BaSi2 Layers Research Articles

Structural design of BaSi2 solar cells with a-SiC electron-selective transport layers

Sputter-deposited polycrystalline BaSi2 films capped with a 5 nm thick a-SiC layer showed high photoresponsivity. This means that the a-SiC layer functions as a capping layer to prevent surface oxidation of BaSi2. Based on the measured absorption edge, the electron affinity of the a-SiC layer, and the work function of the TiN layer, the a-SiC is considered to act as an electron transport layer (ETL) for the BaSi2 light absorber layer/a-SiC interlayer/TiN contact structure in a BaSi2 solar cell. Using a 10 nm thick p+-BaSi2 layer as a hole transport layer, we investigated the effect of the BaSi2/a-SiC layered structure on the device performance of a BaSi2-pn homojunction solar cell by a one-dimensional device simulator (AFORS-HET v2.5). The a-SiC ETL effectively separates photogenerated carriers and allows transport of electrons while blocking holes to achieve an efficiency of 22% for a 500 nm thick BaSi2 light absorber layer.

Improved conversion efficiency of p-type BaSi2/n-type crystalline Si heterojunction solar cells by a low growth rate deposition of BaSi2

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

Zn1-xGexOy Passivating Interlayers for BaSi2 Thin-Film Solar Cells.

BaSi2 is a promising absorber material for next-generation thin-film solar cells (TFSCs). For high-efficiency TFSCs, a suitable interlayer should be found for every light absorber. However, such an interlayer has not been studied for BaSi2. In this study, we investigated amorphous Zn1-xGexOy films as interlayers for BaSi2. The Zn/Ge atomic ratio in the Zn1-xGexOy film and the optical band gap depend on the substrate temperature during sputtering deposition. A suitable i-Zn1-xGexOy/BaSi2 heterointerface with spike-type conduction band offset was achieved when Zn1-xGexOy was deposited on BaSi2 at 50 °C. Furthermore, photoresponsivity measurements revealed that Zn1-xGexOy has an excellent surface passivation effect on BaSi2. When the thickness of Zn1-xGexOy was 2 nm, a high photoresponsivity of 0.9 A/W was obtained for a 500 nm thick BaSi2 layer at a wavelength of 780 nm under an applied bias voltage of 0.5 V between the front and rear electrodes, where the photoresponsivity in the short-wavelength region was significantly improved compared with that of BaSi2 capped with an amorphous Si layer. X-ray photoelectron spectroscopy revealed that the Zn1-xGexOy films suppressed the oxidation of the BaSi2 surface, decreasing the carrier recombination rate. This is the first demonstration of passivation interlayers for BaSi2 with suitable band alignment for carrier transport and surface passivation effects.

Analytical study of electrical performance of SiGe-based n+-p-p+ solar cells with BaSi2 BSF structure

This manuscript reports on an analytical study required to determine and enhance the effect of the high–low junction based back surface field (BSF) of thin n-CdS/p-SiGe/p + -BaSi2 heterojunction solar cell. A 0.3 µm thick BSF layer made of extensively available and low-cost barium silicide (BaSi2) is introduced into the basic SiGe solar cell for the first time. SCAPS-1D simulator is employed to analyze different parameters such as open circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), quantum efficiency (QE) and conversion efficiency (η) of the device. The SiGe absorber layer thickness is varied from 2 to 50 µm to analyze the device performance, however optimum results are observed between 10 and 15 µm. The BSF BaSi2 layer thickness changing from 0.1 to 0.8 µm to check the viability of device for optimal efficiency. The present structure anticipates an efficiency of 17.05% for BSF layer thickness of 0.8 µm. In addition, the use of SiGe earth abundant material instead of CIGS having rare earth metals indium and gallium, reduces the overall fabrication cost of the solar cell.

Effects of molecular beam epitaxy growth conditions on grain size and lattice strain in a-axis-oriented BaSi2 films

We grow 0.5 μm thick a-axis oriented BaSi2 films on Si(111) substrates by a conventional two-step growth method and compare their properties with those grown by a three-step growth method. Both grow methods consist of the initial growth of a BaSi2 template layer and the following molecular beam epitaxy (MBE) to form BaSi2 films. In the two-step growth method, Ba-to-Si deposition rate ratios (RBa/RSi) were varied in the range 0.4–4.7 during MBE. On the other hand, in the three-step growth method, MBE-grown BaSi2 layers are composed of BaSi2 films grown first under Ba-rich conditions (RBa/RSi = 4.0), followed by those grown under Si-rich conditions (RBa/RSi = 1.2). The grain size of BaSi2 films by the three-step growth method was much smaller than those grown by the two-step growth method. To our surprise, however, much higher photoresponsivity was obtained for BaSi2 films grown by the three-step growth method.

