Influence of CH₄–Ar ratios on the composition, microstructure and optical properties of Be₂C films synthesized by DC reactive magnetron sputtering

Abstract

Beryllium carbide (Be₂C) films were first deposited on optical quartz substrates by DC reactive magnetron sputtering on a beryllium target with variable CH₄–Ar ratios. The influence of CH₄–Ar ratios on the composition, microstructure and optical properties were investigated by X-ray photoelectron spectroscopy, X-ray diffraction, high-resolution transmission electron microscope, atomic force microscopy, scanning electron microscope and UV-vis spectrum. The main component in the films prepared at lower CH₄–Ar ratios (<5%) was Be₂C, while hydrocarbon (CH) films were formed at higher CH₄–Ar ratios (>15%). The films exhibited a nanocomposite structure consisting of Be₂C nanocrystals (3 to 5 nm in size) embedded in amorphous hydrocarbon matrices. A smooth surface and columnar structure on the cross-sectional view were revealed. Besides, the depositing rates reached ∼125 nm h⁻¹, which were significantly higher than that of RF reactive magnetron sputtering. High transparency (>50%) of the Be₂C films in the visible region as well as an even higher transparency (>80%) in the near-infrared region were demonstrated. Finally, the dispersion of the optical constants of Be₂C films is presented, and the optical bandgaps were evaluated to be ∼2 eV. The good properties of Be₂C films prepared by DC reactive magnetron sputtering showed that this material could be a potential candidate for application to inertial confinement fusion targets.

---

1. Introduction

Recently, many concerns have been paid to the low Z (atomic number) materials due to their important usages in nuclear fusion research.^1–3 Beryllium carbide (Be₂C), as a typical low Z material, has been reported to be a promising wall material choice for the next generation experimental fusion reactor ITER because it can mitigate not only the pure beryllium but also the pure carbon sputtering yields.^4,5 Another important application is for inertial confinement fusion (ICF) capsule materials. The most principle requirements for capsule materials are a low atomic number, smooth surface, uniform composition and structures and high density.^6–8 Doped beryllium and hydrocarbon (CH) coatings are the most commonly selected target materials at the National Ignition Facility (NIF).^6–11 Currently, the compound Be₂C has caught researchers' interest because it combines the advantages of beryllium and hydrocarbons.^12,13 Compared with many hydrocarbons, Be₂C has a lower atomic number (Z = 4.7), and a higher density (ρ = 2.4 g cm⁻³) and mechanical strength (bulk modulus B₀ = 217.05 GPa and shear modulus C_s = 165.95 GPa).^2,3,14 Compared with beryllium, its antifluorite structure results in a smoother surface and grain refinement. It has yellowish-brown transparent crystals and a bandgap over 1.2–2 eV,^3,14 which might indicate that the deuterium–tritium (D–T) ice layers in Be₂C capsules could be characterized by optical methods and homogenized by infrared heating.¹⁵ However, comprehensive understanding of its optical properties are needed to evaluate this advantage.

Due to its important usages, the structural, electronic and optical properties of beryllium carbide have been adequately investigated by theoretical calculations;^1–3,16 however, there are few literature reports on the experimental side. Beryllium carbide was synthesized and its structural properties were studied in ref. 14 and 17. Thereafter, Xie and Shih et al. deposited beryllium carbide coatings by plasma polymerization of diethyl-beryllium and by RF reactive magnetron sputtering for application to ICF.^13,18,19 The H₂ permeability, mechanical properties, chemical composition and electrical and thermal conductivities of beryllium carbide films were analysed and the results indicated that most of the requirements of the ignition target were satisfied. However, some shortcomings in these two methods have so far limited their application to ICF targets. First, the instability and long recovery time of the saturation vapour pressure of diethyl-beryllium²⁰ makes it hard to control the composition and incurs a high cost. Moreover, doping is difficult to realize in deposition by plasma polymerization. Second, the maximum deposition rate is ∼56 nm h⁻¹ by RF reactive magnetron sputtering,²¹ which is too low for depositing targets. To the best of our knowledge, the optical properties of beryllium carbide have not been researched experimentally to date, even though it is important for application to ICF targets. The scarcity of experimental optical studies is due to the nature of the Be₂C samples: Be₂C is a highly toxic component, which is also difficult to prepare and has a tendency to hydrolyse and oxidize.³

