Atsuhiko Fukuyama's research while affiliated with Miyazaki University and other places
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Publications (159)
The carrier collection efficiencies of InGaAs/GaAsP superlattice photovoltaic structures were optimized by choosing adequate manufacturing parameters, such as the composition and thickness of the quantum wells (QWs) and barrier layers. However, no insights have been observed from the viewpoint of the nonradiative transition of photoexcited carriers...
Using a new implantation technique with multielement molecular ions consisting of carbon, hydrogen, and phosphorus, namely, CH2P molecular ions, we developed an epitaxial silicon wafer with proximity gettering sinks under the epitaxial silicon layer to improve the gettering capability for metallic impurities. A complementary metal-oxide-semiconduct...
We investigated the annealing behavior of hydrogen in a high-dose hydrocarbon-molecular-ion-implanted silicon during rapid thermal annealing (RTA). Gettering sinks in the high-dose hydrocarbon-molecular-ion-implanted region are formed not only at carbon-related defects but also at defects related to the amorphous layer after RTA. The concentration...
The laser heterodyne photothermal displacement (LH-PD) method was used to characterize the nonradiative recombination centers of semiconductors, such as defects and deep-lying electronic levels. When a semiconductor surface is irradiated with a modulated continuous wave laser, the irradiated area is periodically heated and expanded owing to the non...
The role of a GaAs strain-relaxation interlayer inserted into InGaAs/GaAsP superlattice solar cells was evaluated by measuring the piezoelectric photothermal (PPT) signals in the temperature range from 100 K to a device operation temperature of around 340 K. The PPT signals caused by the non-radiative recombination of electrons photo-excited to the...
Photoluminescence (PL) measurements at a wide temperature range, up to room temperature, for high-quality and high-density GaAs/AlGaAs quantum dots (QDs) fabricated by droplet epitaxy were carried out to investigate the anomalous temperature dependence of the PL peak energy from the QD ensemble. In addition to a reported redshift that deviated from...
Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films possess the following specific characteristics: (a) the appearance of additional energy levels in diamond bandgap and (b) large absorption coefficients ranging from visible to ultraviolet, both of which might be due to large number of grain bounda...
We discuss the effect of interlayer insertion into the light absorption region of a carrier escape process for superlattice (SL) solar cells determined by the temperature dependence of photoluminescence (PL) signals. 20-period SL solar cells, with and without interlayers, were prepared. The temperature dependence of the integrated PL intensities du...
Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films possess the following specific characteristics: (a) the appearance of additional energy levels in diamond bandgap and (b) large absorption coefficients ranging from visible to ultraviolet, both of which might be due to large number of grain bounda...
Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films comprising a lot of diamond grains whose diameters are less than 10 nm and an a-C:H matrix are expected to be applied to photovoltaics because of high absorption coefficient in the visible range and production of n and p-type conductions by nitrog...
Ultrananocrystalline diamond(UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films, which comprise a large number of diamond grains whose diameters are less than 10 nm embedded in an amorphous carbon matrix, possess large optical absorption coefficients and the conduction type can be controlled by nitrogen and boron doping, accom...
Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films possess specific characteristics as follows: (a) the appearance of additional energy levels in diamond bandgap [1]; and (b) large absorption coefficients ranging from visible to ultraviolet [2], both of which might be due to large number of grain...
Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films are composed of a large number of UNCD grains with diameters of less than 10 nm and an a-C:H matrix.1,2) The existence of a lot of grain boundaries1) is a structural specific to UNCD/a-C:H, which might be an origin of the appearance of energy leve...
To improve the superlattice (SL) solar cell performance, we carried out an accurate estimation of transition energies and miniband widths and focused on understanding of the optical properties of the SL structure using piezoelectric photothermal (PPT), photoreflectance (PR), and photoluminescence (PL) methods. Solar cell structure samples with diff...
