JP3436076B2 - Method for growing oxide single crystal and method for manufacturing nonlinear optical element using the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide single crystal growth method and an SHG (Second Harmonic Gene) method for growing an oxide single crystal from an oxide melt to which a predetermined impurity is added.
ration: QPM (Quasi-P) with second harmonic generation effect
The present invention relates to a method of manufacturing a nonlinear optical element having a hase Matching (quasi phase matching) structure.

[0002]

[Prior art]

In order to convert a long-wavelength laser beam (1064 nm) generated from a solid-state laser such as Nd: YAG (Nd (neodymium): Y ₃ Al ₅ O ₁₂ (yttrium aluminum garnet)) into a short-wavelength laser beam (532 nm). As the SHG element used, a non-linear optical element in which an oxide single crystal such as LiNbO ₃ (lithium niobate) single crystal is provided with a QPM structure is known. The QPM structure is composed of a plurality of alternating layers in which the impurity concentration is periodically changed and dielectric polarization is inverted, and a phase structure of micron order or less corresponding to the wavelength is required to enable phase matching with incident light. It is a thing.

Conventionally, for manufacturing a nonlinear optical element having a QPM structure, for example, the following manufacturing method is known. (1) After a mask is formed on the surface of a crystal plate of an oxide single crystal, impurities such as Ti (titanium) are thermally diffused to periodically change the impurity concentration in the crystal plate according to the mask pattern of the mask. And forming regions on the crystal plate in which the dielectric polarizations are inverted from each other. (2) By periodically changing the growth rate and the thermal environment during the crystal growth of the oxide single crystal, and periodically changing the concentration of impurities distributed to the crystal, the dielectric polarization inversion of the pattern corresponding to this was performed. A manufacturing method in which alternating layers are periodically formed in the crystal growth direction. (3) A manufacturing method in which a pulse current is periodically applied during the crystal growth of an oxide single crystal to periodically form alternating layers with polarization inversion in the crystal growth direction.

[0004]

However, the above-mentioned conventional manufacturing method has the following problems.
That is, in the manufacturing method (1), a process for reversing polarization after crystal growth is required separately from the crystal growth process, so that the number of steps is increased and the region where polarization is reversed is controlled by the mask pattern. Therefore, it is difficult to obtain high precision, and the limit is submicron. Further, in the manufacturing method (2), the range in which the effective distribution coefficient k can be taken as a guideline for indicating the change in the impurity concentration can be set, but the range simply changes the growth rate and the thermal environment under the conventional growth conditions. If the equilibrium partition coefficient is k
_{If 0} , then k ₀ <k <1 (k ₀ <1) or 1 <k <k ₀ (k ₀ >)
It was impossible to carry out crystal growth so as to produce a region which is limited to 1) and contains no impurities at all. Further, in the manufacturing method (3), there is a disadvantage that the current application becomes difficult as the crystal length increases.

By the way, the present inventors have found that the pulling method (C
In the growth of oxide single crystals from the melt represented by the Z method), the stability of the solid-liquid interface, in particular, the relationship between the temperature fluctuation or composition fluctuation of the melt and the growth stability and the crystal quality is studied. Came. That is, the kink (k
ink, ledge, terrace
The energy required to be consumed by the system is required as a driving force for microscopic dynamics such as the generation and growth of ce), but this instability represented by the degree of supersaturation is due to the newly generated surface energy and stress. − It is the sum of strain field energy, various defect generation energies, and growth kinetics energy, and each energy has an appropriate value. It is generally said that large temperature fluctuations that upset the energy balance should be avoided.

Although the present inventors have not observed such temperature fluctuations, the present inventors have studied the factors of the instability of the solid-liquid interface, which causes compositional fluctuations and point defects in the grown crystal, and thus the present invention Arrived

The present invention has been made in view of the above problems, and provides a method for growing an oxide single crystal capable of obtaining a highly accurate and steep impurity concentration distribution and a method for manufacturing a nonlinear optical element using the same. The purpose is to provide.

