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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 view-points of vapor composition and diffusion time. Microstructural characteristics such as grain size of 30-120nm, 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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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 barrier height for hole transport across the GBs varies in the range from 10 to 55 meV. The potentials are also higher at the BaSi2 GBs grown around Si GBs composed of grains with Si(001) and Si(111) faces. The barrier height for hole transport ranges from 5 to 55meV. These results indicate that BaSi2 GBs formed on (111)-dominant Si surfaces do not have a negative influence on the minority-carrier properties, and thus BaSi2 formed on underlayers, such as (111)-oriented Si or Ge and on (111)-oriented mc-Si, can be utilized as a solar cell active layer. [ABSTRACT FROM AUTHOR]

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 0.53V 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 1.5V 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. [ABSTRACT FROM AUTHOR]

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. 8μs), 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 films. [ABSTRACT FROM AUTHOR]

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 and with atomically smooth surface have been demonstrated. [ABSTRACT FROM AUTHOR]

Grazing-incidence small-angle X-ray scattering (GISAXS) measurements with soft X-rays have been applied to Ge nanodots capped with a Si layer. Spatially anisotropic distribution of nanodots resulted in strongly asymmetric GISAXS patterns in the qy direction in the soft X-ray region, which have not been observed with conventional hard X-rays. However, such apparent differences were explained by performing a GISAXS intensity calculation on the Ewald sphere, i.e. taking the curvature of Ewald sphere into account. [ABSTRACT FROM AUTHOR]

Quantum dots (QDs) have attracted much attention for use in photovoltaic applications because of their potential for overcoming the limits of conventional single-junction devices. One problem associated with solar cells using QDs is that the open-circuit voltage (Voc) always decreases with the addition of QDs with respect to the reference cell without QDs. Here, we report the investigation of current-voltage characteristics in Ge/Si QD solar cells in the temperature range from 100 to 300 K. We show that even thoughVoc decreases with increasing temperature, it depends on the nominal Ge thickness, indicating that Voc reduction is primarily caused by a decrease in the bandgap energy of the cell. From photoluminescence decay measurements, we found that rapid carrier extraction from QDs occurred in the solar cells; this process eliminates the quasi-Fermi energy splitting between the QDs and the host semiconductor and causes Voc reduction in QD solar cells. [ABSTRACT FROM AUTHOR]

We fabricated a three-dimensional (3D) stacked Si nanodisk (Si-ND) array with a high aspect ratio and uniform size by using our advanced top-down technology consisting of bio-template and neutral beam etching processes. We found from conductive atomic microscope measurements that conductivity became higher as the arrangement was changed from a single Si-ND to two-dimensional (2D) and 3D arrays with the same matrix of SiC, i.e., the coupling of wave functions was changed. Moreover, our theoretical calculations suggested that the formation of minibands enhanced tunneling current, which well supported our experimental results. Further analysis indicated that four or more Si-NDs basically maximized the advantage of minibands in our structure. However, it appeared that differences in miniband widths between 2D and 3D Si-ND arrays did not affect the enhancement of the optical absorption coefficient. Hence, high photocurrent could be observed in our Si-ND array with high photoabsorption and carrier conductivity due to the formation of 3D minibands. [ABSTRACT FROM AUTHOR]

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 a 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. [ABSTRACT FROM AUTHOR]

The lateral transport of minority carriers in the surface inversion layer of a crystalline silicon (cSi) surface for a p-aSi:H/i-aSi:H/cSi heterojunction (SHJ) solar cell was experimentally analyzed to study defects/traps at the aSi:H/cSi interface and/or on the cSi surface. To extract the lateral surface inversion layer current, a field-effect transistor type test element group device was designed and fabricated on a cSi wafer where SHJ cells were co-integrated. By analyzing the lateral surface inversion layer current, the effective surface minority carrier mobility was calculated. The extracted mobility was one order of magnitude smaller than that of a reference MoOx/i-aSi:H/cSi structure but it increased by low-temperature post-deposition annealing with respect to the reference. This method is highly sensitive to the quality of the aSi:H/cSi interface and/or the cSi surface. The density of trap sites was estimated to be of the order of 1011 cm−2 at the aSi:H/cSi interface and/or on the cSi surface. [ABSTRACT FROM AUTHOR]

