XRD and XPS studies of room temperature spontaneous interfacial reaction of CeO₂ thin films on Si and Si₃N₄ substrates

Abstract

X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) investigations of interfacial reactions between Ce and Si have been carried out on the same set of as-deposited and 15 month aged films. XRD patterns demonstrate the presence of several peaks associated with CeO₂ planes in aged CeO₂/Si₃N₄ thin film in comparison with as-deposited nanocrystalline film, whereas the peak is broadened in the CeO₂/Si film after aging. XPS studies show that interfacial reaction occurs spontaneously in CeO₂/Si thin film at room temperature. Ce is present as both Ce⁴⁺ and Ce³⁺ oxidation states in as-deposited CeO₂/Si thin film, whereas Ce⁴⁺ is the main species in CeO₂ thin film deposited on the Si₃N₄ substrate. When XPS is recorded after 15 months, the concentration of Ce³⁺ species is observed to increase drastically in the CeO₂/Si thin film. In contrast, interfacial reaction between the CeO₂ and the Si₃N₄ substrate is not significant in the film even after 15 months of deposition. This shows that the initial room temperature spontaneous interfacial reaction observed in the CeO₂/Si film continues at a much higher rate, whereas the nature of the CeO₂/Si₃N₄ interface remains the same after 15 months, proving its stability.

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1. Introduction

In the last few years, CeO₂ based materials have attracted much attention for their applications in catalysis, hydrogen production, H₂–O₂ recombination in batteries and electrodes in fuel cells.^1–5 In microelectronics, they have also been found to be high κ-gate oxide materials due to their unique properties like moderate band gap (3–3.6 eV), high dielectric constant (23–52), high refractive index (2.2–2.8) and high dielectric strength (∼25 MV cm⁻¹).⁶ It is also suitable for Si based metal oxide semiconductor (MOS) devices due to its small lattice mismatch (−0.35%) with Si that favors its epitaxial growth on different silicon surfaces.^6,7 In this regard, understanding of the growth, structure, nature of interface and long term stability between CeO₂ and substrates are very crucial for device applications.

CeO₂ thin films have mainly been grown on Si, Si₃N₄, sapphire, LaAlO₃, SrTiO₃, glass, alloy and yttria-stabilized zirconia (YSZ) substrates using different thin film deposition techniques such as magnetron sputtering, electron beam evaporation, flash evaporation, pulsed laser deposition, ion beam epitaxy, molecular beam epitaxy, plasma enhanced chemical vapor deposition, sol-gel, spray pyrolysis have been employed to deposit CeO₂ films.^7–20 Recently, we have investigated interfacial reactions between CeO₂ and substrates like Si, Al, Ti–6Al–4V alloy, Si₃N₄ and glass using XPS.^8,9 It has been observed that significant interfacial reaction occurs between CeO₂ and Si leading to the formation of Ce₂O₃ and silicate in CeO₂/Si film, whereas reaction is limited in CeO₂/Si₃N₄.⁸ Extent of interfacial reactions of CeO₂ in different types of substrates are found as follows: Si > Al > Ti–6Al–4V alloy > Si₃N₄ > glass.⁹ However, reports on long term stability of CeO₂/Si and CeO₂/Si₃N₄ interfaces lack in the literature. In this communication, we compare the structural changes and reaction at the interfaces of CeO₂/Si and CeO₂/Si₃N₄ films after 15 months of deposition employing XRD and XPS.

2. Experimental methods

Details of growth and characterization of sputter deposited CeO₂ thin films on Si and Si₃N₄ substrates are given in our previous publications.^8,9 CeO₂ thin films were deposited on Si and Si₃N₄ substrates of 10 mm × 10 mm × 0.6 mm dimensions using a CeO₂ target (Allvac, 99.9%) employing magnetron sputtering assisted by inductively coupled plasma generated with 50 W RF power at 13.56 MHz. The substrates were cleaned with acetone and isopropyl alcohol with sonication prior to loading into the vacuum chamber. The chamber was pumped down to a base pressure of 3 × 10⁻⁶ mbar. The substrates were etched with H₂ plasma prior to deposition of thin films. Sputter deposition was carried out at room temperature with Ar atmosphere at a pressure of 8 μbar. The substrate was biased to a constant negative voltage of 150 V and the target was biased with bipolar pulses of 300 V using a pulse generator. Thickness of obtained CeO₂ thin films is around 25 nm. CeO₂/Si and CeO₂/Si₃N₄ thin films were kept in small plastic boxes in ambient conditions.

