VUV Photolysis of silane with nitric oxide in solid neon

The investigation of reactions between silane (SiH₄) and semiconductor-process gases is of great importance in both fundamental and applied chemistry, particularly in semiconductor manufacturing, chemical vapor deposition, and silane combustion.^1–10 Silane serves as a key precursor for the deposition of silicon-based thin films, which are widely used in the modern semiconductor industry. Understanding its reactivity with various agents is crucial for optimizing processes such as plasma-enhanced chemical vapor deposition (PECVD) and for developing new materials with enhanced properties.^11–15 Among the various semiconductor-process gases, nitrogen oxides (NO_x) are particularly intriguing, as it can act as an efficient scavenger of free radicals, controlling the formation of higher silanes in silane-based processes. Additionally, the study of the vacuum-UV (VUV) photochemistry of silane has garnered considerable attention due to the development of photochemical vapor deposition (Photo-CVD) methods for producing amorphous silicon hydride films.^2–5 

Beyond these practical applications, there is also a fundamental interest in studying the reaction between SiH₄ and nitrogen oxides, as limited information exists regarding molecular formations involving these four atoms [H, N, O, Si].^{11,12,16–18} Although these species have been proposed as intermediates in reaction mechanisms, none have been spectroscopically identified. To our knowledge, only a few experiments have explored these reactions, where SiH₄ undergoes either photolysis or discharge to form SiH₃ or SiH₂, which subsequently react with NO, NO₂, or N₂O.^2–6,17,18 However, the main observed products were silicon oxides and/or siloxanes. In contrast, theoretical predictions suggest the formation of various species containing [H, N, O, Si] atoms from the reactions between SiH₃ or SiH₂ with nitrogen oxides.^11,12,14,16 In this work, we performed VUV photolysis of SiH₄ or SiD₄ with NO in solid neon and identified the resulting photoproducts by comparing the observed infrared (IR) spectra with theoretical predictions of harmonic vibrational frequencies for various [H, N, O, Si] species. In addition to previously identified species, we have tentatively characterized the formation of the new species, H₂SiN(H)O.

The matrix-isolation system used in this work has been described elsewhere.^19–21 Photolysis light at 130 nm was generated from the undulator beamline TLS-21A2 at Taiwan Light Source; the photon flux was estimated to be ≈ 1.0⋅10¹⁵ photons⋅s^–1, with a bandwidth of approximately 1 nm. A nickel-plated copper flat was cooled to serve as the cold substrate for the matrix samples, which also served as a mirror to reflect the probed IR beam to the detector. The base pressure of the cryochamber was maintained at less than 1⋅10⁻⁸ torr before the deposition of a matrix. Matrix samples of SiH₄/Ne (1/500), NO/Ne (1/500), and SiD₄/Ne (1/500) were deposited at 3 K for 2 h at a flow rate of 5–8 mmol h⁻¹. IR absorption spectra covering a spectral range from 600 to 4000 cm⁻¹ were recorded on an FTIR spectrometer (Bruker INVENIO R). Typically, 400 scans at a resolution of 0.5 cm⁻¹ were recorded at each stage of the experiment.

SiH₄ (≈ 99.9% purity, Sigma-Aldrich), SiD₄ (≈ 99% deuterium purity, Sigma-Aldrich), Ne (≈ 99.9995% purity, Scott Specialty Gases), and NO (≈ 98.0% purity, AGEM. Inc) were used without further purification, except for a freeze–pump–thaw procedure performed at 77 K. The temperature of the cold substrate was monitored and controlled by a temperature controller (Lakeshore 331S) using a calibrated silicon diode (DT-470). The accuracy of temperature measurements was within ±0.1 K.

The energies, equilibrium structures, vibrational wavenumbers, and IR intensities of the isomers of H₃SiNO, H₂SiNO, and HSiNO were calculated using the Gaussian 16 program.²² All calculations were performed at the B3LYP/aug-cc-pVTZ (abbreviated as B3LYP/AVTZ in following context) level of theory.^23–25 

Figure 1(a) shows the IR absorption spectrum of SiH₄/NO/Ne (1/1/1000) deposited at 3 K for 2 h. The intense bands are easily assigned to SiH₄, with peaks at 2192.9 and 907.1 cm^–1,^26,27 and to NO, with a peak at 1873.4 cm^–1. The weaker bands are identified as t-(NO)₂ (1855.9 cm^–1), (NO₂)₂ (1777.9 cm^–1), and NO₂ (1610.4 cm^–1).²⁸ The VUV absorptions of gaseous SiH₄ and NO have been extensively studied. In the gas phase, NO absorption begins around 210 nm and extends continuously throughout the VUV region, with two prominent band maxima at approximately 175 and 137 nm.²⁹ The absorption cross sections for NO in this region are less than 5⋅10^–18 cm² mol^–1. Several electronically excited states, including A²Σ⁺, B²Π, C²Σ⁺, and D²Σ⁺, are involved in the VUV excitation of NO from its ground state. These excited states primarily lead to fragmentation, producing N(²D) + O(³P) via the B²Π state or N(⁴S) + O(³P) via the ground state.³⁰ In contrast, SiH₄ begins to absorb around 160 nm and exhibits two band maxima at approximately 128 and 118 nm, with significantly higher absorption cross sections, exceeding 8⋅10^–17 cm² mol^–1.³¹ Photodissociation of gaseous SiH₄ results in the formation of Si and SiH, but no SiH₂ or SiH₃ fragments have been observed.³² In contrast, VUV photolysis of SiH₄ in solid Ar confirmed the stabilization of Si₂, SiH, and SiH₂ species indicating that the matrix environment can facilitate the stabilization of these intermediates.²⁶ This stabilization leads to different photochemical reactions compared to those occurring in the gas phase. To explore the VUV photochemical reactions of SiH₄ with NO in solid Ne, we performed photolysis of a SiH₄/NO/Ne matrix sample at 130 nm to study the photoproducts formed from the recombination of photodissociation fragments.