In the present work, Be₂C films were first prepared by DC magnetron sputtering of beryllium into methane plasma. The compositions were controlled by the CH₄–Ar ratio during the sputtering process. The carbon content in the films increased with the increase in the CH₄–Ar ratio, with hydrocarbon films formed at a CH₄–Ar ratio >15%. Based on this finding, the Be₂C films were in situ covered by a layer of hydrocarbons (∼26 nm), which was demonstrated to be effective in preventing oxidation. In addition, the microstructures and morphologies were determined. Finally, the optical properties of Be₂C films were measured by a double-beam spectrophotometer for the first time, indicating an optical bandgap of about 2 eV.

2. Experimental

2.1. Preparation of Be₂C films

Beryllium carbide films were deposited on optical quartz substrates by DC reactive magnetron sputtering of a 76 mm-diameter beryllium target under various CH₄–Ar ratios at room temperature. The flow rates of argon and methane were accurately controlled by mass flow controllers, with the flow rate of Ar fixed at 50 sccm while the flow rate of CH₄ was varied (0.8, 1.0, 1.2, 1.5, 2.0, 2.5, 3.0, 5.0, 7.5 and 10 sccm, corresponding to CH₄–Ar ratios of 1.6%, 2.0%, 2.4%, 3.0%, 4.0%, 5.0%, 6.0%, 10%, 15% and 20%, respectively). An angle of ∼45° existed between the plane of the target and substrate. The distance between the Be target and substrates was set at ∼90 mm. In order to reduce the possibility of target poisoning, argon was introduced near the target and methane was introduced near the substrates. The system was evacuated to a base pressure of 1.0 × 10⁻⁵ Pa. Before the deposition, the substrate shutter was closed and pre-sputtering was conducted on the Be target to remove any contaminants and to guarantee a fresh beryllium surface. During the growth, the working pressure was maintained at 0.5 Pa and the sputtering power was constant at 50 W. The substrate was rotated at a constant rate of 6 rpm to improve the uniformity. The depositing time was controlled at 4 hours. After the deposition, protective layers (∼26 nm hydrocarbon) were in situ covered on the surface of the Be₂C films, which were prepared at a 1.6–5.0% CH₄–Ar ratio, by simply increasing the CH₄ flow rate up to 10 sccm and sustaining this for 8 minutes. A 26 nm hydrocarbon film was deposited on quartz substrate as a reference sample for the morphology and optical property measurements.

2.2. Characterization of the Be₂C films

The elemental composition and the corresponding chemical states of different Be₂C films were studied by X-ray (Mg-Kα 1253.6 eV) photoelectron spectroscopy (XPS). The microstructure was investigated by X-ray diffraction (XRD) with the grazing angle mode using Cu Kα (λ = 0.154056 nm) radiation, as well as high-resolution transmission electron microscope (HRTEM). For the HRTEM measurements, the film (deposited at a 2.4% CH₄–Ar ratio) was scratched with a sharp razor blade in an ethanol environment. Then, it was dispersed in ethanol and placed on the TEM grid. The surface and cross-sectional morphologies were observed by atomic force microscopy (AFM) and scanning electron microscope (SEM), respectively. The root mean square (RMS) surface roughness of the films was evaluated over a 1 × 1 μm² area. The deposition rates were calculated from the thickness (measured by SEM) and deposition time. The average densities were evaluated from the weight gain and thickness of the films with a definite area. The optical properties were analysed from the transmittance and reflectance spectrum measured by a double-beam spectrophotometer, which had two beams of light, one for measurement and the other for reference. When testing, the Be₂C films were placed on the testing sample holder, and the fleshly deposited ∼26 nm hydrocarbon on quartz substrate (reference sample) was placed on the reference sample holder. Thus, the obtained spectrum only showed the response of the Be₂C films.

