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Laser annealing effects on the structural, optical and magnetic properties of sputtered Zn0.95Co0.05O thin film
Materials Science and Engineering B 145 (2007) 6–10
Laser annealing effects on the structural, optical and magnetic properties of sputtered Zn0.95Co0.05O thin film N. Brihi ∗ , Z. Takkouk, A. Bouaine Laboratory of Materials Study, Faculty of Science, Jijel University, 18000 Jijel, Algeria Received 23 May 2007; received in revised form 11 August 2007; accepted 18 August 2007
Abstract We have used reactive magnetron co-sputtering to grow Zn0.95 Co0.05 O dilute magnetic semiconductor (DMS) on 400 nm thick of (0 0 0 1) Al2 O3 substrate at 600 ◦ C. After deposition the films were irradiated by ArF excimer laser (λ = 193 nm, τ = 20 ns) source, with energy density equal to 0.4 J/cm2 for two shoots. The effects of laser annealing on structural, optical and magnetic properties of Zn095 Co0.05 O thin films on Al2 O3 substrate were investigated using X-ray diffraction (XRD), IR absorption spectra, Raman spectroscopy, atomic force microscopy (AFM) and SQUID magnetometry. XRD analysis showed that the annealed film is textured with c-axis of the wurtzite structure along the growth direction. The IR absorption spectra of the sample showed three absorptions peaks before and after laser annealing due to the transitions between the crystal-field-split 3d levels of tetrahedral Co2+ substituting Zn2+ ions. Raman spectra showed the appearance of an additional mode which is an indicator for incorporation of Co2+ ions into the ZnO matrix. The roughness Rms value of the film evaluated by AFM decreased after laser annealing indicates a good surface topography. Room temperature magnetization measurements performed by SQUID magnetometry showed the presence of ferromagnetism with a saturation magnetization increased after ArF excimer laser annealing. © 2007 Elsevier B.V. All rights reserved. Keywords: Co-doped ZnO; Spintronics; Thin films; Excimer laser annealing; Ferromagnetism
1. Introduction In the last years some researchers have come to realize that diluted magnetic semiconductors provide an opportunity to integrate new functionality into the existing semiconductor devices. These materials are promising for spintronics because they possessing two interesting properties; semi conductivity and ferromagnetism [1,2]. The II–VI semiconductor compounds have many novel properties from both fundamental and technological point of view and have been the object of extensive research. Zinc oxide (ZnO) is a II–VI semiconductor compound which has been identified as a promising host material for the realization of diluted magnetic semiconductor, with both a high Tc (above room temperature) and a large magnetization [3,4]. Several groups have obtained ferromagnetic ZnCoO films at room temperature by
∗
Corresponding author. Tel.: +213 34 47 48 97; fax: +213 34 47 48 97. E-mail address: nourb [email protected] (N. Brihi).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.08.009
using either pulsed laser deposition (PLD) [5,6] or sputtering [7]. The Films grown by molecular beam epitaxy (MBE) exhibit ferromagnetism below 4 K [8]. Analysis of the magnetic measurements in thin films is difficult, because of the small magnetic moment and to the large diamagnetic contribution of the substrate. Although several theoretical calculations have pointed out possible ferromagnetism with high Curie temperature [9]. This has been supported by ab initio calculations based on the local density approximation on ferromagnetic ZnO based semiconductors [10]. This has stimulated numerous experimental works to grown ZnO semiconductors doped with ferromagnetic transition metal. Previous works [11–13] have shown that it is possible to locally modify the environment of atoms by light mass and low energy ion irradiation. We have thus used laser irradiation on ZnCoO sputtered magnetic semiconductor to show the change of the magnetic behaviour. The basic principle of laser crystallization is the melting of the semiconductor for a very short time and the recrystalization of it after this instance. Excimer laser crystallization semiconductor thin
N. Brihi et al. / Materials Science and Engineering B 145 (2007) 6–10
7
Fig. 1. Room temperature X-ray diffraction of Zn0.95 Co0.05 O thin film prepared at 600 ◦ C before and after laser annealing.
