Single-crystal LiNbO3 surfaces processed in low-temperature hydrogen plasma: XPS, REELS and AFM study

Single crystals of lithium niobate, which is a widely used material in opto-electronics, holography, surface acoustic wave devices, etc., have been processed in a low-temperature plasma of radiofrequency discharge in hydrogen. The aim was to examine a new kind of surface modification of LiNbO3. The surface modification has been studied by XPS, reflection electron energy-loss spectroscopy (REELS) and atomic force microscopy (AFM). The XPS study via the Nb 3d spectral lineshape revealed the plasma effect: a decomposition of the niobate structure and the appearance of niobium oxides. The subsequent REELS study performed at 4 keV electron energy showed a transfer of characteristic electron-loss energies of relevant transitions. Oxygen and lithium escape from the surface and hydrogen is incorporated from the plasma, producing niobium hydride. The deterioration of the niobate unit cells is accompanied by an enhancement of conduction electrons. The subsequent thermal reoxidation in medical oxygen did not restore the original niobate structure. The surface morphology after the reoxidation is characterized by elongated 100 nm grains situated normally to the surface. The main constituents of this new top layers are niobium oxides Nb2O5 and NbO. The present results form part of a complex study of the plasma modification of lithium niobate. The plasma processing induces lattice defects that are similar to those of ion implantation or metal ion doping: the electrical and optical properties of the surface layer drastically differ from those of the bulk. Copyright © 2002 John Wiley & Sons, Ltd.