JP4940610B2 - Sapphire single crystal growth method - Google Patents

Description

  The present invention relates to a method for growing a sapphire single crystal, and more particularly to a method for growing a sapphire single crystal by a high-frequency heating type Czochralski method.

  A nitride light-emitting film, which is a main component of a blue light-emitting element developed in recent years, is obtained by epitaxially growing the nitride on a sapphire substrate. As a sapphire substrate used for such an epitaxial, a sapphire single crystal cut into a wafer and having a mirror-polished surface is used.

  Incidentally, there is a Czochralski method (hereinafter, simply referred to as “CZ method”) as one method for obtaining an oxide single crystal such as sapphire. In order to obtain an oxide single crystal by the CZ method, for example, a metal iridium crucible is filled with an oxide as a raw material, a high frequency is passed through a work coil provided on the outer periphery of the crucible, and the iridium crucible is heated by this high frequency. The oxide in the crucible is melted by this heat.

  After that, attach the seed crystal to the oxide melt in the crucible, pull up the seed crystal while rotating the melt temperature, and expand the crystal diameter to the target diameter to form the shoulder, then form the straight body Then, an oxide single crystal is grown. During growth, the diameter and growth rate are derived from the change in crystal weight obtained by measuring the crystal weight during growth, and the calculation results are fed back to the work coil input power to control the melt temperature. An oxide single crystal having a good shape is obtained.

  When this CZ method is applied to obtain a sapphire single crystal whose raw material is alumina, the sapphire single crystal solid-liquid interface protrudes into an inverted conical shape (hereinafter, the height of the conical portion is referred to as “convexity” in this specification). A sapphire single crystal having a shape is obtained. If the lower part of the sapphire single crystal grows in an inverted conical shape, the tip reaches the crucible bottom, and the change in the crystal weight cannot be obtained accurately. Cannot be obtained.

Also, if the convexity is large, the stroke for separating the sapphire single crystal from the melt after the crystal growth is finished (the pulling distance of the sapphire single crystal) becomes long, the pulled sapphire single crystal is too far from the heating region, and the sapphire single crystal Is rapidly cooled and the single crystal breaks.
In addition, there is a problem that a substrate cut out from a crystal having a high degree of convexity has a large distortion.

  The tendency to obtain a single crystal in which the lower part of the crystal protrudes in an inverted conical shape is not limited to the growth of sapphire single crystals, but also in the growth of other oxide crystals and compound semiconductor crystals. This is because the lower part of the single crystal has an inverted conical shape because the melt is heated mainly from the side of the crucible. That is, since the melt is heated from the side surface of the crucible, a temperature distribution is generated in the melt such that the melt temperature decreases from the side surface of the crucible toward the center of the melt. As a result, the crystal growth rate at the center of the crucible increases and the crystal growth rate at the outer periphery decreases. As a result, the bottom of the single crystal has an inverted conical shape.

  In order to prevent this, the inversion method or the work coil, which usually flattens the crystal growth interface by increasing the convection of the melt by increasing the number of revolutions of the single crystal to make the melt temperature uniform. By adjusting the relative positions of the crucible and the crucible, the temperature distribution is flattened to obtain a single crystal having a small convexity.

  However, even if these methods are applied to the growth of a sapphire single crystal, sufficient effects are not seen. This is because the alumina melt has a high thermal conductivity. For example, if the temperature of the melt is made uniform by rotating the single crystal, the rotation speed of the single crystal must be greatly increased. This causes a problem that a sapphire single crystal having the required transparency cannot be obtained.

  In order to obtain a sapphire single crystal having a small convexity, the present inventors provided that the main part of the apparatus provided in the chamber is a high-frequency induction coil, a refractory crucible, an iridium crucible, a heat insulating material, and a refractory crucible support cylinder. And a cylindrical heater, and an iridium crucible is housed in a refractory crucible via a heat insulating material, and a refractory crucible support cylinder in which a cylindrical heater is provided at the bottom of the refractory crucible However, the sapphire single crystal growth is characterized by adopting a structure in which the upper surface of the cylindrical heater is provided so as to be in contact with the bottom surface of the refractory crucible, and these are accommodated in a space portion in the high frequency induction coil. An apparatus was proposed (Patent Document 1). And it was shown that the temperature gradient just above the melt can be optimized by using this apparatus, and crystals with low convexity can be obtained.

