JP2001213690A - Growing method of rare earth hexaboron single crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a manufacturing method of fewer defect, high quality crystal of rare earth hexaboron which can grow at higher speed. SOLUTION: At growing single crystal of rare earth hexaboron represented by REB6 - RE represents La, Ce or (La, Ce) solid solution - in the floating zone method, the melted boron content - B/RE (AE) in mol ratio - is kept at 5.9-8 during crystal growth by using the material supply rod into which alkaline earth hexaborom represented by AEB6 - AE represents Ca, Sr, Ba or (Ca, Sr), (Ca, Ba), (Sr, Ba), (Ca, Sr, Ba) solid solution - are added as boron additive. Contamination and bubble in the crystal can be reduced by keeping the melted composition at the determined ratio and the growing speed at 1-3 cm/h during crystal growth.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention
The present invention relates to a method for growing a rare earth hexaboride single crystal by a zone (FZ) method.

[0002]

2. Description of the Related Art Lanthanum hexaboride and cerium hexaboride single crystals are currently used as long-lived, high-brightness electron-emitting materials in scanning electron microscopes and electron drawing apparatuses. When used as this electron emitting material,
A high-quality single crystal with high purity is required. As a method for growing a high-purity rare earth hexaboride single crystal, the growing temperature is high,
The FZ method in which impurities are removed by evaporation is suitable.

However, in the conventional FZ growth, when a rare earth hexaboride is melted, a rare earth element-excess melt zone composition (generally, a B / RE atomic ratio = about 5.5) is obtained by boron evaporation. As a result, it was necessary to reduce the growth rate to 1 cm / h or less ("A survey on the floating z
one crystal growth of the monocarbides of Iva, Vaa
nd Via transition metals '' T.Tanaka, S.Otani Prog.
Crystal Growth and Charct. 16 (1988) 1-18, 16
page).

[0004] The present invention overcomes the above-mentioned disadvantages of the prior art,
It is an object of the present invention to provide a method capable of growing a single crystal at a higher speed and obtaining a high quality rare earth hexaboride single crystal having few defects.

[0005]

Means for Solving the Problems In order to solve the above problems, the present inventors have investigated the factors that determine the growth rate in the conventional FZ method and found the following. In other words, when growing a rare earth hexaboride single crystal, the melt zone composition becomes rare earth element excess (normally, B / RE atomic ratio = about 5.5) from the stoichiometric composition (B / RE atomic ratio = 6). Has determined that the growth rate.

Therefore, by adding boron to the raw material supply rod, that is, by using the raw material supply rod composed of REB ₆ and B, the melt zone composition during the growth is prevented from becoming rare earth excess (ie, boron deficiency). Control the melt zone composition
An attempt was made to grow a lanthanum boride single crystal by the FZ method (Japanese Unexamined Patent Publication No. 9-169597, "Preparation of LaB6
single crystals from a boron-rich molten zone by t
he floating zone method '' S. Otani, S. Honma, Y. Yajim
a, Y. Ishizawa, J. Crystal Growth 126 (1993) 466-47
0.). However, in this method, a semi-molten portion is formed immediately above the melt zone, and an umbrella-shaped material having a eutectic composition of REB ₆ and B is formed thereon, which causes a problem of sudden melting.
Stable breeding was not possible.

Therefore, instead of boron, an alkaline earth metal
Boride AEB₆(AE is Ca, Sr, Ba, or
(Ca, Sr), (Ca, Ba), (Sr, Ba),
(Indicating a solid solution of (Ca, Sr, Ba))
Solved the above problem. AEB ₆Is a rare earth hexaboride
Does not form a eutectic at low temperature with
And the alkaline earth elements evaporate leaving boron,
This is because the boron component in the band increases. As a result,
The growth rate, which was about 1 cm / h at the maximum,
At the speed of h, the breeding became possible. The resulting crystal
Does not contain any inclusions (inclusions).
A high quality single crystal with few boundaries can be obtained. these
The present invention has been made based on the findings described above.

Namely, the present invention is the floating zone method, REB ₆ (RE is La, Ce or (La, Ce) shows a) upon development of rare earth hexaboride single crystal represented by, AEB ₆ (AE is Ca, Sr, B
a, or a (Ca, Sr. Ba) solid solution) is used as a boron feed additive to which an alkaline earth hexaboride is added as a boron additive. / RE (AE), molar ratio) from 5.9 to 8
And growing the crystal while maintaining the temperature of the rare earth hexaboride single crystal.

