JP2003252697A - Boride single crystal, method for producing the same, and semiconductor-layer-forming substrate utilizing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a boride single crystal containing ZrB<SB>2</SB>or TiB<SB>2</SB>, a method for producing the same, and a semiconductor-layer-growing substrate utilizing the same. <P>SOLUTION: The single crystal is produced by growing a starting rod based on XB<SB>2</SB>(wherein X is at least either Ti or Zr) and containing 0.05-3 wt.% at least either MoB<SB>2</SB>or WB<SB>2</SB>into a crystal rod by the floating zone process. The ZrB<SB>2</SB>or TiB<SB>2</SB>single crystal containing 0.05-3 wt.% at least either MoB<SB>2</SB>or WB<SB>2</SB>is freed from a subordinate grain boundary, so that it can be used as a substrate having the principal face for growing a semiconductor layer. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to ZrB ₂ or Ti.
The present invention relates to a boride single crystal containing B ₂ , a method for manufacturing the same, and a semiconductor layer growth substrate using the single crystal.

[0002]

2. Description of the Related Art In recent years, gallium nitride based semiconductors have been put into practical use for light emitting diodes and the like. The gallium nitride-based semiconductor means aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), and a mixed crystal of In _x Ga _y Al _z N (where 0 ≦ x, 0 ≦.
y, x + y ≦ 1, z = 1-x-y).
Conventionally, such a gallium nitride based semiconductor has been produced by epitaxial growth on a sapphire substrate.

Crystals of the compounds TiB ₂ and ZrB ₂ are known, and Japanese Patent Laid-Open No. 10-95699 describes a float zone melt crystallization method (floating zone method, hereinafter simply referred to as F
Z method) is used to make a single crystal of zirconium diboride as a high-brightness electron emitting material. Crystals of TiB ₂ and ZrB ₂ have a melting point of 30.
Since it is a high temperature of 00 ° C or higher, crystallization requires
Besides the floating zone method, the flux method is known.

[0004]

Sapphire as the above substrate material has a large lattice mismatch with the gallium nitride based semiconductor, and crystal defects due to the lattice mismatch are introduced into the epitaxial growth layer, so that the crystalline An excellent gallium nitride based semiconductor layer cannot be obtained. Further, since the sapphire substrate is an insulator, in a semiconductor device structure such as a light emitting diode, electrodes cannot be taken out from the sapphire substrate surface side, and the positive electrode / negative electrode of the gallium nitride based semiconductor is formed only on the surface on which the gallium nitride based semiconductor is formed. It was necessary to form both poles. For this reason, the manufacturing process of the light emitting diode or the like becomes complicated, and the light emitting area becomes smaller than the substrate area or the element area.

Table 1 shows TiB ₂ and ZrB ₂ crystals,
The physical properties of GaN and AlN are summarized, but TiB ₂
And ZrB ₂ crystal are hexagonal, and the above GaN and Al
Since it has substantially the same lattice constant and thermal expansion coefficient as a nitride semiconductor such as N, it can be expected to be used as a substrate crystal for epitaxially growing the gallium nitride-based semiconductor layer, and the crystal matching between the substrate and the semiconductor layer can be expected. Therefore, an epitaxial growth layer with few crystal defects is formed,
A gallium nitride-based semiconductor layer having excellent crystallinity can be obtained. Further, as shown in Table 1, since the ZrB ₂ crystal has a relatively high thermal conductivity, it is a substrate having excellent heat dissipation.
Since both the iB ₂ and ZrB ₂ crystals are electrically good conductors, the crystal substrate can be used as a conductor for electrodes.

[0006]

[Table 1]

In order to utilize these diboride crystals as a substrate crystal of a gallium nitride-based semiconductor, the crystal should be as large as possible and a grain boundary and a sub-grain boundary are not substantially present on the main surface of the substrate. Crystals are needed. However, FZ
When these single crystals of diboride are grown using the method, a large number of crystal bars with a diameter of 10 mm or more generate many crystal grain boundaries and sub-grain boundaries in the crystal, making it difficult to grow high quality single crystals. Met. If sub-grain boundaries exist in the substrate crystal, the density of lattice defects such as dislocations also increases in the semiconductor layer epitaxially grown thereon, and a semiconductor layer with excellent characteristics cannot be obtained.

