The frequencies, intensities, and nature of the vibrations in the IR spectra of the diamond- and polymer-like a-C:H films in the range 4000–1000 cm–1.
Results of the study of structural features and optical properties of thin films of amorphous hydrogenated carbon (a-C:H) films prepared by plasma-activated chemical vapor deposition of various hydrocarbon precursors are reviewed. The effect of different factors on the rate of a-C:H films deposition in a DC glow discharge with the magnetron plasma localized near the anode such as voltage, discharge power, gas pressure, relative content of an inert gas in the mixture with a hydrocarbon and other is analyzed. It is shown that the refractive index of a-C:H films can be changed in the interval 2.35–1.55 by increasing the deposition rate and the choice of the appropriate hydrocarbon precursor. The features of the vibration spectra of the diamond-like and polymer-like films are discussed. The correlations of the structural peculiarities and of the optical absorption edge, gap width, and conductivity as well as the absorption spectra in visible region and the ratio of the fundamental bands in Raman scattering spectra are estimated. Examples of using the optical properties of the a-C:H films are given.
- a-C:H films
- refractive indexes
- vibration spectra
- optical gap
Interest in the study of thin films of amorphous hydrogenated carbon (a-C:H) obtained by a plasma-activated chemical vapor deposition (CVD) is maintained for several decades. The reason for this is the possibility of variations in the properties of the films in a broad interval from diamond-like carbon up to polymer-like films. This opens up great opportunities for their practical application. The glow discharge plasma is produced with RF-biased in [1–6] or DC in [7–9] diode-type systems. The efficiency of gas ionization can be improved by placing a grid negatively biased with respect to the RF plasma near the substrate in , by using a magnetic field perpendicular to the RF plasma electric field in , and also by using a dual microwave electron cyclotron resonance RF plasma by applying an independently controlled RF substrate bias voltage in .
In this chapter, the results of the study of the structural features and optical properties of thin films prepared by CVD process in a DC glow discharge with the magnetron plasma localized near the anode are discussed. The effect of different factors on a rate of a-C:H films deposition such as voltage, discharge power, gas pressure, relative content of an inert gas in the mixture with a hydrocarbon and other is analyzed. Ellipsometry method was used for a comparative analysis of optical constants of a-C:H films prepared from different precursors. Features of the vibration spectra of a-C:H films according to their refractive index are discussed. The correlation of the absorption bands in the visible region of the spectra with of the optical gap and the Raman scattering spectra of the a-C:H films are explained. Examples of the use of diamond-like and polymer-like properties of the a-C:H films are shown.
2. Deposition of films in a DC glow discharge
2.1. Experimental setup for plasma-activated CVD
The a-C:H films are prepared by the method of CVD of hydrocarbons in the DC glow discharge using magnetron plasma localized at the anode in a quasi-closed volume . The schematic representation of the device for the deposition of a-C:H films is shown in Figure 1.
Localized plasma (1), produced by crossed magnetic and electric fields, sustains a glow discharge in volume (2). We used annular permanent magnet (3) and two planar annular electrodes: anode (4) and cathode (5). The magnetic field intensity near the cathode surface was about 600 G. Additional electrode (6) and substrate-carrying electrode (7) were mounted on glass cylinders with a diameter of about 130 mm. The cylinder walls confine the plasma inside the quasi-closed volume of the vacuum chamber. The ring-shaped gap between the magnet and the cathode is used as a gas inlet. Such a design of the device provides uniform gas distribution and efficient gas consumption, as well as reduces the film contamination by foreign impurities. Cathode (4) and anode (5) are placed in the neighborhood of the permanent magnet to provide maximum intensity and uniformity of the magnetic field in the crossed field region. The spacing between cathode 4 and substrate holder (7) is about 50 mm in the absence of the additional electrode. This electrode is placed 30 mm away from the cathode.
