Single crystals of oxides have been used in versatile applications such as optical communications, semiconductor industry and medicine. Since functional property of the oxides strongly relies on the composition, the segregation of ionic species at the solid-liquid interface during growth is critical. In this study, we investigated the redistribution of ionic species during growth by using micro pulling down (μ-PD) method for two types of congruent LNs: conventional congruent-melting c-ZnO:LN and true congruent-melting cs-MgO:LN.
In chapter 1, studies of the melt growth of the LiNbO3 (LN) under the interface electric field was introduced. In the melt, it is known that the intrinsic interface electric field is present, which consists of the crystallization electromotive force (c-EMF) and the Seebeck effect-driven electric field. The conventional and true congruent melting LN crystals were introduced to investigate the partitioning of ionic species in the presence of interface electric field. The superiority of the true congruent melting LN was discussed to obtain highly uniform LN single crystals regardless of growth conditions.
In chapter 2, the effect of interface electric field on partitioning during the growth of conventional and true congruent-melting LN crystals was investigated.
Regarding the presence of the intrinsic interface electric field, the segregation of ionic species at the solid-liquid interface was experimentally observed during the growth of LN crystal by μ-PD technique. When a certain current was injected into the solid-liquid interface to cause Δ𝜙EMF = 0, a homogenous Mg distribution for cs-MgO:LN near the interface was achieved. This confirms that k0 for all melt chemical species is unity for true congruent-melting LN while the conventional is not, suggesting that the true congruent-melting LN is stoichiometric and congruent
83
simultaneously. Due to the non-linear dependence of the potential on the applied current in the solid, the relationship between the melt and the growing LN crystal was revealed to be a metal-n type semiconductor junction.
In chapter 3, the non-steady-state crystal growth of the LN in the presence of an interface electric field was studied. The non-steady-state growth was achieved by an abrupt increase in the growth velocity using the μ-PD technique, yielding a transient decrease of the Mg concentration in the solid. When the intrinsic electric field was compensated by a critical current Ic, no change in the solute concentration occurred during the non-steady-state growth. The true congruent LN crystal maintained compositional uniformity regardless of the growth conditions due to its unity value of k0 for all chemical components, including ionic species.
Also, both c-EMF and the supercooling potential were found to be zero when intrinsic electric field was counterbalanced by the current injection.
We have successfully clarified the partitioning of ionic species for the growth of two types of congruent LNs, conventional congruent-melting c-ZnO:LN and true congruent-melting cs-MgO:LN. The partitioning and transport of ionic species were manipulated by an interface electric field. The detailed process of segregation of ionic species was measured by EPMA. It is confirmed that k0 for all melt chemical species is unity for the true congruent-melting LN, which is simultaneously stoichiometric and congruent. Thus, cs-MgO:LN does not show any compositional change even when it experiences a non-steady-state growth under the injection of a critical current that compensated the intrinsic electric field leading to unity of k0 for all ionic species. The true congruent LN (cs-MgO:LN) exhibits better compositional stability than conventional LN (c-LN, c-ZnO:LN) under any growth conditions.
84 Supplementary
1. Measurement of temperature gradient near the solid-liquid interface
Growth of LN fiber crystal with a diameter of 1 mm or less is typically accompanied by a large temperature gradient near the interface with the μ-PD method. The temperature gradient in the melt at the interface, GL, is even over 3500 °C/cm (S. Uda, et al., J. Cryst. Growth 179 (1997) 567.). Here, the interface position was confirmed by the discontinuity of the temperature distribution in Fig.
S1, due to the different thermal conductivity between the solid and the melt. We obtained the average temperature gradient in the liquid (GL) near the interface, which was measured to be ∆T2/∆L=1020.3 °C/cm.
85
Fig. S1 Distribution of T2 during pulling-down process for cs-MgO:LN.
ΔT2 ΔT2
1205 1215 1225 1235 1245 1255 1265 1275
0 0.01 0.02 0.03 0.04 0.05 0.06
T2/˚C
Distance(L)/cm
Solid Liquid
GL=−1020.3 ˚C/cm ΔL
Interface
GS=−1117.1 ˚C/cm ΔL
86
2. Measurement of resistivity of the LN melt, 𝜌L (= 14.5 Ω cm)
The resistivity of LN melt can be measured via μ-PD furnace with the Pt capillary nozzle (⌀d = 1 mm, l = 2 mm) (Fig. S2). The Pt wire (1) is fixed in the LN melt and the Pt wire (2) is placed in the position 1. Various currents (DC power) are applied to the melt through two Pt wires (0.1 mm ⌀d) and the voltage is recorded with a voltmeter. After the first measurement, Pt wire (2) is pulled down to the position 2 and then we impose currents again. The distance between position 1 and position 2 is controlled to be 0.5 mm. We neglect the temperature difference in these two positions.
