JP4817099B2 - Carbide single crystal and manufacturing method thereof - Google Patents

Description

  The present invention relates to a carbide single crystal in which ZrC is dissolved in TiC and a method for producing the same.

In recent years, gallium nitride-based semiconductors have been used as materials for light-emitting diodes that emit blue to ultraviolet light, and have attracted attention as electronic control elements having performance exceeding silicon and gallium arsenide as wide-gap semiconductors.

Currently, gallium nitride-based semiconductors are mainly formed on sapphire substrates. As shown in Table 1, gallium nitride and sapphire (Al ₂ O ₃ ) differ greatly in lattice constant and thermal expansion coefficient.
The formed gallium nitride based semiconductor layer contains many defects (threading dislocations 10 ⁹ / cm ² ). A gallium nitride substrate is used as a substrate for solving this problem, but the gallium nitride substrate is very expensive.

In addition, as a substrate for forming a high-quality gallium nitride semiconductor, zirconium diboride (Z
rB ₂ ) There is a single crystal (Patent Document 1). As shown in Table 1, this substrate has values close to those of gallium nitride in terms of lattice constant and thermal expansion coefficient, so that the dislocation density (<10 ⁷ / cm 3) is almost the same as that of the substrate crystal.
A good quality nitride film having ² ) is grown. Since this ZrB ₂ melts by coincidence, it is possible to grow crystals from the melt, which is an efficient crystal growth method. Since ZrB ₂ has a high melting point of 3220 ° C., a floating zone melting method (floating zone method, FZ method) without using a crucible is used to grow a single crystal.

However, since ZrB ₂ requires a high growth temperature exceeding 3000 ° C., it has not led to the growth of a large single crystal having an inch size. Therefore, like zirconium diboride,
Attention is focused on titanium carbide, which has a lattice constant and a coefficient of thermal expansion close to those of nitride and has a crystal growth temperature lower than that of zirconium diboride.

As shown in Table 1, this material is not so different in lattice constant and thermal expansion coefficient from that of gallium nitride. Further, as shown in the phase diagram of FIG. A point, 2
776 ° C.), it is possible to grow crystals from a temperature 450 ° C. lower than ZrB ₂ (Patent Documents 2 and 3).

In the vicinity of this temperature, the present inventors have so far developed a TiC single crystal manufacturing technique by a floating zone melting method (floating zone method, FZ method) (Patent Documents 2 to 6). In other words, we tried to improve the quality by increasing the purity of raw materials (Patent Document 4) and to improve the quality of crystals by growing crystals by adding tungsten or the like (Patent Documents 5 and 6). A good-quality TiC single crystal that does not contain subgrain boundaries has not been grown (Non-patent Document 1).

Japanese Patent Laid-Open No. 2002-43223 JP 55-62855 A JP 58-60699 A JP-A 63-256598 JP-A-1-286996 JP-A-2-44100 “Growth conditions of high purity TiC single crystal using the floating zone method” S. Otani, T. Tanaka and Y. Ishizawa J. Crystal growth 92 (1988) 8-12. “Preparation of (La1-xCex) B6single crystals by the floating zone method” S. Otani, T. Tanaka and Y. Ishizawa J. Crystal growth 108 (1991) 425-428.

Since the growth temperature of TiC is as high as 2776 ° C, the floating zone melting method without a crucible is optimal. Compared with crystal growth of zirconium diboride (melting point: 3220 ° C.), the growth temperature is about 45
It can be lowered by 0 ° C. In this temperature range, the energy required for heating is determined by thermal radiation proportional to the fourth power of the temperature, so that the heating power can be reduced to 2/3 due to the reduction of the growth temperature, which is a great merit for growth. However, this TiC single crystal has a problem that subgrain boundaries occur. Therefore, in order to use TiC as a substrate for growing a nitride thin film, it is necessary to find a method for growing a high-quality single crystal without subgrain boundaries.

