1. Introduction
The emission of secondary electrons from surfaces plays a key role on the generation of electron multipacting in high power radio frequency devices and particle accelerators with positively charged beams. In particle accelerators, it can result in the formation of clouds of electrons, causing beam instabilities, deterioration of the vacuum, or heat loads to cryogenics parts of the system, limiting the overall efficiency of the accelerator (i.e. the beam luminosity) [
1,
2,
3,
4,
5,
6]. A possible cure to this problem is to reduce the electron emission by coating the internal surfaces of the vacuum chambers with a thin film of low Secondary Electron Yield (SEY) material. Amorphous carbon (a-C) coatings have been successfully used in the Super Proton Synchrotron at CERN (European Organisation for Nuclear Research) to mitigate the electron multipacting [
7], and it is now in the baseline for the High Luminosity upgrade of the Large Hadron Collider, (CERN, Switzerland), and the Electron Ion Collider, (Brookhaven National Laboratory, USA) [
8,
9].
Secondary electron emission can be conveniently described as follows [
10]. Primary electrons that penetrate a material mainly lose their energy by different types of electron excitations (via plasmon excitations or by a direct energy transfer to valence and core electrons). This results in the generation of secondary electrons inside the material, with energy above the vacuum level. On their way out, these internal secondary electrons can efficiently lose their energy only by excitation of valence electrons. Finally, once they reach the surface, only the internal secondary electrons which still have energy above the vacuum level have a chance to be emitted. The whole process is strongly affected by the electronic structure of the material, which determines the energy loss processes of both primary and internal secondary electrons. Particularly important is the latter, which can be reduced by opening the energy gap. This is the origin of the high SEY of dielectric materials [
11].
The efficiency of a-C coatings for suppressing electron multipacting depends on structural properties: only coatings with pronounced electrical conductivity can have a low SEY. It has been well established that the presence of hydrogen impurities is particularly harmful in that respect [
12,
13]. The effects of hydrogen and nitrogen impurities on the secondary electron emission properties of a-C coatings were a subject of two recent studies [
14,
15]. It was clearly demonstrated that adding hydrogen or deuterium in the discharge gas increases the SEY of carbon films, while the presence of nitrogen has an opposite effect. By combining SEY measurements with Ion Beam Analysis (IBA), a correlation between the deuterium content and the maximum SEY (SEY
max) was established. SEY
max increases linearly, from 1 to 1.4, with the overall hydrogen and deuterium (H + D) relative content up to 47%. Further increase of the H + D amount up to 54% is accompanied by a steep SEY
max growth to 2.2. The samples were also investigated by Optical Absorption Spectroscopy (OAS), enabling estimation of the optical energy gap using the procedure established by Tauc (so-called Tauc gap). These results reveal a strong correlation between the SEY
max and the Tauc gap, apparently offering a straightforward explanation for the SEY
max increase. However, a-C films are often non-uniform, i.e., they may consist of various regions with different composition and electronic structure (e.g., graphitic, diamond-like and hydrocarbon domains, the latter being a result of highly common hydrogen contamination) [
16]. Such materials have different local energy gaps, limiting the interpretation of Tauc plots [
17].
Robertson summarized measurements of the Tauc gap performed on different types of a-C (pure and hydrogenated) in his seminal review paper [
18]. He concluded that the Tauc gap of a film is not determined by the hydrogen content, but by the relative amount and properties of the sp
2 carbon phase. Local energy gaps of diamond-like and various hydrocarbon phases are too large to be related with a Tauc gap below ≈2 eV. The gap is therefore related to the configuration of π states on the sp
2 sites. In a planar cluster model, the band gap of a given cluster is inversely proportional to the square root of the number of the hexagonal rings in the cluster [
19]. Different types of defects (e.g., formation of pentagons or heptagons) will also open the gap. From this perspective, deviation from the linear dependence at low photon energies in Tauc plots should not be attributed to the so-called tail states (i.e., Urbach tail) but to the small quantity of graphitic clusters with small local energy gaps.
