Applied Surface Science 214 (2003) 58–67 Tungsten nitride films grown via pulsed laser deposition studied in situ by electron spectroscopies G. Soto*, W. de la Cruz, F.F. Castillón, J.A. Dı́az, R. Machorro, M.H. Farı́as Centro de Ciencias de la Materia Condensada, UNAM, Apdo. Postal 2681, 22800 Ensenada B.C., Mexico Received 17 June 2002; accepted 30 January 2003 Abstract Tungsten nitride (WNx) films were grown on silicon and glass slide substrates by laser ablating a tungsten target in molecular nitrogen ambient. By in situ Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and electron energy loss spectroscopy (EELS), the films density, elemental composition and chemical state were determined. Ex situ, the films were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Also, the transmittance and resistivity of the film on glass substrates were determined. The results show that the reaction of tungsten and nitrogen is effective; the nitrogen is integrated in the tungsten matrix changing gradually the electronic configuration, chemical states and film properties. Since with this preparation method the obtained films are of high quality, low resistivity and dense, this makes attractive to growth tungsten nitride films for technological applications. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Tungsten nitride; WNx; Pulsed laser deposition; EELS; XPS; AES; Thin films 1. Introduction The thin films of transition metal nitrides are interesting due to their vast range of potential applications. In microelectronics, they can be applied as diffusion barriers, passivating layers and electrodes; in engineering, as wear-resistant and hard protective coatings; and, in optical industry, as selective windows and X-ray mirrors. In particular, tungsten nitride (WNx) films are realistic candidates for its use in all of the above-mentioned applications [1–4]. However, the basic research and the studies on its preparation con* Corresponding author. CCMC-UNAM, P.O. Box 439036, San Ysidro, CA 92143-9036, USA. Tel.: þ52-646-1744602; fax: þ52-646-1744603. E-mail address: gerardo@ccmc.unam.mx (G. Soto). ditions are not as complete as other transition metal nitrides, such as titanium nitride. To grow tungsten nitride films, various plasmabased deposition techniques have been employed, mostly rf-sputtering [3] and dc-sputtering [4]. The laser ablation, also called pulsed laser deposition (PLD) is, at this time, a well-established technique [5], often employed to grow films of refractory elements. PLD looks attractive when compared to other deposition methods because it satisfies the requirements of selectivity for multilayer fabrication and layer-by-layer growth control. However, the properties of a film-to-be-grown are difficult to predict. A slight variation of the deposition parameters (laser fluency, substrate temperature, environmental gas composition and pressure) could result in a great alteration of the film properties. In this work, we show 0169-4332/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0169-4332(03)00343-X G. Soto et al. / Applied Surface Science 214 (2003) 58–67 that nitrogen can react with tungsten by ablating a tungsten target in an N2 atmosphere to grow dense WNx films. This methodology has been applied successfully to deposit other metallic (beryllium [6], titanium [7], etc. [5]) and non-metallic (silicon [8], carbon [9], boron [10]) nitrides. This study is accomplished by in situ using surface analysis techniques, such as X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) and electron energy loss spectroscopy (EELS), and ex situ, by scanning electron microscopy (SEM), X-ray diffraction (XRD). As a final point, we try to relate the film properties, transmittance and resistivity, with the spectroscopic results. 2. Experimental 2.1. Film preparation All films were deposited at room temperature in a modified laser ablation system, Riber LDM-32, with in situ AES, EELS and XPS facilities. The deposition was accomplished by ablating a 99.9 at.% tungsten target in a background of high-purity molecular nitrogen. Nitrogen pressures, PN, were in the 1  108  PN  0:1 Torr range. Layers were deposited on two different substrates, as-received (1 1 1) n-doped silicon wafers, i.e. without any cleaning process and on Corning glass slides. With the aim of preclude the adsorption of adventitious oxygen from H2O evaporating from the walls, the base pressure in the growth chamber was maintained in the 109 Torr range. Target ablation was accomplished by means of a KrF excimer laser (l ¼ 248 nm) focused on a target at 508 off the surface normal. Laser energy, deposition time and pulse repetition rate were fixed at 200 mJ, 60 min and 5 Hz, respectively, for a total of 18,000 laser pulses for each film. 59 from the full-width at half-maximum (FWHM) of backscattered electrons. XPS data were collected after exciting the sample by an unmonochromatized