3. Results and discussion

3.1. Composition analysis

Fig. 1(a) shows the partial XPS survey spectra of the films deposited under different CH₄–Ar ratios from 1.6% to 20% after 30 min Ar⁺ etching to remove any surface contaminants and protective layers. This confirmed the chemical purity of the films, as it only consisted of Be, C and O, and no peaks corresponding to other elements were detected. The binding energy of O_1s peak was about 533.1 eV in all samples, which suggested that its chemical state remained unchanged. Nevertheless, appreciable shifts were observed in the C_1s and Be_1s peaks, indicating changes in the chemical states of carbon and beryllium. In order to identify the chemical states and relative amounts of each element in that state, high-resolution XPS spectra of Be, C and O were conducted, in which the overlapping bands were deconvoluted into separate peaks by Gaussian fitting using XPS peakfit software. The partial typical high-resolution XPS spectra of beryllium and carbon are illustrated in Fig. 1(b) and (c), respectively. The peaks centred at 111.23, 112.42 and 114.55 eV in the Be_1s high-resolution spectra were assigned to metallic Be, Be₂C and BeO, respectively.²² Also, the peaks at 282.46, 284.54, 286.23 and 288.17 eV in the C_1s high-resolution spectra were attributed to Be₂C, CH, C–O and C=O species, respectively.^21,22 It was found that the presence of hydrocarbons increased with increasing CH₄–Ar ratios. When the CH₄–Ar ratios were below 5%, the major chemical state (>50 mol%) of the beryllium and carbon peaks was beryllium carbide. Beryllium was not incorporated in the depositions, and hydrocarbon films were formed when the CH₄–Ar ratios were higher than 15% (shown in Fig. 1(c)). Besides, a small amount of metallic beryllium existed in films prepared at lower CH₄–Ar ratios (2.0% and 1.6%).

Fig. 1 XPS spectra for films prepared at different CH₄–Ar ratios after 30 min Ar⁺ etching: survey spectrum (a), deconvolution of Be_1s peaks (b) and deconvolution of C_1s peaks (c).

The atomic concentrations of elements and their relative amounts in each chemical state were calculated from the peak areas, with corresponding sensitivity factors, as plotted in Fig. 2. It was found that the beryllium content decreased while the carbon content increased gradually with the increasing CH₄–Ar ratios in the range of 1.6–5% (Fig. 2(a)), leading to the reduction of Be–C ratios from 2.11 to 1.26 (Fig. 2(b)). Further increasing the CH₄–Ar ratio resulted in a dramatic decrease of the beryllium content and increase of the carbon content, until pure hydrocarbon films were formed. The results, however, were not consistent with previous research,²⁰ where the Be–C ratios reached 3.5 when the CH₄–Ar ratio was 25% and was maintained at ∼1 even when the CH₄–Ar ratio exceeded 50%. However, the variation trend of the beryllium and carbon content with the CH₄–Ar ratios in our work was the same as in the previous research. However, taking into account the difference in the experiment configuration and parameters, the distinction is understandable. The higher sputtering power (200 W) used in the previous researchers experiments was more effective for beryllium sputtering, resulting in the higher Be–C ratio. The percentage of beryllium (Be*) and carbon (C*) bonded as Be₂C as a function of the CH₄–Ar ratios in the range of 1.6% to 5% are illustrated in Fig. 2(b). The major (>85%) chemical state of beryllium was in Be₂C, meanwhile, the percentage of carbon bonded as Be₂C decreased with the increase in CH₄–Ar ratios. However, the atomic ratios of Be* and C* always remained close to 2, which fitted the stoichiometry of Be₂C, suggesting the accuracy of the XPS analysis.

