JP4518782B2 - Method for producing diboride single crystal - Google Patents

Description

  The present invention relates to a method for producing a diboride single crystal used for a substrate for epitaxial growth of a single crystal containing GaN (GaN-based single crystal).

  Conventionally, various substrate materials have been used as substrates for epitaxial growth of GaN-based semiconductors. The material of the substrate for epitaxial growth which has been generally used so far is sapphire. The reason for this is that sapphire is expensive but easily available, transparent and advantageous for light emission when applied to light emitting diodes (LEDs), and is resistant to heat and chemicals under device process conditions. It is because it is excellent in property.

  However, sapphire has a lattice mismatch of about 14% with a GaN single crystal and is not sufficient for use in highly efficient LEDs and semiconductor laser diodes (LDs).

Therefore, in order to grow a high-quality GaN single crystal epitaxial film, a new material such as SiC having high lattice matching is being developed. Among them, recently, diboride materials such as ZrB ₂ have attracted attention because they are extremely excellent in lattice matching and excellent in thermal conductivity (see, for example, Patent Document 1).
JP 2002-348200 A

  Since diboride-based materials have a very high melting point exceeding 2500 ° C. in any composition, the most popular Czochralski (CZ) method using a crucible cannot be applied to the crystal growth due to crucible restrictions. Therefore, the floating zone (FZ) method or the Bernoulli method without using a crucible can be considered for the crystal growth, and the FZ method by the high frequency heating method is used at the research level.

  However, this growing method generally has difficulty in increasing the size of crystals. For example, even in the case of a Si single crystal in which the FZ method is most established, the diameter is increased to 12 inches in the CZ method, whereas it is only 6 inches in the FZ method. Furthermore, in the case of an oxide other than Si single crystal or a non-oxide single crystal, if the crystal diameter exceeds 2 inches, the thermal conductivity is generally small. The heating temperature far exceeds the melting point, making it difficult to maintain the melt zone and to maintain the composition components, and further increase the size. As described above, the growth of the diboride single crystal by the conventional FZ method has a limit in increasing the size.

  An object of the present invention is to provide a technology for growing a large crystal of a diboride single crystal. Another object of the present invention is to provide a diboride large single crystal substrate for growing a GaN single crystal epitaxial film.

In order to achieve the above object, the method for producing a diboride single crystal according to the present invention has the chemical formula XB ₂ (where X is Zr, Hf, Ti, W, Mo, Cr) provided in a liquid-cooled crucible. a raw material consisting of comprising) at least one, Zr contained in the raw material, Hf, Ti, W, Mo , and the raw material preparation step of addition of either single metal Cr, the raw material and the single metal high frequency Forming a melt by heating to form a melt; forming a solidified portion where the melt is solidified in a portion in contact with the liquid-cooled crucible; and cooling the melt surrounded by the solidified portion. And a single crystal growing step for single crystallization by the method.

In addition, a single crystal having a diameter of 2 inches or more is grown from the melt.

In here, the raw material is particularly preferred formula XB ₂ thin film growth of GaN-based single crystal (where, X is comprises Zr, Hf, Ti, W, Mo, one or more elements selected from among Cr Since the raw material contains a single metal, it is directly heated and melted by high-frequency induction heating, and the melted melt is held in a liquid-cooled crucible with good thermal conductivity. The melted portion in contact with the liquid-cooled crucible remains unmelted, and the solidified portion itself functions as a double crucible, and at the same time, suppresses the temperature rise of the liquid-cooled crucible. Therefore, even if the grown crystal is an ultra-high melting point crystal of 2500 ° C. or higher, crystal growth similar to the conventional crystal growth in the crucible becomes possible, and a large crystal can be grown.

  The method for producing a diboride single crystal of the present invention is characterized in that, in particular, a raw material provided in a liquid-cooled crucible is melted by high-frequency heating to form a melt, and the melt is cooled to form a single crystal. To do. Such forced cooling of the crucible makes it possible to keep the surface temperature of the melt zone low and to prevent the surface temperature from becoming too high, so that a very stable diboride-type large single crystal in the crystal growth process can be obtained. Training is possible. In addition, the Bridgman method, the pulling method, the TSSG method, and the like can be applied as necessary, and a highly versatile large crystal growth technique can be provided.

