Search

We investigated the radiation damage to carrier lifetimes in the neutral and depletion regions of the InGaP and GaAs subcells of an InGaP/GaAs/Ge triple-junction solar cell. It is difficult to clarify the degradation characteristics of the carrier lifetime in each region using the conventional analysis method (e.g., the dark current–voltage characteristic analysis), so we propose a novel method using the internal luminescence efficiency. The radiation damage coefficients for the carrier lifetimes in the neutral region of InGaP and GaAs subcells were two or three orders of magnitude larger than those in the depletion region. This result suggests that the effective radiation-induced defects in the regions, which significantly impact a solar cell's electrical characteristics, differ. This paper discusses the effective radiation-induced defects that contribute to the output degradation in each region. We compare the radiation damage coefficients for the carrier lifetimes obtained in our analysis, the product of the capture cross section, and the defect introduction coefficient of each defect reported in previous studies.

Silicon quantum photonics provides a promising pathway to realize large-scale quantum photonic integrated circuits (QPICs) by exploiting the power of complementary-metal-oxide-semiconductor (CMOS) technology. Toward scalable operation of such silicon-based QPICs, a straightforward approach is to integrate deterministic single-photon sources (SPSs). To this end, hybrid integration of deterministic solid-state SPSs, such as those based on InAs/GaAs quantum dots (QDs), is highly promising. However, the spectral and spatial randomness inherent in the QDs pose a serious challenge for scalable implementation of multiple identical SPSs on a silicon CMOS chip. To overcome this challenge, we have been investigating a new hybrid integration technique called transfer printing, which is based on a pick-and-place operation and allows for the integration of desired QD SPSs on any locations on the silicon CMOS chips at will. Nevertheless, even in this scenario, in-situ fine tuning for perfect wavelength matching among the integrated QD SPSs will be required for interfering photons from the dissimilar sources. Here, we demonstrate in-situ wavelength tuning of QD SPSs integrated on a CMOS silicon chip. To thermally tune the emission wavelengths of the integrated QDs, we augmented the QD SPSs with optically driven heating pads. The integration of all the necessary elements was performed using transfer printing, which largely simplified the fabrication of the three-dimensional stack of micro/nanophotonic structures. We further demonstrate in-situ wavelength matching between two dissimilar QD sources integrated on the same silicon chip. Our transfer-printing-based approach will open the possibility for realizing large-scale QPICs that leverage CMOS technology.

We measured the time profiles of terahertz (THz) radiation emitted from a GaAs-based solar cell under weak excitation with ultrashort optical pulses. The time-domain THz waveform directly reflects the characteristic saturation and overshooting of the drift current under high fields in the thin depletion layer of the solar cell, from which we can evaluate the internal electric field quantitatively. We also measured the time profiles of THz radiation for simultaneous excitation with ultrashort pulses and continuous light and observed the reduction of the internal field in the depletion layer of the solar cell during operation. Since nonlinear responses of the drift current in the depletion layer can be observed, we conclude that THz measurements provide a key technology for characterizing the solar cell performance that can be expected under practical operating conditions.We measured the time profiles of terahertz (THz) radiation emitted from a GaAs-based solar cell under weak excitation with ultrashort optical pulses. The time-domain THz waveform directly reflects the characteristic saturation and overshooting of the drift current under high fields in the thin depletion layer of the solar cell, from which we can evaluate the internal electric field quantitatively. We also measured the time profiles of THz radiation for simultaneous excitation with ultrashort pulses and continuous light and observed the reduction of the internal field in the depletion layer of the solar cell during operation. Since nonlinear responses of the drift current in the depletion layer can be observed, we conclude that THz measurements provide a key technology for characterizing the solar cell performance that can be expected under practical operating conditions.

