JPS5938198B2 - Manufacturing method of semiconductor strontium titanate single crystal - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a semiconductor strontium titanate single crystal.

Generally, in the perovskite type compound ABO3, A
Semiconductorization is achieved by dissolving a small amount of impurity with a different valence in the site or B site, or by further using forced reduction in combination to generate oxygen defects that trap electrons.

Conventionally, strontium titanate single crystals have been produced by the Bernoulli method or the flux method.

To obtain a semiconductor single crystal using these methods, forced reduction must be performed after growing the single crystal, and it is difficult to homogeneously dissolve niobium oxide or tantalum oxide, so it is difficult to obtain a high-quality semiconductor. I couldn't get it.

The present invention eliminates this drawback and provides a method for producing a semiconductor strontium titanate single crystal by homogeneously solid-dissolving niobium oxide or tantalum oxide while growing the strontium titanate single crystal.

As a result of research to achieve the above object, the present inventors found that titanium oxide TiO2 was used as a raw material rod by a floating method.
and strontium oxide SrO and niobium pentoxide Nb2O
When growing a single crystal of strontium titanate using a composition consisting of niobium oxide or niobium pentoxide Ta205, the valence of titanium oxide and niobium oxide or kuntal oxide is adjusted by adjusting the oxygen partial pressure of the atmosphere. The present inventors have found that niobium oxide or mental oxide components can be homogeneously dissolved in a single crystal to easily obtain a semiconductor strontium titanate single crystal, and based on this knowledge, the present invention has been completed.

The present invention uses titanium oxide TiO2 as a raw material rod in a single crystal growth method by floating sea movement in which one end of a raw material rod is melted and the formed melt zone is moved to continue melting of the raw material rod and precipitation of a solid phase. and strontium oxide Sr
A composition consisting of O and niobium pentoxide or quintal pentoxide is used, and the oxygen partial pressure is 10-0 as the growth atmosphere.
Semiconducting titanic acid characterized by growing a single crystal using a reducing atmosphere of 5 atmospheres or less and adjusting the valence of niobium pentoxide or quintal pentoxide contained in the raw material to form a solid solution. It is in the manufacturing method of strontium sentry crystals.

What is the floating zone method used in the present invention?

For example, a raw material rod is placed on top and a seed crystal is placed on the bottom, and a molten zone of solvent is formed between the two and held by surface tension.

Next, when the molten zone is slowly moved downward, melt penetration of the raw material rod occurs on the upper side of the molten zone, and crystal precipitation occurs on the lower side.

At this time, the raw material rod and the seed crystal are rotated at appropriate speeds in opposite directions to give a stirring effect to the molten zone.

In this way, the molten zone is used as an intermediary to connect the materials, and the molten zone is slowly moved with no support other than the raw material rod and the seed crystal to grow a single crystal.

As for the raw material rod, the component ratio of strontium, titanium, niobium or kuntal is molar ratio: l:(1-x):x
(However, it is preferable that X2 is 0.002 to 0.01).

The solvent has a molar ratio of strontium, titanium, niobium or kuntal of approximately 1:(1-Y):Y(Y=
o, ool~0.01) and whose melting point is lower than the melting point of the raw material rod.

As the Nb2O5 or Ta205 component increases, its melting point changes, so depending on the composition of the raw material rod, the Nb in the solvent
The amount of 2O5 or Ta205 may be adjusted.

Although it is possible to precipitate a single crystal without using a solvent, if this is not used, the composition of the initially formed crystal will deviate from the intended one, resulting in poor retention.

In addition, if there is no seed crystal, polycrystals will initially be produced, resulting in a disadvantage that the process is wasted.

Titanium oxide, strontium oxide, niobium oxide, or tantalum oxide as raw materials for the raw material rod and the solvent may be used as they are at ordinary reagent grade, but in order to allow the mutual chemical reaction to proceed quickly, The smaller the particle size, the more preferable.

It is necessary to mix these raw materials as uniformly as possible.

