- Email: [email protected]
Mn-implanted, polycrystalline indium tin oxide and indium oxide films
Nuclear Instruments and Methods in Physics Research B 267 (2009) 1616–1619
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Mn-implanted, polycrystalline indium tin oxide and indium oxide films Camelia Scarlat a,*, Mykola Vinnichenko a, Qingyu Xu b, Danilo Bürger a, Shengqiang Zhou a, Andreas Kolitsch a, Jörg Grenzer a, Manfred Helm a, Heidemarie Schmidt a a b
Forschungszentrum Dresden-Rossendorf, Institut für Ionenstrahlphysik und Materialforschung, Bautzner Landstraße 128, 01328 Dresden, Germany Department of Physics, Southeast University, Nanjing 211189, China
a r t i c l e
i n f o
Article history: Available online 11 February 2009 PACS: 75.50.Pp 73.43.Qt 78.20.Ci Keywords: Diluted magnetic semiconductor ITO Magnetoresistance Optical constants
a b s t r a c t Polycrystalline conducting, ca. 250 nm thick indium tin oxide (ITO) and indium oxide (IO) films grown on SiO2/Si substrates using reactive magnetron sputtering, have been implanted with 1 and 5 at.% of Mn, followed by annealing in nitrogen or in vacuum. The effect of the post-growth treatment on the structural, electrical, magnetic, and optical properties has been studied. The roughness of implanted films ranges between 3 and 15 nm and XRD measurements revealed a polycrystalline structure. A positive MR has been observed for Mn-implanted and post-annealed ITO and IO films. It has been interpreted by considering s–d exchange. Spectroscopic ellipsometry has been used to prove the existence of midgap electronic states in the Mn-implanted ITO and IO films reducing the transmittance below 80%. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
Indium tin oxide (ITO) and indium oxide (IO) thin films exhibit low electrical resistivity, transparency in the visible spectral range, high infrared reflectivity and UV absorption. These properties make ITO and IO thin films good candidates for many technological applications, such as coatings in thermal collectors and mirrors as well as transparent electrodes in several electro-optical set-ups: solar cells and liquid crystal display devices [1]. Recently, hightemperature ferromagnetism has been reported in Mn doped ITO-based systems prepared by reactive thermal evaporation [2], pulsed laser deposition [3] or DC reactive sputtering [4]. This research was focused on the change of structural and magnetic properties upon Mn doping, while the effect of optical and magnetoresistance is rather rarely investigated. On the other hand, ion implantation as a versatile semiconductor-doping method has not been used for magnetic doping of ITO or IO. In this paper, we focus on the structural, optical and magneto-transport properties of Mn-implanted ITO and IO films, and discuss the effect of annealing at different temperatures and ambient pressure.
2.1. Sample preparation The ITO and IO thin films have been deposited by reactive pulsed (4.17 kHz) magnetron sputtering (MS) using two 2-in. unbalanced magnetrons operated in unipolar mode. MS is a low pressure process that permits large scale deposition of high quality films at high rates at 500 °C. Additional technical details are published elsewhere [5]. The ITO films were deposited on heated ntype Si (1 0 0) wafers covered with a 500 nm SiO2 layer, whereas the IO films were deposited on unheated n-type Si (1 0 0) wafers covered with a 250 nm SiO2 layer. The ITO and IO films were implanted with Mn+ ions with a special care taken in creating a homogeneous implantation depth profile. Therefore, three different ion energies (120, 60 and 20 keV) were chosen to produce a fairly uniform ion distribution. A set of samples was produced with Mn atomic concentrations of 1 and 5 at.%. Rapid thermal annealing (RTA) has been performed on implanted ITO and IO films at 650 °C for 10 s in N2. Two other Mn-implanted IO samples were annealed in vacuum (vacuum thermal annealing, VTA) (p = 1.6 106 mbar) at constant temperature of 200 °C for 1 h. 2.2. Characterization methods
