Material combination of Tunnel-SiO₂ with a (sub-)Monolayer of ALD-AlO_x on silicon offering a highly passivating hole selective contact

In this work, we investigate a passivating hole-selective tunnel contact on crystalline silicon (Si) based on an ultrathin thermal SiO₂ and a (sub-)monolayer thick aluminum oxide (AlO_x) deposited by atomic layer deposition (ALD) with 1 to 7 cycles. Rapid thermal annealing (RTA) is essential to activate the passivation and intense visible light is demonstrated to play a key role in the formation and occupation of the Al-induced acceptor states in SiO₂ that provide the passivation. This simple stack provides very good surface passivation on n-type Si with a surface saturation current density as low as J_0s = 25 fA/cm² (S_eff = 2.7 cm/s) and a contact resistivity ρ_c of ~200 mΩ cm², while being Si-dopant-free, fully transparent and firing-stable. The Al-deposition on thermal SiO₂ during the initial ALD-cycles is studied carefully by synchrotron-based reference-free grazing incidence X-ray fluorescence spectrometry (GIXRF). Different metallization and capping approaches are tested and discussed.