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.

Fabrication of SnS/BaSi2 heterojunction by thermal evaporation for solar cell applications

Heterojunction of p-type SnS and n-type BaSi2 is a promising structure for solar cell applications. In this study, we attempted to fabricate SnS/BaSi2 heterojunctions by thermal evaporation and postannealing. Microstructure and composition analyses reveal that when BaSi2 and SnS layers are formed at 650 ◦C and 250 ◦C, which are suitable for single films, the SnS layer is porous and the interface oxygen concentration is high. Postannealing at 400 ◦C is found effective to densify the SnS layer. Combining low-temperature depositions at 150 ◦C with postannealing at 400 ◦C, we succeeded in fabricating a dense SnS/BaSi2 heterojunction by suppressing the oxygen incorporation at the interface. Current–voltage measurement shows that the fabricated SnS/BaSi2 structures are not rectifying, which is discussed by considering the effects of interdiffusion.

Improving the photoresponse spectra of BaSi2 layers by capping with hydrogenated amorphous Si layers prepared by radio-frequency hydrogen plasma

We studied the surface passivation effect of hydrogenated amorphous silicon (a-Si:H) layers on BaSi2 films. a-Si:H was formed by an electron-beam evaporation of Si, and a supply of atomic hydrogen using radio-frequency plasma. Surface passivation effect was first investigated on a conventional n-Si(111) substrate by capping with 20 nm-thick a-Si:H layers, and next on a 0.5 μm-thick BaSi2 film on Si(111) by molecular beam epitaxy. The internal quantum efficiency distinctly increased by 4 times in a wide wavelength range for sample capped in situ with a 3 nm-thick a-Si:H layer compared to those capped with a pure a-Si layer.

Decrease in electrical contact resistance of Sb-doped n+-BaSi2 layers and spectral response of an Sb-doped n+-BaSi2/undoped BaSi2 structure for solar cells

We investigated how the electron concentration n in a 300-nm-thick Sb-doped n+-BaSi2 layer grown by molecular beam epitaxy affected the contact resistance RC to surface electrodes (Al, indium–tin-oxide). As the n of n-BaSi2 increased, RC decreased and reached a minimum of 0.019 Ω cm2 at n = 2.4 × 1018 cm−3 for the Al electrodes. This value was more than 1 order of magnitude smaller than that obtained for Al/B-doped p-BaSi2. We believe that this significant decrease in RC came from Sb segregation. Furthermore, the internal quantum efficiency (IQE) spectrum was evaluated for an Sb-doped n+-BaSi2 (20 nm)/undoped BaSi2 (500 nm)/n+-Si(111) structure. Its IQE reached as high as ∼50% over a wide wavelength range under a small bias voltage of 0.1 V applied between the top and bottom electrodes.

Boron-doped p-BaSi2/n-Si solar cells formed on textured n-Si(0 0 1) with a pyramid structure consisting of {1 1 1} facets

BaSi2 films were fabricated on textured Si(001) substrates that consisted of {111} facets using molecular beam epitaxy. The light-trapping effect of these films and their performance when incorporated into solar cells were measured. X-ray diffraction and reflectivity measurements showed that the BaSi2 films were grown epitaxially on the textured Si(001) substrate and confirmed the light-trapping effect. The critical thickness over which BaSi2 relaxes increased from approximately 50 to 100nm when comparing the BaSi2 films on a flat Si(111) substrate and the textured substrate, respectively. p-BaSi2/n-Si solar cells were fabricated with varying BaSi2 layer thickness and with hole concentrations in the range between 2.0×1018 and 4.6×1018cm−3. These cells exhibited a maximum energy conversion efficiency of 4.62% with an open-circuit voltage of 0.30V and a short-circuit current density of 27.6mA/cm2 when the p-BaSi2 layer was 75nm-thick. These results indicated that the use of BaSi2 films on textured Si(001) substrates in solar cells shows great promise.

