JOURNAL OF APPLIED PHYSICS 107, 103915 共2010兲
Electronic structure of Cu-doped ZnO thin films by x-ray absorption, magnetic circular dichroism, and resonant inelastic x-ray scattering P. Thakur,1,a兲 V. Bisogni,1 J. C. Cezar,1 N. B. Brookes,1 G. Ghiringhelli,2 S. Gautam,3 K. H. Chae,3 M. Subramanian,4 R. Jayavel,4 and K. Asokan5 1
European Synchrotron Radiation Facility, BP 220, 38043 Grenoble Cedex, France CNR-INFM Coherentia and Soft, Dipartimento di Fisica, Politecnico di Milano, I-20133, Milano, Italy 3 Nano Analysis Center, Korea Institute of Science and Technology (KIST), Seoul 136-791, Republic of Korea 4 Crystal Growth Centre, Anna University, Chennai 600 025, India 5 Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110 067, India 2
共Received 21 November 2009; accepted 26 February 2010; published online 25 May 2010兲 The electronic structure of Cu-doped ZnO thin films, synthesized with a nominal composition of Zn1−xCuxO 共x = 0.03, 0.05, 0.07, and 0.10兲 by using spray pyrolysis method, has been investigated using near-edge x-ray absorption fine structure 共NEXAFS兲 experiments at the O K- and the Cu L3,2-edges and resonant inelastic x-ray scattering 共RIXS兲 measurements at Cu L3,2 edge. The Zn1−xCuxO thin films showed single phase wurtzite-hexagonal like crystal structure and ferromagnetic behavior at room temperature 共RT兲. The intensity of the pre-edge spectral feature at the O K-edge increases with the Cu concentration, which clearly reveals that there is strong hybridization of O 2p – Cu 3d orbitals in the ZnO matrix. Spectral features of the Cu L3,2-edge NEXAFS exhibit multiple absorption peaks and appreciable x-ray magnetic circular dichroism signal that persists even at RT. These results demonstrate that Cu is in mixed valence state of Cu2+,3+ / Cu1+, substituting at the Zn site and Cu2+/3+ ions are magnetically polarized. RIXS experiments at Cu L3 edge show strong d-d excitations due to localized nature of Cu ions in the ZnO matrix. © 2010 American Institute of Physics. 关doi:10.1063/1.3372758兴 I. INTRODUCTION
The aim of modern spintronics is to elaborate or fabricate new multifunctional novel devices that can utilize both the charge and spin components of the electrons.1 It is possible to achieve both semiconducting and magnetic properties within a single material by artificial doping of magnetic impurities 共e.g., V, Cr, Mn, Fe, Co, Ni, or Cu兲 into a semiconducting hosts 共such as ZnO, TiO2, GaN, etc兲, producing what is most commonly known as dilute magnetic semiconductors 共DMSs兲. Of all DMSs, ZnO 共direct energy gap ⬃ 3.3 eV and large excitation energy兲 is a potential host material for realizing high Curie temperature 共Tc兲 ferromagnetism by doping a variety of 3d transition metal 共TM兲 ions.2,3 Following the prediction of Dietl et al.4 that ZnO should have a high Tc with hole doping, there were flurry of experimental reports on TM-doped ZnO systems proving or disproving high Tc ferromagnetism. There are still debates on whether the magnetic behavior in DMSs is an intrinsic property or due to the nanoclusters of a magnetic phase or both. Therefore, despite numerous experimental and theoretical reports in the literature, the origin of DMS is still unclear. It has been widely accepted that magnetic interactions, Curie temperature, and electronic properties of DMSs are crucially dependent on the concentration of TM ions and their distribution over different sites 共such as substitutional site, interstitial site, and/or secondary phases兲 in the host lattice.5–7 In order to overcome this bottleneck and to control the materials properties, a clear understanding of the physical a兲
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processes in TM-doped ZnO is necessary in addition to obtaining high Tc ferromagnetism. For better understanding of these controversial results concerning the origin of magnetic properties in DMSs, a detailed investigations involving electronic spectroscopy are increasingly important. In the case of Cu-doped ZnO, there are several reports both theory and experiments concerning the presence or absence of ferromagnetism at room temperature 共RT兲.8–21 Recently, this material has aroused a lot of interest because Cu atoms have no clustering tendency and Cu-based compounds are not ferromagnetic. Cu atoms in ZnO are likely in the 0 − 共i.e., Cu2+兲 or CuZn 共i.e., Cu1+兲 when Cu creates form of CuZn the dominant defects in ZnO matrix. First-principles calculations22–24 have showed that Cu