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Synthesis of NiS2 thin films
Journal of Materials Processing Technology 92±93 (1999) 239±242
Synthesis of NiS2 thin ®lms Electrical and optical properties I.J. Ferrer, C. SaÂnchez* Dpto. de FõÂsica de Materiales, C-IV. U.A.M. Cantoblanco, 28049 Madrid, Spain
Abstract NiS2 thin ®lms for photovoltaic applications have been deposited from the elements S and Ni. Their structural, optical and electrical properties have been investigated and compared with those of single crystals. The deposition technique used here is suitable for growing ®lms on large area substrates and it can be used easily and fruitfully to grow other chalcogenide compounds. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Thin ®lms; Semiconductors
1. Introduction Transition metal sul®des (FeS2, CoS2, NiS2) have received considerable attention due to the large variety of their electrical, optical and magnetic properties [1]. For example, FeS2 is a diamagnetic semiconductor, CoS2, a ferromagnetic metal and NiS2, an anti-ferromagnetic insulator [2,3]. On the other hand, CuS2 is metallic, with a low temperature transition to superconducting state and ZnS2 is a wide-band gap non-magnetic insulator [3]. Among the various transition metal compounds, pyrites have attracted much interest because their 3d electrons are in the low spin con®guration. NiS2 is a transition metal chalcogenide with pyrite structure. In pyrite crystals, the transition metal ion (M2 ) is surrounded by six S2 2ÿ molecules. The 3d levels of the M2 ion are split into d" (t2g) and dg (eg) orbitals, with the d" levels in the lower energy region. NiS2 single crystals have been studied extensively up to now and their main characteristics are well established. In NiS2, the dg levels are partially occupied by two electrons yielding a small band gap of 0.37 eV [4±6]. Although band structure calculations predict NiS2 to be a metal [7±9], however, it is a semiconductor because of the splitting of the eg band due to correlation effects. NiS2 is thus a Mott± Hubbard insulator [10]. A metal±non-metal transition occurs by the substitution of Se for S and Cu or Co for *Corresponding author. Tel.: +34-1-397-4766; fax: +34-1-397-8579 E-mail address: [email protected] (C. SaÂnchez)
Ni, as well as by the application of pressure [11]. Substantial changes in electrical resistivity are also observed by increasing the stoichiometric ratio in pure NiS2 [12]. In contrast to this extensive knowledge on NiS2 single crystals, no information is available yet (as far as we know) on the synthesis and properties of NiS2 thin ®lms. We present in this paper a simple procedure to grow NiS2 thin ®lms. In fact, the experimental method has been very successful in growing FeS2 thin ®lms [13] and has been applied tentatively to obtain Fe1ÿxNixS2 and NiS2 ®lms [14]. 2. Experimental NiS2 thin ®lms were obtained by the sulfuration of metallic Ni layers ¯ash evaporated on glass substrates at room temperature. Flash evaporation was done in a conventional vacuum coating unit at pressures <10ÿ6 Torr. The evaporation source used was Ni powder of 99.9% purity and 50 mm particle size. The sulfuration of Ni layers was carried out in a vacuum-sealed ampoule with sulfur powder inside it at 3508C for 20 h. The heating and cooling rates were 3 and 18C minÿ1, respectively. The sulfur pressure during the annealing treatment was 200 Torr. The experimental procedure is described in detail in [13,14]. The thicknesses of the NiS2 ®lms, measured with a mechanical stylus Sloan Dektak IIA, were 0.6 and 0.4 0.1 mm for the samples identi®ed as 39 and 40, respectively.