  In addition, a seed crystal composed of a diboride single crystal is disposed at the bottom of a liquid-cooled crucible, and a raw material is disposed on the seed crystal, and the raw material is melted by high-frequency heating to form a melt. The melt is cooled by forced cooling to grow a single crystal on the seed crystal, so that it is possible to provide a growth technique for the above-mentioned diboride-type large single crystal as well as extremely excellent crystallinity. This makes it possible to grow a high-quality, large-diameter GaN-based single crystal epitaxial growth substrate by reducing the warpage of the substrate and the distortion of the growth film during the growth of the GaN-based single crystal thin film. .

Further, in the crucible, so characterized that it contains the elemental metal as the raw material, in particular for growing a suitable single crystal for epitaxial growth of GaN-based single crystal, for example, raw material chemical formula XB 2 _(where X is preferably made of a material of at least one of Zr, Hf, Ti, W, Mo, and Cr. Furthermore, the same kind of simple metal effective for high frequency induction is included. As a result, the diboride single crystal made of the material of the chemical formula XB ₂ has a lattice constant very close to that of the GaN single crystal, so the lattice mismatch is significantly smaller than that of conventional sapphire, etc. When the single crystal thin film is grown, the warp of the substrate and the distortion of the growth film are reduced, and a substrate for epitaxial growth of a GaN-based single crystal with higher quality and larger diameter can be provided. Furthermore, by including a single metal, the induction heating concentrates on this single metal first, so that the raw materials around the metal are melted, and then the high frequency is induced in the melt and the whole melts. It is possible to provide a method for producing a diboride single crystal that is improved and has excellent productivity.

  Below, the manufacturing method of the diboride single crystal of this invention is demonstrated. A schematic diagram of a cross-sectional structure of a growth furnace according to the present invention is shown in FIG.

  The structure of the growth furnace is a high-frequency induction heating method, and a quartz tube 200 is installed inside the induction coil 100, and a liquid-cooled crucible 300 such as a water-cooled crucible with high cooling effect capable of forced cooling is provided inside the quartz tube 200. Is arranged.

  Here, in order to further enhance the cooling effect, the liquid-cooled crucible 300 uses a metal material having good thermal conductivity and high workability, such as copper. The liquid-cooled crucible 300 is disposed on the heat-resistant insulating support base 600. The heat-resistant insulating support base 600 can be appropriately selected from ceramic materials such as magnesia and zirconia. When the raw material is melted by heating and the melt 400 is formed, the portion in contact with the liquid-cooled crucible 300 remains without being melted, and the solidified portion 500 is eventually formed. At this stage, after the melt 400 is held for a while, the melt 400 is gradually cooled at a constant cooling rate to perform crystallization. If the slow cooling rate is too high, bubbles may be mixed.

The raw material is made of a material of the chemical formula XB ₂ (where X includes at least one of Zr, Hf, Ti, W, Mo, and Cr), and may contain a single metal of the same type as the grown crystal. This is because the inclusion of a single metal causes the induction heating to first concentrate on this single metal, so that the raw material around the metal melts, and then the high frequency is induced in the melt and the whole melts. It is for improving.

Diboride was cited as the substrate crystal material of the present invention, all of which are highly stable materials having a high melting point, high strength, and high hardness, and a thermal conductivity compared with sapphire. This is because many of them have high electrical conductivity and good electrical conductivity. For this reason, it is possible to use the substrate as an electrode, and there is an advantage that the manufacturing process is simplified when a light-emitting element or a power control element that flows electricity perpendicularly to the semiconductor film is manufactured. In particular, excellent characteristics can be expected as a substrate for a high-power element having excellent heat dissipation. In addition, the material of the above formula XB ₂ was selected from a material having a relatively small lattice mismatch with GaN, specifically, about 1/2 or less of the lattice mismatch of sapphire.

Incidentally, ZrB ₂ shows a good agreement with GaN with a difference of 0.6% in lattice constant and 5% in coefficient of thermal expansion. Thereby, high quality epitaxial film growth with a low defect density is possible.

  As described above, according to the manufacturing method of the present invention, the diboride single crystal having high melting point, high strength, and high hardness can be grown in a very stable state, and the size can be increased.