For this purpose, it may be sufficiently mixed with alcohols, organic reagents such as Atton in a mortar, or by using a ball mill or the like.

These mixed raw material powders are press-molded.

For pressure molding, a molding method using one-way or two-way compression using a mold may be used, but it is preferable to use a rubber press method in which pressure is applied isotropically to prevent bending during heating. .

Rubber press is a method in which raw material powder is placed in rubber, sealed at both ends, and then pressurized with high hydraulic pressure in a closed hydraulic pressure vessel.

The hydraulic pressure is 500 K7/cvfJJ, and any pressure above this may be used, but 1 to 2 ton/cvi is preferable.

The pressurization time is 5 seconds or more, preferably 1 minute.

Molded products that are not sufficiently pressurized are likely to break. The pressure-molded product may be thin and long, but a cylindrical one is preferred.

The size thereof is, for example, a diameter of 1 rrrIn to 10 crn, a length of 1 mm to 5 m, preferably a diameter of 3 mm to 3 cm, and a length of 5 m to 30 crn.

The press-molded product is calcined.

This calcination can be heated by holding it in a crucible in a horizontal furnace, but to prevent the contamination of impurities and bending during calcination, it is recommended to heat it while suspended in a vertical furnace. preferable.

The heating temperature of the press-molded product is 1500°C ~ in the case of raw material rods.
2000°C, preferably 1400°C to 1800°C in the case of a solvent.

The longer the heating time, the better.

Especially in the case of raw material rods, the bulk density after sintering is 80% of the true density.
It is preferable to heat for a sufficiently long time so that

The solvent is preferably fused under the raw material rod or on top of the seed crystal.

The amount of solvent is most preferably a hemisphere having the same diameter as the raw material rod.

The seed crystal may be any solid material that can withstand high temperatures and does not cause a chemical reaction with the solvent, but it is preferable to use the same single crystal as the raw material rod.

To melt the solvent by the floating zone method, for example, a condensing heat method in which light from a high-temperature light source is condensed using a mirror or lens is used to heat the solvent to a temperature above the melting temperature over 3 to 30 degrees above and below.

The crystal growth rate is equal to the downward feeding rate of the raw material rod and the seed crystal, and the feeding rate is at least 0.1 rrv per hour.
It is preferable to maintain n.

A particularly preferred speed is 1 to 1 ON per hour.

The atmosphere for heating solid solution has an oxygen partial pressure of 10-5 to 10
' is best.

For this purpose, the oxygen partial pressure may be adjusted to the desired oxygen partial pressure by appropriately mixing air, nitrogen gas, carbon dioxide gas, or the like.

Due to this atmosphere, the niobium pentoxide contained in the raw material rod,
The valence of tantalum pentoxide is lowered to tetravalent, and it is easily dissolved in a strontium titanate raw material rod and converted into a semiconductor.

The single crystal thus obtained is used as a raw material rod,
If a similar single crystal is grown, a strontium titanate single crystal with better semiconductor properties can be obtained.

According to the method of the present invention, in the conventional Bernoulli method and flux method for growing strontium titanate single crystals, it was necessary to convert the single crystal into a semiconductor by forced reduction after growing the single crystal, but when growing strontium titanate single crystals, However, it can be easily converted into a semiconductor by adjusting the atmosphere, and has an excellent effect of producing a semiconductor superior to conventional ones.

Example 1 Titanium dioxide powder with a purity of 99.9% or more and a purity of 99.9%.
For raw material rods, strontium carbonate powder with a purity of 9% or more and niobium pentoxide powder with a purity of 99.9% or more are used in a molar ratio of strontium, titanium, and niobium of 1.000:0.
1998: Weighed 0.002, added ethyl alcohol in a mortar, mixed thoroughly, and heated to 1100℃ in an aluminium crucible.
After sufficiently removing the carbonic acid from the strontium carbonate, the mixture was ground in a mortar to obtain a fine powder mixture with an average particle size of 1 μm.

