Effect of Silicon Content in Functional Properties of Thin Films

Silicon (Si) has been the paradigm of the electronic industry, because it has been used in fabrication of different electronic circuit elements such as diodes, operational amplifiers, transistors, and in the last few decades, it has been the base material of the development of solar energy industry. However, other research fields in which the physical and chemical properties of Si have demonstrated to be relevant to applications in areas such as the mechanical, optical, electrical, and electrochemical are less known. Therefore, it is relevant to know the technology that has generated materials with new or better physical and chemical properties, such as the nanocomposite materials that have been growing in thin films. These films exhibiting new properties with respect to the bulk material and with the addition of Si, have demonstrated to improve the properties of the transition metal nitride (MeN), obtaining thin films with a high nanohardness, wear resistance, corrosion resistance, and high thermal stability. Therefore, the main objective of this chapter is to know the role that plays the incorporation Si in growth, microstructure, chemical composition, and functional properties of ZrN thin films.


Introduction
Thin films have been widely used in different application fields such as in chemical as diffusion barrier, in electrochemical as films for protection against corrosion, in mechanical as hard and wear resistance coating, in optical as reflection and antireflection coating, and in electronic area as conductor or insulator material [1][2][3][4][5][6][7][8][9][10][11][12].
Different chemical elements or compounds have been used to deposit thin films, which have been shown to improve the surface properties of a material (substrate). Among all these compounds, we find that the transition metal nitrides (MeN) have been widely used in mechanical applications due to their high hardness and wear resistance [2,[13][14][15][16]. However, it has been found that the addition of a third element may improve the physical and chemical properties of MeN due to a change that this new element generates in the microstructure of these materials [1,[17][18][19][20][21][22][23][24][25][26][27][28][29][30]. These structures are called nanocomposite, and there are two different groups of hard nanocomposite films, for instance: (i) crystalline/amorphous nanocomposites and (ii) crystalline/crystalline nanocomposites.
An element used to improve the properties of MeN is the silicon. In the published literature ( Table 1), it has been found that the Si addition generated the formation of two phases: a nanocrystalline of MeN and an amorphous phase of silicon nitride (Si 3 N 4 ), which improved the physical and chemical properties of MeN [17, 19-21,  the MeN crystals or an increase of the thickness of SiN x amorphous phases [48,76]. This process is obtained with high temperature (>550°C) deposition and high nitrogen pressure [64].
The scientific and technologic importance of the development of the nanocomposite films with the chemical and physical performances that those has shown, it is evident if it is considered the great amount the publications that over this theme has been produced. In Table 1, it is summarized the most important works of nanocomposite thin films with silicon, as can see, the deposition method most used to deposit these films is sputtering technique. This technique allows to deposit metallic and insulator elements and compounds at low temperature, maintaining the composition of the target. In addition, the films deposited have shown to be of high quality (homogeneous and dense) and with good adhesion [3,77,78].
In this chapter, we discussed about of a new generation of nanocomposite films composed of two phases: a nanocrystalline embedded in an amorphous matrix, specifically of nanocomposites formed by zirconium nitride and silicon.
Therefore, the content of the chapter is divided into the following topics: Section 1 is dedicated to describe the more usually experimental deposition conditions of the films via magnetron sputtering techniques. In Section 2 of the chapter will present chemical analysis of their bulk through spectroscopy of the X-ray dispersive (EDX) and the chemical surface analysis of the films by means of spectroscopy of photoelectrons (XPS). Section 3 is dedicated to discuss the influence of silicon in the crystalline structure of the films. This analysis is done through X-ray diffraction (XRD) and transmission electron microscopy (TEM). Sections 4-6 will describe the electrical, optical, and mechanical behavior of the deposited films, respectively. Finally, Section 7 will present the corrosion resistance that gives the films on stainless steel substrates. This analysis will be done with potentiodynamic polarization curves (TAFEL).