films on foreign substrates divides into three transformation regimes with respect to the applied laser fluency. The low and high energy density regimes and the so-called super lateral growth (SLG) regime which is resides between those two [14]. The properties of Zn0.95 Co0.05 O thin films, annealed by ArF excimer laser, such as structural, optical and magnetic property were investigated and contrasted to that of the as-grown. In this paper, we report the variations of structural, optical and magnetic properties of Zn0.95 Co0.05 O films in terms of the excimer laser annealing and suggest the utility of laser annealing of the change of various properties of Zn0.95 Co0.05 O thin films. 2. Experiment Zn0.95 Co0.05 O thin films were deposited on Al2 O3 (0 0 0 1) substrates by reactive magnetron Co-Sputtering using pure Zn and Co targets. The base pressure was under 10−7 Torr and the working pressure was a mixture of argon at 5 × 10−3 Torr and oxygen at 1.5 × 10−3 Torr. The thickness of the films was 400 nm. The composition of the film was controlled by adjusting the sputtering power applied of the Co (92 W) and Zn (50 W) targets. During the deposition, the substrate was kept at a constant temperature around 600 ◦ C. After deposition, the sample was slowly cooled down to room temperature and irradiated by ArF excimer laser (λ = 193 nm, τ = 20 ns) source, with energy density equal to 0.4 J/cm2 for two shooting. The structural properties was characterised by X-ray diffraction using a Siemens diffractometer (D 500) through (θ–2θ) configuration with a monochromatic Co K␣1 line radiation at 0.1789 nm. Reflectance optical measurements have been performed at room temperature using a conventional UV–vis–NIR spectrometer. Raman spectra were obtained at room temperature. The surface morphologies of the films are observed by AFM. The magnetic properties of our films were investigated using a superconducting quantum interference magnetometer (SQUID). The irradiating energy density of 0.4 J/cm2 is comprised between the thresholds for surface melting (ESM ) and complete melting (ECM ) of the Zn0.95 Co0.05 O film.
3. Results and discussion Fig. 1 shows XRD measurement results of Zn0.95 Co0.05 O films. The XRD patterns reveal a good crystalline quality of the films. The peaks at about ∼40◦ and 86◦ , corresponding to (0 0 0 2) and (0 0 0 4) planes, respectively, characterise the hexagonal ZnO wurtzite structure. This indicates that the films have c-axis preferential orientation. It can be seen two peaks at about 49◦ and 111◦ characteristic to the (0 0 0 6) Al2 O3 and (0 0 0 1 2) Al2 O3 substrate planes, respectively. No additional diffraction peaks have been appeared this may indicates the absence of the cobalt clusters or the presence of small clusters under the detection limit of the X-ray diffractometer. After ArF excimer laser irradiation the general behaviour of the XRD spectra remains unchanged. The intensity of the peaks is the highest before excimer laser irradiation which indicates that the crystalline quality of the films is very sensitive to the laser irradiation. The crystalline quality of Zn0.95 Co0.05 O films can be evaluated by the full width at half maximum (FWHM) of (0 0 0 2) peak at about 2θ = 40◦ . The FWHM of (0 0 0 2) peak for the annealed sample is greater than that for the as deposited one (0.67◦ after laser annealing and 0.54◦ for as-grown film)
Fig. 2. Room temperature optical transmittance spectrum of Zn0.95 Co0.05 O, before and after laser annealing.