dopants in ZnO favor a spin polarization and ferromagnetic ground state due to large hybridization of the Cu 3d orbitals with the O 2p states. Thus Cu doped ZnO has potential of showing ferromagnetic magnetic behavior. This motivated us to investigate the element-specific electronic structure based on the progressive doping of Cu 共upto a maximum of 10%兲 and thus gives an opportunity to see evolution of spectral features at different concentrations. So far, rather few x-ray spectroscopic studies on Cu-doped ZnO have been reported.9,20 There is no spectroscopic information on how Cu-doping induce changes in the electronic structure of the host ZnO lattices. In this study, we have investigated the electronic and magnetic properties of well characterized Cu-doped ZnO thin films, synthesized by spray pyrolysis method, employing near-edge x-ray absorption fine structure 共NEXAFS兲, x-ray magnetic circular dichroism 共XMCD兲, and resonant inelastic
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x-ray scattering 共RIXS兲 spectroscopy. These techniques are powerful to demonstrate whether the spin polarization of the magnetic ions is consistent with the bulk magnetization of samples and allow for distinguishing between contributions from localized ions 共substitutional or interstitial兲 and metallic clusters. While NEXAFS has been used extensively to identify the valence states of ions in the material, the XMCD is a powerful tool to study the element-specific local magnetic interactions and also it reflects the spin and orbital polarizations of the local electronic states. The XMCD technique, which records the difference in core-level absorption spectra between right- and left-handed circularly polarized x-rays, is sensitive to magnetic polarization of each element, and therefore enables us to directly extract the local electronic structure related a particular magnetic impurity of TM ions in the host matrix. On the other hand, element-specific NEXAFS and RIXS spectroscopy is capable of probing the partial density of unoccupied and occupied states of constituent elements, respectively, and thus are powerful tools for understanding local electronic structure around target atoms. II. EXPERIMENTAL DETAILS
Undoped ZnO and Cu-doped ZnO 共Zn1−xCuxO, x = 0.03, 0.05, 0.07, and 0.10兲, 共hereafter referred as ZnO:Cu兲 thin films having thickness of ⬇350 nm were grown on Si 共100兲 substrate by spray pyrolysis technique. The Zn acetate 关Zn共CH3COO兲2 · 2H2O兴 and Cu acetate 关Cu共CH3COO兲2 · 4H2O兴 were used as the precursors for Zn and Cu, respectively. At first, zinc acetate was dissolved in deionized water 共resistivity= 20 M⍀ cm兲 and stirred for 1.5 h at RT, which forms as a clear spray solution for pure ZnO thin film. For Cu-doping, an appropriate amount of Co acetate was dissolved in deionized water separately, stirred for 1.5 h, and mixed with the starting solution of ZnO precursor as a drop wise addition. The final solution was then continuously stirred for 3 h at RT to get the homogeneous solution. The total concentration of the solution was kept at 0.5 mol. The substrate temperature was maintained at 400 ° C during deposition. Compressed air was used as a carrier gas with the air flow rate of 40 lbs/ in.2. The solution was sprayed on the substrates in several spraying cycles of 3 s, followed by an interval of no spray for 1 min, which avoid the strong cooling of the substrate due to continuous spray. The films were sprayed for 3 h with the above said systematic steps. The single phase and crystal structure of ZnO:Cu films were investigated using high-resolution x-ray diffraction 共HRXRD兲 with = 1.5425 Å, at the 10C1 x-ray scattering 共XRS兲 II bending magnet beamline of the Pohang Accelerator Laboratory. The isothermal magnetization hysteresis measurements were performed at RT using alternating gradient force magnetometer 共AGFM兲 共MicroMag-2900, Princeton Measurements Co.兲 with a sensitivity of 10−11 Am2. The NEXAFS and XMCD experiments were performed at the ESRF⬘s ID08 beamline, which uses an APPLE II type undulator giving ⬃100% linear/circular polarization. All scans were recorded simultaneously in both total electron yield 共TEY兲 and total fluorescence yield 共TFY兲 modes, ensuring both surface 共TEY兲 and bulk 共TFY兲 sensitivities. The spectra
FIG. 1. 共Color online兲 共a兲 HRXRD pattern of Zn1−xCuxO 共x = 0, 0.03, 0.07, and 0.10兲 thin films. As evident, no additional phase are seen. 共b兲 M-H loops of the Zn1−xCuxO 共x = 0.03, and 0.07兲 thin films at RT.
