Technical Report -2

Application of the Optical Floating Zone
Apparatus for Researching Material Sciences

Part-1

Practice of Single Crystal Growth
Using Optical Floating Zone Furnaces

Crystal Systems Corp.
〒408-0011 Kobuchisawa 9633 Yamanashi 408 Japan
TEL:+81-551-36-5271

FAX:+81-551-36-5273

The optical floating zone furnace has a wide range of applications forthe study of both single crystal growth and phase diagram. This type of
furnace needs no crucibles, therefore, the chemical reaction between the melt
and the crucible material should be avoided. Also this type of furnace should
achieve the traveling solvent scheme which is needed for growing
incongruently melting materials and solid solution with uniform
compositions.
During the past 20 years, many successful results have been reported,and many high-quality single crystals have been grown. This report presents
detailed procedures for growing single crystals for younger researchers whowish to grow their own single crystals for the first time.
It is safe to say that the growth of single crystals in an opticalfloating zone furnace is influenced by the following elements;
Feed rod
50% ( almost half )Furnace25% ( quarter )Conversation
25% ( quarter )Feed rodIn the growth of single crystals by the floating zone method, themolten zone is sustained by the feed rod through the surface tension,
therefore, the qualities of the feed rod are very important and have a strong
influence on the stability and successful formation of the molten zone. Thus,
a great deal of effort has to be made to create a straight and uniform
diameter rod.
Furnace
The quality of furnace is also important as stable and uniform
temperature conditions have to be kept throughout the growth of crystals.
The four mirror type furnace can achieve such temperature conditions andcan grow high-quality single crystals.
Conversation
This conversation means that the crystal grower has to ″talk〝with material itself and find its suitable growing conditions.

CONTE NTSPart-1 Practice of the single crystal growth by the optical floating zone
furnace
1. Introduction
2. Flow chart of growing procedure
3. Preparation of starting powders
3.1 Measure the ignition loss
3.2 Weighing
3.3 Mixing
3.5 Drying

2
4
5
5
5
5
5

4. Preparation of feed rod
4.1 Packing
4.2 Hydrostatic pressing
4.3 Make a hole
4.4 Sintering
5. Growth procedure
5.1 Growth of congruently materials
5.2 Growth of peritectic materials
5.2.1 Measure the composition and volume of molten zone
5.2.2 Practice of the growth using molten zone rod

5
5
6
7
7
8
8
9
10
11

2.

Flow chart of the growth of single crystals using optical floating zone
furnace.
Items

Using tools

Ignition loss measurement

Thermo-balance
Platinum crucible

Measure the weight

Balance

MixingAlumina crucible

Drying

Drier

Packing (in rubber tube)

Rubber tube

Air exhaust

Air exhauster

Pressing

Hydrostatic press

WiringDrill(1mmΦ), Platinum wire

Sintering

Vertical furnace
Rotating lifter

Crystal Growth

Floating zone furnace

Remarks

HS-M-1

FZ-T-4000-H
FZ-T-10000-H
FZ-T-12000-X

Circulating chiller
Mass flow controller
Turbo vacuum exhauster
Direction measurement

X-ray Laue method

CuttingFine cutter

CU-M-1

PolishingFine polisher

TP-01

3. Preparation of starting powders
3.1 Measurement of ignition loss of the powders
In the floating zone method using optical image furnaces, the volumeof the molten zone must be limited by its surface tension. Also thecomposition of the starting powders has to be correct, therefore, ignition lossof the powders has to be measured.
3.2

Weighing

An accuracy of 0.01 % is needed, and it is always necessary to watch
the change in the ignition loss value. In the case of the floating zone method,the volume of the molten zone is much less than that of Czochralski pullingmethod, therefore, small compositional changes have a greater influence on
the results.
3.3

Mixing
The wet method of mixing is suitable for reducing the mixing time.
Normally, ethyl-alcohol is added and mixed for approximately 2 hours.
3.4 ..Drying
4.

Preparation of the sintered rod
4.1 Packing
Starting powders are packed into the rubber tube with anappropriate diameter and length. After packing the powders, the packed
rubber tube is pressed by the hydrostatic method.

Φ2mm RodExhaust air
FunnelCottonRubber tube

(1)

Pack the powders
Fig 1.

(2)(3)

Put the cotton on
Exhaust air and bind up
the powders
Procedure for packing powders in a rubber tube

4.2

Pressing
The packed rubber tube is pressed by the hydrostatic press method
using water. In most cases, the oil pressure of the hand press is at 700kg/㎠,therefore, the rubber tube will be pressed at approximately 2,000kg/㎠

Press

Horse

Frame
PistonVesselPressure
gauge

Water
Material
Oil pump

Fig 2.

