Photovoltaic Effect in Ferroelectric LiNbO3 Single Crystal

Abstract

Lithium niobate (LiNbO3) is a human-made dielectric material and was first discovered to be ferroelectric in 1949. Properties and applications of LiNbO3 have been widely studied, resulted in several thousands of papers on this material, since the crystal was successfully grown using the Czochralski method by Ballman in 1965 (Kong et al., 2005). It has been extensively researched for its excellent ferroelectric, piezoelectric, dielectric, pyroelectric, electric-optical and nonlinear optical properties (Wang et al., 2008; Chen et al., 2007; Sarkisov et al., 2000). Now LiNbO3 is a very significant material for optical applications, such as acoustic wave transducers, acoustic delay lines, acoustic filters, optical amplitude modulators, optical phase modulators, second-harmonic generators, Q-switches, beam deflectors, dielectric waveguides, memory elements, holographic data processing devices, and others (Kim et al., 2001; Zhen et al., 2003; Pham et al., 2005; Liu et al., 2002; Zhou et al., 2006). LiNbO3 is a ferroelectric material which has the highest Curie temperature of about 1210 °C up to now and the largest spontaneous polarization of about 0.70 C/m2 at room temperature. LiNbO3 single crystals exhibit paraelectric phases above the Curie temperature and ferroelectric phases below the Curie temperature (Karapetyan et al., 2006; Bermudez et al., 1996). Ferroelectric LiNbO3 crystal is a member of the trigonal crystal system, exhibiting three-fold rotation symmetry about its c axis. Its structure consists of planar sheets of oxygen atoms in a distorted hexagonal close-packed configuration. The octahedral interstices in this structure are one-third filled by lithium atoms, one-third by niobium atoms, and one-third vacant. In the paraelectric phase the Li atoms and the Nb atoms are centered in an oxygen layer and an oxygen octahedral, making the paralelctric phase nonpolar. But in ferroelectric phase the Li atoms and the Nb atoms shifted into new positions along the c axis by the elastric forces of the crystal, making the LiNbO3 crystal exhibiting spontaneous polarization (Bergman et al., 1968). Many methods were reported to determine the +c axis of ferroelectric LiNbO3 single crystal. A standard method is to compress the crystal in the c axis direction. The +c axis is defined as being directed out of the c face that becomes negative upon compression. This can be understood that the Li and Nb ions move closer to their centered positions upon compression, leaving excess negative compensation charges on the +c face, causing the +c face to become negative. Anther method to identify the +c face and –c face of the crystal is an etching technique with HF solution. The etching speed on the –c face is faster than on the +c face (Beghoul et al., 2008; Bourim et al., 2006). Other methods to determine the +c axis