were normalized to incident photon flux and the base pressure of the experimental chamber was better than 3 ⫻ 10−10 Torr. Since the ZnO:Cu films exhibit ferromagnetic behavior at RT with standard magnetometry AGFM, NEXAFS at Fe, Ni, and Co L3,2 edges have also been performed to rule out other ferromagnetic contaminations within the sensitivity of x-ray absorption spectroscopy 共smaller than 0.1% concentration of the 3d metal兲. The RIXS spectra at Cu L3-edge were recorded with the advanced x-ray emission spectroscopy spectrometer equipped with a dedicated monochromator before the sample and a charge coupled device detector.25 III. RESULTS AND DISCUSSION
Figure 1共a兲 presents the HRXRD patterns of Zn1−xCuxO thin films 共x = 0, 0.03, 0.07, and 0.10兲. The diffraction peaks reflect the formation of wurtzite-hexagonal like crystal structure for all Cu concentrations. No traces of Cu-related secondary phases, such as Cu, Cu2O, and CuO, were detected within the experimental limits of the HRXRD beamline. However, all the peaks of ZnO:Cu samples slightly shifts toward lower scattering angle as compare to reference ZnO. This is accountable by considering the difference in ionic radii of Cu and Zn ions. A continuous broadening of diffraction peaks with Cu-doping revels that some defects such as Cu/Zn interstitials or oxygen vacancies are present in the samples. The magnetic property of ZnO:Cu thin films at RT were investigated by an AGFM, with magnetic field applied perpendicular to the sample plane. Figure 1共b兲 shows mag-
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FIG. 2. 共Color online兲 O K edge NEXAFS spectra of Zn1−xCuxO 共x = 0, 0.03, 0.05, 0.07, and 0.10兲 thin films collected at 10 K. 共a兲 Surface sensitive TEY mode. 共b兲 Bulk sensitive TFY mode.
netization versus magnetic field 共M-H兲 curve for x = 0.03 and x = 0.07 samples. As evident, films exhibit a well defined magnetization hysteresis and show ferromagnetic behavior at RT. Since Cu and related oxides are nonmagnetic materials at RT, the observed ferromagnetism is thought to be intrinsic property of the Cu-doped ZnO films. Moreover, absence of any magnetic signal in undoped ZnO film is also confirmed by using same experimental conditions 共e.g., use of nonmetallic tools, subtraction of diamagnetic signal from the substrate, etc.兲, which shows usual diamagnetism with no indication of ferromagnetic signal. In order to understand the role of oxygen O 2p states on the electronic structure of ZnO:Cu as a function of Cudoping, NEXAFS experiments at the O K edge were carried out. This probe the orbital character of the spectral features of the O 2p unoccupied states in the conduction band and its hybridization with different Cu and Zn orbitals. Figures 2共a兲 and 2共b兲 show the normalized spectra of ZnO:Cu 共x = 0, 0.03, 0.05, 0.07, and 0.10兲 thin films acquired at RT in both TEY mode and TFY mode, respectively. According to the published data of similar systems,26–28 the observed spectral features are assigned as follows: 共a兲 the energy region between 528–539 eV 共marked by A1 – C1兲 is mainly attributed to O 2p hybridization with highly dispersive Zn 3d4s / Cu 3d states which form the bottom of the conduction band with peak C1 at ⬃538 eV due to transitions to nondispersive O 2p states and 共b兲 the region between 539–550 eV 共marked by D1兲 can be assigned to O 2p – Zn 4p / Cu 4sp hybridized states and above 550 eV spectrum arise to due the O 2p states that extend to Zn/Cu higher orbitals. The two pre-edge peaks A1 共⬃530 eV兲 and B1 共⬃532.5 eV兲 evolve with Cu-doping in ZnO and their intensity increases monotonically with Cu
