Preparation of air-stable, low recombination velocity Si(111) surfaces through alkyl termination

Abstract

A two-step, chlorination/alkylation procedure has been used to convert the surface Si-H bonds on NH 4 F ͑aq͒ -etched ͑111͒-oriented Si wafers into Si-alkyl bonds of the form Si-C n H 2nϩ1 (nу1). The electrical properties of such functionalized surfaces were investigated under high-level and low-level injection conditions using a contactless rf apparatus. The charge carrier recombination velocities of the alkylated surfaces were Ͻ25 cm s Ϫ1 under high-level and low-level injection conditions, implying residual surface trap densities of Ͻ3ϫ10 9 cm Ϫ2 . Although the carrier recombination velocity of hydrogen-terminated Si͑111͒ surfaces in contact with aqueous acids is Ͻ20 cm s Ϫ1 , this surface deteriorates within 30 min in an air ambient, yielding a high surface recombination velocity. In contrast, methylated Si͑111͒ surfaces exhibit low surface recombination velocities in air for more than 4 weeks. Low surface recombination velocities were also observed for Si surfaces that had been modified with longer alkyl chains.

High-Resolution Soft X-ray Photoelectron Spectroscopic Studies and Scanning Auger Microscopy Studies of the Air Oxidation of Alkylated Silicon(111) Surfaces

“…4,5 Such surfaces have been shown to exhibit very low surface recombination velocities, indicating relatively few electrically active defect sites on the functionalized surface. 1,5 Consistently, no silicon oxides are detectable by surface-sensitive X-ray photoelectron spectroscopy on such initially prepared alkylated Si surfaces. 4,5 Although the rate of oxidation is significantly impeded, core-level photoelectron spectroscopy of the Si 2p region has shown that the alkylated Si(111) surfaces show small but detectable levels of oxide after several weeks of air exposure.…”

“…1,5 The chemical passivation is manifest as a very large suppression in the rate of oxide formation, with only a fraction of an equivalent monolayer formed after months in ambient air, as opposed to after minutes Figure 3. In the spectrum of the oxidized surface (b), the region above 1 eV is shown expanded 10× in signal intensity.…”

“…1,5 The electrical passivation is manifest as a remarkably low number of electrically active defects sites on the alkylated surfaces, as measured by an extremely low surface recombination velocity, S, for extended time periods in an air ambient. 1 The present SXPS studies aid in elucidation of three key mechanistic issues regarding the alkyl passivation process. One question concerns why the surface recombination velocity is small even when alkyl groups are used that cannot sterically terminate every atop site on an unreconstructed Si(111) surface.…”

“…1,2 However, this surface oxidizes rapidly in ambient air, resulting in a surface that has a high surface recombination velocity. 1 A variety of surface passivation schemes have been investigated with the aim of preserving the ideal electrical and electronic properties of the H-terminated Si(111) surface in ambient conditions. 3 One promising technique involves the formation of Si-C bonds to the atop Si sites on the Si(111) surface.…”

“…5 However, the alkylated Si(111) surfaces still show low surface recombination velocities after this time period. 1,5 A question of interest is therefore the nature of the oxide that is formed on these surfaces as compared to the electrically defective oxide that forms during air oxidation of H-terminated Si(111) surfaces.…”