- Email: [email protected]
Investigations on non-stoichiometric zirconium nitrides
Applied Surface Science 200 (2002) 231±238
Investigations on non-stoichiometric zirconium nitrides H.M. Beniaa, M. Guemmaza, G. Schmerberb, A. Mosserb, J.-C. Parlebasb,* a
b
DAC Laboratory, Ferhat Abbas University, Setif 19000, Algeria IPCMS, UMR 7504, CNRS-ULP, 23 rue du Lúss, Strasbourg Cedex, 67037, France
Abstract Non-stoichiometric zirconium nitride thin ®lms were prepared by reactive dc magnetron sputtering. Structural, optical and electrical properties were investigated with respect to nitrogen gas ¯ow. The ®lm characterization was obtained through various techniques: Rutherford back scattering (RBS), X-ray diffraction (XRD), re¯ectometry and a four probe method. We found that the deposition rate decreases while the nitrogen ¯ow increases. Also it was shown that a stoichiometric compound (ZrN) is formed at a 4 sccm nitrogen ¯ow. It has a rock-salt structure and presents a minimum of resistivity and a gold-like color. As nitrogen ¯ow increases, ®lms become more and more amorphous, semi-transparent and electrically resistive. Furthermore, at a nitrogen ¯ow of 9 sccm, the nitrogen to zirconium atomic concentration ratio is near 1.33 and the ®lm exhibits properties close to those of a Zr3N4 phase. # 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55-a; 78.66.Bz; 81.15.Cd Keywords: Zirconium nitrides; dc reactive sputtering; Resistivity; Re¯ectance
1. Introduction Among transition metal nitrides and thanks to their interesting properties (high melting point, chemical stability and corrosion resistance [1]), zirconium nitrides are promising materials in different domains, notably in coating domains [2±6]and cryogenic thermometers [7]. Besides, Schwarz et al. [8] suggested to use them as new materials for Josephson junctions. Also, ZrN compounds were utilized as gates in MESFET transistor technology [9] and as diffusion barriers in p/n junction [10]and in Cu/Si contacts [11,12]. Furthermore, ZrN is considered as an attractive inert * Corresponding author. Tel.: 33-3-88-10-70-72; fax: 33-3-88-10-72-49. E-mail address: [email protected] (J.-C. Parlebas).
matrix in fast reactor fuels for incineration of plutonium and minor actinides [13]. Zirconium nitrides are usually prepared through various techniques such as: magnetron sputtering [9±11,14±17], ion beam assisted deposition (IBAD) [18,19], reactive ion beam sputtering [20], vacuum arc deposition [2], pulsed laser deposition [21], and chemical vapor deposition (CVD) [3,4]. In the ZrN system, the well-known stoichiometric ZrN compound, which is the only stable phase [22], has been widely studied. In opposite, non-stoichiometric domains where other phases exist like Zr3N4 [18,23,24] and ZrN2 [14] are less studied. In this paper, we investigate various properties of non-stoichiometric zirconium nitrides prepared by dc reactive magnetron sputtering technique. Section 2 is devoted to the sample preparation and characterization whereas in Section 3 we present and discuss our experimental results.
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 9 2 5 - X
232
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
2. Experimental analysis 2.1. Sample preparation A schematic view of the sputtering system is shown in Fig. 1. The substrates were centered with respect to the center of the circular target and the target±substrate distance of our sputtering system was ®xed at 200 mm. A pure Zr disc (82.5 mm in diameter, 6 mm thick, 99.16% pure) was used as target. Polished single crystalline Si(1 0 0) and Si(1 1 1) surfaces disoriented by an angle of 88 and 48, respectively were used as substrates and ultrasonically cleaned for 10 min. Each one was successively washed in acetone and ethanol,
then dried in a nitrogen gas jet. Prior to deposit, the system was pumped down to a pressure of about 6 10 8 Torr by a cryopump and the substrates as well as the target were sputter-cleaned for 30 min at 6 10 3 Torr and 20 min at 9 10 3 Torr, respectively. For ZrN deposition, the plasma current was regulated at 800 mA in a constant argon ¯ow of 20 sccm (4 10 3 Torr) and the nitrogen ¯ow was varied from 0 to 11 sccm in order to obtain zirconium nitrides thin ®lms with various nitrogen concentrations. The gas ¯ow was controlled by two speci®c ¯owmeters: one for argon and another one for nitrogen. The results of the characterization will be discussed later on versus nitrogen ¯ow (Section 3). The deposition parameters
Fig. 1. Schematic view of the dc reactive magnetron sputtering system.