Since the crystal grows under a large temperature gradient in the FZ method, the quality of the crystal is determined by the balance between the large thermal stress applied to the growing crystal and the mechanical strength of the crystal itself. That is, if the mechanical strength of the crystal is large, the thermal stress can be overcome and a high-quality crystal can be obtained. For this reason, high-quality single crystals without subgrain boundaries have been grown in crystals in which a two-dimensional three-dimensional network composed of boron-boron covalent bonds exists, such as boride (Non-patent Document 2, Patent Document). 1).

On the other hand, in the case of carbide, mechanical strength is low at a high temperature, for example, it is reduced to the strength of aluminum metal at room temperature at 1100 ° C. For this reason, subgrain boundaries are likely to occur, and so far, high-quality crystals that do not have subgrain boundaries have not been grown. Therefore, even if it is added as in W to form a solid solution to try to improve the quality, it cannot be expected to have high-temperature strength comparable to that of boride. However, it is a situation that has to rely on trial and error.

In order to solve these problems, the present inventors produced a sintered body in which a carbide of a group 4 metal element or a carbide of a group 5 metal element was added to TiC, and a single crystal was grown by the FZ method. The obtained composition range and crystal quality were investigated. As a result, when 5 mol% of ZrC was dissolved, subgrain boundaries were reduced by about an order of magnitude, and by adding 7.5 to 35 mol%, a high-quality single crystal without subgrain boundaries was grown. Furthermore, it was confirmed that the group III nitride was epitaxially grown on the substrate produced from these crystals.

That is, the present invention is as follows.
(1) 5 mol% to 35 mol% of zirconium carbide (ZrC) is dissolved in titanium carbide (TiC) represented by the formula (Ti _1-x Zr _x ) C (where 0.05 ≦ x ≦ 0.35). Carbide single crystal.
(2) The carbon composition (atomic ratio) of the carbide single crystal described in (1) above is C / (Ti + Zr) = 0.92 to 94.
A carbide single crystal characterized by
(3) A nitride growth substrate comprising the carbide single crystal as described in (1) or (2) above.
(4) The above (1) or (2), wherein a single crystal is grown by a floating zone melting method
A method for producing a carbide single crystal.

Examples of Group 4 metal carbides or Group 5 metal carbides include ZrC, HfC, VC, NbC, and TaC, both of which have a NaCl-type crystal structure with a lattice constant difference of about 6%. The lattice constant matches that of gallium nitride. The present inventor has found that only when ZrC is added to TiC among these substances, a high-quality single crystal without subgrain boundaries can be grown by the FZ method.
When 5 mol% of ZrC is dissolved, subgrain boundaries are reduced by about an order of magnitude, and by adding 7.5 to 35 mol%, a high-quality single crystal without subgrain boundaries is grown.

As described above, according to the present invention, a high-quality carbide solid solution single crystal free of subgrain boundaries can be obtained, and can be used as a substrate for growing a nitride thin film.

The present invention is described in further detail below. An example of an apparatus used in the present invention is shown in FIG. This apparatus is a high frequency induction heating FZ furnace designed to allow crystal growth in an inert gas atmosphere of about 1 to 10 atm.

The lower end of the raw material supply rod 5 is heated by causing Joule heat to generate an induced current in the raw material supply rod 5 by passing a high-frequency current through the work coil 4. In this way, the raw material supply rod 5 is fed into the formed melt zone 6 from above, and the single crystal 7 is grown from below.

Below, the procedure of the single crystal growth by this invention is shown.
First, the raw material carbide powder (TiC) and the Group 4 metal carbide or Group 5 metal carbide powder are mixed well, then a small amount of camphor is added as a binder, and a compacting rod is formed by a rubber press (about 2000 kg / cm ² ). Make it. This powder bar is preferably 1500 to 170 in vacuum or in an inert gas.
Heat to about 0 ° C. to produce a raw material sintered rod. The size of the raw material powder is preferably about 7 microns or less and about 1 micron or more on average particle size for ease of sintering and handling.