Subsequent examination revealed that the apparatus had the following problems.
(1) Since such a device must be a special device, it becomes expensive and increases costs.
(2) Even if such an apparatus is used, the temperature gradient inside the growing crystal is extremely unstable, and thus a step of annealing the crystal obtained after the growth is required. And this annealing process tends to be complicated. Therefore, there is a problem in obtaining a sapphire single crystal having a small convexity inexpensively and easily using such an apparatus.
Japanese Patent Application No. 2004-044508

  The present invention has been made in view of the above-described problems, and an object of the present invention is to provide an inexpensive and simple method for growing a sapphire single crystal having a low convexity.

The present inventors tried various studies to solve the above problems. And as a result of analyzing many obtained results, it was found that there was a correlation between the shape of the shoulder formed during the sapphire single crystal growth and the degree of convexity, and the present invention was achieved.
That is, the method of the present invention that solves the above-described problems comprises the length of the shoulder portion from the seed crystal portion to the straight body, that is, the intersection line of the shoulder portion and the straight body portion, when the sapphire single crystal is grown by the CZ method. This is obtained by controlling the ratio (B / A) of the distance A between the intersection of the surface to be passed through the seed crystal center and the straight line drawn in the pulling direction, and the distance A between the seed crystal center and the average diameter B of the straight body portion. The height of the convex shape of the sapphire single crystal solid-liquid interface is controlled, and the B / A is controlled to 2.29 or more.
Then, another aspect of the present invention is to the B / A and from 2.29 to 4.5.

  According to the method of the present invention, in the obtained sapphire single crystal, when the convexity is C, the ratio B / C of the average diameter B to C of the straight body portion is 1.4 or more. Accordingly, it is possible to grow a crystal having a long straight body portion, and it is possible to shorten the stroke at the time of separation, thereby avoiding the problem of crack generation.

  Moreover, although it is a secondary effect, the distortion of the sapphire substrate obtained by cutting and processing the sapphire single crystal obtained by using the method of the present invention without annealing is compared with that of the conventional sapphire substrate. As a result, it is harder to break than a conventional sapphire substrate.

  The present invention controls the shape of the growth interface of a sapphire single crystal by controlling the shoulder shape at the start of crystal growth without greatly changing the configuration of a general crystal growth apparatus.

  In crystal growth by the CZ method, a seed crystal is immersed in a melted alumina melt, and the shoulder is grown by gradually lowering the melt temperature while gradually pulling the seed crystal upward while rotating the seed crystal. Thereafter, the straight body portion is grown by adjusting the rotation speed, the pulling speed, and the melt temperature.

  Here, the aforementioned B / A is defined as the degree of shoulder opening. The larger the shoulder opening value, the larger the shoulder opening angle (opening angle of the shoulder centered on the seed crystal), and the shape of the obtained upper portion of the single crystal is close to a cylinder. On the other hand, if the degree of shoulder opening is small, the shoulder opening angle becomes small, and the shape of the obtained single crystal upper part approaches a cone. As described above, this shoulder opening degree can be adjusted by adjusting the number of rotations of the single crystal, the pulling speed, and the melt temperature.

In the present invention, why the convexity of the sapphire single crystal obtained when the shoulder opening degree is 2.29 or more is a result obtained from a lot of experimental data, and the reason is not clear. However, it can be considered as follows.