The mixture 12f was put into a thick-walled rubber tube with an inner diameter of 11 mm, both ends were sealed, the mixture was inserted into a metal ring with an inner diameter of 11.5 mm, and the mixture was heated at 1 ton/crA in a hydraulic hydrostatic pressure generator.
Pressure molding was carried out for about 1 minute at a pressure of .

The outer shape obtained by the above operation is about 9rIr1n and the length is about 8
A round bar-shaped sample of 0rIr1n was inserted into a vertical electric furnace maintained at 1800°C and calcined.

Insertion into the furnace and withdrawal from the furnace took one hour each, and the atmosphere was oxygen to prevent destruction of the sample due to rapid heating and cooling.

The strontium titanate raw material rod with an outer diameter of about 7.5 rran and a length of about 70 rran obtained by the above operation was fixed to the upper sample rotating shaft of a floating zone method single crystal production device that adopted an infrared concentrated heating method, and the rod was placed at the bottom of the above device. Heating was started using a strontium titanate single crystal grown by the Bernoulli method fixed on the side seed crystal rotation axis as a seed crystal.

The atmosphere was atmospheric.

Adjust the tip of the raw material rod so that it is at the highest temperature point in the single crystal production equipment, and at the same time as this tip melts by heating, the temperature is fixed by constant heating, and the strontium titanate is seeded from the bottom. The crystals were moved upward to bond the seed crystals through the molten zone, and both frames were rotated in opposite directions at a speed of 330 revolutions per minute.

Rotation continued until the end of crystal growth.

After finely adjusting the temperature and the distance between the two frames to obtain a state in which the solvent part is too large or too small,
Move both frames downward at the same speed at a speed of 3 mm/hour,
Crystals of strontium hetitanate were deposited on the solvent bar.

Since a single crystal was used as a seed crystal, the precipitated crystal was a single crystal from the beginning.

When the strontium titanate raw material rod was almost consumed, the grown single crystal and the raw material rod were separated to obtain a round rod-shaped strontium titanate single crystal with a diameter of 6.5 mm and a length of 60 rrrrn. .

The obtained single crystal has a conductivity of 0 x 10” ohm
ffl” and was a semiconductor.

Claims (1)

[Claims] 1. In a single crystal growth method using a floating zone method in which one end of a raw material rod is melted and moved in the formed melt zone to continue melting of the raw material rod and precipitation of a solid phase, Titanium dioxide TiO2, strontium oxide SrO, niobium pentoxide Nb2O5 or tantalum pentoxide Ta20
5 was used as the growth atmosphere,
A single crystal is grown using a reducing atmosphere with an oxygen partial pressure of 10-0°5 atm or less, and the valence of niobium pentoxide or tantalum pentoxide contained in the raw material rod is adjusted to form a solid solution. Characteristic method for manufacturing strontium titanate single crystal semiconductor. 2 The floating zone method places the solvent under the raw material rod,
2. The method according to claim 1, wherein a seed crystal is further provided under the solvent, and the molten portion is heated to melt and moved in a melting zone. 3 The raw material rods are strontium oxide SrO, titanium dioxide TiO2, and niobium pentoxide Nb2O5 or tantalum pentoxide Ta2O, composed of (1-X):X (however, x = 0.002 to 0.01), and the solvent is strontium oxide. , the molar ratio of titanium dioxide and niobium pentoxide or tantalum pentoxide components is 1:(1-
y): y (where y=0.001 to 0.01). 4 In a single crystal growth method using a floating zone method in which one end of a raw material rod is melted and the formed melt zone is moved to continue melting of the raw material rod and precipitation of a solid phase, strontium oxide, titanium dioxide, and niobium pentoxide or tantalum pentoxide, and grow the single crystal by keeping the single crystal growth atmosphere in a reducing atmosphere with an oxygen partial pressure of 10-0°5 atm or less, and grow the single crystal obtained. A method for producing a semiconductor strontium titanate single crystal characterized by repeating the same single crystal growth as a raw material rod.