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Ti-doped copper nitride films deposited by cylindrical magnetron sputtering
Journal of Alloys and Compounds 440 (2007) 254–258
Ti-doped copper nitride films deposited by cylindrical magnetron sputtering X.Y. Fan a , Z.G. Wu a , G.A. Zhang a , C. Li a , B.S. Geng a , H.J. Li a , P.X. Yan a,b,∗ a
b
School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, Republic of China Key Laboratory of Solid Lubrication, Institute of Chemical & Physics, Chinese Academy of Science, Lanzhou 730000, China Received 7 May 2006; received in revised form 29 August 2006; accepted 1 September 2006 Available online 21 December 2006
Abstract Pure and Ti-doped copper nitride films were prepared by cylindrical magnetron sputtering on glass substrates at room temperature. The preferred orientation for copper nitride films changes from [1 1 1] for undoped film to [1 0 0] for Ti-doped films. The variation of surface morphology correlates to that of preferred orientation resulting from the variation of Ti-doped content. The electrical resistivity and optical band gap increases as the Ti-doped content increases. © 2006 Elsevier B.V. All rights reserved. Keywords: Copper nitride; Surface morphology; Electrical resistivity; Optical band gap
1. Introduction Over the past decades, interest in fabrication of copper nitride (Cu3 N) was motivated by its broad applications in the electronic industry, such as write-once optical recording media devices [1–4] and microscopic metal links in integrated-circuit fabrication processes [5,6]. This is due to its prominent properties: nontoxic and stable in air at room temperature, and lower thermal decomposition temperature which is reported in the range of 100–470 ◦ C [1,2,4,7–9]. It can be also used as a barrier in low resistance magnetic tunnel junction [10] and can be a candidate electrode material for rechargeable Li-ion batteries [11]. Copper nitride that is a deficit semiconductor [12] has the cubic anti-ReO3 structure. Strangely, Cu atoms do not occupy completely the “close-packing sites” in the (1 1 1) plane. If different atoms can be inserted into the body center of the cubic unit cell, it will induce significant changes in the optical and electrical properties. In 1991, Zachwieja and Jacobs [13] prepared weakly doped Cu3 NPd0.02 and stoichiometric Cu3 NPd. The resistivity of copper nitride films decreased due to the insertion of Cu atoms into the anti-ReO3 cell, which was reported by Maruyama and Morishite [7]. The resistivity and optical energy gap increased
∗
Corresponding author. Tel.: +86 9318912719; fax: +86 9318913554. E-mail address: [email protected] (P.X. Yan).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.006
sharply to a maximum and then decreased with increasing nitrogen partial pressure. The decreasing of resistivity and optical energy gap were inferred to the excessive nitrogen insertion [14]. Hayashi et al. [15] reported that with hydrogen implantation, the resistivity and the energy gap decreased. Sometimes surplus Cu atoms can also probably occupy an octahedral hole site to form Cu4 N phase [16]. Via calculating the energy band structure by density functional theory, the inserts of Pd [17] and Cu [18] into the anti-ReO3 cell leads to the Cu3 N structure semimetallic and metallic, respectively. In this paper, we prepared copper nitride films with various Ti-doped contents by cylindrical magnetron sputtering method at room temperature. The main aim is to study the variation of structure, morphology, electrical properties and optical properties caused by the doping of Ti. 2. Experiment In preparing copper nitride film, cylindrical magnetron sputtering equipment was used with a hollow target of 10 mm in diameter and 370 mm long. The inside of the target has 8 magnetron cores. In the sputtering procedure, there are 8 arcs which can make the films uniformity above the target plane. Ti filament was wrapped on the 8 arcs of the target to dope the copper nitride films. The Ti-doped content was controlled by the number of Ti rings. The substrate was placed parallel to the target surface at a distance of 130 mm. The working gas was 99.99% pure nitrogen and 99.999% pure argon. Their flow rates were adjusted by independent mass-flow controllers. In the deposition procedure, the nitrogen was set at 40 sccm and 10 sccm for argon for all samples. Common
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glass (CAT. NO.7101) sheets with a size of 30 mm × 100 mm were used as substrates. Before deposition, they were cleaned by ultrasonic waves in acetone for 15 min. The chamber vacuum before deposition was 2 × 10−3 to 2.5 × 10−3 Pa. Pre-sputtering in pure argon was processed for 3 min in order to clear the target surface and the substrate. At room temperature, all samples were deposited with sputtering power of 1000 W, sputtering pressure of 8.5 × 10−1 Pa and the deposition time was 20 min. The Ti-doped contents in all Cu3 N films are investigated by electron probe microanalysis (EPMA) of EPMA-1600. The structure is characterized by X-ray diffraction (XRD, Cu K␣, Philips X Pert Pro.). The morphology is studied in air by HITACHI s-4800 FESEM equipment. Four-probe method is employed to measure electrical resistivity in air at room temperature. The optical properties are measured by UV–VIS spectrometer.
