Improvement of passivation performance of silicon nanocrystal/silicon oxide compound layer by two-step hydrogen plasma treatment

A two-step hydrogen plasma treatment (HPT) is carried out for Si nanocrystals/silicon oxide compound passivating contacts to improve their passivation performance. The 2-step HPT consists of a first HPT at a high temperature of 500 °C and a second HPT at a lower temperature of 300 °C. Etching of n-type polycrystalline Si (n+-poly-Si) is suppressed by using low RF power and short duration as the condition for the second HPT. For the passivating contact with thin n+-poly-Si, the implied open-circuit voltage (i-VOC) was improved to 723 mV after the 2-step HPT, which is higher than the i-VOC after the one-step HPT (718 mV) owing to enhanced hydrogenation of the Si nanocrystals/silicon oxide compound layer. On the other hand, 1.4-fold larger contact resistivity (ρc) was obtained after the 2-step HPT. This increase in ρc is probably attributed to the suppression of trap-assisted tunneling. The largest i-VOC value of 744 mV and the lowest ρc value of 12 mΩ cm2 was observed for the contact with 300-nm-thick n+-poly-Si, possibly due to enhanced field-effect passivation after the 2-step HPT.