[0008]

[Means for Solving the Problems] The present inventors have studied the effect of the electric field generated at the solid-liquid interface (interfacial electric field) on solute partitioning and congruent melting composition (Satoshi Uda, Nippon Crystal) Journal of Growth Society Vol.21 No.2 (1994) 159,
WATiller and S. Uda, J. Crystal Growth 129 (1993) 341
-361 etc.), the following findings were obtained. Laser heated pedestal method (LHPG: Laser Heated Pede
stral Growth) and micro pull-down method (micro-PD
Method: Micro-Pulling down method (hereinafter referred to as μ-PD method) etc., an extremely large temperature gradient is generated at the solid-liquid interface, and the oxide melt generally exhibits ionicity. The Seebeck effect works due to such an extremely large temperature gradient, and a large interfacial electric field is generated at the interface.

This interfacial electric field has a great influence on the segregation of ionic impurities from the melt to the crystal, and a certain critical velocity V
Before and after ^*, the segregation coefficient shifts from a negative value (a state in which no impurities enter the crystal) to a very large positive value (a state in which a large amount of impurities are distributed from the melt to the crystal). I found it. This critical velocity V ^* is obtained from the following relational expression (1) when the temperature gradient G _L in the melt and the impurity diffusion layer thickness δ _C at the crystal growth interface are set to predetermined values.

That is, the relational expression (1) sets the temperature gradient G _L and the impurity diffusion layer thickness δ _C at the crystal growth interface between the oxide melt and the oxide single crystal to predetermined values, and
The diffusion coefficient of the impurities in the oxide melt is D _L , the impurity charge number of the impurities is Z _L , the elementary charge is e, the thermoelectromotive force coefficient in the melt is α _L , and the electromotive force coefficient due to crystallization electromotive force is the alpha _i, the Boltzmann constant k _B, and when the melt temperature was _{^{T, V * = (2D L}} δ C Z L eα L G L) / (D L Z L eα i -2δ C k B T) … (1)

The above relational expression (1) will be roughly described. The impurity concentration C _S of impurities entering the crystal is expressed by the following relational expression (2). C _S = C _{L (0)} k _E0 (2) Here, C _{L (0)} is the impurity concentration of the melt at the growth interface, k
_E0 is the equilibrium distribution coefficient affected by the interface electric field.

The equilibrium distribution coefficient k _E0 is expressed by the following relational expression (3). k _E0 = k ₀ (V / V _E ) (3) Here, k ₀ is the equilibrium distribution coefficient when there is no influence of the electric field, and V _E is the effective velocity affected by the interface electric field.
Note that k ₀ is represented by the following relational expression (4). k ₀ = C _{S (0)} / C _{L (0)} (4) Here, C _{S (0)} is the impurity concentration of the crystal at the growth interface.

Further, the effective speed V _E is expressed by the following relational expression (5). _{V E = V - ((D} L Z L e (E cL + E tL)) / k B T) = V - ((D L Z L e (α i V / 2δ C -α L G L) / k B T) (5) Here, E _cL is an electric field due to crystallization EMF (electromotive force),
_EtL is an electric field due to thermoelectromotive force. Further, E _cL and E _tL are represented by the following relational expressions (6) and (7). E _cL = α _i V / 2δ _C (6) E _tL = −α _L G _L (7)

[0014] In the above equation (5) having such a relationship, when ^{_{V E = 0, V * =}} (2D L δ C Z L eα L G L) / (D L Z L eα i -2δ C k _B T) (1) and the relational expression (1) is obtained.

For the exchange of impurities between the actual crystal and the melt, it is convenient to use the effective distribution coefficient (influenced by the interface electric field) k _E. That is, the impurity concentration C _S and the effective distribution coefficient k _E are expressed by the following relational expression (8).
It is represented by (9). C _S = C _{L (0)} k _E0 = C ₀ k _E (8) k _E = k _E0 / (k _E0 + (1-k _E0 ) exp (-Vδ _C / D _L ) (9) where , C ₀ are bulk melt concentrations of impurities.

Therefore, according to the present invention, the above-mentioned critical velocity V ^*
In order to solve the above problems, the following configuration is adopted. The method for growing an oxide single crystal according to claim 1, wherein the oxide single crystal is grown from an oxide melt to which a predetermined impurity is added. Temperature gradient G _L in the melt and the impurity diffusion layer thickness δ _{C at} the crystal growth interface with the single crystal
Is set to a predetermined value, the diffusion coefficient of the impurity in the oxide melt is D _L , the impurity charge number of the impurity is Z _L , the elementary charge is e, the thermoelectromotive force coefficient in the melt is α _L , When the electromotive force coefficient due to crystallization electromotive force is α _i , the Boltzmann constant is k _B , and the melt temperature is T, the critical velocity V ^* is ^expressed by the following relational expression: V ^* = (2D _L δ _C Z _L eα _L G _L ) / (D _L Z _L eα _i -2δ _C
k _B T), and a technique for controlling the impurity concentration of the oxide single crystal by changing the growth rate V of the oxide single crystal before and after it based on the critical rate V ^*. To be done.

