thin films alloys prepared by pulsed excimer laser deposition

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1 Ž. Applied Surface Science SiC xny thin films alloys prepared by pulsed excimer laser deposition R. Machorro a,), E.C. Samano a, G. Soto b, L. Cota a a CeCiMAC, UNAM, A. Postal 2681, Ensenada BC, Mexico b Centro de InÕestigacion Cientıfica y de Educacion Superior de Ensenada, Programa de Posgrado en Fısica de Materiales, A. Postal 2681, Ensenada BC, Mexico Abstract In this work, thin films of SiCN have been deposited by pulsed laser deposition on silicon substrates by KrF Ž 248 nm. excimer laser ablation of a SiC sintered target in a vacuum system at room temperature. To obtain various stoichiometries, molecular nitrogen is introduced in the deposition chamber in the 5 to 500 mtorr pressure range. The resultant SiC xny films are compared to the one prepared in a high vacuum environment Ž no N gas. 2. The film growth was monitored by real-time kinetic ellipsometry at a single photon-energy Ž 2.5 ev.. The film was analyzed by spectro-ellipsometry in the photon-energy range of 1.5-hÕ-5.0 ev at the end of the deposition process. Tauc s plots are used to estimate the optical band-gap of the films as a function of the N2 gas pressure. High resolution in situ X-ray photoelectron spectroscopy characterization was performed on every film. The bonding character of the elements in the films is obtained by deconvoluting the XPS peaks. The ellipsometric and XPS results suggest that a new phase alloy is present in the SiC xny films. q 1998 Elsevier Science B.V. PACS: je; Fg; 81.40Tv Keywords: SiCN alloys; Ellipsometry; Laser ablation 1. Introduction The quest for wide band-gap materials is an active scientific field. One of the most interesting materials is diamond due to its excellent combination of mechanical, thermal and electronic properties and superior physical stability. A related material is the hypowx thetical b-c N 3 4 1, which has many properties in common with diamond, but is a metastable phase which is difficult to produce. Currently, one of the ) Corresponding author. Tel.: q ; fax: q ; roberto@cecimac.unam.mx. most technologically viable wide band-gap semiconductors is silicon carbide, with properties between those of silicon and diamond. Bendeddouche et al. wx 2 and Gomez et al. wx 3 proposed a SiCN based alloy, whose properties should be between silicon nitride and carbon nitride. In other words, it should be a hard material with a wide band-gap. Previous reports demonstrated that nitrogen-ion implantation in silicon carbide produces a surface layer with an intermediate SiC xny or Si 3N4 state under appropri- ate conditions wx 4. The resulting material could be easily included in a device based on silicon carbide technology as an electric insulator or as a passivating r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. Ž. PII S X

2 R. Machorro et al.rapplied Surface Science layer. Zehnder et al. have successfully grown b-sic films by using Pulsed Laser Deposition Ž PLD. wx 5. Consequently, it is important to produce this new material by the same method. PLD is now a well established technique to produce a wide variety of thin film deposits. High quality ceramic films can be processed at much lower temperatures than other methods like CVD and rf-sputtering. The interaction of intense laser pulses with the target generates particles with non-equilibrium characteristics during deposition; these excited species with high kinetic energies could lead to the formation and growth of films in metastable states. In the present work, we investigate how nitrogen is incorporated into films when a SiC target is ablated in a background of molecular nitrogen gas, resulting in films with a variable stoichiometry SiC xny as a function of gas pressure. We also compare these results with those obtained for films Ž grown in vacuum 4 = 10 Torr. under similar conditions. The atomic concentration of each constituent and bond formation in the films were studied by in-situ XPS. Film growth was monitored by single photon-energy ellipsometry to qualitatively investigate the film absorbance. The resultant films were also analyzed by spectroscopic ellipsometry to determine the optical band-gap as a function of the film stoichiometry; i.e., the nitrogen gas pressure. The experimental setup and procedure are explained in Section 2. The experimental data obtained by XPS and ellipsometry are given in Sections 3.1 and 3.2, respectively. The discussion of results and final conclusions of this investigation are summarized in Section Experimental procedure The experiment was based on the photoevaporation of a commercially available SiC sintered target placed in a laser ablation system Ž Riber, LDM-32.. The system consists of three vacuum chambers: sample loading, film growth and analysis. Each chamber is independently evacuated by an ion pump, and isolated by UHV gate valves. The growth chamber is equipped with two low birefringence fused silica windows suitable for ellipsometry. The target was ablated in the growth chamber by a KrF excimer laser Žl s 248 nm, 30 ns pulse. width, and evacuated to a base pressure of 10 Torr before the actual deposition process begins. The laser beam hits the target at an incident angle of 508 off the surface normal. The substrates were polished single crystal silicon wafers which have well docuwx 6. All depositions were mented optical properties performed using the same laser conditions of 10 pulses per second with an energy of 800 mj per pulse, corresponding to a fluence of 5 Jrcm 2 at the target surface, 10,000 laser pulses, and the substrates always kept at room temperature. The films growth Ž was performed at high vacuum 4 = 10 Torr. and in the presence of high purity N2 gas in the 5 to 500 mtorr pressure range, sustained by a turbo molecular pump. Films growth was monitored with a phase modulated ellipsometer Ž Jobin-Yvon, UVISEL. at a fixed photon-energy Ž 2.5 ev.. The ellipsometric parameters Ž c, D. are measured during film deposition as a function of time. Spectroscopical ellipsometric analysis was performed at the end of the deposition process in the 1.5 to 5 ev photon-energy range. Data fitting was carried out using the effective medium approximation Ž EMA. implemented in the ellipsomewx 7. In situ XPS analysis was per- ter s software formed using the adjacent analysis chamber equipped with a Riber-CAMECA Mac-3 system. XPS data were collected by means of Mg K a radiation Ž ev.. Energy scale was calibrated using the reference binding energies of Cu 2p3r2 at ev and Ag 3d5r2 at ev. The Full Width at Half Maximum Ž FWHM. measured value of Cgraphite 1s re- sulted to be 1.2 ev, being 1 ev the nominal value for high resolution XPS. 3. Results 3.1. Surface analysis The atomic concentration of each component in the SiC xny films as a function of the nitrogen pressure, p N 2, was determined from the XPS measurements by integrating the peak area after linear background subtraction for the N 1s, C 1s, Si 2p and O 1s core levels, as shown in Fig. 1. It can be observed that nitrogen content steadily increased from

