Abstract
The origin of twinning during the Czochralski (CZ) growth of 36°-RY lithium tantalate (LiTaO3) single crystals is examined, and it is shown that lineages composed of dislocation arrays act as an initiation site for twinning. Two types of lineages expand roughly along three different {12¯10} planes and two different {11¯00} planes. The former lineages and some latter lineages are composed of two types of mixed-dislocations with different Burgers vectors, while the other lineages are composed of only one type of edge-dislocation. All the dislocations have the Burgers vector of ⟨12¯10⟩ type with the compression side at the +Z side. Twin lamellae on {101¯2} are generated at a lineage during the CZ growth. We have hypothesized that dislocations in the lineage with b = 1/3⟨12¯10⟩ change their extension direction along a slip plane of {101¯2}, and they dissociate into pairs of partial dislocations with b = 1/6⟨22¯01⟩and 1/6⟨02¯21¯⟩ forming twin lamellae on {101¯2}.
Highlights
Lithium tantalate (LiTaO3 ) crystallizing in the space group R3c [1] represents lithium-based trioxides, and has a unique combination of excellent acoustic, non-linear optical, piezoelectric, pyroelectric, and electro-optical properties depending on its crystallographic direction [2]
Two kinds of LiTaO3 wafers of 0.5 mm thick, both sides of which were polished to a mirror finish by chemical mechanical polishing (CMP) technique [24], were cut out from the single crystals; 36◦ -RY wafers were cut normal to the pulling direction, and {0001}
Most of our non-stoichiometric CZ-LiTaO3 single crystals that are available for commercial wafers contain no macroscopic defect except lineages, and they have high optical uniformity all over the crystals
Summary
Lithium tantalate (LiTaO3 ) crystallizing in the space group R3c [1] represents lithium-based trioxides, and has a unique combination of excellent acoustic, non-linear optical, piezoelectric, pyroelectric, and electro-optical properties depending on its crystallographic direction [2]. The functional crystal is the most attractive material for surface acoustic wave (SAW) filters, especially for mobile phones, as well as for frequency conversion [3], acoustic devices [4], and pyroelectric detectors [5,6]. Nowadays, using Czochralski (CZ) growth techniques [13,14], LiTaO3 single crystals with high optical uniformity, without the macroscopic structural defects of cellular structures, solute striations, domains, and twins [15], can be grown to large diameters more than 4 inches. Even though arrays of dislocations on specific planes (so-called lineages) are occasionally introduced in commercial LiTaO3 single crystals, owing to high thermal stresses during the CZ growth [16], they do not severely influence the performance of SAW filters. From a commercial point of view, Crystals 2020, 10, 1009; doi:10.3390/cryst10111009 www.mdpi.com/journal/crystals
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