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concentrations, suggesting a strong hybridization of O 2p orbitals with Cu 3d states. These pre-edge features carry a substantial amount of information and can be attributed to the unoccupied bands of primary O 2p – Cu 3d character. A continuous evolution of these spectral features with Cu-doping indicate more unoccupied states at the Cu 3d levels and therefore reflects the presence of more charge carriers, electrons, or holes. Similar pre-edge features have been reported in other oxide materials mainly in cuprates and manganites,29–33 and attributed due to the presence of holes at oxygen site. A recent experiment on Mn–ZnO system using O K-edge NEXAFS reported the similar spectral observations for peak A1.27 Considering the energy of peak A1, Chen et al.32 and Asokan et al.33 associated this as a characteristic feature of hole doped system, while in other study of Co–ZnO system peak B1 was attributed due to the oxygen defect states.34 The exact origin of peak B1 is still controversial in TM-doped ZnO systems.35,36 However, a significant enhancement of pre-edge spectral features reveals that the effective doping of Cu in ZnO matrix induces a strong hybridization of s-p-d orbitals. The spectral features above 537 eV are quite similar and nearly independent of Cu concentrations and dominated by the contributions from multiple scattering effects. Therefore, it is reasonable to conclude that Cu ions are incorporated in the system and responsible for change in the electronic structure of this material. Now we comment on the TFY mode spectra, where probing depth is larger by roughly an order of magnitude and the spectra are therefore more representative of the “bulk” composition of the films. Upon inspection, it is clear that the line shape of the TFY mode spectra bears close resemblance to TEY mode spectra. The only difference we find is the peaks B1 forming a broad hump, which is continuously increases in weight with Cu-doping. This can be attributed to the saturation effects which are dominating in the TFY mode and/or stoichiometry of O ions is different on the surface of the films than that of the bulk. Figures 3共a兲 and 3共b兲 show the normalized Cu L3,2 edge spectra of ZnO:Cu thin films in both TEY mode and TFY mode, respectively. CuO and Cu2O spectra have also been collected under the same experimental conditions and are chosen as reference spectra of Cu2+ and Cu1+, respectively. All the spectra stem from excitations of a core electron in the 2p1/2 or 2p3/2 manifold to the unoccupied 3d state, i.e., transitions from a ground sate of 2p63dn to an excited electronic configuration of 2p53dn+1 with different multiplet excitations. As a result of spin-orbit coupling in the 2p state, the spectra display two prominent features in the energy range of 928–940 and 949–955 eV, respectively, corresponding to the L3 共2p3/2 → 3d兲 and L2 共2p1/2 → 3d兲 absorptions. In case of reference samples, Cu2O exhibits asymmetric L3,2 edges, while CuO shows sharp and very symmetric white lines. The small peak at h = 932.2 eV in the Cu2O Cu L3 spectrum 共TEY mode兲 is due absorption by small amount of CuO present at the surface of the sample as a consequence of unavoidable surface oxidation. Cu in CuO is in the nominal Cu2+-ionization state, corresponding to a 3d9 ground state with a small contribution from 3d10Lគ , where Lគ , denotes a hole in the O valence band.37,38 The L3,2 NEXAFS final state
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FIG. 3. 共Color online兲 共a兲 Cu L3,2 edge NEXAFS spectra of Zn1−xCuxO thin films for various concentrations collected at 10 K. 共a兲 Surface sensitive TEY mode. 共b兲 Bulk sensitive TFY mode. Spectra of reference samples of Cu2O and CuO are also shown for comparison.