Al Ka line (1486.6 eV). The energy scale was calibrated against the Cu 2p3/2 and Ag 3d5/2 references, at 932.67 and 368.26 eV, respectively. The elemental XPS quantification is based in a recently proposed scheme [11]. First, it is assumed the film matrix to be homogeneous WNx. Then, the XPS intensity for a core level k can be written as: Ik / I0 nsk lMED ðEk ÞTðEk Þ (1) where n is the atomic density in the film, sk is the photoionization cross-section for the level k [12], lMED ðEk Þ is the mean escape depth for electrons with kinetic energy Ek in the examined material [11], and T(Ek) is the spectrometer transmission function [13]. I0 is a constant factor, which depends on the X-ray radiation intensity. The product sk lMED ðEk ÞTðEk Þ is designated as the relative sensitivity factor, Sr , for the core level k. All these factors were calculated for W 4f7/2, N 1s and O 1s transitions in tungsten nitride using the electron analyzer Cameca Mac-3 transmission function and an Al Ka X-ray source. With this scheme, the film stoichiometry can be evaluated from the XPS peak areas with confidence. The film density was determined by the collective bulk plasma oscillation measured by EELS. The maximum in the energy-loss function, Ep, assigned to the resonant plasma condition at classical frequency, wp, is given by [14]: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 4ph2 ne e2 Ep ¼ (2) me where ne is the number of electrons responsible for plasmon oscillations per volume unit, me is the electron mass and e is the electron charge. 2.2. Film characterization Spectroscopic analyses were performed by means of an electron energy analyzer Mac-3 from Cameca. For the AES measurements, the incident electron beam energy was set to 3000 eV and the data acquisition was performed at a nominal resolution of 1.5 eV. EELS data were collected using an incident electron beam of 1200 eV and a resolution of 2 eV, measured 3. Results 3.1. Quantitative elemental analysis The obtained XPS and AES spectra indicate that all the prepared films contain only tungsten, nitrogen and some unwanted oxygen, as it is shown in the 60 G. Soto et al. / Applied Surface Science 214 (2003) 58–67 Fig. 1. AES spectrum for a film prepared at PN ¼ 75 mTorr. The stoichiometry matches the chemical composition WN2xOx with x ¼ 0:24. representative Auger spectrum of a film prepared at a nitrogen pressure, PN ¼ 75 mTorr, in Fig. 1. In order to know the cause of oxygen on the prepared films, the tungsten target was examined by XPS and Auger. The spectra show the presence of oxygen ( 10 at.%) on the target surface, even after laser ablating its surface under ultrahigh vacuum (UHV) conditions. Since the ablating procedure removes large amounts of material, it is possible the oxygen to be present in the bulk tungsten. On the other hand, given that the oxygen content on the samples did not depend on the nitrogen pressure during growing the films, the possible oxygen contamination resulting from impurities in the nitrogen was considered not important. Consequently, we believe the most likely oxygen contamination source to be the tungsten target and, within the current preparation conditions, the oxygen in our films seems to be unavoidable. Fig. 2 shows the relative atomic concentration of the monitored elements as a function of the nitrogen pressure, PN, during depositing, obtained by means of XPS. At low PN values, we observe a region where the relative nitrogen concentration increases until PN reaches a value close to 75 mTorr, after which, i.e. at nitrogen pressures of PN 75 mTorr, the relative nitrogen concentration remains about constant. In this high PN values region, the calculated relative atomic concentrations of the detected elements are CN ¼ 58 at.%, CW ¼ 34 at.% and CO ¼ 8 at.%. Fig. 2. W (triangles), N (circles) and O (squares) relative atomic concentrations determined by high-resolution XPS as a function of nitrogen depositing pressure. Considering these concentrations, the stoichiometry of the obtained films in these high PN pressures, as determined by XPS with the above mentioned procedure, could correspond to a chemical formula of WN2xOx, where x ¼ 0:24 is the fraction of nitrogen substitution by oxygen. This chemical formula would correspond to a 1:2 atomic concentration ratio of W:N (WN2) if oxygen was not present. On the other hand, quantitative AES measurements result in similar relative atomic concentrations using standard sensitivity factors [15], as can be seen in Fig. 1 for a sample prepared with PN ¼ 75 mTorr, from which CN ¼ 61 at.%, CW ¼ 33 at.% and CO ¼ 6 at.%, in close agreement with XPS results. Likewise, for samples prepared with nitrogen pressures in the vicinity of PN ¼ 25 mTorr, the calculated relative atomic concentrations, from XPS results, are about CN ¼ 37 at.%, CW ¼ 51 at.% and CO ¼ 12 at.