Phosphonic acid monolayers are being considered as versatile surface modification agents due to their unique ability to attach to surfaces in different configurations, including mono-, bi-, or even tridentate arrangements. Tethering by aggregation and growth (T-BAG) of octadecylphosphonic acid (ODPA) on silicon oxide surfaces has proven to be a robust method to establish a strong chemical bond. However, it requires a long processing time (> 48 h) that is a substantial drawback for industrial applications. We demonstrate here that the humidity level during processing is the most important parameter controlling the reaction. Using in situ Fourier Transform Infrared Spectroscopy (FTIR), we first show that the initially physisorbed layer obtained upon immersion in ODPA is composed of well-ordered bilayers and only reacts with the SiO(2) surface at 140 °C. Importantly, we show that the presence of water at the interface (determined by the humidity level) greatly influences the reaction time and completion. In humid environments (relative humidity, RH > 40%), there is no reaction, while in dry environments (RH < 16%), the reaction is essentially instantaneous at 140 °C. Ab initio calculations and modeling confirm that the degree of chemical reaction with the surface OH groups depends on the chemical potential (i.e., concentration) of interfacial water molecules. These findings provide a workable modification of the traditional T-BAG method consistent with many industrial applications.

Deposition of thin films and grafting of organic molecules on semiconductor surfaces, particularly oxide surfaces, are widely studied as means of passivation and functionalization for a variety of applications. However, organic functionalization of silicon oxide is challenging, as the currently used molecules (silanes and phosphonates) do not form layers that are stable in aqueous environments and present challenges during the grafting process. For instance, the chemical grafting of phosphonates requires high temperature (140 °C) to perform. Modification of SiO(2) surfaces with metal oxides is an attractive alternative since strong bonds can be established between metal oxides and relevant molecules (silanes, phosphonates). While such modification is possible using vapor-phase methods, such as atomic layer deposition and physical vapor-phase deposition, wet chemical processing is inexpensive and technologically very attractive. We describe here a simple wet chemical method to deposit an ultrathin layer of metal oxide/hydroxide groups. Further, using a model surface with exactly one-third monolayer OH groups on oxide-free Si surfaces, the precise adsorption geometry on single Al(OH)(3) groups is shown to be bidentate, and the distance between the Al and P atoms is determined to be the main influencing parameter for a thermodynamically stable formation of the Al-O-P bond.

ter.5b04394. Complete information on the kinetics and energetics of the As MLD process, together with the description of the experimental setup and the computational methodology and additional TOF-SIMS and electrical characterization (PDF)

In an effort to grow metal oxide films (e.g., MoO3) at low temperatures, a novel molybdenum precursor, Si­(CH3)3CpMo­(CO)2(η3-2-methylallyl) or MOTSMA, is used with ozone as the coreactant. As is often observed in atomic layer deposition (ALD) processes, the deposition of molybdenum trioxide displays an incubation period (∼15 cycles at 250 °C). In situ FTIR spectroscopy reveals that ligand exchange reactions can be activated at 300 °C, leading to a shorter incubation periods (e.g., ∼ 9 cycles). Specifically, the reaction of MOTSMA with OH-terminated silicon oxide surfaces appears to be the rate limiting step, requiring a higher temperature activation (350 °C) than the subsequent ALD process itself, for which 250 °C is adequate. Therefore, in order to overcome the nucleation delay, the MOTSMA precursor is initially grafted at 350 °C, with spectroscopic evidence of surface reaction, and the substrate temperature then lowered to 250 or 300 °C for the rest of the ALD process. After this initial activation, a standard ligand exchange is observed with formation of surface Si­(CH3)3CpMo­(η3-2-methylallyl) after precursor and its removal after ozone exposures, resulting in Mo­(O)2 formation. Under these conditions, the ALD process proceeds with no nucleation delay at both temperatures. Postdeposition X-ray photoelectron spectroscopy spectra confirm that the film composition is MoO3. This work highlights the critical role of precursor grafting to the substrate as essential to eliminate the nucleation delay for ultrathin ALD grown film deposition.

The reaction pathway for shallow arsenic doping of silicon by methylarsenic acid molecules directly grafted on oxide-free, H-terminated Si(111) surfaces is unraveled combining Infrared absorption spectroscopy, X-ray Photoelectron Spectroscopy, Low Energy Ion Scattering and ab initio Molecular Dynamics simulations. The overall driving force is identified as a thermodynamic instability of As+5 in contact with silicon, which initiates a self-decomposition of chemisorbed methylarsenic molecules at ∼600 K. As the temperature is increased, the As-C bond breaks -- the weakest link of the adsorbed molecule -- with release of the organic ligand and a rearrangement from a monodentate to a bidentate bonding configuration. In this process, oxygen atoms evolve by partial desorption as H2O and partial incorporation into the surface Si atom backbonds. At ∼1050 K, diffusion of As into the sub-surface region of silicon is observed. There is no evidence for As desorption and no remaining C contamination.