Growth of thin films using DC reactive magnetron sputtering technique
Most publication about nanocomposite films has shown that the DC reactive magnetron cosputtering technique is the most used to deposit these materials (see Table 1), and they have reported that the formation and microstructure of the films are determined by the deposition parameters, such as applied power at the target, working pressure, bias voltage, and deposition temperature (see Table 2).
In a sputtering process, the surface target is hitting with ions produced by an electric discharge, which form plasma. Normally, no reactive gas (Argon-Ar) is used to form the plasma. The interaction of these ions with the surface of the target causes the atoms on the surface to be ripped off through a moment exchange between ions and atoms of the target [4,77,79]. These sputtered atoms must transit from target to the substrate surface. During this displacement, the sputtered atoms experience many collisions with the particles that are in this region (sputtered atoms or Ar atoms or reactive atoms in the case of a reactive gas). These collisions change the velocity, direction, and the energy of the sputtered atoms. Therefore, the number of atoms that reach to the substrate surface will depend on the working pressure and the target-substrate distance. Moreover, the formation of the film is related with the condensing energy of the atoms (adatoms) on the surface of the substrate. Different works have found that amorphous films are formed when the adatoms have low energy of diffuse on the surface that does not allow that they may find low energy sites for the nucleation; while, a crystalline structure may be formed when the adatoms have a high surface mobility [8,15,[80][81][82][83][84][85][86][87][88]. However, several works have found that when the growing films are exposed to bombardment Silicon Materials 4 of high-energy particle, the structure and properties of deposited films can be improved. This can be achieved by applying a negative voltage called bias to the surface substrate [18,81,89]. These high-energy particles can improve the diffusion of the adatoms on the substrate surface. Another way to improve adatoms mobility is by increasing the temperature of the substrate [15,83,85]. Therefore, in the deposition of thin films, it is important to be able to find the deposition parameters to determine and understand their physical and chemical properties.
Finally, the composition of the target is another parameter that will affect the characteristics of the films. It has been shown that the addition of silicon to transition metal nitride can affect the structure and properties of the films; due to that the addition of the silicon atoms can block the surface mobility of the adatoms of the metal or the transition metal nitride [28,51,57,63,90]. Therefore, as the amount of silicon is increased, the structure of the film is changed from a polycrystalline, nanocrystalline to amorphous.
Zirconium nitride with silicon films was deposited via DC reactive magnetron sputtering technique. Only one zirconium target was used with silicon pellets on the target surface to tailor the Si content in the Zr▬Si▬N films. The deposition parameters used are shown in Table 3. This sputtering method has been used for depositing different nanocomposite films such as: Ti▬Si▬N [91], W▬Si▬N [72], Zr▬Si▬N [92], Al▬Si▬N [93], and Nb▬Si▬N [71].  The zirconium nitride and silicon nitride were obtained by adding of nitrogen as reactive gas to the deposition chamber. Three series of samples were deposited with different numbers of silicon pellets (X), ZrN + XSi, where X = 0, 1, and 2. Finally, thicknesses of the deposited film were obtained from the cross-sectional scanning electronic microscope images, which are not included in this chapter. These images showed that the thicknesses of the films were 823 ± 2, 955 ± 8, and 1060 ± 6 nm for ZrN, ZrN + 1Si, and ZrN + 2Si, respectively. The results of the chemical and structural characterization and the study of the functional properties are shown in the following subsections.

Chemical characterization by means of spectroscopy of the X-ray dispersive (EDX) and spectroscopy of photoelectrons (XPS)
In the sputtering technique, the elements of target are transferred to the substrate surface; this is verified with EDX analysis. Figure 1a shows the EDX spectrum for the ZrN + 2Si film.
The EDX spectrum evidences the presence of zirconium, nitrogen, and silicon in the film. The elemental chemical composition of the deposited films is listed in Table 4. Figure 1b shows the variation of the zirconium content with the increase of the silicon content in the films. As silicon content increases, the Zr content decreases in the films due to the reduction of the effective sputtering area of the Zr target with the Si pellets. These results are similar to those published by other authors using the same sputtering configuration [59,62,92]. The EDX results also showed that with one Si pellet, the Si content was of 8 at.% and with two pellets was of 15 at.%.
It has been found that when the solubility limit of Si in MeN lattice is exceeded, the Si atoms form a Si 3 N 4 phase [94]. The formation of Si 3 N 4 phase into MeN grain boundaries is typical for the Me▬Si▬N systems [17,19,25,41,71,73]. Therefore, in our case, a chemical analysis for XPS of the ZrN▬Si deposited films with different Si contents was carried out to show the formation of the Si 3 N 4 phase with the Si addition. Figure 2 shows the high-resolution XPS spectrum for the MeN films. The XPS results of Zr 3d peaks (Figure 2a) showed the presence of Zr▬N bond with a binding energy of 179.6 eV [95], and the Si 2p peaks (Figure 2b) showed the presence of Si-N bond to 100.8 eV [24] on the film surface. Additionally, the results
Deposition parameters used to deposit ZrN-Si films via DC reactive magnetron sputtering.

