BaSi2 Thin Films Research Articles

Investigation of defects in BaSi2 thin film on Si prepared by co-sputtering technique

Defects in undoped BaSi2 thin films prepared by co-sputtering technique were investigated. The growth rate ratio between the sputtering of Ba and BaSi2 targets (GRBa/GRBaSi2) was varied between 0 and 0.5. Sample i (GRBa/GRBaSi2 = 0) exhibited a large root-mean-square (rms) roughness of 102 nm due to the presence of high density of 3D features which is attributed to Si precipitates. The rms roughness was significantly reduced to 9–26 nm for samples grown at GRBa/GRBaSi2 > 0.18. Raman data reveal the presence of Si vacancy (VSi) and Ba antisite (BaSi) defects. For GRBa/GRBaSi2 > 0.18, the Ag mode becomes blueshifted while the linewidth becomes narrower, suggesting a reduction in VSi. A similar trend was also observed for BaSi. At 10 K, defect-assisted emissions were observed from sample i with photoluminescence (PL) peak energy of 0.92 and 1.27 eV. Power dependent PL analyses suggested that the emissions were due to VSi-to-BaSi and VSi-to-free hole transitions. However, samples grown at GRBa/GRBaSi2 ≥ 0.18 showed negligible PL emission, possibly due to low density of VSi. Based on the Tauc and Urbach tail analyses, the band gap of all samples is ∼1.31 eV, regardless of the GRBa/GRBaSi2. However, samples with less structural disorder showed smaller characteristic energy of the absorption edge (Eo).

Constructing the composition ratio prediction model using machine learning for BaSi2 thin films deposited by thermal evaporation

A composition ratio prediction model for BaSi2 thin films deposited by thermal evaporation was constructed using machine learning. BaSi2 was prepared by thermal evaporation in a vacuum chamber, and the composition ratio was measured by energy-dispersive X-ray spectroscopy. The results show that the composition ratio is affected by various experimental parameters. To consider these parameters, kernel ridge regression was performed with Si/Ba ratio as the objective variable, and with experimental parameters as explanatory variables. A good fitting result was obtained by kernel ridge regression. The next step was to select a kernel function. We evaluated four types of kernel functions, and confirmed that two of them, the polynomial kernel and the sigmoid kernel, have relatively high prediction accuracy. Then we investigated different combinations of explanatory variables and found the best combination with the highest generalization performance. From the above, a composition ratio prediction model with a mean absolute error of less than 0.2 was obtained.

Fabrication of BaSi2 homojunction diodes on Nb-doped TiO2 coated glass substrates by aluminum-induced crystallization and two-step evaporation method

BaSi2 homojunction diodes on Nb-doped TiO2 (TiO2:Nb) coated glass substrates were fabricated using aluminum-induced crystallization (AIC) and a two-step evaporation method. From Raman scattering spectra, the growth of BaSi2 on TiO2:Nb was confirmed when the thickness of poly-Si grown by AIC (AIC-Si) was more than 150 nm. The partial formation of BaSi2 diodes was confirmed from the samples prepared at temperature during AIC T AIC = 475 °C–525 °C. The long-wavelength edge of photoresponsivity of the diodes was located around 950 nm, which corresponds to the bandgap of BaSi2 of 1.3 eV, suggesting that this photocurrent is derived from BaSi2 thin films. At T AIC = 500 °C, the maximum value of photoresponsivity was obtained. Since the largest grains in AIC-Si were also obtained at T AIC = 500 °C, these results suggest that larger grain of AIC-Si leads to the improvement of the quality of BaSi2 thin films themselves and the performance of BaSi2 diodes.

Recent Progress Toward Realization of High‐Efficiency BaSi2 Solar Cells: Thin‐Film Deposition Techniques and Passivation of Defects

Safe, stable, and earth‐abundant materials for solar cell applications are of particular importance to realize a decarbonized society. Semiconducting barium disilicide (BaSi2), which is composed of nontoxic and earth‐abundant elements, is an emerging material to meet this requirement. BaSi2 has a bandgap of 1.3 eV that is suitable for single‐junction solar cells, a large absorption coefficient exceeding that of chalcopyrite, and inactive grain boundaries. This review is started by describing the recent progress of BaSi2 thin‐film deposition techniques using radio‐frequency sputtering and discuss the high photoresponsivity of BaSi2 thin films. Special attention is paid to passivation of the defects in BaSi2 films by hydrogen or carbon doping. Ab initio studies based on density‐functional theory are then used to calculate the positions of the localized defective states and the Fermi level to discuss the experimentally obtained passivation effects. Finally, the issues that need to be resolved toward realization of high‐efficiency BaSi2 solar cells are addressed.