The structures of as-deposited and aged CeO₂ films were determined by XRD employing a PANalytical X'Pert PRO X-ray diffractometer operated with CuKα radiation of 1.5418 Å wavelength at 40 kV and 30 mA in the 2θ range 20–60° with step size of 0.033°. XPS of CeO₂ thin films were recorded with a SPECS spectrometer using non-monochromatic AlKα radiation (1486.6 eV) as an X-ray source operated at 150 W (12.5 kV and 12 mA). The binding energies reported here were calculated with reference to C1s peak at 284.6 eV. All the spectra were obtained with pass energy of 25 eV and step increment of 0.05 eV. CasaXPS program was employed for curve-fitting of Ce3d core level spectra into several components with Gaussian–Lorentzian peaks after Shirley background subtraction. Peak positions, spin–orbit splitting, doublet intensity ratios and full width at half maximum (FWHM) were allowed to vary slightly as followed in the literature.

3. Results and discussion

XRD patterns of as-deposited and aged CeO₂ thin films on Si and Si₃N₄ substrates are displayed in Fig. 1. Main diffraction peaks associated with substrates that appear after 60° are not shown here. In both as-deposited films, a broad peak observed at 33.4° corresponds to CeO₂ (200) reflection (JCPDS no. 81-0792) indicating the nanocrystalline nature of the films.²¹ It is clear from the XRD patterns that CeO₂ films grow preferentially to its (200) plane on Si and Si₃N₄ substrates. There is a significant change in the XRD pattern of aged CeO₂/Si₃N₄ film. Intense diffraction peaks associated with (111), (200), (220) and (311) planes of CeO₂ can be seen in the pattern of aged film. However, diffraction peak observed at 33.4° in as-deposited CeO₂/Si film appears to be broad after 15 months of deposition.

Fig. 1 XRD patterns of (a) as-deposited CeO₂/Si, (b) as-deposited CeO₂/Si₃N₄, (c) aged CeO₂/Si and (d) aged CeO₂/Si₃N₄.

Ce3d core level spectra of CeO₂/Si and CeO₂/Si₃N₄ films at different conditions are shown in Fig. 2. It is clear from the figure that Ce3d spectral envelop of as-deposited CeO₂/Si film is significantly different from the film after 15 months of deposition. Spectral envelops of the film deposited on Si substrate indicate that Ce is in both +4 and +3 oxidation states and it can be resolved into several Ce3d_5/2,3/2 spin–orbit doublet peaks related to Ce⁴⁺ and Ce³⁺ species along with satellites. Satellite peaks are associated with the charge transfer from ligand (O2p) to metal (Ce4f) during photoionization processes. Appreciable change in Ce3d core level spectrum is observed in the aged CeO₂/Si film. Comparison of spectral envelops indicates that most of Ce⁴⁺ species present in as-deposited film is transformed into Ce³⁺ species during this time span. In contrast, there is no appreciable change in the spectrum of CeO₂/Si₃N₄ film after 15 months of deposition demonstrating that Ce is mostly present in +4 oxidation state. Fig. 3 presents typical curve-fitted Ce3d spectra of CeO₂/Si and CeO₂/Si₃N₄ films after 15 months of deposition. In both the figures, Ce3d spectra consist of 10 peaks that are curve-fitted in 5 doublets corresponding to 3d_5/2 (labeled as v) and 3d_3/2 (labeled as u) components. In curve-fitted Ce3d spectrum of CeO₂/Si film, three doublet peaks labeled as v–u (882.7 and 901.2 eV), v′′–u′′ (888.8 and 907.1 eV) and v′′′–u′′′ (898.1 and 916.5 eV) with spin–orbit separation of 18.5, 18.3 and 18.4 eV, respectively are assigned for the Ce⁴⁺ species, whereas peaks labeled as v_o–u_o (881.6 and 899.6 eV) and v′–u′ (885.5 and 903.9 eV) are associated with Ce³⁺ species.^8,9,22 The u′′′ peak is relatively well separated from the rest of the spectrum and is the characteristic of the presence of tetravalent Ce (Ce⁴⁺) in Ce compounds. It is to be noted that v′′′ and u′′′ spin–orbit peaks are attributed to the primary photoionization from Ce⁴⁺ with Ce3d⁹4f⁰O2p⁶ final state, whereas lower binding energy peaks of v′′–u′′ and v–u correspond to the shake-down satellite features of Ce3d⁹4f¹O2p⁵ and Ce3d⁹4f²O2p⁴ final states of Ce⁴⁺. Spin–orbit doublet peaks of v′–u′ are related to Ce3d⁹4f¹O2p⁶ final state of main photoionization from Ce³⁺ and associated lower binding energy v_o–u_o peaks are characteristic shake-down satellites of Ce3d⁹4f²O2p⁵ final state. Peak areas (A) of Ce⁴⁺ and Ce³⁺ components are commonly used to estimate their relative concentrations (C) in the films using the following equations:^8,23

