Regular domain structures fabricated by an electron beam in stoichiometric LiNbO3 crystals

Abstract

In this work, we study the capability of electron beam writing of regular domain structures (RDSs) in stoichiometric LiNbO 3 crystals. Domain structures were formed in two crystal types. The first type sto ichiometric LiNbO 3 crystal (SLN) was grown by the Czochralski method from a Li 2 O enriched melt (~58.6 mol % Li 2 O) [1]. Such crystals, due to the sig nificant difference in melt and crystal compositions, are characterized by a highly nonuniform refractive index along the growth axis and are hardly applicable in practice. The second crystal was grown from the congruent composition melt containing an additive of 6 wt % K 2 O alkali solvent (flux) [2]. The melt contains almost 58 mol % (in recalculation) of related alkali components (48.6 mol % Li 2 O + 9.3 mol % K 2 O), which determines its structure and makes it possible to obtain LiNbO 3 crystals that are very similar in compo sition and properties to stoichiometric ones (NSLN) but simultaneously are no worse than congruent crys tals (CLN) in homogeneity [2,3]. According to [4], NSLN is slightly more defective than SLN and exhib its a lowered photorefractive effect in comparison with SLN and CLN.To pattern RDSs, optically polished Z cuts 0.75 mm thick, fabricated from SLN and NSLN crys tals, were irradiated in a scanning electron microscope with a controlled electron beam (E = 25 keV, I = 0.1-0.25 nA). To provide a uniform electric field, Al was deposited on the opposite +z surface, and samples were grounded. The local irradiated areas S = 1 × 1 and 1.5 × 1.5 μm were aligned along lines with interval of 1.0 or 1.5 μm. The distances between lines for differ ent structures varied from 6.5 to 10 μm. Periodic lines were patterned in parallel to the X or Y direction on a crystal area of ~500 × 500 μm 2 . After that, samples were etched for ~60 s in a HF + 2HNO 3 solution upon heating. Then RDSs were studied using a Zeiss Axioplan 2 optical microscope and a Nano R2TM atomic force microscope.The threshold charges necessary for individual domain nucleation slightly differed in SLN and NSLN at 25 keV, but were close, Q NSLN ≤ 1 × 10 -11 C and Q SLN ~ 1.2 × 10 -11 C. In both crystals at threshold and close to threshold charges Q, the domain shape was triangular. The average radius of triangular domains, determined by their area, was r d ~ 2.5 μm for SLN and ~4 μm for NSLN. Despite their difference, switching region sizes for both crystals exceed irradi ated area sizes, which is probably caused by electron drift in the irradiated region [5]. In SLN, in contrast to NSLN [6], no gradual transformation of triangular nuclei to the hexagonal form was detected as the intro duced charge increased. In SLN, only an increase in the number of fine triangular nuclei formed in the irra diation region (Figs. 1a and 1b) occurred with increas ing introduced charge. Individual domains in SLN did not grow to the opposite side at distances between irra diations of 30, 20, and 10 μm. In NSLN, the transition from the 30 μm interval to the 20 μm interval resulted in that a significant frac...