8
N. Brihi et al. / Materials Science and Engineering B 145 (2007) 6–10
Fig. 3. Raman spectra of Zn0.95 Co0.05 O before and after laser annealing.
this may be attributed to the reduction of the grain size due to laser treatment. Moreover, the absence of any additional peak in the XRD patterns even after the rapid annealing is a first indication that there is no cobalt metallic cluster in our films under the detection limit of the X- ray diffraction apparatus. In order to be confident that Co2+ has been substituted for 2+ Zn , reflectance optical measurements have been performed at room temperature by conventional UV–vis–IR spectrometer. Fig. 2 shows the optical transmittance spectra of Zn0.95 Co0.05 O films before and after laser annealing. Three absorptions peaks are observed at 570, 615, 655 nm, which have been already explained as electronic transitions involving crystal-field- split 3d levels in Co2+ ions substituting Zn2+ ions. These transitions are assigned as dues to 3 (Co2+ ), T1 (P) ← A2 (F). This confirms that Co2+ substitutes for Zn2+ in the tetrahedral sites [12,15]. The transmittance is strongly reduced after laser irradiation (Fig. 2), this means that the crystalline quality of the layer film is sensitively reduced after the excimer laser annealing. Additional proof of the Co2+ substituted Zn2+ are given by means of Raman spectroscopy. ZnO is a semiconductor with wutrzite crystal structure. This structure belongs to the space group P63 mc with two formula units per primitive cell, where all atoms occupy C3 sites. The Zn2+ ions have a Td geometry with an irreducible representation [16]: = A1 + E + F2 .
Using the site method of Halford we find the optical phonon irreducible representation: opt = 2A1 + 2B1 + 2E1 + 2E2 where the A1 and E1 modes are polar and can split into transverse (TO) and longitudinal (LO) optical phonon both being active in Raman and infrared spectroscopy. Non-polar E2 modes are Raman active only, while B1 modes are Raman inactive. Fig. 3 reports the room temperature Raman spectra of Zn0.95 Co0.05 O films without and with excimer laser irradiation. The spectral range actually investigated was between 300 and 650 cm−1 . The peak at about 437 cm−1 can be assigned to the vibration high mode of E2 (E2 mode). Furthermore, the observation of the E2 high phonon mode in all samples also indicates that the codoped ZnO film still keeps the hexagonal structure of the crystal which is consistent with the XRD results. We can be assigned the Raman band at 402 cm−1 to the TO mode with E1 symmetry and the A1 (LO) at 575 cm−1 mode has not yet been observed in our thin films, likely because it is subsumed by the strong substrate mode [17]. Du et al. [18] have observed some additional vibration modes for different doping, suggested that these modes are related to defect-induced modes and can be used an indicator to check the doping incorporation in the host matrix. The vibration mode at 642 cm−1 (shifts after laser annealing to 636 cm−1 ) can be associated with impurities [19]. The peak at 602 cm−1 appeared before and after laser annealing, can be
Fig. 4. Atomic force microscopy image of Zn0.95 Co0.05 O before and after laser annealing.
N. Brihi et al. / Materials Science and Engineering B 145 (2007) 6–10
attributed to defects due to oxygen distribution in our films. Most major peaks observed here should be assigned to sapphire substrate signals. After excimer laser irradiation, the intensities of the Raman peaks increased due to the decreasing of the surface effect. Fig. 4 shows the surface morphologies of Zn0.95 Co0.05 O films before and after laser irradiation observed by Atomic Force Microscopy (AFM). The surface morphology of these samples was dramatically changed after laser annealing, which indicates that this surface is sensible against the ArF excimer laser annealing. The root-mean-square (Rms ) values for the surfaces are 6.34 and 4.32 nm before and after laser irradiation, respectively. This suggest that the film annealed by laser show better surface morphology compared with that of the as-grown. Magnetization curves M(H) (Fig. 5) of Zn0.95 Co0.05 O thin film before and after laser annealing at room temperature (symbols) are measured. The contribution from the substrate is subtracted from the raw data. All the curves are a typical feature of ferromagnetic behaviour, with small coercive field and small remanence. The saturation magnetization (0.46 emu/Co before laser annealing) increased after laser annealing (0.68 emu/Co), due to the increasing of donors concentration (VO or Zni ) [20]. Electrical measurements showed n-type Zn0.95 Co0.05 O with a donor’s concentration of about 1.7 × 1017 cm−3 , increased to 2.3 × 1018 cm−3 after laser annealing. Ferromagnetism in DMS is due to the exchange interaction between free delocalized carriers (hole or electrons in the valence band) and the localized d spin on Co2+ ions [7]. We note that the concentration of Co ions is greater than the concentration of the free carriers (electrons) this suggests that some Co atoms are not active in ferromagnetism. The presence of free carriers and ferromagnetic elements (F.E.) are a necessary condition for the appearance of ferromagnetism and the enhanced one of the two conditions increased the ferromagnetic behaviour. Fig. 6 shows magnetization M versus temperature (T) curves for Zn0.95 Co0.05 O thin film before and after excimer laser irradiation, between 5 and 350 K, where H = 100 Oe magnetic field was applied. In this range of temperature the films showed ferromagnetic character. After laser irradiation the magnetization was increased. This
9
Fig. 6. Magnetization as a function temperature of Zn0.95 Co0.05 O before and after laser annealing, from 5 to 350 K.
