Comparison between electron-beam and furnace rapid isothermal anneals of phosphorus-implanted solar cells

Solar cells have been fabricated using single crystal silicon wafers, implanted at 10-keV energy with 2×10¹⁵ P⁺/cm². Rapid isothermal heat treatments at temperatures ranging between 500 and 1000 °C were used to anneal the implanted damage. Two different methods have been used: electron-beam and furnace annealing for times ranging between 0.5 and 5 min, and between 2 and 5 min, respectively. Characterization of P⁺-implanted layers and of completed solar cells allowed us to demonstrate that rapid isothermal anneals with furnace and electron beam give equivalent results. The efficiency of solar cells which are annealed using these rapid isothermal processes is fairly good (13.5%), even if slightly lower than that obtained with conventional anneals (750 °C for 30 min). The reduced efficiency is a consequence of the lower value of the open circuit voltage. It is demonstrated that this effect is caused by residual defects in the space-charge region, produced by ion implantation.