High-resolution XPS spectra for ZrN▬Si films with different silicon contents (a) Zr 3d and (b) Si 2p. The Si addition generated the formation of the Si 3 N 4 phase in the films.
showed the formation of zirconium oxide (ZrO 2 ) and oxynitride of zirconium (ZrO x N y ) that can be due to the presence of residual oxygen in the deposition chamber and to high possibility that has the zirconium to react with oxygen according to enthalpy of formation for ZrO 2 that is −1101.3 kJ/mol [96].
Various works have shown that the Si exists as solid solution in the ZrN lattice up to 3.0 at.%, but when the Si content increases, the formation of the Si 3 N 4 phase is observed [17,21]. Therefore, the EDX and XPS results show that with a Si content of 8 at.%, the solubility limit of Si in ZrN lattice is exceeded, generating the formation of Si 3 N 4 into ZrN grain boundaries with the increased Si content, probability of the volume of the phase of Si 3 N 4 is increased, and the phase of ZrN is decreased.

Structural characterization of the films through X-ray diffraction (XRD) and transmission electron microscopy (TEM)
With the addition of Si, it has been found that the microstructure of MeN films changes, and this change will depend on Si. Three types of microstructure have been observed in function of Si content: polycrystalline films with low Si content up to 3 at.%, nanocrystalline films with 3-10 at.% (nanocomposite: nanocrystalline and amorphous phase), and with a Si content above 10 at.%, the films are amorphous. These values on the silicon content are obtained for Ti▬Si▬N deposited films with different Si contents [97], but may change depending on MeN. In our case, the microstructure ZrN▬Si deposited films were characterized by XRD and TEM techniques. Figure 3 shows the XRD pattern of ZrN films with different Si contents deposited on the common glass substrate. Figure 3 exhibits diffraction peaks corresponding to fcc-ZrN (pdf. 01-078-1420) for the ZrN film without silicon (black color). The addition of Si, red and blue color, indicates that the diffraction peak of the ZrN (111) tends to broaden, while the ZrN (200) peak disappears as Si content increased. The broadening of the peak may be due to the formation of nanocrystals of cubic ZrN and tetragonal ZrO 2 , reported in the 2 θ position 33.83° (pdf. 01-078-1420) and 30.27° (pdf. 00-050-1089), respectively. The crystalline size for ZrN films is <10 nm, which was determined for the Scherrer equation, and with the addition of Si, the crystalline size decreased until 5 nm. The XRD evidenced that Si addition generated a refinement of grain, which is related with a broadening of the diffraction peaks. With a high Si content (15 at.%), the film is amorphous.
To study the structure of the ZrN▬Si film with Si content of 8 at.% in more detail, transmission electron microscopy with selected area electron diffraction (SAED) was done. Figure 4 shows the SAED pattern of ZrN + 1Si film. It shows the presence of the (111), (200), and (220) diffraction rings, which indicate a fcc-ZrN structure, but the (111) diffraction ring is very broad, which is in very good agreement with the XRD results in the same d-spacing from 0.295 to 0.262 nm.
In addition, this ring broadens may be related with a mixture of phases, such as: ZrN, ZrO 2 and Si 3 N 4 as we can see in Figure 5. This figure shows the XRD pattern of ZrN + 1Si film and the crystallographic databases for ZrN (pdf. 01-078-1420), ZrO 2 (pdf. 00-050-1089) and Si 3 N 4 (pdf. 00-033-1160). The different crystallographic phases present in the ZrN + 1Si film were analyzed by phase contrast images, Figure 6. The existence of nanocrystals and the Fast Fourier Transform (FTT) in Figure 6b confirms the presence of the diffraction rings observed in Figure 4.
However, this figure allows separating two rings at the lowest d-spacing, which may confirm the hypothesis of a mixture of phases, as observed with XPS and XRD results. According to ZrN (01-078-1420) and ZrO 2 (00-050-1089) pattern diffraction files, the interplanar distances in Figure 6b correspond to the ZrN face center cubic and ZrO 2 tetragonal. Finally, the XPS, XRD, and HRTEM results show that with a Si content 0 at.%, the ZrN film is polycrystalline, with a 8 at.%, the film is nanocrystalline (ZrN and ZrO 2 nanocrystalline), and possibly, with a Si 3 N 4 amorphous matrix and with a 15 at.%, the Zr▬Si▬N film is amorphous.
Various published literatures have reported that electrical, optical, mechanical, and electrochemical properties of the nanocomposite films depend on their nanostructure. These works have found that with the addition of silicon to binary MeN, hardness, thermal stability, and corrosion resistance of the films can be improved. In order to illustrate the relationship between the microstructure and functional properties, measurements of resistivity, reflectance, transmittance, nanohardness, and potentiodynamic polarization were carried out.

