β-FeSi₂ is a promising material for photovoltaics-based applications, as it is eco-friendly with a low carbon footprint and formed from abundant elements. Additionally, it comprises of an extremely high optical absorption coefficient (α > 10⁵ cm^-1 above 1.0 eV), which is more than one order of magnitude than that of Si and GaAs, respectively. This enables the reduction of its absorption layer thickness below 1 µm without compromising the optoelectronic performance of the solar cell. 

In this work, we sputtered 400 nm and 800 nm β-FeSi₂ thin films on a cost-effective stainless steel substrate. This is carried out in a controlled and sequential direct current (DC) and radio frequency (RF) magnetron sputtering processes, followed by post thermal annealing. The phase transformation in β-FeSi₂ is monitored using electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) techniques. To investigate microstructure, spatial phase distribution, and texture, EBSD mapping was applied on cross-sections of the films. Furthermore, a structure-property relationship is established by combining EBSD and electron channelling contrast imaging (ECCI) for microstructure investigations, and cathodoluminescence (CL) for local optoelectronic measurements. 

The results show that the β-FeSi₂ films have good adhesion to the 304SS substrate. Moreover, the formed thin films have high-quality, pore-free structure due to deposition at low temperatures (220-400°C). In both sputtering techniques, the phase distribution is quantified. The formation of the orthorhombic beta phase occurs at low deposition temperatures without post thermal annealing. In contrast, the tetragonal phase developed at a higher substrate temperature of 500°C and annealed the layers at 500-600°C for 45 min. On the other hand, ECCI and EBSD confirm increased lattice defects (mainly dislocations) in the as-deposited grains.