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The impact of hydrogen desorption on the electrical properties of TiOx on crystalline silicon (c-Si) with SiOy interlayers is studied for the development of high-performance TiOx carrier-selective contacts. Compared with the TiOx/c-Si heterocontacts, a lower surface recombination velocity of 9.6 cm/s and lower contact resistivity of 7.1 mΩ cm2 are obtained by using SiOy interlayers formed by mixture (often called SC2). The hydrogen desorption peaks arising from silicon dihydride (α1) and silicon monohydride (α2) on the c-Si surface of the as-deposited samples are observed. The α1 peak pressure of as-deposited heterocontacts with SiOx interlayers is lower than that of heterocontacts without a SiOy interlayer. Furthermore, the hydrogen desorption energies are found to be 1.76 and 2.13 eV for the TiOx/c-Si and TiOx/SC2-SiOy/c-Si heterocontacts, respectively. Therefore, the excellent passivation of the TiOx/SC2-SiOy/c-Si heterocontacts is ascribed to the relatively high rupture energy of bonding between Si and H atoms.

We report on our attempt to perform the three-dimensional (3D) visualization of dislocation clusters in multicrystalline silicon (mc-Si) ingot by processing photoluminescence (PL) images and analysis of dislocation clusters in mc-Si. As-sliced wafers prepared using a high-performance (HP) mc-Si ingot were sequentially measured by PL imaging with intentional superposition of reflection. Then, various image processing techniques were applied to all the PL images to extract dark regions, which most likely correspond to dislocation clusters, as well as microstructures. By 3D reconstruction using a large quantity of 2D images, we could successfully visualize the generation, propagation and annihilation of dislocation clusters in HP mc-Si ingot. In addition, relationship between source region of dislocation clusters and crystal orientation were investigated by combining data scientific and experimental approaches. As a result, it was suggested that small angle grain boundaries with angular deviation of less than 10 degrees cause the generation of dislocation clusters.

We apply NAnocrystalline Transport path in Ultra-thin dielectrics for Reinforced passivating contact (NATURE contact) to crystalline silicon solar cells for simultaneous achievement of passivation and carrier extraction. The Si nanocrystals are formed in silicon oxide by deposition of hydrogenated amorphous silicon oxide with different oxygen concentration and subsequent annealing. NATURE contact with total layer thickness of 8 nm exhibits reasonably good passivation performance. Furthermore, we demonstrate that NATURE contact can be successfully implemented to the both sides of crystalline silicon solar cells. NATURE contact, which permits flexible structural and chemical design of a passivation contact layer, will open a possibility for various functional devices.

Dislocation clusters that would degrade the electric property can be generated from a grain boundary (GB) neighboring a triple junction of GBs. The atomic plane of the GB is bent via the movement of the triple junction, supposedly due to Σ3{111} micro-twins intersecting the GB, and a number of dislocations would be generated nearby the bending corner. Bundles of dislocation arrays expanding nearly parallel to the growth direction and honeycombed dislocation networks lying on a {111} plane nearly normal to the growth direction can coexist, suggesting that multiple slip systems would be operated when the dislocations are tangled.

We investigate an influence of thermal treatment on the properties of tin-doped indium oxide (ITO) and In 2 O 3 on glass and hydrogenated amorphous silicon (a-Si:H)/glass. Work function (WF), electron density and mobility of the In 2 O 3 are dependent on a-Si:H underlayer possibly due to H diffusion from a-Si:H underlayer by annealing. In addition, WF of the In 2 O 3 is higher than that of the ITO regardless of presence of a-Si:H underlayer. Furthermore, the ITO/In 2 O 3 double layers are fabricated for silicon heterojunction (SHJ) solar cells. The electrical and optical properties of the ITO/In 2 O 3 stacks are adequate for transparent conductive oxide (TCO) layer.

We apply silicon nanocrystals as a carrier transport route in silicon oxide for polycrystalline silicon on oxide (POLO) junctions. The Si nanocrystals are formed in silicon oxide by deposition of hydrogenated amorphous silicon oxide with different oxygen concentration and subsequent annealing. From cross-sectional transmission electron microscopy (TEM) images, the Si nanocrystals are observed for the sample at annealing temperature of 750 °C. The highest average effective lifetime of about $630 \ \mu\mathrm{s}$ and contact resistivity of $25.5\ \mathrm{m}\Omega\cdot \text{cm}$ are obtained before hydrogenation process.

Silicon quantum dot (Si-QD) embedded in amorphous silicon oxide is used for p-i-n solar cell on quartz substrate as a photogeneration layer. To suppress diffusion of phosphorus from an n-type layer to a Si-QD photogeneration layer, niobium-doped titanium oxide (TiOx:Nb) is adopted. Hydrofluoric acid treatment is carried out for a part of the samples to remove the thermal oxide layer in the interface of TiOx:Nb/n-type layer. The thermal oxide acts as a photo-generated carrier-blocking layer. Solar cell properties using 10-nm-thick TiOx:Nb without the thermal oxide are better than those with the thermal oxide, notably short circuit current density is improved up to 1.89 mA/cm2. The photo-generated carrier occurs in Si-QD with quantum confinement effect. The 10-nm-thick TiOx:Nb with the thermal oxide layer effectively blocks P; however, P-diffusion is not completely suppressed by the 10-nm-thick TiOx:Nb without the thermal oxide. These results indicate that the total thickness of TiOx:Nb and thermal oxide layer influence the P-blocking effect. To achieve the further improvement of Si-QD solar cell, over 10-nm-thick TiOx:Nb is needed.