0924-0136/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 1 7 2 - 7
240
I.J. Ferrer, C. SaÂnchez / Journal of Materials Processing Technology 92±93 (1999) 239±242
3. Results and discussion 3.1. Structure, morphology and composition From X-ray diffraction (XRD) patterns, NiS2 ®lms are shown to be polycrystalline (see Table 1). XRD analyses of the ®lms show that they have a cubic structure corresponding to vaesite (ASTM 11-99), with maximum intensity in the (2 0 0) direction. Other phases are not present (Fig. 1). The crystallite size, estimated from the half medium width by applying the Scherrer equation, are 317 and Ê for ®lms 39 and 40, respectively. The surface of 414 A the ®lms, as viewed from SEM micrographs, shows some round formations (Fig. 2(a)), but the ground ®lm seems to be formed uniformly by balls of about 0.2 mm diameter (Fig. 2(b)). The stoichiometric ratio, determined by energy dispersive X-ray analyses (EDAX), were 1.963 and 1.905 for NiS2-39 and -40, respectively. S/Ni obtained from RBS measurements of sample 40 agrees well with the EDAX result. Fig. 1. XRD patterns of NiS2 thin films: (a) NiS2-40; (b) NiS2-39.
3.2. Seebeck and Hall effects The positive value of Seebeck coef®cient measured at room temperature, 4±6 mV Kÿ1, suggests that the ®lms are p-type semiconductors. This extremely low value is in contrast to the 350 mV Kÿ1 obtained with single crystals [15]. The Hall effect in these ®lms was found to be extremely small and it was not possible to measure the Hall constant. Previous papers also report a low Hall effect [15±17]. 3.3. Optical energy gap Optical transmission measurements were carried out in the photon energy range between 0.25 and 3.0 eV. The optical absorption coef®cient obtained was about 7 104±7 105 cmÿ1 for photon energies greater than 1.0 eV. The photon energy dependence of the absorption coef®cient near the absorption edge suggests that the energy gap is indirect. By ®tting the absorption coef®cient to the general formula for indirect allowed transitions, a value of the energy gap of 0.33 0.02 eV, was obtained for both samples (Fig. 3). This value is in agreement with the value of 0.37 eV reported previously by other authors for single crystals [15].
Table 1 Structure, morphology and composition of NiS2 films Identification
39
40
Thickness (mm) Ê) Crystallite size (A Seebeck coeffecient (mV Kÿ1) S/Ni
0.6 317 4.5 1.963
0.4 414 6.28 1.905
Fig. 2. SEM micrographs of NiS2-39 thin film.
I.J. Ferrer, C. SaÂnchez / Journal of Materials Processing Technology 92±93 (1999) 239±242
Fig. 3. Estimation of the optical energy gap assuming allowed indirect transition.
3.4. Electrical conductivity Electrical conductivity measurements at different temperatures (230±420 K) were made using the four-probe van der Pauw technique. Despite the large dispersion of the experimental data, the conductivities of both ®lms show a well de®ned behaviour with changing temperature. The room temperature conductivities are similar for both ®lms (55 ÿ1 cmÿ1). However, the conductivity dependence on temperature (T) is quite different. An Arrhenius plot for the conductivities is shown in Fig. 4. Two different dependencies on temperature are observed for the two samples. One of them (NiS2-40) presents two different activation energies,
241
299 and 43 meV, for T > 370 K and T < 370 K, respectively (Fig. 4(a)). Similar behaviour has been reported previously by Kautz [15] in NiS2 single crystals: activation energies of 320 and 68 meV at temperatures above and below 380 K, respectively. From these results, they proposed a model in which at T < 380 K hole conduction dominates and at T > 380 K, electron conduction, based on the assumption of partial carrier compensation which pins the Fermi level in the acceptor band. Our results are in agreement with this interpretation. In fact, the sum of the electrical activation energies (342 meV), which should be equal to the energy gap at 0 K, is in agreement with the optical energy gap obtained experimentally (330 meV). On the other hand, the sign of Seebeck coef®cient at room temperature also con®rms hole conduction at this temperature. Other authors have published slope changes in Arrhenius plots for conductivities and similar activation energies in the low temperature range (75 meV at T < 390 K [18]). However, the other sample (NiS2-39) shows a unique slope in the same range of temperature, with an activation energy of 17 meV. This result can be understood in terms of the stoichiometric ratio. Krill et al. [12] studied the in¯uence of the stoichiometric ratio on the structural, electronic and magnetic properties of different pyrite-structure compounds. They report that, in NiSx, electrical resistivity depends strongly on x. Thus, NiSx behaves as a Mott insulator for 1.91 x 1.93 and shows a metallic behaviour for x 2. These changes in electrical resistivity are accompanied by a lattice parameter reduction [12,19]. From our results, changes in the lattice parameter have not been observed, but the stoichiometric ratio measured by EDAX and RBS analyses con®rms that ®lm 39, with the highest stoichiometry, shows the lowest activation energy for conductivity. Experimentally, Ogawa [11] observed an activation energy of 160 meV in electrical resistivity, which decreased to zero at the boundary of the transition from non-metal to metal. Other authors have reported lower activation energies (120 meV [20]). The conductivity values of both samples are higher than those reported in the literature, probably due to the polycrystalline nature of the samples which are compared here with single crystals. In conclusion, it has been shown that it is possible to prepare NiS2 thin ®lms by a simple technique used previously with FeS2. Their structural, optical and electrical properties have been investigated and compared to those of single crystals already known. Acknowledgements
Fig. 4. Arrhenius plots for the conductivity of NiS2 thin films: (a) NiS2-40; (b) NiS2-39.