Table 1 Deposition parameters (with a constant argon pressure of 4 10 3 Torr and an Ar/N2 ratio varying from in®nite in the case of S0 sample to about 2 in the case of S11 sample) and main characteristics of ZrNx ®lms. Sample
Deposition time (s)
Substrate temperature (8C)
Nitrogen gas flow (sccm)
Nitrogen partial pressure (10 4 Torr)
Thickness Ê) (A
N/Zr atomic ratio (stoichiometry)
S0 S1 S2 S3 S4 S5 S7 S9 S11
600 1200 600 300 600 300 300 300 300
35 150 150 150 150 150 150 150 150
0 1 2 3 4 5 7 9 11
0 2.9 4.3 5.8 7.3 9.1 12.1 15.5 19.0
4940a 10230a 5370a 2350a 3920a 1005b 834b 721b 697b
0 0.15 0.25 0.45 1 1.1 1.2 1.38 1.4
The thickness was measured by two techniques: (a) RBS analysis, (b) X-re¯ectometry.
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
are reported in Table 1 (samples are labeled as S[i] where i indicates the value of nitrogen ¯ow). 2.2. Characterization techniques Rutherford backscattering spectrometry (RBS) [25] was used to determine ®lm composition and thickness. The experiments were carried out using a 2 MeV 4 He ions beam. The backscattered spectra were analyzed using simulation for analysis of materials (SAM) code. The nitrogen and zirconium amounts obtained by RBS allow the determination of the N/Zr atomic ratio (stoichiometry) in the ®lm. The crystal structure of sputtered ®lms was determined by X-ray diffraction (XRD). The XRD spectra were recorded in the y/2y mode, between 2y 30 and 808 with a step of 0.058. Electrical resistivity was measured using a four-probe method. Optical re¯ectance spectra were recorded between 250 and 2500 nm with an UV-VisNIR photospectrometer.
233
3. Results and discussion 3.1. Structure of the ®lms Fig. 2 shows the X-ray diffraction patterns of S0, S2, S4, S5, S7, and S9 samples. For samples S0 and S2, the ®lms crystallize in the a-Zr phase of hexagonal structure, with a (1 0 0) preferential orientation. Peaks of S2 pattern are shifted toward small 2y angles in comparison with S0, which indicates a broadening of the hexagonal cell, due to nitrogen atoms incorporated in interstitial sites [26]. For the other patterns, we remark that the structure becomes cubic of NaCl type (d-ZrN phase) and above the stoichiometry, which is obtained with a 4 sccm nitrogen ¯ow, the ®lms become more and more amorphous. This trend to amorphisation was already reported in references [7,18,23]. It may be explained either by the absence of stable phases in the rich nitrogen zone of ZrN system or by the in¯uence of the deposition parameters
Fig. 2. X-ray diffraction patterns of S[i] samples (where i 1, 2. . ., indicates nitrogen ¯ow).
234
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
and the substrate nature [15]. Besides, peaks of S5, S7 and S9 samples shift toward smaller 2y angles compared to those of S4. This suggests a cell dilation of the stoichiometric compound by the incorporation of nitrogen atoms in interstitial positions and the progressive formation of an other phase like Zr3N4. Several authors have already indicated the shift of the (1 1 1) peak of the NaCl phase toward smaller angles for ®lms which have a composition corresponding to Zr3N4 [14,20,24]. According to Pichon et al. [18,23], the Zr3N4 structure can be described as relaxed NaCl structure with some Zr vacancies. Also, Dauchot et al. [14] found a NaCl structure by grazing angle diffraction for rich nitrogen ®lms (N/Zr atomic ratio x beyond 1.33). 3.2. RBS analysis RBS spectra of samples S0 and S4 are presented in Fig. 3 as an example. The ®lm composition and thickness obtained by RBS spectra simulation with the help of the SAM code are reported in Table 1. We measured the thickness of S5, S7, S9 and S11 samples by the X-ray re¯ectivity technique which is more Ê . Fig. 4 shows deposiaccurate than RBS under 1200 A
tion rate and N/Zr atomic ratio evolution of our ZrNx ®lms with nitrogen ¯ow. The deposition rate decreases when nitrogen ¯ow increases from 0 to 4 sccm. Above this value, the decrease becomes less marked and tends to stabilize beyond 9 sccm at a value near of Ê /s. This effect is due to the irradiation of the 2.5 A target by N-ions and leads to the formation of zirconium nitrides at the target surface which has a smaller sputtering yield than pure Zr. We veri®ed this change of the sputtering yield by means of transport of ions in matter (TRIM) code [27]. Furthermore, the enlarged gas pressure in the sputtering chamber involves a high density of particles between target and substrates which creates an improved dif®culty for sputtered particles to reach the substrates and yields the deposition rate to fall between 2 and 5 sccm. As for the deposition rate overall behavior, two regions are distinguished in the ®lm composition versus nitrogen ¯ow: the ®rst one between 0 and 4 sccm where we observe a strong increase of N/Zr ratio; the second one between 4 and 11 sccm where N/Zr ratio keeps increasing but in a less important way. Similar deposition rate and nitrogen to zirconium atomic ratio behavior in ZrNx ®lms are pointed out in references [16,17].
