Techni cal Repor tNo. H20-FZ-E-1

GAS GUIDEin the Optical Floating Zone Furnace

20th of J an. 2008

Crystal Systems Corporation9633 K obuchisawa Hokuto 408-0044 J APAN
TE L : +81-551-36-5271 FAX: +81-551-36-5273
http://www.crystalsys.co.jp

Introduction
The Optical floating zone furnace has been accepted as the most useful and powerful tool
for the growth of single crystals and had been successfully applied for the growth of various
materials including oxide, metal and inter-metallic compounds. The optical floating zone method
had many advantages, however, this method needed to use the transparent quartz tube, therefore,
keeping the transparency of the quartz tube was the key element to use the optical floating zone
furnace in a best condition. Crystal Systems Corporation had developed a new technique called“Gas Guide” to meet this requirement.

2. Differences among the three types of the focusing system
Single, double and four mirror focusing system had been developed and used for theoptical floating zone furnaces. There were some important differences between these systems. The
effective length of the quartz tube was one of them.
Both the single and double mirror system, for example, the part of the quartz tube, which
was placed to the inside of the ellipsoidal mirror, became an effective zone. Therefore, this zone
needed to be kept in a clean condition. The length of this effective zone of the four mirror system
was shorter than that of both the single and double mirror system (Fig.1).

Single mirror

Double mirror

Four mirror

（A）

(B)

(C)

Fig.1 Three types of the focusing system for the optical floating zone furnace.
In the case of the four mirror system, the center portion of the mirror system has wide space andboth the single and the double mirror system has no such space between the quartz tube and themirrors.

This meant that the four mirror system could prevent the deposition of small particles from thesurface of the quartz tube (Fig. 2).

Fig.2 The Effective zone for the focusing of the infra red rays in the corresponding different
mirror systems. The length of this zone in the four mirror furnace is much shorter than thatof both the single and the double mirror system.

3. Traditional techniques for keeping the quarts tube clean
3.1 Use the large flow of the atmospheric gas
In some cases, the evaporation from the melt occurred during the growth, and many small
particles were condensed to the surface of the quartz tube. In these cases, a large flow of theatmospheric gas; more than 20cm per second inside the quartz tube, would be taken out from thequartz tube chamber, so the surface of the quartz tube was kept in a clean condition. This techniqueseemed to be simple, easy and no problems. However, in the case of pure argon gas, which wasneeded as the atmosphere, the cost became high and conducting a long-time experiment became
difficult.
Gas out
Condensed particles

Transparent quartz tube
Gas inn

Fig.3．Schematic illustration of the optical floating zone furnace.
Small particles of the evaporated materials from the molten zone would condense anddeposit on the surface of the quartz tube, then transparency become poor gradually and insome cases, the condensed materials would react the quartz and the life time of the quartz
3.2 Use the cold metal trap techniquetube become very shortened.

Maeno et al developed the new technique for the use of the cold trap introduced into thequartz tube ( See Japan Tokkyo Kokai 2005-247668 ). The schematic diagram of this technique
was seen in Fig. 4.
Small particles were more condensed on this cold metal trap than the surface of the quartz
tube, but this metal trap shielded the infra red rays, therefore, the position of it influenced thetemperature of the molten zone. The effect of this trap became larger according to the closesetting to the molten zone, while the temperature of the molten zone became lower.
Gas out
Cold trap

Transparent quartz tube
Gas InnFig.4 Schematic diagram of the cold trap inserted in the quartz tube and focusing scheme.
Small particles from the molten zone would be condensed on the cold trap rather than thesurface of the quartz tube, then can keep its clean condition more long time compared to thenormal case without the cold trap.

4.

The use of the gas guideIn the four mirror focusing system, a large flow of the atmospheric gas ( more than 20cm
per second) can take out almost all of small particles from the molten zone to the outside of thequartz tube, so that the surface of the quartz tube in the four mirror focusing system can be kept in aclean condition even under the long-time experiment of the growth of evaporated materials. Thismeans that the smaller cross sectional area in the quartz tube would be more sufficient to keep theinside of the quartz tube in a clean condition even under the use of the same amount of theatmospheric gas. In normally, the 20mm diameter shaft is used for the floating zone furnace, thenthe flow rate on the 2 litter per minutes inside the 24mm inside diameter quartz tube become morethan 20cm/sec. As a good example, the surface of the quartz tube could keep the clean condition
more than 24 hours in the case of the growth of YVO4 under the same atmospheric condition.

Gas out
Gas guide having a horn shaped end
made by transparent quartz

Quartz tube

Gas inn

Fig.5 Schematic diagram of the gas guide having a horn shaped end inserted into the quartztube.
+Gas guide

Quartz tube

Mirror

Fig.6 Photo of the gas guideFig.7

Feature of the gas guide inside thequartz tube