Fig. 2 (a) The atomic content of Be, C and O in the films with various CH₄–Ar ratios. (b) The atomic ratios of Be–C, as well as the percentage of Be and C bonded as Be₂C for the films deposited at different CH₄–Ar ratios.

The XPS analysis also suggested that oxidation had been effectively controlled by the protective coverages since only a little oxygen existed in the films with CH₄–Ar ratios < 5%, compared with the fact that the uncovered Be₂C films were quickly oxidized and delaminated after exposure to ambient air (not shown). This was also confirmed by the films deposited at 6% and 10% (uncovered), where all beryllium was in its oxidation state and the oxygen content reached 34.3 at% and 32.2 at%, respectively.

XRD measurements were also performed on the films as part of our structural investigations. Fig. S1 (ESI†) shows the XRD patterns of the as-deposited Be₂C films with various CH₄–Ar ratios. There were no obvious diffraction peaks in the XRD patterns. Further measurements should thus be introduced to reveal the microstructure. In ref. 23 and 24, the microstructure of Be-related materials (Be_1−xB_x and BeN) were measured by TEM.

HRTEM observations were performed on films deposited at 2.4%. Fig. 3 represents the plane view HRTEM image and its corresponding fast Fourier transform (FFT) pattern (upper-right insert). The HRTEM image demonstrated the nanocomposite structure of the films, with Be₂C nanocrystals (3–5 nm) distributed in amorphous matrices. The interplanar spacing was found to be 0.251 ± 0.005 nm, referring to the (111) plane of the Be₂C phase (PDF-33-0191).²⁵ The FFT pattern exhibited a clear diffraction ring, attributed to the (111) plane of Be₂C. Although there was ∼19% hydrocarbon in the film (as determined by XPS analysis), no diffraction rings corresponding to carbon were detected, which might indicate that the amorphous matrices consisted of hydrocarbons. This nanocomposite structure was widely found in the films prepared by reactive sputtering at room temperature with a low sputtering power.²⁶ Under these conditions, atoms ejected from the target surface with low kinetic energy could not gain sufficient extra energy from the substrates to achieve a competent diffusion rate when they were adsorbed on the surface of the substrates, resulting in the formation of a nanocomposite structure. The influence of the sputtering power and substrate temperature on the deposition was also investigated and will be discussed in other works.

Fig. 3 Plane view high-resolution TEM image and corresponding FFT pattern (inset upper-right) for the film obtained at a 2.4% CH₄–Ar ratio.

Generally, the nanocrystalline phase is detectable in the XRD spectrum.²⁷ However, there were no obvious peaks in the XRD patterns of these nanocomposite Be₂C films. This might be attributed to two possible reasons: (a) the low atomic number of Be₂C resulted in difficulty for XRD detection and²⁸ (b) the films in this work were too thin and the grain size was too small for Be₂C to produce obvious X-ray diffraction peaks.

3.2. Film morphologies and growth characteristics

The surface and cross-sectional morphologies of the films under various CH₄–Ar ratios were studied using AFM and SEM, respectively. The partial typical surface and cross-sectional morphologies are displayed in Fig. 4. The AFM images of Be₂C films at a 2.4% CH₄–Ar ratio clearly show that many small spherical-shaped granules (20–70 nm in diameter and 0–20 nm in height) are homogenously distributed over the surfaces. With the increasing CH₄–Ar ratio, small particles tended to merge into larger ones, resulting in the greater RMS roughness at higher CH₄–Ar ratios. The surface morphologies and RMS roughness of the ∼26 nm hydrocarbon films (i.e. the reference sample) were also examined, and presented a non-characteristic surface with a very low surface roughness of ∼0.22 nm (shown in Fig. 4(a)). Thus, it was shown that the granular morphologies and roughness can be mainly attributed to the Be₂C films instead of a hydrocarbon protective layer. The measured RMS roughness of the films deposited at 1.6%, 2.0%, 2.4%, 3.0%, 4.0% and 5.0% were 2.51, 2.19, 2.63, 2.82, 3.31 and 5.80 nm, respectively. This suggests that the surface roughness of the Be₂C films increased with the increase of CH contents. Since the smooth surface is significant for its application in ICF targets, purer Be₂C films are desirable. The typical surface morphologies (see the 3D views) for the ∼26 nm thick hydrocarbon film (i.e. the reference sample) and films prepared at 2.4% and 5.0% are illustrated in Fig. 4(a–c), respectively. Surface morphologies for the Be₂C films at 1.6%, 2.0%, 3.0% and 4.0% CH₄–Ar ratios are shown in the ESI, Fig. S2.† A typical 2D view of the morphology (deposition at the 2.4% ratio) is also given in Fig. 4(d). Columnar structures always existed when the CH₄–Ar ratio was below 5%, as revealed by the cross-sectional views. As a contrast, a homogeneous morphology, without columns or other characteristic structures, was revealed in hydrocarbon films. Herein, we have only presented the typical cross-sectional morphologies for the samples prepared under 2.4% and 20% ratios, as illustrated in Fig. 4(e) and (f), respectively. The lamination in Fig. 4(f) resulted from a fractionated deposition due target poisoning under high CH₄–Ar ratios.