The current-voltage characteristics in the LN melt under different positions are shown in Fig. S3. The resistance in the melt R, is obtained from the slope value and it is supposed to be 92.143 Ω for 0.5 mm-length LN melt. Thus, the resistivity of LN melt is calculated to be 14.5 Ωcm by Eq. S1
𝜌8 = 𝑅Ž¾, (S1)
where R = 92.143 Ω, A = 0.008 cm2 and d = 0.5 mm.
87
Fig. S2 Schematic illustration of the setup for measurement of the resistivity of the LN melt.
Melt
Pt wire (2)→ mobile
Pt crucible Pt wire (1)→ fixed
DC power (Current)
V
0.5 mm d + 0.5 Pull down 1
2
88
Fig. S3 Current-voltage characteristics for an ohmic contact of LN melt in different positions.
y = 287.69x + 20.338 R² = 0.9923
y = 195.55x + 9.1989 R² = 0.9944 y = 92.143x + 11.14
R² = 0.9822
-150 -100 -50 0 50 100 150 200 250
-0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8
V ol tage ( m V )
Current (mA)
d + 0.5 d
0.5 mm
ρ = 14.5 Ω⋅ cm
89
3. Measurement of the temperature dependence of resistivity of the LN melt The effect of temperature T, on the resistance of LN melt is also measured via μ-PD furnace. Two Pt wires are fixed in the LN melt as shown in Fig. S4 and the distance d, between two wires is maintained at 0.5 mm. The current-voltage characteristics are measured under different LN melt temperature by controlling the main power.
The relationship between resistance and temperature is shown in Fig. S5, which is described by
ln5
Á = ln 5
Á3− cÂ
2pq, (S2)
where EA is the activation energy, equal to 3.44 × 10-19 J.
90
Fig. S4 Schematic illustration of the setup for measurement of the temperature dependence of resistance of the LN melt.
Melt
Pt wire
Pt crucible Pt wire
DC power (Current)
V
d
Change the main power
91
Fig. S5 Temperature dependence of resistance in the LN melt.
y = -24955x + 9.5177 R² = 0.9888
-6.2 -6.1 -6.0 -5.9 -5.8 -5.7
0.00061 0.000615 0.00062 0.000625 0.00063
1/ ln R
1/T
ln 1
R = ln 1
R
0− E
Ak
BT
92 List of Achievements
1. Journal Publications
(1) Qilin Shi, Satoshi Uda, Non-steady-state growth of LiNbO3 in the presence of an interface electric field, Journal of Crystal Growth (Under review).
(2) Qilin Shi, Jun Nozawa, Satoshi Uda, Journal of Crystal Growth, 549 (2020) 125864.
2. Presentations
(1) Oral presentations in international meetings in Japan
[1] “Compositional uniformity in the MgO-doped LiNbO3 crystal that is concurrently congruent and stoichiometric”; 2018 Annual Meeting of Excellent Graduate Schools for Materials Integration Center and Materials Science Center in conjunction with 2018 Russia-Japan Conference “Advanced Materials: Synthesis, Processing and Properties of Nanostructures”; 宮城県 仙台市(秋保); 2018/03/22−23.
(2) Poster presentations in international meetings in Japan
[1] “Compositional uniformity in the MgO-doped LiNbO3 crystal that is concurrently congruent and stoichiometric”; 2018 Annual Meeting of Excellent Graduate Schools for Materials Integration Center and Materials Science Center in conjunction with 2018 Russia-Japan Conference “Advanced Materials: Synthesis, Processing and Properties of Nanostructures”; 宮城県 仙台市(秋保); 2018/03/22−23.
[2] “Study on the interface potential distribution during growth of MgO-doped lithium niobate under an external electric field”; Summit of Materials Science 2018: SMS2018; 宮 城 県 仙 台 市 , 東 北 大 学 金 属 材 料 研 究 所 ;
93 2018/10/29−30.
