Comparison of passivation properties of plasma-assisted ALD and APCVD deposited Al₂O₃ with SiN_x capping

In this paper, we report on the direct comparison of Al₂O₃ films deposited by plasma-assisted atomic layer deposition (ALD) and atmospheric pressure chemical vapor deposition (APCVD) techniques on the passivation properties of both bare and boron diffused Si surfaces. It is found that ALD and APCVD deposited Al₂O₃ layers with SiN_x capping have very similar chemical composition, hydrogen concentration at the Al₂O₃/Si interface, and interface defect density (D_it), but the ALD film has slightly higher negative fixed charge (Q_f) after a high temperature (~750 °C) contact firing cycle typically used for solar cell fabrication. Both films showed excellent surface recombination velocities of <5 cm/s on planar and textured un-diffused Si wafers, but the ALD films gave 1 to 8 fA/cm² lower emitter saturation current densities (J_0e) on implanted boron emitters with sheet resistance in the range of 90–180 Ω/‡. This is attributed to the observed slightly higher negative charge-induced field-effect passivation in the ALD films. However, SiN_x/Al₂O₃/p⁺/n/n⁺ passivated emitter rear totally diffused (PERT) cell structures fabricated with 110 and 150 Ω/‡ emitter in this study showed no appreciable difference in cell efficiency. Device analysis and simulations showed that even for >23% high-performance solar cells design, with very low bulk and rear contact recombination, the observed small difference in J_0e has negligible effect on cell efficiency. Thus, APCVD Al₂O₃ passivation of B emitter can produce comparable efficiencies to their counterpart ALD Al₂O₃ passivated n-type Si solar cells.