Investigations of silicon nitride films for silicon solar cells

Silicon nitride films on crystalline silicon were deposited in a low-temperature (< 400 degrees C) Plasma Enhanced Chemical Vapour Deposition process. The deposition process is monitored with in situ Time Resolved Microwave Conductivity measurements leading to an on-line quality control of the deposited films. It is shown that at the start of the deposition there is a strong decrease of the lifetime of the measured transient signal due to plasma induced damage at the silicon surface. Afterwards an increase of the lifetime is observed due to passivation of the interface. For thin films (< 30 nm), the lifetime and the film composition depend on the film thickness. Furthermore, the him composition has a strong impact on the passivation of thick (100 nm) silicon nitride films. The best passivation is obtained for almost stoichiometric films characterized by a refractive index of 1.95.