The effect of substrate orientation on strain relaxation mechanisms of an InGaAs layer grown on vicinal GaAs substrates was investigated by in situ X-ray diffraction (XRD). The crystallographic tilt and indium segregation in the InGaAs layer were altered depending on the miscut direction and angle. In the case of the substrate tilted 6° toward the...
GaAs quantum nanodisks (QNDs) in nanopillar (NP) arrays are considered to be an attractive candidate for photonic device applications. We report a damageless fabrication technique that can be used to produce large-area lattice-matched GaAs/AlGaAs heterostructure NP arrays through the use of a bio-template and neutral beam etching. We have successfu...
Ultrananocrystalline diamond (UNCD)/hydrogenated amorphous carbon (a-C:H) composite (UNCD/a-C:H) films comprise a large number of grains with diameters of less than 10 nm and an a-C:H matrix [1,2]. Owing to the specific structure, some energy levels generates between the bandgap of diamond [1]. The absorption coefficient in the visible-ultraviolet...
We investigated the effect of built-in electric field on miniband formation and carrier nonradiative recombination in a superlattice structure by using photoreflectance (PR) and photoluminescence (PL) spectroscopy for a strain-balanced InGaAs/GaAsP superlattice inserted in p-i-n GaAs solar cell and n-n GaAs structures. Two critical energies were ob...
By means of high-resolution transmission electron microscopy (HR-TEM) observations, and energy dispersive X-ray analyses performed with a scanning TEM (STEM-EDX), we evaluated the residual Na contents and the induced crystal strains in type-II Si clathrate films grown on a Si substrate and treated with iodine for Na elimination. Cross-sectional TEM...
This New top-down fabrication process using bio-template and neutral beam etching process have succeeded to grow GaAs quantum nano disc (ND) for optical communication devices. The size, the alignment and the density of ND were well controlled. Low temperature photoluminescence measurement confirmed that the quantum energy levels were formed in the...
We demonstrated that the lattice-matched GaAs quantum nanodisks (QNDs) embedded in an AlGaAs matrix were fabricated by our original topdown nanoprocess. Lattice-matched GaAs QNDs are very attractive in quantum cryptography because the spin relaxation time of QNDs might be longer than that of strained quantum dots. Quantum levels of QNDs were invest...
Quantum dot laser diodes are expected to replace conventional semiconductor laser diodes in new high-speed information and communication devices. We successfully fabricated disk-shaped quantum dots using a bio-template and neutral beam etching. Our original top-down process achieved defect-less and a high density of dots from etching process compar...
Thin films of guest-free type-II Si clathrate (Si136) were fabricated on Si(111) wafers in two steps: NaxSi136 thin-film formation by thermal decomposition of NaSi precursor films and Na removal from the NaxSi136 film by a heat treatment with iodine. Cross-sectional TEM observation and XRD and Raman measurements verified the formation of 1-μm-thick...
Multiple Quantum wells (MQWs) have been studied as one promising material for high-efficiency nextgeneration solar cells. However, a portion of photo-excited carriers recombine in MQWs, resulting in the degradation of cell performance. Super-lattice (SL) structures, where quantum states in neighboring quantum wells strongly couple with each other,...
To investigate the carrier collection mechanism in the strain-balanced InGaAs/GaAsP super-lattice (SL)-inserted GaAs p-i-n solar cell structure samples, three non-destructive methodologies, surface photovoltage (SPV), photoluminescence (PL), and piezoelectric photothermal (PPT) spectroscopies, were adopted. They detect the thermal carrier escape fr...
The effects of growth temperature and nitrogen (N) source duration on N, carbon (C), and hydrogen (H) concentrations in GaAsN layers grown by atomic layer epitaxy (ALE) were investigated to understand the incorporation mechanisms of these atoms. In addition, the effects of the above growth conditions on the self-limiting mechanism (SLM) were invest...
Starting on June 2011, NGCPV has been the first coordinated project between the European Commission and Japanese NEDO in order to advance in the science and technology of concentrator photovoltaics (CPV). Research has covered all relevant areas in CPV, from solar cells to complete systems, including multijunction solar cells, novel solar cell conce...