  In general, the shape of the single crystal growth interface is determined by the heat input and output at the solid-liquid interface. The heat flow at the solid-liquid interface goes from the melt side to the crystal side in order for the crystal to grow. If this heat flow is uniform on the growth surface, the shape of the solid-liquid interface is flat, but if it is not uniform, it becomes a convex shape or a concave shape. In semiconductor crystals such as Si and GaAs, the heat flow is determined by the melt temperature, the temperature difference between the crystals, and the thermal conductivity inside the crystals. That is, the heat of the high-temperature melt is transmitted through the inside of the crystal by heat conduction and released outside. However, in single crystal growth such as sapphire single crystal with high transparency, heat is released from the melt by transmitting radiant heat in the crystal in addition to heat conduction in the crystal. In the case of a sapphire single crystal, when comparing the amount of heat released by radiation with the amount of heat released by heat conduction, the former is considered to be overwhelmingly large. When the sapphire single crystal is grown with the shoulder opening degree proposed in the present invention, the upper portion of the single crystal including the shoulder becomes a more cylindrical shape, and as a result, the radiant heat has a higher temperature distribution near the solid-liquid interface. It is considered that the light is radiated from the inside of the crystal to the outside of the crystal in a uniform direction. Thereby, it is considered that the solid-liquid interface shape is closer to a flat shape than in the case of a small degree of shoulder opening.

    The present invention will be further described below using examples.

  Sapphire single crystals having a diameter of about 50 mm and 80 mm were grown using a commercially available apparatus (Daiichi Kiden Co., Ltd.) shown in FIG. The iridium crucible used has an outer diameter of 160 mm, an inner diameter of 156 mm, and a height of 160 mm.

  The iridium crucible was filled with a necessary amount of alumina, and the alumina was melted by high frequency induction heating to obtain a melt. A seed crystal prepared using a sapphire single crystal was brought into contact with this melt, and the seed crystal was pulled up while rotating to obtain a sapphire single crystal. At this time, the opening degree of the shoulder was changed by adjusting the rotation speed, pulling speed and melt temperature of the sapphire single crystal. Table 1 shows the relationship between the degree of shoulder opening, straight body diameter and convexity, and straight body diameter / convexity of the crystals obtained.

  The relationship between the degree of opening of the shoulder and the convexity of Examples 3 to 22 and Comparative Examples 5 to 8 in Table 1 is shown in FIG. The horizontal axis is the degree of shoulder opening, and the vertical axis is the convexity.

From FIG. 2, unlike convex degree evident at 2.29 before and after the shoulder of the opening degree, it is found to be significantly reduced in the 2.29 or more.

  Next, the obtained single crystals were cut without annealing to prepare a substrate, an X-ray Lang method topographic photograph was taken, and the degree of distortion was observed. As a result, it was confirmed that the substrate obtained from the single crystals No. 1 to 22 having a small convexity had less distortion than the substrate obtained from the single crystals No. 23 to 30 having a high convexity.

  The obtained crystal had almost no part containing bubbles, and no particular problem was found in the crystal growth process.

It is sectional drawing of the general purpose type oxide crystal growth apparatus used in the Example. It is the figure which showed the relationship between the opening degree of the shoulder of Examples 3-22 and Comparative Examples 5-8, and convexity. The horizontal axis is the degree of shoulder opening, and the vertical axis is the convexity.

Explanation of symbols

1 --- Refractory crucible 2 --- Refractory crucible support cylinder 3 --- Crucible base 4 --- Insulating material 5 --- Crystal raw material melt 6 --- Single crystal 7 --- Seed crystal 8- ―Work coil 9 ――― Crucible

Claims (2)

When growing a sapphire single crystal by the pulling method, the length of the shoulder from the seed crystal part to the straight body, that is, the surface formed by the intersection of the shoulder part and the straight body part and the center of the seed crystal in the pulling direction The convex shape of the sapphire single crystal solid-liquid interface obtained by controlling the ratio (B / A) of the distance A between the intersection with the drawn straight line and the center of the seed crystal and the average diameter B of the straight body portion A method for growing a sapphire single crystal, wherein the height is controlled and the B / A is controlled to be 2.29 or more. 2. The method for growing a sapphire single crystal according to claim 1, wherein the B / A is controlled to 2.29 to 4.5.

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