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We established a rapid, low-cost, and accurate technique to measure crystallographic orientations in multicrystalline materials by optical images and machine learning. A long short-term memory neural network was trained with pairs of light reflection patterns and the correct orientations of each grain, successfully predicting orientation with an error median of 8.61°. The model was improved by diverse data taken from various incident light angles and by data augmentation. When trained on different incident angles, the model was capable of estimating different orientations. This is related to the geometrical configuration of the incident light angles and surface facets of the crystal. The failure in certain orientations is thought to be complemented by supplementary data taken from different incident angles. Combining data from multiple incident angles, we acquired an error median of 4.35°. Data augmentation was successfully performed, reducing error by an additional 35%. This technique can provide the crystallographic orientations of a 15 × 15 cm2 sized wafer in less than 8 min, while baseline techniques such as electron backscatter diffraction and Laue scanner may take more than 10 h. The rapid and accurate measurement can accelerate data collection for full-sized ingots, helping us gain a comprehensive understanding of crystal growth. We believe that our technique will contribute to controlling crystalline structure for the fabrication of high-performance materials.

We report on the effects of grain boundary (GB) structures on the carrier recombination velocity at GB ( vGB) in multicrystalline Si (mc-Si). The fabricated artificial GBs and an originally developed machine learning model allowed an investigation of the effect of three macroscopic parameters, misorientation angle α for Σ values, asymmetric angle β, and deviation angle θ from the ingot growth direction. Totally, 13 GBs were formed by directional solidification using multi-seeds with controlled crystal orientations. vGB was evaluated directly from photoluminescence intensity profiles across GBs using a pre-trained machine learning model, which allowed a quantitative and continuous evaluation along GBs. The evaluation results indicated that the impact of θ on vGB would be relatively large among the three macroscopic parameters. In addition, the results for the Σ5 and Σ13 GBs suggested that the minimum vGB would be related to the GB energy. These results were discussed in terms of the complexity of the local reconstruction of GB structures. The deviation would make a more complex reconstructed GB structure with local distortion, resulting in an increase in the electrical activity of GBs. The obtained knowledge will contribute to improving various polycrystalline materials through a comprehensive understanding of the relationship between GB structures and their properties.

The effect of low growth rate deposition (LGD) of BaSi2 on the film quality and performance of silicon heterojunction solar cells was investigated. The total thickness of the BaSi2 layer decreased with increasing LGD duration ( tLGD). Analysis using Raman spectroscopy indicated that an amorphous Si (a-Si) phase existed on the surface of the BaSi2 layer. The a-Si on the surface was converted into BaSi2 by post-annealing owing to the diffusion of Ba and Si atoms. X-ray diffraction analysis revealed that LGD improved the rate of a-axis orientation and crystallinity. Post-annealing was also observed to have significantly improved these structural properties. Furthermore, the solar cell performance was observed to be strongly dependent on tLGD, and the highest conversion efficiency of 10.62% was achieved by the p-BaSi2/n-c-Si heterojunction solar cells at a tLGD of 6 min. The improved structure and solar cell properties are attributed to improved atom rearrangement during LGD.

We present a machine learning model to directly predict the carrier recombination velocity, vGB, at the grain boundary (GB) from the measured photoluminescence (PL) intensity profile by training it with numerical simulation results. As the training dataset, 1800 PL profiles were calculated with a combination of random values of four material properties—vGB, the GB inclination angle, and the carrier diffusion lengths in the grains on both sides of the GB. In addition, the measured noise was modeled artificially and applied to the simulated profiles. A neural network was constructed with the inputs of the PL profile and the outputs of the four properties. This served as the solver of the reverse problem of the computational simulation. The coefficient of determination and the root mean squared error of vlog, which is the common logarithm of vGB, for the test dataset were 0.97 and 0.245, respectively. This prediction error was sufficiently low for the practical estimation of vGB. Moreover, the calculation time was reduced by a factor of 198 000 compared to conventional numerical optimization of repeating the computational simulations. By utilizing this fast prediction method, continuous evaluation of vGB along a GB was demonstrated. The finding is expected to advance scientific investigation of the electrical properties of local defects.