Cu2ZnSnS4 (CZTS) is paid attention to the low-cost and nontoxic solar cell material. In this study, CZTS thin film was deposited by using an electrodeposition technique followed by a sulfurization after preheating the deposited metallic precursor layer. We investigated the film properties of CZTS thin films for understanding the effect of preheatin...
The effect of Si-doping on the phase separation of wurtzite (WZ) and zinc-blende (ZB) phases in catalyst-free Si-doped GaAs nanowires (NWs) grown on a Si(111) substrate was investigated using transmission electron microscope (TEM), high-resolution X-ray diffraction (HR-XRD), low-temperature photoreflectance (PR), and photoluminescence (PL) techniqu...
Three non-destructive methodologies, namely, surface photovoltage (SPV), photoluminescence, and piezoelectric photothermal (PPT) spectroscopies, were adopted to detect the thermal carrier escape from quantum well (QW) and radiative and non-radiative carrier recombinations, respectively, in strain-balanced InGaAs/GaAsP multiple-quantum-well (MQW)-in...
To investigate the microstructure of nitrogen-induced localized state (EN) that perturbs the conduction band of GaAs host material, we adopted the photoreflectance measurements to chemical-beam-epitaxy and flow-rate modulated chemical-beam-epitaxy grown GaAsN thin films. We measured both two split subbands to estimate correct values of EN. It was c...
To investigate the microstructure of nitrogen-induced localized state (EN) that perturbs the conduction band of GaAs host material, we adopted the photoreflectance measurements to chemical-beam-epitaxy grown GaAsN thin films. We measured both two split subbands to estimate correct values of EN. It was clearly seen that estimated EN decreased with i...
The fabrication of p-type ZnO crystal is motivated by the need to develop ZnO semiconductor devices. On the other hand, it is well-known that high-quality nano-sized ZnO crystal can be easily obtained by various crystal growth techniques. Consequently, we tried to grow p-type Sb-doped ZnO nanowires by the chemical vapor deposition under various dep...
Since the co-doping method is a key technique for the realization of p-type ZnO films, we grew samples by the co-doping of Li and Ni impurities using a pulsed laser deposition technique to investigate the formation of a possible acceptor level. The effect of the oxygen gas pressure on the physical properties is discussed. The electrical and optical...
To investigate the effect of the miniband formation on the optical absorption spectrum, we adopted two non-destructive methodologies of piezoelectric photothermal (PPT) and photoreflectance (PR) spectroscopies for strain-balanced InGaAs/GaAsP multiple quantum-well (MQW) and superlattice (SL) structures inserted GaAs p-i-n solar cells. Because the b...
The effects of In pre-evaporation on the formation of rotational twin domains (TW) in GaAs layers on Si (111) substrates were systematically investigated. Atomic arrangements of In atoms on the Si substrate during pre-evaporation were studied and resulting formation of TW were discussed. The pre-evaporation of In resulted in the formation of InAs i...
The effect of Be doping on the crystal structure and the optical properties of catalyst-free Be-doped GaAs nanowires (NWs) grown on a (111)Si substrate were investigated by scanning transmission electron microscopy (STEM), X-ray diffraction (XRD) analysis, and photoreflectance (PR) and photoluminescence (PL) techniques. Be-doped NWs sample showed a...
The multiple quantum wells (MQW) structure in the solar cell has a potential to extend the absorption towards the infrared region and enhance the collection of photo-generated carriers. We have studied the external quantum efficiency and non-radiative recombination of the photo-excited carriers in MQW structure by using the surface photovoltage (SP...
Dilute nitride films of GaAsN with high nitrogen homogeneity were grown
by using the chemical beam epitaxy technique. This allowed us to
investigate the electronic structure of the nitrogen-induced localized
level (EN) in GaAsN by photoreflectance measurements. We
found that the EN value decreased as the nitrogen content
increased. Consideration of...
