� .Applied Surface Science 127-129 1998 564–568 SiC N thin films alloys prepared by pulsed excimer laserx y deposition R. Machorro a,), E.C. Samano a, G. Soto b, L. Cota a a CeCiMAC, UNAM, A. Postal 2681, 22800 Ensenada BC, Mexico b Centro de In˝estigacion Cientıfica y de Educacion Superior de Ensenada, Programa de Posgrado en Fısica de Materiales,´ ´ ´ ´ A. Postal 2681, 22800 Ensenada BC, Mexico Abstract � .In this work, thin films of SiCN have been deposited by pulsed laser deposition on silicon substrates by KrF 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 N filmsx y � .are compared to the one prepared in a high vacuum environment no N gas . The film growth was monitored by real-time2 � .kinetic ellipsometry at a single photon-energy 2.5 eV . The film was analyzed by spectro-ellipsometry in the photon-energy range of 1.5-h˝-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 N gas pressure. High resolution in situ X-ray photoelectron spectroscopy characterization was2 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 N films. q 1998 Elsevier Sciencex y B.V. PACS: 81.05.je; 81.15.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 me- chanical, thermal and electronic properties and supe- rior physical stability. A related material is the hypo- w xthetical b-C N 1 , which has many properties in3 4 common with diamond, but is a metastable phase which is difficult to produce. Currently, one of the ) Corresponding author. Tel.: q52-61-744602; fax: q52-61- 744603; e-mail:
[email protected]. most technologically viable wide band-gap semicon- ductors is silicon carbide, with properties between those of silicon and diamond. Bendeddouche et al. w x w x2 and Gomez et al. 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 N or Si N state under appropri-x y 3 4 w xate conditions 4 . The resulting material could be easily included in a device based on silicon carbide technology as an electric insulator or as a passivating 0169-4332r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. � .PII S0169-4332 97 00706-X ( )R. Machorro et al.rApplied Surface Science 127-129 1998 564–568 565 layer. Zehnder et al. have successfully grown b-SiC � . w xfilms by using Pulsed Laser Deposition PLD 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 N as a function of gas pressure. We alsox y compare these results with those obtained for films � y8 .grown in vacuum 4=10 Torr under similar conditions. The atomic concentration of each con- stituent 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 ex- plained 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 summa- rized in Section 4. 2. Experimental procedure The experiment was based on the photoevapora- tion of a commercially available SiC sintered target � .placed in a laser ablation system Riber, LDM-32 . The system consists of three vacuum chambers: sam- ple 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 ls248 nm, 30 ns pulse . y8width , 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 docu- w xmented optical properties 6 . All depositions were 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 Jrcm2 at the target surface, 10,000 laser pulses, and the substrates always kept at room temperature. The films growth � y8 .was performed at high vacuum 4=10 Torr and in the presence of high purity N gas in the 5 to 5002 mTorr pressure range, sustained by a turbo molecular pump. Films growth was monitored with a phase modu- � .lated ellipsometer Jobin-Yvon, UVISEL at a fixed � .photon-energy 2.5 eV . The ellipsometric parame- � .ters c , D are measured during film deposition as a function of time. Spectroscopical ellipsometric anal- ysis 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 EMA implemented in the ellipsome- w xter’s software 7 . In situ XPS analysis was per- 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 1253.6 .eV . Energy scale was calibrated using the reference binding energies of Cu 2p at 932.67 eV and Ag3r2 3d at 368.26 eV. The Full Width at Half Maxi-5r2 � .mum FWHM measured value of C 1s re-graphite 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 N films as a function of the nitrogenx y pressure, p , was determined from the XPS mea-N2 surements 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 ( )R. Machorro et al.rApplied Surface Science 127-129 1998 564–568566 Fig. 1. Atomic concentration, obtained by XPS, vs. chamber pressure, of thin films grown in an N atmosphere obtained from2 a SiC target. 17% at p f5 mTorr up to 43% at p f30N N2 2 mTorr. This curve actually reached a plateau, keep- ing 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= y8 � .10 Torr no N gas down to approximately 25%2 at p f30 mTorr for both. These concentrationsN2 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=10y8 Torr were found to be 283.6 eV and 100.8 eV, respec- tively, which are related to silicon carbide formation w x8 . The binding energies of N 1s, C 1s and Si 2p for the film grown at p s10 mTorr were detected atN2 398.1, 284.2 and 101.6 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 nitro- gen 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 p s80 mTorrN2 were moved to 398.2, 285.9 and 102.8 eV, respec- tively. 