Comparative study of differently grown tunnel oxides for p-type passivating contacts

In this study, boron-doped passivating contacts are investigated. The focus of this study lies on comparing four different methods for growing the thin SiOx tunnel layer, in order to better understand the relationship between the contact's surface passivation and contact resistivity. The thin oxide layers were grown in (i) boiling HNO3 (ChemOx), (ii) by UV induced photo-oxidation (UV/O3), (iii) by wet-chemical oxidation in ozonated water (O3), and by a thermal oxidation (TO) process. All oxides show a similar thickness of around 1.2-1.4 nm. The thermal oxide proved to block the penetration of dopants more effectively than the other three oxides, which resulted in superior passivation. On planar silicon surfaces a saturation current density of 15 fA/cm2 was achieved for the thermally grown oxide. Furthermore, the TO showed tolerance to higher annealing temperatures. The contact resistance of the analyzed samples was within a range where no significant fill factor losses are to be expected for full area cell contacts (< 100 mΩcm2).