Electronic Properties of a-SiO${}_{x}$ N${}_{y}$:H/SiN ${}_{x}$ Stacks for Surface Passivation of P-Type Crystalline Si Wafers

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 τ
_eff
, density of fixed charge Q
_f
, and density of interface defects D
_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 S
_0n
and S
_0p
through fitting τ
_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
_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
_it
, indicating that the firing process enhances chemical passivation of the a-SiO
_x
N
_y
:H/SiN
_x
stacks.