The influence of sodium on the plasma hydrogenation of polycrystalline silicon thin film transistors

It is shown that large negative threshold voltages and high leakage currents can occur in n-channel polycrystalline silicon thin film transistors during the plasma hydrogenation process. This is a result of sodium contamination, with the sodium ions residing in both the silicon dioxide and the grain boundaries of the polycrystalline silicon, close to the silicon-silicon dioxide interface. The sodium ions in the grain boundaries act like donor centres, and a strong electron channel is formed in the device. It is essential that film transistors are passivated against sodium ions before studying the effects of plasma hydrogenation on device characteristics.