%. These values fall close to a stoichiometry of WN1xOx with x ¼ 0:24. In this case, it would correspond to a 1:1 atomic concentration ratio of W:N (WN) if oxygen was not present. Moreover, for samples prepared with even lower nitrogen pressures, at PN ¼ 12:5 mTorr, similar calculations, from XPS data, results in values of about CN ¼ 69 at.%, CW ¼ 26 at.% and CO ¼ 5 at.%. From these calculations, the stoichiometry is now close to W2N1xOx with x ¼ 0:14, corresponding to a 2:1 atomic concentration ratio of W:N (W2N) if oxygen 61 G. Soto et al. / Applied Surface Science 214 (2003) 58–67 Table 1 Stoichiometry and binding energy assignments, according to XPS analysis PN (mTorr) Ideal stoichiometry Calculated stoichiometry N 1s (eV) W 4f7/2 (eV) O 1s (eV) 0 12.5 25 75 W W2N WN WN2 WO0.1 W2N0.86O0.14 WN0.76O0.24 WN1.76O0.24 – 397.9 397.5 397.4 31.6 32.0 32.4 33.3 530.6 530.6 530.6 530.6 was not present. Table 1 shows the results of these calculations and their binding energy assignment for each one of the established stoichiometric relationships. Similar conclusions are obtained by AES measurements considering the relative atomic concentrations using standard sensitivity factors, throughout all the nitrogen pressures utilized during depositing. The chemical states of the W and N atoms in the WNx films were determined by means of XPS. In order to compensate for charging effects and since no carbon was detected, all peaks were shifted using the O 1s signal as reference, fixing it at 530.6 eV [16]. Using this reference, the films prepared at PN ¼ 0 mTorr did not need to be shifted since presented the W 4f7/2 peak at 31.6 eV, close to other standards of pure-W from literature, which contain values between 30.9 and 31.6 eV [16–19]. Figs. 3 and 4 show high resolution W 4f and N 1s XPS results, respectively, acquired on films prepared in the 0  PN  100 mTorr range and deposited on silicon wafers. We can see a gradual evolution in intensities and shifts in peak positions in both cases. Similar trends were observed on the series of samples deposited on glass. The changes in intensity are due to modifications of the relative atomic concentrations between W and N. The energy shifts are considered to be the result of charge transfer between W and N atoms, which are depicted clearly in Figs. 5 and 6. In these figures, the respective W 4f7/2 and N 1s binding energies of the peak maximum are plotted as a function of PN for the two studied series of samples. We observe gradual shifts, indicated by the dashed lines, as a function of depositing nitrogen pressure, PN. We can notice the tungsten to acquire ionic characteristics while PN is incremented, slowly shifting its binding energy from the neutral state, at 31.6 eV, towards a state with certain ionic character, at 33.2 eV. Nitrogen is incorporated with a high ionic character, in all the Fig. 3. XPS result of the development of the W 4f peak as nitrogen is included in the films. Fig. 4. XPS result of the evolution of the N 1s peak as the PN is raised. 3.2. Chemical states 62 G. Soto et al. / Applied Surface Science 214 (2003) 58–67 Fig. 5. W 4f peak position, determined by XPS, as a function of N2 depositing pressure. PN regime, presenting binding energy values far away from the N 1s binding energy value for a non-polar covalent chemical bond with a charge transfer equal to zero, 404 eV, as is reported in reference [20] for N2 implanted in graphite. The relative displacement of the N 1s and W 4f peaks as a function of PN pressure in Figs. 5 and 6 indicate that the nitrogen atoms are gaining electrons at the expense of W atoms. Thus, we can write the following reaction: Wablated þ Ngas ) 0 0 Wxd þNdx , where d0 symbolizes the amount of charge gained by a given N atom and xd0 symbolizes the fraction of charge contributed by a certain W atom. Fig. 6. N 1s peak position, determined by XPS, as a function of N2 depositing pressure. Fig. 7. EELS spectra for films grown at PN of: (a) 0 mTorr; (b) 12.5 mTorr; (c) 25 mTorr; (d) 50 mTorr; (e) 75 mTorr and (f) 100 mTorr. For clarity, curves have been offset vertically after linear background subtraction. 3.3. EELS data analysis In this work, EELS measurements were carried out in order to obtain the plasmon loss structure. In Fig. 7, EELS spectra for films grown on silicon substrates at PN of 0, 12.5, 25, 50, 75 and 100 mTorr are presented. From this experimental EELS results, we take note of some details: (a) The loss structure changes steadily as a function of depositing pressure. (b) The main peak, measured at 24.2 eV for PN ¼ 0 mTorr, gradually shifts to a higher energy, 27.7 eV at PN ¼ 25 mTorr, then declines to 26.3 eV at PN ¼ 100 mTorr. (c) The shoulders at the low-energy-side of the main peak reallocate its position and increment its intensity. From (a) result, we believe there is no abrupt change in the electronic configuration while nitrogen is incorporated in the films; rather, the nitrogen is integrated in the W matrix, changing