Fig. 3. RBS spectra recorded on S0 and S4 samples. The dotted lines represent the experimental spectra and the solid lines the SAM simulation.
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
235
Fig. 4. Evolution of the deposition rate and the N/Zr atomic ratio x of ZrNx ®lms as a function of nitrogen ¯ow.
3.3. Resistivity evolution The electrical resistivity of ZrNx ®lms is shown in Fig. 5 as a function of nitrogen ¯ow. We remark that as the nitrogen ¯ow increases, the resistivity increases too. After reaching a maximum at 2 sccm, the resistivity decreases until it reaches a minimum (65 mO cm) at 4 sccm, then it increases again with higher nitrogen ¯ows. This behavior is quite similar to the results reported by several authors [7,16,17,24]. According to Matthiessen's rule [28], the total resistivity of a metal is a contribution of several independent electron scattering processes due to phonons, impurity atoms, defects and even surface effects when the sample is a ®lm of a smaller thickness than the mean free-electron path. Hence, the initial increase of resistivity may be due to (i) electron scattering by incorporated nitrogen molecules and (ii) atoms in the Zr lattice which play the role of impurities. Also, the stretching out of the cell that increases the distance between Zr atoms leads to the weakness of the metallic bond [29]and thus to the resistivity rise. Between 2 and 4 sccm, especially in the vicinity of 4 sccm, the stoichiometric phase ZrN, which has a metallic behavior due to its special bonds [8,30±32], is going to be
formed. This causes the minimum of resistivity. The increase of resistivity beyond 4 sccm is explained by the trend to amorphization which means a high presence of defects. Also, the increase of nitrogen concentration in ZrN structures induces the stretching out of the cell that decreases the metallic bond contribution. 3.4. Re¯ectance measurement At ®rst glance, observation shows that our zirconium nitride samples have various colors. Actually the color varies from metallic gray for low nitrogen ¯ows to golden yellow (sample S4) which is the normal color of the stoichiometric ZrN compound and to semi-transparent brownish for high nitrogen ¯ows. Fig. 6 shows the re¯ectance evolution of S0, S1, S2, S4, S5, S7, and S9 samples. As far as S7 and S9 samples are concerned (as well as S11 not represented in Fig. 6) since they are semi-transparent, oscillations could be expected due to interferences produced by re¯exions onto the ZrN surface as well as the ZrN/Si interface. Actually those oscillations do not show up for two reasons: (i) the sample absorption is never negligible and (ii) the sample thickness is rather
236
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
Fig. 5. Evolution of electrical resistivity of ZrNx ®lms as a function of nitrogen ¯ow.
Fig. 6. Re¯ectance spectra of ZrNx ®lms, where x indicates nitrogen to zirconium atomic ratio.
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
small (about 80 nm). A more complete discussion will be given on that point in a forthcoming paper. The spectra for S1 and S2 samples are almost the same as the S0 one which indicates a metallic behavior. At 4 sccm, we obtained a spectrum which is characterized by a minimum of re¯ectance in the visible domain and a maximum in the infrared domain. As the ¯ow increases above 4 sccm, the minimum of re¯ectance increases and shifts slightly towards the long waves while the infrared maximum re¯ectance decreases. Several authors observed a similar evolution of re¯ectance spectra in relation with nitrogen content [16,17,20]. 4. Conclusion ZrN ®lms have been sputter-deposited by dc reactive magnetron sputtering. The in¯uence of the nitrogen ¯ow on the ZrN ®lm structure, stoichiometry, resistivity and re¯ectance was investigated. The XRD study shows that under-stoichiometric ®lms present a good crystallization in opposite to overstoichiometric ®lms which become more and more amorphous as the nitrogen ¯ow increases beyond 4 sccm. Simultaneously, their diffraction peaks present a shift towards smaller angles. This suggests either a cell dilatation or the formation of an other phase probably Zr3N4. The RBS analysis shows the formation of zirconium nitrides with compositions which vary from under-stoichiometry to over-stoichiometry. Deposition rates decrease rapidly below 4 sccm, then shows a tendency to stabilize beyond 5 sccm. The electrical resistivity was measured with a four probes method. The results show that the ®lms become in overall more and more resistive as nitrogen ¯ow increases. A minimum of resistivity is however measured for the ®lm synthesized at 4 sccm which con®rms that it is close to stoichiometry. In the vicinity of stoichiometry, the ZrN ®lms have a typical goldlike optical re¯ectance characterized by a minimum in the visible domain and a maximum in the infrared domain. This con®rms the use of these ®lms in heat mirror industry. Outside of stoichiometry, all the optical re¯ectance spectra differ from each other. For the nitrogen rich ®lms, the dielectric behaviours and the semi-transparent colors support the idea that they are most probably formed with a Zr3N4 phase.
