[show abstract][hide abstract] ABSTRACT: XeF2 treatment of aluminum and alumina surfaces is known to produce hydrophilic surfaces. There is however poor knowledge of the chemical nature of these surfaces. Using infrared absorption and X-ray photoelectron spectroscopy, the formation of highly hydrophilic AlF3 and AlOxFy surface layers is identified upon XeF2 exposure, along with strongly bound H2O and other related surface species formed by interactions with trace H2O under typical vacuum conditions (≈10–4 Torr). Surfaces resulting from XeF2 etching of oxide-free aluminum covered by a sacrificial Si layer have a strong affinity for H2O, with a contact-angle of ca. 5–10°. First-principles simulations offer new insight into details of the AlFx surface structure, based on the surface IR characterization by providing reliable assignments for associated AlFx···H2O infrared bands and showing that fluorine is strongly bound to Al, preventing further Al oxidation. The formation of hydrophilic AlF3 surface layers upon fluorine-based etching may pose a fundamental limitation for the use of Al in microelectromechanical (MEMs) applications, precluding the release of low-stiction, low-capillary force components.
[show abstract][hide abstract] ABSTRACT: XeF2 interaction with SiO2/Si stacks has been investigated to understand the role of Si in proximity of SiO2 during XeF2 exposures of Si/SiO2 stacks. In situ Fourier transform infrared absorption spectroscopy, using a custom-made reaction cell compatible with high XeF2 pressures, reveals that, while pure SiO2 is not etched by XeF2, the oxide in SiO2/Si stacks is effectively removed when XeF2 has access to the silicon, i.e., when the Si in close proximity to the oxide is etched. Thick oxides ( ∼ 1–2 μm) are removed if sample edges are accessible, while thinner oxides (50–100 nm) are removed without requiring edge access. This unexpected SiO2 removal is found to be due to the formation of reactive fluorine species (XeF and F) evolved by the reaction of XeF2 with Si, which can, subsequently, etch SiO2. Calculations based on density functional theory provide critical insight into the underlying energetics and reaction pathways controlling XeF2 etching of both Si and SiO2.
Journal of Applied Physics 12/2010; 108(11):114914-114914-11. · 2.21 Impact Factor
[show abstract][hide abstract] ABSTRACT: We demonstrate that interfacial SiO2, usually formed during high-κ oxide growth on silicon using ozone (O3), is suppressed during Al2O3 atomic layer deposition (ALD) by decreasing the O3 flow rate. First-principles calculations indicate that oxygen introduced by the first low-dose O3 exposure is inserted into the surface nucleation layer rather than the Si lattice. Subsequent Al2O3 deposition further passivates the surface against substrate oxidation. Aluminum methoxy [–Al(OCH3)2] and surface Al–O–Al linkages formed after O3 pulses are suggested as the reaction sites for trimethylaluminum during ALD of Al2O3.
[show abstract][hide abstract] ABSTRACT: The initial surface chemistry and growth mechanisms of the atomic layer deposition (ALD) of metallic copper on SiO(2) surfaces are investigated using an amidinate precursor (copper(I) di-sec-butylacetamidinate, [Cu((s)Bu-amd)](2)) and molecular hydrogen. Using in situ Fourier transform infrared spectroscopy together with calculations based on density functional theory, we show that the initial surface reaction of [Cu((s)Bu-amd)](2) with hydroxylated SiO(2) takes place by displacement of one of the sec-butylacetamidinate ligands at a surface -OH site, thus forming a Si-O-Cu-((s)Bu-amd) surface species, evident by the stretching vibrations of Si-O-Cu and the chelating -NCN- bonds. Molecular hydrogen exposure during a subsequent pulse dissociates most of the sec-butylacetamidinate ligands bound to surface Cu, which releases free amidine vapor, leaving Cu atoms free to agglomerate on the surface and thus opening more reactive sites for the next [Cu((s)Bu-amd)](2) pulse. Copper agglomeration is evident in the IR absorbance spectra through the partial recovery of the intensity of SiO(2) optical phonon modes upon H(2) reduction, which was lost after the reaction of [Cu((s)Bu-amd)](2) with the initial SiO(2) surface. The thermally activated ligand rearrangement from a bridging to a monodentate structure occurs above 220 degrees C through hydrogenation of the ligand by surface hydroxyl groups after exposure to a [Cu((s)Bu-amd)](2) pulse. As Cu particles grow with further ALD cycles, the activation temperature is lowered to 185 degrees C, and hydrogenation of the ligand takes place after H(2) pulses, catalyzed by Cu particles on the surface. The surface ligand rearranged into a monodentate structure can be removed during subsequent Cu precursor or H(2) pulses. Finally, we postulate that the attachment of dissociated ligands to the SiO(2) surface during the [Cu((s)Bu-amd)](2) pulse can be responsible for carbon contamination at the surface during the initial cycles of growth, where the SiO(2) surface is not yet completely covered by copper metal.