We propose a method of three-dimensional (3D) visualization of growth interfaces in crystalline ingots prepared by directional solidification and apply the method to cast silicon ingot for solar cells. The method consists of dispersing inclusions along with the growth interface by insertion of a quartz rod followed by measuring inclusions distribution by an infrared brick inspection system. Then, inclusions distribution can be visualized in 3D, which allows extracting growth interfaces as planes. By fitting the plane with a simple polynomial expression, its macroscopic shape and local gradient can be estimated. The method yields us the local changes in growth interfaces and thus contributes to the development of the crystal growth method for high-quality ingots.

Analysis of nucleation sites in multicrystalline silicon (mc-Si) grown by directional solidification is required for further grain refinement to reduce dislocation density. In this study, Voronoi diagrams were utilized to analyze nucleation sites of mc-Si grown by single-layer Si beads (SLSB)-seeding method. The grain distribution at the bottom of the ingot was almost reproduced by the weighted Voronoi diagram with a relaxation method to optimize the positions of generating points of the diagram, which correspond to the nucleation sites, and the difference of nucleation timing of each crystal grain. Comparison of the generating points with the optical image indicated that the nucleation started at the remarkably deep portions of the nucleation layer. Further grain refinement by SLSB-seeding method could be achieved by suppressing the formation of the remarkably deep portions of the nucleation layer.

The fabrication of orthorhombic barium disilicide (BaSi2) thin-films on modified germanium (Ge) substrates by thermal evaporation method was demonstrated, in which the surface modification of Ge substrate was performed by a simple chemical etching method. The effects of etching time on crystalline quality and optical properties of the BaSi2 films were investigated. The results revealed that the substrate modification has positive impact in improving the crystalline quality, reducing the light reflection, and increasing the absorption of the BaSi2 thin-films. Etching time was optimized at 15 min, considering the trade-off between crystalline quality and optical properties. Minority carrier-lifetime of the evaporated film on Ge substrate achieved 3.17 μs, which is among the high value obtained for thin BaSi2 films.

Simple technique of formation of “black silicon” using wet chemical etching of crystalline Si wafers with SiGe self-assembled islands is proposed. The main idea consists of the utilization of SiGe islands as a mask for wet anisotropic etching of Si in alkali-based solution at the first etching stage and further removal of SiGe residuals by etching in a HF:H2O2:CH3COOH mixture at the second etching stage. Initial samples were the crystalline Si wafers with SiGe islands formed on them by deposition of 2.5–14 nm of Ge at 800 °C. After the two above-mentioned etching steps a submicron relief on a Si surface was formed. Investigation of optical properties of fabricated structures revealed significant decrease of reflection (AM 1.5 G weighted reflection ~2–3%) and increase of absorption in the wavelength range of 500–1200 nm. Due to the very small amount of Si removal (

To improve conversion efficiency of silicon nanowire (SiNW) solar cells, it is very important to reduce the surface recombination rate on the surface of SiNWs, since SiNWs have a large surface area. We tried to cover SiNWs with aluminum oxide (Al2O3) and titanium oxide (TiO2) by atomic layer deposition (ALD), since Al2O3 grown by ALD provides an excellent level of surface passivation on silicon wafers and TiO2 has a higher refractive index than Al2O3, leading to the reduction of surface reflectance. The effective minority carrier lifetime in SiNW arrays embedded in a TiO2/Al2O3 stack layer of 94 μsec was obtained, which was comparable to an Al2O3 single layer. The surface reflectance of SiNW solar cells was drastically decreased below around 5% in all of the wavelength range using the Al2O3/TiO2/Al2O3 stack layer. Heterojunction SiNW solar cells with the structure of ITO/p-type hydrogenated amorphous silicon (a-Si:H)/n-type SiNWs embedded in Al2O3 and TiO2 stack layer for passivation/n-type a-Si:H/back electrode was fabricated, and a typical rectifying property and open-circuit voltage of 356 mV were successfully obtained.

We studied the impact of oxidizing pre-treatments (OPT) and post deposition annealing (PDA) on the passivation performance and the hydrogen distribution near the interface between crystalline silicon (c-Si) and hydrogenated amorphous silicon (a-Si:H), the critical functional region in Si heterojunction solar cells. The OPT prior to deposition of the a-Si:H layer consists of immersing the c-Si substrates into hydrogen peroxide solutions, which forms a silicon oxide interlayer. Spectroscopic ellipsometry (SE) indicates that slightly thicker a-Si:H layers result from OPT. The refractive index and the extinction coefficient are increased by inserting the oxide interlayers, suggesting that less deficient and denser a-Si:H layers can be formed. Under optimum conditions, OPT leads to at least 2-fold improvement of the effective photo-generated carrier lifetime. PDA at 200 °C further improves the passivation performance of samples with an interlayer. Hydrogen profiling with nuclear reaction analysis clarifies that higher hydrogen concentrations are present around the heterointerfaces of samples with an interlayer and that these hydrogen concentrations are maintained after PDA. Our results suggest that the oxide interlayer can suppress hydrogen desorption in the initial growth stage of high-quality a-Si:H layers and during subsequent PDA, resulting in excellent passivation performance.