The present invention has been made in view of the above-mentioned problems of grain boundaries and sub-grain boundaries of diboride single crystals, and it uses the FZ method to produce large TiB ₂ and ZrB ₂ single grains without sub-grain boundaries. It is intended to provide a crystal and a manufacturing method thereof. The present invention is intended to provide a substrate for forming a semiconductor layer using such a single crystal with extremely few lattice defects induced by the substrate.

[0009]

The method for producing a boride single crystal according to the present invention comprises an XB ₂ compound (X contains at least one of Ti and Zr) by a floating zone method.
At the time of growing the crystals of, at least 1 boride of molybdenum or tungsten is used as a grain boundary suppressor on the raw material rod.
This is a method of adding a seed to suppress the generation of subgrain boundaries in a large crystal during the growth process to obtain a single crystal. In particular, even in a large crystal rod grown to have a diameter of about 10 mm or more, a single crystal can be obtained in which grain boundaries and sub-grain boundaries do not substantially exist.

In the present invention, the raw material bar contains the molybdenum or tungsten boride as the grain boundary suppressor in an optimum addition amount of 0.05 to 3% by weight. If it is less than 0.05% by weight, the effect of suppressing the generation of sub-grain boundaries in the crystal is small and it is not practical, but the effect of suppressing the generation of sub-grain boundaries becomes large as the compounding amount is increased. Become. If the grain boundary suppressor is blended in an amount of 3% by weight or more in the raw material rod, a layered grain boundary is generated in the crystal along the crystal growth direction, and the crystal is polycrystallized, which is not preferable.

The XB ₂ single crystal of the present invention is XB ₂ (X is T
i) is a single crystal containing at least one of MoB _{2 and} WB _{2 in} an amount of 0.05 to 3% by weight. The present invention includes a substrate for growing a semiconductor layer formed from such a single crystal, and in particular, since a single crystal substrate has no sub-grain boundaries even when cut out from a crystal rod having a diameter of more than 10 mm and formed. It can be used as a growth substrate for growing a gallium nitride based semiconductor layer.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the production method of the present invention, the main component XB ₂ (X contains at least one of Ti and Zr) and 0.05 to 3% by weight of MoB ₂ or WB.
A raw material rod containing at least one of ₂ is prepared, and the raw material rod is locally heated and melted by the floating zone method while gradually solidifying to crystallize into a crystal rod.

The raw material rod is usually formed into a rod shape from a mixture of the above-mentioned XB ₂ compound powder and MoB ₂ powder or WB ₂ powder. Preferably, the raw material rod is a compression-molded green compact, or a sintered rod obtained by further sintering the green compact. The reason for sintering is to impart appropriate mechanical strength and electrical conductivity necessary for melting the floating zone to the raw material rod.

In the process of growing a crystal by the floating zone method, the raw material rod is locally heated from the starting end portion at one end to the terminating end portion at the other end, forming a melting zone locally, When the rod is moved in the longitudinal direction at a constant speed, it is crystallized at the solidification surface of the melting zone and the crystal region grows.

Since the XB ₂ compound is electrically conductive, the raw material rod can be locally energized and heated. In the floating zone method, a high frequency coil is used.
A method of induction heating can be used. Other heating methods include
A laser local irradiation method or heating by plasma irradiation can be used.

In the manufacturing method of the present invention, the floating zone method is used during crystal growth.
The high frequency induction heating floating zone apparatus for growing a crystal | crystallization in a high pressure inert gas atmosphere of several atmospheres is shown. This induction heating floating zone method utilizes the fact that the above-mentioned Ti and Zr diborides have considerable conductivity. The raw material rod is set upright and the high frequency coil is extrapolated to the raw material rod. The induction band is locally melted only in the vicinity of the high-frequency coil to create a narrow zone, and the raw material rod is moved relative to the coil to move the zone, thereby crystallizing behind the zone. Form a phase. In this method, since the end portion side of the raw material rod may be fixed, the melt zone does not come into contact with the refractory material, etc. Therefore, the pollution of the melt zone can be prevented,
Moreover, since there is no cooling by contact, there is an advantage that it can be heated to an extremely high temperature to form a high temperature melt zone. In the initial operation of crystallization, the lower end of the raw material rod 8 is brought close to a seed crystal or the like to form a melting zone as a melting start end portion, and the melting zone is moved relatively upward with respect to the raw material rod so that the melting zone The single crystal 6 is grown below.

