Abstract
The surface passivation quality of plasma-enhanced chemical vapor-deposited silicon oxynitride/silicon nitride (a-SiO ${}_{x}$ N ${}_{y}$ :H/SiN ${}_{x}$ ) stacks has been investigated for p-type float-zone crystalline silicon wafers. The effective lifetime $\tau _{\rm{eff}}$ , density of fixed charge $Q_{{f}}$ , and density of interface defects $D_{\rm{it}}$ were measured as a function of the a-SiO ${}_{x}$ N ${}_{y}$ :H layer thickness both before and after firing at 800 °C for 3 s. Photoluminescence imaging under applied bias has been used to characterize $Q_{{f}}$ and the surface recombination parameters ${\rm S}_{\rm 0n}$ and ${\rm S}_{\rm 0p}$ through fitting $\tau _{\rm{eff}}$ versus voltage curves to an extended Shockley–Read–Hall (SRH) model. The results are in good agreement with values measured using capacitance–conductance–voltage measurements. Good surface passivation has been obtained with a peak effective lifetime of 2.41 ms. For both as-deposited and fired samples, $Q_{{f}}$ and $D_{\rm{it}}$ decrease with increasing a-SiO ${}_{x}$ N ${}_{y}$ :H layer thickness up to ∼18 nm. The results indicate that the field-effect passivation is weakened as the a-SiO ${}_{x}$ N ${}_{y}$ :H thickness increases and that chemical passivation from a-SiO ${}_{x}$ N ${}_{y}$ :H/SiN ${}_{x}$ plays a prominent role. Increasing the a-SiO ${}_{x}$ N ${}_{y}$ :H/SiN ${}_{x}$ thickness to 50 nm produces similar results, indicating that an 18-nm interlayer is enough to obtain the desired passivation properties. Compared with as-deposited samples, fired samples exhibit lower $D_{\rm{it}}$ , indicating that the firing process enhances chemical passivation of the a-SiO ${}_{x}$ N ${}_{y}$ :H/SiN ${}_{x}$ stacks.
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