Negative Ion Implantation Enabled Controlled Oxygen Doping in Iron Pyrite Thin Films

Iron pyrite (FeS₂) is a promising material for next-generation photovoltaic and optoelectronic applications. However, the origin of p-type conductivity in thin films, unlike the n-type behavior of bulk FeS₂, remains unknown and is often attributed to unintentional impurity incorporation, particularly oxygen. This study explores the role of oxygen in tuning the electrical and optical properties of FeS₂ thin films. Phase-pure FeS₂ thin film is deposited on glass substrates via single-step co-sputtering using FeS₂ and S₈ targets at 430°C substrate temperature. The resulting films exhibit p-type conductivity with a carrier concentration and mobility of 4.18 × 10¹⁹ cm^-3 and 5.06 cm² V^-1 s^-1 respectively. Controlled oxygen incorporation is achieved through negative ion implantation at fluences ranging from 9 × 10¹⁴ to 1 × 10¹⁶ ions cm^-2. X-ray photoelectron spectroscopy and time of flight secondary ion mass spectrometry confirm successful oxygen doping, with oxygen atoms preferentially occupying sulfur vacancies for higher doses. This incorporation enhances p-type conductivity and induces direct bandgap widening up to 1.48 eV. The results demonstrate a pathway to fabricate FeS₂ thin films with high hole concentration and offer a strategy for optimizing the optoelectronic properties for advanced semiconductor applications.