The authors thank CIEMAT (Madrid, Spain) for facilities to measure the temperature dependence of conductivity and the Nuclear and Technological Institute (ITN) of Sacavem (Portugal) for the RBS measurements. This work was supported by the DGICyT under contract PB93/0266.
242
I.J. Ferrer, C. SaÂnchez / Journal of Materials Processing Technology 92±93 (1999) 239±242
References [1] K. Sato, Prog. Crystal Growth Characteriz. 11 (1985) 108 and references therein. [2] A.E. Bocquet, K. Mamiya, T. Mizokawa, A. Fujimori, T. Miyadai, H. Takahashi, M. Mori, S. Suga, J. Phys. Condens. Matter 8 (1996) 2389. [3] G.L. Zhao, J. Callaway, M. Hayashibara, Phys. Rev. B 48 (1993) 15781. [4] H.S. Jarret, W.H. Cloud, R.J. Bouchard, S.R. Butler, C.G. Frederick, J.L. Gillson, Phys. Rev. Lett. 21 (1968) 617. [5] K. Adachi, K. Sato, M. Takada, J. Phys. Soc. Jpn. 26 (1969) 631. [6] E.K. Li, K.H. Johnson, D.E. Eastman, J.L. Freeouf, Phys. Rev. Lett. 32 (1974) 470. [7] D.W. Bullet, J. Phys. C: Solid State Phys. 9 (1982) 81. [8] S. Lauer, A.X. Trautwein, F.E. Harris, Phys. Rev. B 29 (1984) 6774. [9] W. Folkerts, G.A. Sawatzky, C. Haas, R.A. de Groot, F.U. Hillebrecht, J. Phys. C: Solid State Phys. 20 (1987) 4135.
[10] N.F. Mott, Proc. Phys. Soc. London Ser. A26 (1949) 416. [11] S. Ogawa, J. Appl. Phys. 50 (1979) 2308. [12] G. Krill, M.F. Lapierre, F. Gautier, C. Robert, C. Czjzek, J. Fink, H. Schmidt, J. Phys. C: Solid State Phys. 9 (1976) 761. [13] I.J. Ferrer, C. SaÂnchez, J. Appl. Phys. 70 (1991) 2641. [14] I.J. Ferrer, C. de las Heras, C. SaÂnchez, J. Phys.: Condens. Matter 7 (1995) 2115. [15] R.L. Kautz, M.S. Dresselhaus, D. Adler, A. Linz, Phys. Rev. B 6 (1972) 2078. [16] T. Thio, J.W. Bennett, Phys. Rev. B 50 (1994) 10574. [17] P. Kwizera, M.S. Dresselhaus, D. Adler, Phys. Rev. B 21 (1980) 2328. [18] J.A. Wilson, G.D. Pitt, Phil. Mag. 23 (1971) 1297. [19] S. Endo, T. Mitsui, T. Miyadai, Phys. Lett. A 46 (1973) 29. [20] T.A. Bither, R.J. Bouchard, W.H. Cloud, P.C. Donohue, W.J. Siemons, Inorg. Chem. 1 (1968) 2208.