Procedure for packing powders in a rubber tube

The packed rubber tube is inserted into the vessel, which is filledwith water, then pressed hydrostatically by using the oil press.

4.3

Drilling
After being pressed, the pressed bar is separated from the rubber, anda small hole of 1mm diameter is bored using a 1mm diameter hand drill.
Thread the small diameter platinum wire through this small hole, andmake a loop as shown in fig.3(right) The pressed bar is easily broken at thisstage, so this process must be done carefully.

Bore a hole using drill( diameter 〜 1mmΦ )4.4

Thread the platinum wire

Sintering

Hang the pressed bar using the loop on the hook at the bottom of thealumina ceramic bar of the rotating lifter and insert into the electric furnace.
In order to make a rod which is straight and of uniform density, the bar has
to be rotated continuously, and be moved up and down to achieve the same
temperature history.
The interval time and number of repeating
up-and-down are set, and then start the sintering.
Atmospheric gas is introduced into the furnace from the bottom.
The top of the ceramic tube is covered by two halved ceramic plates to keep
the good atmospheric condition.
After the repeat of moving up-and-down according to the desiredinterval, the sintered rod will be moved to the top position automatically.

Rotational lifterElectric furnace

Pressed bar

Cooling water

Atmospheric gas inlet

Fig 4.

Sintering of the pressed bar under desired atmospheric gas.

During the sintering process, the bar has to be continuously rotatedand be moved up and down in order to avoid vending and local sintering of
the bar. A straight and uniform sintered rod is needed to achieve and keepa stable floating zone.

5. Crystal Growth
The growth of single crystals by the floating zone method is consideredas the melt growth method; however, if the material melts incongruently, thescheme of the growth will be drastically different from the normal
congruently melting compounds.
5.1 Growth of congruently melting compounds
1) SeedIn many cases of growth, a single crystal is not prepared as the seed.
Therefore, the sintered rod should be used as the seed.
When a polycrystalline sintered rod is used as the seed, grains will be
generated, however, after 30-40 mm length progression, grain will bedecreased gradually. Finally, these grains will become as seed single in somecases ( in the case of Y3Fe5O12, Li2O, for example ).
Seed crystal or rod has to be fixed using nickel wire to the ceramic
seed holder.
Seed rod

Nickel wire

Ceramic seed holder

Fig 5.

Seed rod and its holder

2) Feed rod
The feed rod is hung from the hook of the upper shaft by the platinum wireloop as seen in Fig.6 . This procedure seems to be “rough” and “uncertain”,
however, it will be more safe way because if the feed rod is fixed to the upper
shaft, the rod will be easily broken when it contacts with the seed rod during
the growth.

Upper shaft

Platinum hook

Platinum wire loop
Feed rodFig 6.

Hang the feed rod to the hook of the upper shaft.

3) Length of the molten zone

(a)

(b)

(c)

Fig 7. Length of the molten zone
At the first stage of the heating, both the feed and seed rod are set
separately having a few mm distance ( Fig. 7a ). As the heating progresses,
both ends of the feed and seed rod will begin to melt ( Fig 7b). At this point,
the upper feed rod is moved downwards until it touches the seed rod. Finally,
the length of the molten zone is adjusted, so its diameter will be almost thesame size as that of the seed rod ( Fig 7c ).

4) Rotation rate
In normally, both the upper and the lower shaft should be rotated in
opposite direction at a rate of 30 – 50 rpm. A large rotation rate can mixing
the molten zone and can keep homogeneous compositional condition which
would be good effect to avoid the cellular growth patterns. Furthermore, alarge rotation rate has an effect to decrease the convexity of the solid liquidinterface and can keep the stable molten condition to avoid the touching of
the upper feed rod and the lower grown crystal.
( ref. Kitamura et al, J. crystal Growth 46 (1979) 277, )Molten Zone

20rpm

60rpm

Crystalline

Fig 8. Relation between interface shape and rotation rate.

5) Observation of the growth conditions
One of the most important advantages of the optical floating zone
furnace is that the growth conditions can be observed very easily by using
the CCD camera system during the growth.

C

CCD camera
Fig 9.

CRT

Schematic illustration of the observation system.

6) Grain boundary
In many cases, the shape of the solid-liquid interface is concave (Fig 10).

