The concept of proton affinity on semiconductor surfaces has been explored through an investigation of the chemistry of amines on the Ge(100)-2 x 1, Si(100)-2 x 1, and C(100)-2 x 1 surfaces. Multiple internal reflection Fourier transform infrared (MIR-FTIR) spectroscopy, temperature programmed desorption (TPD), and density functional theory (DFT) calculations were used in the studies. We find that methylamine, dimethylamine, and trimethylamine undergo molecular chemisorption on the Ge(100)-2 x 1 surface through the formation of Ge-N dative bonds. In contrast, primary and secondary amines react on the Si(100)-2 x 1 surface via N-H dissociation. Since N-H dissociation of amines at semiconductor surfaces mimics a proton-transfer reaction, the difference in chemical reactivities of the Ge(100)-2 x 1 and Si(100)-2 x 1 surfaces toward N-H dissociation can be interpreted as a decrease of proton affinity down a group in the periodic table. The trend in proton affinities of the two surfaces is explained in terms of thermodynamics and kinetics. Solid-state effects on the C(100)-2 x 1 surface and the surface proton affinity concept are discussed based on our theoretical predictions.

In-situ attenuated total internal reflection infrared spectroscopy has been used to examine initial adsorption and reaction steps in atomic layer deposition of HfO 2 from tetrakis(diethylamino) hafnium (TDEAHf) on SiO 2 and hydrogen-terminated Si(100) surfaces. At low deposition temperatures (25-250 °C), TDEAHf directly reacts with the Si-H surface, resulting in partial removal of Si-H bonds and formation of a four-membered Si-O-Hf-Si bonding structure that can rapidly oxidize. The hydrogen removal process is observed to continue through many cycles of TDEAHf/H 2 O exposure, signifying continued reactivity of the Hf precursor with the silicon surface. Density functional theory calculations have been performed for various reactions between tetrakis(dimethylamino) hafnium and Si-H surfaces, and several possible reaction pathways for hydrogen removal have been identified and analyzed. The calculations suggest that hydrogen removal proceeds by H abstraction by an amine ligand of the Hf precursor and that the abstraction reaction is made more facile by the presence of OH on the otherwise H-terminated Si surface.

Titanium-oxygen bonds derived from stable nitroxyl radicals are remarkably weak and can be homolyzed at 60 degrees C. The strength of these bonds depends sensitively on the ancillary ligation at titanium. Direct measurements of the rate of Ti-O bond homolysis in Ti-TEMPO complexes Cp2TiCl(TEMPO) (3) and Cp2TiCl(4-MeO-TEMPO) (4) (TEMPO = 2,2,6,6-tetramethylpiperidine-N-oxyl, 4-MeO-TEMPO = 2,2,6,6-tetramethyl-4-methoxypiperidine-N-oxyl) were conducted by nitroxyl radical exchange experiments. Eyring plots gave the activation parameters, deltaH++ = 27(+/- 1) kcal/mol, deltaS++ = 6.9(+/- 2.3) eu for 3 and deltaH++ = 28(+/- 1) kcal/mol, deltaS++ = 9.0(+/- 3.0) eu for 4, consistent with a process involving the homolysis of a weak Ti-O bond to generate the transient Cp2Ti(III)Cl and the nitroxyl radical. Thermolysis of the titanocene TEMPO complexes in the presence of epoxides leads to the Cp2Ti(III)Cl-mediated ring-opening of the epoxide followed by trapping by the nitroxyl radical. The X-ray crystal structure of the Ti-TEMPO derivative, Cp2TiCl(4-MeO-TEMPO) (4), is reported. DFT (B3LYP/6-31G*) calculations and experimental studies reveal that the strength of the Ti-O bond decreases dramatically with the number of cyclopentadienyl groups on titanium. The calculated Ti-O bond strength of the monocyclopentadienyl complex 2 is 43 kcal/mol, whereas that of the biscyclopentadienyl complex 3 is 17 kcal/mol, a difference of 26 kcal/mol. These studies reveal that the strength of these Ti-O bonds can be tuned over an interesting and experimentally accessible temperature range by appropriate ligation on titanium.

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