Aluminum (Al) screen-printed narrow fingers on textured passivated emitter and rear totally diffused (n-PERT) front junction Si solar cells are applied and investigated. Commercial Al paste is screen-printed on symmetric test structures with different Local Contact Opening (LCO) designs by varying the dot-to-dot distances and finger widths. Strong dependency of the metal-silicon saturation current density (J0-met) is found for the considered geometries. Narrow Al grid metallization induced emitter recombination current density can be decreased by optimizing the dot-dot distance. Best results of our test matrix were determined for an 80 µm dot-dot pitch, realizing J0-met values down to 649 fA/cm2 when using narrow fingers of 65 µm compared with the J0-met of 1100 fA/cm2 identified for a commercial Ag/Al paste. For dot-to-dot distances below 40 µm, common pattern of bifacial p-type PERC, fingers with widths exceeding 120 µm are needed to realize J0-met values below 400 fA/cm2. Additionally, the shape and size of the Al-Si alloy and p++ doping layer depth after firing are strongly affected by the combination of dot-to-dot distances and screen-printed finger widths.

Semiconducting barium disilicide (BaSi2) is an emerging material for solar cell applications, and therefore, defect passivation is critical for improving its solar cell performance. Herein, the effect of atomic hydrogen (H) on the photoresponsivity of 500 nm-thick boron (B)-doped p-BaSi2 films was examined. The photoresponsivity reached ∼4 A/W (about twice the highest reported value for H-passivated undoped BaSi2 films) in B-doped p-BaSi2 films exposed to an atomic H supply for 5 − 10 min because of an increased minority-carrier lifetime, as measured by the microwave-detected photoconductivity decay. Furthermore, a ≥15 min atomic H supply was found to degrade photoresponsivity. Ab initio studies were used to interpret and understand experimental observations by analyzing states in the gap region, which can act as traps, in B-doped p-BaSi2 with H incorporation. The effect that atomic H had on the performance of B-doped p-BaSi2/n-Si heterojunction solar cells was also studied. The saturation current density was found to decrease by three orders of magnitude with the atomic H supply, and the conversion efficiency was increased up to 6.2%. Deep-level transient spectroscopy revealed a reduction of defect densities induced by the atomic H supply. Both experimental and theoretical viewpoints show that an atomic H supply is beneficial for BaSi2 solar cells.

We report on the effect of sputtering deposition of indium tin oxide (ITO) as the transparent conductive oxide layer on the passivation performance of hydrogenated amorphous silicon/crystalline silicon heterojunctions. The influence of sputtering damage on passivation performance is studied by varying the ITO layer thickness from 0 nm to 80 nm. The passivation performance decreases considerably up to 10 nm and increases gradually from 20 nm to 80 nm, indicating that damage and recovery stages are present during the sputtering process. We focus on the injection energy as the cause of the recovery phenomenon. To optimize the passivation performance by intentionally enhancing the effect of the recovery stage while minimizing the initial damage at the heterointerface, we develop a two-step sputtering process in which the radiofrequency power is changed from 50 W to 100 W during deposition to prepare ITO double layers. Two step sputtering gives the lower damage, and the properties of ITO double layers are better than those of ITO single layers. These results show that two-step sputtering can realize greater a-Si:H passivation. Furthermore, better optical properties are obtained in ITO double layers compared with conventional ITO single layers. Therefore, modulating the radiofrequency power during ITO deposition can offer higher conversion efficiency.

Barium di-silicide (BaSi2) is a very promising absorber material for high-efficiency thin-film solar cells, due to its suitable bandgap, high light absorption coefficient, and long minority-carrier lifetime. In this study, we compare the nanostructure, layer composition, and point defects of BaSi2 thin films deposited by Radio Frequency (RF) sputtering, Thermal Evaporation (TE), and Molecular Beam Epitaxy (MBE), using Doppler Broadening Positron Annihilation Spectroscopy (DB-PAS) depth profiling, Raman spectroscopy, and x-ray diffraction. Our DB-PAS study on thermally annealed RF-sputter deposited and on TE-deposited BaSi2 layers, in a comparison with high quality BaSi2 films produced by MBE, points to the presence of vacancy-oxygen complexes and Si or Ba mono-vacancies, respectively, in the (poly)crystalline BaSi2 films. The degree of near-surface oxidation increases, going from MBE and TE to the industrially applicable RF-sputtered deposition synthesis. The use of a-Si capping layers on the thermally annealed RF-sputtered BaSi2 films leads to a clear reduction in sub-surface oxidation and improves the quality of the BaSi2 films, as judged from DB-PAS.

