Ayami Takata

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.

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.

Multi-stacked InAs/GaNAs quantum dots with direct Si doping for use in intermediate band solar cell

We investigated the effect of direct doping of quantum dots (QDs) with Si on the performance of QD solar cells (QDSCs). In order to control the Fermi level of intermediate band (IB) region, 25 layers of stacked InAs/GaNAs QDs were directly doped with Si impurity during the self-assembling stage of growth. A QDSC with Si doping shows an improved quantum efficiency (QE) in shorter wavelength region, which is from p-GaAs emitter layer. Further, the fact that applied external bias does not affect QE spectrum as well as photocurrent in QDSC with Si direct doping suggests that carrier collection has been enhanced in QD region as a result of reduction of recombination.

Fabrication of 100 layer-stacked InAs/GaNAs strain-compensated quantum dots on GaAs (001) for application to intermediate band solar cell

In order to demonstrate the predicted high efficiency operation of a quantum dot intermediate band solar cell (QD-IBSC), high-density QD superlattice with good size homogeneity is required. Though multiple stacking is one promising way to increase the total QD density thereby increasing the optical absorption by QD-IB, it is difficult to maintain the size homogeneity and structural quality of QD superlattice. For this, we take advantage of strain-compensation growth technique, in which the compressive strain induced by each InAs QD layer is compensated, or balanced out, by embedding it with a tensile-strained GaNAs strain-compensating layer. In this work, we demonstrate a high quality growth of up to 100 layer-stacked InAs/GaNAs QD superlattice on GaAs (001) substrate. We have also characterized some basic solar cell characteristics.

Multi-stacked InGaAs/GaNAs quantum dot solar cell fabricated on GaAs (311)B substrate

Quantum dot solar cells (QDSCs) comprised of 10 stacked pairs of strain-compensated InGaAs/GaNAs QD structure have been fabricated by atomic hydrogen assisted molecular beam epitaxy (H-MBE). A 3 dimensionally well ordered InGaAs QD array structure with a total density of ∼1012 cm−2 has been achieved on GaAs (311)B substrate. The external quantum efficiencies of InGaAs/GaNAs QDSCs increase in the longer wavelength range due to additive contribution from QD layers inserted in the intrinsic region. We have achieved a higher short-circuit current density of 18.7 mA/cm2 compared to an InAs/GaNAs QDSC fabricated on GaAs (001).

Stacking-layer-number dependence of highly stacked InAs quantum dot laser diodes fabricated using strain-compensation technique

Semiconductor quantum dots (QDs) grown using self-assembly techniques in the Stranski- Krastanov (S-K) mode are expected to be useful for high-performance optical devices such as QD lasers. A significant amount of research has been carried out on the development of highperformance QD lasers because they offer the advantages of a low threshold current, temperature stability, high modulation bandwidth, and low chirp. To realize these high-performance devices, the surface QD density should be increased by fabricating a stacked structure. We have developed a growth method based on a strain-compensation technique that enables the fabrication of a high number of stacked InAs QD layers on an InP(311)B substrate. In this study, we employed the proposed method to fabricate QD laser diodes consisting of highly stacked QD layers and investigated the dependence of the diode parameters on the stacking layer number. We fabricated QD laser diodes with 5, 10, 15, and 20 QD layers in the active region. All of the laser diodes operated at around 1.55 μm at room temperature, and their threshold currents showed clear dependence on the stacking layer number. Laser diodes with more than 10 QD layers showed sufficient gain, i.e., the threshold currents decreased with a decrease in the cavity length. On the other hand, for laser diodes with less than 10 QD layers, the threshold currents increased with a decrease in the cavity length.