is thus dominated by the 2p53d10 configuration. The extra electron that fills the well-localized d shell provides a very effective screening of the core-hole potential. In this way the 2p63d9 → 2p53d10 transitions give rise to a sharp resonance at a photon energy much lower than the ones for which the weak 2p63d9 → 2p53d94sp transitions are excited, and the intermixing between 2p53d10 and 2p53d94sp1 final states is negligible. In Cu2O, Cu appears as Cu1+, with an almost completely full 3d shell. The ground state is a combination of 3d10, 3d94sp1, and 3d104sp1Lគ , where the second configuration gives the largest contribution to the Cu L3,2 absorption spectrum at threshold.38 The NEXAFS final state is then mostly 2p53d104sp1 and the photon energy of the L3 edge is larger than for CuO since some extra energy is needed to promote an electron into the 4sp band. The NEXAFS spectra of ZnO:Cu thin films, specially at the Cu L3 region exhibit multiple absorption peaks. We can identify three dominant peaks, labeled as A2 共930 eV兲, B2 共931.4 eV兲, and C2 共934.5 eV兲, showing significant changes in their intensity. This variation in the peak intensities of the Cu L-edge structure are very remarkable considering the relative small changes in compound composition. A comparison with CuO, Cu2O reference samples, and other similar data reported on cuprates,39–41 we can assign these spectral features as follows: peak A is ascribed to transitions from the Cu共2p3/2,1/2兲3d9O 2p6 ground states 关formally Cu共II兲 sites兴 into the Cu共2p3/2,1/2兲−13d10O 2p6 excited states, in which 共2p3/2,1/2兲−1 represents a 2p3/2 or 2p1/2 hole. The Peak B2 at the higher energy side is either due to Cu 3d8 states or into the originating from the Cu共2p3/2,1/2兲3d9Lគ Cu共2p3/2,1/2兲−13d10Lគ excited states 关formally Cu共III兲 sites兴.
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The peak C2 is due to the presence of the copper ions Cu1+ and can be associated to the oxygen deficiency, i.e., 2p63d10 – 2p53d104s1 transition in the Cu共I兲 sites of the oxygen deficient layer. A similar spectral features have been observed on the Cu L3 edge in other Cu-containing oxides and believed to be attributed to a variety of causes, including multiple Cu valencies, inequivalent Cu sites, and oxygen valencies. Sharma et al.42 attributed peak A2 共B2兲 in YBa2Cu3O7−␦ to Cu2+ 共Cu3+兲. Schofield et al.43 observed a similar splitting in Zn1−xCuxWO4 and attributed it to two divalent Cu sites with inequivalent electronic environments. Recently, Keavney et al.20 assigned this doublet feature 共peak A2 and B2兲 to Cu2+ states having two Cu sites. They have suggested that peak A2 was due to substituted Cu2+ ions, while peak B2 was assigned to CuO precipitates. Their conclusion was based on the fact that peak B2 was not showing any XMCD signal. In the present ZnO:Cu system, both peaks exhibit an appreciable XMCD signal, therefore cannot be attributed to any such CuO related phases. We notice a clear correlation between gain of intensity in peak B2 and relative decrease in peaks A2, and C2 with Cu concentration. Both TEY and TFY spectra of Cu L3,2 edge display similar spectral profile except for the peak C2, which is more prominent at the surface of the samples 共TEY mode兲. A continuous increase in the intensity of peak B2 at the expense of peaks A2 and C2, indicating that Cu3+ components are increasing with Cu concentrations. These behaviors obviously reflect that Cu L3,2 NEXAFS spectra exhibit divalent Cu apart from mixed valent Cu3+ / Cu1+ states and Cu valence changes with increase in Cu concentrations. It is important to point out that previously a similar variation in Mn valence states has been observed in ZnO:Mn system.27 The XMCD experiments were performed at different temperatures 共10–300 K兲 and external applied magnetic fields 共0.5–5 T兲 to probe the element-specific local magnetic interaction of Cu 2p orbitals. Figure 4 show the NEXAFS spectra 共+ and −兲 in the photon energy region of the Cu L3,2 absorption collected in both TEY mode 共upper panel兲 and TFY mode 共bottom panel兲 for x = 0.03 sample at 10 K with an applied magnetic field of 5 T. The samples were aligned at an angle of 45° between the surface normal and the incident beam. The corresponding XMCD spectra, defined as + − −, are also shown in the figure. Here, +共−兲 refers to the absorption coefficient for the photon helicity parallel 共antiparallel兲 to the Cu 3d majority spin direction. We find a negative XMCD structure at the L3 edge followed by a positive XMCD structure at the L2 edge, which illustrates the magnetic polarization of Cu 2p orbitals. It is observed that both the peaks A2 共substituted Cu2+ components兲 and B2 共Cu3+ components through ligand-field hybridization兲, exhibit a well define XMCD signals. The absence of magnetic signal at peak C3 clearly reflects the nonmagnetic nature of Cu1+ ions in the system. The fact that peak B2 shows an appreciable XMCD signal along with peak A2 supports the assumption that it represents the Cu3+ ions rather than CuO phase, which is not ferromagnetic. Figure 5 provides the XMCD spectra for all the studied compositions in both TEY mode 共upper panel兲 and TFY mode 共bottom panel兲 at 10 K. These observations suggest that all the ZnO:Cu
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FIG. 4. 共Color online兲 Cu L3,2 edge NEXAFS and XMCD spectra of Zn1−xCuxO 共x = 0.03兲 thin film collected at 10 K in both TEY mode 共top panel兲 and TFY mode 共bottom panel兲. As evident, magnetic polarization of Cu 2p orbitals are clearly seen.
