Molecular self-assembly at bare semiconductor surfaces: Investigation of the chemical and electronic properties of the alkanethiolate-GaAs(001) interface

High-resolution X-ray photoelectron spectroscopy (HRXPS), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and Raman scattering have been used to characterize the bonding and electronic properties of the interfaces formed by ambient-temperature, solution self-assembly of octadecanethiol and dodecanethiol under oxygen-free conditions on GaAs(001) surfaces. The combination of HRXPS and ToF-SIMS shows that both monolayers form direct S-[GaAs] attachment to the bare substrate with dominant As-S bonding and some fraction of Ga-S bonding, but the densely packed octadecanethiolate self-assembled monolayers (SAMs) are significantly more effective in protecting the interface from oxide regrowth compared to the less dense dodecanethiolate SAMs. Raman scattering measurements of the GaAs LO phonon modes indicate that the formation of these direct, oxide-free S-[GaAs] interfaces does not induce any significant changes in the GaAs surface electronic states, whereas control experiments with inorganic sulfide treatment does indicate a significant reduction in the surface traps, consistent with earlier reports. Overall these experiments show that the formation of direct alkane-S-[GaAs(001)] interfaces of high quality provides a method to passivate the GaAs surface against oxide regrowth but does not necessarily lead to changes in the surface electronic states.