We present device performance of solar cells based on multicrystalline SiGe (mc-SiGe) bulk crystal with microscopic compositional distribution grown by the casting method. The average Ge composition was systematically changed in the range between 0% and 10%. A small addition of Ge to multicrystalline Si (mc-Si) was found to be very effective to increase the short-circuit current density without affecting the open-circuit voltage. As a consequence, the overall efficiency of a solar cell based on mc-SiGe was improved compared with that based on mc-Si. This result demonstrates that mc-SiGe is a promising candidate to replace mc-Si since it could achieve higher conversion efficiency without drastic increase of the production cost.

Networks consisting of one-dimensional ZnO nanowires and two-dimensional ZnO nanowalls were synthesized using a catalyst-free low-temperature approach. The size of the nanostructure was much smaller than that obtained by the previous catalyst-assisted method. The nanostructures exhibited stable excitonic states at room temperature, and emission due to exciton-exciton scattering was observed.

We report on a theoretical investigation of the elastic strain in an ellipsoidal SiGe inclusion coherently embedded in Si and its influence on the band structure of SiGe. The strain was calculated as a function of the Ge fraction in SiGe and the aspect ratio of the ellipsoid, and utilized to derive the shift of the band edge. When the principal axis of the ellipsoid was chosen to be parallel to [001], the band structure of SiGe was predicted to be Si like regardless of the aspect ratio. The band gap of strained SiGe was also calculated, and the deviation of the aspect ratio from unity was found to be effective to decrease the band gap due to the breaking of the crystal symmetry. These results suggest the importance of controlling strain, shape, and local Ge fraction in multicrystalline SiGe, which we propose as a promising material for solar cell applications.

We report on growth and characterizations of Si/multicrystalline-SiGe (mc-SiGe) heterostructure as a promising candidate to surpass mc-Si solar cells. Spatial distribution of the status of strain in Si was investigated using microscopic Raman spectroscopy. The strain was found to be strongly influenced by the composition and microstructure of underlying mc-SiGe. Spatial variation of the strain as well as strain relaxation was found to be suppressed by decreasing average Ge composition of underlying SiGe.

A detailed experimental study of the optical properties of separation by implanted oxygen (SIMOX) substrates is performed using photoluminescence techniques. The photoluminescence properties of SIMOX substrates are compared with those of Si substrates. The silicon-on-insulator (SOI) layer is found to be free of any strain. High-quality SiGe quantum wells are grown on SOI using gas source molecular beam epitaxy. It is found that the photoluminescence properties of these quantum wells on SOI differ significantly from those of the SiGe quantum wells grown on a Si substrate under identical growth conditions. Intense photoluminescence and higher operation temperature are obtained for quantum wells grown on SOI substrates. It is shown that growth of a few hundred A Si buffer layer on SOI is essential in order to achieve a high quality SiGe epitaxy on SOI. Finally, a SiO2/Si/SiO2 optical cavity is proposed on SIMOX substrates, and it is employed to further enhance the photoluminescence intensity of SiGe quantum ...

We fabricated high-quality strain-relaxed thin SiGe layers by Ar ion implantation into Si substrates before epitaxial growth. The surface of 100-nm-thick Si0.8Ge0.2 layers, the relaxation ratio of which was more than 80%, was found to be very smooth, with a rms roughness of 0.34 nm. Cross-sectional transmission electron microscopy analysis confirmed that strain-relieving dislocations were effectively generated due to the ion-implantation-induced defects and confined in the vicinity of the heterointerface, resulting in a dislocation-free SiGe surface. Moreover, in-plane strain-field fluctuation was found to be largely reduced by this ion implantation method.

We investigated the microscopic strain fluctuation in strained-Si grown on SiGe-on-insulator (SGOI) and SiGe virtual substrates, and clarified the origins of the strain fluctuation in the strained-Si film. A periodic strain fluctuation, which reflects a cross-hatch pattern of the substrate, was observed in the sample on the virtual substrate. On the other hand, a featureless strain fluctuation with suppressed amplitude was observed in the sample on SGOI substrate. By analyzing the correlation of the Raman peak positions of the Si–Si modes in strained-Si and SiGe, the dominant mechanism of the strain fluctuation in the strained Si film was found to be the compositional fluctuation in underlying SiGe for the sample on SGOI, and the strain fluctuation reflecting the cross-hatch pattern for the sample on the virtual substrate, respectively.