3 566 R. Machorro et al.rapplied Surface Science Fig. 1. Atomic concentration, obtained by XPS, vs. chamber pressure, of thin films grown in an N2 atmosphere obtained from a SiC target. 17% at pn f 5 mtorr up to 43% at pn f mtorr. This curve actually reached a plateau, keeping the concentration of nitrogen almost constant for pressures higher than 30 mtorr. On the contrary, the concentrations of carbon and silicon simultaneously decreased from 50% and 45%, respectively, at 4= 10 Torr Ž no N gas. 2 down to approximately 25% at pn 2 f 30 mtorr for both. These concentrations remain unchanged for pressures higher than 30 mtorr. A small oxygen contamination, never higher than 8%, was always present in the films. The presence of oxygen can be attributed to residual water in our system. High resolution XPS spectra around the N 1s, C 1s and Si 2p core levels are shown in Fig. 2 for three typical as-deposited films. The binding energies of C 1s and Si 2p of the film grown at 4=10 Torr were found to be ev and ev, respectively, which are related to silicon carbide formation wx 8. The binding energies of N 1s, C 1s and Si 2p for the film grown at pn 2 s10 mtorr were detected at 398.1, and ev, respectively. The energy shift of C 1s and Si 2p XPS peaks for this film with respect to films grown in vacuum suggest that nitrogen starts to be part of a new compound, agreeing with the results shown in Fig. 1. The N 1s, C 1s and Si 2p peaks for the film grown at pn 2 s80 mtorr were moved to 398.2, and ev, respectively. The FWHM values became significantly broader at 80 mtorr referred to 10 mtorr, they grew from 1.8 to 2.6 ev for the N 1s peak, from 2.2 to 5.0 ev for C 1s and from 2.0 to 2.6 ev for Si 2p. The energy shift and the broadening of XPS peaks in Fig. 2 are a clear indication that the film is not longer composed only by SiC but a rather complex material is formed as nitrogen is incorporated into the films, according to the literature w3,4 x. The actual film composition can be inferred by deconvoluting the C 1s and Si 2p spectra of films grown at pn 2 s80 mtorr, they have to be composed by more than one peak. Fig. 3 shows that the C 1s peak is quite asymmetric and broad revealing the presence of at least three distinct bonds of carbon. The peaks were determined to be at 284.1, and ev, which correspond to the reported values for C C, C N and C O bonds w3,4 x, respectively. The overlap of these three peaks fit well to the experimental data of C 1s, as observed from Fig. 3. Although the Si 2p peak is not as broad as the C 1s peak, it can be noticed that at least two distinct bonds of silicon constitute the peak. The peaks were found to be and ev, which correspond to Si N and Si O, respectively, according to the literature w3,4 x. The superposition of these two peaks agree well to the measurements of Si 2p. The difference in binding energy between Si 2p and N 1s peaks Ž D E. B for this film is equal to ev, which is lower by 0.4 ev over the typical value of Fig. 2. High resolution XPS spectra of the N 1s, C1s and Si 2p transitions of SiC xny films grown in vacuum at 4=10 Torr Ž q., and in an N environment at 10 mtorr Ž =. 2 and 80 mtorr Ž )..