237
Acknowledgements This work was supported by the French-Algerian CMEP Accord-Programme No. 00-MDU-468. References [1] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. [2] V.N. Zhitomirsky, I. Grimberg, R.L. Boxman, N.A. Travisky, S. Goldsmith, B.Z. Weiss, Surf. Coat. Technol. 94/95 (1997) 207. [3] W.C. Russell, J. de Phys. II 5 (1995) C5±C127. [4] B. Karlsson, R.P. Shimshok, B.O. Seraphin, J.C. Haygarth, Solar Energy Mater. 7 (1983) 401. [5] W.D. Sproul, Thin Solid Films 107 (1983) 141. [6] C. Mitterer, P.H. Mayerhofer, W. Waldhauser, E. Kelesoglu, P. Losbichler, Surf. Coat. Technol. 108/109 (1998) 230. [7] T. Yotsuya, M. Yoshitake, T. Kodama, Cryogenics 37 (1997) 817. [8] K. Schwarz, A.R. Williams, J.J. Cuomo, J.H.E. Harper, H.T.G. Hentzell, Phys. Rev. B 32 (12) (1985) 8312. [9] M. Wautelet, J.P. Dauchot, F. Debal, S. Edart, M. Hecq, J. Mater. Res. 11 (4) (1996) 825. [10] Y. Igarashi, T. Yamaji, S. Nishikawa, Jpn. J. Appl. Phys. 29 (1990) L2337. [11] B.M. Takeyama, T. Itoi, E. Aoyagi, A. Noya, Appl. Surf. Sci. 190 (2002) 450. [12] B.M. Takeyama, A. Noya, K. Sakanishi, J. Vac. Sci. Technol. B 18 (2000) 1333. [13] M. Burghartz, G. Ledergerber, H. Hein, R.R. vander Laan, R.J.M. Konings, J. Nucl. Mater. 288 (2001) 233. [14] J.P. Dauchot, S. Edart, M. Wautelet, M. Hecq, Vacuum 46 (1995) 927. [15] D. Wu, Z. Zhang, W. Fu, X. Fan, H. Guo, Appl. Phys. A 64 (1997) 593. [16] S. Inoue, K. Tominaga, R.P. Howson, K. Kusaka, J. Vac. Sci. Technol. A 13 (1995) 2808. [17] M. Yoshitake, T. Yotsuya, S. Ogawa, Jpn. J. Appl. Phys. 31 (1992) 4002. [18] L. Pichon, T. Girardeau, A. Straboni, F. Lignou, J. PerrieÁre, J.M. FrigeÂrio, Nucl. Instr. Meth. Phys. Res. B 147 (1999) 378. [19] S. Horita, T. Tujikawa, H. Akahori, M. Kobayashi, T. Hata, J. Vac. Sci. Technol. A 11 (1993) 2452. [20] M. Yoshitake, T. Nosaka, A. Okamoto, S. Ogawa, Jpn. J. Appl. Phys. 32 (1993) L113. [21] M.B. Lee, M. Kawasaki, M. Yoshimoto, M. Kumagai, H. Koinuma, Jpn. J. Appl. Phys. 33 (1994) 6308. [22] L. Gribaudo, D. Arias, J. Abriata, J. Phase Equilib. 15 (1994) 441. [23] L. Pichon, T. Girardeau, A. Straboni, F. Lignou, Ph. GueÂrin, J. PerrieÁre, Appl. Surf. Sci. 150 (1999) 115. [24] B.O. Johansson, H.T.G. Hentzell, J.M.E. Harper, J.J. Cuomo, J. Mater. Res. 1 (1986) 442. [25] W.-K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectroscopy, Academic Press, Orlando, 1978.
238
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
[26] M. Yoshitake, T. Yotsuya, K. Takiguchi, S. Ogawa, Jpn. J. Appl. Phys. 29 (12) (1990) 2800. [27] J.F. Ziegler, J.P. Biersack, U. Littmark (Eds.), The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1986. [28] Milton Ohring, The Materials Science of Thin Films, Academic Press, New York, 1992.
[29] P. Schwarzcopf, R. Kieffer, Refractory Hard Metals, Macmillan, New York, 1953, p. 37. [30] A. Dunaud, H.D. Flack, K. Yvon, Phys. Rev. B 31 (1985) 2316. [31] P. Blaha, J. Redinger, K. Schwarz, Phys. Rev. B 31 (1985) 2316. [32] A. Neckel, Intern. J. Quantum Chem. 23 (1983) 1317.