The cathode was under the ground potential. The substrate holder was either negatively biased with a DC power unit or under the ground potential. The additional electrode was used as a substrate holder when a-C:H films were deposited at a glancing angle in . The vacuum chamber was evacuated to 1–5 × 10–3 Pa by a rotary backing pump and a turbo-molecular pump. When the voltage is applied to the electrodes, the high-density toroidal plasma arises as a result of the effective electron capture by a magnetic trap, followed by gas ionization in the discharge gap between the anode and the substrate holder. Ion collisions with the solid surface confining the plasma generate extra electrons which also take part in gas ionization. a-C:H films were deposited on polished copper and glass substrates covered by thin conducting and semiconducting layers. This device allows the deposition of films with a thickness nonuniformity of not more than 20% on substrates of diameter up to 100 mm. Dependences of the ion current
When the plasma is sustained by the plasma localized in the device, the ion current varies only slightly with the gas pressure (varying from 0.01 to 0.04 Pa) at a constant voltage
2.2. Factors governing the rate of a-C:H film deposition
Chemical reactions and physical processes at the surface of a-C:H films during their deposition in a low-temperature hydrocarbon plasma were considered in terms of the adsorbed layer model in . This model assumes that CH3 plasma radicals are physically adsorbed on the surface and then pass into the chemisorbed state as a result of cross-linking due to energetic ions. The surface coverage depends on the number of surface states and surface temperature. When a-C:H films are deposited with the described device, their surface is continuously bombarded by positive ions whose energy depends largely on the voltage. The ion energy may be high enough to consolidate the condensate either by cross-linking or by decomposing weakly bound particles with subsequent surface diffusion and desorption of the decomposition products. The balance of the processes conducive to film deposition and etching defines the deposition rate. The film deposition rate can also be varied by properly selecting the glow discharge parameters (gas pressure, ion current to the substrate, and voltage), which control the number of ions and their energy.
The deposition rate was defined as the ratio of the a-C:H film thickness to the deposition time. The thickness was measured by an MII-4M (Russia) micro-interferometer with an accuracy of 10%. The thickness of a-C:H films was found to be 0.1–0.3 μm. As the voltage increases from 700 to 1300 V, the deposition rate varies from 0.25 to 2 Å/s. The rate
Figure 4 illustrates the effect of surface conductivity on the deposition rate at an acetylene pressure in the vacuum chamber of about 0.05 Pa. For the same discharge power, the rate of film deposition on the surface of a transparent conducting indium–tin oxide (ITO) layer is one order of magnitude higher than that on the surface of a-Si:C:H semiconductor layer with a resistivity of about 1012 Ω cm.
Space charge is produced on the substrate surface when it is bombarded by positive ions in a DC glow discharge plasma. Charge leakage from the surface depends on the substrate conductivity, as well as on the thickness and resistivity of the growing film. As the resistivity grows, the critical thickness of the film and the deposition rate decrease. Therefore, the surface conductivities of the substrate and condensate in glow discharge plasma should be considered as important factors affecting the kinetics of a-C:H film deposition. The dependence of the deposition rate on the acetylene volume concentration in a mixture with krypton is shown in Figure 5 for a constant discharge power of 1.8 W and a pressure of 0.05 Pa.
The addition of an inert gas to acetylene decreases the deposition rate of the a-C:H films on the copper substrates for the same pressure in the vacuum chamber from 4 to 0.5 Å/s. A film bombardment by inert gas ions etches the surface during the condensation and lowers the deposition rate.
3. Optical constants of a-C:H films
The method of ellipsometry enables to investigate the optical constants of thin films in the visible region. Ellipsometric studies have shown that the a-C:H films can have a high refractive index and transparent in the UV–visible range in [16, 17]. The dependences of the optical constants of a-C:H films on the parameters of the condensation process have been established by ellipsometry in [18–21]. The optical constants for the wavelength of 632.8 nm of the a-C:H films obtained by the method described above were determined using LEF-3M ellipsometer (Russia) . The experimental setup consisted from polarizer–compensator–sample analyzer. A dual-zone null method at angles of incidence f of 50°, 60°, and 70° was used to calculate the ellipsometric parameters. To calculate a refractive index
The refractive index of a-C:H films obtained from acetylene and toluene decreases monotonically with increasing the deposition rate in Figure 6. Decreasing the extinction coefficients for these films can be seen in Figure 7. The films obtained from acetylene have the highest refractive indexes n = 2.35 and the highest extinction indexes k = 0.3 under the deposition rate equal to 0.5 Å/s. For films prepared from toluene, these indexes were equal to n = 1.8 and k = 0.1 in the same deposition rate. The films obtained from octane under the same conditions have the lowest values of n = 1.55–1.6 and k < 0.01 at a wavelength of 632.8 nm, and no dependence on the deposition rate is observed in Figure 6. These points to the dependence of the optical constants of the films from hydrocarbon precursors used for their deposition in the glow discharge plasma. It can be concluded that the optical constants of a-C:H films can be varied by changing the deposition rate in the plasma, as well as by selecting the precursor.