[show abstract][hide abstract] ABSTRACT: Hydrogen termination of oxidized silicon in hydrofluoric acid results from an etching process that is now well understood and accepted. This surface has become a standard for studies of surface science and an important component in silicon device processing for microelectronics, energy, and sensor applications. The present work shows that HF etching of oxidized silicon carbide (SiC) leads to a very different surface termination, whether the surface is carbon or silicon terminated. Specifically, the silicon carbide surfaces are hydrophilic with hydroxyl termination, resulting from the inability of HF to remove the last oxygen layer at the oxide/SiC interface. The final surface chemistry and stability critically depend on the crystal face and surface stoichiometry. These surface properties affect the ability to chemically functionalize the surface and therefore impact how SiC can be used for biomedical applications.
Journal of the American Chemical Society 11/2009; 131(46):16808-13. · 10.68 Impact Factor
[show abstract][hide abstract] ABSTRACT: Passivation of semiconductor surfaces is conveniently realized by terminating surface dangling bonds with a monovalent atom such as hydrogen using a simple wet chemical process (for example, HF treatment for silicon). However, the real potential of surface chemical passivation lies in the ability to replace surface hydrogen by multivalent atoms to form surfaces with tailored properties. Although some progress has been made to attach organic layers on top of H-terminated surfaces, it has been more challenging to understand and control the incorporation of multivalent atoms, such as oxygen and nitrogen, within the top surface layer of H-terminated surfaces. The difficulty arises partly because such processes are dominated by defect sites. Here, we report mechanistic pathways involved in the nitridation of H-terminated silicon surfaces using ammonia vapour. Surface infrared spectroscopy and first-principles calculations clearly show that the initial interaction is dominated by the details of the surface morphology (defect structure) and that NH and NH(2) are precursors to N insertion into Si-Si bonds. For the dihydride-stepped Si(111) surface, a unique reaction pathway is identified leading to selective silazane step-edge formation at the lowest reaction temperatures.
Nature Material 09/2009; 8(10):825-30. · 35.75 Impact Factor
[show abstract][hide abstract] ABSTRACT: Mechanisms of atomic layer deposition (ALD) growth of lanthanum oxide on H-terminated Si(111) using lanthanum tris(N,N′-diisopropylacetamidinate) (La(iPr-MeAMD)3) are investigated using infrared (IR) absorption spectroscopy. The reactivity of this amidinate precursor is high, with almost all surface Si−H bonds consumed after 5 ALD cycles at 300 °C. Gas phase IR spectra show that, although most of the precursor (La(iPr-MeAMD)3) remains intact, a strong feature at 1665 cm−1, characteristic of a hydrogenated and dissociated free ligand with localized electrons in the N−CN bonds, is present. Such partial precursor dissociation in the gas phase is due to hydrolysis by traces of water vapor remaining in the reactor, even after purging. As a result, some Si−O−La bonds are formed upon reaction with the surface during the first La(iPr-MeAMD)3 pulse, prior to any water pulse. During film growth, acetate/carbonate and hydroxyl impurities are incorporated into the film. Annealing to 500 °C in dry N2 removes these impurities but fosters the growth of interfacial SiO2. Deposition at 300 °C leads to decomposition of adsorbed ligands, as evidenced by the formation of cyanamide or carbodiimide vibrational bands (or both) at 1990 and 2110 cm−1, respectively. Despite this decomposition, ideal self-limited ALD growth is maintained because the decomposed ligands are removed by the subsequent water pulse. Growth of pure lanthanum oxide films is often characterized by nonuniform film thickness if purging is not complete because of reversible absorption of water by the La2O3 film. Uniform ALD growth can be maintained without a rigorous dry purge by introducing alternating trimethylaluminum (TMA)/D2O ALD cycles between La/D2O cycles. Chemistry and Chemical Biology Version of Record
[show abstract][hide abstract] ABSTRACT: The growth of of metallic copper by atomic layer deposition (ALD) using copper(I) di-sec-butylacetamidinate ([Cu(sBu-amd)]2) and molecular hydrogen (H2) on SiO2/Si surfaces has been studied. The mechanisms for the initial surface reaction and chemical bonding evolutions with each ALD cycle are inferred from in situ Fourier transform infrared spectroscopy (FTIR) data. Spectroscopic evidence for Cu agglomeration on SiO2 is presented involving the intensity variations of the SiO2 LO/TO phonon modes after chemical reaction with the Cu precursor and after the H2 precursor cycle. These intensity variations are observed over the first 20 ALD cycles at 185°C.
