Photovoltaic effect and photoconductivity in Sc-doped near-stoichiometric LiNbO3 crystals

The photorefractive damage (optical damage) process in Sc-doped near-stoichiometric LiNbO3 (Sc:SLN) crystals was investigated by measuring the photocurrents in these crystals for several Sc concentrations (up to approximately 1 mol%). The photovoltaic current density and photoconductivity were then estimated using the measured photocurrents. The saturated space-charge field, which is the ratio between the photovoltaic current density and photoconductivity and which causes the photorefractive damage, was then estimated. The photovoltaic current density decreased with an increasing Sc concentration while the photoconductivity increased with the concentration. This combination of decreased photovoltaic current density and increased photoconductivity means that the saturated space-charge field decreases with an increasing Sc concentration. This decrease in the saturated space-charge field apparently reduced the photorefractive damage in the Sc:SLN crystals. These behaviors are similar to those of Mg-doped LN (Mg:LN) crystals. The saturated space-charge field of the Sc:SLN crystals with no photorefractive damage was less than 100 V/cm (104 V/m), as it is for Mg:LN crystals. This means that the Sc concentration needed to obtain a less than 100-V/cm saturated space-charge field in SLN crystals is less than the corresponding value for Mg doping.