3. Results and discussion The Ti-doped content in all Cu3 N films are investigated by electron probe microanalysis (EPMA). The Ti-doped content and other chemical compositions of Cu3 N films are shown in Table 1. The XRD spectra shown in Fig. 1 exhibit the Ti-doped effect on the texture of Cu3 N films. First of all, it can be confirmed that all the films are composed of Cu3 N crystallites with antiReO3 structure, and no Cu peaks in the undoped Cu3 N film and especially no TiN peaks in Cu3 N films with different Ti-doped content can be found. It is obvious that the growth trend of the Cu3 N films is affected by the doping of Ti. As shown in Fig. 1(a), the preferred orientation of the undoped film is [1 1 1] which corresponds to the Cu-rich plane of (1 1 1). With a few Ti-doped content introducing, the texture of the film is influenced significantly. Fig. 1(b) shows that the film with Ti-doped content of 0.136 mol% has no preferred orientation. Clearly, the preferred growth of Nrich plane (1 0 0) occurs in Sample C with Ti-doped content of 0.232 mol% and this trend becomes more intensive as the Tidoped content increases, which is seen from the XRD spectra of Samples D and E with Ti-doped content of 0.59 and 1.066 mol% in Fig. 1(d and e). In conclusion, the doping of Ti in the Cu3 N films affects the texture of the film significantly. This is because the Ti atoms deposited on the substrate prevent the Cu atoms from diffusing. This makes sufficient nitrogen reactive with copper. If the Ti content reaches a given value (0.232 mol%), the preferred orientation will change from (1 1 1) to (1 0 0). In addition, the slight reduction of the Cu sputtering rate caused by the increase of the number of the Ti rings on the Cu target may be another reason. The changes may also occur under the combined action of the two effects. Compared with the undoped Cu3 N film, the FWHM of Peak [1 1 1] for the film with Ti-doped content of 0.136 mol% becomes narrow suddenly from 0.8663 to 0.5609, and that of Peak [1 0 0] from 1.1519 to 0.4803. This implies that the dop-
Fig. 1. XRD spectra of Cu3 N films undoped and with various Ti-doped contents.
Table 1 Chemical composition in all samples Sample
A
B
C
D
E
Ti (mol%) Cu (mol%) N (mol%)
0 84.316 15.684
0.136 83.22 16.644
0.232 83.050 16.718
0.59 81.401 18.009
1.066 80.144 19.79
Fig. 2. The mean grain size of Cu3 N films deposited with various Ti-doped content.
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ing of Ti improved the crystalline quality of Cu3 N films which can be further enhanced by increasing the Ti-doped content in Cu3 N films. It is found that the position of Peak [1 0 0] divagates from 23.04◦ for the undoped film to 23.32◦ by increasing the Ti-doped content to 1.066 mol%. For Peak [1 1 1], it is from 47.51◦ to 47.62◦ . The mean grain size of the films is estimated by the Debye–Scherrer formula [19] and the results are shown in Fig. 2.