4. Vibration spectra of the a-C:H films
Infrared spectroscopy is widely used to study the optical absorption and structural features of a-C:H films in [22–29]. However, this method is not sufficiently sensitive for systems composed of a-C:H films and semiconductors with high refractive indices (Si and Ge) due to the significant interference effect. Comparative analysis of the IR spectra of multiple attenuated total internal reflection (MATIR) in the range 4000–1000 cm–1 was performed in . The use of the MATIR method excludes the influence of the interference effect and makes it possible to record the vibration spectra of thin a-C:H films. A single-crystal germanium prism, which provided 12 reflections of IR radiation from a plane surface at an angle of 45°, served as the MATIR element. Figure 8 shows the IR spectra of a-C:H films with the refractive index
As can be seen from Figures 8 and 9, an increase in the power by a factor of 5 (from 2 to 10 W) leads to significant changes in the IR spectra of the a-C:H films. The higher the discharge power, the higher the energy of the positive ions and their effect on the deposition process increase. The four functional branching structures are formed during the deposition of diamond-like films by decomposition of hydrocarbons in glow discharge plasma in the interaction of ions with the surface of the growing film. There are the stretching vibrations of C–H groups, carbonyl groups, single (C–C) and double (C=C) bonds, and bending vibrations of C–H groups in the spectra in Figures 8 and 9. In addition, weak peaks at 2100 and 1900 сm–1 (Figure 8) and 2080 сm–1 (Figure 9) due to the stretching vibrations of the C≡C bonds are observed in the spectra. The diamond-like films in Figure 8 and polymer-like films in Figure 9 have specific features in the IR absorption spectra. It should be noted that the IR spectra of the diamond-like a-C:H films (Figure 8) prepared from different hydrocarbons (octane, toluene, and cyclohexane) do not demonstrate any significant differences. They are similar to the spectra of the a-C:H films prepared from acetylene in . The IR spectra of the polymer-like a-C:H films prepared from toluene and octane (Figure 9) are similar to previously measured spectra of polymer-like films prepared from benzene, toluene in , and acetylene in [29, 30]. The symmetric (νs) and asymmetric (νas) stretching vibrations and the symmetric (δs) and asymmetric (δas) bending vibrations of aromatic and olefin compounds, which observed in the IR spectra of the diamond- and polymer-like a-C:H films, are shown in Table 1.
|Nature of vibration||Intensity of band|
|Diamond-like films||Polymer-like films|
|3100||ν (=CH) aromatic ||low||low|
|3000||ν(=CH) olefin ||low||low|
|2850||νs (−CH2) ||medium||medium|
|1900–2100||ν (C≡C)||very low||very low|
|1600||ν (С−С) aromatic||medium|
|1250||ν (С−С) complex branching||medium|
A feature of the IR spectra of these films is the presence of bands at ~1250 cm−1 due to the stretching vibrations of C–C bonds at the branch points of four functional structures (Table 1). This band weakly manifests itself or is almost absent in the spectra of the polymer-like a-C:H films. The spectra of these films show strong absorption bands due to the stretching (~2920 cm–1) and bending (~1450 cm–1) vibrations of CH groups, as well as of carbonyl (~1700 cm–1) and hydroxyl (~3400 cm–1) groups in [27–30]. The reason of occurrence of the stretching vibrations of carbonyl and hydroxyl groups in the IR spectra is in the chemisorptions of water and oxygen from the environment into micropores of a-C:H films. The porosity of a-C:H films with a low refractive index can be up to 7% as the ellipsometric investigation of the films was shown in .
