A Detailed Chemical Model for the Diffusion of Phosphorus Into the Silicon Wafer During POCl3 Diffusion

The POCl
₃
diffusion is the main technology to form the p-n junction of industrial silicon solar cells. However, the diffusion mechanism of phosphorus (P) into the silicon wafer is not fully understood. In this article, we study the P diffusion mechanism during drive-in by systematically varying the drive-in time in the oxygen (O
₂
) atmosphere and subsequently in nitrogen (N
₂
). When increasing the drive-in time in O
₂
from 0 to 120 min, the sheet resistance R
_sheet
stays constant at 485±30 Ω/sq. Hence, we demonstrate for the first time that the phosphorus diffusion can be completely suppressed in the O
₂
atmosphere. When adding a drive-in in the N
₂
atmosphere directly after the drive-in in O
₂
, we find that the SiO
₂
thickness dSiO
₂
,O
₂
changes from initially 2 to 10 nm after O
₂
drive-in to an equilibrium SiO
₂
thickness dSiO
_2,eq
of 4.7 nm after N
₂
drive-in. We prove for the first time that if dSiO
₂
,O
₂
> dSiO
_2,eq
, no P diffuses into the silicon wafer even in the N
₂
atmosphere. Only if dSiO
₂
,O
₂
<; dSiO
_2,eq
, phosphorus diffuses into the silicon wafer in the N
₂
atmosphere. We propose a detailed chemical model to explain our experimental results, which assumes that the diffusion of Si from the wafer through the SiO
₂
interface toward the PSG plays a key role. In this model, P can only diffuse into the Si wafer if P
₂
O
₅
in the PSG is reduced by the Si from the wafer to P and SiO
₂
.