Chemical characterization and charge carrier dynamics of crystalline silicon(111) surfaces modified with surface-bound organic functional groups

Abstract

Investigations of the chemical structure and charge carrier properties of alkylated crystalline silicon(111) surfaces are presented. Hydrogen-terminated Si(111) surfaces were alkylated with a series of saturated hydrocarbons through a chlorination/alkylation procedure and characterized using surface-sensitive techniques. High-resolution soft X-ray photoelectron spectroscopy (SXPS) identified 1 monolayer of H, Cl, and C on the H-, Cl- and CH3-terminated surfaces, respectively. Surfaces functionalized with bulkier alkyls showed Si 2p binding energy shifts that suggest that unalkylated Si atoms are bonded to hydrogen. Alkylated Si(111) surfaces were further characterized using transmission infrared spectroscopy (TIRS). The Si-Cl stretching and bending motions were identified at 583 and 528 cm-1, respectively. On the methyl-terminated Si(111) surface, a CH3 symmetrical bending vibrational mode at 1257 cm-1 polarized perpendicular to the surface was observed. On the C2H5-terminated surface, a Si-H stretching and bending motion at 2080 and 627 cm-1, respectively, were observed, confirming that unalkylated Si atoms are terminated with H atoms. Structural morphology of the CH3-terminated Si(111) surface was investigated by low energy electron diffraction (LEED), which found that this surface retained a flat, unreconstructed (1 x 1) structure. Scanning tunneling microscopy (STM) images of CH3-Si(111) were obtained at 4.7 and 77 K. The functionalized surface preserved the atomically flat morphology of freshly etched H-Si(111), and at 4.7 K individual methyl H atoms were clearly resolved for the first time. Electronic passivation of the alkylated Si(111) surface was investigated through measurement of surface charge carrier recombination velocities (S), through time-resolved radio frequency (rf) photoconductivity decay methods. While unpassivated H- and Cl-terminated surfaces reacted rapidly in an air ambient to yield S>1400 cm s-1, alkylated surfaces preserved S<200 cm s-1 even when exposed to air for a period of weeks. Finally, the two-step chlorination/alkylation route was compared to three other Si(111) surface functionalization techniques: (1) chlorination with Cl2(g) followed by alkylation with an alkylmagnesium halide reagent, (2) Lewis acid-mediated reduction of a terminal alkene, and (3) anodization of the H-terminated Si(111) surface in diethyl ether containing 3.0 M CH3MgI. The chemical properties and charge carrier recombination rates of each surface were measured as a function of time exposed to air.