ZnO tubes were epitaxially grown on sapphire (0001) substrates by metalorganic chemical vapor deposition. The tubes grew along the substrate normal and were characterized by hexagon-shaped cross sections. All of the tubes possessed the same epitaxial relationships with respect to the substrate. Both reactor pressure and growth temperature were found to play an important role in the formation of ZnO tubes. Spiral column growth mode was found to be responsible for the formation of ZnO tubes.

In order to control the electrical properties of an evaporated BaSi2 film, which is an emerging candidate for the absorber-layer material of earth-abundant thin-film solar cells, we have investigated the effects of deposition rate on the produced phases, microstructure, and carrier density of the thin films grown by thermal evaporation of BaSi2. X-ray diffraction results show that a high substrate temperature is necessary for BaSi2 formation at a high deposition rate, which is discussed from viewpoints of vapor composition and diffusion time. Microstructural characteristics such as grain size of 30–120 nm, oxide particle arrays present around the interface, and partial oxidation at a low substrate temperature are revealed by cross-sectional transmission electron microscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy combined with an energy-dispersive X-ray spectroscopy. With increasing deposition rate, the crystalline quality of BaSi2 is found to improve, as evidenced by a decrease in full-width at half maximum of a [Si4]4− vibration band in Raman spectra. At the same time, electron density, which is determined by Hall measurement, decreases with deposition rate. The variation of electron density is discussed on the basis of microstructural characteristics and BaSi2 formation mechanism. The most probable reason is concluded to be composition deviation from stoichiometry.

In-plane strain fluctuation in the strained-Si/relaxed-SiGe heterostructure was studied by micro-Raman spectroscopy. It was found that misfit dislocation, which is necessarily induced by strain relaxation of SiGe buffer layers, caused micrometer-scale inhomogeneous strain field in the strained-Si layer as well as SiGe buffer, which may degrade device performance. After annealing, the fluctuation was found to be enhanced due to partial strain relaxation of strained Si, particularly in the region where tensile strain was relatively high before annealing. From homoepitaxial growth of SiGe on planarized SiGe buffer layers, it was confirmed that the growth rate also fluctuated laterally, in correspondence with the in-plane strain variation.

High-quality ZnO rods were formed directly on sapphire (0001) substrates by metalorganic chemical vapor deposition. The rods exhibited free exciton and very sharp bound exciton emissions at low temperatures. By increasing the excitation intensity, biexciton emission was observed. Temperature dependence of the emission spectra suggested that the emission peak at ∼3.315 eV, which had been attributed to neutral acceptor-bound exciton emission, is due to donor-acceptor pairs. The acceptor binding energy was determined to be about 107 meV, which agrees well with that estimated from a hydrogen-atom-like acceptor model.

We have studied the local structure of GeSi nanocrystals embedded in SiO2 prepared by co-sputtering of Ge, Si, and SiO2 targets onto a Si(100) substrate. From Raman scattering, we conclude that the formation of the isotropic crystalline Ge phase starts at about 800 °C followed by the formation of a GeSi phase at higher temperatures. The formed nanocrystals, whose size depends on the annealing temperature, are randomly oriented. The local structure of the nanocrystals has been studied by x-ray absorption fine structure spectroscopy. They are found to consist of a relaxed Ge core with a typical diameter of ∼4 nm and the Ge–Ge bond length of 2.45 A and of a GeSi outer shell, the Ge–Si bond length being 2.39 A. The average composition of the grown nanocrystals is estimated to be Ge0.75Si0.25.

Strain-balanced Si/SiGe multiple quantum wells (MQWs), which are designed to overcome the limitation of the number of wells coming from the strain accumulation, were fabricated, and their optical properties were investigated. X-ray diffraction spectra and cross-sectional transmission-electron-microscope images showed a high-crystalline quality of samples and excellent uniformity of the well width. Well-resolved no-phonon and TO-phonon-assisted transitions from strain-balanced MQWs were observed by low-temperature photoluminescence spectroscopy, and both their temperature and excitation power dependence showed blueshifts due to the delocalization of excitons, the band bending, and/or the band-filling effect.

