We recently discovered that in an AlF3/SiO2/Si structure extrinsic electrons are trapped at fluorine (F) vacancies in AlF3 at the interface with SiO2, generating a high sheet density of fixed negative charges. p- and n-Type Si substrates were oxidized using rapid thermal oxidation (RTO) or furnace oxidation (th); some samples were passivated in hydrogen (H-2). AlF3 was deposited onto oxidized Si wafers by a modified PVD process, leading to a F deficiency (AlFx). Samples were characterized by mercury probe (Hg) CV and microwave photo conduction decay (muW-PM), determining charge and trap densities and effective carrier lifetime tau(eff), respectively. An effective charge density of up to \Q(eff) = -6.9 x 10(12) cm(-2) is reached due to electrons tunneling from Si into AlF3, occupying F vacancies. Lifetime scans of p-type float zone (FZ) Si samples with 1.5 nm RTO and 20 nm AlF3 show an increase in effective minority carrier lifetime by a factor of 8.4 compared to samples with 1.5 nm RTO only. The fixed negative charge density increases with exposure time to sunlight or at simulated ageing by a 24 h anneal at 200degreesC in air. (C) 2004 Elsevier B.V. All rights reserved.