Local electronic states due to characteristic surface islands and pits formed on an atomically flat (110) GaAs interface of a quantum well fabricated by a cleaved-edge-overgrowth method with high-temperature growth-interrupt annealing are characterized by micro-photoluminescence (PL) imaging and spectroscopy. With the overall observation of the 6-nm-thick (110) quantum well with the area of 6.8-μm width and 3-mm length, we found local PL signals due to isolated 2- or 3-monolayer (ML) islands, isolated 1-ML pits, and 1-ML islands and pits formed along the cleavage atomic-step lines, and gradual change of the states across the full range of the sample.

Planar-type Lambertian light-emitting diodes (LEDs) with a circular aperture of several tens of μm to a few mm in diameter were developed for use as radiant-flux standard light sources, which have been in strong demand for applications such as quantitative or absolute intensity measurements of weak luminescence from solid-state materials and devices. Via pulse-width modulation, time-averaged emission intensity of the LED devices was controlled linearly to cover a wide dynamic range of about nine orders of magnitude, from 10 μW down to 10 fW. The developed planar LED devices were applied as the radiant-flux standards to quantitative measurements and analyses of photoluminescence (PL) intensity and PL quantum efficiency of a GaAs quantum-well sample. The results demonstrated the utility and applicability of the LED standards in quantitative luminescence-intensity measurements in Lambertian-type low radiant-flux level sources.

We demonstrate the sensitive measurement of photocarriers in an active layer of a GaAs-based photovoltaic device using time-resolved terahertz reflection spectroscopy. We found that the reflection dip caused by Fabry-Perot interference is strongly affected by the carrier profile in the active layer of the p-i-n structure. The experimental results show that this method is suitable for quantitative evaluation of carrier dynamics in active layers of solar cells under operating conditions.

Microphotoluminescence (micro-PL) characterization was performed for T-shaped InxGa1−xAs quantum wires (T-QWRs) fabricated by the cleaved edge overgrowth method with molecular beam epitaxy. The spatial distribution of optical properties in InxGa1−xAs T-QWRs was examined by means of PL intensity imaging and scanning micro-PL spectroscopy with about 1 μm spatial resolution. In the successfully fabricated 3.5-nm-scale In0.17Ga0.83As T-QWRs with Al0.3Ga0.7As barriers, uniform PL image and PL spectra from QWRs along the wire direction were observed, which indicates spatially uniform and high quality QWRs were formed. The effective lateral confinement energy of one-dimensional excitons was found to be 34 meV, showing the usefulness of InxGa1−xAs as a well material in T-QWR structures. On the other hand, in the unsuccessfully fabricated 4-nm-scale In0.09Ga0.91As T-QWRs with Al0.3Ga0.7As barriers, PL image and spectra were inhomogeneous. These results demonstrate the importance of flat cleaved surface and optimiz...

We have fabricated GaAs T‐shaped edge quantum wire (T‐QWR) structures by the cleaved edge overgrowth method. The geometries of the samples were designed in such a way that we can separately detect photoluminescence (PL) from T‐QWRs as well as two adjacent quantum wells (QWs) from spatially resolved PL measurements. Each PL peak is unambiguously assigned and analyzed to precisely determine the quantized energy of excitons in T‐QWRs and the adjacent QWs. Special care is made in the cleavage and the growth procedures to realize sharp and efficient PL from these structures.

In this work, we propose and demonstrate a durable and distributable Lambertian light-emitter secondary standard using the electroluminescence (EL) of a Si solar cell. This standard is useful for calibration of the absolute sensitivity of an EL-imaging infrared camera used to acquire quick on-site measurements of the absolute EL efficiencies of individual Si solar cells in modules and arrays. The developed method enables the realization of quantitative open-circuit voltage mapping.

We determined potential-energy surfaces for Ga and As adatoms on a GaAs (110) surface by first-principles calculations in order to understand the epitaxial growth mechanism. We found small migration barrier energies for Ga and As, which explain the long atom-migration length suggested by experiments. We also found that Ga migration is one dimensional and As migration is two dimensional, and that, for both Ga and As adatoms, the sites near As of the topmost layer are stable while those near Ga are unstable.