Fabrication of heterojunction crystalline Si solar cells with BaSi2 thin films prepared by a two-step evaporation method

We have fabricated p-type BaSi2 (p-BaSi2)/n-type crystalline Si (n-c-Si) heterojunction solar cells using thermal evaporation. To control the defect around the heterointerface, samples were fabricated by two methods using different current profiles during evaporation. The one-step method was designed to avoid supplying Ba-rich vapor, and the two-step method was designed to intentionally introduce Ba-rich vapor at the first step. Deep-level transient spectroscopy measurements revealed that defect densities in the n-c-Si side were almost the same for both samples. Open-circuit voltage (V oc) was successfully improved from 319 to 463 mV by the two-step method. This may be due to the increase of carrier density in p-BaSi2 prepared using a two-step method. As a result of optimization, the conversion efficiency of 6.23% was achieved by the heterojunction solar cells.

Epitaxial growth of BaSi2 thin films by co-sputtering of Ba and Si for solar cell applications

A new growth method for BaSi2 thin film has been developed by co-sputtering Ba and Si to solve the problem that it is difficult to sputter BaSi2 epitaxial films by a single BaSi2 target. A template layer was first optimized for the subsequent BaSi2 deposition. X-ray diffraction results revealed that BaSi2 epitaxial films with high crystalline quality have been achieved under different growth temperatures between 500 °C–600 °C and different growth rates from 9.37–16.7 nm min−1. Compared with molecular beam epitaxy, the growth rate of BaSi2 was increased by more than one order of magnitude. The new growth method provides a high-speed, low-cost way for the growth of high-quality BaSi2 thin films. BaSi2-based devices such as an n-BaSi2/p-Si heterojunction diode and a Ag/n-BaSi2 Schottky junction diode were also fabricated. We demonstrated the obvious rectifying properties in these junctions, which will be a guide to design and fabricate BaSi2 thin-film solar cells.

Structural and electronic properties of BaSi2(100) thin film on Si(111) substrate

BaSi2, which can be grown on Si(111) substrate by molecular beam epitaxy experimentally, holds great promise for solar-cells. Here, we report a detailed ab initio study on the structural and electronic properties of BaSi2(100) thin films on Si(111) substrate. A high stable interface structure with bond breaking of Si4-tetrahedra at interface is obtained by ab initio molecular-dynamics simulations. We find that the bond breaking of Si4-tetrahedra at interface play a key role to saturate the dangling bonds of Si(111) substrate. Electronic band structures and band-decomposed charge density distributions reveal that such BaSi2(100) thin film structures are semiconductor with an interface band gap of 0.71–0.75 eV and a large surface band gap of 1.17–1.27 eV, closing to the bulk BaSi2 band gap of 1.25 eV. These results provide an excellent explanation for the recent experimental observations on the BaSi2-based thin films on Si(111) substrate.

Effect of Si substrate modification on improving the crystalline quality, optical and electrical properties of thermally-evaporated BaSi2 thin-films for solar cell applications

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.

Influence of the time-dependent vapor composition on structural properties of the BaSi2 thin films fabricated by vacuum evaporation

We developed an original method to measure the time-dependent vapor composition, and applied this method to vacuum evaporation of BaSi2 thin films at three source heating currents. At high heating current, the vapor was found to change from Ba-rich to Si-rich, and the resultant film is of high surface crystallinity. On the other hand, Si-rich vapor did not appear at low heating current, and the film was more a-axis oriented. Based on these results, the formation mechanisms of BaSi2 films and a guideline for further improvement are discussed.