The mean size of grains is in nanoscale and increases from 8.50 to 19.11 nm with increasing the Ti-doped content. The results conflict with those of FE-SEM, which is interpreted later. Fig. 3 shows the FE-SEM images of Cu3 N films with different Ti-doped content. The surface of undoped Cu3 N film is composed of pyramid-like grains, as seen from Fig. 3a. With a small quantity of Ti doping into the film, the surface morphology changes noticeably. The crystalline grains vary from pyramid-like to rugby-like as shown in Fig. 3b. While the Tidoped content in Cu3 N film reaches 1.066 mol%, it shows a spherical-like morphology, which is reported by Pierson [20] as nodular-like morphology. The variation of the surface morphology is due to the fact that Ti doping changes the preferred orientation of the films. The un-doped film preferring to grow along the [1 1 1] orientation shows a pyramid-like morphology while the Ti-doped film preferring to grow along the [1 0 0] orientation shows a spherical-like morphology. Here, we confirm that the surface morphology is correlated with the film texture, which was reported previously. Increasing the Ti-doped content, the films become more and more compact, and the mean grain size changes from 130–150 to 50–80 nm with Ti-doped content increasing from 0 to 1.066 mol%. Compared with the results of average grain size calculated from the Debye–Scherrer formula, it is much bigger. This is because: (1) the Debye–Scherrer formula that relies on translational symmetry assumption fails to give even a rough estimation of the size for nanocrystals and (2) each particle of the FE-SEM image contains many single-crystal grains. And the variation of the average grain size dependence of the Ti-doped content is just the opposite, which results from the weakened crystalline grains’ agglomeration. The electric resistivity of the film was measured by four-point probe techniques. Fig. 4 shows the Ti-doped content dependence of electrical resistivity. The electrical resistivity of the undoped Cu3 N film is 1.63 × 10−3 cm. The pure stoichiometric Cu3 N film is insulated; the lattice constant is 0.3817 nm, while the asprepared undoped Cu3 N film is a semiconductor and the lattice constant is 0.3835 nm. Due to the expansion of a lattice constant
Fig. 3. FE-SEM images of Cu3 N films with different content of Ti-doped: (a) Sample A; (b) Sample B; (c) Sample E.
Fig. 4. Plot of electrical resistivity of the Cu3 N films as a function of Ti doping content.
X.Y. Fan et al. / Journal of Alloys and Compounds 440 (2007) 254–258
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Fig. 5. Plot of (Ahν)1/2 vs. hν of Samples A and E.
and the decrease of electrical resistivity, it is confirmed that Cu atoms insert into the body of electrical lattice. The inserted Cu atom becomes a donor which releases a free electron as a carrier. While the film is doped with a few Ti atoms of 0.136 mol%, the electrical resistivity increases rapidly to 4.464 cm. With increasing the Ti content, the electrical resistivity has no substantial variations. The electrical resistivity of the doped film with 1.066 mol% Ti is only 9.123 cm. The changes of electrical resistivity suggest that all the Ti atoms in the film do not contribute to dopants. The Ti atoms do not insert into the body of the lattice or substitute for the Cu atoms, but segregate at the grain boundary. The rapid increase in electrical resistivity of Cu3 N films doped with Ti is probably due to a decrease in the mobility and carrier concentration by the increased grain boundary barrier [21]. According to the band structure of copper nitride calculated by Ma Guadalupe Moreno-Armenta, etc. and Hahn and Weber, copper nitride films are indirect transition-type semiconductors. The optical band gap (Eg ) of the film can be obtained by plotting ( αhν)1/2 versus hν (α is the absorption coefficient and hν is the photon energy). The photon energy at the point where (αhν)1/2 = 0 is Eg . The Eg value is determined by extrapolation method. Here, we obtained absorbency A. A is defined as A = αρd for the solid thin film as normal incidence, where ρ is the density of the film and d is the thickness of the film. Because ρ and d are constants for the same film, the curve of
(Ahν)1/2 ∼hν is equivalent to that of (αhν)1/2 ∼hν. So we can obtain Eg by extrapolating the full line of the curve (Ahν)1/2 ∼hν to the abscissa. Fig. 5 shows the (Ahν)1/2 versus hν of Samples A and E. As seen from Fig. 6, the absorption edge shifted to a shorter wavelength region with increasing Ti content. That is, the band gap widens (blue shift) with increasing Ti content, which is consistent with the variation of electrical resistivity. This broadening effect can be understood based on the Burstein [22] effect. Burstein pointed out that an increase in the Fermi level in the conduction band of degenerate semiconductors leads to widening of the energy band (blue shift Eg ) which is related to carrier concentration. Further study on the relation between Eg and carrier concentration is needed. 4. Conclusion High quality Ti-doped copper nitride films were prepared on glass substrates by cylindrical magnetron sputtering at room temperature. The preferred orientation of undoped Cu3 N film is [1 1 1]. By increasing the Ti-doped content, it converts to [1 0 0] orientation. Calculated by the Debye–Scherrer formula, the mean size of grains is in nanoscale and increases from 8.50 to 19.11 nm with increasing Ti-doped content. With the doping of Ti, the surface morphology changes noticeably from pyramid-like to spherical-like. The shape of the grains is correlated with the proffered orientation which is determined by the Ti-doped content. The doping of Ti results in the abrupt increase of electrical resistivity, which is probably due to a decrease in both the mobility and carrier concentration by the increased grain boundary barrier. The band gap widens (blue shift) with increasing Ti content. It is interpreted as an increase in the Fermi level in the conduction band of degenerate semiconductors. Acknowledgements
Fig. 6. The variation of optical bandgap as a function of Ti content.