Atomically flat (110) GaAs surfaces fabricated by the cleaved-edge overgrowth method and high-temperature growth-interrupt annealing are characterized at the atomic scale. We observe atomically flat (110) surfaces extending over areas more than 100 μm in size. Moreover, deposition of slightly less or more than integral monolayers (MLs) causes the appearance of characteristic step-edge shapes such as 1-ML-deep pits, or 2-to-3-ML-high isolated islands. Statistical analysis on the size and shape distribution of the 1-ML-deep pits supports a simple model based on the stability of Ga and As atoms on step edges with different bonding configurations, and reveals driving force to form an atomically flat (110) surface.

We experimentally and theoretically study the effects of interface roughness and phonon scattering on intersubband absorption linewidth in a modulation-doped GaAs/AlAs quantum well. Quantitative comparisons between experimental results and theoretical calculations make it clear that interface roughness scattering is the dominant scattering mechanism for absorption linewidth in the temperature range below 300 K. Even at room temperature, phonon scattering processes contribute little to linewidth, while polar-optical phonon scattering limits electron mobility.

Distribution of electronic states inside ridge quantum-wire (QWR) laser cavity was investigated using spatially and spectrally resolved top-view photoluminescence (PL) imaging method. PL inhomogeneity in QWR has shown that the electronic states were perturbed by the fluctuation of vertical thickness in the scale of ∼nm, while the optical waveguide was not. The PL images of QWR were traced up to the lasing condition to examine the lasing origin.

High efficiency of light collection was demonstrated by applying solid immersion lenses (SILs) with refractive indices n=1.845 and 1.687 to fluorescence microscopy of 0.11-μm-radius dye-doped polystyrene sphere beads. This was analyzed with theories on the radiation of dipoles near the surface of the dielectric medium. The estimated collection efficiency with the NA=0.8 objective and the n=1.845 SIL was about 60%–70%, which well exceeds the diffraction-limit value of 50% for 2π-solid angle collection in conventional methods.

A promising method for studying intraband carrier dynamics in semiconductors is monitoring the evolution of far-infrared (FIR) absorption induced by photoexcited carriers. By monitoring the photoinduced FIR absorption as a function of magnetic field, we performed time-resolved cyclotron resonance of photocreated electrons in InSb with picosecond resolution.

We have analyzed the aberrations and allowances for an aspheric error, a thickness error, and an air gap in a hemisphere solid immersion lens for photoluminescence microscopy with submicron resolution beyond the diffraction limit, where light with large ray angle and the effect of an evanescent field play important roles.

Conventional external quantum-efficiency measurement of solar cells provides charge-collection efficiency for approximate short-circuit conditions. Because this differs from actual operating voltages, the optimization of high-quality tandem solar cells is especially complicated. Here, we propose a contactless method, which allows for the determination of the voltage dependence of charge-collection efficiency for each subcell independently. By investigating the power dependence of photoluminescence decays, charge-separation and recombination-loss time constants are obtained. The upper limit of the charge-collection efficiencies at the operating points is then obtained by applying the uniform field model. This technique may complement electrical characterization of the voltage dependence of charge collection, since subcells are directly accessible.

Carrier diffusion and drift in facet-growth quantum wells (QWs) on mesa-patterned substrates by molecular beam epitaxy was studied by high-resolution microscopic photoluminescence spectroscopy and imaging using a solid immersion lens at low temperatures. Under point excitation, excitation-position-dependent anisotropic carrier migration was observed, which was explained by carrier diffusion and drift due to spatial change in the quantization energy in QWs.