Electrical properties
Electrical resistivity and sheet resistance measurements were obtained through the van der Pauw method for the Zr▬Si▬N deposited films, and their values were calculated and listed in Table 5. The results evidence that the electrical resistivity increases from 4.40 × 10 −4 Ω cm (free Si) to 77.99 Ω cm (with 8 at.% Si) with the addition of silicon. This increase on the resistivity has been reported by other authors in different nanocomposites [54,98]. They have found that depending on the chemical composition and electrical nature of the amorphous phase and nanocrystalline phase, the resistivity of Me▬Si▬N nanocomposite films can change. The electrical resistivity increases with increasing Si content, and the nanocomposite films have showed to have a structure of MeN nanocrystalline (conductor) surrounded of a SiN x amorphous phase (insulator). However, when the electrical resistivity behavior is independent to Si content, the resistivity is due to a direct percolation of the MeN 1−x nanocrystalllines (conductors) separated by low degree of nitration of the SiN x grain boundary phase [98].
Therefore, the results obtained evidenced the formation of Zr▬Si▬N nanocomposite films with ZrN nanocrystallites embedded in the amorphous phase of SiN x , and the increase in the electrical resistivity with the Si addition is due to an increase in the thickness of SiN x layer that covers the nanocrystallites. The grain boundary scattering model is used for explaining the electrical conductivity in nanocomposite films [98]. The sample with a 15 at.% Si was not possible to measure the electrical resistivity due to high resistivity.  Reflectance spectra of ZrN and ZrN + 1Si films from 300 to 2500 nm. As Si content increases, the transmittance optical of the films also increase.

Figure 8.
Transmittance spectra of ZrN▬Si films with different Si contents and of the bare substrate (common glass). As the silicon content increases, the transmittance of the deposited films increases.

Optical properties
The optical properties were investigated using a UV-Vis-NIR spectrophotometer. Reflectance and transmittance measurement were carried out from 300 to 2500 nm. The reflectance spectra of ZrN (black line) and ZrN + 1Si (red line) films are shown in Figure 7. In this figure, the typical reflectance of ZrN is observed [85]. It exhibits a maximum reflectance in the infrared region, which decreases as wavelengths decrease, and for wavelengths <500 nm, the reflectance slightly increases again. The ZrN films exhibit a similar Drude-like behavior [53,85]. At longer wavelengths, the high electromagnetic absorption in this optical region is due to the conduction electrons and to the absorption at shorter wavelengths is due to the inter-band transitions of the bounded electron [85]. However, with the addition of silicon, the reflectance decreases drastically. At longer wavelengths, the film without silicon has a reflectance <80%, but the film with a Si content of 8 at.% has a reflectance <20%.
Therefore, transmittance measurements were done to investigate the effect of silicon in the ZrN films. The transmittance spectra of ZrN + 1Si and ZrN + 2Si films are shown in Figure 8. The transmittance spectra show that as silicon content increases, the transmittance also increases. With the addition of Si, new phases are generated according to previous results (TEM and XPS), such as: SiN x , ZrN, and ZrO 2 phases. These phase mixtures change the optical behavior of the deposited films due to SiN x and ZrO 2 that have been reported as optically transparent materials, while ZrN is an optically reflective material. Nowadays, there is not a theoretical model that may explain this behavior. However, the increase in the transmittance in films may be due to an increase in the volume of SiN x phase and a decrease of ZrN nanocrystalline phase with the increase of Si content.

