Hydrogen fluoride adsorption and reaction on the <i>α</i>-Al2O3(0001) surface: A density functional theory study

Jie-Li, Quan; Teng, Botao; Wen, Xiaodong; Zhao, Yüe; Liu, Rui; Luo, Ming

doi:10.1063/1.3694102

Cited by 17 publications

(11 citation statements)

References 50 publications

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“…From the figure, it can be seen that the electronegative F atom gains electrons that are polarized toward the bound Al atom at the surface and that the H atom forms a covalent bond with a surface oxygen atom ( d O–H = 0.99 Å). The optimized geometry and charge density difference plots are similar to those of one HF adsorbed on the α-Al 2 O 3 (0 0 0 1) surface reported by Quan et al We also performed Bader charge analysis and found that the surface O–H and Al–F have charges of −0.85 and +1.62, respectively. More information about this is given in section S6 of the Supporting Information.…”

Section: Resultssupporting

confidence: 81%

“…For example, a first-principles simulation of ALD of alumina on the α-Al 2 O 3 (0 0 0 1) surface has been published . The mechanism of HF adsorption on the α-Al 2 O 3 (0 0 0 1) surface has been studied in detail by Quan et al However, ALD-deposited alumina is found to transform under annealing into θ-Al 2 O 3 , which is the most stable transition phase of alumina. We therefore use θ-Al 2 O 3 as a periodic model for ALD-grown alumina in this study.…”

Section: Methods and Computational Detailsmentioning

confidence: 99%

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Modeling the Chemical Mechanism of the Thermal Atomic Layer Etch of Aluminum Oxide: A Density Functional Theory Study of Reactions during HF Exposure

Natarajan

Elliott

2018

Chem. Mater.

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Thermal atomic layer etch, the reverse of atomic layer deposition, uses a cyclic sequence of plasma-free and solvent-free gas surface reactions to remove ultrathin layers of material with a high degree of control. A theoretical investigation of the hydrogen fluoride pulse in the thermal atomic layer etch of monoclinic alumina has been performed using density functional theory calculations. From experiments, it has been suggested that the HF pulse forms a stable and nonvolatile layer of AlF 3 on an alumina surface. Consistent with this, the desorption of an AlF 3 molecule from an HF-saturated surface was computed to be energetically unfavorable. HF molecules adsorbed on the alumina surface by forming hydrogen bonds and either remained intact or dissociated to form Al−F and O−H species. At higher coverages, a mixture of molecularly and dissociatively adsorbed HF molecules in a hydrogen-bonded network was observed. Binding energies converged as the coverage of dissociated F became saturated, consistent with a self-limiting reaction. The formation of H 2 O molecules in the HF pulse was found to be endoergic with an energy barrier of at least +0.9 eV, but their subsequent desorption was computed to cost as little as +0.2 eV. On the basis of a model of the saturated Al−F surface, the theoretical maximum of the etch rate was estimated to be −0.57 ± 0.02 Å/ cycle (−20.0 ± 0.8 ng cm −2 cycle −1 ), which matches the range of maximum experimental values. The actual etch rate will, however, be dependent on the specific reagent used in the subsequent step of the atomic layer etch cycle.

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Section: Resultssupporting

confidence: 81%

Section: Methods and Computational Detailsmentioning

confidence: 99%

Modeling the Chemical Mechanism of the Thermal Atomic Layer Etch of Aluminum Oxide: A Density Functional Theory Study of Reactions during HF Exposure

Natarajan

Elliott

2018

Chem. Mater.

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“…The loss of HF is believed to be the result of surface interactions with the α-alumina reactor tube. Indeed, there is experimental evidence in the literature for HF adsorption under dry conditions on α-alumina. − This phenomenon was further observed at temperatures above 550 °C, where it was noted that HF desorption from the α-alumina tube surface started to occur. Consequently, the concentration of SO 2 was used as a proxy for PFOS conversion in the estimation of the rate constant k 1 for the α-alumina tube.…”

Section: Resultsmentioning

confidence: 93%

Kinetics of Decomposition of PFOS Relevant to Thermal Desorption Remediation of Soils

Weber

Stockenhuber

Delva

et al. 2021

Ind. Eng. Chem. Res.

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Kinetics of pyrolysis of the pollutant perfluorooctanesulfonic acid (PFOS) in inert bath gases have been studied in two flow reactors constructed of α-alumina and of stainless steel (SS) at temperatures between 400 and 615 °C. Results from the SS reactor give support to previous and our own quantum chemical calculations based on smaller perfluorinated sulfonates, according to which initiation of decomposition of PFOS first takes place by elimination of HF to form an unstable α-sultone with a rate constant, k 1 . The sultone then rapidly liberates SO 2 and forms perfluorooctanoyl fluoride with a rate constant, k 2 with k 2 ≫ k 1 such that the overall rate constant k′ ≈ k 1 . Products observed from both reactors in the above temperature range comprised HF, SO 2 , and perfluorooctanoyl fluoride. The value of the rate constant for the formation of HF and SO 2 measured in the SS reactor was found to be k 1 = (1.3 ± 0.5) × 10 14 exp(−253 ± 5 kJ/mol/RT) s −1 .

