynamics, diffusion and drift, which cannot be probed by the optical pump-probe technique, was successfully visualized in real space.