The impact of the implementation of magnesium interlayer and the layer thickness (tTiOx) of titanium oxide on the electrical properties of TiOx/SiOy/Si heterojunctions is investigated to improve electron transport for use in silicon heterojunction solar cells. The passivation performance is improved with increasing tTiOx. For the samples with Mg interlayer, ohmic contact can be attained for the thicker TiOx layer compared with the sample without Mg interlayer. Schottky contact is mitigated by the TiOx/SiOy stacking layers and Mg interlayer, attributed to the reduction of interfacial energy level by TiOx/SiOy stacking layers and enhanced downward band bending by Mg interlayer. The open-circuit voltage and fill factor of the solar cells are improved by inserting the TiOx/SiOy stack and the Mg interlayer, indicating that the electron selectivity is enhanced. Excellent surface passivation and transport properties can be achieved by controlling TiOx layer thickness and using the Mg interlayer.

Low-cost aluminum (Al) metallization process on p+ side of n-type silicon (Si) substrate with low recombination current density (J0met) is presented. Narrow-line Al screen-printed fingers on symmetrically passivated p+ -n-p+ Si precursors with local contact opening (LCO) are investigated. The Si content of the Al pastes, LCO patterns and Al finger widths are varied to evaluate the formation quality of the Al-doped p+ region. By increasing the pitch distance of the dot-shaped LCOs, the inverted pyramid cavities are found after removing the Al-Si eutectic part by HCl. The tendency of the reaction during the sintering process is controlled by fritting the Al paste with Si, which allows a faster Si saturation in the paste matrix and facilitates faster return of Si to the LCO region. With a combination of narrow-line fingers of 90 μm and a dot-shaped LCO pitch of 140 μm, J0met value of 420 fA/cm2 is realized.

Dependences of the etch rates for KOH and HF:H2O2:CH3COOH solutions on SiGe layer composition were investigated. The obtained results has been proposed to use for formation of the submicron relief on the silicon surface via selective etching of the structures with Ge(Si) self-assembled nanoislands. In the framework of the proposed approach the Ge(Si) nanoislands serve as a mask for selective etching of Si in a mixture of an aqueous solution of KOH with isopropyl alcohol, followed by the islands removal from the surface by the selective etching in HF:H2O2:CH3COOH. It was demonstrated experimentally that such approach allows to produce the submicron relief on a silicon surface, which leads to the significant decrease of the reflectivity in a wide spectral range. It is believed that the proposed method of surface relief formation can be used to improve the efficiency of the thin-film solar cells based on the crystalline silicon.

We studied on surface morphology, optical properties and passivation performance of copper iodide (CuI) on crystalline silicon (c-Si) deposited by spin-coating for application to hole-selective contacts to realize high performance and low-cost c-Si solar cells. Absorbance was increased by depositing CuI on c-Si owing to absorption and antireflection effect of CuI. From surface images by scanning electron microscope measurements, discontinuous layer was observed for CuI deposited Si. Effective lifetime was characterized for c-Si with ultra-thin oxide covered by CuI both sides by using a microwave photoconductive decay. The lifetime increased from 2 μs to order of 10 μs by introducing CuI layer possibly due to large conduction band offset.

The photovoltaic (PV) nature of the silicon (Si) quantum dot super lattice (QDSL) is studied with an atomic-layer-deposited aluminum oxide film (ALD-Al2O3) and a conventional sputtered-grown amorphous silicon carbide film (a-SiC). The QDSL structures act as an intermediate layer in a p/i/n+ Si solar cell. The QDSL consists of 4-nm Si on 2-nm SiC nanodisks (NDs) arrayed in an ALD-Al2O3 and a-SiC passivation matrix. Formation of Si-NDs was confirmed by bright field scanning transmission electron microscope. A significant PV response in generating a high photocurrent density $\mathit {J_{sc}}$ of 30.15 mA/cm $^{2}$ , open circuit voltage $\mathit {V_{oc}}$ of 0.50 V, fill factor FF of 0.61, and efficiency $\eta $ of 9.12% was observed in ALD-Al2O3/QDSL solar cell with respect to a-SiC/QDSL solar cell with $\mathit {J_{sc}}$ of 26.94 mA/cm $^{2}$ , $\mathit {V_{oc}}$ of 0.50 V, FF of 0.47, and $\eta $ of 6.42%. A wide range of photo-carrier transports by the ALD-Al2O3/QDSL structure is possible in the external quantum efficiency spectra with respect to a-SiC/QDSL solar cell. The enhanced PV performance of the QD solar cells was clarified in terms of simulating the absorption contributions for all possible transitions in the nanostructure with different passivation films.

Single layer silicon beads (SLSB) and chunks (SLSC) coated with silicon nitride were used as seed layer to grow high-quality multicrystalline silicon (mc-Si) ingots. The mc-Si ingots were grown using various sizes of the Si beads and chunks in the range of

The conversion efficiency of crystalline silicon heterojunction solar cells is increased by carrier selective contacts (CSCs) thanks to the combination of conductive and passivating layers. In this work, we propose the titanium oxide (TiO TiOx) bilayer to consist of TiO TiOx layerlayers prepared at 100 and 150°C by atomic layer deposition ALD). The TiO TiOx bi layer shows higher electrical properties in comparison with a single TiO TiOx layer. The enhanced electrical properties are origin ated from high passivation performance and low contact resist resistivity of TiO TiOx layerlayers prepared at 150 and 100 °C, respectively respectively, suggesting modulation of the deposition temperature can improve the functionality of ALD ALD-materials.