The abrupt increase of SEY
max, when the H+D relative concentration changes from 47% to 54%, suggests that the hydrogen content may not be directly responsible for the change of the secondary electron emission (SEE) properties of a-C. This doubt is further supported by the high correlation between the SEY
max and the Tauc gap [
14,
15], knowing that the Tauc gap can be independent of the hydrogen concentration [
18]. This aspect motivates our study: since the Tauc gap in a-C is known to be affected by the concentration and size of graphitic domains, could the latter also influence the SEE?
In our recent work we performed deposition of a-C coatings using magnetron sputtering in Ar discharges with several fractions of D
2, identified the mechanism behind the incorporation of deuterium in the films during the production phase and its impact on the SEY [
15]. In this work we report detailed analysis of the same samples by means of different electron and vibrational spectroscopic techniques. X-ray and UV Photoelectron Spectroscopies (XPS and UPS, respectively) were used to determine the surface composition, to identify different phases in the a-C films, and obtain information on their electronic structure. Further insights are provided by Raman scattering, Fourier Transform Infrared Spectroscopy (FTIR) and High Resolution Electron Energy Loss Spectroscopy (HREELS). Raman scattering, enabling the detection of the presence of graphitic carbon, was particularly valuable in supporting the XPS results. FTIR and HREELS were used to identify the character of C-D bonds in the most contaminated samples. These results are then compared with the corresponding SEY, OAS and IBA measurements, enabling us to reveal the actual mechanism behind the SEY increase by the hydrogen contamination.
2. Materials and Methods
All details of the thin film deposition procedure are explained in [
15]. Briefly, the films were deposited by magnetron sputtering from a 50 mm diameter graphite target. The total operating pressure was set to 2 Pa, consisting of Ar and small quantities of D
2, to study deuterium incorporation into the films. The substrates were mounted 93 mm away from the graphite target and the discharge power fixed at 30 W. Five sets of samples were deposited on Si single crystal and quartz substrates for different characterisations: the reference (without D
2 added to the discharge gas), 0.2D (0.2 vol% of added D
2), 0.5D (0.5 vol% of D
2), 1D (1.3 vol% of D
2) and 10D (10.9 vol% of D
2). D
2 was added to distinguish deliberately introduced contaminants from hydrogen contamination from the residual gas. The system was baked prior to each run for 24 h at 230 °C to minimize the natural contamination, keeping the residual gas pressure in the low 10
-6 Pa range or better (N
2 equivalent). A new graphite target was used for each run. The deposition rate was 10-15 nm/h, generally growing with the D
2 partial pressure, pD
2. The thin film thicknesses are in the range from 486–719 nm (measured by SEM). The samples were transferred to different laboratories in stainless steel vacuum chambers, pre-evacuated by a turbomolecular pump and filled with N
2 gas.
UPS and XPS measurement s of ‘as-received’ samples were performed on AXIS SUPRA setup (Kratos Analytical), containing both UPS and monochromatic XPS sources. XPS measurements were performed using the monochromatic Al Kα line (photon energy of 1486.7 eV), and the spectrometer pass energy of 80 eV (survey spectra) and 5 eV (high resolution spectra). UPS measurements were performed by means of the He I α line (photon energy of 21.22 eV) and the spectrometer pass energy of 5 eV. The binding energy scale for UPS and XPS measurements were both calibrated using a sputter-cleaned Ag sample, based on the position of the Ag 3d5/2 line and of the top of the valence band. Fitting of graphitic contributions was based on the XPS measurements of freshly cleaved Highly Oriented Pyrolytic Graphite (HOPG). A correction of the raw UPS spectra was made by removing the contributions of the He I β line (energy shift of 1.87 eV, intensity of 1.2% with respect to the He I α line). All samples were conductive, apart from the sample 10D, which was measured with charge compensation using an electron flood gun.
Raman spectroscopy measurements were carried out at room temperature in a backscattering geometry on an alpha 300 R confocal Raman microscope (WITec) using a 532 nm Nd:YAG laser (2nd harmonic) as well as a 633 nm He-Ne laser for excitation. The laser beam with a power of 0.7 mW was focused on the sample by a ×50 lens (Zeiss), providing a spot with a diameter of about 1 μm. The spectra were collected with a 600 groove/mm grating using 5 acquisitions with a 2 s acquisition time. The same setup was used to perform photoluminescence measurements with two excitation wavelengths, 532 nm and 633 nm. For each sample, the spectra were acquired on several spots to check their lateral uniformity.