(3) Oral presentations in domestic meetings in Japan
[1] “Compositional uniformity in the MgO-doped LiNbO3 crystal that is concurrently congruent and stoichiometric”; 第 46回結晶成長会議; 静岡県 浜松市; 2017/11/27−29.
[2] “Compositional uniformity in the MgO-doped LiNbO3 crystal that is concurrently congruent and stoichiometric”; 第 65 回応用物理学会春季学 術 講 演 会 ; 東 京 都 新 宿 区, 早 稲 田 大 学 西 早 稲 田 キ ャ ン パ ス; 2018/03/17−20.
[3] “Study on the interface potential distribution during growth of MgO-doped lithium niobate under an external electric field”; 第 47回結晶成長会議; 宮 城県仙台市; 2018/10/31−11/02.
[4] “Study on the electrical relationship at the interface between LN melt and LN
crystal”; 第 66 回応用物理学会春季学術講演会; 東京都目黒区, 東京工
業大学大岡山キャンパス; 2019/03/09−12.
[5] “Effect of interface electric field on ionic species segregation during the growth of LiNbO3”; 第 48回結晶成長会議; 大阪府吹田市, 大阪大学銀杏 会館; 2019/10/30−11/01.
[6] “Unsteady-state crystal growth in the presence of interface electric field”; 第 67 回応用物理学会春季学術講演会; 東京都千代田区, 上智大学四谷キ ャンパス; 2020/03/12−15.
[7] “Unsteady-state crystal growth in the presence of interface electric field”; 第 49回結晶成長会議; オンライン開催; 2020/11/09−11.
(4) Poster presentations in domestic meetings in Japan
[1] “Compositional uniformity in the MgO-doped LiNbO3 crystal that is concurrently congruent and stoichiometric”; 東北大学&理研 第 1回ワーク
94
ショップ [テラヘルツ光研究の新展開と産業応用への展望]; 宮城県仙 台市; 2019/10/23.
[2] “Effect of interface electric field on ionic species segregation during the
growth of LiNbO3”; 東北大学大学院 理学・生命科学 2 研究科合同シン
ポジウム 2020; 宮城県仙台市; 2020/02/14.
3. Awards
(1) 日 本 結 晶 成 長 学 会 第 17 回 講 演 奨 励 賞(第 49 回 結 晶 成 長 国 内 会 議), 2020/12/21.
(2) Outstanding Performance of Poster Presentation in 2018 Annual Meeting of Excellent Graduate Schools for Materials Integration Center and Materials Science Center in conjunction with 2018 Russia-Japan Conference “Advanced Materials:
Synthesis, Processing and Properties of Nanostructures”, 2018/03/23.
(3) Japanese Government (MEXT) Scholarship, 2015/10−2020/09.
95 Acknowledgements
I would like to express my gratitude to all those who helped me during the process of writing of this thesis and during my study years in Tohoku University.
First of all, my heartiest thanks flow to my supervisor, Professor Satoshi Uda for his helpful guidance, valuable suggestions and constant encouragement both in my study and in my life. His profound insight and accurateness about my thesis taught me so much that they are engraved on my heart. He provided me with beneficial help and offered me precious comments during the whole process of my writing, without which the thesis would not be what it is now.
Also, I would like to express my sincere gratitude to all the professors, Junpei Okada, Jun Nozawa and Niinomi Hiromasa who greatly broadened my horizon and enriched my knowledge in my study. Their inspirational and conscientious teaching have provided me with a firm basis for the composing of this thesis and will always be of great value to my future academic research. Especially thanks to Professor Nozawa during my whole doctoral course, he was so patient and kind to assist me in experiments and paper drafts seriously.
I am deeply thankful to Chiharu Nagura, our secretary to Professor Uda’s research group, for her careful and thoughtful help regardless of work or life. I am grateful to Issei Narita of IMR, Tohoku University for his EPMA measurement.
My thanks also go to Chihiro Koyama, Suxia Guo, Bo Peng, Yusuke Horie, Naoki Ihara, Yuki Honda, Keiya Sato, Jie Wang, Masato Arimitsu and Ea Nakagawara who have instructed and helped me a lot in the past years. Koyama taught me a lot in my research and often had delightful discussions with me, which encouraged me to go through my doctoral course.
Finally, I would like to extend my deep gratefulness to my family and friends,