Investigations on non-stoichiometric zirconium nitrides H.M. Beniaa, M. Guemmaza, G. Schmerberb, A. Mosserb, J.-C. Parlebasb,* a
b
DAC Laboratory, Ferhat Abbas University, Setif 19000, Algeria IPCMS, UMR 7504, CNRS-ULP, 23 rue du Lúss, Strasbourg Cedex, 67037, France
Abstract Non-stoichiometric zirconium nitride thin ®lms were prepared by reactive dc magnetron sputtering. Structural, optical and electrical properties were investigated with respect to nitrogen gas ¯ow. The ®lm characterization was obtained through various techniques: Rutherford back scattering (RBS), X-ray diffraction (XRD), re¯ectometry and a four probe method. We found that the deposition rate decreases while the nitrogen ¯ow increases. Also it was shown that a stoichiometric compound (ZrN) is formed at a 4 sccm nitrogen ¯ow. It has a rock-salt structure and presents a minimum of resistivity and a gold-like color. As nitrogen ¯ow increases, ®lms become more and more amorphous, semi-transparent and electrically resistive. Furthermore, at a nitrogen ¯ow of 9 sccm, the nitrogen to zirconium atomic concentration ratio is near 1.33 and the ®lm exhibits properties close to those of a Zr3N4 phase. # 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55-a; 78.66.Bz; 81.15.Cd Keywords: Zirconium nitrides; dc reactive sputtering; Resistivity; Re¯ectance
1. Introduction Among transition metal nitrides and thanks to their interesting properties (high melting point, chemical stability and corrosion resistance [1]), zirconium nitrides are promising materials in different domains, notably in coating domains [2±6]and cryogenic thermometers [7]. Besides, Schwarz et al. [8] suggested to use them as new materials for Josephson junctions. Also, ZrN compounds were utilized as gates in MESFET transistor technology [9] and as diffusion barriers in p/n junction [10]and in Cu/Si contacts [11,12]. Furthermore, ZrN is considered as an attractive inert * Corresponding author. Tel.: 33-3-88-10-70-72; fax: 33-3-88-10-72-49. E-mail address: [email protected] (J.-C. Parlebas).
matrix in fast reactor fuels for incineration of plutonium and minor actinides [13]. Zirconium nitrides are usually prepared through various techniques such as: magnetron sputtering [9±11,14±17], ion beam assisted deposition (IBAD) [18,19], reactive ion beam sputtering [20], vacuum arc deposition [2], pulsed laser deposition [21], and chemical vapor deposition (CVD) [3,4]. In the ZrN system, the well-known stoichiometric ZrN compound, which is the only stable phase [22], has been widely studied. In opposite, non-stoichiometric domains where other phases exist like Zr3N4 [18,23,24] and ZrN2 [14] are less studied. In this paper, we investigate various properties of non-stoichiometric zirconium nitrides prepared by dc reactive magnetron sputtering technique. Section 2 is devoted to the sample preparation and characterization whereas in Section 3 we present and discuss our experimental results.
0169-4332/02/$ ± see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 2 ) 0 0 9 2 5 - X
232
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
2. Experimental analysis 2.1. Sample preparation A schematic view of the sputtering system is shown in Fig. 1. The substrates were centered with respect to the center of the circular target and the target±substrate distance of our sputtering system was ®xed at 200 mm. A pure Zr disc (82.5 mm in diameter, 6 mm thick, 99.16% pure) was used as target. Polished single crystalline Si(1 0 0) and Si(1 1 1) surfaces disoriented by an angle of 88 and 48, respectively were used as substrates and ultrasonically cleaned for 10 min. Each one was successively washed in acetone and ethanol,
then dried in a nitrogen gas jet. Prior to deposit, the system was pumped down to a pressure of about 6 10 8 Torr by a cryopump and the substrates as well as the target were sputter-cleaned for 30 min at 6 10 3 Torr and 20 min at 9 10 3 Torr, respectively. For ZrN deposition, the plasma current was regulated at 800 mA in a constant argon ¯ow of 20 sccm (4 10 3 Torr) and the nitrogen ¯ow was varied from 0 to 11 sccm in order to obtain zirconium nitrides thin ®lms with various nitrogen concentrations. The gas ¯ow was controlled by two speci®c ¯owmeters: one for argon and another one for nitrogen. The results of the characterization will be discussed later on versus nitrogen ¯ow (Section 3). The deposition parameters
Fig. 1. Schematic view of the dc reactive magnetron sputtering system.