Further, in order to particularly made suitable epitaxial growth of GaN-based single crystal material of the formula XB ₂ (where, X is comprises Zr, Hf, Ti, W, Mo, one or more elements selected from Cr) A single crystal made of Such a single crystal of the chemical formula XB ₂ has a lattice constant that is very close to that of a GaN-based single crystal, so that the lattice mismatch is significantly smaller than that of conventional sapphire and the like. This reduces the warpage of the substrate and the distortion of the growth film during the growth of the GaN-based single crystal thin film, and can provide a higher quality and superior epi single crystal growth substrate.

  As described above, an unprecedented crystal substrate for epitaxial growth of a GaN-based single crystal excellent in high quality and large diameter crystallinity can be provided at low cost. That is, in the method for producing a diboride single crystal of the present invention, a raw material provided in a liquid-cooled crucible is melted by high-frequency heating to form a melt, and the melt is cooled to obtain a single crystal by forced cooling of the crucible. Since it crystallizes, forced cooling of the liquid-cooled crucible makes it possible to grow a diboride-type large single crystal that is very stable in the crystal growth process. In addition, the Bridgman method, the pulling method, the TSSG method, and the like can be applied as necessary, and a large crystal growth technique with high versatility can be provided.

  In addition, a seed crystal composed of a diboride single crystal is disposed at the bottom of a liquid-cooled crucible, and a raw material is disposed on the seed crystal, and the raw material is melted by high-frequency heating to form a melt. By cooling this melt and forcing a single crystal on the seed crystal, a single crystal having particularly excellent crystallinity can be grown. In addition, the warpage of the substrate and the distortion of the growth film are reduced, and a higher quality and larger diameter GaN-based single crystal epitaxial growth substrate can be provided.

Furthermore, by including a metal simple substance as a raw material in a liquid-cooled crucible, for example, the raw material is a chemical formula XB ₂ (where X includes at least one of Zr, Hf, Ti, W, Mo, Cr). The same kind of metal that is effective for high-frequency induction is included. When growing a diboride single crystal made of the material of Formula XB _2, because the lattice constant is the same very close GaN-based single crystal, lattice mismatch is significantly smaller than the conventional sapphire. Further, when a GaN-based single crystal thin film is grown, the warp of the substrate and the distortion of the grown film are reduced, and a substrate for epitaxial growth of a GaN-based single crystal having a higher quality and a larger diameter can be provided. Furthermore, because the raw material contains a simple metal, the induction heating first concentrates on this simple metal, so the raw material around the metal melts, and then the high frequency is induced in the melt, so that the whole melts. Thus, it is possible to provide a method for producing a diboride single crystal with improved productivity.

Next, an embodiment in which the present invention is embodied will be described using ZrB ₂ as a representative example.

A raw material powder obtained by adding a very small amount of Zr to commercial ZrB ₂ powder (purity of about 98%, average particle size of about 2 μm) was pressed with a rubber press at a pressure of 9.8 × 10 ⁷ Pa, and then vacuumed at 1700 ° C. , And heated for 30 minutes to produce a sintered body having a diameter of 90 mm and a height of 90 mm. The density of this sintered body was 58% of the theoretical value.

This sintered body was filled in a copper-cooled crucible made of copper having a diameter of 100 mm and a height of 100 mm, and crystals were grown by a slow cooling method in an argon (Ar) atmosphere (0.8 MPa). The slow cooling rate was 50 ° C./hr. The resulting crystals are covered with sintered bodies of ZrB _2, size had exhibited approximately diameter 60 mm × height 50mm massive.

The orientation of the grown crystal was measured with X-rays and cut into a thickness of about 1 mm in parallel with the C-plane with a wire saw. Then, lapping is performed using GC abrasive grains # 600, lapping is performed using a diamond grinding wheel, and finally polishing is performed using colloidal silica, and the thickness is such that the (0001) plane of ZrB ₂ is the main surface. A 500 μm 2-inch wafer was prepared.

  When this wafer was crystallized by the X-ray topography method and the X-ray rocking curve, crystal grain boundaries such as low-angle grain boundaries were hardly seen, and the FWHM value showed an almost homogeneous distribution at 80 arcsec.

  For comparison, when a sintered rod having a diameter of 10 mm and a length of 140 mm was prepared under the same conditions and grown using the high frequency FZ method, the growth was possible, but there was a grain boundary in the entire crystal, and the value of FWHM The good part was about 380 arcsec and the variation was large.