A_Ce³⁺ = A_v0 + A_u0 + A_v′ + A_u′ (1)

A_Ce⁴⁺ = A_v + A_u + A_v′′ + A_u′′ + A_v′′′ + A_u′′′ (2)

Fig. 2 XPS of Ce3d core levels in CeO₂/Si and CeO₂/Si₃N₄ films: (a) as-deposited and (b) aged.

Fig. 3 Curve-fitted XPS of Ce3d core levels in aged CeO₂ films deposited on (a) Si and (b) Si₃N₄ substrates [v_o – Ce³⁺3d_5/2, v – Ce⁴⁺3d_5/2, v′ – Ce³⁺3d_5/2, v′′ – Ce⁴⁺3d_5/2, v′′′ – Ce⁴⁺3d_5/2, u_o – Ce³⁺3d_3/2, u – Ce⁴⁺3d_3/2, u′ – Ce³⁺3d_3/2, u′′ – Ce⁴⁺3d_3/2, u′′′ – Ce⁴⁺3d_3/2].

Concentration of Ce³⁺ in as-deposited CeO₂/Si film is evaluated to be 32% with respect to the total amount of Ce species.⁸ On the other hand, mainly Ce³⁺ related peaks along with a small amount of Ce⁴⁺ can be seen in the film after 15 months of deposition and its concentration is 67% which is more than double in comparison with as-deposited film. In case of CeO₂/Si₃N₄ film, Ce⁴⁺ species is predominant after 15 months of deposition. A comparison of concentrations of Ce⁴⁺ and Ce³⁺ evaluated from Ce3d spectra in CeO₂/Si and CeO₂/Si₃N₄ thin films at different conditions are given in Table 1. It is important to note that Ce³⁺ in CeO₂/Si film can be related to the formation of cerium silicate or Ce₂O₃ at the interface.⁷ Drastic increase in the concentration of Ce³⁺ in CeO₂/Si film compared to CeO₂/Si₃N₄ film after 15 months of deposition indicates that interfacial reaction between CeO₂ and Si occurs continuously in CeO₂/Si film at room temperature.

Table 1 Relative surface concentrations of Ce⁴⁺ and Ce³⁺ species in as-deposited and aged CeO₂/Si and CeO₂/Si₃N₄ thin films as evaluated from XPS

Ce species	As-deposited		Aged
	CeO2/Si	CeO2/Si3N4	CeO2/Si	CeO2/Si3N4
Ce^4+	68 ± 1.73	89 ± 0.95	33 ± 4.4	82 ± 1.93
Ce^3+	32 ± 1.73	11 ± 0.95	67 ± 4.4	18 ± 1.93