A solid immersion lens (SIL) was applied to microscopic photoluminescence (PL) imaging at low temperatures. The spatial resolution corresponding to a numerical aperture of 1.0 was achieved and confirmed to be independent of temperature from 5 to 300 K. We performed PL imaging with the SIL for a GaAs quantum well and obtained stable PL images with high-spatial resolution at a wide temperature range from 5 to 200 K. From the temperature dependence of the PL images with high-spatial resolution, we can directly reveal the diffusion of photocarriers in the sample, which indicates the usefulness of the SIL technique.

We report the observation of stimulated emission in ridge quantum wire (QWR) structures at temperatures from 4.7 to 290 K. To examine the origin of the stimulated emission, the spatially and spectrally resolved microscopic images of the emission were measured. It was most likely attributed to the optical transition between the excited states in QWRs.

Intersubband light absorption measurements have been performed on a series of GaAs/AlAs quantum wells with heavy delta doping of Si atoms in the range 9.0×1011–6.5×1012 cm−2. The increase of intersubband transition energy by as much as 38 meV has been observed, and attributed to the deepening of the V-shaped potential by the Si layer and to the depolarization effects.

We report on time‐resolved photoluminescence study of AlxGa1−xAs spontaneous vertical quantum well (SVQW) structures on GaAs V‐grooved substrates. Four distinct photoluminescence peaks are observed originating from the spatial nonuniformity of the alloy compositions spontaneously formed during metalorganic vapor phase epitaxial growth. The decay time of the (111)A sidewall AlGaAs decreased with increasing temperature, while that of the SVQW increased. The rise time of the SVQW was found to be longer than the typical value of the exciton formation and increases with increasing temperature, indicating that the exciton formation is not limiting factor of the rise time. These results are explained in terms of the exciton diffusion toward the SVQW from the outer AlGaAs layers with less Ga compositions. In addition, two dimensionality of the SVQW was evidenced by temperature dependence of the radiative lifetime.

Temporal decay characteristics of dislocation‐related luminescence bands (D1–D4) were explored in strain‐relieved epitaxial SiGe/Si(100). Close similarity of the decay profiles was observed not only between D1 and D2 bands but also between D3 and D4 bands. The decay transients of the D1 and D2 bands at low temperatures are characterized by long decay times, τ≳200 ns, whereas the D3 and D4 bands exhibit even sharper transients with τ

Influence of the surface on the radiative recombination is studied in near‐surface (NS) strained Si1−xGex/Si quantum wells (QWs). The luminescence intensity of NSQWs is clearly diminished as the Si cap thickness decreases due to carrier capture to the surface. However, the luminescence diminution discloses a saturable behavior with increasing excitation density. Time‐resolved measurements reveal selective electron capture to surface traps as being the controlling process in the luminescence attenuation, and there is no signature of quantum mechanical hole tunneling to the surface. Results of potential‐biased luminescence lend further support to the electron‐controlled picture of the luminescence attenuation in Si‐based near‐surface geometry.

We have performed lateral diffusion measurements of photo‐excited electron‐hole pairs in a GaAs/AlAs multiple quantum well structure under magnetic fields applied perpendicular to the wells, by using an all‐optical time‐of‐flight technique with a single‐mode optical fiber probe. Remarkable suppression of exciton diffusion is observed, and is ascribed to the magnetic perturbation by which moving excitons are affected proportionally to the square of wave vectors.

A remarkable change in photoluminescence spectra was observed when CO2 laser light was incident to induce an intersubband transition in a novel asymmetric double quantum well structure where the second subbands of both wells are resonantly coupled, while the ground subbands are not. It is found that the observed change originates from the CO2‐laser‐induced transfer of electrons from one well to the other, demonstrating the controllability of electron population by means of infrared light.

A ridge quantum wire structure has been successfully fabricated on a patterned (001) GaAs substrate by first growing a (111)B facet structure with a very sharp ridge and then depositing a thin GaAs quantum well on its top. Electron microscope study has shown that a GaAs wire with the effective lateral width of 17–18 nm is formed at the ridge top. Photoluminescence and cathodoluminescence measurements indicate that one of the luminescence lines comes from the wire region at the ridge and its blue shift (∼60 meV) agrees with the quantum confined energy calculated for the observed wire structure.

