Optical Properties of Sputtered Iron Sulfide for Photonic Device Applications
Authors: Zaka Awais , Abdul Hadi Sabina , Alhassan Saeed , Nayfeh Ammar 
Published: September 2023

Iron Pyrite is an earth-abundant, non-toxic, low-cost material commonly referred to as fool’s gold due to its golden-yellow appearance. It belongs to the family of transition metal dichalcogenides (TMDs) and gained fame as a potential solar material during the 70s and 80s. The reason was its suitable band gap and excellent absorption coefficient. However, the performance of pyrite never achieved a beneficial place among other materials and remained a mysterious path of research so far. With advanced technology and better characterization techniques and equipment available, the focus is again shifting towards exploring pyrite as a potential photovoltaic material which can compete with other materials, such as silicon, primarily in terms of cost. However, the true nature of pyrite and its application in photovoltaics cannot be adequately defined without looking deeply into its optical properties and interaction with light [1-3].
Here in this work, we explored the photosensitivity of pyrite thin films at different thicknesses. These thin films were fabricated using an RF-powered sputtering method using a 99.9% FeS2 target. P-doped silicon and fused silica were used as substrates. The pressure and RF power were kept at 03 mTorrs and 70 watts, respectively, and the procedure took place at room temperature with an argon gas flow rate of ~24 cm3/min. The thickness was controlled by varying the sputtering time from 20 to 90 mins. As-prepared pyrite films were characterized through variable angle spectroscopic ellipsometry (VASE) and UV-vis spectrophotometer. An optical model was first developed for VASE in which the thin pyrite film showed light-absorbing characteristics (B-spline). Based on that model, the film thickness was determined. It was observed that the thickness of pyrite increased linearly with time, having grown to ~100 nm after 90 minutes of sputtering.
UV-vis spectrophotometry was used to collect the transmission, reflectance and absorbance data for the films deposited onto a fused silica substrate. It was observed that the films showed very high absorbance in the UV range, and the absorbance tended to increase with the increase in thickness. The direct band gap was then calculated using absorbance data and building Tauc’s plot, which was observed to be around 2.25 eV.
In conclusion, owing to its suitable properties and optical characteristics, iron pyrite opens many paths to scientists and researchers to explore its utilization not only in photovoltaics but also in other opto-electronic fields. However, further studies are also required to deeply understand its mechanism and capability. Nonetheless, high absorption of light in UV and visible range at very thin layers, suitable band gap and easy accessibility could be breakthrough parameters in near future for pyrite.

References

[1] A. Zaka, S. M. Alhassan, and A. Nayfeh, "Iron Pyrite in Photovoltaics: A Review on Recent Trends and Challenges," ACS Applied Electronic Materials, vol. 4, no. 9, pp. 4173-4211, 2022/09/27 2022, doi: 10.1021/acsaelm.2c00489.
[2] S. Khalid, E. Ahmed, Y. Khan, K. N. Riaz, and M. A. Malik, "Nanocrystalline pyrite for photovoltaic applications," Chemistryselect, vol. 3, no. 23, pp. 6488-6524, 2018.
[3] M. Rahman, G. Boschloo, A. Hagfeldt, and T. Edvinsson, "On the Mechanistic Understanding of Photovoltage Loss in Iron Pyrite Solar Cells," Advanced Materials, vol. 32, no. 26, p. 1905653, 2020.

Figure 1