In this method for growing an oxide single crystal, the temperature gradient G _{L in the} melt and the impurity diffusion layer thickness δ _C are set in advance, and the growth rate V is determined based on the critical rate V ^* calculated by the above relational expression. Since it is changed before and after that, the segregation coefficient changes from negative to large positive, so that it is possible to continuously grow a single crystal with an extremely high impurity concentration from a single crystal with an almost zero impurity concentration. An oxide single crystal having a distribution is grown.

The method for manufacturing a nonlinear optical element according to claim 2 is the method for manufacturing a nonlinear optical element having a quasi phase matching structure composed of alternating layers of oxide single crystals in which dielectric polarizations are inverted from each other. Based on the growth method of a single crystal of a single crystal, the growth rate V is cyclically changed before and after the critical speed V ^* as a reference to form the alternating layers having a large difference in the impurity concentration. To be done.

In this method of manufacturing a non-linear optical element, the growth rate V is periodically changed before and after the critical rate V ^* as a reference to form an alternate layer having a large difference in impurity concentration, so that the impurity concentration is increased. A periodic structure composed of a region of almost zero and a region of extremely high impurity concentration is produced, and a domain-inverted structure can be easily obtained.

According to a third aspect of the present invention, there is provided a method of manufacturing the non-linear optical element according to the second aspect, wherein the oxide melt is a lithium niobate melt. Is a single crystal of lithium niobate formed into a fiber.

In this method of manufacturing a non-linear optical element, since the oxide single crystal is a lithium niobate single crystal and is formed into a fiber shape, the SHG effect with high conversion efficiency is obtained, and the sharpness and accuracy are high. The domain-inverted structure can be easily built in the optical waveguide.

[0022]

BEST MODE FOR CARRYING OUT THE INVENTION An embodiment of a method for growing an oxide single crystal and a method for manufacturing a nonlinear optical element using the same according to the present invention will be described below with reference to FIGS. 1 and 2. In FIG. 1, reference numeral 1 is a crystal growth apparatus for growing a fiber-shaped oxide single crystal by the μ-PD method.

The crystal growth apparatus 1 is, as shown in FIG.
Platinum crucible 2 arranged in a sealed container, and the platinum crucible 2
Auxiliary heaters 4 arranged above and below the platinum crucible 2 for heating the oxide melt 3 stored in the platinum crucible 2 to a predetermined temperature, and an oxide single crystal 5 grown below the platinum crucible 2 and grown from the platinum crucible 2. And an after-heater 6 for heating to a predetermined temperature. In addition, the platinum crucible 2 is directly heated by electricity.
Functions as the main heater.

A crucible nozzle 2a having a hole opened to grow an oxide single crystal 5 is formed below the platinum crucible 2, and the oxide single crystal 5 is grown in a fiber shape from the crucible nozzle 2a. . In addition, the auxiliary heater 4
The after-heater 6 adjusts and controls the temperature gradient G _L in the melt at the solid-liquid interface to be a predetermined value.

Next, a method for growing an oxide single crystal and a method for manufacturing a non-linear optical element by the crystal growth apparatus 1 will be described.

First, 3 wt% of Mn (manganese) as an impurity was added to a lithium niobate powder raw material having a congruent melt composition (Li / (Li + Nb) = 0.4838), and the mixture was stirred well to obtain platinum. It is put in the crucible 2 and the temperature is raised to 1260 ° C., which is slightly higher than the melting point, and melted and held. That is, the lithium niobate oxide melt 3 is stored in the platinum crucible 2 at a predetermined temperature.

On the other hand, a prismatic lithium niobate crystal cut out in the X-axis direction is used as a seed crystal X, and the crucible nozzle 2a is used.
Contact the tip of the and start pulling down. During the pulling, the diameter of the crystal is adjusted and the temperature gradient _GL in the melt at the interface is adjusted.
Is set to 4000 ° C./cm. Further, the impurity diffusion layer thickness δ _C is 50 μm, which is almost equal to the thickness of the melting zone.