[show abstract][hide abstract] ABSTRACT: The possibility of forming endohedral nanomaterials by introducing guest species into the inner phase of a carbon nanotube may give rise to altered composite system properties through spontaneous inner phase charge transfer and electrostatic interactions. Density functional calculations have been carried out in an effort to illustrate the criterion for guest species/discrete carbon nano-tube inner phase charge transfer and for determining the donor region for external ionization of the composite guest-host system. As confining host systems, a series of discrete (7,7) nanotubes of various lengths, as well as fixed length (8,8) and (9,0) nanotubes were used; in neutral and charged states. The specific cases of charge transfer and electron donor regions are identified and characterized through the examination of a set of endohedral probe species (Na, HF, Br, CN−) and point charge model calculations.
Journal of Computational and Theoretical Nanoscience 05/2006; 3(3):398-404. · 0.67 Impact Factor
[show abstract][hide abstract] ABSTRACT: Tris(8-hydroxyquinoline)aluminum(III), AlQ3, is used in organic light-emitting diodes (OLEDs) as an electron-transport material and emitting layer. The reaction of AlQ3 with trace H2O has been implicated as a major failure pathway for AlQ3-based OLEDs. Hybrid density functional calculations have been carried out to characterize the hydrolysis of AlQ3. The thermochemical and atomistic details for this important reaction are reported for both the neutral and oxidized AlQ3/AlQ3+ systems. In support of experimental conclusions, the neutral hydrolysis reaction pathway is found to be a thermally activated process, having a classical barrier height of 24.2 kcal mol(-1). First-principles infrared and electronic absorption spectra are compared to further characterize AlQ3 and the hydrolysis pathway product, AlQ2OH. The activation energy for the cationic AlQ3 hydrolysis pathway is found to be 8.5 kcal mol(-1) lower than for the neutral reaction, which is significant since it suggests a role for charge imbalance in promoting chemical failure modes in OLED devices.
Physical Chemistry Chemical Physics 04/2006; 8(12):1371-7. · 3.83 Impact Factor
[show abstract][hide abstract] ABSTRACT: Peptide nucleic acid (PNA) is a synthetic analogue of deoxyribonucleic acid (DNA) capable of tightly binding to itself and DNA with high specificity. Using hybrid density functional methods, hydrogen-bond (H-bond) strengths have been evaluated for isolated Watson-Crick base pairs, PNA base pairs, and charged as well as neutral DNA base pairs. Heterogeneous base pairs of PNA with charged and neutral DNA have also been investigated. The competing effects of short-range H-bonding and long-range Coulombic repulsions in charged DNA base pairs have been analyzed. Polarizable continuum models have been employed to evaluate solvation effects on the binding energies.
The Journal of Physical Chemistry B 03/2006; 110(7):3336-43. · 3.61 Impact Factor
[show abstract][hide abstract] ABSTRACT: Density functional calculations have been carried out to investigate the nature of the inner phase of a (6,6) carbon nanotube, using the Cl(-) exchange S(N)2 reaction as an indicator. Inside the carbon nanotube the classical barrier height increases by 6.6 kcal/mol due to the nanotube polarizability. This suggests that the inner phase environment can be considered a form of solid solvation, offering the possibility of obtaining altered guest properties and reactivity through dielectric stabilization.
[show abstract][hide abstract] ABSTRACT: Hybrid density functional calculations have been carried out using cluster models of the H/Si(100)-2 x 1 surface to investigate the mechanistic details of the initial surface reactions occurring in the atomic layer deposition of hafnium and zirconium oxides (HfO2 and ZrO2). Reaction pathways involving the metal precursors ZrCl4, Zr(CH3)4, HfCl4, and Hf(CH3)4 have been examined. Pathways leading to the formation of a Zr-Si or Hf-Si linkage show a significant sensitivity to the identity of the leaving group, with chloride loss reactions being both kinetically and thermodynamically less favorable than reactions leading to the loss of a methyl group. The energetics of the Zr(CH3)4 and Hf(CH3)4 reactions are similar with an overall exothermicity of 0.3-0.4 eV and a classical barrier height of 1.1-1.2 eV. For the reaction between H2O and the H/Si(100)-2 x 1 surface, the activation energy and overall reaction enthalpy are 1.6 and -0.8 eV, respectively. Due to contamination, trace amounts of H2O may be encountered by metal precursors, leading to the formation of minor species that can lead to unanticipated side-reaction pathways. Such gas-phase reactions between the halogenated and alkylated metal precursors and H2O are exothermic with small or no reaction barriers, allowing for the possibility of metal precursor hydroxylation before the H/Si surface is encountered. Of the contaminant surface reaction pathways, the most kinetically favorable corresponds to the surface -OH deposition. Interestingly, for the hydroxylated metal precursors, a unique reaction pathway resulting in the direct formation of Si-O-Zr and Si-O-Hf linkages has been identified and found to be the most thermodynamically stable pathway available, being exothermic by approximately 1.0 eV.