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.

Variable-angle spectroscopic ellipsometry analysis is performed to study the impact of post-deposition annealing on the passivation performance of the heterocontacts consisting of titanium oxide and silicon oxide on crystalline silicon (c-Si) prepared by atomic layer deposition (ALD) for the development of high-performance ALD-TiO x /SiO y /c-Si heterocontacts. The highest lifetime of 1.8 ms is obtained for the TiO x /SiO y /c-Si heterocontacts grown at 175 °C after annealing at 275 °C for 3 min. With increasing annealing temperature, the TiO x layers of the TiO x /SiO y /c-Si heterocontacts become dominant. Furthermore, the amplitude of dielectric functions of the ALD-TiO x layer decreases as annealing temperature increases, which suggests that enhanced diffusion of Ti into SiO y interlayers at higher annealing temperature. The sufficient diffusion of Ti atoms into SiO y interlayers is caused by annealing at 275 °C for 3 min, yielding high-quality interface passivation.

A comparative experimental and theoretical study of the role of H incorporation in As-doped BaSi2 films has been carried out based on the experimental results that an optimal time of H treatment for the increase in photoresponsivity and carrier lifetime was in the range of 1 – 20 min. Adequate theoretical representation of the decay curves in the framework of the model for non-radiative processes accounted for various trap-related recombination mechanisms to estimate the trap concentration to be in the range of 1.9 × 1013 to 1.7 × 1014 cm−3. Additionally, the extended theoretical ab initio quantum-chemical simulation of the electronic structure of the studied systems was performed. It was revealed that interstitial As atoms can mostly provide trap states in the gap while H atoms neutralize such traps. The experimentally observed unexpected switching in conductivity from n-type to p-type and vice versa in As-doped BaSi2 with H incorporation was explained to specific configurations of point defects (an As impurity with a H atom in different positions and various interatomic As-H distances) which affect the position of states in the gap.

The theoretical lateral current of the surface inversion layer in a crystalline silicon (cSi) surface for a p-aSi:H/i-aSi:H/cSi heterojunction (SHJ) solar cell was calculated using computer simulation and was compared with the experimental one to study defects/traps at the aSi:H/cSi interface and/or in the cSi surface and to detect the acceptor concentration (N a) in p-aSi:H. To experimentally extract the lateral surface inversion layer current, a field-effect transistor type test element group device was co-integrated with SHJ cells on the same wafer. From the correlation between the experimental and calculated lateral surface inversion layer current, the density of defects/traps (D it) at the aSi:H/cSi interface and/or in the cSi surface and the value of N a were extracted. The calculated lateral surface inversion layer current stayed unchanged for various minority carrier lifetimes in the substrate, suggesting that this method is not suffered from the variation in the material parameters in the substrate.

The introduction of directional solidified cast mono silicon promised a combination of the cheaper production via a casting process with monocrystalline material quality, but has been struggling with high concentration of structural defects. The SMART approach uses functional defects to maintain the monocrystalline structure with low dislocation densities. In this work, the feasibility of the SMART approach is shown for larger ingots. A G2 sized crystal with SMART and cast mono silicon parts has been analyzed regarding the structural defects via optical analysis, crystal orientation, and etch pit measurements. Photoluminescence measurements on passivated and processed samples were used for characterization of the electrical material quality. The SMART approach has successfully resulted in a crystal with mono area share over 90% and a confinement of dislocation structures in the functional defect region over the whole ingot height compared to a mono area share of down to 50% and extending dislocation tangles in the standard cast mono Si. Cellular structures in photoluminescence measurements could be attributed to cellular dislocation patterns. The SMART Si material showed very high and most homogeneous lifetime values enabling solar cell efficiencies up to 23.3%.

The joint impact of the Si/Al layer thickness on the growth kinetics, the crystalline orientation and the size of the poly-Si grains resulting from aluminum-induced crystallization process is analyzed. It is shown that the surface coverage of resulting poly-Si layers rapidly decreases together with annealing temperature and the Si/Al ratio. The surplus of a-Si over the Al needed to ensure continuity of the poly-Si thin film is in the range of 35%–50% for Al layers thicker than 225 nm, but rapidly goes up to 200% as the thickness of the Al layer decreases below 50 nm. It is demonstrated that the angular distribution of grain orientations is discrete and shifts towards the {111} direction as the Si/Al increases. It is reported that during an isothermal annealing, the nucleation of Si grains occurs in two steps. Finally, a simple model of the aluminum-induced crystallization process explaining the two-step nucleation is proposed.