Fig. 4 Typical surface morphologies (AFM 3D images) of: (a) ∼26 nm hydrocarbon film on the quartz substrate as a reference sample, Be₂C films deposited at a (b) 2.4%, (c) 5.0% CH₄–Ar ratio and (d) AFM 2D image for Be₂C films deposited at a 2.4% CH₄–Ar ratio. Cross-sectional morphologies (SEM images) for films deposited at a (e) 2.4% and (f) 20% CH₄–Ar ratio.

The coating thickness was evaluated from the cross-sectional views and the deposition rates were calculated from the thickness in a defined deposition time. The variation of the deposition rates with CH₄–Ar ratios could imply changes in the sputtering modes, which thus should also be monitored by the target voltages. Fig. 5 shows the deposition rates and target voltages as a function of CH₄–Ar ratios. The introduction of a small volume of methane (1.6%) resulted in a remarkable deterioration of the deposition rates. The deposition rates were relatively stable (∼125 nm h⁻¹) over the ratio range of 1.6% to 5.0% and then increased to ∼182 nm h⁻¹ in the ratio range of 15–20%. The deposition rates in our work (DC reactive magnetron sputtering) were much higher that reported in ref. 21 (RF reactive magnetron sputtering), and could be even further improved by increasing the sputtering power (not shown in this work). The higher deposition rate was beneficial to depositing ICF capsules. The oxidation in films deposited at 6% and 10% ratios led to an increasing thickness, resulting in the abnormally high deposition rates. Combined with XPS analysis, we suspected that there were three sputtering modes during the depositions as the CH₄–Ar ratio varied from 0–20%: (a) a metallic sputtering mode, in which the depositions mainly consisted of metallic beryllium, (b) a compound sputtering mode, where a thin layer of carbide was formed on the target surface and the depositions mainly consisted of beryllium carbide, (c) a non-metallic sputtering mode, in which hydrocarbon was coated on the target surface and the depositions comprised hydrocarbons. The formation of hydrocarbons made it more complicated than other reactive sputtering techniques (involving oxides or nitrides). The variations of target voltages followed a similar trend to that of the deposition rates. Target voltages in the metallic mode were higher than that in the compound mode but lower than that in the non-metallic mode, as shown in Fig. 5. The results had good repeatability. Therefore, we suspected that the target voltage can be used as a proxy for judging the sputtering status.

Fig. 5 Deposition rates and target voltages versus CH₄–Ar ratios.

Based on the thickness and weight gain of the films in a defined area (15 × 15 mm²), the average density was calculated. The average densities were found to decrease from 1.94 to 1.68 g cm⁻³ with the increasing CH₄–Ar ratios from 1.6% to 5.0%, which were smaller than for Be₂C films prepared by PECVD¹³ but higher than that of hydrocarbon films.⁹ The comparative low density might be caused by the incorporation of hydrocarbons, but could be improved by optimizing the experiment parameters, such as the sputtering power and annealing temperature (not studied in this work).

