Cobalt doped ZnO prepared by electrochemistry: chemistry, morphology, and magnetism

I. Enculescu*, E. Matei*, V. Vasilache**, and C.M. Teodorescu*

*National Institute of Materials Physics, P.O.Box MG-7, 077125 Bucharest-Magurele, Romania, teodorescu@infim.ro

**University of Suceava, University Str. 13, 720229 Suceava, Romania, violetav@usv.ro

ABSTRACT ZnO:Co thin films are prepared by electrochemical

deposition at different overpotentials and investigated by scanning electron microscopy, X-ray photoelectron spectroscopy and Kerr magnetometry (MOKE). Increse of structural disorder and crystallite re-orientation are observed for cobalt doped samples (especially for those obtained at - 750 and at - 800 mV overpotentials). MOKE magnetometry revealed room temperature ferromagnetic behaviour only for ZnO:Co deposited at - 850 mV. These are the samples with the less cobalt content and lowest structural perturbation of the ZnO matrix by cobalt.

Keywords: diluted magnetic semiconductors, ZnO:Co, scanning electron microscopy, X-ray photoelectron spectroscopy, magneto-optical Kerr effect

1. INTRODUCTION Transition metal doped zinc oxide has been revealed as

a promising diluted magnetic semiconductor (DMS) material [1]. One of the main features of such materials is the fact that ferromagnetism occurs due to indirect exchange between localized magnetic moments via the free carriers in the semiconductors, an interaction proposed four decades ago by Ruderman, Kittel, Kasuya and Yosida (RKKY) [2]. Hence, possibility of ferromagnetism control is offered, by modulating the carrier density via electrical or optical injection [3]. However, to date the results reported for ZnO:Co are still controversial. Some papers report on the absence of room temperature ferromagnetism (RTFM) on this system [1,4], some other papers report RTFM, but attribute it to the formation of metal or metal oxide clusters, or some other secondary phases with ferromagnetic properties [5]; a majority of recent papers report RTFM, but attribute it to the presence of defect (oxygen vacancies, metal interstitials) [6], whereas recent works point on the DMS-RKKY (or carrier-mediated) character of RTFM [7]. Still, the magnetic properties are strongly dependent on the preparation method employed: sol-gel [1], magnetron sputtering, molecular beam epitaxy, pulsed laser deposition (PLD), and electrochemistry. The latter method is amongst the chepest and allows easy implementation in an industrial process. At the same time, to our knowledge, to date there was no sound report on the occurence of RTFM in thin

layers of ZnO:Co prepared by this method. In this Contribution we report the electrochemical synthesis of ZnO:Co, along with characterization of the deposited films by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and magneto-optical Kerr effect (MOKE), which evidences RTFM in some samples. These combined measurements allowed us to infer the relationship between composition, chemical state, structure, morphology and magnetism.

2. EXPERIMENTAL

Pure and cobalt-doped ZnO films were electrodeposited

on platinum substrates. A typical electrochemical aqueous nitrate bath [0.1 M Zn(NO3)2] was employed for the process at a working temperature of 90°C. For doping, cobalt nitrate (0.06 M) was added to the sollution. The deposition was performed at different overpotentials. In Fig. 1 one may observe the electrochemical polarization curves for both pure and doped ZnO deposition. The increase of the deposition current indicates the range where Co2+ cations are transported to the substrate. Hence, depositions were performed at overpotentials between - 850 and - 700 mV. Table 1 summarizes the preparation conditions for ZnO and ZnO:Co samples.

-1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3

0.0

5.0x10-3

1.0x10-2

1.5x10-2

2.0x10-2

2.5x10-2

Cur

rent

(A)

Potential vs. SCE (V)

ZnO ZnCoO

Figure 1: Electrochemical polarization curves for pure

ZnO (full line) and cobalt-doped ZnO (dashed). Electron micrographs were performed by using a Zeiss

EVO 50 scanning electron microscope with LaB6 cathode, enabling a 2 nm resolution. XPS was employed to derive

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the elemental composition and chemical states, by using a VG ESCA Mk-II installation, upgraded with a Specs X-ray gun and a Specs electron flood gun for sample neutralization. The Al Kα unmonochromatized radiation (1486.7 eV) was used for excitation. Photoelectrons were collected at a takeoff angle of 50°. The magnetic characteristics have been inferred from MOKE hysteresis loops (AMACC Anderberg & Modéer Accelerator) with the magnetic field parallel to sample surface.