The obtained raw material sintered rod 5 is set on the upper shaft 2 via the holder 3, and the lower shaft 2 'is seeded with a seed crystal (
Alternatively, a sintered rod for initial bandage formation) 8 is set through the holder 3 '. Next, after filling with an inert gas of several atmospheres, preferably about 5 atmospheres, the lower end of the raw material sintered bar 5 is melted by heating to form a melt zone 6, and the upper shaft 2 and the lower shaft 2 'are slowly moved. , Preferably 0.5-1.5cm
The single crystal 7 is grown by moving downward at about / h.

At this time, the feed rate of the raw material sintered rod 5 to the melt zone 6 is such that the density of the feed raw material rod is low, so that a single crystal having substantially the same diameter as the raw material feed rod is grown to compensate for it. Set. As an atmosphere, an inert gas such as helium having a pressure of several atmospheres, preferably about 3 to 7 atmospheres is used. This is to prevent electric discharge generated in the portion of the high frequency work coil 4.

In the above, the method for producing a raw material sintered rod by mixing TiC powder and ZrC powder has been described, but other methods, for example, a method of adding Zr and C to TiC, ZrO ₂ and C to TiC Various combinations, such as the method of adding, are possible.

From the obtained crystal rod, a (111) plane was formed by an X-ray back surface Laue method and cut by an electric discharge machine. The cut surface was polished with diamond paste, and the mirror finish was performed using colloidal silica. Etching was performed with hydrofluoric acid (HF: HNO ₃ : H ₂ 0 = 3: 1: 1), and the crystallinity was examined under an optical microscope.

Table 2 shows the composition ranges in which single crystals were grown from the raw material rods of various compositions by the FZ method. HfC,
When NbC and TaC were added, the solid solution was no more than a few mol% and no effect of suppressing subgrain boundaries was observed. VC was the most solid solution, and single crystals were obtained up to 40 mol%. At that time, the addition of 15 mol% or more reduced the sub-boundary to about 1/10 as compared with the case of no addition, but it could not be completely removed.

When ZrC was added, the subgrain boundaries were reduced to 1/10 by addition of 5 mol%, and disappeared by addition of 7.5 mol% or more. Single crystals could not be obtained by addition of 40% or more. The results are shown in FIG. The lattice constant changed in accordance with Vegard's law with TiC and ZrC as end components in the composition range where single crystals were obtained. There was a tendency for cracks to occur in the crystals as the amount of ZrC added increased, and the amount of ZrC added was most preferably 7.5 to 20 mol%.

Furthermore, in order to suppress the generated cracks, crystals were grown from a raw material rod produced by adding 2 to 8 atomic% of titanium metal in the raw material powder. If the amount of metal added to the raw material rod increases during the growth, the evaporation of the metal becomes violent, adheres to the work coil, and causes troubles such as contact with the sample. Therefore, 2 to 6 atomic% was the optimum value as the amount added to the raw material bar. As a carbon composition (atomic ratio) of the obtained crystal, C / (Ti + Zr) = 0.92 to 94 was a preferable composition. The method for reducing the carbon component is not limited to the above method, and for example, a method of adding Ti0 ₂ , adding Zr, adding ZrO ₂ , or the like may be used.

In order to use the obtained single crystal as a nitride semiconductor growth substrate, it is preferably cut out parallel to the (111) plane by electric discharge machining and mirror-polished by chemical mechanical polishing.
Since there is an oxide layer on the polished surface, it is preferable to remove the oxide layer by heat treatment at about 1300 ° C. in a vacuum to obtain an atomically clean surface. Further, a nitride semiconductor film is grown by a normal crystal growth method, for example, a plasma assisted molecular beam epitaxy method (PA-MBE).
That is, a group 3 atom beam is supplied from a Knudsen cell and a nitrogen radical is supplied from a high-frequency radical source onto the substrate surface held at a certain temperature between 500 ° C. and 900 ° C. to react with each other. A nitride thin film such as aluminum is grown.

Next, examples of the present invention will be described.
10% by mole of ZrC powder (average particle size 4.2 microns) and 2% by mole of Ti metal powder (<particle size 44 microns) are mixed with TiC powder (average particle size 1.7 microns), and then a binder. A small amount of camphor was added and packed into a rubber bag with a diameter of 12 mm to form a cylinder. A green compact was obtained by applying a hydrostatic pressure of 2000 kg / cm ² thereto. This green compact was heated in vacuum at 1800 ° C. to obtain a sintered rod having a diameter of 1 cm and a length of 12 cm. The density was about 55%.