3.3. Optical properties

The deposited Be₂C films varied from deep reddish-brown (1.6%) to light orange (5.0%) in colour. In order to evaluate the optical properties of the Be₂C films, the optical transmittance and reflectance spectra were measured over the wavelength range of 200–2000 nm. Fig. 6 shows the optical transmittance spectrum of the Be₂C films deposited with various CH₄–Ar ratios (1.6–5.0%). An apparent interference feature could be observed in every spectrum, which was universal in transparent films and implied a smooth and homogeneous surface. The optical transmittance increased with the increasing CH₄–Ar ratios. For Be₂C films without metallic beryllium, the average transmittance was greater than 55% in the visible region (500 to 760 nm) and over 80% in the near-infrared region. This suggested that it was feasible to characterize the deuterium–tritium (D–T) ice layers by optical methods and to homogenize them by infrared heating the Be₂C capsules.

Fig. 6 Optical transmittance spectra of the Be₂C films prepared with different CH₄–Ar ratios.

The dispersion of the absorption coefficient was calculated using transmittance and reflectance data combined with the thickness data. The transmittance and reflectance spectra were fitted by envelope methods before being adopted to eliminate interferences; one typical fitting curve is shown in the inset of Fig. 6. The absorption coefficient was computed by:²⁹

where d is the thickness measured by SEM, R(λ) is the reflectance and T(λ) is the transmittance.

The dispersion behaviour of the optical constants, as is well known, plays an important role in the research of optical materials and can be readily calculated from the following equations:^29,30

ε_r(λ) = n²(λ) − k²(λ) (4)

ε_i(λ) = 2n(λ)k(λ) (5)

where α(λ) is the absorption coefficient calculated from eqn (1), k(λ) is the extinction coefficient, n(λ) is the refractive index and ε_r(λ) is the real parts and ε_i(λ) is the imaginary parts of the complex dielectric constant. The dispersion curves of the extinction coefficient (k), the refractive index (n) and the variation of the real (ε_r) and imaginary (ε_i) parts of the dielectric constant are illustrated in Fig. 7. As can be seen, the extinction coefficient (k) and refractive index (n) tend to increase when the CH₄–Ar ratios decrease. The variations in the refractive index could be attributed to the difference in densities. According to the Lorentz–Lorenz formula,³¹ (where α(λ) is the mean polarizability, N is the number density of molecules), the refractive index (n) increases with the increase in densities. In addition, for the purer Be₂C films (deposited at 2.4%) the refractive index (n) and the real parts of the dielectric constant (ε_r) in the infrared region are 2.52 and 6.34, respectively, which are extraordinarily close to the theoretical calculation values of 2.50 for n, and 6.25 for ε_r.²

Fig. 7 (a) Dispersion curves of the extinction coefficient (k), refractive index (n) and (b) the variation of the real (ε_r), imaginary (ε_i) parts of the dielectric constant with wavelength for Be₂C films deposited at 2.0–5.0% CH₄–Ar ratios.

The optical bandgaps were determined from the absorption coefficient via the Tauc relation²⁶ by extrapolating the linear region of the (αhν)^1/2–hν plot to the energy axis, where the exponent 1/2 is utilized for the indirect bandgap. Fig. 8 shows the Tauc analysis, which gives optical bandgaps of 2.43, 2.21, 2.10, 2.03, 1.7 and 0.37 eV for films deposited with CH₄–Ar ratios of 5.0%, 4.0%, 3.0%, 2.4%, 2.0% and 1.6%, respectively. Theoretical calculations on Be₂C using the GGA and LDA functionals revealed an indirect bandgap of about 1.2 eV, located between Γ and Χ.^1,2,16 However, the electron energy loss spectra (EELS) data determined the need for an experimental correction to the gap of ∼2 eV.³ This value was close to the obtained optical bandgaps. A decrease in the optical bandgaps with a decrease in the CH₄–Ar ratio was found, which might be related to the compositions. The larger optical bandgaps for films with CH₄–Ar ratios of 5.0% and 4.0% might result from the presence of beryllia (bandgap ∼ 10.6 eV (ref. 32)). The Be₂C film deposited at 1.6% has the smallest bandgap (0.37 eV), which is probably due to the existence of metallic beryllium in the films, as revealed by XPS analysis. The comparison with the other experimental literature has proven to be difficult as the optical properties of beryllium carbide films have been barely investigated.