* Corresponding author. E-mail address: [email protected] (C. Scarlat). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.158
The structure of as-prepared and ion implanted IO and ITO film has been studied by means of grazing incidence X-ray diffraction
C. Scarlat et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1616–1619
(XRD). A SIEMENS D5000 X-ray diffractometer using Cu Ka (k = 0.154 nm) radiation has been used for XRD. The size of coherently diffracting domains, or the crystallites size d of the films has been determined from the broadening of the corresponding XRD peaks using Scherrer’s formula (d = 0.9 k/b cos h, k = 0.15406 nm, b – peak width at half maximum height, h – Bragg angle). Using this parameter the grain size in the direction normal to the film surface may be evaluated. The ITO and IO films were characterized ex situ by spectroscopic ellipsometry (SE) measurements using M-2000 spectroscopic ellipsometer from J.A. Woolam Co. at an angle of incidence of 70°. The energy of the probing photons was in the range of 1.24–3.3 eV. The data were evaluated using WVASE software following a procedure described in more detail elsewhere [6,7]. The SE data analysis yields the film thickness, surface roughness, and optical constants. For this purpose, the film dielectric function was parameterized in the Drude–Lorentz approach [8], Eq. (1) and the SE data were fitted simultaneously with reflectance data: 1=2
e
¼
A1 A2 e1 þ 2 hxðhx þ iB1 Þ E2 þ ðhxÞ2 þ iB2 hx
1617
.
.
!1=2 ;
ð1Þ
where the parameters A2, B2, and E2 refer to the oscillator amplitude, damping constant, and resonant energy, respectively. They describe the contributions of interband transitions to the optical absorption. Here e1 is the high frequency dielectric constant, A1 ¼ ð hxp Þ2 and B1 ¼ ð hxs Þ are the squared plasma frequency and the relaxation frequency, respectively, expressed in units of energy. The latter two parameters represent the free electron contribution to the optical constants according to the Drude model. From A1 the free electron density, Ne, is obtained using known relations. Magnetotransport measurements with the field applied perpendicular to the film were performed in van de Pauw configuration and fields up to 6 T were applied at temperature 5 K.
.
3. Results and discussion 3.1. Structure Fig. 1(a) shows XRD patterns of unimplanted, as deposited film of IO, and 1 and 5 at.% Mn-implanted films after RTA. The XRD pattern of unimplanted IO (Fig. 1(a)) can be indexed based on the cubic bixbyite structure of In2O3. Both, for unimplanted and implanted IO films, the lattice parameter are close to that of bulk In2O3 (1.0117 nm [9]). Ion implantation itself and subsequent RTA lead to a decrease of the (4 0 0) and (4 4 0) XRD peak intensity in case of IO films. This indicates the deterioration of the film crystalline structure due to the implantation not being remedied by annealing. Fig. 1(b) shows the XRD patterns of an unimplanted, as deposited ITO film, and of 1 and 5 at.% Mn-implanted films after RTA. The lattice parameter of unimplanted and implanted ITO films amounts to 1.0060 nm being smaller than the lattice parameter of bulk In2O3. This is expected, if Sn ions substitutionally occupy the In sites [2]. The (2 2 2), (4 4 0), and (6 2 2) XRD peak amplitude of ITO (Fig. 1(b)) also decreases after implantation and RTA. Peaks corresponding to Mn oxides were only found for ITO 5% Mn, RTA. XRD yields the crystallites size d of 21 and 28 nm for virgin IO and ITO, respectively, in direction normal to the film surface. After implantation and RTA the crystallites size d for IO amounts to 18 nm (1 at.% Mn) and to 17 nm (5 at.% Mn) and for ITO it amounts to 20 nm (1 at.% Mn) and to 18 nm (5 at.% Mn). Transmission Electron Microscopy (TEM) [5,7] showed that the ITO films exhibit columnar crystalline structure with elongated crystals oriented normally to the film surface.
