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 (E_s) 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 E_s = 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. E_s = 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: E_s = 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.

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/28932 must be provided.

1. Growth and EPR properties of NdVO₄, HoVO₄ and NdVO₄ single crystals
2. Pyrochlores Grown by Optical Floating Zone Method
3. EPR and optical properties of PbMoO₄:Co single crystals
4. Temperature dependence of PL and EPR spectra of Sr_0.33Ba_0.67Nb₂O₆:Cr (0.02mol.%) single crystals
5. Magnetic and optical properties of  Li_0.28Na_1.72Ge₄O₉:Cr, Mn (0.1mol%) single crystals
6. High-pressure diffraction study of structural and elastic properties of zircon-type and scheelite-type RVO₄ (R = Nd, Eu)
7. Structural instabilities in LnGaO₃ perovskites at high pressures
8. Growth and characterization of Li₂B₄O₇ single crystals pure and doped with Co ions
9. Temperature Raman scattering study of CaAl_0.5Ta_0.5O₃ crystal
10. Growth and Magnetic properties of rubidium rare earth double tungstates single crystals.
11. Strain relaxation in thin films of LaSrCuO grown by pulsed laser deposition
12. Origin of the negative thermal expansion in perovskite-type PrGaO₃
13. The effect of film-substrate mismatch on the properties of superconducting YBa₂Cu₃O_7-δ films
14. The enhancement of vortex pinning in ferromagnet-superconductor bilayers
15. Use of element selective methods for characterization of thin films
16. Gallium nitride surface formation and modification by Mn deposition - photoemission studies with use of synchrotron radiation
17. Transport and magnetic properties of La_2/3Pb_1/3MnO₃ thin films
18. Crystal structure and thermal expansion of PrGaO₃ in the temperature range 10-1253 K
19. Twin structure of LSGMO crystals studied by Laue method