High-quality GaAs film grown on Si substrate is a promising structure
for high-efficiency, low-cost, and lightweight multi-junction solar
cells. However, the formation of rotational twins degrades the cell
efficiency. This limitation could be circumvented by evaporating In
atoms onto Si (111) substrate before the molecular beam epitaxial (MBE)
grow...
Starting on June 2011, NGCPV is the first project funded jointly between the European Commission (EC) and the New Energy and Industrial Technology Development Organization (NEDO) of Japan to research on new generation concentration photovoltaics (CPV). The Project, through a collaborative research between seven European and nine Japanese leading re...
We organized a project of solar cells in University of Miyazaki in order to develop the technologies for solar cells. We have four kinds PV module systems, such as hybrid Si Thin film (1 kW), Poly-Si (50 kW), CIS (100 kW) and CPV (14 kW×2). Their total is 179 kW. These take appoximately 5% of total electricity of the university.
Fabrication of superlattice (SL) structures embedded in an absorption layer of solar cell is a promising idea for developing higher efficient devices. This is because the quantum well can extend the absorption to longer wavelength region and enhance the short-circuit current. However, recombination centers for photo-excited carriers are simultaneou...
Fabrication of superlattice (SL) structures embedded in an absorption layer of solar cell is a promising idea for developing higher efficient devices. This is because the quantum well can extend the absorption to longer wavelength region and enhance the short-circuit current. However, recombination centers for photo-excited carriers are simultaneou...
The acceptor levels and their concentration in GaAsN films grown by the
chemical beam epitaxy technique were investigated through detailed
analysis of the temperature dependence of hole concentration. Two
acceptor levels A1 and A2 were found, and their
energy levels were fixed at 130 ± 20 and 55 ± 10 meV,
respectively, from the valence band maximum...
To optimize the multiple quantum well (QW) structure of the
strain-balanced InGaAs/GaAsP inserted into GaAs p-i-n solar cell,
carrier escaping process from QW, carrier radiative and non-radiative
recombination processes in QW were investigated by using surface
photovoltage (SPV), photoluminescence (PL) and piezoelectric
photothermal (PPT) spectrosc...
For establishing a new methodology for evaluating an effect of the grain boundaries, both the piezoelectric photo-thermal (PPT) and the surface photo-voltage (SPV) measurements of polycrystalline Si p–n junction samples with different volume fractions of grain boundaries were carried out. We could define the signal intensity ratio of SPV/PPT as the...
The temperature dependences of the piezoelectric photo-thermal (PPT) signals from unintentionally doped p-type GaAsN films grown on semi-insulating GaAs substrate were measured in the temperature range from 80 to 300 K. From the theoretical analysis based on the rate equation for the recombination of photo excited carriers to the localized levels,...
Anisotropies in misfit dislocations (MDs) at the interface of InGaAs/GaAs (001) are investigated by monochromatic X-ray topography (XRT) technique. Single MD line or several MD lines (MD bunching) are observed as white lines in XRT. Distribution, density, and number of MDs in one MD bunching are evaluated. The density of α-MDs is larger than that o...
By using a sequential evaporation method, Cu(In,Ga)Se2 (CIGS)
thin films with a high Ga/(Ga+In) mole ratio were fabricated on
Mo/soda-lime glass substrates. The bandgap energy (Eg)
estimated by the photoreflectance (PR) and piezoelectric photothermal
(PPT) methods shifted to the higher photon energy side with increasing
Ga/(Ga+In) mole ratio. Altho...
Temperature dependences of the optical gap of diamond-like carbon (DLC) films were investigated by using a piezoelectric photothermal (PPT) spectroscopy, that detects the nonradiative transition of photoexcited electrons. Since the PPT signal intensity is expected to be proportional to its optical absorption coefficient, the optical gap was estimat...
Dye-sensitized solar cells (DSCs) are expected to be used for future clean energy. In general, when the titania porous electrode in DSCs is made, a polyethylene glycol (PEG) is added to obtain the porous structure. Although the conversion efficiency of the DSC became high when the high molecular weight of PEG was used, its reason was not clear. In...