FTIR measurements were performed in vacuum in the wave number range 500-5000 cm-1, using a Vertex 80v system (Bruker) in conventional reflection and transmission mode, as well as in the attenuated total reflection (ATR) configuration using a Ge crystal.
HREELS measurements were performed in a UHV system (operating pressure in the 10
-7 Pa range) using a LK Technologies 2000R spectrometer. Further details on this experimental setup are provided in the
Appendix A, together with the experimental results.
4. Discussion
The XPS results and the mismatch between the Tauc plot measurements and UPS spectra evidence that the samples are generally non-uniform. Hence, the films should be considered as mixtures of different carbon phases (being a frequent situation [
19]), each characterised with their own local energy gaps. The steady increase of the films’ density with the amount of incorporated deuterium up to the sample 1D can be understood as the increase of the sp
3 phases (cf.
Table 1). At the same time the sample with the lowest density is 10D, despite the similar relative composition with that of 1D, clearly indicating formation of a polymeric phase [
20]. These estimations are fully supported by the results of the XPS, UPS and Raman analyses, revealing that increase of the deuterium content is accompanied by a decrease of the overall relative amount of the graphitic regions and their size reduction (i.e., transition from nanocrystalline graphitic phase into amorphous carbon). This is particularly evident when comparing the samples 1D and 10D, characterized with the similar relative content of deuterium (
Table 1) and practically the same character of the C-D bonds. The abrupt increase of the SEY and the Tauc gap from sample 1D to 10D can be only explained by the dramatic diminishing of the graphitic component (
Table 1).
A similar conclusion has been obtained by Robertson [
18], demonstrating a nearly linear dependence of the Tauc gap with the sp
2 fraction
csp2 for its relative contents below 80%, practically independent from the hydrogen relative amount. Since the data presented in [
18] are compiled from the works of different authors and measured on various a-C and a-C:H samples, the points are rather scattered. Nevertheless, the line
ET = 3 – 2.5∙
csp2 represents well the observed trend with the Tauc gap uncertainty bar of about
ΔET = ±0.25 eV [
37]. The dependence of the Tauc gap and the
SEYmax vs. the graphitic content, obtained from the XPS measurements, is shown in
Figure 4. It can be observed that both
ET and
SEYmax also decrease with the relative amount of graphitic regions. Moreover, the trend of the Tauc plot fits well with that of Robertson, presented by a dashed blue line, for the graphitic contents below 70%. The systematic shift between the two trends, which is within the uncertainty
ΔET, can be explained by different means of measuring the graphitic content in [
37] (using electron energy loss spectroscopy and nuclear magnetic resonance). Besides, at
csp2 > 70% we observe faster drop of
ET, as expected having the non-existent band gap of graphite. These agreements imply that the methodology used to extract the graphitic fraction from the XPS spectra is quite reliable and can be also used to correlate the SEY with the graphitic content.
The decrease of the SEY with the increase of the graphitic content (observed in
Figure 4) is in accordance with the expectations: higher the fraction of the sp
2 phase, lower the SEY. A simple model, based on the same assumptions as those used to derive the well-known semi-empirical equation for SEY [
38], was developed to support the observed experimental dependence. This equation, originally derived for uniform samples, was recently modified to encompass multilayer systems [
39]. The latter approach was used as a starting point to model non-uniform a-C films as mixtures of two different phases: a graphitic one, and a polymeric one. Therefore, the polymeric phase is considered as a representative of all non-graphitic phases in the films.
In the frame of the semi-empirical theory, the number of internal secondary electrons generated in a depth range (
z,
z+dz) is equal to
S(
z)∙
dz/
ε(
z), where
S and
ε represent the stopping power of the primary electrons and the effective energy required to create one internal secondary electron, respectively. The secondary electrons created at depth z will be then emitted with a probability equal to 0.5∙exp(-
z/
λ) [
40], where
λ is the mean escape depth of secondary electrons, taken from the literature [
11]. Since
S,
λ and
ε are material dependent, they will be changing from point to point of the sample interior.