By employing a long growth interruption (GI) in molecular beam epitaxy, we have successfully determined the concentration of oxygen incorporated in GaAs/AlAs interfaces and AlAs layers by secondary ion mass spectrometry. The concentration of oxygen atoms incorporated on AlAs surfaces during GI is found to be proportional to the period of GI when the incoming fluxes of residual oxygen‐related species reach steady‐state values. The net incorporation rate of oxygen on the AlAs surface is found to be constant for a wide range of substrate temperatures from 540 to 620 °C, indicating that the oxygen desorption is negligible.

We measured the quantitative spectra of firefly (Photinus pyralis) bioluminescence at various temperatures to investigate the temperature dependence of the luciferin-luciferase reaction at 15–34 °C. The quantitative spectra were decomposed very well into red (1.9 eV), orange (2.0 eV), and green (2.2 eV) Gaussian components. The intensity of the green component was the only temperature sensitive quantity that linearly decreased as the temperature increased at pH 7 and 8. We found the quantitative bioluminescence spectra to be robust below 2.0 eV against temperature and other experimental conditions. The revealed robustness of the red emissions should be useful for quantitative applications such as adenosine-5′-triphosphate detection.

To develop a realistic design principle, we calculated the maximum conversion efficiency ηsc and optimized sub-cell band-gap energies Eg in double-junction tandem solar cells via a detailed-balance theory, paying particular attention to their dependence on internal luminescence quantum yields yint of the top and bottom sub-cell materials. A strong drop in the maximum ηsc occurs when yint slightly drops from 1 to 0.9, where the drop in yint of the bottom cell causes a stronger effect than that of the top cell. For low values of yint, the maximum ηsc has a simple logarithmic dependence on the geometric mean of the two sub-cells'yint.

The transient optical gain was quantitatively determined from the rise time of output pulses of a gain-switched semiconductor laser via impulsive optical pumping. The optical gain for a high carrier density during gain switching was greatly reduced from that during steady-state operation because of hot carriers.

We report the demonstration of the picosecond-pulse lasing in blue region from an optically pumped gain-switched InGaN vertical-cavity-surface-emitting laser (VCSEL). Through 150-fs optical pulse excitations at room temperature, multimode pulse lasing with a main mode at 436.5 nm from the InGaN VCSEL was observed. The output pulse widths were measured to be as short as 9.8 ps for the total lasing spectra and 6.0 ps for the main mode. Since the obtained short pulses were still not limited by its photon lifetime of 0.7 ps or band-width of 0.8 nm, possible ways to generate even shorter pulses have been examined.

The absolute strength of two-dimensional continuum absorption in semiconductor quantum wells (QWs) was investigated for application to an absorption-strength standard. We presented an equation for the continuum-state absorbance in a QW, showing that the absorbance near the band edge is essentially described by a universal value proportional to the fine-structure constant with correction terms for band-mixing effects and Coulomb enhancement effects. By using k·p equations with the established band parameters, we calculated the absorbance with quantitative estimations of the correction terms in typical semiconductor QWs. The results indicate that the absorbance of the QW is applicable to an absorption-strength standard.

An absorption-spectroscopy method that utilizes internal emissions as the source of the probe light was used to measure the absorption spectra of quasi-one-dimensional (q-1D) excitons in T-shaped quantum wires embedded in an optical waveguide. The modal absorption area of the 1D ground-state excitons was estimated to be 0.39 eV cm−1 and was almost independent of temperature in the range 4–150 K. Quantitative evaluation using the absorption spectra revealed that the absorption cross-section per unit length at resonance peak and the spectrally integrated absorption cross-section area per unit length of the 1D ground-state excitons were 1.0 nm and 2.5 × 10−3 eV nm, respectively.

