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

Surface passivation schemes based on Al₂O₃ have enabled increased efficiencies for silicon solar cells. The key distinguishing factor of Al₂O₃ is the high fixed negative charge density (Q_f = 10¹²-10¹³ 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 Al₂O₃ surface passivation films synthesized with plasma atomic layer deposition (ALD), thermal ALD (using H₂O as oxidant) and PECVD. We will show that with all three methods a high level of surface passivation can be obtained for Al₂O₃ 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 Al₂O₃ is relatively insensitive to variations in structural properties. Al₂O₃ is therefore a very robust solution for silicon surface passivation.