Synthesis of NiS2 thin ®lms Electrical and optical properties I.J. Ferrer, C. SaÂnchez* Dpto. de FõÂsica de Materiales, C-IV. U.A.M. Cantoblanco, 28049 Madrid, Spain
Abstract NiS2 thin ®lms for photovoltaic applications have been deposited from the elements S and Ni. Their structural, optical and electrical properties have been investigated and compared with those of single crystals. The deposition technique used here is suitable for growing ®lms on large area substrates and it can be used easily and fruitfully to grow other chalcogenide compounds. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Thin ®lms; Semiconductors
1. Introduction Transition metal sul®des (FeS2, CoS2, NiS2) have received considerable attention due to the large variety of their electrical, optical and magnetic properties [1]. For example, FeS2 is a diamagnetic semiconductor, CoS2, a ferromagnetic metal and NiS2, an anti-ferromagnetic insulator [2,3]. On the other hand, CuS2 is metallic, with a low temperature transition to superconducting state and ZnS2 is a wide-band gap non-magnetic insulator [3]. Among the various transition metal compounds, pyrites have attracted much interest because their 3d electrons are in the low spin con®guration. NiS2 is a transition metal chalcogenide with pyrite structure. In pyrite crystals, the transition metal ion (M2 ) is surrounded by six S2 2ÿ molecules. The 3d levels of the M2 ion are split into d" (t2g) and dg (eg) orbitals, with the d" levels in the lower energy region. NiS2 single crystals have been studied extensively up to now and their main characteristics are well established. In NiS2, the dg levels are partially occupied by two electrons yielding a small band gap of 0.37 eV [4±6]. Although band structure calculations predict NiS2 to be a metal [7±9], however, it is a semiconductor because of the splitting of the eg band due to correlation effects. NiS2 is thus a Mott± Hubbard insulator [10]. A metal±non-metal transition occurs by the substitution of Se for S and Cu or Co for *Corresponding author. Tel.: +34-1-397-4766; fax: +34-1-397-8579 E-mail address: [email protected] (C. SaÂnchez)
Ni, as well as by the application of pressure [11]. Substantial changes in electrical resistivity are also observed by increasing the stoichiometric ratio in pure NiS2 [12]. In contrast to this extensive knowledge on NiS2 single crystals, no information is available yet (as far as we know) on the synthesis and properties of NiS2 thin ®lms. We present in this paper a simple procedure to grow NiS2 thin ®lms. In fact, the experimental method has been very successful in growing FeS2 thin ®lms [13] and has been applied tentatively to obtain Fe1ÿxNixS2 and NiS2 ®lms [14]. 2. Experimental NiS2 thin ®lms were obtained by the sulfuration of metallic Ni layers ¯ash evaporated on glass substrates at room temperature. Flash evaporation was done in a conventional vacuum coating unit at pressures <10ÿ6 Torr. The evaporation source used was Ni powder of 99.9% purity and 50 mm particle size. The sulfuration of Ni layers was carried out in a vacuum-sealed ampoule with sulfur powder inside it at 3508C for 20 h. The heating and cooling rates were 3 and 18C minÿ1, respectively. The sulfur pressure during the annealing treatment was 200 Torr. The experimental procedure is described in detail in [13,14]. The thicknesses of the NiS2 ®lms, measured with a mechanical stylus Sloan Dektak IIA, were 0.6 and 0.4 0.1 mm for the samples identi®ed as 39 and 40, respectively.