This work was supported by the Nature Science Foundation of Gansu Province, People’s Republic of China. The project number is ZS021-A25-022-C. The authors would like to thank the Laboratory of Field Emission Scanning Electron Microscope (FE-SEM, Hitachi S-4800) platform of information material and technology innovation, Lanzhou University.
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References [1] T. Maruyama, T. Morishita, Appl. Phys. Lett. 69 (7) (1996) 890. [2] M. Asano, K. Umeda, A. Tasaki, Jpn. J. Appl. Phys. 29 (1990) 1985. [3] R. Cremer, M. Witthaut, D. Neuschutz, C. Trappe, M. Laurenzis, O. Winkler, H. Kurz, Mikrochim. Acta 133 (2000) 299. [4] T. Nosaka, M. Yoshitake, A. Okamoto, S. Ogawa, Y. Nakayama, Appl. Surf. Sci. 169–170 (2001) 358. [5] L. Maya, J. Vac. Sci. Technol. A11 (1993) 604. [6] L. Maya, Mater. Res. Soc. Symp. Proc. 282 (1993) 203. [7] T. Maruyama, T. Morishita, J. Appl. Phys. 78 (1995) 4104. [8] D.Y. Wang, N. Nakamine, Y. Hayashi, J. Vac. Sci. Technol. A16 (1998) 2084. [9] Z.Q. Liu, W.J. Wang, S. Chao, S.K. Zheng, Thin Solid Films 325 (1998) 55. [10] D.M. Borsa, S. Grachev, C. Presura, D.O. Boerma, Appl. Phys. Lett. 80 (2002) 1823. [11] N. Pereira, L. Dupont, J.M. Tarascon, L.C. Klein, G.G. Amatucci, J. Electrochem. Soc. 150 (2003) A1273.
[12] Y. Du, A.L. Ji, L.B. Ma, Y.Q. Wang, Z.X. Cao, J. Cryst. Growth 280 (2005) 490. [13] U. Zachwieja, H. Jacobs, J. Less-Common Met. 170 (1991) 185. [14] T. Nosaka, M. Yoshitake, A. Okamoto, S. Ogawa, Y. Nakayama, Thin Solid Films 348 (1999) 8–13. [15] Y. Hayashi, T. Ishikawa, D. Shimokawa, J. Alloys Comp. 330–332 (2002) 348–351. [16] J. Blucher, K. Bang, Mater. Sci. Eng. A117 (1989) L1–L3. [17] U. Hahn, W. Weber, Phys. Rev. B 53 (1996) 12684. [18] M.G. Moreno-Armenta, A. Mart´ınez-Ruiz, N. Takeuchi, Solid State Sci. 6 (2004) 9–14. [19] P. Klug, L.E. Alexander (Eds.), X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials, second ed., Wiley, New York, 1974, pp. 687–780. [20] J.F. Pierson, Vacuum 66 (2002) 59–64. [21] K.C. Park, D.Y. Ma, K.H. Kim, Thin Solid Films 305 (1997) 201. [22] E. Burstein, Phys. Rev. 93 (1954) 632.
