JPS5849517B2 - Single crystal growth method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for growing bismuth-euritite group single crystals such as Bi4Ge301 and Bi4Si3012 using the Czochralski method.

For example, Bi4Ge301 single crystals are used as optical crystals and crystals for γ-ray scintillators.

In that case, the crystal used must have high light transmittance and must not contain any scatterers.

When a Bi4Ge301 single crystal is grown by the Czochralski method, what happens during the crystal growth process? A number of voids are generated in a band shape, and these are likely to be incorporated into the crystal.

These voids act as light scatterers, degrading crystal quality and decreasing crystallization yield.

Such voids are also observed in single crystals of sapphire (α-Al203), and the cause of their occurrence is thought to be as follows.

That is, fluctuations in temperature occur at the solid-liquid interface of a growing crystal and fluctuations in crystal growth rate due to accumulation of impurities.

When the liquid incorporated into the crystal solidifies,
Voids are created due to the difference in density between solid and liquid.

In the case of sapphire, the generation of voids has been prevented by making the solid-liquid interface highly convex.

However, when the relationship between the shape of the solid-liquid interface and the generation of voids in crystal growth of B i4 G e 3 01 was investigated, it was found that when the solid-liquid interface is flat, the amount of voids generated is small.

FIG. 1 shows the crystal shape when the solid-liquid interface of the single crystal being grown, that is, the growth end face is convex.

Here, if the crystal diameter is D and the height of protrusion at the solid-liquid interface is H, then the degree of protrusion is expressed as H/D.

This value has a positive sign if it is convex and a negative sign if it is concave.

It was found that in order to prevent the generation of voids in the Bi4Ge301 single crystal, the value of H/D should be set to 0.2 or less.

However, if the solid-liquid interface changes from flat to too concave, crystal growth becomes unstable and inclusions are easily incorporated, so the H/D value decreases to 10.2 or less. I can't.

The present invention will be explained in detail below using examples.

Example 1 An example will be described in which a Bi4Ge3012 single crystal with a diameter of 25 mm was grown by the Czochralski method using a platinum crucible with a diameter of 50 mm.

What is the relationship between the crystal rotation speed and the H/D value when crystals are grown by changing the crystal rotation speed, which is one of the growth conditions? is shown in Figure 2.

From the figure, when the crystal rotation speed is 3 Orpm, the solid-liquid interface becomes convex, but when the rotation speed is increased to 40, 50, or 6 6 rpm, the shape of the interface changes from flat to concave.

However, when the rotational speed was further increased to 70 rpm, the solid-liquid interface became greatly concave, the crystal length was unstable, and it was not possible to grow the crystal.

The length of the parallel portion of the grown crystal is 80 mm, and the voids generated in the crystal at this time are distributed in a band shape along the position of the solid-liquid interface, as shown in FIG.

The number of these void bands is investigated, and H/
When plotted against the value of D, it becomes as shown in Fig. 4.

As is clear from the figure, when the solid-liquid interface is flat,
The number of Void Emperors has decreased, and the H/D value is approximately -0.2 ~
If it is within the range of 0.2, the number of poid bands is 5.
It can be seen that the number is less than 1.

In order to collect a scintillator crystal from the grown single crystal, it is essential that the number of void bands be 5 or less in order to improve the yield.

Therefore, the shape of the solid-liquid interface of the crystal, that is, H/D
If the crystal rotation speed is selected so that the value of is within the above range, high quality crystals can be obtained with a high yield.

Similar results are obtained with Bi4Si301.

Bi4Ge with a diameter of approximately 25 mm was grown as described above.
3012 or Bi4Si3012 single crystal was cut into wafers for practical use, and conditions for etching the wafer surface were investigated.

The cut wafers were first given a mirror finish using alumina powder and mechanical/chemical polishing, and then etched.

The etching solution was made by adding water to a 36% by weight HCl solution.
The concentration is 2.5-36% by weight, and the solution is further reduced to 20-36% by weight.
It was heated in the range of 80°C.

The wafer was rotated 60 times per minute in an etching solution, and the optimal hydrochloric acid concentration and solution temperature range for etching were determined.

The results are shown in FIG. In this figure, Tatoeha,
In a 9% by weight HCI solution, the etching rate at a solution temperature of 48°C is approximately 10μ/mI! At t, clear etch pits are observed, but a mirror surface is not obtained.

When the liquid temperature is raised to 75° C., an etching rate of 50 μ/min is obtained, and the wafer becomes mirror-finished.

On the other hand, when heated to 80° C., the etching rate is too fast, a large amount of white deposits tend to adhere to the wafer, and the etched surface becomes rough.

When the liquid temperature is raised to 80°C or higher, a large amount of hydrogen chloride gas is generated.

FIG. 5 summarizes the etching conditions for obtaining a mirror surface using an aqueous solution having a hydrochloric acid concentration of 2.5 to 36% by weight.

In the figure, curve 1 is the condition under which etch pits appear and a mirror surface can be obtained, and curve 2 is the limit at which a mirror surface can be obtained.

Straight line 3 is a condition under which a large amount of hydrogen chloride gas is generated.

In the end, it was found that by performing etching under the conditions of the shaded region in FIG. 5, etch pits for examining crystal lattice defects could be created and at the same time a mirror surface could be obtained.

[Brief explanation of drawings]

Figure 1 shows the crystal shape when the solid-liquid interface of the growing single crystal is convex, and Figure 2 shows the relationship between the crystal rotation speed during growth and the shape of the growth end of the grown single crystal. Figure 3 is a diagram showing the band distribution of voids in the grown single crystal, Figure 4 is a diagram showing the relationship between the growth end shape of the grown single crystal and the number of void bands, and Figure 5 is a diagram showing the relationship between the shape of the growth end of the grown single crystal and the number of void bands. FIG. 3 is a diagram showing etching conditions for obtaining a mirror surface using an aqueous hydrochloric acid solution.

Claims (1)

[Claims] 1 In the method of growing a single crystal of a bismuth-euritite group compound by the Czochralski method, when the diameter of a certain long crystal is D and the protrusion height of the solid-liquid interface of the crystal is H, H/D is A method for growing a single crystal, characterized in that the rotation speed of the crystal is selected so that the rotation speed is within the range of -0.2 to 0.2.