FIG. 5. 共Color online兲 共a兲 Comparison of XMCD spectra of Zn1−xCuxO 共x = 0, 0.03, 0.07, and 0.10兲 thin films at 10 K in both TEY mode 共top panel兲 and TFY mode 共bottom panel兲. Inset in the bottom panel provides the temperature dependence of XMCD signal for x = 0.03 film in both TEY mode and TFY mode.
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FIG. 6. 共Color online兲 Magnetic field dependence of Cu L3,2 edge XMCD spectra of Zn1−xCuxO 共x = 0.03兲 thin film collected at 10 K in TFY mode. Inset: hysteresis curves at the peak A2 of the Cu L3 edge. Here, both polarization of the photons are used to extract the curves. As evident, at low magnetic field 共⬍1 T兲 an observable ferromagnetic Cu spins are clearly seen, which are over ruled by strong paramagnetic Cu spins at higher magnetic field.
films show a magnetic polarization of Cu 2p orbitals at 10 K, which persists even at RT. The inset in Fig. 5 共bottom panel兲 displays the temperature dependence of Cu L3 XMCD 共in percentage with respect to the edge jump兲 for x = 0.03 sample. The relative intensity of the Cu L3 XMCD signal remains to be constant with Cu-doping indicating that magnetic ordering is independent of Cu concentrations. Hence the XMCD signal of Cu L3,2 edges, as seen in Figs. 4 and 5, is indicative of magnetism associated with the Cu ions. Since Cu1+ ions with a full 3d shell should not exhibit any magnetism, the observed XMCD signal is attributed to the Cu2+ ions along with a significant contribution from Cu3+ components. It is well known that XMCD signal usually a “representative” of both ferromagnetic and paramagnetic components of the functioning ions in a system. To elucidate the nature of Cu spins in ZnO matrix, we have carried out the XMCD experiments at different applied magnetic fields followed by an element-specific polarization dependent M-H curves at Cu L3 edge. Figure 6 show the magnetic field dependence of XMCD signal for x = 0.03 sample at 10 K collected in TFY mode, while inset in Fig. 6 displays the corresponding hysteresis curves at the peak A2 of the Cu L3 edge. Upon close inspection of the hysteresis curves, it is observed that at low magnetic field 共⬍1 T兲, ZnO:Cu film shows a signature of ferromagnetic components associated with the Cu spins in ZnO matrix. At higher field indeed magnetic signal increases, but does not show any saturation up to 5 T of applied magnetic field, indicating that paramagnetic components of Cu spins are dominating with increase in mag-
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FIG. 7. 共Color online兲 Measured Cu L3 edge RIXS spectra of Zn1−xCuxO 共x = 0.03, 0.05, and 0.10兲 thin films at RT, plotted on an energy-loss scale 共raw data兲.
