Juan Carlos F. Rodríguez-Reyes

Tuning the reactivity of semiconductor surfaces by functionalization with amines of different basicity

Surface functionalization of semiconductors has been the backbone of the newest developments in microelectronics, energy conversion, sensing device design, and many other fields of science and technology. Over a decade ago, the notion of viewing the surface itself as a chemical reagent in surface reactions was introduced, and adding a variety of new functionalities to the semiconductor surface has become a target of research for many groups. The electronic effects on the substrate have been considered as an important consequence of chemical modification. In this work, we shift the focus to the electronic properties of the functional groups attached to the surface and their role on subsequent reactivity. We investigate surface functionalization of clean Si(100)-2 × 1 and Ge(100)-2 × 1 surfaces with amines as a way to modify their reactivity and to fine tune this reactivity by considering the basicity of the attached functionality. The reactivity of silicon and germanium surfaces modified with ethylamine (CH3CH2NH2) and aniline (C6H5NH2) is predicted using density functional theory calculations of proton attachment to the nitrogen of the adsorbed amine to differ with respect to a nucleophilic attack of the surface species. These predictions are then tested using a model metalorganic reagent, tetrakis(dimethylamido)titanium (((CH3)2N)4Ti, TDMAT), which undergoes a transamination reaction with sufficiently nucleophilic amines, and the reactivity tests confirm trends consistent with predicted basicities. The identity of the underlying semiconductor surface has a profound effect on the outcome of this reaction, and results comparing silicon and germanium are discussed.

Mechanisms of adsorption and decomposition of metal alkylamide precursors for ultrathin film growth

Atomic layer deposition film growth is usually characterized by the presence of a transient (nonlinear) regime, where surface reactions of precursors take place on the substrate, resembling the first stages of chemical vapor deposition and affecting the composition of the forming interface. Here, the adsorption and decomposition of tetrakis(dimethylamido)titanium, Ti[N(CH3)2]4, tetrakis(dimethylamido)zirconium, Zr[N(CH3)2]4, tetrakis(dimethylamido)hafnium, Hf[N(CH3)2]4, pentakis(dimethylamido)tantalum, Ta[N(CH3)2]5, and bis(t-butylimido)-bis(dimethylamido)tungsten, [(CH3)3CN]2W[N(CH3)2]2, on a silicon substrate are investigated using density functional methods. These alkylamides are widely used for deposition of both diffusion barriers and high-permittivity (high-κ) materials. Adsorption is found to be dissociative, with scission of metal-ligand bonds being more feasible than scission of N–C bonds, suggesting that decomposition of ligands is not favored at low temperatures. However, decomposition through ...

Competing reactions during metalorganic deposition: Ligand-exchange versus direct reaction with the substrate surface

Surface-mediated reactions of metalorganic compounds on solid substrates are key processes in film deposition technology, especially in atomic layer deposition (ALD) or chemical vapor deposition. Since most applications of thin films require high purity, understanding and controlling the mechanisms of desired and undesired surface reactions are of the utmost importance. This work outlines a general approach to understand potential surface reactions during deposition through density functional theory calculations, considering precursors containing the most commonly used types of ligands, namely alkyl (Al(CH3)3), alkoxide (Ti[OC3H7]4), alkylamide (Hf[N(CH3)2]4), diketonate (Cu(acac)2), amidinate (Ni[Pr-amd]2), and cyclopentadienyl (Hf(Cp)2(CH3)2). In all cases, the “desired” ligand-exchange reaction (the basis of most ALD processes) is compared to “undesired” surface reactions, where the ligands of the precursor interact with reactive surface sites and can undergo uncontrolled decomposition pathways, incorp...

Interpretation of temperature-programmed desorption data with multivariate curve resolution: Distinguishing sample and background desorption mathematically

Efficient interpretation of thermal desorption data for complex surface processes is often complicated further by species desorbing from heating elements, support materials, and sample holder parts. Multivariate curve resolution (MCR) can be utilized as an unbiased method to assign specific temperature-dependent profiles for evolution of different species from the target surface itself as opposed to traces evolving from the surroundings. Analysis of thermal desorption data for iodoethane, where relatively low exposures are needed to form a complete monolayer on a clean Si(100)-2 × 1 surface in vacuum, provides convenient benchmarks for a comparison with the chemistry of chloroethane on the same surface. In the latter set of measurements, very high exposures are required to form the same type of species as for iodoethane, and the detection and analysis process is complicated by both the desorption from the apparatus and by the presence of impurities, which are essentially undetectable during experiments wi...