Table 1 Deposition parameters (with a constant argon pressure of 4 10 3 Torr and an Ar/N2 ratio varying from in®nite in the case of S0 sample to about 2 in the case of S11 sample) and main characteristics of ZrNx ®lms. Sample
Deposition time (s)
Substrate temperature (8C)
Nitrogen gas flow (sccm)
Nitrogen partial pressure (10 4 Torr)
Thickness Ê) (A
N/Zr atomic ratio (stoichiometry)
S0 S1 S2 S3 S4 S5 S7 S9 S11
600 1200 600 300 600 300 300 300 300
35 150 150 150 150 150 150 150 150
0 1 2 3 4 5 7 9 11
0 2.9 4.3 5.8 7.3 9.1 12.1 15.5 19.0
4940a 10230a 5370a 2350a 3920a 1005b 834b 721b 697b
0 0.15 0.25 0.45 1 1.1 1.2 1.38 1.4
The thickness was measured by two techniques: (a) RBS analysis, (b) X-re¯ectometry.
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
are reported in Table 1 (samples are labeled as S[i] where i indicates the value of nitrogen ¯ow). 2.2. Characterization techniques Rutherford backscattering spectrometry (RBS) [25] was used to determine ®lm composition and thickness. The experiments were carried out using a 2 MeV 4 He ions beam. The backscattered spectra were analyzed using simulation for analysis of materials (SAM) code. The nitrogen and zirconium amounts obtained by RBS allow the determination of the N/Zr atomic ratio (stoichiometry) in the ®lm. The crystal structure of sputtered ®lms was determined by X-ray diffraction (XRD). The XRD spectra were recorded in the y/2y mode, between 2y 30 and 808 with a step of 0.058. Electrical resistivity was measured using a four-probe method. Optical re¯ectance spectra were recorded between 250 and 2500 nm with an UV-VisNIR photospectrometer.
233
3. Results and discussion 3.1. Structure of the ®lms Fig. 2 shows the X-ray diffraction patterns of S0, S2, S4, S5, S7, and S9 samples. For samples S0 and S2, the ®lms crystallize in the a-Zr phase of hexagonal structure, with a (1 0 0) preferential orientation. Peaks of S2 pattern are shifted toward small 2y angles in comparison with S0, which indicates a broadening of the hexagonal cell, due to nitrogen atoms incorporated in interstitial sites [26]. For the other patterns, we remark that the structure becomes cubic of NaCl type (d-ZrN phase) and above the stoichiometry, which is obtained with a 4 sccm nitrogen ¯ow, the ®lms become more and more amorphous. This trend to amorphisation was already reported in references [7,18,23]. It may be explained either by the absence of stable phases in the rich nitrogen zone of ZrN system or by the in¯uence of the deposition parameters
Fig. 2. X-ray diffraction patterns of S[i] samples (where i 1, 2. . ., indicates nitrogen ¯ow).
234
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
and the substrate nature [15]. Besides, peaks of S5, S7 and S9 samples shift toward smaller 2y angles compared to those of S4. This suggests a cell dilation of the stoichiometric compound by the incorporation of nitrogen atoms in interstitial positions and the progressive formation of an other phase like Zr3N4. Several authors have already indicated the shift of the (1 1 1) peak of the NaCl phase toward smaller angles for ®lms which have a composition corresponding to Zr3N4 [14,20,24]. According to Pichon et al. [18,23], the Zr3N4 structure can be described as relaxed NaCl structure with some Zr vacancies. Also, Dauchot et al. [14] found a NaCl structure by grazing angle diffraction for rich nitrogen ®lms (N/Zr atomic ratio x beyond 1.33). 3.2. RBS analysis RBS spectra of samples S0 and S4 are presented in Fig. 3 as an example. The ®lm composition and thickness obtained by RBS spectra simulation with the help of the SAM code are reported in Table 1. We measured the thickness of S5, S7, S9 and S11 samples by the X-ray re¯ectivity technique which is more Ê . Fig. 4 shows deposiaccurate than RBS under 1200 A
tion rate and N/Zr atomic ratio evolution of our ZrNx ®lms with nitrogen ¯ow. The deposition rate decreases when nitrogen ¯ow increases from 0 to 4 sccm. Above this value, the decrease becomes less marked and tends to stabilize beyond 9 sccm at a value near of Ê /s. This effect is due to the irradiation of the 2.5 A target by N-ions and leads to the formation of zirconium nitrides at the target surface which has a smaller sputtering yield than pure Zr. We veri®ed this change of the sputtering yield by means of transport of ions in matter (TRIM) code [27]. Furthermore, the enlarged gas pressure in the sputtering chamber involves a high density of particles between target and substrates which creates an improved dif®culty for sputtered particles to reach the substrates and yields the deposition rate to fall between 2 and 5 sccm. As for the deposition rate overall behavior, two regions are distinguished in the ®lm composition versus nitrogen ¯ow: the ®rst one between 0 and 4 sccm where we observe a strong increase of N/Zr ratio; the second one between 4 and 11 sccm where N/Zr ratio keeps increasing but in a less important way. Similar deposition rate and nitrogen to zirconium atomic ratio behavior in ZrNx ®lms are pointed out in references [16,17].
