Twinning Defects in Near-stoichiometric Lithium Niobate Single Crystals

doi:10.15541/jim20240343

Abstract

Abstract:

Lithium niobate (LN) single crystals have emerged as one of the most valuable materials for integrated photonics materials due to their exceptional properties, including non-linear and electro-optical effects. Compared to congruent lithium niobate (CLN) crystals, near-stoichiometric lithium niobate (nSLN) crystals exhibit more pronounced non-linear and electro-optical properties, offering higher application value. nSLN crystals with high compositional uniformity can be prepared using a vapor transport equilibration method. However, large-size LN single crystals are highly susceptible to twinning defects and wafer cracks during diffusion processing. Here, large size nSLN single crystals were prepared using the vapor transport equilibration method to address the aforementioned defects and cracking. The twinning defects within wafers after diffusion processing were characterized, the mechanisms of twinning formation were analyzed, the wafer placement method to produce complete 4-inch (100 nm) and 6-inch (153 nm) wafers was modified, and the composition and transmittance of wafers were tested. Results indicate that composition of the wafers is at least 49.94% (in mole), approaching stoichiometric ratio, and their transmittance is greater than 71% across the 600-3300 nm range. Both Z-cut and X-cut wafers prepared by vapor transport equilibration method exhibited twinning defects. However, cracks were observed when twinning defects intersected on Z-cut wafers, whereas no cracks were present on X-cut wafers. The twinning planes on both Z-cut and X-cut wafers were depending on ${01 1 ¯ 2}$ , which they were identified as deformation twins. According to the mechanism of deformation twin formation, the non-uniform deformation of lithium-rich materials is identified as the primary driving force beneath the twin formation. Ultimately, it was proposed to mitigate twin activation by modifying the diffusion treatment process, thereby increasing the yields of 4-inch (100 nm) and 6-inch (153 nm) nSLN wafers.

Key words: near-stoichiometric lithium niobate, vapor transport equilibration, twinning defect, Li-rich atmosphere

CLC Number:

O782

HAO Yongxin, SUN Jun, YANG Jinfeng, ZHAO Chencheng, LIU Ziqi, LI Qinglian, XU Jingjun. Twinning Defects in Near-stoichiometric Lithium Niobate Single Crystals[J]. Journal of Inorganic Materials, 2025, 40(2): 196-204.

Figures/Tables 14

Fig. 1 Schematic diagram of sample locations for wafer Li content testing

Table 1 Results of crystals diffusion under different conditions

Experiment	Sample	Crystal		Diffusion treatment condition			Results
Experiment	Sample	Orientation	Size/mm	Temperature/℃	Time/h	Wafer placement	Results
1	VTE-1-1	Z-cut	ϕ100×0.86	1100	120	Horizontal	Cracked, twins
1	VTE-1-2	Z-cut	ϕ100×0.86	1100	120	Horizontal	Cracked, twins
2	VTE-2-1	Z-cut	ϕ100×0.86	1100	120	Vertical	Uncracked, twins
2	VTE-2-2	Z-cut	ϕ100×0.86	1100	120	Vertical	Uncracked, twins
3	VTE-3-Z	Z-cut	ϕ100×1.02	1100	300	Vertical	Uncracked, twins
3	VTE-3-X	X-cut	ϕ76.2×0.50	1100	300	Vertical	Uncracked, twins
4	VTE-4-1	Z-cut	ϕ100×0.86	1100	120	Vertical	Uncracked, twins
4	VTE-4-2	Z-cut	ϕ100×0.86	1100	120	Vertical	Uncracked, twins
5	VTE-5-1	Z-cut	ϕ100×0.86	1100	120	Vertical	Uncracked, twins
5	VTE-5-2	Z-cut	ϕ100×0.86	1100	120	Vertical	Uncracked, without twins
6	VTE-6-1	Z-cut	ϕ100×0.86	1100	120	Horizontal	Uncracked, without twins
6	VTE-6-2	Z-cut	ϕ100×0.86	1100	120	Horizontal	Uncracked, without twins
7	VTE-7	Z-cut	ϕ153×1.02	1100	120	Horizontal	Uncracked, without twins