little-by-little the film properties. The (b) result could mean the electronic density to be changing while nitrogen is being included in the films. If we consider the assumption that all atoms contribute with their corresponding valence band G. Soto et al. / Applied Surface Science 214 (2003) 58–67 63 Additionally, as noted in (Fig. 7(c)) there is a shoulder in the low-energy side of the bulk plasmon, which reallocate its position and increment its intensity. These peaks were assigned to surface plasmons. In a clean metal surface, the dispersion relation of the surface-to-bulk plasmon is given by: op (4) os ¼ pffiffiffi 2 Fig. 8. Electronic (filled circles) and mass (empty squares) densities determined by the free-electron model using EELS peak position energies. electrons [18], then the total electronic density can be written as: ne ¼ 6nW þ 5nN þ 6nO (3) where nW, nN and nO are the tungsten, nitrogen and oxygen atomic concentrations, respectively. The evaluation of Eqs. (2) and (3) for the series of films deposited on silicon are shown in Fig. 8 for the dependence of the electronic (filled circles, right axis) and mass (empty squares, left axis) densities as a function of depositing pressure. For the error bars, a tolerance of 0.2 eV in the measured peak position energy was applied. The electronic density presents a maximum at PN ¼ 25 mTorr, and lower values for films with substoichiometry (WNx, x < 1) and over-stoichiometry (WNx, x > 1). Mass density shows a constant decrement while nitrogen is included in the films. For the case of growing a tungsten film under UHV, PN ¼ 0 mTorr, we obtain a mass density, r, of 19:5 0:4 g cm3, in very good agreement with the known density of metallic tungsten. For films grown with PN ¼ 12:5 mTorr, where it is estimated to be nearly W2N stoichiometry, we calculate r ¼ 17:5 0:3 g cm3. This value is close to 18.048 g cm3 of crystalline b-W2N [21]. For films grown at PN ¼ 25 mTorr, the mass density is calculated to be r ¼ 15:8 0:3 g cm3, also not far from r ¼ 16:040 g cm3 of d-WN [22]. However, for films grown at PN 75 mTorr we get r ¼ 11 0:3 g cm3, relatively larger than the r value for crystalline WN2 of 8.897 g cm3 [23]. However, we do not observe any characteristic losses at this particular energy in our films, neither in the tungsten film prepared at UHV conditions. A wellrecognized fact is that the energy of the surface plasmon should be affected by the existence of a charge-confining dielectric layer. In general, the dielectric layer is due to a very thin oxide overlayer. Taking into account the existence of this dielectric layer, the dispersion relation between the surface-tobulk-plasmon is now given by [24]:  1=2 1 os ¼ op (5) 1 þ eo where eo is the dielectric constant of the surface overlayer. For the tungsten film prepared at UHV conditions, the surface plasmon energy is being affected by a WO3 overlayer (eo  2:9, as it was measured by ellipsometry), consistent with the observed shoulder at the low energy side, 13 eV, and with the AES and XPS results showing the existence of that oxide on the film surface. Thereafter, we propose that the modification in energy and strength of the surface plasmon in our films is due to the modification of overlayer characteristics. However, in the WNx films, the dielectric overlayer seems to be of complex constitution, something resembling a tungsten oxinitride: WNxO1x. To verify this hypothesis, more detailed optical studies are needed in order to find the likewise overlayer dielectric constants, beyond the scope of this work. 3.4. Film properties Scanning electron micrographs (not shown here) indicate that all prepared films are smooth and uniform. The tungsten films, prepared at PN ¼ 0 mTorr, show the inclusion of some droplets or splashing often encountered in these type of films processed by PLD. There are also scarce dendrites, possibly indicative of 64 G. Soto et al. / Applied Surface Science 214 (2003) 58–67 Fig. 9. XRD y –2y scans for the films grown on silicon substrates. Curves have been offset vertically for clarity. the early stages of crystalline growth. However, for the rest of films, grown under nitrogen environments, the density of droplets is very small. XRD was used to examine the crystalline structure of WNx films deposited on the (1 1 1)-Si substrates at different pressures. Fig. 9 shows representative y–2y diffractograms for films grown at PN ¼ 0, 5, 12.5 and 25 mTorr. For films grown at PN ¼ 0 mTorr we can see the (1 1 0)-W planes [25], as evidence of some crystalline structure. As nitrogen is incorporated in the films, this peak shifts slightly, to lower diffraction angles, increasing its width and decreasing its intensity. It disappears completely for films prepared at PN ¼ 25 mTorr, indicative of a complete loss of the crystalline character. Fig. 10 shows the variations in the spectral transmittance for the series of WNx films deposited