3. RESULTS AND DISCUSSION

Figure 2 presents XPS analysis of the obtained layers: O

1s and Zn Auger LMM electron spectra (Fig. 2a), Zn 2p (Fig. 2b), and Co 2p (Fig. 2c). From these core level spectra, the atomic composition of the sample is estimated, as given in Table 1. One may observe that the ZnO samples present excess of Zn (or, equivalently, oxygen deficit) which is really strong at - 750 mV (1.5 Zn atoms for an O atom) and decreases when increasing the overpotential; this unbalance disappears, however, for the sample obtained at - 850 mV overpotential. When doping with cobalt, the oxygen deficit stays almost constant (~ 1.14 Zn for an O in average) with the value of the overpotential. This suggests a competition between two routes for Zn2+ reduction, with formation of ZnO and NO2

- in one case, and of metal Zn + NO3

- in the other. The Co concentration varies smoothly with the overpotential (from 15 to 11 at. % when increasing the absolute value of the overpotential from 750 to 850 mV). This presents an easy way to control the Co concentration via just the deposition conditions.

The Co 2p3/2 peak position is found at 779.3 - 779.6 eV, which seems to indicate the formation of compounds like Co3O4 [9], so with a mixture of Co3+ and Co2+. The pure contribution of Co2+ have to be ruled out, since it results in a considerable higher binding energy, by about 1 eV [9].

Figure 3 presents SEM micrographs of the six samples, ZnO and Co-doped ZnO. Increse of structural disorder and crystallite re-orientation are observed for cobalt doped samples. This trend is most prononced for samples obtained at - 750 and at - 800 mV overpotentials, whereas a lower perturbation of the ZnO morphology by insertion of cobalt is observed for the samples deposited at - 850 mV.

MOKE magnetometry (Figure 4) revealed an open hysteresis cycle at room temperature only for ZnO:Co deposited at - 850 mV. This is a non trivial result, in view of the existing data in literature. To our knowledge, this is a first report on occurence of RTFM in cobalt doped ZnO obtained electrochemically.

The samples exhibiting RTFM are the samples with the less cobalt content (Table 1) and where the structural perturbation by cobalt is the lowest (Fig. 3 c, f). ________________________________________________ Figure 2 (right): Al Kα X-ray photoelectron spectroscopy (XPS) of ZnO and ZnO doped with Co: (a) represents O 1s and Zn LMM regions, (b) represents the Zn 2p region, (c) represents the Co 2p region.

(a)

(b)

(c)

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(a) (b) (c)

(d) (e) (f)

Figure 3: Scanning electron micrographs (magnification: x 85,000) of pure ZnO [(a): - 750 mV; (b): - 800 mV; (c): - 850 mV] and of cobalt doped ZnO [(d): - 750 mV; (e): - 800 mV; (f): - 850 mV]. Table 1: Composition analysis of ZnO and ZnO:Co samples. For each level O 1s, Co 2p3/2, or Zn 2p3/2 the following data are given: background-corrected XPS line intensities, corrected intensities by atomic sensitivity factors [8], atomic percentage, and estimated compositions.

O 1s Zn 2p Co 2p Sample V(mV) intensity corrected at.% intensity corrected at.% intensity corrected at.% Estimated

composition ZnO:Co - 750 1012 1533 45 7829 1631 48 586 234 7 Co0.15Zn1.06O ZnO:Co - 800 796 1207 43 6934 1445 52 328 131 5 Co0.12Zn1.21O ZnO:Co - 850 923 1399 45 7519 1566 50 361 144 5 Co0.11Zn1.11O ZnO - 750 841 1274 40 9337 1945 60 0 0 0 Zn1.5O ZnO - 800 795 1205 43 7606 1585 57 0 0 0 Zn1.32O ZnO - 850 997 1511 50 7111 1481 50 0 0 0 ZnO

4. CONCLUSION

Cobalt doped zinc oxide thin layers were grown by electrochemistry and some of the films, grown at highest overpotential (- 850 mV) exhibited room temperature ferromagnetism. To our knowledge, this is the first report of such a result for electrochemically grown ZnO:Co films. Films exhibiting room temperature ferromagnetism are those where the cobalt content is the lowest, the morphological perturbation induced by cobalt of the ZnO is also the lowest, and also grown in conditions where, in absence of cobalt, the films presented the closest stoichiometry to bulk ZnO. The Co-containing films exhibit a slight deviation in stoichiometry, having about 20 % more metal atoms (Zn + Co) for an oxygen atom. Although the XPS data seem to indicate the formation of Co3O4 complexes, it is yet not clear whether the observed ferromagnetism belongs to such complexes [5], to Zn interstitials and/or to oxygen vacancies [6]. Future X-ray absorption fine structure (XAFS) experiments are planned

to ellucidate the placement of Co ions in the ZnO lattice and the local atomic structure of the films [1]. Acknowledgements: This work was funded bt the National Authority for Scientific Research in Romania through Contract No. 71-063/2007 MAMAINCOPAE.

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