We investigated the influence of Sn and Ge ratio in Al paste to fabricate Si-based alloy semiconductor on large-area Si substrate using a conventional screen-printing process and high-temperature treatment steps. From the X-ray diffraction patterns, crystalline SiSn peaks with applying Al-Sn paste have been detected and Sn content in the SiSn layer was estimated around 0.35%, close to the level of solid solubility of Sn in Si. In addition, the Sn depth profile showed similar behavior with Al, which confirms the concept of a liquid epitaxial growth using Al. On the other hand, the SiGe peaks with applying Al–Sn–Ge paste were clearly observed and their main peaks were shifted to a higher angle linearly with decreasing the Ge ratio in the paste. It was suggested that Sn was segregated easily to the surface if in coexistence with Ge due to its lower solubility in SiGe systems. [ABSTRACT FROM AUTHOR]

We propose to utilize artificial neural network (ANN) to optimize positions of a limited number of sensors for accurate monitoring, and demonstrate its effectiveness by a case study of four thermocouples in a directional solidification furnace. Our concept consists of choosing the positions with ANN that has the lowest loss from a multiplicity of ANNs, which were trained by the simulated temperature distributions along the outer crucible wall. Interestingly, the top ten ranks of accurate predictions contain positions around the crucible’s bottom to suggest the importance of measuring temperatures carefully around high-temperature gradients that is the boundary between different materials. [ABSTRACT FROM AUTHOR]

The impact of the size distributions of Ge islands on the structural and optical characteristics of anti-reflection structures was investigated. The variation of island size distribution was achieved through the variation of growth temperature in gas-source molecular beam epitaxy at 700 °C–800 °C. Ge islands were utilized as etching masks to form the anti-reflection structures. By using lower growth temperature of 700 °C and subsequent etching, larger texture without many hollows was formed in contrast to that using 800 °C. Broader size distribution of islands formed at 700 °C was found to lead to larger texture size. Smaller texture is formed by smaller islands and short etching, whereas larger texture is formed by erosion of smaller texture during long etching. A potential short-circuit current density of 36.75 mA cm−2 was obtained for the sample by the islands grown at 700 °C with reduced etching margins, comparable to that of conventional pyramid textures. [ABSTRACT FROM AUTHOR]

We report on a simple liquid-phase epitaxy (LPE) of SiGe on Si (100) substrate based on printing and firing. LPE was performed using an Al–Ge mixed paste screen-printed on a Si (100) substrate followed by annealing above the Al–Ge eutectic temperature in air or in an Ar atmosphere. In the case of annealing in air at 800 °C, SiGe was formed between the mixed paste and the surface of the Si substrate. However, a wave-like oxide film was confirmed at the SiGe/Si interface, which hindered the growth of SiGe. On the other hand, annealing in the Ar atmosphere at 800 °C led to the successful formation of a more continuous, thicker SiGe film by suppressing the oxidation of Al. It can be concluded that LPE growth using printing and firing of Al–Ge mixed paste is a suitable method to grow SiGe with a low-cost and simple high-speed process. [ABSTRACT FROM AUTHOR]

Because of their >95% (111) surface orientation and very large (10–300 μm) grain sizes, sub-50 nm Group IV thin films fabricated by metal-induced crystallization are promising seed layers for epitaxy. However, methods for evaluating Group IV film quality for subsequent homo-and heteroepitaxial growth have not been widely reported. Here, we show how pole figures obtained by electron backscatter diffraction allow for texture analysis and measurement of grain misorientation. Correlations with Group IV thin film processing parameters such as annealing temperature and film quality in heteroepitaxially grown GaN films are developed. [ABSTRACT FROM AUTHOR]

We report on the activation mechanism of titanium oxide (TiOx) prepared by atomic layer deposition on crystalline silicon (c-Si), which is regarded as a prominent electron-selective contact with ideal band alignment and low carrier recombination for Si heterojunction solar cells. The activation energy of surface passivation was revealed to be about 0.37 eV, which was shown to be associated with the marked transformation of the chemical bonding states at the TiOx/c-Si interface. Detailed analysis confirmed that Si–O bonds are formed after postannealing, owing to the dissociation of the Si–H and Si–H2 bonds at the interface and subsequent termination by oxygen atoms. [ABSTRACT FROM AUTHOR]

