The Influence of Cadmium Sulfide and Contact Annealing Configuration on the Properties of High-Performance Cadmium Stannate

The sensitivity of cadmium stannate (CTO) performance to both sputtering and annealing conditions was investigated. Films treated by the standard proximity anneal in contact with a CdS film displayed an electrical resistivity of ∼2.2×10^-4 Ω cm, high mobility (∼57 cm ²/V s), and >90% transmission throughout the near infrared (λ≤1350 nm). Film properties were insensitive to annealing temperature and sputtering ambient when O₂ was present during deposition. Next, we demonstrated process modifications to the proximity anneal. CTO and CTO/CdS bilayer films were annealed either uncovered or covered with a bare glass plate. CTO/CdS bilayers annealed in the covered configuration had comparable or superior conductivity to the proximity anneal, with optimal performance achieved with 10 nm of CdS. The resistivity of uncovered films and films produced without CdS was insensitive to CdS thickness (∼3×10^-4 Ω cm), and displayed higher mobility and improved transparency, particularly in the near infrared. The electrical properties were well correlated with X-ray diffraction measurements of film crystallinity and purity. These high-conductivity films are promising for photovoltaic applications, transmitting 92-95% of solar radiation > 1 eV.