Ti-doped copper nitride films deposited by cylindrical magnetron sputtering X.Y. Fan a , Z.G. Wu a , G.A. Zhang a , C. Li a , B.S. Geng a , H.J. Li a , P.X. Yan a,b,∗ a
b
School of Physical Science and Technology, Lanzhou University, Lanzhou 730000, Republic of China Key Laboratory of Solid Lubrication, Institute of Chemical & Physics, Chinese Academy of Science, Lanzhou 730000, China Received 7 May 2006; received in revised form 29 August 2006; accepted 1 September 2006 Available online 21 December 2006
Abstract Pure and Ti-doped copper nitride films were prepared by cylindrical magnetron sputtering on glass substrates at room temperature. The preferred orientation for copper nitride films changes from [1 1 1] for undoped film to [1 0 0] for Ti-doped films. The variation of surface morphology correlates to that of preferred orientation resulting from the variation of Ti-doped content. The electrical resistivity and optical band gap increases as the Ti-doped content increases. © 2006 Elsevier B.V. All rights reserved. Keywords: Copper nitride; Surface morphology; Electrical resistivity; Optical band gap
1. Introduction Over the past decades, interest in fabrication of copper nitride (Cu3 N) was motivated by its broad applications in the electronic industry, such as write-once optical recording media devices [1–4] and microscopic metal links in integrated-circuit fabrication processes [5,6]. This is due to its prominent properties: nontoxic and stable in air at room temperature, and lower thermal decomposition temperature which is reported in the range of 100–470 ◦ C [1,2,4,7–9]. It can be also used as a barrier in low resistance magnetic tunnel junction [10] and can be a candidate electrode material for rechargeable Li-ion batteries [11]. Copper nitride that is a deficit semiconductor [12] has the cubic anti-ReO3 structure. Strangely, Cu atoms do not occupy completely the “close-packing sites” in the (1 1 1) plane. If different atoms can be inserted into the body center of the cubic unit cell, it will induce significant changes in the optical and electrical properties. In 1991, Zachwieja and Jacobs [13] prepared weakly doped Cu3 NPd0.02 and stoichiometric Cu3 NPd. The resistivity of copper nitride films decreased due to the insertion of Cu atoms into the anti-ReO3 cell, which was reported by Maruyama and Morishite [7]. The resistivity and optical energy gap increased
∗
Corresponding author. Tel.: +86 9318912719; fax: +86 9318913554. E-mail address: [email protected] (P.X. Yan).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.006
sharply to a maximum and then decreased with increasing nitrogen partial pressure. The decreasing of resistivity and optical energy gap were inferred to the excessive nitrogen insertion [14]. Hayashi et al. [15] reported that with hydrogen implantation, the resistivity and the energy gap decreased. Sometimes surplus Cu atoms can also probably occupy an octahedral hole site to form Cu4 N phase [16]. Via calculating the energy band structure by density functional theory, the inserts of Pd [17] and Cu [18] into the anti-ReO3 cell leads to the Cu3 N structure semimetallic and metallic, respectively. In this paper, we prepared copper nitride films with various Ti-doped contents by cylindrical magnetron sputtering method at room temperature. The main aim is to study the variation of structure, morphology, electrical properties and optical properties caused by the doping of Ti. 2. Experiment In preparing copper nitride film, cylindrical magnetron sputtering equipment was used with a hollow target of 10 mm in diameter and 370 mm long. The inside of the target has 8 magnetron cores. In the sputtering procedure, there are 8 arcs which can make the films uniformity above the target plane. Ti filament was wrapped on the 8 arcs of the target to dope the copper nitride films. The Ti-doped content was controlled by the number of Ti rings. The substrate was placed parallel to the target surface at a distance of 130 mm. The working gas was 99.99% pure nitrogen and 99.999% pure argon. Their flow rates were adjusted by independent mass-flow controllers. In the deposition procedure, the nitrogen was set at 40 sccm and 10 sccm for argon for all samples. Common
X.Y. Fan et al. / Journal of Alloys and Compounds 440 (2007) 254–258
255
glass (CAT. NO.7101) sheets with a size of 30 mm × 100 mm were used as substrates. Before deposition, they were cleaned by ultrasonic waves in acetone for 15 min. The chamber vacuum before deposition was 2 × 10−3 to 2.5 × 10−3 Pa. Pre-sputtering in pure argon was processed for 3 min in order to clear the target surface and the substrate. At room temperature, all samples were deposited with sputtering power of 1000 W, sputtering pressure of 8.5 × 10−1 Pa and the deposition time was 20 min. The Ti-doped contents in all Cu3 N films are investigated by electron probe microanalysis (EPMA) of EPMA-1600. The structure is characterized by X-ray diffraction (XRD, Cu K␣, Philips X Pert Pro.). The morphology is studied in air by HITACHI s-4800 FESEM equipment. Four-probe method is employed to measure electrical resistivity in air at room temperature. The optical properties are measured by UV–VIS spectrometer.