Further, the present invention is characterized in that the melt zone composition is brought close to the stoichiometric composition, the crystal is grown at a growth rate of 1 to 3 cm / h, and the content and bubbles in the crystal are reduced. This is a method for growing rare earth hexaboride single crystals.

Hereinafter, the present invention will be described in more detail. FIG. 1 shows an example of an apparatus used in the present invention. This apparatus is a high-frequency induction heating FZ furnace designed so that crystals can be grown in an inert gas atmosphere of several atmospheres. The lower end of the raw material supply rod 10 is heated by causing a high frequency current to flow through the work coil 4 to generate an induced current in the raw material supply rod 10 and using Joule heat. In this way, the raw material supply rod 10 is fed into the formed melt zone 7 from above, and the single crystal 6 is grown from below.

A procedure for growing a single crystal according to the present invention will be described. First, after mixing the raw material rare earth hexaboride powder (REB ₆ ) and alkaline earth hexaboride (AEB ₆ ) powder well, a small amount of camphor was added as a binder, and a rubber press (2000 kN) was used.
g / cm ² ) to produce a dust bar. This powder bar is heated in a vacuum or an inert gas to a temperature of several hundreds of degrees Celsius to produce a raw material sintered bar.

The obtained raw material sintered rod 10 is set on the upper shaft 2 via the holder 3, and a <100> seed crystal (or a sintered rod for forming an initial melt zone) 5 is mounted on the lower shaft 2 ′. Set via 3 '. Next, the lower end of the raw material sintered rod 10 is melted by heating to form a fusion zone 7, and the upper shaft 2 and the lower shaft 2 'are slowly moved downward to grow the single crystal 6. At this time, the feed rate of the raw material sintering rods 10 to the melt zone 7 is set so that the density of the raw material sintering rods is low. Set. As the atmosphere, an inert gas such as argon or helium at several atmospheres is used. This is to prevent discharge generated in the high-frequency work coil 4 portion.

If the melt zone composition becomes a rare earth element excess (B / RE atomic ratio <6), the phase containing the rare earth element in the crystal (ie, LaB ₄ phase or CeB ₄ phase) is required unless the growth rate is reduced.
Is included as a content (generally, a content of several hundred μm or less) and cannot be used as an electron-emitting material. Therefore, in the case of the conventional method using a rare earth hexaboride as a raw material rod,
The melt zone composition had a composition near B / RE = 5.5, and the growth rate was reduced to 1 cm / h or less.

As a solution to the above problem, a method of growing a crystal by using a raw material rod containing an excessive amount of boron to bring the melt zone composition closer to a stoichiometric composition (B / RE atomic ratio = 6) has been attempted. In this case, a semi-molten portion 8 is formed immediately above the fusible zone 7, and an umbrella-shaped material 9 having a eutectic composition of REB ₆ and B is formed thereon. This may come into contact with the work coil 4 during growth, making it impossible to move the melt zone thereafter, or the umbrella-shaped portion may be suddenly melted before melting into the melt zone due to its low melting point, and may become a raw material rod. It was sucked in and the volume of the melt constantly fluctuated, and stable crystal growth was impossible.

In the present invention, boron is not added to the raw material rod, but an alkaline earth hexaboride is added, so that the rare earth hexaboride does not become eutectic at a low temperature, and the alkaline earth metal is not added. It has been found that alkaline earth hexaboride acts as a boron supply agent because boron remains and evaporates immediately after melting into the melt zone, and has a function of increasing the boron content in the melt zone.

The relationship between the amount of alkaline earth hexaboride added and the heating power was examined. FIG. 2 shows the results obtained by adding CaB ₆ to LaB ₆ and growing crystals at a growth rate of 2 cm / h. A circle in the figure indicates a crystal containing neither bubbles nor inclusions at a growth rate of 2 cm / h. The heating power was increased by the addition of CaB _6, the heating power had a maximum in the vicinity of the addition of 7 to 10 mol%, and the heating power was decreased by further increasing the amount.

In the region where the heating power was maximized (CaB ₆ of 5 mol% or less was added), a LaB ₄ phase was confirmed in the melt zone solidified after the growth. When CaB ₆ was added at 5 mol%, inclusions were confirmed in the crystal, and
/RE(AE)=5.7. 7 mol of CaB ₆
When% was added, neither inclusions nor bubbles were found in the crystals. The melt zone composition was B / RE (AE) = 5.9.