To obtain high efficiency in crystalline silicon thin-film solar cells, it is necessary to develop a novel light-trapping structure that not only has high light absorptance and can be fabricated with a small etching margin but also has a small surface area for a small effect of surface recombination. In this study, optical properties of silicon nanowires (SiNWs) with submicron diameters were investigated by the finite-difference time-domain (FDTD) method in the infrared wavelength region, in which the absorption coefficient of crystalline Si is quite low and the increase in optical path length is very important. To verify the simulation results, SiNWs with submicron diameters were successfully fabricated by the metal-assisted chemical etching method using two types of etching masks. The optical properties and minority carrier lifetime of the obtained structures were measured, and the results suggest that SiNWs with submicron diameters such as ∼700 nm are effective for both light trapping and the suppression of surface recombination. [ABSTRACT FROM AUTHOR]

We investigated the influence of the barrier layer height in silicon quantum dot superlattice (Si-QDSL) solar cells on the cell performance by two-dimensional device simulation taking into account the quantum size effect. Although at Si-QD diameters (d) of 5 and 10 nm better cell performance was obtained as the barrier height increased, in the case of d = 3 nm, the efficiency decreased as the barrier height increased. Further investigation showed that the band mismatch at the interface between Si-QDSL and the n-type layer degraded open-circuit voltage (Voc) at d = 3 nm. After the optimization of the physical properties of the n-type layer, Voc was improved to 1.43 V and the bandgap-Voc (Woc) offset became about 0.4 V, suggesting that the band mismatch is the significant factor for the Voc loss. From these results, it was found that careful selection of n- or p-type materials is necessary when the strong quantum confinement effect occurs in Si-QDSL. [ABSTRACT FROM AUTHOR]

A light-trapping structure was fabricated on crystalline Si wafers. In this method, a Ge/Si(001) self-assembled dot structure was etched using KOH and a commercial alkaline solution including nanomask particles. Many nanoscale islands were formed by adding the nanomask particles. The enhancement of light absorption was successfully confirmed in samples with nanostructures after etching. Since the etching margin of a nanostructure was less than 2 µm, the nanostructure could be applied to an ultrathin Si substrate. To investigate the effect of surface passivation on the nanostructure, the nanostructure was passivated using hydrogenated amorphous Si thin films by plasma-enhanced chemical vapor deposition. By quasi-steady-state photoconductance measurement, the suitable implied open-circuit voltage and implied fill factor for heterojunction solar cells were obtained. From calculations based on these parameters, solar cells with nanostructures at a substrate thickness of 100 µm are expected to have a conversion efficiency of 23.21%. [ABSTRACT FROM AUTHOR]

We report on our attempt to increase the carrier density in the p-type polycrystalline silicon (poly-Si) layers grown by aluminum-induced crystallization (AIC) employing a B-doped Si target. Hall measurement revealed that we could obtain a lower-resistance and heavily doped p-type continuous poly-Si thin layer formed by AIC using B-doped a-Si. According to our evaluation of tunnel oxide passivated contact (TOPCon) solar cells with AIC-grown poly-Si, the AIC-TOPCon solar cells fabricated at 570 °C using B-doped a-Si showed higher conversion efficiency of 13.5% than that of 12.8% when using nondoped a-Si. It is considered that the cell characteristics, particularly FF and Voc, were improved owing to the lower series resistance and higher carrier density since both Al and B were successfully incorporated into the AIC-grown poly-Si. [ABSTRACT FROM AUTHOR]