3. Results and discussion The Ti-doped content in all Cu3 N films are investigated by electron probe microanalysis (EPMA). The Ti-doped content and other chemical compositions of Cu3 N films are shown in Table 1. The XRD spectra shown in Fig. 1 exhibit the Ti-doped effect on the texture of Cu3 N films. First of all, it can be confirmed that all the films are composed of Cu3 N crystallites with antiReO3 structure, and no Cu peaks in the undoped Cu3 N film and especially no TiN peaks in Cu3 N films with different Ti-doped content can be found. It is obvious that the growth trend of the Cu3 N films is affected by the doping of Ti. As shown in Fig. 1(a), the preferred orientation of the undoped film is [1 1 1] which corresponds to the Cu-rich plane of (1 1 1). With a few Ti-doped content introducing, the texture of the film is influenced significantly. Fig. 1(b) shows that the film with Ti-doped content of 0.136 mol% has no preferred orientation. Clearly, the preferred growth of Nrich plane (1 0 0) occurs in Sample C with Ti-doped content of 0.232 mol% and this trend becomes more intensive as the Tidoped content increases, which is seen from the XRD spectra of Samples D and E with Ti-doped content of 0.59 and 1.066 mol% in Fig. 1(d and e). In conclusion, the doping of Ti in the Cu3 N films affects the texture of the film significantly. This is because the Ti atoms deposited on the substrate prevent the Cu atoms from diffusing. This makes sufficient nitrogen reactive with copper. If the Ti content reaches a given value (0.232 mol%), the preferred orientation will change from (1 1 1) to (1 0 0). In addition, the slight reduction of the Cu sputtering rate caused by the increase of the number of the Ti rings on the Cu target may be another reason. The changes may also occur under the combined action of the two effects. Compared with the undoped Cu3 N film, the FWHM of Peak [1 1 1] for the film with Ti-doped content of 0.136 mol% becomes narrow suddenly from 0.8663 to 0.5609, and that of Peak [1 0 0] from 1.1519 to 0.4803. This implies that the dop-
Fig. 1. XRD spectra of Cu3 N films undoped and with various Ti-doped contents.
Table 1 Chemical composition in all samples Sample
A
B
C
D
E
Ti (mol%) Cu (mol%) N (mol%)
0 84.316 15.684
0.136 83.22 16.644
0.232 83.050 16.718
0.59 81.401 18.009
1.066 80.144 19.79
Fig. 2. The mean grain size of Cu3 N films deposited with various Ti-doped content.
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ing of Ti improved the crystalline quality of Cu3 N films which can be further enhanced by increasing the Ti-doped content in Cu3 N films. It is found that the position of Peak [1 0 0] divagates from 23.04◦ for the undoped film to 23.32◦ by increasing the Ti-doped content to 1.066 mol%. For Peak [1 1 1], it is from 47.51◦ to 47.62◦ . The mean grain size of the films is estimated by the Debye–Scherrer formula [19] and the results are shown in Fig. 2.
