The ability to selectively chemically functionalize silicon nitride (Si3N4) or silicon dioxide (SiO2) surfaces after cleaning would open interesting technological applications. In order to achieve this goal, the chemical composition of surfaces needs to be carefully characterized so that target chemical reactions can proceed on only one surface at a time. While wet-chemically cleaned silicon dioxide surfaces have been shown to be terminated with surficial Si-OH sites, chemical composition of the HF-etched silicon nitride surfaces is more controversial. In this work, we removed the native oxide under various aqueous HF-etching conditions and studied the chemical nature of the resulting Si3N4 surfaces using infrared absorption spectroscopy (IRAS), x-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and contact angle measurements. We find that HF-etched silicon nitride surfaces are terminated by surficial Si-F and Si-OH bonds, with slightly subsurface Si-OH, Si-O-Si, and Si-NH2 groups. The concentration of surficial Si-F sites is not dependent on HF concentration, but the distribution of oxygen and Si-NH2 displays a weak dependence. The Si-OH groups of the etched nitride surface are shown to react in a similar manner to the Si-OH sites on SiO2, and therefore no selectivity was found. Chemical selectivity was, however, demonstrated by first reacting the -NH2 groups on the etched nitride surface with aldehyde molecules, which do not react with the Si-OH sites on a SiO2 surface, and then using trichloro-organosilanes for selective reaction only on the SiO2 surface (no reactivity on the aldehyde-terminated Si3N4 surface).

Amine termination of surfaces constitutes a core platform for fields as diverse as microelectronics and bioengineering, and for nanotechnology in general. Diamines are particularly attractive for surface amination because unlike ammonia or simple amine molecules, they have a metal chelating capability useful in fabricating heterostructures. They can act as a linker molecule between inorganic electronic materials and biomolecules or photoactive quantum dots for applications in microelectronic, photonics, and biosensing. In contrast to ammonia modification of silicon surfaces, the direct grafting of diamine on silicon surfaces has been less explored. In this work, the attachment of liquid and vapor-phase ethylenediamine (EDA) on three types of oxide-free (H-, 1/3 ML F-, and Cl-terminated) Si( 111) surfaces is therefore examined by infrared absorption spectroscopy and X-ray photoelectron spectroscopy in conjunction with first-principle calculations. We find that EDA chemisorption is only possible on 1/3 ML F-and Cl-terminated Si(111) surfaces: EDA only physisorbs on Hterminated Si(111) surfaces. On Cl-terminated Si(111) surfaces, EDA molecules adsorb in a mixture of monodentate and bridging configurations (chemical reaction of both EDA end groups), while on 1/3 ML F-terminated Si(111) surfaces the adsorption occurs primarily at one end of the molecule. EDA reaction with Cl-terminated Si(111) surfaces is also characterized by complete removal of Cl and partial Si−H (∼25% ML) formation on the surface. This unexpected Si−H product suggests that a proton−chlorine exchange may take place, with the endothermic barrier possibly reduced via concerted chemical reactions after an initial attachment of EDA to the surface.

Amination of surfaces is useful in a variety of fields, ranging from device manufacturing to biological applications. Previous studies of ammonia reaction on silicon surfaces have concentrated on vapor phase rather than wet chemical processes, and mostly on clean Si surfaces. In this work, the interaction of liquid and vapor-phase ammonia is examined on three types of oxide-free surfaces-passivated by hydrogen, fluorine (1/3 monolayer) or chlorine-combining infrared absorption spectroscopy, X-ray photoelectron spectroscopy and first-principles calculations. The resulting chemical composition highly depends on the starting surface; there is a stronger reaction on both F-and Cl-terminated than on the H-terminated Si surfaces, as evidenced by the formation of Si-NH 2. Side reactions can also occur, such as solvent reaction with surfaces, formation of ammonium salt by-products (in the case of 0.2 M ammonia in dioxane solution), and nitridation of silicon (in the case of neat and gas phase ammonia reactions for instance). Unexpectedly, there is formation of Si-H bonds on hydrogen-free Cl-terminated Si(111) surfaces in all cases, whether vapor phase of neat liquid ammonia is used. First principles modeling of this complex system suggests that step edge surface defects may play a key role in enabling the reaction under certain circumstances, despite the endothermic nature for Si-H bond formation.