Fig. 1. XRD patterns of unimplanted (0 at.% Mn) and implanted (1 at.% Mn, 5 at.% Mn), RTA treated (a) IO films and (b) ITO films. d is the crystallites size.
3.2. Optical properties Spectroscopic ellipsometry data show that the implantation of 1 at.% of Mn into IO and subsequent VTA lead to a decrease of the film thickness from 233 to 182 nm. In the case of ITO implantation of the same amount of Mn followed by RTA a much less significant decrease of the film thickness from 122 to 113 nm has been observed. The decrease of the film thickness is due to the fact
1618
C. Scarlat et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1616–1619
that the crystalline size became smaller after implantation and annealing such that the films became more dense. The complex refractive index of the films was calculated as N ¼ ðeÞ1=2 , where e is determined according to the Eq. (1). The refractive indices (n) and extinction coefficients (k) for the unimplanted and implanted ITO and IO films are shown in Fig. 2. One can observe that n and k of ITO and IO significantly differ from each other. In case of unimplanted films, the refractive index of the ITO sample is lower than that of the unimplanted IO sample in the whole measured photon energy range, while the extinction coefficient of the ITO in the near IR is significantly higher than that of IO. It is explained by higher free electron absorption in ITO due to a higher free electron density which is of 9.9 1020 cm3 for ITO compared to 9.9 1018 cm3 for IO sample, according to Hall effect data. The lower carrier concentration in IO is also the reason for its higher refractive index compared to ITO that is consistent with other authors’ results [7,8]. The extinction coefficient k of ITO
a
1 at.% Mn and IO 1 at.% Mn-implanted and annealed samples increases compared to unimplanted ones. In case of IO film, ion implantation and VTA leads to a significant increase of the k values in the whole spectral range, although in the near IR it increases even stronger. The latter is in good agreement with an increase of the free electron concentration from 9.9 1018 to 1.9 1020 cm3 due to this treatment. In contrast, for ITO 1 at.% Mn implantation and RTA the free electron absorption does not increase in the near IR, although the optical absorption significantly increases in the visible and UV spectral range. The latter may be explained in terms of the defect states formation in the band gap. An increase of the refractive index values in ion implanted and annealed samples compared to untreated ones may be related to an increased packing density in the films [10] due to the treatment. A higher packing density has also been probed by XRD (Section 3.1). A better understanding of the optical properties modification of ITO and IO requires further research which is in progress now.
2.2
a
2.0
.
1.8
.
1.6
.
1.4
1.2 1.0
.
.
1.5
2.0
2.5
3.0
3.5
b
b
0.15
0.10
0.05
0.00 1.0
1.5
2.0
2.5
3.0
3.5
Fig. 2. (a) Refractive index n and (b) extinction coefficient k of unimplanted and implanted IO and ITO films.
Fig. 3. 5 K magnetoresistance versus magnetic field (open symbols) for (a) IO 1 at.% Mn, RTA and (b) IO 1 at.% Mn, VTA 5 K. The solid lines are the fitting curves. l is mobility, q is resistivity and n is electron concentration.
