Method of growing doped crystals of lithium niobate with composition close stoichiometric, and device to this end

FIELD: metallurgy.

SUBSTANCE: invention relates to the technology of growing monocrystals using Chokhralsky method. Growth of doped crystals of lithium niobate with composition close to stoichiometric is done on an inoculating crystal from molten mixture of lithium niobate of identical composition with ratio Li/Nb equal to 0.938-0.946 and containing 9-13 mol % K2O and 0.5-2.5 mol % MgO or ZnO, in conditions of applied electric field with current density of 0.2-40 A/m2. A device is provided for realising the method, comprising a housing with a growth station and a cooling chamber, crucible 1, placed in the growth station, induction heater, top metallic heating shield 4, fitted above the crucible 1, mechanism for moving the crystal with a coupling rod, a rod with a holder 3 for the inoculating crystal 2. The device is also provided with a regulated direct current source 10 with electrodes; under the inoculating crystal 2 there is an additional load from electrically conducting material, separated from the wall of the holder by electrically insulating material. One of the electrodes is connected to the crucible 1, and the second - to the load.

EFFECT: invention allows for growing large optically homogenous crystals of lithium niobate with composition close to stoichiometric Li/Nb>0,994, additionally doped with MgO or ZnO, composition of which in the top and bottom parts of the crystal is virtually the same, without destroying the inoculating crystal.

5 cl, 2 ex, 2 dwg

 

The invention relates to a method of growing single crystals by the Czochralski method, in particular, to a method of growing large-size (diameter and a length of more than 60 mm) oxide crystals of lithium niobate (LiNbO3) composition close to stoichiometric, which are widely used in electro-optic modulators, electro-optic switches, optical waveguides and other optoelectronic devices instead of the previously used lithium niobate crystals congruent composition.

There is a method of growing single crystals of lithium niobate from the melt mixture containing oxygen compounds of niobium and lithium, in order to improve optical uniformity crystal composition close to the stoichiometric by increasing the speed of growing in charge of impose additional potassium and components take in the amount determined from the ratio of the oxides of niobium pentoxide and lithium oxide, mol.%, equal(51,0-51,4)/(48,6-49,0), and potassium oxide (4-6) % by weight of the sum of the oxides of niobium and lithium (SU # 845506, SW 15/00, published 27.02.2000.)

There is a method of growing single crystals of lithium niobate almost stoichiometric composition (more than 49 mole% Li2O) from the melt mixture congruent lithium niobate containing K2O, in order to improve the relations Li/Nb in the grown crystal in the melt impose additional oxide ka the Oia and the growth process lead by the Czochralski method in terms of the gradient of 1-2 .5°C/mm at speeds of extrusion of 0.1-0.5 mm/h and rotation speeds of 15-30 rpm (CN 1362545, C30B 29/30, C30B 15/00 published 08.07.2002.).

The disadvantage of the considered methods is that they do not allow to grow close to stoichiometric lithium niobate crystal with a uniform composition along the direction of cultivation. This is due to the fact that the composition of the lithium niobate crystal, namely the ratio of the molar fractions of Li2O/Nb2O5in the grown crystal according to the state diagram of the ternary system K2O-Li2O-Nb2O5[1] depends on the ratio of K2O/(Li2O+Nb2O5in the melt, which varies during the crystallization process. The last statement is easily explained by the fact that the content (Li2O+Nb2O5in the crucible decreases due to the use of these components to the growing crystal, and K2O not spent on the construction of the crystal.

It is known that doping of lithium niobate crystals of composition close to the stoichiometric impurities (MgO, ZnO)used to reduce optical damage, leading to the emergence of their heterogeneous domain structure and non-uniform distribution of impurities MgO and ZnO [2], which does not allow the use of such crystals in optical devices.

It is known that application of an electric field in the process of growing crystals of lithium niobate due to the high current density in the seed crystal is proishodit its decomposition due to electrolysis [3]. The decomposition of the seed crystal begins at the point of his last touch with the metal bracket of the seed crystal that connects the upper electrode, and develops on the sphere from a point of extreme touch with intensity proportional to the current density. Thus, when using conventional (cylindrical) structures of the holders of the seed crystals decomposition of the seed crystal is on a circle in place of the output of the seed crystal from the holder. This leads to loss of mechanical strength of the seed crystal, its subsequent destruction, the fall of the grown crystal in the melt and, consequently, to the termination of the crystallization process.

