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Some peculiarities in ultraviolet absorption spectra for amorphous hydrogenated carbon coatings are presented. The coatings were prepred by the rf magnetron sputtering method, with a gas pressure between 0.1 and 0.3 Torr. The most significant result found was that the films deposited at high pressures (greater than 0.2 Torr) and small thickness (bellow 400 nm) presents some peculiarities in the UV absorption spectrum. Tht re corresponding to a diamond-like structure. The Tauc plot for these films, compared with the films with thickness greater than 1000 nm, presents two different regions corresponding to two different optical transitions. Films deposited under similar conditions, with a thickness greater than 1 Î¼m, present an optical band gap around 2.5 eV and a large sp3 ratio (greater than 85%). The results suggest a dimensional effect indicating optical absorption behavior closer to diamond.
Over the last two decades amorphous hydrogenated carbon films (a-C:H) have acquired growing interest, both scientifically and technologically, by virtue of their unique combination of useful and adjustable properties like high hardness, chemical inertness and infrared transparency [1,2]. A wide variety of methods, such as ion beam deposition, plasma deposition from hydrocarbons, magnetron sputtering, ion beam sputtering, laser plasma deposition, have been used to prepare amorphous carbon films . The properties of amorphous carbon films have been correlated with the energy of the incident species during deposition . The study of the deposition techniques used to obtain a-C films shows that the film properties are dependent on the deposition system used and the deposition parameters. The ion bombardment at the surface and processes in the plasma bulk can be separately controlled in a circular magnetron sputtering system. The magnetic field confines the electrons and improves gas decomposition and the bias voltage applied to substrate provides a selective control of the ion energies. The advantage of the magnetron sputter source is that this technique is widely established in industry and allows the deposition onto large areas with relatively high deposition rates.
The absorption spectrum in UV-VIS range is commonly used to calculate the value of the optical band gap energy. Basically, the optical gap is a measure of the gap between the extended state in the valence band and the conduction band. According to Cohen et al., a localized state is one were an optical transition probability amplitude decrease exponentially with the distance from the center of localization. These states are known as tails of the localized state density. The model applies to intense enough tails of the localized state density. This paper presents some peculiarities of UV absorption spectra for a-C:H thin films deposited by sputtering.
Thin films were deposited by magnetron sputtering technique, using a pure graphite target, 7.5 cm in radius and 99.99 % pure Ar/CH4 gas mixture. The magnetron installation was built in our laboratory in order to deposit thin films by dc and rf planar and circular magnetron sputtering [6-8]. The energy and the flux of the ions (mainly Ar+ and CHn+) reaching the growing film surface was varied by applying an external bias voltage (from -400 V to +400 V) to the substrate. The films were deposited by r.f. circular magnetron sputtering, using a r.f.-source at 13,56 MHz. A stabilized dc power supply with maximum ratings 500 V and 240 mA was used for negative or positive substrate polarization. Films were deposited on quartz substrates at a target-to-substrate distance of 3 cm. The volumetric proportion between Ar and methane was maintained constant at 1/1. The pressure during deposition was between 0.1 and 0.3 Torr. The substrate temperature remains below 100°C for all samples. Optical absorption of the films was measured in the wavelength range 200-350 nm using an UV-VIS SPECORD spectrophotometer. The absorption coefficient ± was calculated using Lambert's Law and was explored as a function of photon energy hÎ½. The optical band gap is determinate from a Tauc plot:
with n=2 for indirect band transitions and n=1/2 for extend to extended states transitions in amorphous material.
3. RESULTS AND DISSCUTION
The structure of electronic states in amorphous materials is considered to be formed by parabolic bands (extended states) with tail states (localized states).
The transitions from extended to extended states are very similar to the ones known from crystalline materials, with direct band transitions. The assumption of parabolic bands leads to a dependence between absorption coefficient and photon energy of the form presented in eq.(1), with n = ½. Hence a plot of eq.(1) should lead to a straight line whose intersection with the y-axis gives the gap energy, the so-called "Tauc gap" or "optical gap". This gap is used to characterize the optical properties of amorphous materials and gives information on the energy separation of the extended states of valence and conduction bands.
In amorphous materials is possible an electronic transition without a correspondent in crystalline materials, between localized to extended states and for extended to localized ones. For these transitions, with an exponential decay of the density of states into the gap, one finds an exponential relation between absorption coefficient and photon energy:
where EU is called "Urbach energy".
The transitions between two localized states are not very important since the number of states involved is low and the transition matrix elements are significantly smaller compared to those of the transitions mentioned above.
Table 1. Deposition and films parameters
Deposition pressure (Torr)
Substrate applied bias voltage (V)
In Fig.1 are presented on separate layers the Tauc plots for all five samples, separated after the pressure deposition. The deposition and films parameters are indicated in Table 1. The samples were obtained for different gas pressures and applied bias voltages, with thickness between 0.1 and 0.4 Î¼m. For the two samples deposited at 0.1 Torr gas pressure, the shape of the plot is similar to the dependencies usually presented in the literature. The curve for samples deposited at 0.2 and 0.3 Torr reveals some energy distinctive features, more evident for higher pressure. The presence of the two regions in the absorption spectrum is not dependent by the film thickness or the applied bias voltage during deposition.
Tauc plots for five samples deposited at three different pressures, indicated in Table 1
In order to explain these peculiarities, in Fig.2 is presented the Tauc plot for sample E deposited at 0.2 Torr gas pressure, with 0.21 Î¼m thickness. The figure shows the presence of two different regions. For smaller energy values, the plot presents an almost linear region, corresponding to Tauc energy around 3.3 eV. The films deposited in similar conditions, with a thickness greater than 1 Î¼m, presents an optical band gap around 2.5 eV and a great sp3 proportion (greater than 85%).
Tauc plot for n = ½ for a sample deposited at 0.2 Torr, with 0.21 Î¼m thickness
The large amount of sp3 bonds, correlated with the small thickness, can be the explanation for the great value of the optical gap for investigated films. Because the Ï€ states of sp2 bonds lie closer o the Fermi level EF than do the Ïƒ states, so the Ï€ states form both the valence and the conduction band edge states and they govern the optical gap of amorphous carbon films9. When the thickness increases, the density of the localized states increases to a "saturation" value, and the optical band gap decreases. The deposition pressure must influence the film growth process, favouring the sp3 bonds formation. The second region in the Tauc plot can be the result of other electronic transition.
Tauc plot for n = 2 for a sample deposited at 0.2 Torr, with 0.21 Î¼m thickness
One possible explanation is the contribution of the indirect band to band transitions that appears in diamond crystals. The plot from Fig. 3 confirms this assumption. The plot presents a large linear region, corresponding to an energy transition of 5.45 eV, very close to the 5.5 eV value for crystalline diamond.
In summary, we have prepared a-C:H coatings by rf magnetron sputtering. The most significant result was that the films deposited at high pressures (greater than 0.2 Torr) and small thickness (bellow 0.4 Î¼m) presents some peculiarities in the UV absorption spectrum that may be related to diamond like structure. The Tauc plot for these films compared with the films with thickness greater than 1 Î¼m, present two different regions and a greater optical band that confirm the diamond like behaviour. The results suggest a dimensional effect indicating optical absorption behaviour closer to diamond.