C. Scarlat et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1616–1619
3.3. Magnetotransport properties Negative MR was observed in unimplanted ITO and IO films. A large positive MR at 5 K and low field has been observed in implanted IO films. Also the MR in magnetic ITO and IO films depends on the electron concentration [11,12]. In this paper, we extend the modeling of s–d spin splitting on the disorder-modified electron– electron interaction to explain the positive MR in Mn-implanted ITO and IO. The type of the conducting carriers was confirmed to be n-type by Hall measurements for all samples at 5 K. The product of the Fermi wave vector kF and mean free path l can be calculated hð3p2 Þ2=3 =ðe2 qn1=3 Þ [13], where h is the Planck constant, using kF l ¼ e the electron charge, q the resistivity, and n the electron concentration. Values of kF l are less than 1 calculated with n and q at 5 K, hinting towards the strong localization regime. Because only the topmost 100 nm of the film have been implanted, we applied a two layer model to separate the MR of the Mn-implanted IO or ITO layer from the MR of the unimplanted layer Fig. 3. The fitting procedure is described in detail in [13]. The sample IO 1 at.% Mn, RTA is in the weak localized regime, kF l > 1 and the sample IO 1 at.% Mn, VTA is in the strong localized regime, kF l < 1. The fitting parameter for positive MR, xeff, called the effective mole fraction of the magnetic ions is 10 times larger for IO 1 at.% Mn, VTA then IO 1 at.% Mn, RTA. The s–d exchange interaction is much stronger in IO 1 at.% Mn, VTA. 4. Summary After implantation and annealing the grain size and lattice parameter decreased for all samples. Thus the Mn-implanted and RTA/VTA treated IO and ITO films are more dense compared to the virgin IO and ITO films. The annealing process also results in an increase of refractive index. Films with high refractive index
1619
also possess a high packing density. For implanted and annealed IO the extinction coefficient increases mostly in IR range, while for implanted and annealed ITO the optical absorption increases in the visible and UV spectral range. The positive MR in the Mn-implanted ITO and IO films has been modeled by accounting for the quantum correction of s–d spin splitting on the disorder-modified electron–electron interaction. Compared to RTA, VTA is a better annealing technique in the point of view of large s–d exchange. Acknowledgment This work is partially (C.S.) supported by DFG (Grant No. Schm1663/1-1) and (Q.X., S.Z., H.S.) by BMBF (Grant No. FKZ03N8708). References [1] D.C. Paine, T. Whitson, D. Janiac, R. Beresford, C.O. Yangv, B. Lewis, J. Appl. Phys. 85 (1999) 8445. [2] J. Philip, N. Theodoropoulou, G. Berera, J.S. Moodera, B. Satpati, Appl. Phys. Lett. 85 (2004) 777. [3] M. Venkatesan, R.D. Gunning, P. Stamenov, J.M.D. Coey, J. Appl. Phys. 103 (2008) 07D135. [4] S.R. Sarath Kumar, P. Malar, Thomas Osipowicz, S.S. Banerjee, S. Kasiviswanathan, Nucl. Instr. and Meth. B 266 (2008) 1421. [5] A. Rogozin, M. Vinnichenko, A. Kolitsch, W. Möller, J. Vac. Sci. Technol. A 22 (2004) 349. [6] H. Fujiwara, M. Kondo, Phys. Rev. B 71 (2005) 075109. [7] A. Rogozin, M. Vinnichenko, N. Shevchenko, A. Kolitsch, W. Möller, Thin Solid Films 496 (2006) 197. [8] A. Rogozin, M. Vinnichenko, N. Shevchenko, L. Vazquez, A. Mücklich, U. Kreissig, R.A. Yankov, A. Kolitsch, W. Möller, J. Mater. Res. 22 (2007) 8. [9] I. Elfallal, R.D. Pilkington, A.E. Hill, Thin Solid Films 223 (1993) 303. [10] N. Mehan, V. Gupta, K. Sreenivas, A. Mansingh, J. Appl. Phys. 96 (2004) 3134. [11] Q. Xu, L. Hartmann, H. Schmidt, H. Hochmuth, M. Lorenz, R. Schmidt-Grund, C. Sturm, D. Spemann, M. Grundmann, Phys. Rev. B 73 (2006) 205342. [12] Q. Xu, L. Hartmann, H. Schmidt, H. Hochmuth, M. Lorenz, R. Schmidt-Grund, C. Sturm, D. Spemann, M. Grundmann, Y. Liu, J. Appl. Phys. 101 (2007) 063918. [13] Q. Xu, L. Hartmann, H. Schmidt, H. Hochmuth, M. Lorenz, D. Spemann, M. Grundmann, J. Phys. Rev. B 76 (2007) 134417.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