We report on the enhancements of effective carrier lifetime by light-induced “recovery” instead of “degradation” in oxygen-rich boron-doped Czochralski-grown silicon (Cz-Si) wafers at room temperature. The highest measured lifetime of τ=550 μs was obtained under light illumination at room temperature. We found that the increasing rate of the lifetimes depends on the firing condition and the illumination intensity. These dependencies are very similar to those of permanent recovery caused by hydrogenation of B-O defects or so called regeneration induced by illumination at higher temperatures. Therefore, it can be presumed that the significant increase of minority carrier lifetime is caused by hydrogenation of recombination active defects. However, the defect-inactive state is not stable and the lifetime was found to decrease slowly after the illuminated recovery step. The thermal activation energy of this defect reactivation reaction was obtained to be 0.36 eV from the Arrhenius plot, which is much smaller than that of the thermal dissociation of the hydrogenated B-O complex of 1.25 eV. This result indicates that the measured defect transitions are markedly different from those of B-O related defects. Possible mechanisms to control defect deactivation are discussed based on a given fractional concentration of charged hydrogen. By combining this deactivation process and a regeneration process, a very high lifetime value could be achieved in Cz-Si for solar cells.

We investigated the effect of the grain boundary (GB) character of multicrystalline Si (mc-Si) on the efficiency of external and internal gettering of impurities during phosphorus diffusion gettering (PDG). We utilized seed crystals with an artificially designed GB configuration to grow mc-Si ingots with different artificial GB characters. PDG combined with an originally developed multiple-cycle gettering technique at low temperature was introduced on intentionally Fe-contaminated mc-Si samples to enhance external and internal gettering. A significant positive PDG effect was observed after PDG combined with the multiple-cycle technique, as evidenced by the increase in lifetimes after PDG. A bright cloud-like photoluminescence signal around contaminated GBs was observed for artificial Σ5-GBs and tilt-GBs after PDG, suggesting the enhancement of the internal gettering efficiency by leaving a cleaner area around the GBs. This result suggests the importance of the control of crystal defect character as well as impurities in mc-Si ingots, which could strongly affect the PDG efficiency. Copyright © 2016 John Wiley & Sons, Ltd.

We propose to utilize single layer silicon beads (SLSB) coated with silicon nitride as cost-effective seed layer to grow high-quality multicrystalline silicon (mc-Si) ingot. The texture structure of silicon nitride provides a large number of nucleation sites for the fine grain formation at the bottom of the crucible. No special care is needed to prevent seed melting, which would lead to decrease of red zone owing to decrease of feedstock melting time. As we expected, mc-Si ingot seeded with SLSB was found to consist of small, different grain orientations, more uniform grain distribution, high percentage of random grain boundaries, less twin boundaries, and low density of dislocation clusters compared with conventional mc-Si ingot grown under identical growth conditions. These results show that the SLSB seeded mc-Si ingot has enhanced ingot quality. The correlation between grain boundary structure and defect structure as well as the reason responsible for dislocation clusters reduction in SLSB seeded mc-Si wafer are also discussed.

Exploration of impurity elements for carrier density control is important for device application of the BaSi 2 semiconductor, which is a promising candidate for an earth-abundant solar cell absorber. In this study, we have investigated the doping of BaSi 2 films with alkali metals (Li, Na, and K) by deposition of alkali-metal fluorides followed by rapid thermal annealing. Electrical characterization by Hall measurement shows that LiF treatment increases electron density in BaSi 2 up to 10 20 cm − 3 after annealing at 500 °C while NaF and KF treatments have limited effects with electron densities lower than 10 18 cm − 3 . The mechanisms of electrical property modification are discussed from structural viewpoints. Secondary ion mass spectroscopy shows that Li atoms slightly diffuse into the film at 400 °C by prolonged annealing, which nevertheless does not accordingly increase the carrier density. Formation of barium fluoride, oxide, and silicon at 500 °C, which is revealed by energy-dispersive X-ray and Raman spectroscopies, brings about the conclusion that a high temperature is needed for the dissociation of LiF through the reaction with BaSi 2 and the incorporation of Li atoms probably into the interstitial sites of BaSi 2 , which would generate electrons. On the other hand, Na and K atoms are found to evaporate at the same time as the dissociation of NaF and KF, respectively, evidenced by energy-dispersive X-ray spectroscopy. BaF 2 formation accompanied by the NaF dissociation is detected by X-ray diffraction.

Non-coherent Σ3{111} grain boundaries (GBs) with a positive deviation in the tilt angle (θ 〈110〉 > 70.5°) exhibit a high recombination activity in high-performance multicrystalline silicon ingots. Most of the GB segments are composed of edge-type dislocations with the Burgers vector b of a/3〈111〉, unlike Lomer dislocations with b = a/2〈110〉 observed for negative deviations, arranged on coherent Σ3{111} GB segments. Stretched 〈110〉 reconstructed bonds along the tilt axis are introduced so as not to form dangling bonds, and large strains are generated around the dislocation cores. Oxygen and carbon atoms segregating due to the strains would induce the recombination activity.

Three-dimensional distribution of grain boundaries (GBs) and generation sources of dislocation clusters is examined in a cast-grown high-performance multicrystalline silicon ingot for commercial solar cells. A significant number of dislocations are generated nearby some triple junctions of random angle GBs, although it is believed that such non-coherent GBs would not induce large strain during the cast growth. This explosive generation of dislocations would take place when the triple junctions are interacted with multiple Σ3{111} GBs. A segment of the random angle GB connected with a pair of Σ3{111} GBs nearby the triple junction would act as a dislocation source.