The exciton binding energies (Exb) of a dilute nitride Ga1-yInyN0.012As0.988 layer (y = 0.0 to 4.5%) with the thickness of 100 nm were determined by both piezoelectric photothermal (PPT) and photoreflectance (PR) spectroscopies. Curve-fitting analyses were carried out using a three-dimensional direct allowed-transition model with the Voigt function...
The exciton binding energies (Exb) of a dilute nitride Ga1-yInyN0.012As0.988 layer (y = 0.0 to 4.5%) with the thickness of 100 nm were determined by both piezoelectric photothermal (PPT) and photoreflectance (PR) spectroscopies. Curve-fitting analyses were carried out using a three-dimensional direct allowed-transition model with the Voigt function...
The optical properties of catalyst-free GaAs nanowires (NWs) grown on a
(111)Si substrate were investigated by low-temperature photoreflectance
(PR) and photoluminescence (PL) techniques. Although the bandgap energy
(Eg) of non-doped NWs agreed well with that of
liquid-encapsulated Czochralski-grown semi-insulating bulk, a
distinctive PL peak cause...
The optical properties of catalyst-free GaAs nanowires (NWs) grown on a (111)Si substrate were investigated by low-temperature photoreflectance (PR) and photoluminescence (PL) techniques. Although the bandgap energy (Eg) of non-doped NWs agreed well with that of liquid-encapsulated Czochralski-grown semi-insulating bulk, a distinctive PL peak cause...
Shallow carrier trap levels in GaAs1-xNx (0.0010 <= x <= 0.0038) were investigated by photoluminescence (PL) and photoreflectance (PR) ranging from 4.2 to 300 K. The band gap energies of the GaAsN were clearly determined in the whole temperature range by the PR fitting analysis. It is clarified by peak decomposing that there were three emission pea...
Shallow carrier trap levels in GaAs1-xNx (0.0010≤x≤0.0038) were investigated by photoluminescence (PL) and photoreflectance (PR) ranging from 4.2 to 300 K. The band gap energies of the GaAsN were clearly determined in the whole temperature range by the PR fitting analysis. It is clarified by peak decomposing that there were three emission peaks in...
It has been reported that Zn atoms diffused from the Zn-doped p-InAlP cladding to the active layer in InGaAlP visible-light-emitting diodes cause a degradation of light output efficiency. A doping effect of the Zn atoms was then investigated using a Piezoelectric Photothermal Spectroscopy from a nonradiative transition point of view. The results in...
The effect of the secondary light illumination of hv = 0.9 eV on the photoquenched and the enhanced states in semi-insulating GaAs are investigated by using piezoelectric photoacoustic (PPA) measurements at 80 K. It is found that the secondary light causes an optical recovery from EL2* to EL2° and this is in agrezment with the result reported by us...
The piezoelectric photoacoustic (PPA) spectra for Cu-rich CuGaSe2 (CGS)/GaAs epitaxial layers were successfully observed between liquid nitrogen and room temperatures for the first time. Bandgap energy of CGS (A band) is estimated to be 1.72 eV at liquid nitrogen temperature. The activation energies of three possible intrinsic defect levels are est...
The photoluminescence measurements of alkaline metal doped zincselenide (ZnSe) single crystals were carried out to investigate how the impurity atoms substitute the intrinsic vacancy. Sample are grown by the sublimation method and the Li and Na with the concentrations of 0.1, 0.25 and 0.5 mol% were incorporated simultaneously. In the PL spectra wit...
Phosphorpus-doped ZnO nanowires were synthesized by our newly developed nanoparticle-assisted pulsed-laser deposition (NAPLD) technique under various growth conditions. This NAPLD technique, which does not require a catalyst for crystal growth, is expected to synthesize high-quality nanostructured ZnO crystals. The luminescence properties were inve...
