Surface analysis of LiNbO3 single crystals modified by radiofrequency hydrogen plasma

Abstract

Lithium niobate (LiNbO3) is a widely used ferroelectric material with high electro-optic, non-linear optic, birefringent and photorefractive activities. Modification of the surface of LiNbO3 single crystals is essential for its application in optoelectronics. We accomplished the surface modification of single-domain LiNbO3 wafers (z-cut) in the hydrogen plasma of radiofrequency (r.f.) discharge (13.56 MHz) under pressures of 0.5–3 mbar. Modification of the wafer surface is demonstrated mainly by oxygen and lithium depletion. The overall depth of the modified surface region is ∼0.5 µm. There is a thin near-surface layer (<10 nm) where the niobate structure is destroyed and only niobium oxide remains there. Towards the bulk the niobate structure is preserved, although with many point defects. From a depth of 1 µm the genuine niobate composition remains. A double inversion of the spontaneous polarization in the layer was found after reoxidation of the plasma-processed sample. The polarity inversion is adjoined to the internal fields induced by point defect gradients in the plasma-modified surface. The following methods are used: neutron depth profiling for lithium depth distribution; RBS for heavier element depth profiles; x-ray diffraction for evaluation of crystallinity changes; and XPS for composition and chemical bonding of elements in the actual surface. Copyright © 2000 John Wiley & Sons, Ltd.