Fabrication and characterizations of nitrogen-doped BaSi2 epitaxial films grown by molecular beam epitaxy

Nitrogen doped BaSi2 layers are grown on high-resistivity n-Si (111) substrates by molecular beam epitaxy using a radio-frequency nitrogen plasma. The nitrogen concentration measured by secondary ion mass spectrometry is homogeneous throughout the grown layers. The carrier concentration is measured by Hall measurement using the van der Pauw method. Nitrogen-doped BaSi2 shows n- or p-type conductivity, depending on the intensity of nitrogen plasma. The hole concentration is of the order of 1016–1017cm−3 at room temperature. The acceptor level is estimated to be approximately 64meV from the temperature dependence of hole concentration. The temperature dependence of resistivity is explained by variable-range hopping conduction in p-BaSi2. First-principle calculation suggests that the nitrogen atoms are most likely to occupy the interstitial site in BaSi2.

Growth of BaSi2 continuous films on Ge(111) by molecular beam epitaxy and fabrication of p-BaSi2/n-Ge heterojunction solar cells

We grew BaSi2 films on Ge(111) substrates by various growth methods based on molecular beam epitaxy (MBE). First, we attempted to form BaSi2 films directly on Ge(111) by MBE without templates. We next formed BaSi2 films using BaGe2 templates as commonly used for MBE growth of BaSi2 on Si substrates. Contrary to our prediction, the lateral growth of BaSi2 was not promoted by these two methods; BaSi2 formed not into a continuous film but into islands. Although streaky patterns of reflection high-energy electron diffraction were observed inside the growth chamber, no X-ray diffraction lines of BaSi2 were observed in samples taken out from the growth chamber. Such BaSi2 islands were easily to get oxidized. We finally attempted to form a continuous BaSi2 template layer on Ge(111) by solid phase epitaxy, that is, the deposition of amorphous Ba–Si layers onto MBE-grown BaSi2 epitaxial islands, followed by post annealing. We achieved the formation of an approximately 5-nm-thick BaSi2 continuous layer by this method. Using this BaSi2 layer as a template, we succeeded in forming a-axis-oriented 520-nm-thick BaSi2 epitaxial films on Ge substrates, although (111)-oriented Si grains were included in the grown layer. We next formed a B-doped p-BaSi2(20 nm)/n-Ge(111) heterojunction solar cell. A wide-spectrum response from 400 to 2000 nm was achieved. At an external bias voltage of 1 V, the external quantum efficiency reached as high as 60%, demonstrating the great potential of BaSi2/Ge combination. However, the efficiency of a solar cell under AM1.5 illumination was quite low (0.1%). The origin of such a low efficiency was examined.

Evaluation of band offset at amorphous-Si/BaSi2 interfaces by hard x-ray photoelectron spectroscopy

The 730 nm-thick undoped BaSi2 films capped with 5 nm-thick amorphous Si (a-Si) intended for solar cell applications were grown on Si(111) by molecular beam epitaxy. The valence band (VB) offset at the interface between the BaSi2 and the a-Si was measured by hard x-ray photoelectron spectroscopy to understand the carrier transport properties by the determination of the band offset at this heterointerface. We performed the depth-analysis by varying the take-off angle of photoelectrons as 15°, 30°, and 90° with respect to the sample surface to obtain the VB spectra of the BaSi2 and the a-Si separately. It was found that the barrier height of the a-Si for holes in the BaSi2 is approximately −0.2 eV, whereas the barrier height for electrons is approximately 0.6 eV. This result means that the holes generated in the BaSi2 layer under solar radiation could be selectively extracted through the a-Si/BaSi2 interface, promoting the carrier separation in the BaSi2 layer. We therefore conclude that the a-Si/BaSi2 interface is beneficial for BaSi2 solar cells.

Measurement of valence-band offset at native oxide/BaSi2 interfaces by hard x-ray photoelectron spectroscopy

Undoped n-type BaSi2 films were grown on Si(111) by molecular beam epitaxy, and the valence band (VB) offset at the interface between the BaSi2 and its native oxide was measured by hard x-ray photoelectron spectroscopy (HAXPES) at room temperature. HAXPES enabled us to investigate the electronic states of the buried BaSi2 layer non-destructively thanks to its large analysis depth. We performed the depth-analysis by varying the take-off angle (TOA) of photoelectrons as 15°, 30°, and 90° with respect to the sample surface and succeeded to obtain the VB spectra of the BaSi2 and the native oxide separately. The VB maximum was located at −1.0 eV from the Fermi energy for the BaSi2 and −4.9 eV for the native oxide. We found that the band bending did not occur near the native oxide/BaSi2 interface. This result was clarified by the fact that the core-level emission peaks did not shift regardless of TOA (i.e., analysis depth). Thus, the barrier height of the native oxide for the minority-carriers in the undoped n-BaSi2 (holes) was determined to be 3.9 eV. No band bending in the BaSi2 close to the interface also suggests that the large minority-carrier lifetime in undoped n-BaSi2 films capped with native oxide is attributed not to the band bending in the BaSi2, which pushes away photogenerated minority carriers from the defective surface region, but to the decrease of defective states by the native oxide.