Fig. 3. RBS spectra recorded on S0 and S4 samples. The dotted lines represent the experimental spectra and the solid lines the SAM simulation.
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
235
Fig. 4. Evolution of the deposition rate and the N/Zr atomic ratio x of ZrNx ®lms as a function of nitrogen ¯ow.
3.3. Resistivity evolution The electrical resistivity of ZrNx ®lms is shown in Fig. 5 as a function of nitrogen ¯ow. We remark that as the nitrogen ¯ow increases, the resistivity increases too. After reaching a maximum at 2 sccm, the resistivity decreases until it reaches a minimum (65 mO cm) at 4 sccm, then it increases again with higher nitrogen ¯ows. This behavior is quite similar to the results reported by several authors [7,16,17,24]. According to Matthiessen's rule [28], the total resistivity of a metal is a contribution of several independent electron scattering processes due to phonons, impurity atoms, defects and even surface effects when the sample is a ®lm of a smaller thickness than the mean free-electron path. Hence, the initial increase of resistivity may be due to (i) electron scattering by incorporated nitrogen molecules and (ii) atoms in the Zr lattice which play the role of impurities. Also, the stretching out of the cell that increases the distance between Zr atoms leads to the weakness of the metallic bond [29]and thus to the resistivity rise. Between 2 and 4 sccm, especially in the vicinity of 4 sccm, the stoichiometric phase ZrN, which has a metallic behavior due to its special bonds [8,30±32], is going to be
formed. This causes the minimum of resistivity. The increase of resistivity beyond 4 sccm is explained by the trend to amorphization which means a high presence of defects. Also, the increase of nitrogen concentration in ZrN structures induces the stretching out of the cell that decreases the metallic bond contribution. 3.4. Re¯ectance measurement At ®rst glance, observation shows that our zirconium nitride samples have various colors. Actually the color varies from metallic gray for low nitrogen ¯ows to golden yellow (sample S4) which is the normal color of the stoichiometric ZrN compound and to semi-transparent brownish for high nitrogen ¯ows. Fig. 6 shows the re¯ectance evolution of S0, S1, S2, S4, S5, S7, and S9 samples. As far as S7 and S9 samples are concerned (as well as S11 not represented in Fig. 6) since they are semi-transparent, oscillations could be expected due to interferences produced by re¯exions onto the ZrN surface as well as the ZrN/Si interface. Actually those oscillations do not show up for two reasons: (i) the sample absorption is never negligible and (ii) the sample thickness is rather
236
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
Fig. 5. Evolution of electrical resistivity of ZrNx ®lms as a function of nitrogen ¯ow.
Fig. 6. Re¯ectance spectra of ZrNx ®lms, where x indicates nitrogen to zirconium atomic ratio.
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
small (about 80 nm). A more complete discussion will be given on that point in a forthcoming paper. The spectra for S1 and S2 samples are almost the same as the S0 one which indicates a metallic behavior. At 4 sccm, we obtained a spectrum which is characterized by a minimum of re¯ectance in the visible domain and a maximum in the infrared domain. As the ¯ow increases above 4 sccm, the minimum of re¯ectance increases and shifts slightly towards the long waves while the infrared maximum re¯ectance decreases. Several authors observed a similar evolution of re¯ectance spectra in relation with nitrogen content [16,17,20]. 4. Conclusion ZrN ®lms have been sputter-deposited by dc reactive magnetron sputtering. The in¯uence of the nitrogen ¯ow on the ZrN ®lm structure, stoichiometry, resistivity and re¯ectance was investigated. The XRD study shows that under-stoichiometric ®lms present a good crystallization in opposite to overstoichiometric ®lms which become more and more amorphous as the nitrogen ¯ow increases beyond 4 sccm. Simultaneously, their diffraction peaks present a shift towards smaller angles. This suggests either a cell dilatation or the formation of an other phase probably Zr3N4. The RBS analysis shows the formation of zirconium nitrides with compositions which vary from under-stoichiometry to over-stoichiometry. Deposition rates decrease rapidly below 4 sccm, then shows a tendency to stabilize beyond 5 sccm. The electrical resistivity was measured with a four probes method. The results show that the ®lms become in overall more and more resistive as nitrogen ¯ow increases. A minimum of resistivity is however measured for the ®lm synthesized at 4 sccm which con®rms that it is close to stoichiometry. In the vicinity of stoichiometry, the ZrN ®lms have a typical goldlike optical re¯ectance characterized by a minimum in the visible domain and a maximum in the infrared domain. This con®rms the use of these ®lms in heat mirror industry. Outside of stoichiometry, all the optical re¯ectance spectra differ from each other. For the nitrogen rich ®lms, the dielectric behaviours and the semi-transparent colors support the idea that they are most probably formed with a Zr3N4 phase.
