| AVS 57th International Symposium & Exhibition | |
| Thin Film | Monday Sessions |
| Session TF+EN-MoM |
| Session: | ALD: Energy Applications |
| Presenter: | E.W.M.M. Kessels, Eindhoven University of Technology, Netherlands |
| Authors: | G. Dingemans, Eindhoven University of Technology, Netherlands N.M. Terlinden, Eindhoven University of Technology, Netherlands M.L.C. Adams, Eindhoven University of Technology, Netherlands H.B. Profijt, Eindhoven University of Technology, Netherlands M.M. Mandoc, Eindhoven University of Technology, Netherlands M.C.M. van de Sanden, Eindhoven University of Technology, Netherlands E.W.M.M. Kessels, Eindhoven University of Technology, Netherlands |
| Correspondent: | Click to Email |
Ultrathin films of Al2O3 synthesized by ALD at low substrate temperatures (200 ºC) have recently demonstrated their potential as surface passivation material in c-Si photovoltaics enabling solar cell efficiencies up to 23.4%. So far, the best results were obtained by plasma ALD Al2O3 but recently it was shown that similar excellent results can be obtained by thermal ALD [1]. For both methods, Al2O3 yield excellent lifetimes after annealing (at 400 ºC) on both p- and n-type Si wafers with the best results corresponding to ultralow surface recombination velocities < 1 cm/s (for 3.5 Ohm cm n-type c-Si). However, also several striking differences have been observed in terms of the underlying passivation mechanism as will be addressed in this contribution. From capacitance-voltage (C-V) measurements, corona charging experiments and optical second harmonic generation (SHG) it was found that Al2O3 deposited with thermal ALD contains a lower negative fixed charge density (~1012 cm-2) than plasma ALD (> 5·1012 cm-2). This indicates that field-effect passivation (i.e., shielding of one type of charge carriers from the surface) is less important for thermal ALD. On the other hand, it was observed that chemical passivation (i.e., reduction of interface defect states) is very important for both thermal and plasma ALD Al2O3. Relatively low interface defect densities Dit of <1011 eV-1 cm-2 were found for both methods after anneal. However, for as-deposited Al2O3, the interface defect density (and correspondingly the surface recombination velocity) is significantly poorer for plasma ALD Al2O3 than for thermal ALD Al2O3. From vacuum ultraviolet (VUV) emission measurements, it has been found that the high Dit can be attributed to plasma radiation damage by photons with an photon energy of ~9.5 eV. Furthermore, it will be shown that the difference in importance of field-effect passivation explains several differences observed in the passivation performance for thermal and plasma ALD Al2O3 (e.g., the minimum thickness required for excellent passivation).
[1] G. Dingemans, M. C. M. van de Sanden, W. M. M. Kessels, Electrochem. Solid-State Lett., 13, H76 (2010).