Figure 1(b) display the IR difference spectra of the matrix sample after photolysis at 130 nm. Upward peaks indicate the formation of new species, while downward peaks correspond to the depletion of existing species. As expected, many more species were generated upon photolysis at 130 nm. Along with the formation of silicon hydrides, various silicon oxides were also clearly observed, including SiO,³⁴ SiO₂,³⁴ SiO₃,³⁵ Si₂O₂,³⁴ Si₂O₄,³⁶ and (SiO)₃.³⁷ This suggests that photolysis at 130 nm promotes a broad range of reactions, leading to the formation of both simple and complex silicon oxide species. Weak peaks at 1241.6 and 1068.1 cm^–1 are assigned to t-HNOH,³⁸ that was previously observed in our earlier work involving VUV photolysis of NO in solid H₂. Additionally, the peak observed at 1343.7 cm^–1 was unable to match the literature values of the known silicon oxides and silicon hydrides. The observed peak positions along with their corresponding assignments are summarized in Table I.

We further performed photolysis of SiD₄ and NO in solid neon at 130 nm, and the resulting spectrum is depicted in Fig. 2(b), alongside the spectrum of the natural isotopic sample shown in Fig. 2(a) for comparison. The peak positions along with their corresponding assignments observed in the D-substituted experiment are summarized in Table I. In the D-substituted experiment, the absorptions due to photoproducts containing H atoms were observed to shift, while those of photoproducts containing no H atoms remained at their original band positions. This result provides further confirmation of our assignments for most of the photoproducts. In the D-substituted experiment, the band intensities of silicon oxides were observed to be much weaker, while those of silicon hydrides were significantly stronger. This difference may be attributed to the varying dissociation efficiencies between SiH₄ and SiD₄. Additionally, the unknown peak did not change in intensity but was slightly blue-shifted (+2.9 cm^–1), suggesting that this vibrational mode predominantly involves the less motion of H atoms. Because SiH₄ has significantly larger absorption cross sections than NO at 130 nm, its photodissociation predominates under irradiation at this wavelength. Consequently, the unknown species is likely formed through reactions involving SiH₃, SiH₂, or SiH fragments with NO. To better understand its origin, quantum chemical calculations will be performed to assist in identifying and assigning this unknown species.

According to the reported reaction potential energy surface,¹⁶ four intermediates are involved in the reaction SiH₃ + NO → HSiN + H₂O, and the energy were predicted to be H₃SiNO (–5.26 kcal mol^–1), H₂SiN(H)O (6.88 kcal mol^–1), and HSiN(H)OH (–11.7 kcal mol^–1), relative to H₂SiNOH predicted at the level of theory QCISD(T)/6-311++G**//MP2/6-31G**. Additionally, six stable isomers of [SiH₃NO] were previously predicted using the HF/6-31G* method in the earlier work Ref. 11. In this study, we performed B3LYP/AVTZ calculations to predict the stable isomers of [SiH₃NO], and we identified a total of seven stable structures, including those from the previous studies.^11,16 The structures and their relative energies are depicted in Fig. 3. Based on our calculations, the relative energies of the intermediates are H₃SiNO (–2.78 kcal mol^–¹), H₂SiN(H)O (1.40 kcal mol^–¹), and HSiN(H)OH (–12.85 kcal mol^–¹) relative to H₂SiNOH. These results are qualitatively consistent with earlier studies,^11,16 confirming the general stability trends of these intermediates. Moreover, the most stable isomer of [SiH₃NO] was predicted to be H₂Si(O)NH, featuring a tetrahedral bonding structure around the central Si atom. In contrast, the highest energy isomer was predicted to be H₃SiON. The higher energy of H₃SiON is likely due to its unfavorable bonding configuration, including a relatively long Si–O bond and a staggered conformation with repulsive Si–H⋅⋅⋅N interactions, which weaken the overall stability of the molecule.

