Ultrathin SiO₂layers on Si(111): preparation, interface gap states and the influence of passivation

Abstract

An essential prerequisite for the successful application of Si/SiO(2) nanostructures in photovoltaics is the realization of well-defined and abrupt interfaces with low densities of interface gap states. Here, a complete in situ process from preparation and hydrogen passivation to interface gap state analysis by near-UV photoelectron spectroscopy without breaking ultrahigh vacuum (UHV) conditions is introduced. It is demonstrated that by RF plasma oxidation of Si(111) substrates with thermalized neutral oxygen atoms, ultrathin SiO(2) layers can be realized with compositionally and structurally abrupt Si/SiO(2) interfaces and a minimal amount of intermediate oxidation states bridging the transition from Si to SiO(2). Plasma oxidized samples have significantly lower interface gap states than samples oxidized by thermal oxidation at 850 °C. Interface gap state densities were further reduced by in situ hydrogen plasma passivation with nearly thermalized H atoms. The resulting reduction of interface recombination velocity and the increase of effective majority and minority carrier lifetimes are revealed by constant photocurrent measurements and quasi-steady-state photoconductance, respectively.

Effect of wet-chemical substrate smoothing on passivation of ultrathin-SiO2/n-Si(111) interfaces prepared with atomic oxygen at thermal impact energies

“…It has been demonstrated that structural abruptness is not inherent with an increase of the density of interface states [2]. The resulting oxide layer thickness can be adjusted in the range by appropriate variation of the oxidation time (5,…, 60 min), the substrate temperature (300,…, 600°C), and the plasma power (200,…, 600 W) [25].…”

“…4, curve 2). These extrinsic states, forming a Gaussian distribution in the lower half of the forbidden gap are caused by dangling bond defects (Si−) 2 (O−)Si• which are correlated with Si atoms of a lower state of oxidation, Si +1 [32]. The occurrence of oxygen backbonded Si dangling bond defects indicates that the surface is not completely hydrogen terminated.…”

“…In photovoltaics, ultrathin SiO 2 layers have been successfully demonstrated to act as functional elements in various innovative solar cell concepts: e.g., as passivation layers [1,2], as substrates for the deposition of organic molecules [3], and (iii) as barrier in Si/SiO 2 quantum well structures [4]. Main challenge for those applications is the question of whether stable, homogeneous, ultrathin SiO 2 layers produced on silicon with both structurally abrupt interfaces and low density of defect states.…”

“…It was shown, that the substrate surface conditioning has to be carefully optimized with respect to the substrate morphology and the subsequent layer preparation [13]. In this contribution we report on the preparation of ultrathin SiO 2 layers in the range of 9 -15 Å on c-Si substrates by wet-chemical treatment with various specific oxidizing solutions and successive novel oxidation with neutral atomic oxygen from a remote plasma cracker source under ultrahigh vacuum conditions, which has proven to produce high-quality Si/SiO 2 interfaces [1,2]. The effects of wet-chemical substrate pre-treatment, oxidation and annealing processes on the electronic properties of the Si/SiO 2 interface were investigated.…”

Improved Si/SiOx interface passivation by ultra-thin tunneling oxide layers prepared by rapid thermal oxidation

“…Moreover, SPV allows the determination of surface band-bending and the recombination behavior [42,43]. It has been proven to be well-suited for the characterization of hydrogenated Si wafers, and ultrathin oxide nanolayers [44,45], as well as a-Si:H/c-Si interfaces [46].…”