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, w xaccording to the literature 3,4 . The actual film composition can be inferred by deconvoluting the C 1s and Si 2p spectra of films grown at p s80 mTorr, they have to be composedN2 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, 286.0 and 287.7 eV, which correspond to the reported values w xfor C–C, C–N and C–O bonds 3,4 , 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 102.6 and 103.7 eV, which correspond to Si–N and Si–O, respectively, according to the w xliterature 3,4 . The superposition of these two peaks agree well to the measurements of Si 2p. The differ- ence in binding energy between Si 2p and N 1s � .peaks D E for this film is equal to 295.4 eV,B which is lower by 0.4 eV over the typical value of Fig. 2. High resolution XPS spectra of the N , C and Si1s 1s 2p transitions of SiC N films grown in vacuum at 4=10y8 Torrx y � . � .q , and in an N environment at 10 mTorr = and 80 mTorr2 � .) . ( )R. Machorro et al.rApplied Surface Science 127-129 1998 564–568 567 Fig. 3. Detailed C and Si transitions from a SiC N film1s 2p x y as-deposited at p s80 mTorr. The deconvoluted peaks areN2 shown as slashed lines, while their corresponding XPS peaks are shown as solid lines. w xSi N 3 . This is a signal that not only silicon3 4 nitride was formed in the film, but the difference of 0.4 eV in D E is an indication that a small changeB in the ionicity of the bonds occurred when nitrogen w xwas incorporated into SiC 3 . 3.2. Optical properties The film growth was monitored by real-time ellip- sometry. Real-time or kinetic ellipsometry was used to qualitatively determine the films absorbance at a fixed photon energy of 2.5 eV. An absorbent mate- rial is recognized by an open loop locus when the time evolution of the ellipsometric parameters are plotted, but a perfect transparent material is identi- w xfied by a closed loop locus 9 . Fig. 4 shows the � .ellipsometric trajectories in the c , D plane for films deposited under different N gas pressures. As2 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 p s10 mTorr wasN2 less absorbent than the previous one, as observed from Fig. 4. As the pressure was raised up to p sN2 80 mTorr, the deposited film turned into the trajec- tory of a high internal transmittance material, as shown in Fig. 4. The films absorbance was confirmed by spectro- � .scopic ellipsometry. The n, k optical parameters of � .the film are determined from the 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 N2 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, by as 4prhc 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 , is obtained by inter-g secting the straight line behavior at the high absorp- � .1r2 w xtion region, aE , with the E-axis 10 , as shown in Fig. 5. The values of E for films grown atg vacuum, 10 mTorr and 80 mTorr of N are 1.6, 1.852 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 ( )R. Machorro et al.rApplied Surface Science 127-129 1998 564–568568 and 2.3 eV, respectively. Again, the film grown at 80 mTorr had the highest E , 2.3 eV, corresponding tog a low-absorbent material. 4. Conclusions SiC N thin films have been grown by pulsedx y excimer laser deposition of a SiC target in a N2 atmosphere at different gas pressures. The film stoi- chiometry can be varied as the gas pressure is in- creased. The nitrogen content is strongly increased into the films in the 5 mTorrFp F30 mTorrN2 range, as observed from Fig. 1. The stoichiometry determined by XPS for films grown at p G30N2 mTorr resulted to be unchanged and given by SiCN O . The small content of oxygen is an un-2 0.2 wanted contamination in the films. The formation of this new compound cannot be attributed to the inter- mixing of two binaries phases like Si N and SiC,3 4 nor the presence of graphite immersed in a Si N3 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 E is an indication that a new SiCNB 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 ellipso- metric 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 p s80 mTorr. Unfortunately,N2 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 Nx y material as a function of photon energy is still unknown. Nevertheless, PLD combined with ellip- sometry is recognized as an excellent tool to control the deposition of SiC N films.x y Acknowledgements The assistance of Israel Gradilla, Armando Reyes, and Jesus Nieto is gratefully appreciated. Financial´ support was provided by CONACYT through re- search grant No. 4228-E. References w x � .1 A.Y. Liu, M.L. Cohen, Science 245 1989 841. w x2 A. Bendeddouche, R. Berjoan, E. Beche, S. Schamm, V.´ � .Serin, R. Carles, R. Hillel, J. Phys. 5 1995 C5–793. w x3 F.J. Gomez, P. Prieto, E. Elizalde, J. Piqueras, Appl. Phys.´ � .Lett. 69 1996 773. w x4 A. Nakao, M. Iwaki, H. Sakairi, K. Terasima, Nucl. Instr. � .Meth. B 65 1992 352. w x5 T. Zehnder, A. Blatter, A. Bachli, Thin Solid Films 241 � .1994 138. w x6 E.D. Palik, Handbook of Optical Constants of Solids, Aca- demic Press, Orlando, FL, 1985. w x7 Spectroscopic phase modulated ellipsometer, ISA Jobin- Yvon, France, 1995. w x � .8 D.N. Belton, S.J. Schmieg, J. Vac. Sci. Technol. A 8 1990 2353. w x � .9 J.B. Theeten, D.E. Aspnes, Annu. Rev. Mater. Sci. 11 1981 97. w x10 R.A. Smith, Semiconductors, Cambridge Univ. Press, Cam- bridge, UK, 1978.