Eu doped rare-earth orthovanadates are known to be good red phosphor materials. In particular, LaVO{sub 4}:Eu is a promising candidate due to the low Eu-site point symmetry, and thus high dipole transition probability within Judd-Ofelt theory. However, the low solubility limit (< 3 mol %) of Eu in LaVO{sub 4} prevents its efficient use as a phosphor. We present optical evidence of enhanced Eu solubility as high as 10 mol % in LaVO{sub 4}:Eu thin films grown by pulsed laser deposition and postannealing. The photoluminescent intensity exceeded that of YVO{sub 4}:Eu thin films when excited below the host bandgap, indicating stronger direct emission of Eu in LaVO{sub 4}.

We characterized the micro-photoluminescence (micro-PL) of GaInAsP/InP single quantum wires (Q-wires) at 6 K. The Q-wires had 6–39-nm lateral widths and a 6-nm vertical thickness and were fabricated by dry etching and regrowth. Micro-PL spectra were clearly observed for all single Q-wires. The spectra revealed a high degree of uniformity along the Q-wires. The PL peak energy showed a systematic blueshift with the reduction of the Q-wire lateral width. The blueshift was attributed to the lateral quantum confinement and strain from a lateral direction, and was 100 meV in a 6-nm-wide Q-wire. Systematic analysis on the PL widths of the Q-wires showed that the PL spectra were broadened by fluctuations of 3 nm in the lateral width of the Q-wire after the dry etching and the regrowth in addition to the broadening caused by the fluctuation in the vertical thickness during the initial film growth and that caused by ion bombardment through a Ti metal mask in the dry etching process. The decreased PL intensities in ...

We report on the electronic structure, efficient carrier injection, and quantitative lasing characteristics of T-shaped GaAs/AlGaAs quantum-wire-laser diodes with parallel p- and n-doping layers grown by a cleaved-edge-overgrowth method with molecular-beam epitaxy. Continuous single-mode lasing from the ground subband of the quantum wires was demonstrated between 30 and 70 K in laser diodes with high-reflectivity Au coating on both cavity facets. The lowest threshold of 0.27 mA and the highest differential quantum efficiency of 12% were achieved at 30 K. Micro-photoluminescence measurements demonstrated the high optical quality of the quantum wires with narrow linewidth of 0.9 meV and provided electronic structures of surrounding layers. Microscopic electroluminescence (EL) imaging measurements demonstrated the efficient carrier injection into the quantum wires at 30 K. These two factors, i.e., high material quality and efficient carrier injection, contribute to the low threshold current and high efficien...

Temperature and surface-coverage dependence of the evolution of surface morphology during growth-interrupt in situ annealing on GaAs epitaxial layers grown on the singular (110) cleaved edges by the cleaved-edge overgrowth method with molecular-beam epitaxy has been studied by means of atomic force microscopy. Annealing at substrate temperatures below 630 °C produced atomically flat surfaces with characteristic islands or pits, depending on the surface coverage. The atomic flatness of the surfaces is enhanced with increasing annealing temperature owing to the enhanced adatom migration. At a higher annealing temperature of about 650 °C, however, 2-monolayer-deep triangular pits with well-defined step edges due to Ga-atom desorption from the crystal appeared in the atomically flat surface. The growth-interrupt annealing temperature optimal for the formation of atomically flat GaAs(110) surfaces is therefore about 630 °C.

The authors measured the temperature dependence of the lasing properties of current-injection T-shaped GaAs∕AlGaAs quantum-wire (T-wire) lasers with perpendicular p- and n-doping layers. The T-wire lasers with high-reflectivity coatings on both cleaved facets achieved continuous-wave single-mode operation between 5 and 110K. The lowest threshold current was 2.1mA at 100K. The temperature dependences of differential quantum efficiency and threshold current were attributed mainly to that of current-injection efficiency.