indicates that the ferromagnetism is more important in the film subjected to laser irradiation than the as-grown one. On the light of these results, we can conclude that the introduction of defects, affects the magnetic properties of Zn0.95 Co0.05 O thin film, indicates a directly relationship between a structural property and a magnetic property. 4. Conclusion We have shown the laser annealing induced defects in ZnO dilute magnetic semiconductor leads to change the structural, optical and magnetic properties of the film. The rutile structure of ZnO was confirmed by X-ray diffraction, and no presence of metallic cobalt or other impurity phase under the limit detection of the spectrometer. Raman and UV–vis–NIR have been demonstrated the presence of Co2+ in substitution of Zn2+ before and after excimer laser annealing. The surface morphology of the film observed by AFM was changed by laser irradiation. Nevertheless, the magnetization measurements have shown the presence of ferromagnetism at room temperature for Zn0.95 Co0.05 O dilute magnetic semiconductor grown by reactive co-sputtering technique before and after excimer laser annealing. The annealing can increase the magnetization in Zn0.95 Co0.05 O DMS film. References
Fig. 5. Room temperature magnetization curves versus field of Zn0.95 Co0.05 O before and after laser annealing.
[1] S.J. Pearton, C.R. Abernathy, M.E. Overberg, G.T. Thaler, D.P. Norton, N. Theodoropoulou, A.F. Hebard, Y.D. Park, F. Ren, J. Kim, L.A. Boatner, J. Appl. Phys. 93 (2003) 1. [2] Z. Yin, N. Chen, F. Yang, S. Song, C. Chai, J. Zhong, H. Qian, K. Ibrahim, Solid State Commun. 135 (2005) 430. [3] P. Sharma, A. Gupta, K.V. Rao, F.J. Owens, R. Sharma, R. Ahuja, J.M. Osorio Guillen, G.A. Ghring, Nat. Mater. 2 (2003) 673. [4] J.J. Wu, S.C. Liu, M.H. Yang, Appl. Phys. Lett. 85 (2004) 1027. [5] K. Rode, A. Anane, R. Mattane, J.P. Contour, O. Durand, R. Lebourgeois, Appl. Phys. Lett. 93 (2003) 7676. [6] S. Ramachandran, A. Tiwari, J. Narayan, Appl. Phys. Lett 84 (2004) 5255.