In addition, we also considered the possibility of unknown species from the reactions of SiH₂ + NO and SiH + NO. Becerra et al. demonstrated that H₂SiNO is the initial adduct in the SiH₂ + NO reaction and highlighted its ability to undergo ring closure or isomerization via low-energy barriers, forming products like cyclo-H₂SiNO.³⁹ Raghunath et al. focused on the potential energy surfaces of silicon-oxygen-nitrogen compounds, particularly emphasizing the stability of chain-like isomers such as HNSiO over cyclic or branched configurations.⁴⁰ In this work, the calculated structures and relative energies of the [SiH₂NO] and [SiHNO] isomers are shown in Figs. 4 and 5. Among the [SiH₂NO] isomers, HNSiOH is the most stable structure, with the lowest relative energy. This chain-like isomer features silicon bonded to nitrogen and hydrogen, while oxygen is terminally attached to hydrogen. The cyc-H₂Si(N)O isomer, which forms a three-membered Si–N–O ring, also has a negative relative energy, indicating that it is more stable than the reference H₂SiNO. This stability is somewhat surprising given the typical strain associated with three-membered rings and suggests favorable electronic interactions in this configuration. Similarly, the HSi(N)OH isomer, where hydrogen migrates to nitrogen, is also more stable than H₂SiNO. For the [HSiNO] isomers, HSiNO, defined as the reference point (ΔE = 0 kcal mol^–1), is more stable than the HSiON isomer, where oxygen bridges silicon and nitrogen, has a significantly higher relative energy (ΔE = 40.49 kcal mol^–1), indicating that its bonding arrangement is far less favorable. The most stable isomer in the set is HNSiO, with a significantly lower relative energy. Its chain-like structure, with silicon bonded to both nitrogen and oxygen, demonstrates highly favorable bonding and delocalization, contributing to its exceptional stability.

We subsequently calculated the harmonic vibrational frequencies and corresponding IR intensities of the isomers of [SiH₃NO], [SiH₂NO], and [SiHNO], which are summarized in Tables II, III, and IV, respectively. Among the possible isomers, H₂SiN(H)O was predicted to exhibit the strongest NO stretching mode at 1363 cm^–1, closely matching the experimental observation. While SiNOH was also predicted to exhibit the NO stretching mode at 1320 cm^–1, its second strongest mode was predicted at 745 cm^–1 with comparable intensity, which was absent in the experimental spectra. Additionally, the NO stretching mode of SiN(H)OH was predicted to appear at 1359 cm^–1, but its corresponding IR intensity is relatively weak compared to the other modes, making it unlikely to be the possible carrier. In contrast, other isomers were predicted to have no significant IR lines within the 1400–1300 cm^–1 spectral region. Further comparison with the vibrational shift of the same mode in D₂SiN(D)O shows that the corresponding vibrational mode was predicted to shift to 1366.5 cm^–1, yielding an isotopic shift ratio of 1.0024. This unusual blue-shift and the D-isotopic-shift ratio align well with the experimental observation, where the frequency shifted from 1343.7 cm^–1 in natural isotopic abundance to 1346.6 cm^–1 in D-isotopic substitution. Taking the experimental and theoretical results together, we tentatively assign the band at 1343.7 cm^–1 to the NO stretching mode of H₂SiN(H)O. A complete list of prediction of harmonic vibrational frequencies of D₂SiN(D)O is also listed in Table II for comparison.

Photolysis of SiH₄ with NO in solid neon at 130 nm results in the formation of a diverse array of species. Both SiH₄ and NO undergo photodissociation at this wavelength, generating a range of reactive fragments including SiH_1,2,3 and atomic nitrogen and oxygen. These silicon hydrides can subsequently undergo further photolysis, yielding Si atoms. These Si atoms, along with the diffusive oxygen atoms, then participate in various reactions, leading to the formation of a series of silicon oxides such as SiO_1,2,3 and Si₂O₄. A large variety of photoproducts observed at 130 nm indicates that the high-energy photons not only dissociate SiH₄ and NO but also facilitate a greater variety of recombination reactions between the photodissociation fragments, resulting in the formation of both simple and more complex silicon oxide species.

In this study, we investigated the VUV photolysis of SiH₄ and NO in a solid neon matrix at 3 K, using photolysis wavelengths at 130 nm. The experimental results revealed photolysis at 130 nm led to a complex set of products, including various silicon hydrides and oxides. The detection of a new species at 1343.7 cm^–1, tentatively assigned to H₂SiN(H)O is supported by the D-isotopic substitution experiments and quantum chemical calculations. These findings not only provide new insights into the VUV photochemistry of SiH₄ and NO in the solid matrix, but also offer potential pathways for the formation of [H, N, O, Si] compounds in various applications. Future work involving additional spectroscopic and theoretical studies will help to further refine our understanding of these reaction pathways and the roles of the identified intermediates.

The data supporting the findings of this study are available in the article and supplementary material. The raw data supporting the findings of this study are available from the corresponding author upon reasonable request.

The authors declare that they have no competing interests in this paper.

This paper is dedicated to the memory of Professor John F. Ogilvie, who passed away on November 22, 2023. He was our friend, mentor, and collaborator for over 20 years, and his guidance and contributions have a lasting impact on our work.

The authors acknowledge financial support from the National Science and Technology Council of the Republic of China (Grant No. NSTC 113-2113-M-213-001 and 113-2639-M-A49-002-ASP) and the National Synchrotron Radiation Research Center.