The metastable zincblende (ZB) phase in N-polar (0001¯) (−c-plane) InGaN/GaN multiple quantum wells (MQWs) grown by metalorganic vapor phase epitaxy is elucidated by the electron backscatter diffraction measurements. From the comparison between the −c-plane and Ga-polar (0001) (+c-plane), the −c-plane MQWs were found to be suffered from the severe ZB-phase inclusion, while ZB-inclusion is negligible in the +c-plane MQWs grown under the same growth conditions. The ZB-phase inclusion is a hurdle for fabricating the −c-plane light-emitting diodes because the islands with a triangular shape appeared on a surface in the ZB-phase domains. To improve the purity of stable wurtzite (WZ)-phase, the optimum conditions were investigated. The ZB-phase is dramatically eliminated with decreasing the V/III ratio and increasing the growth temperature. To obtain much-higher-quality MQWs, the thinner InGaN wells and the hydrogen introduction during GaN barriers growth were tried. Consequently, MQWs with almost pure WZ phase...

Heterojunction solar cells that consist of 15 nm thick molybdenum trioxide (MoOx, x

We suggested the possibility to suppress dislocation generation by controlling the microstructure of dendrite crystals in practical size Si wafers grown by the floating cast method. With the floating cast method, the contact angle between adjacent dendrite crystals can be used as a structural parameter to define grain boundaries (GBs). We fabricated a practical size silicon ingot fully covered with dendrite crystals and investigated dislocation density near the GBs as a function of the contact angle. The dislocation density was found to decrease with decreasing contact angle. This result can be explained by differences in shear stress on {111} slip surface around the GBs, as supported by numerical calculations considering various structural parameters in multicrystalline Si. These results confirm our previous results with laboratory-scale ingots, and we believe this concept can be applied to commercial growth processes.

Electron cyclotron resonance plasma oxidation of strained Si on relaxed Si1−xGex buffer layers in O2 ambient at room temperature is reported. The electrical properties of grown oxide have been characterized and compared with thermally grown oxides using a metal-oxide semiconductor structure. At a low field, the accumulation of holes in the buried Si1−xGex layer, due to the type-II band offset, has been observed. The experimental results from thermally grown oxides have been compared with the simulation results obtained using a heterostructure Poisson solver.

An ultrashort lifetime of photocarriers as short as 600 fs has been obtained in ion‐implanted Ge thin films grown on sapphire substrates. The photocarrier mobility determined by photoconductivity measurements is found to be reasonably high (∼100 cm2/V s). We have observed terahertz (THz) radiation from a photoconductive dipole antenna geometry.

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.

The origin of the no‐phonon (NP) luminescence enhancement was explored in Si‐based neighboring confinement structures (NSCs). Post growth annealing was found to lead to quenching of the NP line. Time‐resolved photoluminescence study clarified that the radiative lifetime of the annealed sample increases with increasing temperature, while that of the as‐grown sample shows no significant change at low temperatures (

Spontaneous formation of step‐edge Ge quantum wires and lateral quantum confinement effect are clearly observed in Si submonolayer‐Ge/Si heterostructures grown on Si(100). By depositing submonolayer equivalent Ge (Q), the Ge atoms are found to line up in a wire‐like fashion at the 〈011〉 step edges as revealed by plan‐view transmission electron microscopy. Photoluminescence (PL) peak energy shift with Q due to lateral quantum confinement of holes is clearly observed, and PL features characteristic of the quasi‐one‐dimensional system are also addressed.

SiGe quantum wire (QWR) structures grown by gas source molecular beam epitaxy have been characterized by cathodoluminescence (CL) imaging and spectroscopy. At T≊5 K, the CL spectra obtained with a focused beam contain SiGe luminescence features associated with electron‐hole plasma recombination. When the electron beam is defocused three separate SiGe no‐phonon luminescence features and their transverse optic phonon replicas are observed. These features are related to recombination in the SiGe (100) quantum wells (QWs), SiGe (111) QWs, and the SiGe QWRs. The high generation rates associated with the focused electron beam quenches the luminescence and results in electron‐hole plasma recombination in the adjacent SiGe (100) QWs, and saturation of the radiative processes. Defocusing the electron beam reduces the injection level and nonradiative processes, enabling the SiGe bound exciton features to be observed. Monochromatic CL imaging of the SiGe (100) QWs show that at low temperatures the CL image is broade...

We present a photoluminescence (PL) study on the growth mode changeover during growth of Ge on Si(100) substrates. Intense PL signals originating from both the flat Ge layer and the three‐dimensional (3D) Ge islands are observed from Si/Ge/Si quantum wells with various Ge coverage. The onset of the 3D island formation is determined to be 3.7 monolayers (ML). It is also found that the 3D islands grow with only 3.0 ML of the flat Ge layer retained. This implies that only the 3.0 ML Ge is thermodynamically stable on Si(100) and hence corresponds to the ‘‘equilibrium’’ critical thickness.