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[7] A. Dinia, G. Schmerber, V. Pierron-bohnes, C. M´eny, P. Panissod, E. Beaurepaire, J. Magn. Magn. Mater 286 (2005) 37. [8] Z. Jin, T. Fukumura, M. Kawasali, K. Ando, H. Saito, T. Sekiguchi, Y.Z. Yoo, M. Murakami, Y. Matsumoto, T. Hasegawa, H. Koinuma, Appl. Phys. Lett. 78 (2001) 3824. [9] T. Dielt, H. Ohno, F. Matsukura, J. Cibert, D. Ferrand, Science 287 (2000) 1019. [10] K. Sato, H. Katayama-Yoshida, Jpn. J. Appl. Phys. 39 (2000) L555. [11] M. Abes, O. Ersen, D. Muller, M. Acosta, C. Ulhaq-Bouillet, A. Dinia, V. Pierron-Bohnes, Mater. Sci. Eng. C 23 (2002) 229. [12] P. Koidl, Phys. Rev. B 15 (1977) 2493. [13] A. Dinia, J.P. Ayoub, G. Schmerber, E. Beaurepaire, D. Muller, J.J. Grob, Phys. Lett. A 333 (2004) 152. [14] J.S. Im, H.J. Kim, M.O. Thompson, Appl. Phys. Lett. 63 (1993) 1969.
[15] M. Bouloudenine, N. Viart, S. Colis, A. Dinia, Chem. Phys. Lett. 397 (2004) 73. [16] C.A. Arguello, D.L. Rousseau, S.P.S. Porto, Phys. Rev. 181 (1969) 1351. [17] N. Ashkenov, B.N. Mbenkum, C. Bundesmann, V. Riede, M. Lorentz, D. Spemann, E.M. Kaidashev, A. Kasic, M. Schubert, M. Grundmann, G. Wagner, H. Neumann, V. Darakchieva, H. Arwin, B. Monemar, J. Appl. Phys. 93 (2003) 126. [18] C.L. Du, Z.B. Gu, M.H. Lu, J. Wang, S.T. Zhang, J. Zhao, G.X. Cheng, H. Heng, Y.F. Chen, J. Appl. Phys. 99 (2006) 123515. [19] N. Hasuike, H. Fukumura, H. Harima, K. Kisoda, H. Matsui, H. Saeki, H. Tabata, J. Phys.: Condens. Matter 16 (2004) 5807. [20] N. Khare, M.J. Kappers, M. Wie, M.G. Blamire, J.L. MacManus-Driscoll, Adv. Mater. 18 (2006) 1449.
Laser annealing effects on the structural, optical and magnetic properties of sputtered Zn0.95Co0.05O thin film N. Brihi ∗ , Z. Takkouk, A. Bouaine Laboratory of Materials Study, Faculty of Science, Jijel University, 18000 Jijel, Algeria Received 23 May 2007; received in revised form 11 August 2007; accepted 18 August 2007
Abstract We have used reactive magnetron co-sputtering to grow Zn0.95 Co0.05 O dilute magnetic semiconductor (DMS) on 400 nm thick of (0 0 0 1) Al2 O3 substrate at 600 ◦ C. After deposition the films were irradiated by ArF excimer laser (λ = 193 nm, τ = 20 ns) source, with energy density equal to 0.4 J/cm2 for two shoots. The effects of laser annealing on structural, optical and magnetic properties of Zn095 Co0.05 O thin films on Al2 O3 substrate were investigated using X-ray diffraction (XRD), IR absorption spectra, Raman spectroscopy, atomic force microscopy (AFM) and SQUID magnetometry. XRD analysis showed that the annealed film is textured with c-axis of the wurtzite structure along the growth direction. The IR absorption spectra of the sample showed three absorptions peaks before and after laser annealing due to the transitions between the crystal-field-split 3d levels of tetrahedral Co2+ substituting Zn2+ ions. Raman spectra showed the appearance of an additional mode which is an indicator for incorporation of Co2+ ions into the ZnO matrix. The roughness Rms value of the film evaluated by AFM decreased after laser annealing indicates a good surface topography. Room temperature magnetization measurements performed by SQUID magnetometry showed the presence of ferromagnetism with a saturation magnetization increased after ArF excimer laser annealing. © 2007 Elsevier B.V. All rights reserved. Keywords: Co-doped ZnO; Spintronics; Thin films; Excimer laser annealing; Ferromagnetism
1. Introduction In the last years some researchers have come to realize that diluted magnetic semiconductors provide an opportunity to integrate new functionality into the existing semiconductor devices. These materials are promising for spintronics because they possessing two interesting properties; semi conductivity and ferromagnetism [1,2]. The II–VI semiconductor compounds have many novel properties from both fundamental and technological point of view and have been the object of extensive research. Zinc oxide (ZnO) is a II–VI semiconductor compound which has been identified as a promising host material for the realization of diluted magnetic semiconductor, with both a high Tc (above room temperature) and a large magnetization [3,4]. Several groups have obtained ferromagnetic ZnCoO films at room temperature by
∗
Corresponding author. Tel.: +213 34 47 48 97; fax: +213 34 47 48 97. E-mail address: nourb [email protected] (N. Brihi).