Mn-implanted, polycrystalline indium tin oxide and indium oxide films Camelia Scarlat a,*, Mykola Vinnichenko a, Qingyu Xu b, Danilo Bürger a, Shengqiang Zhou a, Andreas Kolitsch a, Jörg Grenzer a, Manfred Helm a, Heidemarie Schmidt a a b
Forschungszentrum Dresden-Rossendorf, Institut für Ionenstrahlphysik und Materialforschung, Bautzner Landstraße 128, 01328 Dresden, Germany Department of Physics, Southeast University, Nanjing 211189, China
a r t i c l e
i n f o
Article history: Available online 11 February 2009 PACS: 75.50.Pp 73.43.Qt 78.20.Ci Keywords: Diluted magnetic semiconductor ITO Magnetoresistance Optical constants
a b s t r a c t Polycrystalline conducting, ca. 250 nm thick indium tin oxide (ITO) and indium oxide (IO) films grown on SiO2/Si substrates using reactive magnetron sputtering, have been implanted with 1 and 5 at.% of Mn, followed by annealing in nitrogen or in vacuum. The effect of the post-growth treatment on the structural, electrical, magnetic, and optical properties has been studied. The roughness of implanted films ranges between 3 and 15 nm and XRD measurements revealed a polycrystalline structure. A positive MR has been observed for Mn-implanted and post-annealed ITO and IO films. It has been interpreted by considering s–d exchange. Spectroscopic ellipsometry has been used to prove the existence of midgap electronic states in the Mn-implanted ITO and IO films reducing the transmittance below 80%. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
2. Experiment
Indium tin oxide (ITO) and indium oxide (IO) thin films exhibit low electrical resistivity, transparency in the visible spectral range, high infrared reflectivity and UV absorption. These properties make ITO and IO thin films good candidates for many technological applications, such as coatings in thermal collectors and mirrors as well as transparent electrodes in several electro-optical set-ups: solar cells and liquid crystal display devices [1]. Recently, hightemperature ferromagnetism has been reported in Mn doped ITO-based systems prepared by reactive thermal evaporation [2], pulsed laser deposition [3] or DC reactive sputtering [4]. This research was focused on the change of structural and magnetic properties upon Mn doping, while the effect of optical and magnetoresistance is rather rarely investigated. On the other hand, ion implantation as a versatile semiconductor-doping method has not been used for magnetic doping of ITO or IO. In this paper, we focus on the structural, optical and magneto-transport properties of Mn-implanted ITO and IO films, and discuss the effect of annealing at different temperatures and ambient pressure.
2.1. Sample preparation The ITO and IO thin films have been deposited by reactive pulsed (4.17 kHz) magnetron sputtering (MS) using two 2-in. unbalanced magnetrons operated in unipolar mode. MS is a low pressure process that permits large scale deposition of high quality films at high rates at 500 °C. Additional technical details are published elsewhere [5]. The ITO films were deposited on heated ntype Si (1 0 0) wafers covered with a 500 nm SiO2 layer, whereas the IO films were deposited on unheated n-type Si (1 0 0) wafers covered with a 250 nm SiO2 layer. The ITO and IO films were implanted with Mn+ ions with a special care taken in creating a homogeneous implantation depth profile. Therefore, three different ion energies (120, 60 and 20 keV) were chosen to produce a fairly uniform ion distribution. A set of samples was produced with Mn atomic concentrations of 1 and 5 at.%. Rapid thermal annealing (RTA) has been performed on implanted ITO and IO films at 650 °C for 10 s in N2. Two other Mn-implanted IO samples were annealed in vacuum (vacuum thermal annealing, VTA) (p = 1.6 106 mbar) at constant temperature of 200 °C for 1 h. 2.2. Characterization methods
