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 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 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
_oc
of 705 mV and J
_0e
as low as 14 fA/cm
²
on symmetrically diffused boron emitters on 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.