0921-5107/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2007.08.009
using either pulsed laser deposition (PLD) [5,6] or sputtering [7]. The Films grown by molecular beam epitaxy (MBE) exhibit ferromagnetism below 4 K [8]. Analysis of the magnetic measurements in thin films is difficult, because of the small magnetic moment and to the large diamagnetic contribution of the substrate. Although several theoretical calculations have pointed out possible ferromagnetism with high Curie temperature [9]. This has been supported by ab initio calculations based on the local density approximation on ferromagnetic ZnO based semiconductors [10]. This has stimulated numerous experimental works to grown ZnO semiconductors doped with ferromagnetic transition metal. Previous works [11–13] have shown that it is possible to locally modify the environment of atoms by light mass and low energy ion irradiation. We have thus used laser irradiation on ZnCoO sputtered magnetic semiconductor to show the change of the magnetic behaviour. The basic principle of laser crystallization is the melting of the semiconductor for a very short time and the recrystalization of it after this instance. Excimer laser crystallization semiconductor thin
N. Brihi et al. / Materials Science and Engineering B 145 (2007) 6–10
7
Fig. 1. Room temperature X-ray diffraction of Zn0.95 Co0.05 O thin film prepared at 600 ◦ C before and after laser annealing.
films on foreign substrates divides into three transformation regimes with respect to the applied laser fluency. The low and high energy density regimes and the so-called super lateral growth (SLG) regime which is resides between those two [14]. The properties of Zn0.95 Co0.05 O thin films, annealed by ArF excimer laser, such as structural, optical and magnetic property were investigated and contrasted to that of the as-grown. In this paper, we report the variations of structural, optical and magnetic properties of Zn0.95 Co0.05 O films in terms of the excimer laser annealing and suggest the utility of laser annealing of the change of various properties of Zn0.95 Co0.05 O thin films. 2. Experiment Zn0.95 Co0.05 O thin films were deposited on Al2 O3 (0 0 0 1) substrates by reactive magnetron Co-Sputtering using pure Zn and Co targets. The base pressure was under 10−7 Torr and the working pressure was a mixture of argon at 5 × 10−3 Torr and oxygen at 1.5 × 10−3 Torr. The thickness of the films was 400 nm. The composition of the film was controlled by adjusting the sputtering power applied of the Co (92 W) and Zn (50 W) targets. During the deposition, the substrate was kept at a constant temperature around 600 ◦ C. After deposition, the sample was slowly cooled down to room temperature and irradiated by ArF excimer laser (λ = 193 nm, τ = 20 ns) source, with energy density equal to 0.4 J/cm2 for two shooting. The structural properties was characterised by X-ray diffraction using a Siemens diffractometer (D 500) through (θ–2θ) configuration with a monochromatic Co K␣1 line radiation at 0.1789 nm. Reflectance optical measurements have been performed at room temperature using a conventional UV–vis–NIR spectrometer. Raman spectra were obtained at room temperature. The surface morphologies of the films are observed by AFM. The magnetic properties of our films were investigated using a superconducting quantum interference magnetometer (SQUID). The irradiating energy density of 0.4 J/cm2 is comprised between the thresholds for surface melting (ESM ) and complete melting (ECM ) of the Zn0.95 Co0.05 O film.