4 R. Machorro et al.rapplied Surface Science Fig. 3. Detailed C1s and Si 2p transitions from a SiC xny film as-deposited at pn 2 s80 mtorr. The deconvoluted peaks are shown as slashed lines, while their corresponding XPS peaks are shown as solid lines. Si N wx This is a signal that not only silicon nitride was formed in the film, but the difference of 0.4 ev in D EB is an indication that a small change in the ionicity of the bonds occurred when nitrogen was incorporated into SiC wx Optical properties The film growth was monitored by real-time ellipsometry. Real-time or kinetic ellipsometry was used to qualitatively determine the films absorbance at a fixed photon energy of 2.5 ev. An absorbent material is recognized by an open loop locus when the time evolution of the ellipsometric parameters are plotted, but a perfect transparent material is identiwx 9. Fig. 4 shows the fied by a closed loop locus ellipsometric trajectories in the Ž c, D. plane for films deposited under different N2 gas pressures. As can be seen, the ellipsometric curve for the film grown in vacuum follows a trajectory typically found for strongly absorbing films; in fact, a spiral can be observed. The film grown at pn 2 s10 mtorr was less absorbent than the previous one, as observed from Fig. 4. As the pressure was raised up to p N2 s 80 mtorr, the deposited film turned into the trajectory of a high internal transmittance material, as shown in Fig. 4. The films absorbance was confirmed by spectroscopic ellipsometry. The Ž n, k. optical parameters of the film are determined from the Ž c, D. set of data Fig. 4. Kinetic ellipsometric trajectories obtained at a fixed photon energy of 2.5 ev for a SiC target ablated in vacuum and in an N 2 atmosphere at the pressures shown in the figure. as a function of photon energy. In particular, the absorption coefficient a is related to the extinction coefficient, k, byasž 4prhc. Ž Ek., where h is the Planck s constant, c is the speed of light and E is the photon energy. The coefficient a is known to follow the Tauc s equation for amorphous materials, where the optical band-gap, E g, is obtained by inter- secting the straight line behavior at the high absorption region, Ž ae. 1r2, with the E-axis w10 x, as shown in Fig. 5. The values of Eg for films grown at vacuum, 10 mtorr and 80 mtorr of N2 are 1.6, 1.85 Fig. 5. Tauc s plots used to determine the optical band-gap from ellipsometric parameters for a SiC thin film deposited in vacuum and different N pressures. 2

5 568 R. Machorro et al.rapplied Surface Science and 2.3 ev, respectively. Again, the film grown at 80 mtorr had the highest E g, 2.3 ev, corresponding to a low-absorbent material. 4. Conclusions SiC xny thin films have been grown by pulsed excimer laser deposition of a SiC target in a N 2 atmosphere at different gas pressures. The film stoichiometry can be varied as the gas pressure is increased. The nitrogen content is strongly increased into the films in the 5 mtorrf pn 2 F 30 mtorr range, as observed from Fig. 1. The stoichiometry determined by XPS for films grown at pn 2 G 30 mtorr resulted to be unchanged and given by SiCN2O 0.2. The small content of oxygen is an unwanted contamination in the films. The formation of this new compound cannot be attributed to the intermixing of two binaries phases like Si 3N4 and SiC, nor the presence of graphite immersed in a Si 3 N 4 matrix. In fact, Figs. 2 and 3 show that nitrogen is bonded to silicon and carbon, besides a difference of 0.4 ev in D EB is an indication that a new SiCN alloy has been formed. The mechanisms about how this bonding occurs are still uncertain. We conjecture that collisions between highly ionized SiC ejected species and nitrogen molecules produce the chemical reaction. Optical spectroscopy of the plume might help to clarify this supposition. These results were corroborated by the ellipsometric measurements. Figs. 4 and 5 show that the films tend to be transparent as the nitrogen content in the film is increased. In fact, the optical band-gap value of 2.3 ev estimated from Fig. 5 is very close to the photon energy of 2.5 ev used to obtain the almost close ellipsometric trajectory shown in Fig. 4 for films grown at pn 2 s 80 mtorr. Unfortunately, the ellipsometric data could not be used to determine the film composition because the contribution to the dielectric response of each component of the SiC x N y material as a function of photon energy is still unknown. Nevertheless, PLD combined with ellipsometry is recognized as an excellent tool to control the deposition of SiC N films. Acknowledgements x y The assistance of Israel Gradilla, Armando Reyes, and Jesus Nieto is gratefully appreciated. Financial support was provided by CONACYT through research grant No E. References wx 1 A.Y. Liu, M.L. Cohen, Science 245 Ž wx 2 A. Bendeddouche, R. Berjoan, E. Beche, S. Schamm, V. Serin, R. Carles, R. Hillel, J. Phys. 5 Ž C wx 3 F.J. Gomez, P. Prieto, E. Elizalde, J. Piqueras, Appl. Phys. Lett. 69 Ž wx 4 A. Nakao, M. Iwaki, H. Sakairi, K. Terasima, Nucl. Instr. Meth. B 65 Ž wx 5 T. Zehnder, A. Blatter, A. Bachli, Thin Solid Films 241 Ž wx 6 E.D. Palik, Handbook of Optical Constants of Solids, Academic Press, Orlando, FL, wx 7 Spectroscopic phase modulated ellipsometer, ISA Jobin- Yvon, France, wx 8 D.N. Belton, S.J. Schmieg, J. Vac. Sci. Technol. A 8 Ž wx 9 J.B. Theeten, D.E. Aspnes, Annu. Rev. Mater. Sci. 11 Ž w10x R.A. Smith, Semiconductors, Cambridge Univ. Press, Cambridge, UK, 1978.