The raw material diboride powder is TiB ₂ or ZrB ₂ or a mixture thereof, and molybdenum diboride (MoB ₂ ) powder or tungsten diboride (WB ₂ ) powder or these diboride powders are used. The boride is well mixed into a raw material mixed powder, and a small amount of camphor as a binder is added thereto, and compression molding is carried out to prepare a dust bar. Molding of dust bar is
Preferably, it is hydrostatically pressurized using a rubber press or the like. In order to sinter this green powder rod, it is heated to about 1100 to 1600 ° C. in vacuum or in an inert gas to form a sintered rod.

The prepared raw material rod 8 is mounted in an induction heating floating zone device. As shown in the outline of the floating zone device in FIG. 1, the upper end of the raw material rod 8 is fixed to the upper shaft 2 via a holder 3. Then, the seed crystal 5 (this may be another sintered rod for forming the initial melt zone) is set on the lower shaft 10 via the holder 11 so as to be close to the starting end portion of the raw material rod 8. Next, the starting end portion of the raw material rod 8 is energized and heated in the high frequency coil to locally melt to form a fusion zone 7, and the upper shaft 2 and the lower shaft 10 are connected to the high frequency coil.
The single crystal 6 is gradually moved downward and the single crystal 6 is grown into a crystal rod with the movement of the melt zone. At this time, the feed rate of the raw material rod 8 to the melting zone 7 is determined by the density of the feed raw material rod (the relative density with respect to the crystal phase is usually about 55%) and the diameter of the crystal to be grown with respect to the diameter of the raw material rod. Considering the size of
Is set.

The atmosphere during crystal growth has a high pressure (3 to 1).
0 atm) of an inert gas such as argon or helium is used. This is to prevent discharge that occurs in the high-frequency coil 4 and to suppress evaporation from the melt zone 7.

FIG. 2 shows the appearance of a crystal rod containing the crystal 6 thus formed. By mixing and diluting the above-mentioned Mo or W diboride, a single crystal 60 substantially free of subgrain boundaries can be formed in the crystal rod. In the initial stage of crystallization, the seed crystal or the crystal first grown from another sintered body is a polycrystal in which grain boundaries are always generated.
As shown in the polished surface of the longitudinal section in (A), the grain boundary 12 is discharged out of the crystal when the melt zone 7 moves about 10 mm or more, and a single crystal 60 is obtained by the movement of the melt zone thereafter. Subgrain boundaries do not occur in the crystal. In the initial growth process, addition of molybdenum or tungsten tends to delay the discharge of grain boundaries generated in the initial stage of crystal growth, as compared with the case where crystals are grown without addition. In FIG. 3 (A, B), the length L12 of the polycrystal region at the starting end of the crystal is compared. When Mo or W diboride is added, L1
2 becomes longer than the case of no addition. Therefore, in order to obtain a good quality single crystal without subgrain boundaries, a longer zone transfer distance is required as compared with the case of no addition. On the other hand, it is effective not to add the diboride of Mo and W to the starting end portion of about 0.5 to 3 cm from the tip of the raw material rod. When the starting rod is melted in the floating zone using a starting rod prepared so as not to contain Mo and W diboride at the starting end, the length L12 of the polycrystal region of the starting end 61 at which melting starts in contact with the seed crystal is It can be made smaller, and the crystals that follow it can be reduced by the addition of Mo or W diboride.
Since a single crystal having no subgrain boundaries can be obtained, the production yield of the single crystal can be increased.