on corning glass slides. The mean thickness is 20 nm and the thickness variation among samples is less than 10%, as it was determined with perfilometric measurements. Accordingly, we can make an estimation of the optical properties starting from that graph. Large changes in the spectral response can be noted as a function of nitrogen depositing pressure. For film grown at PN ¼ 0 mTorr, there is a transmittance of 12% at l ¼ 1500 nm and a lower transmittance of 8% for samples grown at PN ¼ 5 mTorr. After that pressure, there is a transmittance increase as the depositing pressure is raised. It is interesting to note that films grown at PN ¼ 15, 20 and 25 mTorr are Fig. 10. Spectral transmittance of the series WNx coated on Corning glass slides. (a) PN ¼ 0 mTorr; (b) PN ¼ 5 mTorr; (c) PN ¼ 12:5 mTorr; (d) PN ¼ 15, 20, 25 mTorr; (e) PN ¼ 50 mTorr and (f) PN ¼ 75 mTorr. The mean film thickness is 20 nm. almost indistinguishable in the transmittance spectra, while films grown at PN ¼ 50 and 75 mTorr show a large transmittance, becoming semi-transparent in the studied wavelength range. Particularly, films grown at PN ¼ 12:5 mTorr show a dissimilar spectral behavior to the rest of the prepared films, however this behavior is reproducible since the same result was obtained from repeated films using the same experimental configuration. Fig. 11. Film sheet resistance and resistivity (insert) of the series of WNx films deposited on glass substrates as a function of nitrogen depositing pressure. G. Soto et al. / Applied Surface Science 214 (2003) 58–67 Resistivity of the prepared WNx films was measured from sheet resistance using the standard four-point probe method. Resistance and calculated resistivity of the series of films deposited on glass substrates are shown in Fig. 11. The average resistivity for a tungsten film, prepared at PN ¼ 0 mTorr, is 135 mW cm. The resistivity drops to 42 mW cm for films prepared at PN ¼ 12:5 mTorr. After that pressure, it increases as a function of PN, as indicated in the inset of Fig. 11. 4. Discussion The notorious increase in the electron density as a function of nitrogen depositing pressure, with an increase until a maximum at a PN value of 25 mTorr, and then a slow decrease, is singular. The mass density calculated from the electron density results by assuming that each atom in the film contributes with all their valence band electrons to the free-electron cloud, agrees well with the expected values for low and medium PN, however appears to be overestimated for high PN values. By using the same line of calculations, Shen and Mai [18] report a density of 16.7 g cm3 for films with a W2N stoichiometry prepared by means of sputtering, which is lower than the theoretical density and below the densities calculated in this work. Shen assumes incorporation of 5–10% void fraction in his films. In the sputtering case, the growing films are subjected to energetic Arþ bombardment, which could induce roughness, generate some voids and even incorporate some gas [26]. In contrast, the density values that are obtained by PLD are very close to the densities of the corresponding crystalline compounds. This situation allows us to believe WNx films processed by PLD to contain a higher packing factor than those produced by means of DC magnetron sputtering. As a result, the quality of WNx films prepared by PLD could be of interest in microelectronic applications. On regards to resistivity measurements, Takeyama reports a tendency to increase the resistivity of WNx films as a function of nitrogen concentration incorporated into the films [27]. Films reported in this work present the same tendency, except for films prepared at low PN values, since the sheet resistance and resistivity of WNx films prepared at PN ¼ 5 and 65 12.5 mTorr are lower than the corresponding values of W films (PN ¼ 0 mTorr). Since the thickness of all those films are comparable and considering the fact that the optical transmittance of those films is lower than the corresponding of W films, these results indicate a metallic character to be present in all films prepared at low PN values. This metallic character is more significant in the low PN tungsten nitride films than in the tungsten films. The observed decrease of electron density for PN > 25 mTorr, a corresponding increase of optical transmittance and resistivity and the observed gradual shifts in binding energy of W 4f and N 1s transitions agree with the idea that the N atoms are gaining electrons at the expense of W atoms. Considering the N atoms as attractive poles for electrons, to incorporate more nitrogen into the films would cause a decrease in the number of free electrons in the solid provided by tungsten. This situation would, for high nitrogen concentration, cause some localization of the tungsten free carriers. Contrasting the results for high nitrogen depositing pressures, for low PN values, films