Effect of deposition temperature on chemical composition and electronic properties of amorphous carbon nitride (a-CNx) thin films grown by plasma assisted pulsed laser deposition
Introduction
Carbon nitride (CNx) is a very interesting material for many different applications, ranging from tribological type, like wear-resistant coating, to optical and electronic engineering. The synthesis of a crystalline material, however, has been proved to be most elusive, and most attempts resulted in an amorphous a-C:N alloy, with a variable range of nitrogen concentrations [1]. The amorphous structure (a-C:N) exhibits good and exploitable properties, like high hardness value, very low friction coefficient, chemical inertness, and variable controllable electrical conductivity and band-gap, as a function of relative N/C composition [2]. Many experiments have been made in the past to deposit carbon nitride thin films using physical vapour deposition techniques, direct current (DC) and radio frequency (RF) magnetron sputtering [3], electron cyclotron resonance [4], ion beam or ion beam assisted deposition [5], [6] and reactive pulsed laser deposition (PLD) [7], [8].
PLD is one of the most widely used methods of deposition for a-CN, because of its specific features. The carbon bond configuration in PLD films of a-C and a-CN type is highly dependent on the energy (wavelength) of the laser radiation, viz. ultraviolet, visible and infrared (UV, Vis, and IR), used for the ablation [9], [10]. The average kinetic energy of the particles in the UV (~ 6.2 eV) radiation "plume", is very suitable to generate sp3-bonded carbon [9], even if the high energetic particles striking the growing film can produce high compressive stress and problems in film stability and adhesion [11], quite similar to problems observed for sputtered films [32].
Nitrogen incorporation in the films can be improved using a RF or DC nitrogen glow discharge with a proper bias voltage, since the negative voltage plays a dominant role in the tetrahedral C–N bond formation and suppression of graphite-like CN state [12], [13], [14], [15], [16], [17], [18].
In our work, we studied the effect of deposition temperature and N2 plasma on physical–chemical and electronic properties of carbon nitride thin films. We used a specific DC biased plasma configuration, to improve adhesion and film quality. Selected samples were analyzed by x-ray photoelectron spectroscopy (XPS), and evaluated for electronic properties by the four-contact in-line probe measurements, as a function of deposition temperature.
Section snippets
Preparation of films
Carbon nitride thin films were prepared by PLD technique. We used an ArF excimer laser (Lambda Physik COMPex 102), 10 ns duration, pulse energy of ~ 57 mJ, with a corresponding fluence of ~ 2.7 J/cm2 on the target. The laser beam was focused at an angle of 45° on a pyrolytic graphite target (99.999% purity). High-purity N2 (99.999%) was used as reactance gas. During the deposition, the target was rotated to ensure a uniform erosion of the surface. The substrates were mounted on a heated holder, at a
Spectroscopic characterization (XPS)
A study of the C–N chemical binding states has been performed, in order to characterize the deposited C–Nx compounds, trying also to correlate the chemical properties to electrical behavior. The chemistry of carbon and nitrogen compounds is very complex and it is not an easy point to ascribe the chemical shifts measured to well-defined and unique compounds. In addition, the XPS technique allows determining the chemical states of the C and N atoms at the very surface of the films, that is,
Electrical characterization
All the deposited films have ohmic, current/voltage (I/V), characteristics, under low bias conditions and show the expected semiconducting behavior (Fig. 5), characterized by the decrease of electrical resistivity as a function of measurement temperature (T). Fig. 6 shows that the resistivity dependence on T−1 follows an exponential function for all the films. That implies an Arrhenius conduction mechanism described by ρ(T) = ρ0 eEA/(kT), where EA is defined as the activation energy, k the
Conclusion
The high reactivity of nitrogen species, generated in the RF plasma, colliding with the activated carbon species generated in the "laser plume", was able to induce the formation of a larger quantity of C–N bonds, both sp2 and sp3-types, resulting in an overall increase of nitrogen insertion in the deposited film (> 30%), compared to a simple expansion of the carbon plume in a N2 atmosphere. Nitrogen incorporation in the films is also a function of deposition temperature, as well as the C–N bond
Acknowledgements
The authors are very grateful to G. Piciacchia for the fundamental technical support in the excimer laser experiments, to A. Bellucci for electronic set-up assistance and measurements and to Nello Vitulano for the RF and DC plasma assisted PLD apparatus design and realization.
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