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O1s core level spectrum of respective CeO₂ thin film can also reveal the oxidation states of Ce in the film. In Fig. 4, O1s core level spectra of CeO₂/Si and CeO₂/Si₃N₄ films at different conditions are shown. It is to be mentioned that O1s core level spectral envelop of as-deposited CeO₂/Si film is different from that of aged film, whereas both spectra look similar in CeO₂/Si₃N₄ film. Peak at 529.9 eV corresponds to O²⁻ species in CeO₂, whereas peak at 531.6 eV is associated with Ce³⁺ species originated from silicate or Ce₂O₃ species.⁸ It has been found from the figure that Ce⁴⁺ related peak along with strong peak associated with Ce³⁺ species are present in as-deposited CeO₂/Si film. Intensity of the peak related to Ce⁴⁺ decreases drastically in the film after 15 months of deposition. On the other hand, intensity of the peak corresponding to Ce⁴⁺ remains unchanged in CeO₂/Si₃N₄ film after 15 months of deposition.

Fig. 4 XPS of O1s core levels in CeO₂ films deposited on Si and Si₃N₄ substrates: (a) as-deposited and (b) aged.

High resolution Si2p core level spectra of CeO₂/Si and CeO₂/Si₃N₄ films at different conditions are shown in Fig. 5. Broad envelops of Si2p core level spectra in CeO₂/Si films indicate the presence of elemental Si as well as oxidized Si species that can be curve-fitted into several component peaks. A peak at 99.3 eV is related to the contribution from elemental Si present in Si substrate. On the other hand, observed peaks at 101.2 and 102.4 eV correspond to Si²⁺ and Si³⁺ species, respectively.^24,25 The appearance of these species at the interface of CeO₂ and Si indicates the interaction between them resulting the formation of cerium silicate species.^7,8 and concentrations of different oxide species increase in the film to some extent after 15 months of deposition demonstrating the continuous interfacial reaction over time. In contrast, a weak single peak at 101.5 eV in Si2p core level spectrum in CeO₂/Si₃N₄ film is attributed to Si–N bond in Si₃N₄ and it remains same after 15 months of deposition.²⁶

Fig. 5 XPS of Si2p core levels in CeO₂ films deposited on Si and Si₃N₄ substrates: (a) as-deposited and (b) aged.

It has been observed from XPS studies that concentration of Ce⁴⁺ species in CeO₂/Si film decreases after 15 months of deposition indicating the continuous reduction of Ce⁴⁺ species to Ce³⁺ at room temperature. Decrease in the u′′′ peak intensity and increase in v′ and u′ intensities in the film after 15 months of deposition confirms the Ce⁴⁺ reduction. On the other hand, Ce⁴⁺ species predominates in CeO₂/Si₃N₄ film after 15 months of deposition demonstrating that CeO₂/Si₃N₄ film is stable even after 15 months of deposition at room temperature. XRD results also support this finding as intense peaks related to CeO₂ are observed only in CeO₂/Si₃N₄ film indicating the lack of intermixing between CeO₂ and Si in this system. Thus, spontaneous interfacial reaction at room temperature increases CeO₂ reduction in CeO₂/Si film. It has also been demonstrated earlier that interfacial reaction in CeO₂/Si film can occur as change in standard molar free energy of formation of products and reactants (ΔG°) is negative (3Si + 8CeO₂ → SiO + Si₂O₃ + 4Ce₂O₃, ΔG° = −340.8 kJ mol⁻¹) indicating that reaction is thermodynamically favorable to proceed to their products.⁹ Therefore, this reaction continuously goes on at room temperature over the time.

4. Conclusions

CeO₂ related prominent peaks are observed in XRD pattern of aged CeO₂/Si₃N₄ film with respect to as-deposited film, whereas only peak observed in as-deposited film becomes broad in aged CeO₂/Si film. XPS studies show that most of Ce⁴⁺ species gets reduced to Ce³⁺ in CeO₂/Si film after 15 months of deposition, whereas mainly Ce⁴⁺ species is present in CeO₂/Si₃N₄ film of same time period. Extent of spontaneous interfacial interaction at room temperature over time is found to be very high in CeO₂/Si film, whereas it is significantly less in CeO₂/Si₃N₄ film in the same condition. Thus, CeO₂ film deposited on Si₃N₄ is stable in comparison with CeO₂/Si film over time.

Acknowledgements

Authors would like to thank the Director, CSIR-National Aerospace Laboratories for giving permission to publish this work.

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