Purity of about 99.0% as a powder raw material, the average particle commercial ZrB ₂ using a diameter 1.5 [mu] m, the press pressure 2.0 × 10 ⁸ Pa, 2000 ℃ sintering temperature in vacuum, diameter 100mm height 100mm and heated for 30 minutes A sintered body was prepared. The density of this sintered body was 80% of the theoretical value.

The sintered body was filled in a copper crucible having an inner diameter of 120 mm and a height of 120 mm, and crystal growth was carried out under the same conditions as in Example 1. The obtained crystals were covered with a sintered body of ZrB _{2 in} the same manner as in Example 1, and the size was a lump of about 80 mm diameter x 70 mm height.

The grown crystal was subjected to wafer processing similar to that in Example 1 to produce a 3-inch wafer having a thickness of 350 μm with the (0001) plane of ZrB ₂ as the main surface.

  Further, when the same crystal evaluation as in Example 1 was performed on the wafer, no misorientation such as a crystal grain boundary was observed on the wafer, and the FWHM value was 60 arcsec, indicating further improvement in crystallinity.

  For comparison, when a sintered rod having a diameter of 30 mm and a length of 140 mm was produced under the same conditions and grown by the high-frequency FZ method, it could not be grown with an output of 40 kw.

  For this reason, growth was possible when the output was increased to 120 kw, but the growth was possible. However, the grown crystal had a large compositional deviation due to the evaporation of the B element due to an increase in the melt surface temperature. The number of grain boundaries and the minimum value of FWHM were 980 arcsec.

Using a commercial ZrB ₂ having a purity of 99.5% and an average particle size of about 1.0 μm as a powder raw material, press pressure 2.0 × 10 ⁸ Pa, heating temperature in vacuum at 1900 ° C. for 30 minutes, sintering of the same size as in Example 2 The body and the bottom of the crucible were prepared with a (0001) ZrB ₂ seed crystal having a size of 2 inches in diameter and a thickness of 1 mm. The growth conditions were a growth atmosphere of helium (He) gas (0.9 MPa), and a cooling rate of 25 The crystal growth was carried out under the same conditions as in Example 2 except for the temperature of ° C./hr.

  The obtained crystal was covered with a ZrB2 sintered body in the same manner as in Example 2, but it showed a facet growth type with the (0001) plane as the main surface, and the size was approximately 85 mm in diameter × 75 mm in height.

  Three wafers similar to those of Example 2 were produced from the grown crystals, and crystal evaluation was performed in the same manner. As a result, the crystal grain boundary was almost free over the entire crystal, and the FWHM value was 30 arcsec, indicating a very good crystallinity.

  For comparison, a sintered rod having a diameter of 2 inches was produced under the same conditions as in Example 3 and crystal growth was performed by the high-frequency FZ method. However, it could not be grown even with a power supply with an output of 120 kW.

In these examples, ZrB ₂ was described as an example, but other borides such as HfB _{2 having} a melting point of 3380 ° C. and a lattice mismatch of 1,6%, a melting point of 2790 ° C. and a lattice mismatch of 5.2% It became clear that the same crystal manufacturing method and wafer process as ZrB ₂ can be applied to TiB ₂ and the like.

It is sectional drawing which shows typically the single crystal growth furnace which concerns on this invention.

Explanation of symbols

100: induction coil
200: Quartz tube
300: Liquid-cooled crucible
400: Melt
500: Solidified part
600: Heat-resistant insulation support base

Claims (2)

In the raw material consisting of chemical formula XB ₂ (wherein X includes at least one of Zr, Hf, Ti, W, Mo, Cr) provided in the liquid-cooled crucible, Zr, Hf, Ti, A raw material preparation step of adding a single metal of W, Mo, or Cr ;
A melt forming step of melting the raw material and the metal simple substance by high-frequency heating to form a melt;
Forming a solidified portion where the melt is solidified in a portion in contact with the liquid-cooled crucible, and a single crystal growing step of cooling the melt surrounded by the solidified portion to form a single crystal. A method for producing a compound single crystal. The method for producing a diboride single crystal according to claim 1, wherein a single crystal having a diameter of 2 inches or more is grown from the melt.

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