Contactless analysis of surface passivation and charge transfer at the TiO₂-Si interface

Transition metal oxides are pivotal in enhancing surface passivation and facilitating charge transfer (CT) in silicon based photonic devices, improving their efficacy and affordability through interfacial engineering. This study investigates TiO₂/Si heterojunctions prepared by atomic layer deposition (ALD) with different pre-ALD chemical and post-ALD thermal treatments, exploring their influence on the surface passivation and the correlation with the CT at the TiO₂-Si interface. Surface passivation quality is evaluated by the photoconductance decay method to study the effective carrier lifetime, while CT from Si to TiO₂ is examined by transient reflectance spectroscopy. Surprisingly, the as-deposited TiO₂ on HF-treated n-Si (without interfacial SiO_x) demonstrates superior surface passivation with an effective lifetime of 1.23 ms, twice that of TiO₂/SiO_x/n-Si, and a short characteristic CT time of 200 ps, tenfold faster than that of TiO₂/SiO_x/n-Si. Post-ALD annealing at temperatures approaching the TiO₂ crystallization onset re-introduces the SiO_x layers in HF-treated samples and induces chemical and structural changes in all the samples which decrease passivation and prolong the CT time and are hence detrimental to the photonic device performance.