The Journal of Physical Chemistry B 04/2005; 109(11):4969-76. · 3.61 Impact Factor
[show abstract][hide abstract] ABSTRACT: Internal photoemission (IPE) studies were performed on molecular diodes in which the alkanedithiol [HS(CH(2))(n)SH, n = 8, 10] molecular layer is sandwiched between Au and GaAs electrodes. The results are compared to those from Au-GaAs Schottky diodes. An exponential energy dependence in the IPE yield was observed for the molecular diodes, in contrast to the quadratic energy dependence characteristic of metal-semiconductor Schottky diodes, indicating that Au is not the source of electrons in the IPE process in the molecular diodes. From the GaAs dopant density dependence, we also can rule out GaAs being the source of these electrons. Compared with the results of cluster electronic structure calculations, we suggest that IPE is probing the occupied levels of GaAs-molecular interfacial states.
The Journal of Physical Chemistry B 04/2005; 109(12):5719-23. · 3.61 Impact Factor
[show abstract][hide abstract] ABSTRACT: Infrared absorption spectroscopy was used to investigate the chlorination of hydrogen-terminated Si(111) surfaces by three different methods: (a) exposure to a saturated solution of phosphorus pentachloride (PCl5) in chlorobenzene; (b) exposure to chlorine gas, Cl2(g), and (c) exposure to Cl2(g) under UV illumination. X-ray photoelectron spectroscopy and first principles model (clusters) calculations were used to explore the structure and dynamics of these surfaces. The infrared spectra exhibited sharp chlorine-related vibrations at 586 and 527 cm^–1. The narrow full width at half maximum of these vibrations for all three preparation methods indicated that all functionalization schemes produced a nearly complete monolayer of Cl with little surface roughening or introduction of step edges. The 527 cm^–1 mode was at a much higher frequency than might be expected for the bending vibration of Si monochloride. Theoretical calculations show, however, that this vibration involves the displacement of the top Si atom parallel to the surface, subject to a relatively stiff potential, shifting its frequency to a value fairly close to that of the Si–Cl stretching mode on a Si(111) surface.
Journal of Vacuum Science & Technology A Vacuum Surfaces and Films 01/2005; · 1.43 Impact Factor
[show abstract][hide abstract] ABSTRACT: Hybrid density functional calculations have been carried out on cluster models of the pristine and oxidized hydrogen-terminated Si(100)-2×1 surface to investigate the changes in the surface infrared absorption spectrum upon incorporation of oxygen. Due to a combination of geometric and electronic effects, a dramatic increase in the ν(Si−H) band intensity is found for the oxidized surface. An increase in infrared intensity of up to 62% is predicted for multiply oxidized sites. The results presented here may prompt the reinterpretation of infrared studies examining silicon surface chemistry using the ν(Si−H) region to probe changes in surface structure and coverage.
Journal of Physical Chemistry B - J PHYS CHEM B. 11/2004; 108(50).
[show abstract][hide abstract] ABSTRACT: Cluster calculations employing hybrid density functional theory have been carried out to examine the atomistic details and thermochemistry of the early stages of Al2O3 atomic layer deposition (ALD) on the Si(100)-2×1 surface using the gas-phase precursors, trimethylaluminum (TMA) and H2O. The critical point structures and enthalpies characterizing both the Al- and O-deposition half-reactions were investigated. Both sets of ALD half-reactions were found to be thermodynamically favorable and kinetically uninhibited. For all reactions the transition states and reaction products were determined to be lower in energy than the starting reactants. The H2O ALD half-reactions were found to have an overall reaction enthalpy between −1.45 and −1.63 eV, with a transition state energy of −0.13 to −0.21 eV. The TMA ALD half-reactions were found to be exothermic by 1.85−1.88 eV. The transition state energy for the Al deposition half-reactions were determined to be −0.19 to −0.27 eV. Careful comparison of the reaction enthalpies suggests a small reactivity dependence on neighboring −OH* groups, activating and suppressing the Al- and O-deposition ALD half-reactions, respectively.