A study on the microstructure formation mechanisms and functional properties of new β-FeSi₂-based polycrystalline thin films for photovoltaic applications using correlative electron microscopy

RWTH Publications (RWTH Aachen)

Abstract

Our research endeavors are directed toward developing sustainable materials that can tackle the challenges of current photovoltaic commercial structures. Amid the variety of potential materials, β-FeSi2 stands out as one of the promising materials for photovoltaics-based applications because it is eco-friendly with a low carbon footprint in its processing and ubiquitous nature. Also, it has an extreme optical absorption coefficient (α > 105 cm-1 above 1.0 eV) of more than one order of magnitude larger than that of Silicon and Gallium Arsenide. This enables the reduction of its absorption layer thickness below 1 µm, without affecting the optoelectronic performance of the solar cell. Nevertheless, Iron disilicide has many distinct features, but it still faces a critical challenge with the phase transformations. Fe-Si has four primary intermetallic phases: Fe3Si, Fe5Si3, FeSi, and FeSi2. Among them, the β-FeSi2 phase with orthorhombic crystal structure exhibits semiconductive properties, making it a potential candidate for solar cell applications. Typically, the β-FeSi2 phase is formed at low temperatures in PVD. In contrast to the β-FeSi2, the α-FeSi2 formed at high substrate temperatures has a tetragonal crystal structure, which is metallic, making it suitable for thermoelectric devices. In the present thesis, β-FeSi2 thin films (~800 nm thick) on stainless-steel (304SS) besides silicon ((100)-oriented Si) and sapphire ((0001)-oriented Al2O3) substrates were produced through controlled direct current (DC) sputtering. A series of DC-PVD experiments were performed to produce the highest quality of FeSi2 thin films with respect to the morphological, textural, and spatial phase distribution. We investigated the combined sputtering parameters affecting the desired morphological features, texture formation, and phase transformation. Also, we explored other factors besides the sputtering parameters affecting the β phase stability and the strength of the texture formed on different substrates. Subsequently, the interplay of nucleation- and growth-controlled mechanisms on the preferred microstructural features was studied. For these purposes, a correlative study of β-FeSi2 thin films was conducted using complementary electron microscopy techniques. Surface roughness measurements were also performed using the confocal laser scanning microscope. The electron backscatter diffraction (EBSD), X-ray diffraction (XRD), and energy dispersive X-ray spectroscopy (EDS) measurements were conducted to monitor the phase formation for β-FeSi2. The EBSD mapping in two dimensions was employed for the cross-sectional and plane-view thin films to observe the microstructural features, spatial phase distribution, and texture evolution. The analysis of the EBSD patterns was challenging using the classical Hough transform approach (HT) due to the complex crystallography of tetragonal and orthorhombic phases, the nano-sized grains (i.e., on SS, 60 nm ± 30 nm at 0.5 Pa, and 45 nm ± 25 nm at 1 Pa), and the high defect density in the crystal structure. Therefore, spherical indexing (SI) was utilized to index the EBSD patterns accurately and identify the phases. The results show that 800 nm thick β-FeSi2 thin films have a high-quality and continuous microstructure with a good adhesion to the 304SS substrate. It exhibits a uniform grain size of 45 nm ± 25 nm, and a moderately smooth surface of Ra= 32 nm. On these films we revealed for the first time a sharp (001) and (010)-oriented texture and the highest fraction of β phase (90% ± 10%), which was achieved under the following optimal conditions: deposition time of 1h, deposition rate of 12.12 nm/min, substrate temperature of 500 °C, and sputtering pressure of 1 Pa. We investigated also the correlation between both DC sputtering parameters and substrate on the crystallographic phase fraction and texture formation. Therefore, we investigated the spatial phase transformation issue by providing a case study of a methodology to control the desired phase distribution across FeSi2 thin films on different substrates. Accordingly, we found that the highest β phase fraction on (100)-oriented Si was developed at 1h, 0.5 Pa, and 500 °C, while on (0001)-oriented Al2O3, the highest fraction of β phase was (98% ± 2%). The deposition temperature plays a critical role in facilitating the interdiffusion process of the Fe or Si atoms at the interface at temperatures above 600 ⁰C. Additionally, it is found that the thin film on Si substrate shows the strongest and on SS substrate shows the weakest textures. However, thin films on both Si and SS substrates have the (010) poles being strongest, while on Al2O3 the (001) peak is strongest. It is found also that the substrate position in PVD chamber influences the texture formation as the directionality and uniformity of the arriving atoms are changed (i.e., it was found that the texture depends on the position of the sample inside the PVD chamber. Samples in the centre show a sharp fibre texture parallel to the surface normal, while samples outside the sample show inclined fibre directions). The correlative study also shows that the direction of the ion beams affects the texture formation, but the substrate nature has the strongest impact on the texture, while the impact of the primary beam direction is of secondary importance. This was observed on SS as it has a polycrystalline nature and thus weakens the texture. Therefore, future work should focus on employing a textured SS substrate or coating of iron-based alloy (i.e., by selecting high-silicon steel with optimal silicon content and mechanically processing). This provides a better controlling over the texture formation and evolution.

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