Thermal evaporation is a simple and rapid method to fabricate semiconducting BaSi2 films. In this study, to elucidate the BaSi2 formation mechanism, the microstructure of a BaSi2 epitaxial film fabricated by thermal evaporation has been investigated by transmission electron microscopy. The BaSi2 film is found to consist of three layers with different microstructural characteristics, which is well explained by assuming two stages of film deposition. In the first stage, BaSi2 forms through the diffusion of Ba atoms from the deposited Ba-rich film to the Si substrate while in the second stage, the mutual diffusion of Ba and Si atoms in the film leads to BaSi2 formation. On the basis of the BaSi2 formation mechanism, two issues are addressed. One is the as-yet unclarified reason for epitaxial growth. It is found important to quickly form BaSi2 in the first stage for the epitaxial growth of upper layers. The other issue is the high oxygen concentration in BaSi2 films around the BaSi2–Si interface. Two routes of oxygen incorporation, i.e., oxidation of the Si substrate surface and initially deposited Ba-rich layer by the residual gas, are identified. On the basis of this knowledge, oxygen concentration is decreased by reducing the holding time of the substrate at high temperatures and by premelting of the source. In addition, X-ray diffraction results show that the decrease in oxygen concentration can lead to an increased proportion of a-axis-oriented grains. [ABSTRACT FROM AUTHOR]

A strained Si/relaxed SiGe heterostructure grown on Si(110) substrate is attractive as a platform for high-hole-mobility Si-based electronic devices. To improve the electrical property, a smoother surface is desirable. In this study, we investigated surface morphology and microstructural aspects of strained Si/relaxed SiGe/Si(110) heterostructures grown by solid-source (SS) molecular beam epitaxy (MBE). It was revealed that SSMBE provides a way to grow strained Si/relaxed SiGe heterostructures with smooth surfaces. In addition, it was found that the strain in the SiGe layer of the SSMBE-grown sample is highly anisotropic whereas that of the GSMBE-grown sample is almost biaxially relaxed. Along with the surface morphology, the symmetry in degree of strain relaxation has implications for the electrical property. Results of a calculation shows that anisotropic strain is preferable for device application since it confines holes solely in the strained Si layer where hole mobility is enhanced. [ABSTRACT FROM AUTHOR]

We have fabricated Si-based photonic nanostructures with submicron sizes by the maskless wet etching of Ge quantum dot (QD) multilayers and demonstrated that the photonic nanostructures result in the enhanced optical absorption in the near-infrared light owing to light trapping. In this study, the optical properties of Si-based photonic nanostructures with surface morphology randomness were calculated by the finite-difference time-domain (FDTD) method. The obtained results indicate that as the degree of randomness increased, the absorption in a near-infrared light range enhanced, suggesting that the enhancement of optical absorption in the near-infrared light by photonic nanostructures is due to the randomness of the nanostructures. [ABSTRACT FROM AUTHOR]

We investigated the effects of grain boundary (GB) misorientation and the change in GB energy on the generation of dislocations during the directional growth of a crystalline silicon ingot. For this purpose, two ingots were grown using artificially designed seeds to introduce plural GBs with and without misorientation from an ideal coincident site lattice boundary. We revealed that dislocations are frequently generated from Σ3 GB with misorientation. On the other hand, dislocations are hardly observed around naturally formed perfect Σ3 GBs. In addition, we found that Σ5 and Σ25 GBs with higher GB energies are not sensitive to misorientation, and few dislocations are found around Σ25 GBs. [ABSTRACT FROM AUTHOR]

The growth mechanism of multicrystalline silicon ingots in directional solidification using single-layer silicon beads (SLSB) coated with Si3N4 was investigated. The grains in the SLSB-seeded ingot were smaller than those in a nonseeded ingot, but still larger than those in a polycrystalline Si (poly-Si)-seeded ingot. The dislocation density in the SLSB-seeded ingot was lower than that in the nonseeded ingot, but higher than that in the poly-Si-seeded ingot. The minority carrier lifetime mapping showed that a higher production yield was obtained in the SLSB-seeded ingot than in the poly-Si-seeded ingot. Grain refinement by the SLSB-seeding method was associated with the increase in nucleation rate. The increase in the surface area of the bottom of the crucible cannot quantitatively explain the grain refinement. Therefore, it is considered that SLSB may supply many nucleation sites because of their uneven structure. [ABSTRACT FROM AUTHOR]