* Corresponding author. E-mail address: [email protected] (C. Scarlat). 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.01.158
The structure of as-prepared and ion implanted IO and ITO film has been studied by means of grazing incidence X-ray diffraction
C. Scarlat et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1616–1619
(XRD). A SIEMENS D5000 X-ray diffractometer using Cu Ka (k = 0.154 nm) radiation has been used for XRD. The size of coherently diffracting domains, or the crystallites size d of the films has been determined from the broadening of the corresponding XRD peaks using Scherrer’s formula (d = 0.9 k/b cos h, k = 0.15406 nm, b – peak width at half maximum height, h – Bragg angle). Using this parameter the grain size in the direction normal to the film surface may be evaluated. The ITO and IO films were characterized ex situ by spectroscopic ellipsometry (SE) measurements using M-2000 spectroscopic ellipsometer from J.A. Woolam Co. at an angle of incidence of 70°. The energy of the probing photons was in the range of 1.24–3.3 eV. The data were evaluated using WVASE software following a procedure described in more detail elsewhere [6,7]. The SE data analysis yields the film thickness, surface roughness, and optical constants. For this purpose, the film dielectric function was parameterized in the Drude–Lorentz approach [8], Eq. (1) and the SE data were fitted simultaneously with reflectance data: 1=2
e
¼
A1 A2 e1 þ 2 hxðhx þ iB1 Þ E2 þ ðhxÞ2 þ iB2 hx
1617
.
.
!1=2 ;
ð1Þ
where the parameters A2, B2, and E2 refer to the oscillator amplitude, damping constant, and resonant energy, respectively. They describe the contributions of interband transitions to the optical absorption. Here e1 is the high frequency dielectric constant, A1 ¼ ð hxp Þ2 and B1 ¼ ð hxs Þ are the squared plasma frequency and the relaxation frequency, respectively, expressed in units of energy. The latter two parameters represent the free electron contribution to the optical constants according to the Drude model. From A1 the free electron density, Ne, is obtained using known relations. Magnetotransport measurements with the field applied perpendicular to the film were performed in van de Pauw configuration and fields up to 6 T were applied at temperature 5 K.
.
3. Results and discussion 3.1. Structure Fig. 1(a) shows XRD patterns of unimplanted, as deposited film of IO, and 1 and 5 at.% Mn-implanted films after RTA. The XRD pattern of unimplanted IO (Fig. 1(a)) can be indexed based on the cubic bixbyite structure of In2O3. Both, for unimplanted and implanted IO films, the lattice parameter are close to that of bulk In2O3 (1.0117 nm [9]). Ion implantation itself and subsequent RTA lead to a decrease of the (4 0 0) and (4 4 0) XRD peak intensity in case of IO films. This indicates the deterioration of the film crystalline structure due to the implantation not being remedied by annealing. Fig. 1(b) shows the XRD patterns of an unimplanted, as deposited ITO film, and of 1 and 5 at.% Mn-implanted films after RTA. The lattice parameter of unimplanted and implanted ITO films amounts to 1.0060 nm being smaller than the lattice parameter of bulk In2O3. This is expected, if Sn ions substitutionally occupy the In sites [2]. The (2 2 2), (4 4 0), and (6 2 2) XRD peak amplitude of ITO (Fig. 1(b)) also decreases after implantation and RTA. Peaks corresponding to Mn oxides were only found for ITO 5% Mn, RTA. XRD yields the crystallites size d of 21 and 28 nm for virgin IO and ITO, respectively, in direction normal to the film surface. After implantation and RTA the crystallites size d for IO amounts to 18 nm (1 at.% Mn) and to 17 nm (5 at.% Mn) and for ITO it amounts to 20 nm (1 at.% Mn) and to 18 nm (5 at.% Mn). Transmission Electron Microscopy (TEM) [5,7] showed that the ITO films exhibit columnar crystalline structure with elongated crystals oriented normally to the film surface.