3. Results and discussion Fig. 1 shows XRD measurement results of Zn0.95 Co0.05 O films. The XRD patterns reveal a good crystalline quality of the films. The peaks at about ∼40◦ and 86◦ , corresponding to (0 0 0 2) and (0 0 0 4) planes, respectively, characterise the hexagonal ZnO wurtzite structure. This indicates that the films have c-axis preferential orientation. It can be seen two peaks at about 49◦ and 111◦ characteristic to the (0 0 0 6) Al2 O3 and (0 0 0 1 2) Al2 O3 substrate planes, respectively. No additional diffraction peaks have been appeared this may indicates the absence of the cobalt clusters or the presence of small clusters under the detection limit of the X-ray diffractometer. After ArF excimer laser irradiation the general behaviour of the XRD spectra remains unchanged. The intensity of the peaks is the highest before excimer laser irradiation which indicates that the crystalline quality of the films is very sensitive to the laser irradiation. The crystalline quality of Zn0.95 Co0.05 O films can be evaluated by the full width at half maximum (FWHM) of (0 0 0 2) peak at about 2θ = 40◦ . The FWHM of (0 0 0 2) peak for the annealed sample is greater than that for the as deposited one (0.67◦ after laser annealing and 0.54◦ for as-grown film)
Fig. 2. Room temperature optical transmittance spectrum of Zn0.95 Co0.05 O, before and after laser annealing.
8
N. Brihi et al. / Materials Science and Engineering B 145 (2007) 6–10
Fig. 3. Raman spectra of Zn0.95 Co0.05 O before and after laser annealing.
this may be attributed to the reduction of the grain size due to laser treatment. Moreover, the absence of any additional peak in the XRD patterns even after the rapid annealing is a first indication that there is no cobalt metallic cluster in our films under the detection limit of the X- ray diffraction apparatus. In order to be confident that Co2+ has been substituted for 2+ Zn , reflectance optical measurements have been performed at room temperature by conventional UV–vis–IR spectrometer. Fig. 2 shows the optical transmittance spectra of Zn0.95 Co0.05 O films before and after laser annealing. Three absorptions peaks are observed at 570, 615, 655 nm, which have been already explained as electronic transitions involving crystal-field- split 3d levels in Co2+ ions substituting Zn2+ ions. These transitions are assigned as dues to 3 (Co2+ ), T1 (P) ← A2 (F). This confirms that Co2+ substitutes for Zn2+ in the tetrahedral sites [12,15]. The transmittance is strongly reduced after laser irradiation (Fig. 2), this means that the crystalline quality of the layer film is sensitively reduced after the excimer laser annealing. Additional proof of the Co2+ substituted Zn2+ are given by means of Raman spectroscopy. ZnO is a semiconductor with wutrzite crystal structure. This structure belongs to the space group P63 mc with two formula units per primitive cell, where all atoms occupy C3 sites. The Zn2+ ions have a Td geometry with an irreducible representation [16]: = A1 + E + F2 .
Using the site method of Halford we find the optical phonon irreducible representation: opt = 2A1 + 2B1 + 2E1 + 2E2 where the A1 and E1 modes are polar and can split into transverse (TO) and longitudinal (LO) optical phonon both being active in Raman and infrared spectroscopy. Non-polar E2 modes are Raman active only, while B1 modes are Raman inactive. Fig. 3 reports the room temperature Raman spectra of Zn0.95 Co0.05 O films without and with excimer laser irradiation. The spectral range actually investigated was between 300 and 650 cm−1 . The peak at about 437 cm−1 can be assigned to the vibration high mode of E2 (E2 mode). Furthermore, the observation of the E2 high phonon mode in all samples also indicates that the codoped ZnO film still keeps the hexagonal structure of the crystal which is consistent with the XRD results. We can be assigned the Raman band at 402 cm−1 to the TO mode with E1 symmetry and the A1 (LO) at 575 cm−1 mode has not yet been observed in our thin films, likely because it is subsumed by the strong substrate mode [17]. Du et al. [18] have observed some additional vibration modes for different doping, suggested that these modes are related to defect-induced modes and can be used an indicator to check the doping incorporation in the host matrix. The vibration mode at 642 cm−1 (shifts after laser annealing to 636 cm−1 ) can be associated with impurities [19]. The peak at 602 cm−1 appeared before and after laser annealing, can be
Fig. 4. Atomic force microscopy image of Zn0.95 Co0.05 O before and after laser annealing.