To achieve heterojunction thin film solar cells with high conversion efficiency using BaSi2 as a photoabsorption layer, we investigated the effects of electron affinity (χ) and the band gap (Eg) of a p-type semiconductor on the performance of BaSi2 thin film solar cells with a one-dimensional device simulator, Afors-HET ver. 2.5. By simulation, we found that χ should be less than 3.5 eV and Eg should be in the range of 1.5–2.5 eV to achieve a conversion efficiency of more than 20% in the case of p-type semiconductor/n-type BaSi2 active layer/n++-type BaSi2/electrode. Ultimately, we selected Zn3P2 (Eg = 1.5 eV, χ = 3.2 eV) and SnS (Eg = 1.3 eV, χ = 3.6 eV) as options for p-type materials. In particular, Zn3P2 maintains high efficiency even if a very thin oxide layer exists at the pn heterointerface. The highest conversion efficiency of p-type Zn3P2/n-type BaSi2 thin film solar cells was 23.17% without any light-trapping structure. [ABSTRACT FROM AUTHOR]

Semiconducting BaSi2 is expected as an alternative material for solar cell application because of large absorption coefficients and a suitable bandgap for single-junction solar cells. In this study, the electrical properties of undoped BaSi2 films grown by a simple vacuum evaporation method are explored. The obtained results show that an undoped evaporated film is an n-type semiconductor with a high carrier density of ∼1019 cm−3 and exhibits a metallic behavior. The quality of BaSi2 films is significantly improved by postannealing at 1000 °C with surface covering to prevent oxidation. After annealing, the carrier density of BaSi2 films markedly decreases and the temperature dependence of carrier density changes from being metallic to being semiconducting. By comprehensive structural analysis, it is speculated that postannealing improves the film quality, for example, by decreasing the density of grain boundaries. [ABSTRACT FROM AUTHOR]

600-nm-thick B-doped p-BaSi2 layers were grown on (111)-oriented n-Si substrates by molecular beam epitaxy, and the dependences of the minority carrier lifetime τ and photoresponsivity on the hole concentration p were investigated. p was varied from 1.4 × 1016 to 3.9 × 1018 cm−3. The highest τ of 2 µs was obtained for the sample with the lowest p of 1.4 × 1016 cm−3, reaching two orders of magnitude higher than that of the sample with the highest p of 3.9 × 1018 cm−3. The low-concentration-doped sample also exhibited an excellent external quantum efficiency (EQE) as large as 80% at a wavelength of approximately 800 nm at a reverse bias voltage of 0.2 V. This value is higher than any other EQEs we have ever achieved for BaSi2, showing the great potential of p-BaSi2 as a light absorber in solar cells. [ABSTRACT FROM AUTHOR]

We have successfully grown a polycrystalline orthorhombic BaSi2 film on a Ge(100) substrate by an evaporation method. Deposition of an amorphous Si (a-Si) film on the Ge substrate prior to BaSi2 evaporation plays a critical role in obtaining a high-quality BaSi2 film. By controlling substrate temperature and the thickness of the a-Si film, a crack-free and single-phase polycrystalline orthorhombic BaSi2 film with a long carrier lifetime of 1.5 µs was obtained on Ge substrates. The photoresponse property of the ITO/BaSi2/Ge/Al structure was clearly observed, and photoresponsivity was found to increase with increasing substrate temperature during deposition of a-Si. Furthermore, the BaSi2 film grown on Ge showed a higher photoresponsivity than that grown on Si, indicating the potential application of evaporated BaSi2 on Ge to thin-film solar cells. [ABSTRACT FROM AUTHOR]

To improve the surface quality for photovoltaic applications, we have investigated the effects of post-annealing on the surface structure and carrier lifetime of evaporated BaSi2 films. Structural characterizations by Raman spectroscopy and X-ray diffraction analysis show that there is an optimum post-annealing duration for fabricating a homogeneous film up to around the surface. By detailed surface analysis by X-ray photoelectron spectroscopy, the existence of a surface oxidation layer consisting of BaCO3 and barium silicate is revealed, and the thickness of the oxidation layer is found to be smallest for the optimum post-annealing duration. These surface structural changes are discussed from a thermodynamic viewpoint. Carrier lifetime is also investigated by the microwave-detected photoconductivity decay method, which shows that the structural change around the surface by post-annealing has negligible effects on carrier lifetime, possibly because the silicate layer covers the BaSi2 surface irrespective of post-annealing duration. [ABSTRACT FROM AUTHOR]

