Study and manipulation of charges present in silicon nitride films

As crystalline silicon solar cells continue to get thinner, the surfaces of the cell play an ever important role in controlling the cell efficiency. One tool to minimize surface recombination is field effect passivation from the charges present in the thin films applied on the cell surfaces. Basic PC 1D simulations were carried to understand the relation between the amount and sign of charge on cell efficiencies with varying emitter-doping levels. Silicon nitride (SiN_x) thin films are known to carry net positive fixed charges that originate from specific silicon nitrogen dangling bonds (•SiN₃) known as K centers. The properties of fixed positive charges present in as-deposited SiN_x films are studied by capacitance - voltage (CV) and electron spin resonance (ESR) techniques. We report that the as-deposited SiN_x films also carry neutral defects (K0 centers) that can easily be manipulated to either positive (K⁺) or negative (K) charge states depending on the end application. Corona charging was used to change the net charge in the film to either positive or negative and high energy (sub-300 nm) UV light was used to neutralize or annihilate the charges. ESR measurements showed that the neutral K0 defects are distributed throughout the bulk of the nitride film. A high temperature annealing step decreases the amount of neutral defects possibly due to bonding of hydrogen with the K center. First order effects of both positive and negative nitride charges on test structures were studied by photoconductance measurements.