The mean size of grains is in nanoscale and increases from 8.50 to 19.11 nm with increasing the Ti-doped content. The results conflict with those of FE-SEM, which is interpreted later. Fig. 3 shows the FE-SEM images of Cu3 N films with different Ti-doped content. The surface of undoped Cu3 N film is composed of pyramid-like grains, as seen from Fig. 3a. With a small quantity of Ti doping into the film, the surface morphology changes noticeably. The crystalline grains vary from pyramid-like to rugby-like as shown in Fig. 3b. While the Tidoped content in Cu3 N film reaches 1.066 mol%, it shows a spherical-like morphology, which is reported by Pierson [20] as nodular-like morphology. The variation of the surface morphology is due to the fact that Ti doping changes the preferred orientation of the films. The un-doped film preferring to grow along the [1 1 1] orientation shows a pyramid-like morphology while the Ti-doped film preferring to grow along the [1 0 0] orientation shows a spherical-like morphology. Here, we confirm that the surface morphology is correlated with the film texture, which was reported previously. Increasing the Ti-doped content, the films become more and more compact, and the mean grain size changes from 130–150 to 50–80 nm with Ti-doped content increasing from 0 to 1.066 mol%. Compared with the results of average grain size calculated from the Debye–Scherrer formula, it is much bigger. This is because: (1) the Debye–Scherrer formula that relies on translational symmetry assumption fails to give even a rough estimation of the size for nanocrystals and (2) each particle of the FE-SEM image contains many single-crystal grains. And the variation of the average grain size dependence of the Ti-doped content is just the opposite, which results from the weakened crystalline grains’ agglomeration. The electric resistivity of the film was measured by four-point probe techniques. Fig. 4 shows the Ti-doped content dependence of electrical resistivity. The electrical resistivity of the undoped Cu3 N film is 1.63 × 10−3 cm. The pure stoichiometric Cu3 N film is insulated; the lattice constant is 0.3817 nm, while the asprepared undoped Cu3 N film is a semiconductor and the lattice constant is 0.3835 nm. Due to the expansion of a lattice constant
Fig. 3. FE-SEM images of Cu3 N films with different content of Ti-doped: (a) Sample A; (b) Sample B; (c) Sample E.
Fig. 4. Plot of electrical resistivity of the Cu3 N films as a function of Ti doping content.
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257
Fig. 5. Plot of (Ahν)1/2 vs. hν of Samples A and E.
and the decrease of electrical resistivity, it is confirmed that Cu atoms insert into the body of electrical lattice. The inserted Cu atom becomes a donor which releases a free electron as a carrier. While the film is doped with a few Ti atoms of 0.136 mol%, the electrical resistivity increases rapidly to 4.464 cm. With increasing the Ti content, the electrical resistivity has no substantial variations. The electrical resistivity of the doped film with 1.066 mol% Ti is only 9.123 cm. The changes of electrical resistivity suggest that all the Ti atoms in the film do not contribute to dopants. The Ti atoms do not insert into the body of the lattice or substitute for the Cu atoms, but segregate at the grain boundary. The rapid increase in electrical resistivity of Cu3 N films doped with Ti is probably due to a decrease in the mobility and carrier concentration by the increased grain boundary barrier [21]. According to the band structure of copper nitride calculated by Ma Guadalupe Moreno-Armenta, etc. and Hahn and Weber, copper nitride films are indirect transition-type semiconductors. The optical band gap (Eg ) of the film can be obtained by plotting ( αhν)1/2 versus hν (α is the absorption coefficient and hν is the photon energy). The photon energy at the point where (αhν)1/2 = 0 is Eg . The Eg value is determined by extrapolation method. Here, we obtained absorbency A. A is defined as A = αρd for the solid thin film as normal incidence, where ρ is the density of the film and d is the thickness of the film. Because ρ and d are constants for the same film, the curve of
(Ahν)1/2 ∼hν is equivalent to that of (αhν)1/2 ∼hν. So we can obtain Eg by extrapolating the full line of the curve (Ahν)1/2 ∼hν to the abscissa. Fig. 5 shows the (Ahν)1/2 versus hν of Samples A and E. As seen from Fig. 6, the absorption edge shifted to a shorter wavelength region with increasing Ti content. That is, the band gap widens (blue shift) with increasing Ti content, which is consistent with the variation of electrical resistivity. This broadening effect can be understood based on the Burstein [22] effect. Burstein pointed out that an increase in the Fermi level in the conduction band of degenerate semiconductors leads to widening of the energy band (blue shift Eg ) which is related to carrier concentration. Further study on the relation between Eg and carrier concentration is needed. 4. Conclusion High quality Ti-doped copper nitride films were prepared on glass substrates by cylindrical magnetron sputtering at room temperature. The preferred orientation of undoped Cu3 N film is [1 1 1]. By increasing the Ti-doped content, it converts to [1 0 0] orientation. Calculated by the Debye–Scherrer formula, the mean size of grains is in nanoscale and increases from 8.50 to 19.11 nm with increasing Ti-doped content. With the doping of Ti, the surface morphology changes noticeably from pyramid-like to spherical-like. The shape of the grains is correlated with the proffered orientation which is determined by the Ti-doped content. The doping of Ti results in the abrupt increase of electrical resistivity, which is probably due to a decrease in both the mobility and carrier concentration by the increased grain boundary barrier. The band gap widens (blue shift) with increasing Ti content. It is interpreted as an increase in the Fermi level in the conduction band of degenerate semiconductors. Acknowledgements
Fig. 6. The variation of optical bandgap as a function of Ti content.