Furthermore, when CaB ₆ was added in an amount of 30 mol%, neither inclusions nor bubbles were found in the crystal.
The melt zone composition was B / RE (AE) = 7.9. Although the boron is a boron-excess melt zone, boron has a rare earth element (L
a and Ce) are smaller by one order of magnitude or more and diffusion at the growth interface is easy, so that deviation from the stoichiometric composition does not matter,
It is presumed that high-speed growth of 2 cm / h was also possible. 40
When mol% CaB ₆ was added, a CaB ₆ phase released on the surface of the crystal was confirmed. Melt zone composition is B / RE
(AE) = 8.9.

Further, 10 mol% at which the heating power is maximized
Using the added raw material rod, the relationship between the growth rate and the crystal quality was examined. At 1 cm / h, 2 cm / h, and 3 cm / h, crystals containing no inclusions or bubbles were grown. However, at a speed of 3 cm / h or more, cracks began to form in the crystal. Therefore, the preferred growth rate was 2-3 cm / h. Ca in the resulting crystals
The content was 180 ppm.

Next, in growing the CeB ₆ crystal,
Added CaB _6, a similar experiment was performed. LaB ₆ above
As in the case of the above, when 10 mol% CaB ₆ was added, the heating power was maximized, and when the addition amount was further increased, the heating power was decreased. In addition, a high-quality single crystal containing no inclusions or bubbles was grown even at a growth rate of 2 cm / h at a composition where the heating power was maximized.

[0021] as additives other than CaB _6, SrB _6,
The case where BaB ₆ was used was examined. The heating power was maximized by adding 10 atomic%. In addition, 2cm /
High quality LaB ₆ crystal was obtained even at the speed of h. Therefore, as a boron additive, CaB ₆ , SrB ₆ , BaB
_No difference was found between the _six .

The above growing method can be applied to a heating method other than high-frequency heating, for example, to grow a single crystal by the FZ method using infrared concentrated heating. The use of alkaline earth hexaboride as a boron additive can be applied not only to crystal growth by the FZ method but also to sample preparation by a melting method.

[0023]

Next, examples of the present invention will be described. Example 1 Commercially available LaB ₆ powder was mixed with 10 mol% of CaB ₆ powder, mixed with a small amount of camphor as a binder, and had a diameter of 12 mm.
And packed in a rubber bag. 2000kg / c
Pressing was performed with a rubber press of m ² to obtain a green compact. This green compact is heated in vacuum at 1800 ° C.,
A sintered rod having a length of about 12 cm was obtained. The density was about 55%.

The sintered rod was fixed to the upper shaft of the FZ growth furnace shown in FIG. 1 via a holder, and the LaB ₆ sintered rod was fixed to the lower shaft. After filling the growth furnace with 5 atm of argon, the lower end of the boron sintering rod is melted by high-frequency induction heating to form an initial melt zone, which is moved downward at a speed of 2 cm / h for 3 hours, and has a total length of 6 cm. A single crystal having a diameter of 0.9 cm was grown. The amount of Ca impurities in the obtained crystal was 180 ppm. The amount of Ca in the melt zone was 500 ppm, and the melt zone composition was B / La = 6.6.

The (100) plane was cut out from the obtained crystal, and after mirror polishing, etching was performed to observe the surface. As a result, it was confirmed that there were no bubbles and no other phase was contained.

[0026]

As described above, according to the present invention,
The rare earth hexaboride single crystal is 2
Obtained at ~ 3 times speed. In addition, high-quality crystals with fewer defects are grown at the growth rate in the conventional method.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing an example of a single crystal growing apparatus used in the present invention.

FIG. 2 is a graph showing the relationship between the amount of Ca added and heating power.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Upper shaft drive part 1 'Lower shaft drive part 2 Upper shaft 2' Lower shaft 3 Holder 3 'Holder 4 Work coil 5 Sintered rod for seed crystal or initial fusion zone formation 6 Single crystal 7 Fusion zone 8 Semi-molten part 9 Umbrella-shaped part 10 Raw material sintered rod

Claims (2)

[Claims] 1. A floating zone method,
When growing a rare earth hexaboride single crystal represented by B ₆ (RE represents La, Ce, or (La, Ce) solid solution), AEB ₆ (AE represents Ca, Sr, Ba, or (C
a, Sr), (Ca, Ba), (Sr, Ba), (C
a, Sr, Ba) which indicates a solid solution), using a raw material supply rod to which an alkaline earth hexaboride represented by the following formula (1) is added as a boron additive:
/ RE (AE), molar ratio) in the range of 5.9 to 8 to grow the crystal. 2. A hexahedron according to claim 1, wherein the melt zone composition is brought close to the stoichiometric composition, the crystal is grown at a growth rate of 1 to 3 cm / h, and the content and bubbles in the crystal are reduced. Method for growing rare earth single crystals.