0924-0136/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 1 7 2 - 7
240
I.J. Ferrer, C. SaÂnchez / Journal of Materials Processing Technology 92±93 (1999) 239±242
3. Results and discussion 3.1. Structure, morphology and composition From X-ray diffraction (XRD) patterns, NiS2 ®lms are shown to be polycrystalline (see Table 1). XRD analyses of the ®lms show that they have a cubic structure corresponding to vaesite (ASTM 11-99), with maximum intensity in the (2 0 0) direction. Other phases are not present (Fig. 1). The crystallite size, estimated from the half medium width by applying the Scherrer equation, are 317 and Ê for ®lms 39 and 40, respectively. The surface of 414 A the ®lms, as viewed from SEM micrographs, shows some round formations (Fig. 2(a)), but the ground ®lm seems to be formed uniformly by balls of about 0.2 mm diameter (Fig. 2(b)). The stoichiometric ratio, determined by energy dispersive X-ray analyses (EDAX), were 1.963 and 1.905 for NiS2-39 and -40, respectively. S/Ni obtained from RBS measurements of sample 40 agrees well with the EDAX result. Fig. 1. XRD patterns of NiS2 thin films: (a) NiS2-40; (b) NiS2-39.
3.2. Seebeck and Hall effects The positive value of Seebeck coef®cient measured at room temperature, 4±6 mV Kÿ1, suggests that the ®lms are p-type semiconductors. This extremely low value is in contrast to the 350 mV Kÿ1 obtained with single crystals [15]. The Hall effect in these ®lms was found to be extremely small and it was not possible to measure the Hall constant. Previous papers also report a low Hall effect [15±17]. 3.3. Optical energy gap Optical transmission measurements were carried out in the photon energy range between 0.25 and 3.0 eV. The optical absorption coef®cient obtained was about 7 104±7 105 cmÿ1 for photon energies greater than 1.0 eV. The photon energy dependence of the absorption coef®cient near the absorption edge suggests that the energy gap is indirect. By ®tting the absorption coef®cient to the general formula for indirect allowed transitions, a value of the energy gap of 0.33 0.02 eV, was obtained for both samples (Fig. 3). This value is in agreement with the value of 0.37 eV reported previously by other authors for single crystals [15].
Table 1 Structure, morphology and composition of NiS2 films Identification
39
40
Thickness (mm) Ê) Crystallite size (A Seebeck coeffecient (mV Kÿ1) S/Ni
0.6 317 4.5 1.963
0.4 414 6.28 1.905
Fig. 2. SEM micrographs of NiS2-39 thin film.
I.J. Ferrer, C. SaÂnchez / Journal of Materials Processing Technology 92±93 (1999) 239±242
Fig. 3. Estimation of the optical energy gap assuming allowed indirect transition.
3.4. Electrical conductivity Electrical conductivity measurements at different temperatures (230±420 K) were made using the four-probe van der Pauw technique. Despite the large dispersion of the experimental data, the conductivities of both ®lms show a well de®ned behaviour with changing temperature. The room temperature conductivities are similar for both ®lms (55 ÿ1 cmÿ1). However, the conductivity dependence on temperature (T) is quite different. An Arrhenius plot for the conductivities is shown in Fig. 4. Two different dependencies on temperature are observed for the two samples. One of them (NiS2-40) presents two different activation energies,
241
299 and 43 meV, for T > 370 K and T < 370 K, respectively (Fig. 4(a)). Similar behaviour has been reported previously by Kautz [15] in NiS2 single crystals: activation energies of 320 and 68 meV at temperatures above and below 380 K, respectively. From these results, they proposed a model in which at T < 380 K hole conduction dominates and at T > 380 K, electron conduction, based on the assumption of partial carrier compensation which pins the Fermi level in the acceptor band. Our results are in agreement with this interpretation. In fact, the sum of the electrical activation energies (342 meV), which should be equal to the energy gap at 0 K, is in agreement with the optical energy gap obtained experimentally (330 meV). On the other hand, the sign of Seebeck coef®cient at room temperature also con®rms hole conduction at this temperature. Other authors have published slope changes in Arrhenius plots for conductivities and similar activation energies in the low temperature range (75 meV at T < 390 K [18]). However, the other sample (NiS2-39) shows a unique slope in the same range of temperature, with an activation energy of 17 meV. This result can be understood in terms of the stoichiometric ratio. Krill et al. [12] studied the in¯uence of the stoichiometric ratio on the structural, electronic and magnetic properties of different pyrite-structure compounds. They report that, in NiSx, electrical resistivity depends strongly on x. Thus, NiSx behaves as a Mott insulator for 1.91 x 1.93 and shows a metallic behaviour for x 2. These changes in electrical resistivity are accompanied by a lattice parameter reduction [12,19]. From our results, changes in the lattice parameter have not been observed, but the stoichiometric ratio measured by EDAX and RBS analyses con®rms that ®lm 39, with the highest stoichiometry, shows the lowest activation energy for conductivity. Experimentally, Ogawa [11] observed an activation energy of 160 meV in electrical resistivity, which decreased to zero at the boundary of the transition from non-metal to metal. Other authors have reported lower activation energies (120 meV [20]). The conductivity values of both samples are higher than those reported in the literature, probably due to the polycrystalline nature of the samples which are compared here with single crystals. In conclusion, it has been shown that it is possible to prepare NiS2 thin ®lms by a simple technique used previously with FeS2. Their structural, optical and electrical properties have been investigated and compared to those of single crystals already known. Acknowledgements
Fig. 4. Arrhenius plots for the conductivity of NiS2 thin films: (a) NiS2-40; (b) NiS2-39.