Compositional abruptness of strained Si/Ge heterointerfaces grown by solid source Si molecular beam epitaxy under supply of atomic hydrogen (AH) was investigated using secondary ion mass spectrometry and reflection high‐energy electron diffraction. Systematic variation of growth temperature and AH exposure pressure revealed that Ge segregation length is a steadily decreasing function of AH coverage on the growth surface.

A study of the photoluminescence properties of Si/SiGe/Si quantum wells grown on separation by oxygen implantation (SIMOX) substrate by gas source molecular beam epitaxy is presented. Intense photoluminescence and carrier confinement in the quantum well are demonstrated. It is found that buried SiO2 isolates the top silicon on insulator from the back Si of SIMOX, and alters the photoluminescence properties of SIMOX compared to those of bulk Si. The SOI layer is found to be free of any strain. A SiO2/Si/SiO2 optical cavity is proposed by depositing SiO2 on SIMOX. A substantial enhancement of the photoluminescence intensity of SiGe quantum well is found, which is attributed to the optical confinement of incident beam in the cavity.

We report on the successful fabrication of SiGe quantum wire structures on a V‐groove patterned substrate by gas‐source selective epitaxial growth technique, and their optical properties. Optical anisotropy, showing the realization of luminescent SiGe layers with wire geometry, was clearly observed in electroluminescence from the SiGe layers grown inside the groove.

SiGe/Si quantum wire structures were successfully fabricated on a V‐groove patterned Si substrate by using gas‐source Si molecular beam epitaxy (GS‐SiMBE). A cross sectional image of transmission electron microscope clarified a crescent‐shaped SiGe layer at the bottom of the V‐groove owing to anisotropy of the growth rate on the different crystal orientations in GS‐SiMBE.

An n‐type Si contact layer for an electroluminescent (EL) diode was successfully grown on a Si/Si1−xGex/Si single‐quantum‐well (SQW) structure by ‘‘hybrid’’ Si molecular beam epitaxy (MBE) for the first time. The hybrid MBE was performed by regrowing the Si contact layer in a solid‐source MBE chamber after transferring the SQW sample through air from a gas‐source (GS) MBE chamber, in which the starting SQW structure was grown. A (2×1) reconstruction was observed on a GSMBE‐prepared Si(100) surface even after the SQW sample was exposed to air for up to 15 h. An excellent quality of the EL device was evidenced by the sharpest emission lines ever reported in the EL spectra of SiGe system. The spectral features of the EL and photoluminescence were found to be almost identical, and a well‐resolved acoustic phonon replica was observed.

Optical investigation of interdiffusion at Si1−xGex/Si heterointerfaces has been performed for the first time in strained Si1−xGex/Si single quantum wells (SQWs). Photoluminescence (PL) peak energy blue shift of up to 22 meV due to interdiffusion‐induced potential profile modulation was observed after annealing in vacuum. The diffusion coefficients obtained were found to closely follow an Arrhenius behavior with an activation energy of 2.47±0.4 eV. Dramatic increase in the integrated PL intensity was observed in the annealed samples, as a result of the elimination of effective nonradiative centers. Strain relaxation was hardly observed even after 900 °C annealing, indicating the unprecedented structural stability of SQWs in contrast to rather vulnerable thick alloy layers. Anomalous peak red shift, probably due to surface oxidation, was observed by annealing in N2 ambient.

Electroluminescence (EL) was observed at temperatures up to 60 °C in p‐type strained Si0.65Ge0.35/Si multiple‐quantum‐well (MQW) diode structures grown on Si(111) substrates by Si molecular‐beam epitaxy. No‐phonon line and its transverse optical phonon replica were well‐resolved in the room‐temperature EL spectrum for the first time. EL spectrum was dominated by a broad alloy band located below the band‐edge state by ≊100 meV at lower temperatures, which was taken over by clear band‐edge emissions at elevated temperatures. The emission intensity of the alloy band exhibited a saturation behavior with increasing injection current while the band‐edge emission was found to develop with a power exponent of 1.4.

Luminescent Si0.8Ge0.2/Si single quantum well structures with abrupt interfaces were successfully grown by segregant assisted growth (SAG) using Sb adlayer. The emission energy shifted to the lower side compared to the sample grown without Sb adlayer due to the suppression of the surface segregation of Ge atoms. Improved abruptness of interfaces was confirmed by sputter depth profile measurement, where no segregation edge was seen in the sample grown by SAG technique. Light emitting p‐i‐n diode structure can be fabricated by utilizing the incorporation of Sb during SAG.