237
Acknowledgements This work was supported by the French-Algerian CMEP Accord-Programme No. 00-MDU-468. References [1] L.E. Toth, Transition Metal Carbides and Nitrides, Academic Press, New York, 1971. [2] V.N. Zhitomirsky, I. Grimberg, R.L. Boxman, N.A. Travisky, S. Goldsmith, B.Z. Weiss, Surf. Coat. Technol. 94/95 (1997) 207. [3] W.C. Russell, J. de Phys. II 5 (1995) C5±C127. [4] B. Karlsson, R.P. Shimshok, B.O. Seraphin, J.C. Haygarth, Solar Energy Mater. 7 (1983) 401. [5] W.D. Sproul, Thin Solid Films 107 (1983) 141. [6] C. Mitterer, P.H. Mayerhofer, W. Waldhauser, E. Kelesoglu, P. Losbichler, Surf. Coat. Technol. 108/109 (1998) 230. [7] T. Yotsuya, M. Yoshitake, T. Kodama, Cryogenics 37 (1997) 817. [8] K. Schwarz, A.R. Williams, J.J. Cuomo, J.H.E. Harper, H.T.G. Hentzell, Phys. Rev. B 32 (12) (1985) 8312. [9] M. Wautelet, J.P. Dauchot, F. Debal, S. Edart, M. Hecq, J. Mater. Res. 11 (4) (1996) 825. [10] Y. Igarashi, T. Yamaji, S. Nishikawa, Jpn. J. Appl. Phys. 29 (1990) L2337. [11] B.M. Takeyama, T. Itoi, E. Aoyagi, A. Noya, Appl. Surf. Sci. 190 (2002) 450. [12] B.M. Takeyama, A. Noya, K. Sakanishi, J. Vac. Sci. Technol. B 18 (2000) 1333. [13] M. Burghartz, G. Ledergerber, H. Hein, R.R. vander Laan, R.J.M. Konings, J. Nucl. Mater. 288 (2001) 233. [14] J.P. Dauchot, S. Edart, M. Wautelet, M. Hecq, Vacuum 46 (1995) 927. [15] D. Wu, Z. Zhang, W. Fu, X. Fan, H. Guo, Appl. Phys. A 64 (1997) 593. [16] S. Inoue, K. Tominaga, R.P. Howson, K. Kusaka, J. Vac. Sci. Technol. A 13 (1995) 2808. [17] M. Yoshitake, T. Yotsuya, S. Ogawa, Jpn. J. Appl. Phys. 31 (1992) 4002. [18] L. Pichon, T. Girardeau, A. Straboni, F. Lignou, J. PerrieÁre, J.M. FrigeÂrio, Nucl. Instr. Meth. Phys. Res. B 147 (1999) 378. [19] S. Horita, T. Tujikawa, H. Akahori, M. Kobayashi, T. Hata, J. Vac. Sci. Technol. A 11 (1993) 2452. [20] M. Yoshitake, T. Nosaka, A. Okamoto, S. Ogawa, Jpn. J. Appl. Phys. 32 (1993) L113. [21] M.B. Lee, M. Kawasaki, M. Yoshimoto, M. Kumagai, H. Koinuma, Jpn. J. Appl. Phys. 33 (1994) 6308. [22] L. Gribaudo, D. Arias, J. Abriata, J. Phase Equilib. 15 (1994) 441. [23] L. Pichon, T. Girardeau, A. Straboni, F. Lignou, Ph. GueÂrin, J. PerrieÁre, Appl. Surf. Sci. 150 (1999) 115. [24] B.O. Johansson, H.T.G. Hentzell, J.M.E. Harper, J.J. Cuomo, J. Mater. Res. 1 (1986) 442. [25] W.-K. Chu, J.W. Mayer, M.-A. Nicolet, Backscattering Spectroscopy, Academic Press, Orlando, 1978.
238
H.M. Benia et al. / Applied Surface Science 200 (2002) 231±238
[26] M. Yoshitake, T. Yotsuya, K. Takiguchi, S. Ogawa, Jpn. J. Appl. Phys. 29 (12) (1990) 2800. [27] J.F. Ziegler, J.P. Biersack, U. Littmark (Eds.), The Stopping and Range of Ions in Solids, Pergamon Press, New York, 1986. [28] Milton Ohring, The Materials Science of Thin Films, Academic Press, New York, 1992.
[29] P. Schwarzcopf, R. Kieffer, Refractory Hard Metals, Macmillan, New York, 1953, p. 37. [30] A. Dunaud, H.D. Flack, K. Yvon, Phys. Rev. B 31 (1985) 2316. [31] P. Blaha, J. Redinger, K. Schwarz, Phys. Rev. B 31 (1985) 2316. [32] A. Neckel, Intern. J. Quantum Chem. 23 (1983) 1317.