The optical properties of ZnO nanorods and nanowires grown by our newly developed nanoparticle-assisted pulsed-laser deposition (NAPLD) technique were studied by photoluminescence (PL) spectroscopy. This NAPLD technique, which does not require a catalyst for crystal growth, is expected to synthesize high-quality nanostructured ZnO crystals. The gre...
We investigate the confinement, radiative, and nonradiative recombinations of photoexcited carriers in each subband of GaAs/AlAs multiple quantum wells (MQWs) using two nondestructive techniques, namely, the surface photovoltage (SPV) and the piezoelectric photothermal (PPT) techniques. In the temperature range from 160 to 300 K, there is no differ...
To investigate the photodecomposition mechanism of poly(di-n-hexylsilane) (PDHS), the ultraviolet (UV) light irradiation time dependence of the optical absorption (OA) and the piezoelectric photothermal (PPT) signals were measured in vacuum. A strong OA peak at 370 nm and a broad OA band at approximately 320 nm related to the all-trans and helical...
Dilute nitrides are novel III--V semiconductors that have been developed for photonic and electronic devices. Although they have been investigated, some of their characteristics have not been established yet. In this study, the static dielectric constant of dilute nitrides was experimentally estimated from the exciton binding energy. The results im...
We investigated the photogenerated carrier leakage from a Ga0.65In0.35N0.005As0.995/GaAs single quantum well (SQW) by piezoelectric photothermal (PPT) spectroscopy. The PPT measurement can detect the heat generated by a nonradiative recombination of photogenerated carriers. Hence, we could clarify the carrier confinement and leakage processes in th...
The optical properties of ZnO nanorods and nanowires grown by a newly developed nanoparticle-assisted pulsed-laser deposition were studied by using photoluminescence (PL) and a surface photovoltage (SPV) spectroscopy. SPV spectroscopy can detect an optical absorption spectrum by the illumination-induced change in the surface potential due to the dr...
By analysing exciton binding energies, we have experimentally investigated static dielectric constant of GaInNAs. Contrary to conventional semiconductors, it was found that both static dielectric constant and bandgap decrease by adding nitrogen.
The band gap and exciton binding energies of dilute nitride Ga1−yInyN0.012As0.988 films with the thickness of 100 nm were determined as a function of indium composition for investigating the effect of strain on the electronic band structure. The high sensitive piezoelectric photothermal (PPT) methodology was used for measuring the optical absorptio...
In this study, non-radiative recombination center in InGaN films with varying III/N ratio grown by rf-molecular beam epitaxy was investigated directly by piezoelectric photothermal spectroscopy (PPTS). To clarify the influence of the III/N supply ratio during the InGaN layer growth, the nitrogen flow rate was changed from 1.0 to 2.0 sccm. Since met...
The piezoelectric photo-thermal (PPT) and the surface photo-voltage (SPV) spectra of GaAs/AlAs short-period superlattices (SPS)-confined GaAs single-quantum well (SQW) were measured. The electron nonradiative transitions within GaAs-SQW and subband of SPS were observed in the room-temperature PPT spectrum. At low temperature (81K), the PPT and SPV...
Two nondestructive techniques, surface photovoltage (SPV) and piezoelectric photothermal (PPT) spectroscopies, were adopted to investigate a GaAs single quantum well (SQW) confined by GaAs/AlAs short-period superlattices (SPSs) fabricated on a semi-insulating (SI) GaAs substrate, whose absorption spectra cannot be obtained easily using conventional...
Two nondestructive techniques of surface photovoltage (SPV) and piezoelectric photothermal (PPT) spectroscopies were adopted to investigate GaAs/AlAs multiple quantum well (MQW) heterostructures fabricated on a GaAs substrate, which is difficult to obtain the absorption spectra using a conventional technique. Excitonic and two-dimensional step-like...
Room-temperature piezoelectric photothermal spectroscopy (PPTS) and surface photo-voltage spectroscopy (SPVS) were carried out on two types of Si p-n junction sample to investigate carrier recombination processes at the interface. It was found that the SPVS peak positions of each spectrum depend on the direction of illumination: a 1.18 eV peak for...