Time-resolved tunneling current measurement in the subpicosecond range was realized byultrashort-pulse laser combined scanning tunneling microscopy, using the shaken-pulse-pairmethod. A low-temperature-grown GaNxAs1−x (x=0.36%) sample exhibited two ultrafast transientprocesses in the time-resolved tunnel current signal, whose lifetimes were determined to be0.653±0.025 and 55.1±5.0 ps. These values are of the same order of magnitude as those measuredin the conventional pump–probe reflectivity measurement

It is crucial to develop a low-cost growth system to implement large-scale terrestrial modules in III–V solar cells. Hydride vapor-phase epitaxy (HVPE) has emerged as a low-cost alternative for the fabrication of III–V solar cells. However, it is difficult to utilize HVPE to fabricate abrupt GaAs-on-InGaP heterointerfaces. During growth interruption, indium segregation and exchange between arsenic and phosphorous atoms occur easily on the InGaP surface. The growth sequence is dependent on the reactor configuration. In this study, we investigated the growth sequence in a triple-chamber HVPE system to improve the abruptness of the heterointerfaces of GaAs solar cells. It was found that a short interruption of growth under flowing phosphine is critical in eliminating the InGaAsP interlayer at the interface between the p-InGaP back surface layer and the p-GaAs base layer. This improved cell efficiency to 20.87% for the upright cell and to 22.10% for the inverted front-junction cell.

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.

The elevated manufacturing cost of highly efficient III–V multijunction devices limits them to space and high-concentration terrestrial applications. Hydride vapor-phase epitaxy (HVPE), in contrast to metal–organic vapor-phase epitaxy, can reduce the manufacturing costs due to the use of cheaper group III metal sources, capability to grow crystals under a low arsenic overpressure, and its low installation cost. In this study, we characterized the abruptness of the heterointerface between the InGaP and GaAs layers in GaAs solar cells grown by HVPE. InGaP passivation layers, such as the back-surface field (BSF) and window layers, were introduced in order to enhance the performance of the GaAs solar cells. Owing to the reduction of the surface recombination, the devices fabricated with the incorporated window layer showed a significant improvement in the short-wavelength range of the external quantum efficiency response, compared with that obtained for the unpassivated cells. This provided a cell efficiency improvement from 9.25 to 20.75%. However, the introduction of the BSF layer degrades the cell efficiency to 17.92%, owing to the formation of an anomalous interlayer at the GaAs-on-InGaP heterointerface.