Fig. 2 4-inch (100 nm) Z-cut LN wafers obtained from VTE experiments 1-6 (a) VTE-1-1; (b) VTE-1-2; (c) VTE-2-1; (d) VTE-2-2; (e) VTE-3-Z; (f) VTE-3-X; (g) VTE-4-1; (h) VTE-4-2; (i) VTE-5-1; (j) VTE-5-2; (k) VTE-6-1; (l) VTE-6-2

Fig. 3 Schematic diagram of experimental device for the 4th VTE experiment

Fig. 4 6-inch (153 nm) Z-cut nSLN wafer obtained from the 7th VTE experiment (VTE-7)

Table 2 Curie temperatures (TC) and Li contents (CLi) of different wafers

Sample		T_C/℃	Average of T_C/℃	C_Li/% (in mole)
VTE-5-2	1	1200	1200.2	49.94
	2	1201
	3	1200
	4	1201
	5	1199
VTE-6-1	1	1202	1201.6	49.98
	2	1201
	3	1202
	4	1201
	5	1202
VTE-7	1	1202	1201.0	49.96
	2	1201
	3	1200
	4	1201
	5	1201

Fig. 5 Transmission spectra of CLN wafer and nSLN wafers (a) VTE-5-2; (b) VTE-6-1; (c) VTE-7

Fig. 6 Birefringence changes in the twinning regions of wafers VTE-3-Z and VTE-3-X rotated at different angles within 180° (a) 0°-VTE-3-Z; (b) 45°-VTE-3-Z; (c) 90°-VTE-3-Z; (d) 135°-VTE- 3-Z; (e) 0°-VTE-3-X; (f) 45°-VTE-3-X; (g) 90°-VTE-3-X; (h) 135°-VTE- 3-X

Fig. 7 Direction of the twinned lamella on the Z-cut wafer after VTE process (a) Wafer VTE-3-Z; (b) Schematic diagram of VTE-3-Z; (c) Angle between the twinned lamella and the Z-plane is 57°; (d) Angle between the twinned lamella and the X-plane is 90°

Fig. 8 X-ray diffraction pattern of the twinning plane within the wafer VTE-1-1

Table 3 Angles between families of crystal planes and (0001) plane of LN crystal[22]

Family of LN crystal planes	Angle	Family of LN crystal planes	Angle
${10 1 ¯ 0}$	90°	${11 2 ¯ 0}$	90°
${10 1 ¯ 1}$	72°10′	${22 4 ¯ 3}$	74°26′
${10 1 ¯ 4}$	37°52′	${01 1 ¯ 2}$	57°15′
${11 2 ¯ 3}$	60°53′	${01 1 ¯ 8}$	21°14′
${11 2 ¯ 6}$	41°55′	${02 2 ¯ 1}$	80°52′

Table 3 Angles between families of crystal planes and (0001) plane of LN crystal[22]

Family of LN crystal planes	Angle	Family of LN crystal planes	Angle
${10 1 ¯ 0}$	90°	${11 2 ¯ 0}$	90°
${10 1 ¯ 1}$	72°10′	${22 4 ¯ 3}$	74°26′
${10 1 ¯ 4}$	37°52′	${01 1 ¯ 2}$	57°15′
${11 2 ¯ 3}$	60°53′	${01 1 ¯ 8}$	21°14′
${11 2 ¯ 6}$	41°55′	${02 2 ¯ 1}$	80°52′

Fig. 9 Cracks at crossing of twins on Z-cut wafers before and after polishing (a) Two twinned lamella (before polishing); (b) Three twinned lamella (before polishing); (c) Two twinned lamella (after polishing); (d) Three twinned lamella (after polishing)

Fig. 10 Direction of the twinned lamella on the X-cut wafer after VTE process (a) Wafer VTE-3-X; (b) Schematic diagram of VTE-3-X; (c) Angle between the wider twinned lamella and the Z-plane is 37°; (d) Angle between the narrower twinned lamella and the Z-plane is 57°; (e) Angle between the wider twinned lamella and the Y-axis is 30°; (f) Angle between the projection of the narrower twinned lamella and the Y-axis is 90°; (g) Projections of ( 1 1 ¯ 02 ) and ( 1 ¯ 012 ) on the X-plane; (h) Schematic diagram of observation directions ( 1 1 ¯ 02 ) and ( 1 ¯ 012 )

Fig. 11 Slip of niobium atoms (or lithium atoms) during twinning deformation of LN crystal

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