Potential variations across the grain boundaries (GBs) in a 100 nm thick undoped n-BaSi2 film on a cast-grown multicrystalline Si (mc-Si) substrate are evaluated using Kelvin probe force microscopy (KFM). The θ-2θ X-ray diffraction pattern reveals diffraction peaks, such as (201), (301), (410), and (411) of BaSi2. Local-area electron backscatter diffraction reveals that the a-axis of BaSi2 is tilted slightly from the surface normal, depending on the local crystal plane of the mc-Si. KFM measurements show that the potentials are not significantly disordered in the grown BaSi2, even around the GBs of mc-Si. The potentials are higher at GBs of BaSi2 around Si GBs that are formed by grains with a Si(111) face and those with faces that deviate slightly from Si(111). Thus, downward band bending occurs at these BaSi2 GBs. Minority carriers (holes) undergo a repelling force near the GBs, which may suppress recombination as in the case of undoped n-BaSi2 epitaxial films on a single crystal Si(111) substrate. The b...

Potential variations around the grain boundaries (GBs) in antimony (Sb)-doped n-type and boron (B)-doped p-type BaSi2 epitaxial films on Si(111) were evaluated by Kelvin probe force microscopy. Sb-doped n-BaSi2 films exhibited positively charged GBs with a downward band bending at the GBs. The average barrier height for holes was approximately 10 meV for an electron concentration n ≈ 1017 cm−3. This downward band bending changed to upward band bending when n was increased to n = 1.8 × 1018 cm−3. In the B-doped p-BaSi2 films, the upward band bending was observed for a hole concentration p ≈ 1018 cm−3. The average barrier height for electrons decreased from approximately 25 to 15 meV when p was increased from p = 2.7 × 1018 to p = 4.0 × 1018 cm−3. These results are explained under the assumption that the position of the Fermi level Ef at GBs depends on the degree of occupancy of defect states at the GBs, while Ef approached the bottom of the conduction band or the top of the valence band in the BaSi2 grain ...

Precipitation free boron (B)-doped as-grown p+-BaSi2 layer is essential for the BaSi2 p-n junction solar cells. In this article, B-doped p-BaSi2 layers were grown by molecular beam epitaxy on Si(111) substrates, and the influence of substrate growth temperature (TS) and B temperature (TB) in the Knudsen cell crucible were investigated on the formation of B precipitates and the activation efficiency. The hole concentration, p, reached 1.0 × 1019 cm−3 at room temperature for TS = 600 and TB = 1550 °C. However, the activation rate of B was only 0.1%. Furthermore, the B precipitates were observed by transmission electron microscopy (TEM). When the TS was raised to 650 °C and the TB was decreased to 1350 °C, the p reached 6.8 × 1019 cm−3, and the activation rate increased to more than 20%. No precipitation of B was also confirmed by TEM.

Current status and future prospects towards BaSi2 pn junction solar cells are presented. As a preliminary step toward the formation of BaSi2 homojunction diodes, diodes with a Cr/n-BaSi2 Schottky junction and an n-BaSi2/p-Si hetero-junction have been fabricated to investigate the electrical properties of the n-BaSi2. Clear rectifying properties were observed in the current density versus voltage characteristics in both diodes. From the capacitance-voltage measurements, the build-in potential, VD, was 0.53 V in the Cr/n-BaSi2 Schottky junction diode, and the Schottky barrier height was 0.73 eV calculated from the thermoionic emission theory; the VD was about 1.5 V in the n-BaSi2/p-Si hetero-junction diode, which was consistent with the difference in the Fermi level between the n-BaSi2 and the p-Si.

We have fabricated approximately 0.5-μm-thick undoped n-BaSi2 epitaxial films with various average grain areas ranging from 2.6 to 23.3 μm2 on Si(111) by molecular beam epitaxy, and investigated their minority-carrier lifetime properties by the microwave-detected photoconductivity decay method at room temperature. The measured excess-carrier decay curves were divided into three parts in terms of decay rate. We characterized the BaSi2 films using the decay time of the second decay mode, τSRH, caused by Shockley-Read-Hall recombination without the carrier trapping effect, as a measure of the minority-carrier properties in the BaSi2 films. The measured τSRH was grouped into two, independently of the average grain area of BaSi2. BaSi2 films with cloudy surfaces or capped intentionally with a 3 nm Ba or Si layer, showed large τSRH (ca. 8 μs), whereas those with mirror surfaces much smaller τSRH (ca. 0.4 μs). X-ray photoelectron spectroscopy measurements were performed to discuss the surface region of the BaSi2...