netic field. These investigations suggest that Cu ions in ZnO matrix are having two spin sites 共a small ferromagnetic and large paramagnetic site兲 contributing to the total magnetic moment of the system. However, this small ferromagnetic contribution to the total magnetic moment is difficult to compare with that of the magnetic signal obtained with bulk magnetometry at RT. It is also noted that XMCD signal neither observed at O K edge nor at Zn L-edge, therefore, inconsistency of magnetic measurements between bulk magnetometry and XMCD spectroscopy need to be clarified. As all the ZnO:Cu thin films show a ferromagnetic behavior with standard magnetometery 共AGFM兲 and are having convincing XMCD signal at RT, therefore, it would be useful to study local Cu 3d-3d interactions by using RIXS spectroscopy. Since the magnetism of Cu-doped ZnO correlates with the electronic exchange interactions of the Cu atom with its neighboring atoms and also the surrounding carriers, the local electronic structure of the Cu dopants provides important information for understanding this DMS material. In RIXS experiments local excitations of valence electrons are revealed by means of photon scattering. The system is left either in the initial ground state 共Rayleigh or elastic scattering兲 or in an excited state corresponding to local electronic excitations 共inelastic scattering兲. To obtain the spectral information of occupied Cu 3d states from RIXS measurements, we chose three excitation energies 共marked as A2, B2, and C2兲 at the Cu L3 threshold spectra, indicated on the NEXAFS spectrum in the top panel of the Fig. 7 for x = 0.03 film. The resulting RIXS spectra of ZnO:Cu thin films
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collected at RT are displayed in the bottom panels of the Fig. 7. The RIXS spectra are plotted as a function of the energy loss, i.e., the incident energy position is set to be 0 eV of RIXS with a combined energy resolution of 0.5 eV. As evident, RIXS spectra exhibit multiple spectral features when excitation energy is tuned at different regions of Cu L3-edge NEXAFS. RIXS at A2, which is an indicative of Cu2+ ions at substitutional site in ZnO matrix consists of two main features. The shoulder like peak located around ⫺0.5 eV below the elastic peak 共at 0 eV兲 can be associated with the low energy loss d-d excitations due to crystal field splitting or spin flip excitations.44 The main peak located around ⫺1.8 eV is due to the local Cu d-d excitation loss feature and corresponds to transitions to 3d9 multiplet states. It is also noted that this feature gains its spectral weight with Cu concentrations indicating a strong correlations between magnetic and electronic properties of this DMS. These spectral features agree fairly well with those found for other TM oxide systems.45,46 RIXS at B2 is mainly dominated by an intense spectral feature at ⫺2.2 eV. Since this feature represents the d-d excitations from Cu3+ states 共real Cu3+ or actually Cu2+ plus an oxygen p hole Lគ 兲, the unchanged peak intensity with Cu concentrations is very intriguing. As it has been shown above that the NEXAFS intensity corresponding to this feature varies considerably with Cu-doping. Moreover, the relative intensity of the d-d excitations compared to the elastic peak is almost opposite to that of excitation energy A2. We also notice a small chemical shift of center of mass toward higher energy loss side, which clearly signify different d-d excitations than those of Cu2+ states. In another Mn-doped GaAs DMS,47 it has been suggested that d-d excitations strongly depend upon the valency of the functioning ions 共in that case Mn2+ / Mn3+兲. If this feature is contributed from the charge-transfer 共CT兲 excitations from the O ligand states to Cu 3d states then at high energy loss some CT excitations should be observed, which is not the case here. Usually, CT excitations are predicted to be occur below ⬃5 – 12 eV to the elastic recombination peak in various oxide compounds.48 Hence, we tentatively suggest that RIXS at B2 is a combined contributions from d8 共real Cu3+兲 and d9Lគ 共 ⬃formally Cu3+兲 states, hybridized with O 2p6 orbitals. In other words, indirectly d-d excitations at ⫺2.2 eV reflects the hole states at oxygen site, confirming the O K edge absorption, which reveal doping-induced lower-lying O 2p “impurity” states. If the excitation energy is tuned at C2 共Cu1+兲, a new pronounced energy loss feature is observed at ⬃−4.3 eV and can be attributed to the on-site interband excitations from Cu 3d states at the top of the valence band into empty conduction-band states which have both Cu 3d and Cu 4s characters. A similar structure for Cu1+ states has been observed in RIXS spectra of Cu2O and CuAlO2 where energy loss features are occurred at ⫺4.5 eV and ⫺5.4 eV, respectively.49,50 However, resonance at ⬃−2.2 eV is still visible due to the local d-d transitions from Cu2+ / Cu3+ states because of contribution from leading tails of A2 and B2 excitation energies. Hence the overall RIXS energy loss features exhibits a strong d-d excitations, which are at variance with excitation energy and confirming the mixed-valent states of localized Cu ions.