The basis of the invention is to provide a method of growing large optically homogeneous crystals of lithium niobate composition close to the stoichiometric Li/Nb>0,994, optionally doped with MgO or ZnO, which at the top and bottom of the crystal is almost the same, and device for growing crystals, allowing you to grow crystals in terms of the applied electric field through the crystal-melt without destruction of the seed crystal by separating the upper electrode from the holder of the seed crystal and the use of additional contact devices is to the seed crystal, which is separated from the holder of the seed crystal space filled with material, non-conductive electrical shock. The above result is due to the fact that the process of growing by Czochralski method from a melt mixture of congruent lithium niobate composition (the ratio Li/Nb=0,938-0,957) and containing 9 to 13 mol.% K2O and 0.5 to 2.5 mol.% MgO or ZnO in terms of the gradient of 0.5-2.5°C/mm at speeds of extrusion of 0.1-0.5 mm/h and rotation speeds of 10-30 rpm, in order to improve the relations Li/Nb to 0,994-0,997 in the grown crystal and removing the inhomogeneous domain structure the process of growing lead in terms of the applied electric field through the melt - crystal density of 0.2 to 40 a/m2.

Thus the technical problem of the preservation of the seed crystal is solved due to the fact that in the known device, taken as a prototype [4], for growing crystals, comprising a housing with a growing chamber and the cooling chamber, the crucible, placed in the cell growth, induction heater, the upper metal heating screen installed above the crucible, and the movement of the crystal rod, the rod from the holder of the seed crystal, the structure of the device includes additional stable source of direct current with electrodes; the holder tetrasociological made in the form of a cylinder, above the seed crystal has the additional burden of electrically conductive material, separated from the walls of the holder material, non-conductive electrical shock, this one of the electrodes is connected to the crucible, and the second electrode is connected to the load.

The proposed solution to the design of the holder of the seed crystal allows you to move the plot decomposition and subsequent destruction of the seed crystal due to electrolysis of the lower part, which must be able to withstand the weight of the grown large crystal in the upper end part, not subject to mechanical load.

The invention is illustrated figure 1 and figure 2. Figure 1 shows the growth site for growing crystals by the Czochralski method. The growth node contains the crucible 1, the melt 8 grown crystal (seed crystal) 2, ceramic screens 7, the metal shield 4, the rod 5 with the holder of the seed crystal 3 and the electrode 6. The growth node is placed in the induction heater (not shown in figure 1). The positive electrode 9 a stabilized constant current source 10 is applied to the crucible, and a negative 6 - to the holder of the seed crystal 3.

2 shows the design of the holder of the seed crystal with cylindrical cargo. The holder 1 is made in the form of a cylinder, on the bare cu is a steel 2 has an additional cylindrical cargo 3 of electrically conductive material attached to the electrode 4, separated from the body of the holder 1 material not conducting electric current 5.

The crystallization process is performed as follows. In the cavity of the crucible 1 is melted mixture of congruent lithium niobate composition (the ratio Li/Nb=0,938-0,957), containing 9 to 13 mol.% K2O and 0.5 to 2.5 mol.% MgO or ZnO. Provide the necessary axial temperature gradient at the interface of the liquid and solid phases (0.5 to 2.5°C/mm). After melting of the charge set the speed of 10-30 rpm and carry out the persecution of the crystal on the seed crystal 2, which is mounted with the bracket of the seed crystal 3 on the rod 5 with the electrode 6. Upon completion of persecution include the pulling speed of 0.1-0.5 mm/h and the current source 10, which stabilizes the current level of 0.2-40 a/m2(where m2- the surface area of the section of the crystal - melt)

Under the influence of an electric field generated by the current source there are two parallel processes:

1. Transfer of positively charged lithium ions from the melt in the crystal, which leads to the enrichment of crystal lithium ions, i.e. closer to the stoichiometric composition. The degree of enrichment of a crystal of lithium ions is proportional to the density of the electric current supplied to Rostova site. Thus, by changing the current density applied is suspended electric field it is possible to control the concentration of lithium in the growing crystal.

2. The electric field polarizes the lithium niobate crystal, thereby preventing the process of beginning a heterogeneous domain structure and aligning the distribution of impurities MgO and ZnO.

When the current density is limited by the interval of 0.2-40 a/m2. At the current density below 0.2 a/m2the migration of positively charged lithium ions from the melt in the crystal is not fixed, and at current densities of more than 40 a/m2there is a substantial evolution of heat on solidification front, the decomposition of the melt and the deterioration of the optical properties of the crystal by reducing its optical homogeneity [3].

Examples of the production method of the lithium niobate.

Option 1.

1. The melt in the crucible with a diameter of 120 mm and a height of 110 mm, consisting of a mixture of lithium niobate congruent composition (the ratio Li/Nb=0,945)containing 10,7 mol.% K2O and 1.0 mol.% MgO, heated to 30°C above the melting temperature (1190°C) and heated for 3 hours. Next, the temperature of the melt is lowered to 28°C, the seed crystal is brought into contact with the melt and the standard image is selected stationary regime of the phase transition.

