Introduction of negative charges in nitride for PV applications

Negative charges were tunneled from Si surface into nitride film in the nitride/oxide/Si stacks by bias or corona charging. The tunneled charges appear to have linear relationship with the applied electrical field. A maximum negative charge density exists, when all K centers in nitride film are negative charged. At high bias condition, Si interface will take the risk of high energy electron damage. The damage is thermal unstable and can be annealed out at 300-400°C for very short time. Linear relationship is discovered with nitride thickness and threshold voltage, from which the electrical field strengths across nitride (3.4 MV/cm) and oxide (6 MV/cm) layers for tunneling were calculated. The tunneled charges are stable at room temperature, but will decay at elevated temperatures. At the cell operation conditions (around 90°C), 20% of the tunneled charges will disappear. At higher annealing conditions (350°C to 500°C), all tunneled charges vanish rapidly. Negative bias can also introduce negative charges around the top of the nitride layer through electrons injection. However, the negative charges are in very small amount and of little interest for PV applications.