Under this condition, the critical velocity V ^* is ^calculated by the relational expression (1).
V ^* = 3.6 × 10 ⁻⁴ cm / s. The constants used for the calculation at this time are as follows. _{k 0 = 0.5 Z L = 4} D L = 5.8 × 10 -6 cm 2 / s α i = 1.25mVsμm -1 α L = -0.4mVK -1 T = 1530K

A graph of the equilibrium distribution coefficient k _E0 at different temperature gradients G _L when the growth rate V is used as a variable is shown in FIG. When the growth rate V is the critical rate V ^* , the effective rate V _E becomes zero, and as can be seen from FIG. 2, when the growth rate V becomes smaller than the critical rate V ^* at the boundary of the critical rate V ^* , the equilibrium distribution coefficient k _E0. Is a negative value. Further, it can be seen that as the temperature gradient G _L increases, the critical velocity V ^* also increases. In addition, in the growth by the normal CZ method,
In contrast to the comparatively small temperature gradient G _L , the growth is carried out by the μ-PD method in the large temperature gradient G _{L in} the present embodiment.

Based on the critical velocity V ^* thus obtained, the rate of change ΔV is set to ΔV = ± 5.0 × 10 ⁻⁵ cm / s, and the growth rate V is set at regular intervals. The values are periodically changed to V ^* -ΔV and V ^* + ΔV. In this embodiment, the speed change interval is set to 2 seconds. That is, when the growth rate V is V ^* -ΔV, V _E = −3.9 × 10 ⁻⁵ cm / s k _E0 = −4.0, and V _E <0 and k _E0 <0. Therefore, Mn does not enter the grown crystal.

On the other hand, when the growth rate V is V ^* + ΔV, V _E = 3.9 × 10 ⁻⁵ cm / s k _E0 = 5.3 k _E = 4.6 and V _E > 0, k _E0> 0, and with becomes k _E> 0,
Mn having the following impurity concentration C _S will enter the grown crystal. C _S = C ₀ k _E = 13.8 wt%

As a result of the above growth, the Mn content is almost 0 w.
Since the t% layer and the 13.8 wt% layer are continuously formed in a 7-micron cycle, the mutual difference in the impurity concentration C _S is very large, and the alternating layers in which the dielectric polarization inversions occur are longer than the oxide single crystal 3. It is produced in the direction. When the velocity change interval was set to 0.1 seconds, a domain-inverted periodic structure of 0.3 micron could be produced. By cutting and processing the thus-grown oxide single crystal 3 into a predetermined length, it is possible to obtain a nonlinear optical element having a QPM structure composed of the alternating layers.

This periodic structure can have a concentration difference of 5 times or more than the concentration difference obtained by the conventional method, and the period is adjusted from the submicron to the nanoscale by adjusting the interval of velocity change. It is possible to cover up to dimensional accuracy. Further, since the oxide single crystal 3 is a lithium niobate single crystal and is formed into a fiber shape, the SHG effect of high conversion efficiency is obtained, and
A steep and highly accurate domain-inverted structure can be easily formed in the optical waveguide.

The present invention also includes the following embodiments. (1) Although a LiNbO ₃ single crystal was grown as an oxide single crystal and Mn was doped as an impurity thereof, other oxide single crystals and impurities may be used. For example, as an oxide single crystal, LiTaO ₃ (lithium tantalate), B
a ₂ NaNb ₅ O ₁₅ (barium sodium niobate),
A single crystal such as (Sr, Ba) Nb ₂ O ₄ (strontium barium niobate) is adopted, and Cr (chromium), Mg (magnesium), Zn (zinc) and Ni are used as impurities.
(Nickel) or the like may be used.

(2) Although the μ-PD method is used as the crystal growth method, other crystal growth methods such as LHPG may be adopted. (3) In this embodiment, the nonlinear optical element having the quasi phase matching structure used as the SHG element is manufactured.
Since the impurity concentration can be controlled with dimensional accuracy from submicron to nanoscale, it may be used for other materials as long as it uses a crystal having a sharp and highly accurate concentration distribution. For example, it may be applied to a technique for growing a semiconductor crystal used for a quantum well structure semiconductor laser.