Fig. 1. XRD patterns of unimplanted (0 at.% Mn) and implanted (1 at.% Mn, 5 at.% Mn), RTA treated (a) IO films and (b) ITO films. d is the crystallites size.
3.2. Optical properties Spectroscopic ellipsometry data show that the implantation of 1 at.% of Mn into IO and subsequent VTA lead to a decrease of the film thickness from 233 to 182 nm. In the case of ITO implantation of the same amount of Mn followed by RTA a much less significant decrease of the film thickness from 122 to 113 nm has been observed. The decrease of the film thickness is due to the fact
1618
C. Scarlat et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1616–1619
that the crystalline size became smaller after implantation and annealing such that the films became more dense. The complex refractive index of the films was calculated as N ¼ ðeÞ1=2 , where e is determined according to the Eq. (1). The refractive indices (n) and extinction coefficients (k) for the unimplanted and implanted ITO and IO films are shown in Fig. 2. One can observe that n and k of ITO and IO significantly differ from each other. In case of unimplanted films, the refractive index of the ITO sample is lower than that of the unimplanted IO sample in the whole measured photon energy range, while the extinction coefficient of the ITO in the near IR is significantly higher than that of IO. It is explained by higher free electron absorption in ITO due to a higher free electron density which is of 9.9 1020 cm3 for ITO compared to 9.9 1018 cm3 for IO sample, according to Hall effect data. The lower carrier concentration in IO is also the reason for its higher refractive index compared to ITO that is consistent with other authors’ results [7,8]. The extinction coefficient k of ITO
a
1 at.% Mn and IO 1 at.% Mn-implanted and annealed samples increases compared to unimplanted ones. In case of IO film, ion implantation and VTA leads to a significant increase of the k values in the whole spectral range, although in the near IR it increases even stronger. The latter is in good agreement with an increase of the free electron concentration from 9.9 1018 to 1.9 1020 cm3 due to this treatment. In contrast, for ITO 1 at.% Mn implantation and RTA the free electron absorption does not increase in the near IR, although the optical absorption significantly increases in the visible and UV spectral range. The latter may be explained in terms of the defect states formation in the band gap. An increase of the refractive index values in ion implanted and annealed samples compared to untreated ones may be related to an increased packing density in the films [10] due to the treatment. A higher packing density has also been probed by XRD (Section 3.1). A better understanding of the optical properties modification of ITO and IO requires further research which is in progress now.
2.2
a
2.0
.
1.8
.
1.6
.
1.4
1.2 1.0
.
.
1.5
2.0
2.5
3.0
3.5
b
b
0.15
0.10
0.05
0.00 1.0
1.5
2.0
2.5
3.0
3.5
Fig. 2. (a) Refractive index n and (b) extinction coefficient k of unimplanted and implanted IO and ITO films.
Fig. 3. 5 K magnetoresistance versus magnetic field (open symbols) for (a) IO 1 at.% Mn, RTA and (b) IO 1 at.% Mn, VTA 5 K. The solid lines are the fitting curves. l is mobility, q is resistivity and n is electron concentration.