Structural and electrical characterizations of crack-free BaSi2 thin films fabricated by thermal evaporation

The BaSi2 semiconductor is a promising candidate for an earth-abundant solar cell absorber. In this study, we have realized a crack-free BaSi2 film by a simple thermal evaporation technique on a CaF2 substrate at a growth temperature of 500°C for electrical characterization. A Si layer preliminarily formed on the substrate by sputtering is a key to obtain stoichiometric BaSi2 film. Detailed structural characterization of the evaporated films with different Si layer thicknesses by X-ray diffraction, scanning and transmission electron microscopy demonstrates that a crack-free 370-nm-thick BaSi2 film is formed by consuming the Si layer. It is observed that the 90-nm-thick bottom part is microcrystalline and contains Ar atoms, which come from Si deposition atmosphere. The surface of the BaSi2 layer is found to be covered by an amorphous Si layer due to Si-rich vapor at the last stage of evaporation. Electrical properties of the BaSi2 film are revealed by Hall measurement and the electron density and mobility are found to be 6×1020cm−3 and 0.04cm2/V⋅s, respectively. Owing to a better crystalline quality contributed by the preliminarily-deposited Si layer, minority-carrier lifetime of the evaporated film (0.6μs) is twenty times longer than previous films on glass substrates.

N-type doping of BaSi2 epitaxial films by arsenic ion implantation through a dose-dependent carrier generation mechanism

Arsenic doping by ion implantation and thermal annealing of the BaSi2 epitaxial films grown on Si(111) substrates has been studied. Raman spectroscopy shows that a structural change can occur by annealing at 500°C after As implantation with as high dose as 1.0×1015cm−2. This structural change is shown to result in the formation of an altered layer after annealing at the surface by cross-sectional scanning electron microscopy. Secondary ion mass spectroscopy reveals that the altered layer contains a significant amount of O atoms. With the altered layer present, average electron density up to 6.0×1019cm−3 has been realized, which is revealed by the Hall measurement. On the other hand, without the altered layer, high resistance of the BaSi2 layer prevented the electrical characterization. Current path is discussed by theoretically calculating the band alignment and resistance, and possible highest electron density without the altered layer is extracted to be less than 2×1017cm−3.

Precipitation control and activation enhancement in boron-doped p+-BaSi2 films grown by molecular beam epitaxy

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

Photoresponse properties of undoped BaSi2 epitaxial layers on n+-BaSi2/p+-Si(001) by molecular beam epitaxy

We investigated the photoresponse properties of 500-nm-thick undoped BaSi2 epitaxial layers formed on Sb-doped n+-BaSi2/p+-Si heterostructures by molecular beam epitaxy using Si(001) substrates. The external quantum efficiency (EQE) reached approximately 5% when 1 V was applied between the top and bottom electrodes. However, the EQE decreased to approximately 0.2% after inserting a solid-phase-epitaxy crystalline Si (c-Si) layer between the undoped and Sb-doped n+-BaSi2 layers to prevent Sb diffusion during the growth of the undoped BaSi2 overlayer. This result was unexpectedly different from that obtained for BaSi2 layers on Si(111), where the c-Si layer improved the EQE significantly.

Fabrication and characterization of BaSi2 epitaxial films over 1 µm in thickness on Si(111)

We attempted to fabricate a-axis-oriented BaSi2 epitaxial films up to 2180 nm in thickness. First, we investigated the influence of growth temperature and growth rate on the crystalline quality of approximately 400-nm-thick BaSi2 layers, and then optimized the above two growth conditions based on X-ray diffraction measurements. We next grew BaSi2 films with various layer thicknesses at 580 °C in the range between 100 and 2180 nm, and characterized their properties. The a-axis-oriented BaSi2 thick epitaxial films had three epitaxial variants rotating 120° with each other around the surface normal. The microwave photoconductive decay measurements for the 1640-nm-thick BaSi2 epitaxial film showed that the minority-carrier lifetime was approximately 8 µs at room temperature. These achievements open up the possibilities of thin-film solar cell applications of BaSi2.