Barium disilicide (BaSi 2 ) is an emerging light-absorbing material for earth-abundant thin-film solar cells. Here, we report scalable and rapid deposition of BaSi 2 films by close-spaced evaporation. In this method, BaAl 4 –Ni evaporation source generates Ba gas, which is deposited on a closely-placed Si(001) substrate. The reaction between deposited Ba atoms and Si substrates yields compositionally homogeneous BaSi 2 films through the depth, as evidenced by X-ray diffraction and Auger electron spectroscopy. Although the present BaSi 2 films suffer from cracking and exfoliation issues caused by thermal strain, exfoliation is successfully suppressed by lowering the substrate temperature using a reflector. Electron backscatter diffraction and X-ray diffraction analyses show that the dominant orientation of the BaSi 2 films is (001). A considerably large single-orientation area more than 68 μ m was observed, which is the largest among previously reported BaSi 2 films.

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.

Silicon isotope engineering invokes a new paradigm into material science for nanoscale device applications [1]. The development of planer CMOS devices confronts an immediate issue of its downscaling limit, requiring a basal changeover to three-dimensional (3D) vertical transistors as a possible solution. General gate-all-around transistors involve formation of reliable gate-oxide film surrounding Si nanopillar structures. For the precise 3D process simulator, silicon isotopes are used as a self-diffusive marker for experimental determination of its model parameters. Recent advancement of atom probe tomography enables nearly atomic-resolution mass spectroscopy for 3D limited areas, together with great sensitivity to different Si isotopic mass [2]. By extending this technique to isotopically programmed Si nanopillars, individual self-diffused Si isotopes were visualized at MBE-grown isotopic layer interface [3, 4]. Recently, our quantitative evaluation revealed that Si self-diffusivity in gate-oxidized nanopillars was the same as the one measured for standard planar oxidation despite of its high interface-to-volume ratio [4]. In addition, silicon isotope engineering provides a crucial solution to eliminate unwanted decoherence for silicon quantum computers (QCs). Strong compatibility with industrial CMOS technology is expected to satisfy a scalability criterion significant for realizing practical QCs. In order to support the Si-QC research, we adopted silicon isotope engineering to tailor high-quality CVD-grown 28Si epi-wafers [1], where state-of-the-art Si-MOS nanoelectronic devices were implemented to demonstrate two-qubits operations encoded by single donor electron and nuclear spins [5]. Following an original Kane’s proposal, selective control of the electron spins was successfully achieved by adding a local gate bias thanks to the Stark tuning [6], while the nuclear-spin resonance has been recently shown to be driven by local electric fields in the presence of lattice strain [7]. Additionally, the isotopically enriched wafers were, in parallel, employed to demonstrate single and two spin-qubit operations in 28Si-MOS quantum dots (QDs) that were defined by standard CMOS lithography [8]. A series of proof-of-concepts experiments has given a boost to several proposals of Si-QC architectures based on advanced manufacturing technology [9]. Meanwhile, an alternative approach following conventional III-V QDs was made to obtain buried-type QDs utilizing strained-Si/SiGe quantum-well (QW) heterostructures, which are placed away from gate-oxide interface traps that can work as sources for charge noise to disturb spin-qubit operation. In fact, we accomplished isotope enrichment of Si-QW layer to efficiently remove magnetic nuclear-spin noise. As a consequence, the electron-spin dephasing time was prolonged to T 2 * ~ 20 ms for 28Si/SiGe QDs, where local single-spin controllability was introduced by a magnetic-field gradient from an on-chip micro-magnet [10]. These 28Si-QD devices amenable to several technical improvements have recorded the highest single-spin gate fidelities of 99.93% for 28Si/SiGe QDs [10] and 99.96% for 28Si-MOS QDs [11], respectively. Concurrently, it was revealed that inherent charge noise and/or residual nuclear-spin noise induced spin-resonance frequency fluctuations during the benchmark test executions. For higher-fidelity operations, including spin-qubit readout and initialization, low-energy excited states responsible for Si valley degeneracy should be also well-lifted by rendering the QD confinement atomically sharp. In these contexts, we will present recent development of high-quality 28Si/SiGe QWs appropriate for the silicon quantum information research. This work has been in part supported by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant Number JPMXS0118069228 and the Center for Spintronics Research Network, Keio University. The authors acknowledge fruitful collaborations with Yasuyoshi Nagai’s group, Seigo Tarucha’s group, Mark Eriksson’s group, Andrew Dzurak’s group and Andrea Morello’s group. [1] K. M. Itoh and H. Watanabe, MRS Communications 4, 143 (2014). [2] Y. Shimizu et al., J. Appl. Phys. 106, 076102 (2009). [3] T. Südkamp et al., J. Appl. Phys. 123, 161515 (2018). [4] R. Kiga et al., J. Appl. Phys. under review. [5] J.T. Mohonen et al., Nature Nanotechnol. 9, 986 (2014). [6] A. Laucht et al., Sci. Adv. 1, e1500022 (2015). [7] S. Asaad et al., arXiv:1906.01086. [8] M. Veldhorst et al., Nature Nanotechnol. 9, 981 (2014); M. Veldhorst et al., Nature 526, 410 (2015). [9] L.M.K. Vandersypen et al., npj Quantum Information 3, 34 (2017); M. Veldhorst et al., Nature Comm. 8, 1766 (2017); R. Li et al., Sci. Adv. 4, eaar3960 (2018). [10] J. Yoneda et al., Nature Nanotechnol. 13, 102 (2018). [11] C.H. Yang et al., Nature Electronics 2, 151 (2019).