Fig. 8 Plots of (αhν)^1/2 versus photon energy hν and extrapolating for optical bandgaps (E_g) for Be₂C films under various CH₄–Ar ratios.

4. Conclusions

In summary, Be₂C films were firstly prepared by DC reactive magnetron sputtering on optical quartz substrates with various CH₄–Ar ratios and characterized by suitable analytical techniques. The XPS results revealed that the hydrocarbon content in films increased with increasing CH₄–Ar ratios; meanwhile, beryllium carbide was the main component (>50 mol%) in films prepared with lower CH₄–Ar ratios (<5%), while hydrocarbon films were obtained when the CH₄–Ar ratios exceeded 15%. Based on this finding, protective coatings (∼26 nm CH) were in situ deposited on Be₂C films, which was proven to be effective in preventing oxidation. HRTEM images clearly showed that nanocrystal Be₂C particles were embedded in the amorphous hydrocarbon matrices. The films exhibited a smooth surface and columnar structure on the cross-sectional view as presented by AFM and SEM. In our experiments, the deposition rates (∼125 nm h⁻¹) were higher than that of RF sputtering, which was favourable for depositing inertial confinement fusion (ICF) capsules. The optical measurements revealed the high transparency of Be₂C films in visible light to the near-infrared region (500–2000 nm), suggesting the feasibility of characterizing the deuterium–tritium (D–T) ice layers by optical methods and homogenizing by infrared heating in Be₂C capsules. In addition, the dispersion of the optical constant was investigated. The obtained optical bandgap (E_g), refractive index (n) and real part of the dielectric constant (ε_r) for the purer Be₂C films (deposited at a CH₄–Ar ratio of 2.4%) were 1.98 eV, 2.52 and 6.34, respectively, and are in accordance with the theoretical calculations. The results showed that this material could be a potential candidate for application to ICF capsules.

Acknowledgements

This study was financially supported by the National Key Science Foundation of China (Grant NO. 2014YQ090709). The authors would like to acknowledge Yong Zeng and Dr Qi Yang for their help in testing. The authors also are grateful to Xiulan Tan and Dr Zhengwei Xiong for their beneficial discussions.

