Comparison of the Electrical Properties and Chemical Stability of Crystalline Silicon(111) Surfaces Alkylated Using Grignard Reagents or Olefins with Lewis Acid Catalysts

Abstract

Four methods were used to functionalize crystalline Si(111) surfaces with alkyl groups (C_nH_(2n+1), n = 1, 2, 6, 8):  chlorination with PCl_5 followed by alkylation with C_nH_(2n+1)MgX (X = Cl, Br), chlorination with Cl_2(g) followed by alkylation with C_nH_(2n+1)MgX, Lewis acid-mediated reduction of a terminal alkene, and anodization in diethyl ether containing 3.0 M CH_3MgI. The chemical properties of each surface were characterized as a function of time exposed to air using X-ray photoelectron spectroscopy, and the electrical properties of the various surfaces were probed using time-resolved radio frequency (rf) photoconductivity decay methods. Both chlorination/alkylation routes produced alkylated Si surfaces that displayed low (<200 cm s^(-1)) initial charge carrier surface recombination velocities (S); furthermore, the recombination velocities of these functionalized surfaces were stable during >600 h of exposure to air. Surfaces functionalized through this route also displayed a significantly lower rate of oxidation than did unalkylated, H-terminated or Cl-terminated Si(111) surfaces. In contrast, surfaces modified by the Lewis acid-catalyzed reduction of 1-hexene and 1-octene exhibited high S values (S > 400 cm s^(-1)) when initially exposed to air and oxidized as rapidly as H-terminated Si(111) surfaces. Methyl-terminated Si(111) surfaces functionalized by anodization in a solution of CH_3MgI in ether exhibited stable, albeit high, S values (460 cm s^(-1)), indicating that the surface had been partially modified by the anodization process. The fractional monolayer coverage of oxide on the alkylated surface after exposure to air was determined for each functionalization technique. Although all four of the functionalization routes studied in this work introduced alkyl groups onto the Si surface, subtle changes in the extent and quality of the alkyl termination are significant factors in determining the magnitude and degree of chemical and electrical passivation of the resulting functionalized Si surfaces.