Crystalline silicon (c-Si) wafers have found extensive use in photovoltaic applications. In this regard, to enable advanced light manipulation in thin-wafer c-Si solar cells, we demonstrate the fabrication of double-side-textured Si wafers composed of a front-surface photonic nanotexture fabricated with quantum dot arrays and a rear-surface microtexture. The addition of the rear-surface microtexture to a Si wafer with the front-surface photonic nanotexture increases the wafer’s optical absorption in the near-infrared region, thus enabling enhanced light trapping. Excitation spectroscopy reveals that the photoluminescence intensity in the Si wafer with the double-sided texture is higher than that in the Si wafer without the rear-surface microtexture, thus indicating an increase in true optical absorption in the Si wafer with the double-sided texture. Our results indicate that the double-sided textures, i.e., the front-surface photonic nanotexture and rear-surface microtexture, can effectively reduce the surface reflection loss and provide enhanced light trapping, respectively. [ABSTRACT FROM AUTHOR]

Semiconducting barium disilicide (BaSi2), which is composed of earth-abundant elements, has attractive features for thin-film solar cell applications. Both a large absorption coefficient comparable to copper indium gallium diselenide and a minority-carrier diffusion length much larger than the grain size of BaSi2 can be used to improve solar cell properties. In this review article, we explore the potential of semiconducting BaSi2 film for thin-film solar cell applications. We start by describing its crystal and energy band structure, followed by discussing thin-film growth techniques and the optical and electrical properties of BaSi2 films. We use a first-principles calculation based on density-functional theory to calculate the position of the Fermi level to predict the carrier type of impurity-doped BaSi2 films using either a group 13 or 15 element, and compare the calculated results with the experimental ones. Special attention was paid to the minority-carrier properties, such as minority-carrier lifetime, minority-carrier diffusion length, and surface passivation. The potential variations across the grain boundaries measured by Kelvin-probe force microscopy allowed us to detect a larger minority-carrier diffusion length in BaSi2 on Si(1 1 1) compared with BaSi2 on Si(0 0 1). Finally, we demonstrate the operation of p-BaSi2/n-Si heterojunction solar cells and discuss prospects for future development. [ABSTRACT FROM AUTHOR]

We performed numerical calculations of temperature distributions in a furnace and clarified that a simple modification of heat insulators allows the realization of a complex temperature distribution for a parallel arrangement of adjacent dendrite crystals at the initial stage of the floating cast method. The temperature distribution included a unidirectional temperature gradient on the Si melt surface, which led to the preferential nucleation on one side of a square crucible. Numerical simulation was utilized to design crystal growth experiments, and we demonstrated the preferential formation of dendrite crystals on the expected side of the crucible. [ABSTRACT FROM AUTHOR]

We demonstrated the fabrication of modulated surface nanostructures as a new surface texture design for thin wafer solar cells. Using a combination of conventional alkali etching and colloidal lithography, we fabricated surface textures with micrometer and nanometre scales on a Si substrate. These modulated surface nanostructures exhibit reduced surface reflection in a broad spectral range, compared with conventional micrometer textures. We investigated optical absorption using a rigorous coupled wave analysis simulation, which revealed a significant reduction in surface reflection over a broad spectral range and efficient light trapping (comparable to that of conventional micrometer-scale textures) for the modulated nanostructures. We found that the modulated surface nanostructures have a high potential of improving the performance of thin wafer crystalline Si solar cells. [ABSTRACT FROM AUTHOR]

We demonstrate that compressively strained Si/Si1−xCx heterostructures are epitaxially grown on Ar ion implanted Si substrates. The ion-implantation-induced defects are found to promote strain relaxation in the Si1−xCx layers, which accompanies an increase in the substitutional C concentrations. The top Si layers are strained on the Si1−xCx layers for all samples, and thus the increase in the substitutional C concentration based on strain relaxation in the Si1−xCx layers is very important for the control of the compressive strain in the top Si layer. [ABSTRACT FROM AUTHOR]