References

1. C. H. Lee, W. R. L. Lambrecht and B. Segall, Phys. Rev. B: Condens. Matter Mater. Phys., 1995, 51(16), 10392–10398.
2. S. Laref and A. Laref, Comput. Mater. Sci., 2008, 44(2), 664–669.
3. C.-T. Tzeng, K.-D. Tsuei and W.-S. Lo, Phys. Rev. B: Condens. Matter Mater. Phys., 1998, 58(11), 6837–6843.
4. M. Mehine, C. Björkas, K. Vörtler, K. Nordlund and M. I. Airilaet, J. Nucl. Mater., 2011, 414(1), 1–7.
5. R. Mateus, P. A. Carvalho, N. Franco, L. C. Alves, M. Fonseca, C. Porosnicu and E. Alves, J. Nucl. Mater., 2013, 442(1), S320–S324.
6. A. N. Simakov, D. C. Wilson, A. Y. Sunghwan, J. L. Kline, D. S. Clark and S. H. Batha, Phys. Plasmas, 2014, 21(2), 022701.
7. J. E. T. Team, J. Nucl. Mater., 1990, 176, 3–13.
8. A. J. MacKinnon, N. B. Meezan, J. S. Ross, S. L. Pape and L. B. Hopkins, et al., Phys. Plasmas, 2014, 21, 056318.
9. S. W. Haan, J. D. Salmonson, D. S. Clark, D. D. Ho, B. A. Hammel and D. A. Callahan, et al., Fusion Sci. Technol., 2011, 59(1), 1–7.
10. B. C. Luo, K. Li, X. L. Tan, J. Q. Zhang, J. S. Luo, W. D. Wu and Y. J. Tang, J. Alloys Compd., 2014, 607, 150–156.
11. B. A. Hammel and The National Ignition Campaign Team, Plasma Phys. Controlled Fusion, 2006, 48(12B), B497–B506.
12. R. Brusasco, S. Letts, P. Miller, M. Saculla and R. Cook, J. Vac. Sci. Technol., A, 1996, 14(3), 1019–1024.
13. W. S. Shih, R. B. Stephens and W. J. James, Fusion Sci. Technol., 2000, 37(1), 24–31.
14. M. W. Mallett, E. A. Durbin, M. C. Udy, D. A. Vaughan and E. J. Center, J. Electrochem. Soc., 1954, 101(6), 298–305.
15. B. C. Luo, K. Li, Y. D. He, G. Niu, J. Q. Zhang, J. S. Luo, W. D. Wu and Y. J. Tang, High Power Laser Part. Beams, 2013, 25(12), 2359–2365.
16. F. Kalarasse and B. Bennecer, J. Phys. Chem. Solids, 2008, 69(7), 1775–1781.
17. J. H. Coobs and W. J. Koshuba, J. Electrochem. Soc., 1952, 99(3), 115–120.
18. Y. X. Xie, R. B. Stephens, N. C. Morosoff and W. J. James, J. Fusion Energy, 1998, 17(3), 259–260.
19. W. S. Shih, N. E. Barr, W. J. James, N. C. Morosoff and R. B. Stephens, Fusion Sci. Technol., 1997, 31(4), 442–448.
20. R. Brusasco, S. Letts, P. Miller, M. Saculla and R. Cook, J. Vac. Sci. Technol., A, 1996, 14(3), 1019–1024.
21. Y. X. Xie, Plasma deposition of beryllium carbide via magnetron sputtering [D], Univ. of Missouri, Rolla, 1998.
22. Y. X. Xie, N. C. Morosoff and W. J. James, J. Nucl. Mater., 2001, 289, 48–51.
23. A. F. Jankowski, M. A. Wall and T. G. Nieh, J. Non-Cryst. Solids, 2003, 317(1), 129–136.
24. A. F. Jankowski, M. A. Wall, A. W. Van Buuren, T. G. Nieh and J. Wadsworth, Acta Mater., 2002, 50, 4791–4800.
25. Powder Diffraction File (PCPDFWIN v.2.02), JCPDS-International Centre for Diffraction Data, 33–0191, 1999.
26. J. Nazon, J. Sarradin, V. Flaud, J. C. Tedenac and N. Frety, J. Alloys Compd., 2008, 464(1), 526–531.
27. S. Mehraj, M. S. Ansari and Alimuddin, Thin Solid Films, 2015, 589, 57–65.
28. Y. D. He, J. S. Luo, J. Li, L. B. Meng, B. C. Luo, J. Q. Zhang, Y. Zeng and W. D. Wu, Fusion Eng. Des., 2016, 103, 118–124.
29. Y. Xin, H. Zhou, X. Ni, Y. Pan, X. Zhang, J. Zheng, S. Bao and P. Jin, RSC Adv., 2015, 5(71), 57757–57763.
30. J. M. Khoshman, A. Khan and M. E. Kordesch, J. Appl. Phys., 2007, 101, 103532.
31. K. E. Oughstun and N. A. Cartwright, Opt. Express, 2003, 11(13), 1541–1546.
32. A. L. Ivanovskii, I. R. Shein, Y. N. Makurin, V. S. Kiiko and M. A. Gorbunova, Inorg. Mater., 2009, 45(3), 223–234.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra02141g

---