The first step is to determine the parameters that characterize the electron emission properties of the graphitic and polymeric regions. For that purpose, the SEY dependencies on the primary electron energy of the reference sample and the sample 10D [
15] were used as representatives of pure graphitic and pure polymeric material, respectively. Then, for a defined relative graphitic content, it is possible to estimate the range of primary electrons
R. The samples were then modelled as a multilayer system with predefined graphitic content. However, the depth distribution of different phases within each sample is unknown. Another difficulty in establishing this distribution is in the lateral non-uniformity of the samples. To overcome this problem, a set of M depth distributions were generated (here designated as configurations), each containing the same amount of graphitic carbon, using a Monte Carlo approach. Finally, for each of the configuration, the
SEY(
E) was calculated by generating
S(
E,
z)∙
dz/
ε(
z) electrons at all depths
z in the range [0,
R], and calculating their escape probabilities affected by the multilayer structure in the range [0,
z]. The whole procedure is detailed in the
Appendix B.
The result of the SEY model, obtained using M = 1000 configurations and for the primary electron energies in the range 50-1000 eV is presented in Fig. 4. Having in mind the simplifications introduced in the frame of the model, and that the calculated SEY curves were obtained without any adjustments to the existing experimental data, the agreement with the measured values is very good. There is also a built-in systematic error in the model due to the consideration that the reference sample would be purely graphitic, while its actual graphitic content is only 90%. This approximation is responsible for an overestimation of the SEY for the highest graphitic contents. Although the effective energy ε is indeed smaller in the polymeric (40.8 eV) than in the graphitic (60.4 eV) regions, the major difference between the characteristics of the two regions is the mean escape depth λ, being more than two times smaller in graphite (4.9 nm) than in polymers (11 nm). That is probably the reason why the theoretical SEYmax(sp2 content) dependence can be described by an exponential decay function.
In a sample with sufficiently high content of the graphitic phase, the regions with large energy gap (e.g., diamond-like or hydrocarbon regions) will not contribute to the Tauc gap: photons that may penetrate large energy gap regions will be most likely absorbed once they enter a sufficiently large graphitic region. The reduction of the amount and size of the graphitic phase increases the optical transparency of the films and, consequently, the Tauc gap. A similar effect takes place during the transport of secondary electrons through the samples. The escape depth of internal secondary electrons, which is directly related with their energy loss mechanisms, is strongly affected by the electronic structure of a material. The efficient energy loss is secured by sufficiently long trajectories of the secondary electrons through graphitic regions characterised by very narrow energy gap. As in the case of light transmission, the graphitic phase serves as energy absorber. The increase of the graphitic content directly reduces the escape depth of internal secondary electrons and, therefore, the secondary electron yield.
Although the presence of hydrogen and deuterium in the thin films is not directly responsible for the SEY increase, it plays a key role in reducing the graphitic amount. Indeed, the H/D incorporation will naturally reduce the relative contents of both graphitic and diamond-like phases in a-C:H(D). It was revealed in the previous work that the deuterium incorporation is a consequence of target poisoning, yielding in the sputtering of CD and CD
2 molecules in parallel with C atoms [
15]. At the same time, pure carbon phases can only be deposited by magnetron sputtering of C atoms. The relative amount of pure carbon phases is directly related with the flux ratio of C and CD
x particles leaving the target and travelling towards the substrate. In the case of the sample 10D, this ratio was very low due to the strong target poisoning, resulting in a typical polymeric film.
Author Contributions
“Conceptualization, N.B. and P.C.P.; methodology, N.B and P.C.P; software, N.B.; validation, A.F., M.H. and M.V.; formal analysis, N.B., J.D., M.C., A.R.; investigation, C.A., J.D., M.C., A.R., N.P.B, Y.D., H.N., M.R.; writing—original draft preparation, N.B. and P.C.P.; writing—review and editing, M.H., O.T., C.A., A.R.; visualization, C.A.; supervision, M.V., I.F., E.A. All authors have read and agreed to the published version of the manuscript.”