The content of bound hydrogen in polymer films is much more than the diamond-like films. The intensity of the absorption band in the range 3400–2600 cm–1 in the spectra of the polymer-like a-C:H films (Figure 9) is much higher than the spectra of the diamond-like films (Figure 8). The correlation between the refractive index of the a-C:H films and the integrated intensity of the band peaked at 2900 cm–1 has been shown in . With a change in the refractive index from 1.55 to 2.4, the integrated intensity of the band attributed to the stretching vibrations of the CH groups exponentially decreases by an order of magnitude. Hence, a decrease in the content of bound hydrogen is accompanied by an increase in the refractive index of the a-C:H films, which indirectly proves that the film structure becomes denser.
5. Structural features of the a-C:H films
The known forms of amorphous carbon, including the various modifications of a-C:H, consist of carbon atoms in the
Increasing the hydrogen concentration in an amorphous carbon structure leads simultaneously to a reduction in its equilibrium density and to a substantial change in the character of the clustering. This has been shown by studies of the stability of a-C:H systems and of their atomic and electronic structure as functions of the mass density and concentration of hydrogen using the molecular-dynamic density method in [37, 38]. The extent of the clusters is reduced by the introduction of
5.1. Relationship of the Tauc parameters to the absorption spectra of a-C:H films
The optical gap in the various modifications of a-C:H depends on the conditions, under which it was produced in glow discharge plasma. The gap is made smaller when the energy delivered to the condensate is increased by raising the temperature of the carrier gas in  or substrate in [4, 20], by raising the power of the RF discharge , and by raising the voltage in [21, 40–42]. The conductivity of the a-C:H films increased when the optical gap decreased in .
For studying absorption spectra of a-C:H in the visible region, thin films were deposited from pure acetylene (C2H2) or a mixture of it with argon (Ar) at the ambient temperature. Table 2 shows the conditions of preparation of the samples of the films. Samples
The spectral variations in the absorption α (λ) at wavelengths in the range 400–2400 nm were obtained from the reflection spectra of films with thicknesses of 0.3–0.5 μm deposited on polished copper substrates. These spectra can be resolved into a series of Gaussian-type bands. In the wavelength interval from 400 to 1000 nm, two absorption bands were identified with peaks at 600 ± 5 and 800 ± 40 nm in . The ratio of the integrated intensities of these absorption bands (
The absorption edge of this a-C:H films, like that of other amorphous semiconductors, was adequately fit by the Tauc equation in . The Tauc optical gap
The impact of the ions of the inert gas on the growing film (sample
The atomic structure of the diamond-like can be described as a stressed-bond rigid lattice, in which mixed bonds predominate with local electronic systems of π-bound atoms imbedded in it. The presence of CH-groups in the
The optical gap of the a-C:H films is determined by the energy of the most probable electronic π–π* transition. It should be noted a transition of a valence π-electron localized either on isolated linear chains or on complicated π-bound structural elements to a corresponding free π*-level in the conduction band. The spectra of these films contain weak discrete absorption bands, which lie below the energy
5.2. Studying the a-C:H nanostructure using resonant Raman scattering spectroscopy
The Raman scattering spectra of amorphous carbon in its different modifications consisted of a broad band and can easily be resolved into two Gaussian-type bands. The first has a peak in the interval 1530–1580 cm−1 and was initially attributed to an active 1585 cm−1 line of single-crystal graphite, while the second band near 1300–1400 cm−1, with a line at 1355 cm−1, corresponds to a disordered mode in . However, a high-frequency shifts of the principal maximum observed in the Raman scattering spectra of various modifications of a-C:H when the excitation energy is raised that has cast doubt on this interpretation in [47, 48]. It is the result of scattering from π-bond elements of the a-C:H structure, which is enhanced resonantly at photon energies approaching the π–π * resonance. The bands near 1400 and 1530 cm−1 may be associated with scattering on large- and small-sized π-bond clusters, respectively, in .
The absence of graphite clusters in the structure of fresh and thermally worked at a temperature of 400 °C in vacuum a-C:H films has been confirmed by comparing resonant Raman scattering spectra of a-C:H and graphite in . The observed features of the a-C:H Raman scattering spectra lead to the conclusion that their structure includes a set of scattering centers characterized by different excitation energies of π–π * electronic transitions and vibration energies. The different sizes of the π-bond elements of the structure lead to different values of the coupling parameter. The strong disordered interaction among them causes the large spread in their electronic and vibration spectra. Resonant excitation of the Raman spectra produces a selective enhancement in the scattering at the frequencies of those centers for which these conditions are optimal. The width and shape of the Raman scattering bands are determined by the dependencies of the vibration frequency and location of an absorption band on the length of the coupling chain and on the size distribution of the elements. In this case, the procedure of expanding the complicated Raman scattering band and absorption bands in the electronic spectra into Gaussian profiles is arbitrary.