The integration of gallium nitride with low-cost, transparent substrates such as glass and crystallographically incompatible substrates such as Si(0 0 1) has been long desired for III-N electronics. Here the authors demonstrate how GaN growth on uniformly 〈1 1 1〉 oriented silicon seed layers fabricated by aluminum-induced crystallization provide a straightforward method for realizing large area GaN growth on these substrate by adopting well-developed GaN on Si(1 1 1) growth processes. Critical factors for promoting high quality GaN growth are identified, and remaining steps necessary for realizing GaN devices are discussed.

We have grown orthorhombic barium disilicide ([Formula: see text]) thin-films on modified silicon (Si) substrates by a thermal evaporation method. The surface modification of Si substrate was performed by a metal-assisted chemical etching method. The effects of etching time [Formula: see text] on crystalline quality as well as optical and electrical properties of the [Formula: see text] films were investigated. The obtained results showed that substrate modification can enhance the crystalline quality and electrical properties; reduce the light reflection; and increase the absorption of the [Formula: see text] thin-films. The [Formula: see text] of 8 s was chosen as the optimized condition for surface modification of Si substrate. The achieved inferred short-circuit current density, Hall mobility, and minority carrier lifetime of the [Formula: see text] film at [Formula: see text] of 8 s were [Formula: see text], [Formula: see text], and [Formula: see text]s, respectively. These results confirm that the [Formula: see text] thin-film evaporated on the modified Si substrate is a promising absorber for thin-film solar cell applications.

We report on the synthesis of Mg2Si thin films from a Mg-Si solution by thermal treatment of Mg on a Si substrate under an Ar gas atmosphere. The Mg was prepared by two methods. One is to deposit Mg films by thermal evaporation, and another is to simply place Mg ribbons on Si. After thermal treatment, a two-layer structure consisting of oxide and ~1 μm-thick Mg2Si films with {100} preferential orientation was observed for the sample prepared by thermal evaporation. On the other hand, ~10 μm-thick single layer Mg2Si with more random orientations and cracks was formed for the Mg ribbon contact samples. Possible mechanisms for different structures, crystal orientations, and crack formation are discussed.

The authors report on the effect of hydrogen plasma treatment (HPT) on the passivation performance of titanium oxide (TiOx) on crystalline silicon (c-Si) fabricated by atomic layer deposition. Recently, TiOx has gathered attention as an electron-selective contact material for silicon heterojunction (SHJ) solar cells due to its preferable work function and band lineup. Moreover, TiOx has excellent light transmission properties due to its large bandgap energy. In order to improve the power conversion efficiency of SHJ solar cells with TiOx, it is necessary to enhance the passivation performance. The effective carrier lifetime representing the passivation performance is enhanced by HPT and amounts to 407.2 μs after HPT at 200 °C for 1 min. This value is twice as high as after forming gas annealing, which is the standard method to enhance the passivation performance of TiOx/c-Si heterostructures. Nuclear reaction analysis clarifies that the hydrogen concentration (CH) at the TiOx/c-Si interface of the HPT-processed sample is higher than that of an as-deposited sample and that the peak position of the CH distribution is shifted closer to the TiOx/c-Si heterointerface after HPT. Moreover, thermal desorption spectroscopy shows that Si–H and Si–H2 hydrogen bonds increase with HPT. These results indicate that the atomic hydrogen produced by the hydrogen plasma diffuses toward the TiOx/c-Si interface and terminate the local dangling bonds, which is responsible for the improved passivation performance.The authors report on the effect of hydrogen plasma treatment (HPT) on the passivation performance of titanium oxide (TiOx) on crystalline silicon (c-Si) fabricated by atomic layer deposition. Recently, TiOx has gathered attention as an electron-selective contact material for silicon heterojunction (SHJ) solar cells due to its preferable work function and band lineup. Moreover, TiOx has excellent light transmission properties due to its large bandgap energy. In order to improve the power conversion efficiency of SHJ solar cells with TiOx, it is necessary to enhance the passivation performance. The effective carrier lifetime representing the passivation performance is enhanced by HPT and amounts to 407.2 μs after HPT at 200 °C for 1 min. This value is twice as high as after forming gas annealing, which is the standard method to enhance the passivation performance of TiOx/c-Si heterostructures. Nuclear reaction analysis clarifies that the hydrogen concentration (CH) at the TiOx/c-Si interface of the HPT-pro...

Silicon thin films with uniform Si(111) surface orientation (>95%) have been fabricated on amorphous substrates through aluminum-induced crystallization. However, Si(100) oriented films have not been fabricated with nearly the same degree of uniformity. For Al and Si thicknesses below 50 nm, Si nucleation has been previously proposed to initiate at the Al/substrate interface, suggesting that a cubic (100) oriented substrate could promote highly uniform Si(100) oriented thin film fabrication. Using two single crystalline (100) oriented substrates, strontium titanate and germanium, we demonstrate how substrate surface orientation and surface termination affect the crystallization and preferential orientation of silicon thin films fabricated by aluminum-induced crystallization.

