The ZnO samples were grown by atomic layer deposition (ALD) in the Institute of Physics Polish Academy of Sciences in Warsaw. The main advantage of this method is a self limitation and sequential growth process, which enables using very reactive precursors and reducing growth temperature, while keeping good crystallographic and optical parameters. The polycrystalline and epitaxial ZnO films were deposited at 300ºC on a Si and a GaN/Al₂O₃ template, respectively.

Deposition of Sm atoms onto the ZnO surface and photoemission measurements were performed at the FLIPPER II beamline in HASYLAB synchrotron radiation laboratory (Hamburg, Germany).  Samarium was deposited from a Knudsen cell under ultra-high vacuum conditions at room temperature. Sm atoms were evaporated sequentially to obtain capping layers with the thickness of 1 ML, 3 ML and 7 ML. After the last deposition the samples were annealed for 0.5 and 4 hours at 300ºC. The set of energy distribution curves (EDC) was acquired after each step of the deposition and annealing processes. The investigations of Sm/ZnO system by resonant photoemission spectroscopy were carried out for the photon energy range of 130-160 eV that corresponds to the Sm4d→Sm4f Fano resonance region [4]. For the 4f⁶ (Sm²⁺) and the 4f⁵ (Sm³⁺) configurations, two resonant photoemission processes occur:

Sm4d¹⁰4f⁶ + hν → [Sm4d⁹4f⁷]* → Sm4d¹⁰4f⁵ + e        (1)

and

Sm4d¹⁰4f⁵ + hν → [Sm4d⁹4f⁶]* → Sm4d¹⁰4f⁴ + e        (2)

where []* denotes an excited state. Each of the processes described above is accompanied by the classical photoemission process as follows:

Sm4d¹⁰4f⁶ + hν → Sm4d¹⁰4f⁵ + e                                  (3)

and

Sm4d¹⁰4f⁵ + hν → Sm4d¹⁰4f⁴ + e                                  (4)

As a result of the interference between resonant and classical photoemission channels, we observe two separated resonances for divalent (channels (1) and (3)) and trivalent (channels (2) and (4)) samarium states [5]. The photoemission resonance for Sm³⁺ configuration is observed at photon energy 141 eV, whereas the resonance for Sm²⁺ configuration occurs at photon energy 136 eV [4]. Photoemission Energy Distribution Curves (EDCs) taken after each step of deposition and annealing at photon energy of both Fano resonances are shown in Fig. 1 and 2.

Fig. 1. Photoemission spectra EDCs for Sm/ZnO measured at hν = 136 eV to emphasize the 4f⁶ (Sm²⁺) emission. The spectra show the valence band and the nearest core levels.

After each step of deposition we observed photoemission enhancement at the valence band edge, i.e. in the energy region characteristic of Sm²⁺ configuration. We also see a small increase of photoemission intensity in the energy region between 5 eV and 10 eV below the valence band edge, i.e. characteristic of Sm³⁺ configuration. After 30 min. annealing at 300ºC the peak at the valence band edge (characteristic of Sm²⁺) disappears and the feature of Sm³⁺ markedly increases, which is the fingerprint that samarium atoms become trivalent. We also observe that the maximum of the Zn3d core level shifts to lower binding energy after samarium deposition, but returns to the previous position characteristic of ZnO crystal after 30 min. annealing. This indicates that chemical reaction between ZnO and Sm takes place at the interface region.

Fig. 2. Photoemission spectra EDCs for Sm/ZnO measured at hν = 141 eV to emphasize the 4f⁵ (Sm³⁺) emission. The spectra show the valence band and the nearest core levels.

Acknowledgements: The authors acknowledge support by MSHE of Poland research Projects DESY/68/2007 and by the European Community via the Research Infrastructure Action under the FP6 Structuring the European Research Area" Programme (through the Integrated Infrastructure Initiative "Integrating Activity on Synchrotron and Free Electron Laser Science") at DESY. Partially supported by Innovative Economy  (POIG.01.01.02-00-008/08).

References

[1] W. M. Jadwisienczak et al., J. Electron. Mater. 31, 776 (2002)

[2] L. Armelao et al., J. Phys. Chem. C  111, 10194 (2007)

[3] C.T.M. Ribeiro, F. Alvarez, A.R. Zanatta, Adv. Mat. 14, 1154 (2002)

[4] E. Guziewicz et al., Surf. Sci. 551, 132 (2004)

[5] F. Gerken, et al., in: B. Craseman (Ed.), Proceedings of the International Conference on X-ray and Atomic Inner-Shell Physics, AIP Conference Proceedings no. 94, AIP, New York, 1982, p. 602.

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