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Surface charging of crystalline oxides in X-ray photoelectron spectroscopy - examples of LiNbO3 , Lu2SiO5 and LuVO4
|Ryszard J. Iwanowski 1, Markku Heinonen 2, Marek Berkowski 1, Michał M. Głowacki 1, Izabela Pracka 3, Hanna A. Dabkowska 4
1. Polish Academy of Sciences, Institute of Physics, al. Lotników 32/46, Warszawa 02-668, Poland
X-ray photoelectron spectroscopy (XPS) is one of the major research tools for studying solid surfaces, either of bulk materials or of thin films. For conducting solids it enables direct qualitative- and quantitative chemical analysis of the surface and subsurface region, also providing information about electronic structure of material (valence band, core-level structure). On the other hand, a positive surface charge induced by X-ray beam within the photoemission process is a main problem in the XPS study of insulating materials, including the large-gap oxide compounds.
The goal of this work was to exhibit certain aspects of surface charging by taking into account the results of XPS investigations of the following single crystalline oxides doped with rare earths: LiNbO3 :(Er,Yb), Lu2SiO5 :Dy and LuVO4 :Yb (the dopant symbols will be omitted further in the text).
XPS measurements were performed with a monochromatized Al Kalpha (E= 1487 eV) X-ray excitation source, where charging of the sample is very important. The spectra were acquired in two modes: (1) with use of surface charge neutralization by low-energy electron beam (electron flood gun), (2) without a charge neutralization. Their comparison enabled us to determine the binding energy (BE) shift (Es) of the particular XPS spectrum and thus, the charge induced on the X-rayed surface of the crystal studied.
For LiNbO3 we obtained the magnitude of BE shift equal Es = 230 eV, i.e. the higher value than the relevant ones reported earlier (e.g. F.Bart et al., J. El. Spectr. Rel. Phenom. 69). It was accompanied by a modification of the spectral lineshapes. For example, in neutralization conditions (mode 1) the Nb 3d spectrum indicated a purely 5+ valence state of Nb ion, whereas in a lack of charge compensation additional contribution of the Nb4+ and Nb3+ centers appeared in the core-line. This could be ascribed to partial surface reduction of Nb5+ ions to Nb4+ and Nb3+ ions, although in this case the process is not irreversible.
The analogous XPS experiment for Lu2SiO5 gave unexpectedly high value of BE shift, i.e. Es = 650 eV, which indicated very strong surface charging. A huge broadening of the most intensive XPS lines led to disappearance of the other spectral lines that excluded even a qualitative chemical analysis. On the contrary, the same experiment with charge neutralization mode (1) provided a high quality XPS spectrum. This has been compared with the relevant XPS data for LuVO4 , where much weaker surface charging occurred, namely: Es = 130 eV. In the neutralization conditions both these Lu-based oxides exhibited good consistency between their valence-band spectra as well as between their Lu 4f and Lu 4d core-level spectra. In the latter case, we observed slightly higher (but not exceeding 7 %) widths of these core-lines in Lu2SiO5 , compared to the relevant ones of LuVO4. It could suggest an indirect influence of stronger surface charging in Lu2SiO5.
The above results illustrate well particular problems related to surface charging (and its neutralization) as an inevitable effect in the X-ray photoelectron spectroscopy of insulating crystalline oxides.
Presentation: Poster at 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17, General Session 5, by Ryszard J. Iwanowski
See On-line Journal of 17th International Conference on Crystal Growth and Epitaxy - ICCGE-17
Submitted: 2013-03-27 11:45 Revised: 2013-04-12 14:16