present a metallic character. The split point between the ionic–metallic behavior appears to be at a nitrogen depositing pressure of 25 mTorr, where the atomic concentration ratio W:N is about 1:1. Considering the atomic size difference, for low nitrogen concentrations, the large tungsten atoms could easily accommodate the small N atoms with a slight distortion of the lattice. As the N concentration increases, it becomes more difficult to accommodate more N atoms and the lattice will be destroyed until becoming a completely amorphous compound. It is noteworthy to observe the beginning of decrease in metallic character at the point where the atomic concentrations of W and N are about the same, where the number of interstitial sites associated to the W atoms would be very small since most of them have been already occupied by N atoms. After that critical point, the introduction of more N atoms in the film is expected to be in detriment of the atomically dense structure in the solid and it starts acquiring an ionic character. Considering this scheme, we assume the nitrogen incorporation process in the tungsten matrix to occur following the three stages described in Table 2, where, as before, d symbolizes the charge gained by the N atoms at expenses of the W metallic matrix, being the N valence state a little-bit smaller for 66 G. Soto et al. / Applied Surface Science 214 (2003) 58–67 Table 2 Stages of nitrogen incorporation in WNx PN range Reaction Result xd0 þ 0 Ndx  Þmetallic Low PN Wablated þ Ngas ) ðW Medium PN Wablated þ Ngas ) ðWxd þ Nxd  Þsemi-metallic High PN 00 xd000 þ Wablated þ Ngas ) ðW 00 000 Nxd  Þionic low PN, d000 > d00 > d0 , as it is suggested by the XPS results in Fig. 6. The fact that the W 4f and N 1s transitions present a continuous shift in energy as a function of nitrogen concentration in the films, instead of the appearance of a new peak (or peaks) at a different energy and the disappearance of the initial peak, makes this nitridation process different from others. Beryllium nitride [6] and silicon nitride [28], prepared using a similar methodology as here, present two well distinguished peaks in the main transitions, which evolve in intensity with little or no change in binding energy as a function of nitrogen content in the film. In the current study, we notice that the W 4f binding energy is dynamically adjusted as nitrogen is incorporated. This result seems to indicate that there are not two electronically or chemically different states for the tungsten atoms in the films. In other words, a W atom with an N neighbor is electronically equivalent to a W atom without an N neighbor. This explanation is in accordance with our hypothesis that the N atoms are developing their chemical bondings by taking electrons from the matrix free-electrons, provided by the W atoms, and not from the nearest neighbor. More experiments are underway in order to understand this point more clearly. 5. Conclusions From the composition, chemical states, morphology, structure, electrical resistivity and optical transmittance results obtained by several techniques on WNx films with various compositions prepared by reactive pulsed laser ablation under nitrogen environment, we can obtain the following conclusions. The reaction between tungsten and nitrogen is very effective; the nitrogen is incorporated in the tungsten matrix and changes chemical states and film properties. N is incorporated in W interstices; the solid preserve its metallic character W interstices become depleted The compact arrangement is lost and N is incorporated in a solid with some ionic character Nitrogen works as the electron-acceptor atom, taking electrons from the matrix free-electrons provided by the tungsten atoms (electron-donor). Differently from other nitrides, electronic density and chemical state of W and N present steady changes. From the XPS point of view, a W atom with an N neighbor and a W atom without an N neighbor appear to be equivalent. W2N, WN and WN2 stoichiometries are obtained for 12.5, 25 and 75 mTorr N2 depositing pressures, respectively. Two combined effects result in the observed behavior as a function of nitrogen depositing pressure through this work (resistivity, transmittance, chemical states, electronic and mass densities): (a) the gradual depletion of interstitial sites while nitrogen is being incorporated in the films and (b) the localization of electrons, where N acts as an electron trap, taking away the free-electrons provided by W. Since with this preparation method the obtained films are of high quality, low resistivity and dense, this makes attractive to growth tungsten nitride films for technological applications. Acknowledgements The authors are grateful to I. Gradilla, F. Ruiz, C. González, E. Aparicio, J. Peralta, V. Garcı́a and P. Casillas for valuable technical assistance. 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