Point defects in BaSi2 thin films for photovoltaic applications studied by positron annihilation spectroscopy

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

Properties of sputtered BaSi2 thin films annealed in vacuum condition

As a potential absorber candidate for high-efficient solar cell applications, BaSi2 films are confronted with issues of surface oxidation associated with the high-temperature annealing. Herein, BaSi2 films are deposited by the sputtering technique. A vacuum annealing process is subsequently carried out to crystallize sputtered BaSi2 films. Raman spectroscopy is used to study surface structures and crystalline quality. Elemental depth profile is measured by Auger Electron spectroscopy to understand the compositions of films. Optical and electrical properties are further investigated to reveal the effects of annealing condition. Applying vacuum annealing condition can effectively suppress diffusions of Ba and ensures a stochiometric BaSi2 layer. However, surface oxidation still occurs even in the vacuum environment owing to the high reactivity of Ba. Further attempts to prevent BaSi2 surface oxidation may focus on the combination of other methods, such as capping layer and reducing atmosphere, with vacuum (or low-pressure) annealing condition.

Formation of poly-crystalline BaSi2 thin films by pulsed laser deposition for solar cell applications

A new growth method for BaSi2 thin films by using pulsed laser deposition (PLD) on transparent SiO2 and CaF2 substrates has been developed. X-ray diffraction and Raman spectroscopy revealed the poly-crystalline property of the deposited films. By introducing a thin Si buffer layer on the SiO2 substrate, the crystalline quality of BaSi2 thin films were improved. BaSi2 thin films exhibited a Ba/Si ratio very close to 0.5, indicating the good stoichiometry control of PLD growth. The absorption coefficient of the poly-BaSi2 thin film reached 105 cm−1 and its band gap was deduced to be 1.32 eV, which are similar to those grown by molecular beam epitaxy or sputtering. A maximum photoresponsivity of 12.5 mA/W was achieved in the BaSi2 thin film, which implies the potential of PLD-deposited BaSi2 for thin film solar cell applications.

Effects of lattice parameter manipulations on electronic and optical properties of BaSi2

We present comprehensive experimental and theoretical investigation on a band-gap engineering and modification in optical properties of BaSi2 – a new and very promising material for solar cell fabrication. BaSi2 thin films have been synthesized by molecular beam epitaxy with various deposition rates of Si and Ba. Changes in their band gaps and shifts in absorption edges with respect to alteration in the a lattice parameter have been investigated by optical measurements. It is possible to shrink a by about 0.003 nm (or 0.3%), while the other lattice parameters are locked by the epitaxial relationship with a Si(111) substrate, that leads to the gap reduction from 1.28 eV to 1.20 eV. By means of ab initio calculations we explore a possibility to manipulate band-gap values in BaSi2 along with the corresponding shift in the absorption edge by changing its a, b and c lattice parameters. It is revealed that an increase in any of the lattice parameters provides band-gap enlargement while the opposite trend is observed when decreasing the lattice parameters. Numerically uniaxial lattice strain of 3% can provide variations in the band gap up to 0.1 eV. We also discuss possible reasons for a variation and applicability of the band-gap engineering in BaSi2 by strain.

Fabrication and properties characterization of BaSi2 thin-films thermally-evaporated on Ge (100) modified substrates

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.

Oxidation-Induced Structure Transformation: Thin-Film Synthesis and Interface Investigations of Barium Disilicide toward Potential Photovoltaic Applications.

Barium disilicide (BaSi2) has been regarded as a promising absorber material for high-efficiency thin-film solar cells. However, it has confronted issues related to material synthesis and quality control. Here, we fabricate BaSi2 thin films via an industrially applicable sputtering process and uncovered the mechanism of structure transformation. Polycrystalline BaSi2 thin films are obtained through the sputtering process followed by a postannealing treatment. The crystalline quality and phase composition of sputtered BaSi2 are characterized by Raman spectroscopy and X-ray diffraction (XRD). A higher annealing temperature can promote crystallization of BaSi2, but also causes an intensive surface oxidation and BaSi2/SiO2 interfacial diffusion. As a consequence, an inhomogeneous and layered structure of BaSi2 is revealed by Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). The thick oxide layer in such an inhomogeneous structure hinders further both optical and electrical characterizations of sputtered BaSi2. The structural transformation process of sputtered BaSi2 films then is studied by the Raman depth-profiling method, and all of the above observations come to an oxidation-induced structure transformation mechanism. It interprets interfacial phenomena including surface oxidation and BaSi2/SiO2 interdiffusion, which lead to the inhomogeneous and layered structure of sputtered BaSi2. The mechanism can also be extended to epitaxial and evaporated BaSi2 films. In addition, a glimpse toward future developments in both material and device levels is presented. Such fundamental knowledge on structural transformations and complex interfacial activities is significant for further quality control and interface engineering on BaSi2 films toward high-efficiency solar cells.