[0036]

The present invention has the following effects. (1) According to the method for growing an oxide single crystal according to claim 1.
For example, the temperature gradient G in the melt in advance _LAnd impurity diffusion layer thickness δ_CTo
Critical velocity calculated by the above relational equation while setting
V^*Change the growth rate V before and after
, The impurity concentration is almost zero from the single crystal with almost zero impurity concentration.
Large single crystal can be grown continuously, sharp and high precision
Easy growth of oxide single crystals with various impurity concentration distributions
You can

(2) According to the method of manufacturing a non-linear optical element described in claim 2, the growth rate V is periodically changed before and after the critical rate V ^* as a reference, and the difference in impurity concentration is large. Since alternating layers are formed, a periodic structure composed of a region having almost zero impurity concentration and a region having extremely high impurity concentration can be formed in the crystal growth direction, and a domain-inverted structure corresponding to the periodic structure can be easily formed. Can be obtained. Therefore, by adjusting the interval of velocity change, it is possible to form a periodic structure with a large concentration difference, which was not obtained by the conventional method, with dimensional accuracy from submicron to nanoscale. An excellent nonlinear optical element with less wave loss can be obtained.

(3) According to the method for manufacturing a non-linear optical element of claim 3, since the oxide single crystal is a lithium niobate single crystal and is formed into a fiber shape, the SHG effect of high conversion efficiency is obtained. In addition to being obtained, a sharp and highly accurate domain-inverted structure can be easily formed in the optical waveguide.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing a crystal growth apparatus of one embodiment according to the present invention.

FIG. 2 is a graph showing an equilibrium distribution coefficient k _E0 at different temperature gradients G _L when the growth rate V is used as a variable.

[Explanation of symbols]

1 Crystal growth equipment 3 Oxide melt 5 oxide single crystal

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-5-270992 (JP, A) JP-A-9-67189 (JP, A) JP-A-10-279379 (JP, A) Satoshi Uda, solid-liquid The effect of the electric field generated at the interface on solute distribution and congruent melting composition, Journal of the Japan Society for Crystal Growth, June 25, 1994, Vol. 21, No. 2, pp. 159-165 William A. TILLER et al. , Intrinsic L iNbO3 melt types partitioning at the congruent melt composition II. Dynamic ... case, Journal of Crystal Growth, March 2, 1993, Vol. 129, Nos. 1/2, pp. 341-361 (58) Fields investigated (Int.Cl. ⁷ , DB name) C30B 1/00-35/00 G02F 1/35 JSTPlus (JOIS)

Claims (3)

(57) [Claims] 1. A method for growing an oxide single crystal, which comprises growing an oxide single crystal from an oxide melt to which a predetermined impurity has been added, the method comprising the steps of forming a crystal growth interface between the oxide melt and the oxide single crystal. The temperature gradient G _{L in the} melt and the impurity diffusion layer thickness δ _C are set to predetermined values, the diffusion coefficient of the impurities in the oxide melt is D _L , the impurity charge load of the impurities is Z _L , and the elemental charge is the e, the melt UchinetsuOkoshi power factor alpha _L, the electromotive force coefficient by crystallization electromotive force alpha _i, the Boltzmann constant k _B, and the critical speed V ^* when the melt temperature is T, the following _{^{relation; V * = (2D L δ}} C Z L eα L G L) / (D L Z L eα i -2δ C
k _B T), and the impurity concentration of the oxide single crystal is controlled by changing the growth rate V of the oxide single crystal before and after it based on the critical rate V ^*. Method for growing oxide single crystal. 2. A method for manufacturing a non-linear optical element having a quasi phase matching structure composed of alternating layers of oxide single crystals in which dielectric polarizations are inverted from each other, wherein the method for growing an oxide single crystal according to claim 1 comprises: A method for manufacturing a non-linear optical element, characterized in that the growth rate V is periodically changed before and after the critical rate V ^* as a reference to form the alternating layers having a large difference in impurity concentration. 3. The method for manufacturing a nonlinear optical element according to claim 2, wherein the oxide melt is a melt of lithium niobate, and the oxide single crystal is formed in a fiber shape. A method for manufacturing a non-linear optical element, which is a single crystal of the above.