The effect of passivation films on a Si quantum dot superlattice (QDSL) was investigated to generate high photocurrent in solar-cell applications. Three types of passivation films, sputter-grown amorphous silicon carbide (a-SiC), hydrogenated a-SiC (a-SiC:H), and atomic-layer-deposited aluminum oxide (ALD-Al2O3), were used to passivate the Si QDSLs containing a stack of four 4 nm Si nanodisks (NDs) and 2 nm silicon carbide (SiC) films fabricated by neutral beam etching (NBE). Because of the high surface-to-volume ratio typically present in quantum Si-NDs formed in the top-down NBE process, there is a tendency to form larger surface dangling bonds on untreated Si-ND surfaces as well as to have short distance (<10 nm) between high-aspect-ratio nanopillars of stacked 4 nm Si-NDs/2 nm SiC films, which conventionally sputter SiC films cannot uniformly cover. Therefore, we optimized the passivation techniques with an ALD-Al2O3 film. Scanning electron microscopy (SEM) analysis helped to explain the surface morphology before and after the passivation of the QDSLs. After the completion of the passivation process, the quality of the top surface films of the QDSLs was analyzed from the surface roughness by atomic force microscopy (AFM) analysis, which revealed that ALD-Al2O3 passivated films had the smallest roughness (RMS) of 1.09 nm with respect to sputter-grown a-SiC (RMS: 1.75 nm) and a-SiC:H (RMS: 1.54 nm) films. Conductive atomic force microscopy (CAFM) revealed that ALD-Al2O3 passivation decreased the surface-leakage current as a result of proper passivation of side-wall surface defects in the QDSLs. The carrier transport characteristics were extracted from the QDSLs using the photovoltaic (PV) properties of p++/i/n+ solar cells, where the QDSLs consisted of different passivation layers acting as intermediate layers (i-layers) between the high-doping-density p++ Si (1 × 1020 cm−3) and n+ Si (1 × 1019 cm−3) substrates. High-doping-density p++ Si acted as a hole conductor instead of a photocarrier generator, hence, we could observe the PV properties of the i-layers. The highest short-circuit current density of 4.75 mA cm−2 was generated from the QDSL with the ALD-Al2O3-passivated surface, which is suitable for high-efficiency QD solar cells compared with a-SiC-passivated (0.04 mA cm−2) and a-SiC:H-passivated (0.37 mA cm−2) QDSL surfaces. [ABSTRACT FROM AUTHOR]

We propose a new growth method for quasi-monocrystalline Si that achieves high-quality ingots and a high yield ratio. This method induces defect regions with compose dislocations surrounded by grain boundaries. These functional defects benefit the crystalline quality through impurity gettering, dislocation-propagation blocking, and stress relaxation with plastic deformation. Functional defect regions were grown from designed seeds and introduced to ingot edges where crystalline Si was disposed because of contamination. Preliminary experimentation demonstrated that functional defects could effectively form from the manipulated seeds. We call this method the Seed Manipulation for Artificially Controlled Defect Technique. [ABSTRACT FROM AUTHOR]

We report on the selective growth of vertical silicon nanowire arrays guided by an anodic aluminum oxide (AAO) template without the introduction of any metallic catalyst. Gas-source molecular beam epitaxy using disilane as a source gas was carried out. The growth conditions such as flow rate and growth temperature were changed to optimize the Si nanowire growth. It was found that the selective growth was promoted at a flow rate of 0.5 sccm, whereas the selective growth was poor at high flow rates of 1 and 2 sccm. One-micrometer-long Si nanowire arrays were obtained at a low flow rate of 0.5 sccm only at the growth temperature of 700 °C. The obtained Si grown at a temperature of 650 °C exhibited conglomerated structures with Si grains piled up inside the nanopores of the AAO template. We found that increasing the growth temperature and decreasing the flow rate are useful for improving the growth selectivity. [ABSTRACT FROM AUTHOR]

We fabricate photonic nanostructures by combination of growth of Ge quantum dot (QD) multilayer structures and maskless wet etching, and demonstrate that surface dip diameters in photonic nanostructures can be tuned by changing the growth temperature of QDs. Increase of growth temperature from 700 to 800 °C leads to the enlargement of surface dip diameters by a factor of more than two through the increase of QDs lateral sizes. In addition, the wet etching rate of the stacked Ge QDs depends on the Ge composition in the QDs and the dislocation generation in the Ge QD multilayer structures. The surface dips with larger diameters leads to an enhanced optical absorption in the near-infrared region. These results show that varying the growth temperature of QDs is effective to control the photonic nanostructures geometry and optical properties. [ABSTRACT FROM AUTHOR]

