Radiation Damage Effects in Ferroelectric Litao 3 Single Crystals
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ABSTRACT Z-cut lithium tantalate (LiTaO3) ferroelectric single crystals were irradiated with 200 keV Art ions. LiTaO 3 possesses a structure that is a derivative of the corundum (A12 0 3 ) crystal structure. A systematic study of the radiation damage accumulation rate as a function of ion dose 2 ions/m 2 was performed using ion-beam channeling experiments. An ion fluence of 2.501018 Ar + was sufficient to amorphize the irradiated volume of a LiTaO3 crystal at an irradiation temperature of -120K. This represents a rather exceptional susceptibility to ion-induced amorphization, which may be related to a highly disparate rate of knock-on of constituent lattice ions, due to the large mass difference between the Li and Ta cations. We also observed that the c end of the ferroelectric polarization exhibits slightly higher ion dechanneling along with an apparent greater susceptibility to radiation damage, as compared to the c' end of the polarization. INTRODUCTION Lithium tantalate (LiTaO3) is a ferroelectric oxide possessing large nonlinear optical coefficients [1] as well as large electro-optic coefficients [2]. As such, LiTaO 3 crystals are of interest for nonlinear optical applications as well as for surface acoustic wave and piezoelectric devices [3]. LiTaO 3 is a derivative structure of corundum (cx-A120 3) with space group R3c. Recently, there has been considerable interest in the radiation damage tolerance of corundum structural derivative rhombohedral oxides [4-7]. The purpose of the study presented here is to assess the ion beam radiation damage resistance of lithium tantalate, especially in the context of ion beam irradiations under cryogenic conditions. EXPERIMENT Synthetic Z-cut LiTaO 3 crystals were obtained from Yamaju Ceramics Co., Ltd., (Crystal Technology Division, No. 1123 Sango-Cho, Owariasashi-shi Aichi-ken, Japan). Crystal wafers were 51 mm diameter and 0.5 mm thick, polished to a mirror finish on both sides. Samples were irradiated with 200 keV Ari' ions at an inclination of a few (-10) degrees to the c crystallographic axis, in order to minimize channeling effects. The ferroelectric polarization of these crystals, P8, is oriented along the c axis, i.e. normal to the front and back surfaces of each LiTaO 3 wafer. The c' axis is defined as the surface normal parallel to Ps, while the c axis is antiparallel to P,. Samples were irradiated at cryogenic temperature using a sample stage cooled to -120K by liquid nitrogen conduction cooling. TRIM simulations [8] were performed to determine the projected range of 200 keV Ar ions in LiTaO 3 and to determine the instantaneous displacement damage induced by this irradiating 159
Mat. Res. Soc. Symp. Proc. Vol. 504 © 1998 Materials Research Society
species. Simulations indicate that the longitudinal range of Ar ions in LiTaO 3 is -120 nm, while the longitudinal straggling is -51 nm. The peak Ar concentration is about 0.8 at.% per 10'9 Ar/m2 , while the peak number of displacements per atom (dpa) is approximately 0.8 dpa / 10' 9 Ar/m 2. Unirradiated and irradia
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