Se

Se

Se

Initial

Initial

Initial

stage

stage

stage

Fig 10.

Schematic illustration of the grains.

In conclusion, the number of grains will be decreased, then a single
gain (single crystal) will be obtained in some cases when a sinteredpolycrystalline rod is used as the seed. In such cases, the convex shape of thesolid liquid interface is an important element to decrease the number ofgrains as the crystal growth proceeds.

5.2

The growth of incongruently melting or peritectic compounds

Temp.

A

C0

C1

C2

B

composition

Fig 11.

Schematic diagram including an incongruently melting compound.

An incongruently melting compound C0 can be precipitated from asolvent of composition between C1 and C2. Therefore, a traveling solvent
scheme in which the feed is dissolved into solvent, and the relative
proportions of crystal is precipitated from solvent, has to be kept during the
growth.

feed

Polycrystalline
sintered rod is oncefeed
solvent

meltedandthensolidified

with

the

same composition ofsolvent

the sintered rod.
crystalcrystalFig 12.

Schematic illustration of the traveling solvent floating zone method.

5.2.1

Measurement of the volume and the composition of the solvent
In order to grow a single and uniform compositional crystal ofincongruently melting materials, the volume and the composition of thesolvent zone under equilibrium condition has to be measured, and the chip
having the measured volume and composition named as the solvent chip isset at the initial stage of the growth. In the first experiment, these values are
not known, therefore, the growth is started without the solvent. After a
30-50mm long crystal is grown, all electric powers of the rotation shafts andhalogen lamps are cut off to quench the molten zone.
The quenched bole is halved along the longitudinal direction. The
volume of the quenched molten zone can be estimated, and its composition
can be determined by an electron probe micro analyzer or other chemical
methods.

Feed
Quenched
molten zone
Grown crystal

Fig 13.

Halved bar after quenching

The quenched molten zone is very soft and easily collapses; therefore,
organic resin would be very useful to fix it, and the corresponding procedures
such cutting and polishing would become more easy and safety.
5.2.2

Growth of uniform compositional crystal using a solvent chip.
After the measurement of volume and composition of the molten zone,
a sintered rod of this composition is made separately and is cut with thedesired volume. This solvent zone chip is set on the top of the seed rod asshown in the following figure.
First, the top of the solvent chip and the bottom of the feed rod arepartially melted and connected. Then, the top of the seed rod and the bottomof the solvent chip are melted and connected. This procedure needs a very
stable and uniform temperature condition around the horizontal plane. The
four mirror image furnace can meet this requirement.

First stage

Fig 14.

Second stage

Third stage

Final stage

How to use the solvent zone chip1) First stage
Set the solvent zone chip on the top of the seed rod. A small amount ofstarch paste can be used to fix it. The starch will be burnt off as thetemperature increases, and a small amount of smoke will appear. If thissmoke stains the center portion of the quartz tube, it can block off the light.
Therefore, a larger amount of atmospheric gas (10ℓ/min) has to be used tocarry the smoke away from the center of the quartz tube.
2) Second stage
Both the top of the solvent zone chip and the end of the feed rod arepartially melted, and connected without rotation of either the upper orlower shafts. After that, the lamp power will be slightly decreased and theupper shaft will be retracted to separate the solvent zone.
3) Third stage
Both the end of the solvent zone chip and the top of the seed rod arepartially melted, and are brought into contact with rotation of both shafts.
3) Final stageAdjust the lamp power to keep the molten zone stable and to melt a few
millimeters of the top portion of the seed. After a few minutes, the crystal
growth will begin.
5.2.3

Preparation of a solvent zone attached feed rod
The technique of the solvent zone chip described in former section
requires much time and careful handling. The following simple techniquewill be more effective in some cases. Firstly the desired volume of thesolvent zone material is put into a rubber tube for pressing, then the feedmaterial is also put into the rubber tube as shown in the Fig.14. .Afterhydrostatic pressing, a feed rod with an attached solvent zone can beobtained.

Feed material

Fig 14.

Solvent zone

Feed rod attached solvent zone

Although this technique is simple and uncomplicated, it can only beapplied to some specific cases in which the sintered temperature of the goodquality feed rod is lower than that of the solvent zone material. In normally,
the sintered temperature of the feed rod and the solvent zone material is not
the same level, therefore, the feed rod and the solvent zone rod have to bemade separately, and the proper amount of the solvent zone should be cut
and used as the solvent chip which will be inserted into between the feed rod
and the seed rod before melting.