2. Grown crystal by the following program: initial radius (the radius of the seed crystal) of 3 mm, the shape of the generatrix of the cone - hyperbolic, the angle between the generatrix of the cone at the point of inflection and the axis vitaliani is 67°, the radius of the crystal 30 mm, the rotation speed of the crystal 20 rpm, the speed of extrusion of 0.2 mm/h.

3. When the radius of the crystal 20 mm installed electric current through the melt - crystal equal to I=0,015A.

4. If the length of the cylindrical part of the crystal 60 mm electric current through the melt - crystal is removed and the gap of the crystal from the melt surface, the crystal is placed in the cooling zone, the temperature in the growing node linearly decreases to room at 40°C/hour.

Thus, the grown lithium niobate crystal composition close to the stoichiometric Li/Nb=0,995 - in the middle part doped with 1.0 mol.% MgO has a homogeneous domain structure.

Option 2.

1. The melt in the crucible with a diameter of 120 mm and a height of 110 mm, consisting of a mixture of lithium niobate congruent composition (Li/Nb=0,945)containing 10,7 mol.% K2O and 1.0 mol.% MgO is heated to 30°C above the melting temperature (1190°C) and heated for 3 hours. Next, the temperature of the melt is lowered to 28°C, the seed crystal is brought into contact with the melt and the standard image is selected thermal regime of the phase transition.

2. Grown crystal by the following program: initial radius (the radius of the seed crystal) of 3 mm, the shape of the generatrix of the cone - hyperbolic, the angle between the generatrix of the cone at the point p is Regina and the axis of extrusion 67°, the radius of the crystal 30 mm, the rotation speed of the crystal 20 rpm, the speed of extrusion of 0.2 mm/h.

3. When the radius of the crystal 10 mm is the initial electric current through the melt-crystal IR=0,03 A. Since the transition of the crystal on the cylindrical part of the current changes in the law I=In-k·L, where IR is the current through the crystal-melt when the current length of the cylindrical part (A), L is the current length of the cylindrical part (m), k is the coefficient of proportionality equal to 0.45 (a/m).

4. If the length of the cylindrical part of the crystal 60 mm electric current through the melt-crystal is removed and the gap of the crystal from the melt surface, the crystal is placed in the cooling zone, the temperature in the growing node linearly decreases to room at 40°C/hour.

Thus, the grown lithium niobate crystal composition close to the stoichiometric with the same ratio of Li/Nb=0,995 along the length of the crystal, doped with 1.0 mol.% MgO with homogeneous domain structure.

Sources of information

1. Polgar K. et al. Chemical and thermal conditions for the formation of stoichiometric LiNbO3// Journal of Crystal Growth - 2002. - Vol.237-239 - P.682-686.

2. Niwa K. et al Growth and characterization of MgO doped near stoichiometric LiNbO3crystals as a new nonlinear optical material // Journal of Crystal Growth - 2000. - Vol.208 - P.493-500.

3. Balasanyan P.H., Gabrielian V.T., Makanan EP, Foldvari I. the Composition and homogeneity of the crystals LibO 3in their interaction with the growing conditions. 1. The influence of the electric field. // Crystallography - 1990. - T.35. - 6 - S-1544.

4. Patent JP 10045497 And 17.02.1998 (prototype).

1. The method of growing doped lithium niobate crystals of composition close to the stoichiometric by the Czochralski method from a melt of the charge on the seed crystal, characterized in that use a mixture of congruent lithium niobate composition with the ratio of Li/Nb 0,938-0,946 and containing 9-13 mol. % K2O and 0.5 to 2.5 mol. % MgO or ZnO, and the process of growing lead in terms of the applied electric field and the current density of 0.2-40 a/m2.

2. The growing method according to claim 1, characterized in that the density of electric current change in the growth process.

3. Device for growing doped lithium niobate crystals of composition close to the stoichiometric to implement the method according to claim 1, comprising a housing with a growing chamber and the cooling chamber, the crucible, placed in the cell growth, induction heater, the upper metal heating screen installed above the crucible, the movement of the crystal rod, the rod from the holder of the seed crystal, wherein the device is further provided with a stable source of direct current with electrodes; above the seed crystal is an additional burden and the electrically conductive material, separated from the walls of the holder are not conducting electric current material, with one of the electrodes is connected to the crucible, and the second electrode is connected to the load.

4. The device according to claim 3, characterized in that the holder of the seed crystal is made in the form of a cylinder.

5. The device according to claim 3, characterized in that the positive electrode is connected to the crucible.



 

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