Comparison between Al2O3 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 (Qf = 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 150-250oC. 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.