C. Scarlat et al. / Nuclear Instruments and Methods in Physics Research B 267 (2009) 1616–1619
3.3. Magnetotransport properties Negative MR was observed in unimplanted ITO and IO films. A large positive MR at 5 K and low field has been observed in implanted IO films. Also the MR in magnetic ITO and IO films depends on the electron concentration [11,12]. In this paper, we extend the modeling of s–d spin splitting on the disorder-modified electron– electron interaction to explain the positive MR in Mn-implanted ITO and IO. The type of the conducting carriers was confirmed to be n-type by Hall measurements for all samples at 5 K. The product of the Fermi wave vector kF and mean free path l can be calculated hð3p2 Þ2=3 =ðe2 qn1=3 Þ [13], where h is the Planck constant, using kF l ¼ e the electron charge, q the resistivity, and n the electron concentration. Values of kF l are less than 1 calculated with n and q at 5 K, hinting towards the strong localization regime. Because only the topmost 100 nm of the film have been implanted, we applied a two layer model to separate the MR of the Mn-implanted IO or ITO layer from the MR of the unimplanted layer Fig. 3. The fitting procedure is described in detail in [13]. The sample IO 1 at.% Mn, RTA is in the weak localized regime, kF l > 1 and the sample IO 1 at.% Mn, VTA is in the strong localized regime, kF l < 1. The fitting parameter for positive MR, xeff, called the effective mole fraction of the magnetic ions is 10 times larger for IO 1 at.% Mn, VTA then IO 1 at.% Mn, RTA. The s–d exchange interaction is much stronger in IO 1 at.% Mn, VTA. 4. Summary After implantation and annealing the grain size and lattice parameter decreased for all samples. Thus the Mn-implanted and RTA/VTA treated IO and ITO films are more dense compared to the virgin IO and ITO films. The annealing process also results in an increase of refractive index. Films with high refractive index
1619
also possess a high packing density. For implanted and annealed IO the extinction coefficient increases mostly in IR range, while for implanted and annealed ITO the optical absorption increases in the visible and UV spectral range. The positive MR in the Mn-implanted ITO and IO films has been modeled by accounting for the quantum correction of s–d spin splitting on the disorder-modified electron–electron interaction. Compared to RTA, VTA is a better annealing technique in the point of view of large s–d exchange. Acknowledgment This work is partially (C.S.) supported by DFG (Grant No. Schm1663/1-1) and (Q.X., S.Z., H.S.) by BMBF (Grant No. FKZ03N8708). References [1] D.C. Paine, T. Whitson, D. Janiac, R. Beresford, C.O. Yangv, B. Lewis, J. Appl. Phys. 85 (1999) 8445. [2] J. Philip, N. Theodoropoulou, G. Berera, J.S. Moodera, B. Satpati, Appl. Phys. Lett. 85 (2004) 777. [3] M. Venkatesan, R.D. Gunning, P. Stamenov, J.M.D. Coey, J. Appl. Phys. 103 (2008) 07D135. [4] S.R. Sarath Kumar, P. Malar, Thomas Osipowicz, S.S. Banerjee, S. Kasiviswanathan, Nucl. Instr. and Meth. B 266 (2008) 1421. [5] A. Rogozin, M. Vinnichenko, A. Kolitsch, W. Möller, J. Vac. Sci. Technol. A 22 (2004) 349. [6] H. Fujiwara, M. Kondo, Phys. Rev. B 71 (2005) 075109. [7] A. Rogozin, M. Vinnichenko, N. Shevchenko, A. Kolitsch, W. Möller, Thin Solid Films 496 (2006) 197. [8] A. Rogozin, M. Vinnichenko, N. Shevchenko, L. Vazquez, A. Mücklich, U. Kreissig, R.A. Yankov, A. Kolitsch, W. Möller, J. Mater. Res. 22 (2007) 8. [9] I. Elfallal, R.D. Pilkington, A.E. Hill, Thin Solid Films 223 (1993) 303. [10] N. Mehan, V. Gupta, K. Sreenivas, A. Mansingh, J. Appl. Phys. 96 (2004) 3134. [11] Q. Xu, L. Hartmann, H. Schmidt, H. Hochmuth, M. Lorenz, R. Schmidt-Grund, C. Sturm, D. Spemann, M. Grundmann, Phys. Rev. B 73 (2006) 205342. [12] Q. Xu, L. Hartmann, H. Schmidt, H. Hochmuth, M. Lorenz, R. Schmidt-Grund, C. Sturm, D. Spemann, M. Grundmann, Y. Liu, J. Appl. Phys. 101 (2007) 063918. [13] Q. Xu, L. Hartmann, H. Schmidt, H. Hochmuth, M. Lorenz, D. Spemann, M. Grundmann, J. Phys. Rev. B 76 (2007) 134417.