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“…This study also complements previous theoretical and experimental examinations of the fluorination of Al 2 O 3 . Earlier theoretical investigations have studied the fluorination of α-Al 2 O 3 by HF using density functional theory (DFT) methods. , Recent theoretical investigations have also examined the fluorination of Θ-Al 2 O 3 surfaces by HF and compared the fluorination to measured etch rates during Al 2 O 3 ALE . Many experimental studies have also explored the fluorination of Al 2 O 3 because of the effect of fluorination on heterogeneous catalysis. , Fluorination is known to create strong Lewis acid sites that are important in a variety of reactions.…”

Section: Introductionmentioning

confidence: 99%

Effect of HF Pressure on Thermal Al₂O₃Atomic Layer Etch Rates and Al₂O₃Fluorination

et al. 2019

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Thermal Al2O3 atomic layer etching (ALE) can be accomplished using sequential fluorination and ligand-exchange reactions. HF can be employed as the fluorination reactant, and Al(CH3)3 can be utilized as the metal precursor for ligand exchange. This study explored the effect of HF pressure on the Al2O3 etch rates and Al2O3 fluorination. Different HF pressures ranging from 0.07 to 9.0 Torr were employed for Al2O3 fluorination. Using ex situ spectroscopic ellipsometry (SE) measurements, the Al2O3 etch rates increased with HF pressures and then leveled out at the highest HF pressures. Al2O3 etch rates of 0.6, 1.6, 2.0, 2.4, and 2.5 Å/cycle were obtained at 300 °C for HF pressures of 0.17, 0.5, 1.0, 5.0, and 8.0 Torr, respectively. The thicknesses of the corresponding fluoride layers were also measured using X-ray photoelectron spectroscopy (XPS). Assuming an Al2OF4 layer on the Al2O3 surface, the fluoride thicknesses increased with HF pressures and reached saturation values at the highest HF pressures. Fluoride thicknesses of 2.0, 3.5, 5.2, and 5.5 Å were obtained for HF pressures of 0.15, 1.0, 4.0, and 8.0 Torr, respectively. There was an excellent correlation between the Al2O3 etch rates and fluoride layer thicknesses versus HF pressure. In addition, in situ Fourier transform infrared spectroscopy (FTIR) vibrational studies were used to characterize the time dependence and magnitude of the Al2O3 fluorination. These FTIR studies observed the fluorination of Al2O3 to AlF3 or AlO x F y by monitoring the infrared absorbance from the Al–O and Al–F stretching vibrations. The time dependence of the Al2O3 fluorination was explained in terms of rapid fluorination of the Al2O3 surface for initial HF exposures and slower fluorination into the Al2O3 near surface region that levels off at longer HF exposure times. Fluorination into the Al2O3 near surface region was described by parabolic law behavior. The self-limiting fluorination of Al2O3 suggests that the fluoride layer on the Al2O3 surface acts as a diffusion barrier to slow the fluorination of the underlying Al2O3 bulk. For equal fluorination times, higher HF pressures achieve larger fluoride thicknesses.

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Hydrogen fluoride adsorption and reaction on the α-Al2O3(0001) surface: A density functional theory study

Cited by 17 publications

References 50 publications

Modeling the Chemical Mechanism of the Thermal Atomic Layer Etch of Aluminum Oxide: A Density Functional Theory Study of Reactions during HF Exposure

Modeling the Chemical Mechanism of the Thermal Atomic Layer Etch of Aluminum Oxide: A Density Functional Theory Study of Reactions during HF Exposure

Kinetics of Decomposition of PFOS Relevant to Thermal Desorption Remediation of Soils

Effect of HF Pressure on Thermal Al₂O₃Atomic Layer Etch Rates and Al₂O₃Fluorination

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Hydrogen fluoride adsorption and reaction on the α-Al2O3(0001) surface: A density functional theory study

Cited by 17 publications

References 50 publications

Modeling the Chemical Mechanism of the Thermal Atomic Layer Etch of Aluminum Oxide: A Density Functional Theory Study of Reactions during HF Exposure

Modeling the Chemical Mechanism of the Thermal Atomic Layer Etch of Aluminum Oxide: A Density Functional Theory Study of Reactions during HF Exposure

Kinetics of Decomposition of PFOS Relevant to Thermal Desorption Remediation of Soils

Effect of HF Pressure on Thermal Al2O3Atomic Layer Etch Rates and Al2O3Fluorination

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Effect of HF Pressure on Thermal Al₂O₃Atomic Layer Etch Rates and Al₂O₃Fluorination