This work was supported by the Nature Science Foundation of Gansu Province, People’s Republic of China. The project number is ZS021-A25-022-C. The authors would like to thank the Laboratory of Field Emission Scanning Electron Microscope (FE-SEM, Hitachi S-4800) platform of information material and technology innovation, Lanzhou University.
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References [1] T. Maruyama, T. Morishita, Appl. Phys. Lett. 69 (7) (1996) 890. [2] M. Asano, K. Umeda, A. Tasaki, Jpn. J. Appl. Phys. 29 (1990) 1985. [3] R. Cremer, M. Witthaut, D. Neuschutz, C. Trappe, M. Laurenzis, O. Winkler, H. Kurz, Mikrochim. Acta 133 (2000) 299. [4] T. Nosaka, M. Yoshitake, A. Okamoto, S. Ogawa, Y. Nakayama, Appl. Surf. Sci. 169–170 (2001) 358. [5] L. Maya, J. Vac. Sci. Technol. A11 (1993) 604. [6] L. Maya, Mater. Res. Soc. Symp. Proc. 282 (1993) 203. [7] T. Maruyama, T. Morishita, J. Appl. Phys. 78 (1995) 4104. [8] D.Y. Wang, N. Nakamine, Y. Hayashi, J. Vac. Sci. Technol. A16 (1998) 2084. [9] Z.Q. Liu, W.J. Wang, S. Chao, S.K. Zheng, Thin Solid Films 325 (1998) 55. [10] D.M. Borsa, S. Grachev, C. Presura, D.O. Boerma, Appl. Phys. Lett. 80 (2002) 1823. [11] N. Pereira, L. Dupont, J.M. Tarascon, L.C. Klein, G.G. Amatucci, J. Electrochem. Soc. 150 (2003) A1273.
[12] Y. Du, A.L. Ji, L.B. Ma, Y.Q. Wang, Z.X. Cao, J. Cryst. Growth 280 (2005) 490. [13] U. Zachwieja, H. Jacobs, J. Less-Common Met. 170 (1991) 185. [14] T. Nosaka, M. Yoshitake, A. Okamoto, S. Ogawa, Y. Nakayama, Thin Solid Films 348 (1999) 8–13. [15] Y. Hayashi, T. Ishikawa, D. Shimokawa, J. Alloys Comp. 330–332 (2002) 348–351. [16] J. Blucher, K. Bang, Mater. Sci. Eng. A117 (1989) L1–L3. [17] U. Hahn, W. Weber, Phys. Rev. B 53 (1996) 12684. [18] M.G. Moreno-Armenta, A. Mart´ınez-Ruiz, N. Takeuchi, Solid State Sci. 6 (2004) 9–14. [19] P. Klug, L.E. Alexander (Eds.), X-Ray Diffraction Procedures for Polycrystalline and Amorphous Materials, second ed., Wiley, New York, 1974, pp. 687–780. [20] J.F. Pierson, Vacuum 66 (2002) 59–64. [21] K.C. Park, D.Y. Ma, K.H. Kim, Thin Solid Films 305 (1997) 201. [22] E. Burstein, Phys. Rev. 93 (1954) 632.