Surface Passivation of Boron-Diffused Junctions by a Borosilicate Glass and In Situ Grown Silicon Dioxide Interface Layer

An in situ oxidation during the boron tribromide diffusion process to form $\text{p}^{+}$ -doped junctions on crystalline Si solar cells leads to the formation of a layer stack system consisting of a borosilicate glass (BSG) (a binary B $_{2}$ O $_{3}$ –SiO $_{2}$ system) and a SiO $_{2}$ layer at the Si interface. We present a method to passivate the $\text{p}^{+}$ -doped regions by using this in situ grown SiO $_{2}$ in combination with a plasma-enhanced-chemical-vapor-deposited SiN $_{x}$ layer. We show that the etching rate of the BSG layer, in a hydrofluoric acid solution, varies over the wafer. The etching rate depends on its local B $_{2}$ O $_{3}$ content in the BSG and is markedly higher than that of the SiO $_{2}$ layer. This difference in the etching rates can be used to controllably etch back the BSG layer in order to obtain a thin and uniform passivating oxide layer for solar cell application. Using this oxide/SiN $_{x}$ stack, we obtained implied $V_{\text{oc}}$ of 705 mV and $J_{0e}$ as low as 14 fA/cm $^{2}$ on symmetrically diffused boron emitters on $\text{n}$ -type Czochralski wafers. These passivation results are comparable, on similar structure and boron emitters, with state-of-the-art Al $_{2}$ O $_{3}$ -based passivation methods. Moreover, we have successfully implemented this passivation method in the fabrication of n-type passivated emitter and rear totally diffused and interdigitated back contact solar cells, avoiding the need of adopting additional process steps and costs for boron emitter passivation.