Simple method for significant improvement of minority-carrier lifetime of evaporated BaSi2 thin film by sputtered-AlOx passivation

A novel surface passivation of AlOx on BaSi2 thin films fabricated by vacuum evaporation for solar cell application has been developed. The minority-carrier lifetime of evaporated BaSi2 film is shorter than that of epitaxial film. This lifetime has been improved by forming AlOx passivation layer on the BaSi2 surface by RF sputtering of Al followed by oxidation in air. A drastic improvement of the lifetime upon the passivation of AlOx was observed up to 15μs with 3-nm of AlOx layer after rapid thermal annealing at 475°C for 15min. The dependence of lifetime on passivation layer thickness disappeared for thickness higher than 9nm, especially after annealing, showing the chemical passivation effect of AlOx. The result confirmed that AlOx is a promising passivation-layer for improving properties of BaSi2 film.

Donor and acceptor energy levels in impurity Sb-, In-, Ag- and Cu-doped semiconducting BaSi2 thin films for device applications

In this article donor and acceptor energy levels due to the impurity Sb-, In-, Ag- and Cu-doped BaSi2 films grown in the ultra-high vacuum, UHV molecular beam epitaxy (MBE) were investigated. The temperature dependence of electron or hole concentrations indicated that the acceptor energy levels in impurity, In-, and Ag-doped BaSi2 are 86 meV, and 126 meV, respectively, and the donor energy levels in impurity Cu-, and Sb-doped BaSi2 are 35 meV, and 47 meV respectively. Two shallow donor energy levels of 47 meV and 35 meV respectively due to Sb and Cu impurity atoms in n-type BaSi2 were successfully identified for the device applications including photovoltaic.

Fabrication of BaSi2 thin films capped with amorphous Si using a single evaporation source

The capping of a BaSi2 film with amorphous Si (a-Si) is a key technology for the solar cell applications of BaSi2. In this study, we have investigated the two-step evaporation process of BaSi2 to fabricate the BaSi2 film capped with an a-Si layer on a Si(100) substrate. The deposition of Si is possible as the second step because a small amount of Si remains after the first BaSi2 evaporation step. Raman and X-ray photoelectron spectroscopies show that a-Si is formed by the two-step evaporation on the sample surface while Ba and carbonate species are not present on the surface, which are detected when the BaSi2 film is fabricated without the second step. Depth composition profiles determined by X-ray photoelectron spectroscopy clearly demonstrate the formation of the layered structure of a-Si and BaSi2. Owing to the a-Si capping, O concentration in the BaSi2 film is lower than that without a-Si, as evidenced by the composition profiles and energy-dispersive X-ray spectroscopy results. In addition, the microwave-detected photoconductivity decay analysis shows that the carrier lifetime in the a-Si/BaSi2 samples is similar to or slightly higher than the BaSi2 film without a-Si layer possibly due to the surface passivation effect and the reduction of oxygen concentration.

Investigation of p-type emitter layer materials for heterojunction barium disilicide thin film solar cells

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.

Preferred orientation of BaSi<sub>2</sub> thin films fabricated by thermal evaporation

Thermal evaporation is a simple and high-speed method to grow a BaSi2 thin film, which is an emerging candidate for an absorber-layer material of thin-film solar cells. In this study, we have investigated the preferred orientation of BaSi2 films grown at substrate temperatures of 600–700 °C by thermal evaporation using X-ray diffraction. 2θ–ω scans show that peaks derived from (100) orientation grow steadily with increasing substrate temperature. By X-ray pole figure analysis, the (100)-oriented crystals are proven to be epitaxially grown on Si(100) with two variants. The reason of epitaxial growth is discussed from the epitaxial temperature.