Mechanical properties
The nanohardness (H) values as a function of Si content for the deposited films are shown in Figure 9 and Table 6. The H for the ZrN film is 29.55 ± 3.70 GPa, which decreases with the addition of silicon to 18.12 ± 2.65 and 15.92 ± 1.23 GPa at 8 and 15 at.% of Si, respectively. The value obtained for ZrN was similar to the reports from other authors [99,100]. The decrease of the nanohardness with the Si addition is related with the increase amorphous phase of SiN x and decreasing of the crystalline size of ZrN [101].
The mechanical properties of nanocomposite films depend on the chemical composition of each one of the phases present, of the crystallite size, crystallographic orientation, lattice structure, and the thickness grain boundary phase [94,101]. Different works have reported that the main mechanisms that allow to explain the hardness enhancement in the nanocomposites are three: (i) the dislocation-induced plastic deformation when the crystalline size is >10 nm, (ii) the nanostructure of materials when the crystalline size is ≤ 10 nm, and (iii) cohesive force between atoms when the crystalline size is <10 nm. However, when the thickness of amorphous phase is larger than the crystalline size, the nanohardness of the films decreases due to a deformation mechanism reported as grain boundary sliding [24,94].
According to the XPS and electrical resistivity value results, the Si addition generated the formation of an amorphous phase of SiN x , which increases its thickness with the silicon content in the film. It has been reported that when  the SiN x phase thickness is larger than the crystallite size of the ZrN phase, the nanohardness of the films decreases due to an increase of the volume fraction of the amorphous soft phase. The deformation mechanism, in this case, is grain boundary sliding [24].

Electrochemical properties
Potentiodynamic polarization curves of the 316l stainless steel (substrate) and deposited film with different Si contents were carried out for studying the Si effect in electrochemical properties of the films. They were tested in 3.5 wt.% NaCl solution, and the results are shown in Figure 10. For each curve, corrosion potential (E corr ) and corrosion current density (J corr ) were determined and are reported in Table 7. The results show that the deposited film with a Si content of 15 at.% has lower J corr than that of the uncoated SS 316 L substrate, indicating that with the Si addition or with the coating, the corrosion resistance increases.
Several research groups have found that the addition of silicon to MeN films can improve the corrosion resistance due to the formation of nanocomposite films. The increase on the corrosion resistance could be attributed to the formation of a dense structure, which can block the paths of corrosion medium to the substrate. It has been demonstrated to nanocomposite films, such as: Ti▬Si▬N [102], Al▬Si▬N [103], Nb▬Si▬N [71], and W▬Si▬N and Zr▬Si▬N [104].

Conclusions
The addition of a third element as silicon affects the microstructure and functional properties of the transition metal nitride films, specifically those of zirconium nitride as observed in this chapter. Therefore, it is important has a control of the content of this element in the deposited film to understand and relate the microstructure and their properties.
With a silicon content of 8 at.% and with the deposition parameter used, the microstructure changed from a polycrystalline structure of fcc-ZrN (free silicon) to a mixture of nanocrystalline (ZrN) and amorphous (SiN x ) phases, and with an increase in the Si content (15 at.%), the films were amorphous. However, these films showed the formation of two crystalline phases corresponding to zirconium nitride and zirconium oxide due to the base pressure used, which is not high enough to remove oxygen in the deposited chamber, and high enthalpy of formation for ZrO 2 .
These changes in the microstructure and the mixture of phases present in the films generated changes in the functional properties of zirconium nitride: • With the addition of silicon, the electrical resistivity increased various orders of magnitude comparison with the resistivity of ZrN. The electrical measurements allowed to determine that the films have a nanocomposite structure: nanocrystalline of ZrN conducting embedded in an amorphous SiN x insulating phases.
• As silicon content increases, the optical response changed from a high reflectance in the infrared region (ZrN) to a high transmittance in the infrared region (ZrN▬Si coatings).
• The nanohardness values decrease from 29.55 GPa (free silicon) to 15.92 GPa (15.0 at.% Si), due to an increase in the thickness of amorphous phase (SiN x ) and a decrease in crystalline size (ZrN).
• The potentiodynamic polarization curves showed that the coated substrate has higher corrosion resistance than the uncoated substrate due to a decrease in I corr .