Scanning Kelvin Probe Microscopy: A Tool to Investigate Nano-Scale Doping Non-Uniformities in Poly-Si/SiOx Contacts: Preprint
Monocrystalline Si (c-Si) solar cells with passivated contacts based on the ultrathin SiOx and doped polycrystalline Si (poly-Si) layers in a poly-Si/SiOx/c Si structure show high solar cell efficiencies that are ~26%. Excellent surface passivation using these contacts is achieved via the combined effects of chemical passivation of the SiOx/c-Si interface by the SiOx layer and field-effect passivation from the heavily doped poly-Si layer. These contacts give best performance only when annealed to temperatures higher than 850 degrees C. Structural changes in the SiOx layer and dopant diffusion from poly-Si into the underlying c-Si wafer occur during this step which are hard to investigate using conventional characterization techniques. In this work we investigate poly-Si/SiOx contacts with both a 1.5 (tunneling transport) and 2.2 (pinhole transport) nm SiOx layer using atomic force microscopy techniques. Conductive AFM on n+-poly-Si/SiOx/p-Si structures show significant spatial variations for both contact types, likely due to non-uniformities in the poly-Si layer itself. The electrical and structural variations deeper into the contact were revealed by scanning Kelvin probe microscopy after precisely etching away the poly-Si and SiOx layers and few nanometers of c-Si surface. This etching was performed using tetramethylammonium hydroxide and dilute HF solutions. The resulting surface potential maps appear similar for both contacts, and show less than 500 nm size heavily-doped regions. However, further etching of the c-Si surface reveals these heavily-doped regions to be less than 200 nm deep for the 2.2 nm SiOx contact and greater than 200 nm deep for the 1.5 nm SiOx contact.