The way the relative intensity of the Raman scattering bands of fresh and thermally processed a-C:H depended on the excitation wavelength has been shown that these bands correspond to different types of structural elements, which are polyene chains of various lengths and polycyclic aromatic groups with different numbers of rings. The former predominates in the intensity of the band at ~1540 cm−1 and the latter, to that of the band at ~1340 cm−1, is extremely probable, since in the Raman scattering spectra of the corresponding molecular structures, the most intense bands lie in the corresponding range of vibration frequencies in .
The existence of olefin chains in the structure is confirmed by studies of a-C:H films by elastic neutron scattering in  and by nuclear magnetic resonance and neutron diffraction in [51, 52]. Theoretical calculations of resonant Raman scattering spectra of amorphous carbon have shown that the band shapes are determined by a complex of
5.3. Comparison of Raman scattering and absorption spectra in the visible region
Resonant Raman scattering spectroscopy is an extremely informative and highly sensitive method for investigating the characteristics of middle-range π-bond elements in a-C:H structures. Increasing the energy of the ions involved in the condensation of a-C:H in RF in [54, 55], and DC glow discharge plasmas in  leads to a rise in the intensity ratio of the main bands at 1340 and 1540 cm−1 (
It can be seen that as the ratio
6. Application of optical properties of the films
The a-C:H films with
The transmission of a Ge sample that has both sides coated with a-C:H films does not exceed 90% at a wavelength of 10 μm due to the absorption loss in the a-C:H coatings in . Due to their combination of chemical and mechanical durability, radiation resistance, and transparency in the IR region, diamond-like a-C:H films can be used as protective coatings for IR optics, in particular, for copper mirrors. Higher breakdown thresholds were observed on a-C:H coatings obtained with deposition rates <2 Å/s. Such coatings have the depths of trap levels not exceeding ~1.5 eV and by resistivity of 107–108 Ω cm in . The a-C:H layer with the absorption coefficient about 4 × 104 cm−1, and having resistivity in the interval from 1010 to 1011 Ω cm, and
Results of studies on the preparation of the a-C:H films by plasma-activated CVD in various hydrocarbon precursors and their structure and optical properties are reviewed. The dependences of the deposition rate of the films obtained by CVD process in a DC glow discharge with the magnetron plasma localized near the anode on the voltage, relative content of an inert gas in the mixture with a hydrocarbon, were discussed. Knowing them, one can control the process of condensation. One can vary the refractive index of a-C:H films in the interval 2.35–1.55 and modify their structure and properties from diamond-like carbon to polymer by increasing the deposition rate and the choice of the appropriate hydrocarbon precursor. Identifying the a-C:H film as a diamond-like or polymer-like film can use their vibration spectra, since they have specific features. A characteristic feature of the IR spectra of the diamond-like films is the presence of a band at ~1250 cm–1 due to the stretching vibrations of the C–C bonds in four-functional branching points of the structure. The spectra of polymer-like films show strong absorption bands due to the stretching and bending vibrations of CH groups, as well as of carbonyl and hydroxyl groups. The integrated intensity of the band of CH vibrations peaked at ~2900 cm–1 decrease exponentially by an order of magnitude with an increase in refractive indices from 1.55 to 2.4. Nanostructure of the a-C:H films can characterize using the absorption spectra in the visible region and the Raman scattering. Absorption in the range 400–600 nm is caused by π−π* transitions in polyene chains, while the absorption in the range 600–800 nm is related to similar transitions in more complex π-bond combinations, including polycyclic aromatic groups. A conductivity a-CH films increase with decreasing a width of the optical gap. Diamond-like films can be used as an optical antireflection coating for infrared optical elements, and light-blocking layers in the visible spectral region. Transparent in the visible region polymer-like films can be used as alignment layer in LC devices due to their surface properties. A wide range of optical properties of the film holds great promise for application in different optical devices.
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