Comparison between aluminum oxide surface passivation films deposited with thermal ALD, plasma ALD and PECVD

Surface passivation schemes based on Al
₂
O
₃
have enabled increased efficiencies for silicon solar cells. The key distinguishing factor of Al
₂
O
₃
is the high fixed negative charge density (Q
_f
= 10
¹²
-10
¹³
cm
^-2
), which is especially beneficial for p- and p+ type c-Si, as it leads to a high level of field-effect passivation. Here we discuss the properties of Al
₂
O
₃
surface passivation films synthesized with plasma atomic layer deposition (ALD), thermal ALD (using H
₂
O as oxidant) and PECVD. We will show that with all three methods a high level of surface passivation can be obtained for Al
₂
O
₃
deposited at substrate temperatures in the range of 150-250°C. Furthermore, the role of chemical and field-effect passivation will be briefly addressed. It is concluded that the passivation performance of Al
₂
O
₃
is relatively insensitive to variations in structural properties. Al
₂
O
₃
is therefore a very robust solution for silicon surface passivation.