We attempt to clarify the effect of phosphorus diffusion gettering (PDG) on the differences in microstructures of multicrystalline silicon (mc-Si) wafers. From the results of the floating cast method, the obtained microstructure of mc-Si was determined to contain unique microstructures, consisting of large faceted dendrites and small grains with low and high dislocation densities. The PDG efficiency was evaluated through the change in minority carrier lifetime. As a result, a stronger positive PDG effect was found for large faceted dendrites with low dislocation density, attributed to a small number of defects that can act as recombination centers of carriers. Lifetime improvement was also found in dislocation regions possibly owing to the redistribution of interstitial metals. These results suggest the importance of controlling the microstructure of mc-Si, which could strongly affect the PDG efficiency and existence of incorporated impurities. [ABSTRACT FROM AUTHOR]

We have recently developed a new structure for solar cells that consists of photonic nanostructures coupled with vertically aligned Ge quantum dots on a crystalline Si substrate. For the fabrication of a solar cell device, a heterojunction with a-Si:H was chosen because its processing temperature is less than 200 °C, at which point Ge atoms cannot diffuse into Si layers. In this study, we developed a guideline for the most appropriate cell structures to take advantage of the Ge layer by fabricating heterojunction solar cells with various structures. As a result, we confirmed that the carriers absorbed in Ge quantum dots contributed output current when Ge quantum dots are fabricated on the pn-junction side. Hence, the presence of built-in potential in a Ge dot layer was found to be necessary to extract the carriers generated in the Ge layer. In addition, carrier transport in Ge quantum dots is improved under conditions of reverse bias. Therefore, the electrical field in the Ge layer is a key parameter to improve solar cell performance in our proposed structure. [ABSTRACT FROM AUTHOR]

We report on effects of anodization process of anodic aluminum oxide (AAO) template on selective growth of Si nanowires using gas source molecular beam epitaxy. By switching off the anodization currents at several points, the structure of Si surface was altered, which was critical factor for the Si nanowire array growth. With reasonable switched-off point of anodization current at R of 10%, the selective growth of Si nanowire growth was favorable and 1-µm-long Si nanowire arrays were successfully grown. It was interesting that their structures were epitaxial structures; whereas, long anodization of AAO was found to be unfavorable for growth of Si nanowires due to anodization of Si surface. It caused the density of Si nanowire arrays to be modest and their structures were polycrystal structure. [ABSTRACT FROM AUTHOR]

We report on the successful fabrication of single-phase polycrystalline BaSi2 films on Si(111) substrates by a simple vacuum evaporation method using BaSi2 granules. Substrate heating was found to be the key, and high substrate temperatures are required to realize single-phase BaSi2. The underlying mechanism is discussed considering the composition of vapor flux, and we confirmed that the vapor flux is Ba-rich at the initial stage of evaporation. The reevaporation of excess Ba atoms or their reaction with Si substrates would be responsible for the realization of stoichiometric single-phase BaSi2, which explains why high substrate temperatures are necessary. [ABSTRACT FROM AUTHOR]

We have realized BaSi2 films by a simple vacuum evaporation technique for solar cell applications. X-ray diffraction analysis shows that single-phase BaSi2 films are formed on alkali-free glass substrates at 500 and 600 °C while impurity phases coexist on quartz or soda-lime glass substrates or at a substrate temperature of 400 °C. The mechanism of film growth is discussed by analyzing the residue on the evaporation boat. An issue on the fabricated films is cracking due to thermal mismatch, as observed by secondary electron microscopy. Optical characterizations by transmittance and reflectance spectroscopy show that the evaporated films have high absorption coefficients, reaching 2 × 104 cm−1 for a photon energy of 1.5 eV, and have indirect absorption edges of 1.14–1.21 eV, which are suitable for solar cells. The microwave-detected photoconductivity decay measurement reveals that the carrier lifetime is approximately 0.027 µs, corresponding to the diffusion length of 0.84 µm, which suggests the potential effective usage of photoexcited carriers. [ABSTRACT FROM AUTHOR]