By controlling the Si thickness and the annealing temperature used for Al-induced crystallization, we controlled the fraction of (100) and (111) orientations of polycrystalline Si (poly-Si) grains grown on glass. Changing the proportions of crystal orientation strongly influenced the average grain size of the poly-Si layer. By growing a 99% (111)-oriented poly-Si layer, formed with a 50-nm-thick Si layer at 375 °C, we produced a Si layer with grains nearly 40 μm in size. We discuss the growth mechanism from the perspective of competition between (100)- and (111)-oriented nuclei. This achievement holds promise for fabricating high-efficiency thin-film solar cells on inexpensive glass substrates.

The Al-induced crystallization (AIC) yields a large-grained (111)-oriented Ge thin film on an insulator at temperatures as low as 180 °C. We accelerated the AIC of an amorphous Ge layer (50-nm thickness) by initially doping Ge in Al and by facilitating Ge diffusion into Al. The electron backscatter diffraction measurement demonstrated the simultaneous achievement of large grains over 10 μm and a high (111) orientation fraction of 90% in the polycrystalline Ge layer formed at 180 °C. This result opens up the possibility for developing Ge-based electronic and optical devices fabricated on inexpensive flexible substrates.

Well‐resolved band edge luminescence with no deep level emissions is reported for Si0.8Ge0.2/Si single quantum well (SQW) structures grown at high substrate temperatures (Ts≊620 °C) by solid source Si molecular beam epitaxy (MBE). No‐phonon (NP) transitions due to symmetry‐breaking alloy disordering in SiGe layers and transverse optical (TO) phonon replicas were clearly identified. With decreasing well width, NP and TO emissions were found to show systematic blue shift due to quantum confinement effect. Excellent crystal quality was evidenced by total absence of defect‐related deep level emissions characteristic of photoluminescence spectra of samples grown at lower temperatures, Ts

We have derived the formula to calculate the open-circuit voltage in a pn-junction solar cell from carrier densities by considering the driving force of charge separation without using the equation for current. The excess amount of chemical potential of charge carriers is proposed as the origin of the driving force of charge separation, and the voltage formula is derived from the gradient of excess chemical potential. The calculated voltage is shown to agree with the result of a rigorous device simulation for symmetrical pn-homojunction devices with band gaps of 0.6–1.8 eV and majority-carrier densities of 1015–1019 cm-3. The developed formula is, therefore, valid for the pn-homojunction devices, indicating that the driving force of charge separation stems from the excess chemical potential.

We investigated the effects of different crystal defects and their interactions with impurities on the electrical properties of multicrystalline Si (mc-Si) using samples with unique defect patterns and impurities. By using the floating cast method, a single grain boundary (GB), identified as a Σ27 boundary, was first formed with a high density of impurities from atmosphere, leading to an inefficient external gettering of impurities during phosphorus (P) diffusion. During crystal growth, the Σ27 GB splits into the Σ3 and Σ9 GBs with accompanying generation of dislocations and reduction in the density of impurities. The external gettering of impurities became efficient for removing impurities as evidenced by an increase in average minority carrier lifetime. At the final stage of crystal growth, the decrease in minority carrier lifetime was significant, which could not be improved by phosphorus diffusion because of the high densities of segregated impurities and crystal defects originating from the strong contact with the crucible. The increase in Σ number was found to result in more enhanced precipitation of impurities, which led to the poor gettering effect of P diffusion. These results further confirmed the importance of the reduction in the densities of impurities and dislocations for the quality and yield improvement of mc-Si ingots for solar cells.

B-doped p-BaSi2 layer growth by molecular beam epitaxy and the influence of rapid thermal annealing (RTA) on hole concentrations were presented. The hole concentration was controlled in the range between 1017 and 1020 cm−3 at room temperature by changing the temperature of the B Knudsen cell crucible. The acceptor level of the B atoms was estimated to be approximately 23 meV. High hole concentrations exceeding 1 × 1020 cm−3 were achieved via dopant activation using RTA at 800 °C in Ar. The activation efficiency was increased up to 10%.