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IV. CONCLUSIONS
In conclusion, the element-specific electronic and magnetic characterizations were done on single phase ZnO:Cu thin films by employing NEXAFS/XMCD and RIXS spectroscopy. NEXAFS at O K edge displays the evolution of pre-edge spectral features which are very similar to the hole doped cuprates and manganites, and also shows the strong hybridization of O 2p states with Cu 3d orbitals. NEXAFS at Cu L edges provide the evidence of mixed valent states 共Cu2+,3+ / Cu1+兲 of Cu ions substituting at Zn site. XMCD experiments at Cu L3,2 edge suggests that magnetism in this system is associated with the Cu2+/3+ ions and absence of any Cu-related oxide phases or clusters. The hysteresis curves at the Cu L3 edge demonstrate that Cu spins in ZnO matrix have combined contribution of magnetic signals 共a small ferromagnetic and large paramagnetic兲 to the total magnetic moment of the system. RIXS at Cu L3 edge supports the localized nature of Cu ions in ZnO matrix followed by an intense local Cu 3d-3d excitations. It is important to note that coexistence of Cu3+ / Cu1+ ions along with Cu2+ which may have important implications on the electronic structure of Cu-doped ZnO and possibly conclusive role through double exchange mechanism in order to stabilize the Cu2+ ions leading to ferromagnetic order. Any mechanism proposed to understand the origin of ferromagnetism in this material should account for the presence of holes at the oxygen sites and mixed valent states on Cu. These results show that Cu is doped into ZnO matrix in the mixed valent states that enhances carrier density apart from being in a highly correlated state of ferromagnetic ordering. H. Ohno, Science 281, 951 共1998兲. F. Pan, C. Song, X. J. Liu, Y. C. Yang, and F. Zeng, Mater. Sci. Eng. R. 62, 1 共2008兲. 3 Ü. Özgür, Y. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, and H. Morkoç, J. Appl. Phys. 98, 041301 共2005兲. 4 T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science 287, 1019 共2000兲. 5 K. Potzger and S. Zhou, Phys. Status Solidi B 246, 1147 共2009兲. 6 Q. Wang, Q. Sun, P. Jena, and Y. Kawazoe, Phys. Rev. B 79, 115407 共2009兲. 7 M. D. McCluskey and S. J. Jokela, J. Appl. Phys. 106, 071101 共2009兲. 8 M. Ferhat, A. Zaoui, and R. Ahuja, Appl. Phys. Lett. 94, 142502 共2009兲. 9 Q. Ma, D. B. Buchholz, and R. P. H. Chang, Phys. Rev. B 78, 214429 共2008兲. 10 K. Samanta, P. Bhattacharya, and R. S. Katiyar, J. Appl. Phys. 105, 113929 共2009兲. 11 F.-Y. Ran, M. Imaoka, M. Tanemura, Y. Hayashi, T.-S. Herng, and S.-P. Lau, Phys. Status Solidi B 246, 1243 共2009兲. 12 K. Ando, H. Saito, Z. Jin, T. Fukumura, M. Kawasaki, Y. Matsumoto, and H. Koinuma, J. Appl. Phys. 89, 7284 共2001兲. 13 H.-J. Lee, B.-S. Kim, C. R. Cho, and S.-Y. Jeong, Phys. Status Solidi B 241, 1533 共2004兲. 14 D. B. Buchholz, R. P. H. Chang, J. H. Song, and J. B. Ketterson, Appl. Phys. Lett. 87, 082504 共2005兲. 15 D. Chakraborti, J. Narayan, and J. T. Prater, Appl. Phys. Lett. 90, 062504 共2007兲. 16 X. Wang, J. B. Xu, W. Y. Cheung, J. An, and N. Ke, Appl. Phys. Lett. 90, 212502 共2007兲. 17 M. S. Seehra, P. Dutta, V. Singh, Y. Zhang, and I. Wender, J. Appl. Phys. 101, 09H107 共2007兲. 18 A. Tiwari, M. Snure, D. Kumar, and J. T. Abiade, Appl. Phys. Lett. 92, 062509 共2008兲. 19 C. Sudakar, K. Padmanabhan, R. Naik, G. Lawes, B. J. Kriby, S. Kumar, and V. M. Naik, Appl. Phys. Lett. 93, 042502 共2008兲. 20 D. J. Keavney, D. B. Buchholz, Q. Ma, and R. P. H. Chang, Appl. Phys. 1 2
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