This sintered rod was fixed to the upper shaft of the FZ growth furnace shown in FIG. 2 via a holder, and the TiC sintered rod was fixed to the lower shaft. After filling the growth furnace with 6 atmospheres of helium, the lower end of the sintered bar is melted by high frequency induction heating to form an initial melt zone and moved downward at a speed of 1 cm / h for 6 hours. The total length is 6 cm and the diameter is 1 cm. Samples were obtained.

The analysis results at that time are shown in Table 3. From the comparison of the composition of the raw material rod and the crystal, the Ti during evaporation during growth
However, it can be seen that crystals having the same composition as the raw material rod can be obtained. In addition, it can be seen that the addition of ZrC lowers the crystal growth temperature because the Zr component increases from the band composition. That is, it can be seen that TiC and ZrC are in a eutectic relationship.

The (111) plane was cut out from the obtained crystal with an electric discharge machine and polished to a mirror surface. The sample was etched with hydrofluoric acid (HF: HNO3: H20 = 3: 1: 1), and it was confirmed that there were no subgrain boundaries.

After polishing the (111) surface of a solid solution single crystal in which 20 mol% of ZrC was dissolved in TiC made in the same way as in Example 1, it was cleaned by heating at 1300 ° C or higher in vacuum to grow a nitride thin film A substrate was used. The substrate was kept at 600 ° C., and Ga molecular beam and N radical were supplied to grow a GaN crystal for 270 minutes. The post-growth RHEED image showed a surface superstructure peculiar to 3 × 3 Ga-rich GaN (000-1).

Only Ga and N signals appeared in the AES spectrum of this surface, and it was found that GaN single crystals were grown epitaxially. The orientation relation of epitaxy is (0001) GaN // (111) Zr
TiC, <10-10> GaN // <11-2> ZrTiC. From the AFM image of the grown film, it was found that it was growing as a uniformly continuous film. The film thickness was 280 nm.

After polishing the (111) surface of a solid solution single crystal in which 20 mol% of ZrC was dissolved in TiC made in the same way as in Example 1, it was cleaned by heating at 1300 ° C or higher in vacuum to grow a nitride thin film A substrate was used. The substrate was kept at 810 ° C. and Al molecular beam and N radical were supplied to grow AlN crystal for 270 minutes. The grown RHEED image showed a 1x1 pattern, indicating that the AlN film was grown in the same orientation as the underlying layer.

In the AES spectrum of this surface, only Al and N signals appeared, and it was confirmed that AlN single crystal was grown epitaxially. The orientation relation of epitaxy is (0001) AlN // (111) Zr
TiC, <10-10> AlN // <11-2> ZrTiC. Looking at the AFM image of the grown film, a uniformly continuous film was grown. The film thickness was 230 nm.

Ti-C phase diagram (A point is TiC-C eutectic point). The schematic diagram of the single crystal growth apparatus used for this invention. The graph which shows the subgrain boundary density in a (Ti, Zr) C single crystal.

Explanation of symbols

1 Upper shaft drive
1 'Lower shaft drive part 2 Upper shaft
2 'Lower shaft 3 holder
3 'holder 4 work coil
5 Raw material sintered bar 6 Fusion zone 7 Single crystal 8 seed crystal or sintering rod for initial zone formation

Claims (4)

A _single carbide comprising titanium carbide (TiC) represented by the formula (Ti _1-x Zr _x ) C (where 0.05 ≦ x ≦ 0.35) is dissolved in 5 mol% to 35 mol% of zirconium carbide (ZrC). crystal. A carbide single crystal having a carbon composition (atomic ratio) of the carbide single crystal according to claim 1 of C / (Ti + Zr) = 0.92 to 94. A nitride growth substrate comprising the carbide single crystal according to claim 1. The method for producing a carbide single crystal according to claim 1 or 2, wherein the single crystal is grown by a floating zone melting method.

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