Phosphorus-doped silicon nanocrystals (Si-NCs)/silicon oxide (SiOy) multilayers were prepared by plasma enhanced chemical vapor deposition and post-annealing. After annealing, Si-rich amorphous silicon oxide (a-SiOx) and a-SiOy layer were converted into Si-NCs layer and lowly thermal conductive layer, respectively. The diameter of Si-NCs were changed by varying the thickness of a-SiOx layer from 3 to 10 nm. Electrical and thermal conductivity of Si-NCs/SiOy multilayers were decreased with decreasing the diameter (d) of Si-NCs. Thermal conductivity at d=3 nm was estimated at 1.48 W•m-1•K-1, which is much lower than that of bulk silicon. These results show that phosphorus doped Si-NCs/SiOy multilayers are promising for thermoelectric materials.

To obtain silicon quantum dot (Si-QD) multilayers, amorphous silicon oxide (a-SiO x ) films have been prepared by plasma enhanced chemical vapor deposition (PECVD) using SiH4 and CO 2 . As increasing CO 2 flow rate, crystal volume of Si was decreased and optical gap was increased. This results suggest the increasing of oxygen content in samples. From this analysis, Si-QD multilayers having different Si-QD size were fabricated and were evaluated for structural properties. A cross-sectional transmission electron microscopy image showed the formation of Si-QDs with the diameter of about 5 nm. PL spectra showed the bandgap was blue-shifted to 1.3 eV with decreasing the size of Si-QDs to 5 nm, indicating the quantum size effect appeared in these samples.

We investigated the local structure at the interface between titanium oxide (TiO x ) and crystalline Si (c-Si) with silicon oxide (SiOx) interlayers by employing the electron energy loss (EEL) spectroscopy to clarify the origin of the high performance TiO x passivating layer deposited by atomic layer deposition. Five kinds of TiO x /SiO x /c-Si structures were fabricated and the best passivation performance was achieved by the SiO x interlayer formed by nitric acid at room temperature. EEL spectra revealed that Ti-contained more stoichiometric SiO x including few oxygen vacancies and Ti-O-Si bonding would be formed after forming gas annealing, resulting in the enhancement of passivation performance.

The photovoltaic effect of the silicon (Si)/silicon carbide (SiC) quantum dot super lattice (QDSL) and multi-quantum well (QW) strucutres is presented based on numerical simulation and experimental studies. The QDSL and QW structures act as an intermediate layer in a p-i-n Si solar cell. The QDSL consists of a stack of four 4-nm Si nano disks and 2-nm SiC barrier layers embedded in a SiC matrix fabricated with a top-down etching process. The Si nano disks were observed with bright field-scanning transmission electron microscopy. The simulation results based on the 3D finite element method confirmed that the quantum effect on the band structure for the QDSL and QW structures was different and had different effects on solar cell operation. The effect of vertical wave-function coupling to form a miniband in the QDSL was observed based on the solar-cell performance, showing a dramatic photovoltaic response in generating a high photocurrent density Jsc of 29.24 mA/cm2, open circuit voltage Voc of 0.51 V, fill factor FF of 0.74, and efficiency η of 11.07% with respect to a i-QW solar cell with Jsc of 25.27 mA/cm2, Voc of 0.49 V, FF of 0.69, and η of 8.61% and an i-Si solar cell with Jsc of 27.63 mA/cm2, Voc of 0.55 V, FF of 0.61, and η of 10.00%. A wide range of photo-carrier transports by the QD arrays in the QDSL solar cell is possible in the internal quantum efficiency spectra with respect to the internal quantum efficiency of the i-QW solar cell. Copyright © 2015 John Wiley & Sons, Ltd.

Producing large-grained polycrystalline Si (poly-Si) film on glass substrates coated with conducting layers is essential for fabricating Si thin-film solar cells with high efficiency and low cost. We investigated how the choice of conducting underlayer affected the poly-Si layer formed on it by low-temperature (500°C) Al-induced crystallization (AIC). The crystal orientation of the resulting poly-Si layer strongly depended on the underlayer material: (100) was preferred for Al-doped-ZnO (AZO) and indium-tin-oxide (ITO); (111) was preferred for TiN. This result suggests Si heterogeneously nucleated on the underlayer. The average grain size of the poly-Si layer reached nearly 20 µm for the AZO and ITO samples and no less than 60 µm for the TiN sample. Thus, properly electing the underlayer material is essential in AIC and allows large-grained Si films to be formed at low temperatures with a set crystal orientation. These highly oriented Si layers with large grains appear promising for use as seed layers for Si light-absorption layers as well as for advanced functional materials.

This work provides a rapid overview for the current state of surface passivation layer schemes for thin film solar cells: From its fundamentals to solar cell applications, and their perspective. It provides an overview of important literature and prospect considerations based on simulations. European Research Council (ERC) under the European Union [715027]

We report on structural and optical properties of copper iodide (CuI) on crystalline Si (c-Si) and properties of the diode with intrinsic type (i-type) hydrogenated amorphous Si (a-Si:H) at CuI/c-Si interface. The i-type a-Si:H layer was deposited on n-type c-Si by catalytic chemical vapor deposition (Cat-CVD) prior to deposition of CuI by spin-coating method. Higher rotational speed of 4000 rpm was found to be useful to suppress generation of large particles and absorption by CuI. The effective carrier lifetime was characterized for CuI/a-Si: H/c-Si/a-Si: H/CuI structures and it strongly depended on a-Si:H. The CuI/a-Si: H/c-Si diodes showed rectification behavior.