N. Brihi et al. / Materials Science and Engineering B 145 (2007) 6–10
attributed to defects due to oxygen distribution in our films. Most major peaks observed here should be assigned to sapphire substrate signals. After excimer laser irradiation, the intensities of the Raman peaks increased due to the decreasing of the surface effect. Fig. 4 shows the surface morphologies of Zn0.95 Co0.05 O films before and after laser irradiation observed by Atomic Force Microscopy (AFM). The surface morphology of these samples was dramatically changed after laser annealing, which indicates that this surface is sensible against the ArF excimer laser annealing. The root-mean-square (Rms ) values for the surfaces are 6.34 and 4.32 nm before and after laser irradiation, respectively. This suggest that the film annealed by laser show better surface morphology compared with that of the as-grown. Magnetization curves M(H) (Fig. 5) of Zn0.95 Co0.05 O thin film before and after laser annealing at room temperature (symbols) are measured. The contribution from the substrate is subtracted from the raw data. All the curves are a typical feature of ferromagnetic behaviour, with small coercive field and small remanence. The saturation magnetization (0.46 emu/Co before laser annealing) increased after laser annealing (0.68 emu/Co), due to the increasing of donors concentration (VO or Zni ) [20]. Electrical measurements showed n-type Zn0.95 Co0.05 O with a donor’s concentration of about 1.7 × 1017 cm−3 , increased to 2.3 × 1018 cm−3 after laser annealing. Ferromagnetism in DMS is due to the exchange interaction between free delocalized carriers (hole or electrons in the valence band) and the localized d spin on Co2+ ions [7]. We note that the concentration of Co ions is greater than the concentration of the free carriers (electrons) this suggests that some Co atoms are not active in ferromagnetism. The presence of free carriers and ferromagnetic elements (F.E.) are a necessary condition for the appearance of ferromagnetism and the enhanced one of the two conditions increased the ferromagnetic behaviour. Fig. 6 shows magnetization M versus temperature (T) curves for Zn0.95 Co0.05 O thin film before and after excimer laser irradiation, between 5 and 350 K, where H = 100 Oe magnetic field was applied. In this range of temperature the films showed ferromagnetic character. After laser irradiation the magnetization was increased. This
9
Fig. 6. Magnetization as a function temperature of Zn0.95 Co0.05 O before and after laser annealing, from 5 to 350 K.
indicates that the ferromagnetism is more important in the film subjected to laser irradiation than the as-grown one. On the light of these results, we can conclude that the introduction of defects, affects the magnetic properties of Zn0.95 Co0.05 O thin film, indicates a directly relationship between a structural property and a magnetic property. 4. Conclusion We have shown the laser annealing induced defects in ZnO dilute magnetic semiconductor leads to change the structural, optical and magnetic properties of the film. The rutile structure of ZnO was confirmed by X-ray diffraction, and no presence of metallic cobalt or other impurity phase under the limit detection of the spectrometer. Raman and UV–vis–NIR have been demonstrated the presence of Co2+ in substitution of Zn2+ before and after excimer laser annealing. The surface morphology of the film observed by AFM was changed by laser irradiation. Nevertheless, the magnetization measurements have shown the presence of ferromagnetism at room temperature for Zn0.95 Co0.05 O dilute magnetic semiconductor grown by reactive co-sputtering technique before and after excimer laser annealing. The annealing can increase the magnetization in Zn0.95 Co0.05 O DMS film. References
Fig. 5. Room temperature magnetization curves versus field of Zn0.95 Co0.05 O before and after laser annealing.
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