The authors thank CIEMAT (Madrid, Spain) for facilities to measure the temperature dependence of conductivity and the Nuclear and Technological Institute (ITN) of Sacavem (Portugal) for the RBS measurements. This work was supported by the DGICyT under contract PB93/0266.
242
I.J. Ferrer, C. SaÂnchez / Journal of Materials Processing Technology 92±93 (1999) 239±242
References [1] K. Sato, Prog. Crystal Growth Characteriz. 11 (1985) 108 and references therein. [2] A.E. Bocquet, K. Mamiya, T. Mizokawa, A. Fujimori, T. Miyadai, H. Takahashi, M. Mori, S. Suga, J. Phys. Condens. Matter 8 (1996) 2389. [3] G.L. Zhao, J. Callaway, M. Hayashibara, Phys. Rev. B 48 (1993) 15781. [4] H.S. Jarret, W.H. Cloud, R.J. Bouchard, S.R. Butler, C.G. Frederick, J.L. Gillson, Phys. Rev. Lett. 21 (1968) 617. [5] K. Adachi, K. Sato, M. Takada, J. Phys. Soc. Jpn. 26 (1969) 631. [6] E.K. Li, K.H. Johnson, D.E. Eastman, J.L. Freeouf, Phys. Rev. Lett. 32 (1974) 470. [7] D.W. Bullet, J. Phys. C: Solid State Phys. 9 (1982) 81. [8] S. Lauer, A.X. Trautwein, F.E. Harris, Phys. Rev. B 29 (1984) 6774. [9] W. Folkerts, G.A. Sawatzky, C. Haas, R.A. de Groot, F.U. Hillebrecht, J. Phys. C: Solid State Phys. 20 (1987) 4135.
[10] N.F. Mott, Proc. Phys. Soc. London Ser. A26 (1949) 416. [11] S. Ogawa, J. Appl. Phys. 50 (1979) 2308. [12] G. Krill, M.F. Lapierre, F. Gautier, C. Robert, C. Czjzek, J. Fink, H. Schmidt, J. Phys. C: Solid State Phys. 9 (1976) 761. [13] I.J. Ferrer, C. SaÂnchez, J. Appl. Phys. 70 (1991) 2641. [14] I.J. Ferrer, C. de las Heras, C. SaÂnchez, J. Phys.: Condens. Matter 7 (1995) 2115. [15] R.L. Kautz, M.S. Dresselhaus, D. Adler, A. Linz, Phys. Rev. B 6 (1972) 2078. [16] T. Thio, J.W. Bennett, Phys. Rev. B 50 (1994) 10574. [17] P. Kwizera, M.S. Dresselhaus, D. Adler, Phys. Rev. B 21 (1980) 2328. [18] J.A. Wilson, G.D. Pitt, Phil. Mag. 23 (1971) 1297. [19] S. Endo, T. Mitsui, T. Miyadai, Phys. Lett. A 46 (1973) 29. [20] T.A. Bither, R.J. Bouchard, W.H. Cloud, P.C. Donohue, W.J. Siemons, Inorg. Chem. 1 (1968) 2208.