Mo and W to Ti or Zr diboride
With respect to the addition of the diboride, the larger the amount added, the more strain remains in the crystal rod, and the tensile residual stress easily causes cracks. In particular, cracks may occur during the process of cutting or polishing a single crystal from a grown crystal rod.
In order to prevent this, annealing that removes strain by heating after crystal growth is effective. This annealing is 1
It is effective to carry out at 600 to 2000 ° C. for about 3 to 10 hours.

The Ti or Zr diboride single crystal obtained by this method can be used as a substrate for semiconductor growth. In such a growth substrate, a single crystal portion of a crystal rod is cut so that a specific crystal plane becomes the main surface of the substrate, and the cut surface is polished to be used as the main surface. . For example, in the case of a substrate used for growing a nitride semiconductor such as GaN, a plate surface is cut out parallel to the (0001) plane from the grown single crystal of diboride,
Be on the board.

When the diboride of Ti or Zr is polycrystalline, grain boundaries and subgrain boundaries can be visually observed on the polished surface when chemical polishing is performed using an abrasive of alumina or silica.
Such a grain boundary is not recognized in the single crystal cut out from the crystal rod manufactured by the method of the above embodiment. Above Ti
Diborides such as can be confirmed by X-ray topography or acid etching of the polished surface if there are grain boundaries and subgrain boundaries, but they can be easily visually identified when the main surface is chemically polished. I can.

[0024]

Example 1 Commercially available ZrB ₂ compound powder was mixed with commercially available Mo ₂ B ₅ compound powder (in terms of MoB ₂ ) of 0% by weight, 0.1% by weight, 0.9% by weight, 2.5 wt% and 4.2 wt% were added at 5 levels to form a mixed powder, which was molded into a green compact and then sintered at 1600 ° C. to prepare a raw material rod having a diameter of 13 mm. Using this raw material rod, a crystal rod having a diameter of 13 mm and a length of the crystal 6 of about 50 mm was manufactured using the high frequency induction heating FZ apparatus. The cross-section macrostructure of the crystal end portion 62 was observed from the crystal rod, and an analysis sample was taken from the raw material rod 8, the rapidly solidified portion of the melt zone 7, and the end portion 62 of the crystal 6 for composition analysis. . The test results are shown in Table 2.

[0025]

[Table 2]

From the above results, it can be predicted that the composition change due to the evaporation of MoB _{2 in} the melt zone is negligibly small. From this data, in the distribution ratio of approximately 0.3 MoB ₂ for ZrB _2, it can be seen that quite small. Here, the distribution ratio is the ratio MoB ₂ content in the molten zone of MoB ₂ content of ZrB ₂ crystal. If this ratio is small, the amount of solute impurities remains in the melt zone to some extent, and it is expected that the solidus temperature will decrease due to the melting point drop. Thus, although the addition of molybdenum lowers the growth temperature, the effect of reducing the heating power during the growth by adding molybdenum was further recognized.

Similarly, a crystal growth test was conducted by adding WB ₂ to ZrB ₂ . The test results are shown in Table 3. W
The addition of B ₂ gave similar results to MoB ₂ .

[0028]

[Table 3]

As can be seen from these tables, long ZrB
₂ to the production of single crystals, it is understood that it suffices contained in a content of at least one of MoB ₂ or WB ₂ 0.1 to 3 wt%. From the above results, MoB ₂ and WB ₂ are
They can be replaced with each other, and they can be mixed and added within the above-mentioned blending range.

MoB ₂ and WB mixed in XB _2
2 and also in high XB ₂ melting point of the, for the evaporation from the XB ₂ is very small, most of the added quantity has remained in the single crystal grown, MoB ₂ in the obtained crystal or WB ₂ The content was in the range of 0.1 to 3% by weight.

The sub-grain boundaries formed during the single crystal growth process are those in which dislocations move and are arranged in parallel or form a network structure due to the thermal stress received immediately after the growth.
Impurity Mo which has a different atomic radius from Ti or Zr in ₂ crystal
It is considered that or W suppresses the movement of dislocation and prevents the formation of sub-grain boundaries. Further, it is considered that one of the causes of the prevention of sub-grain boundaries is that the temperature gradient between the melting zone and the crystal growth region is relaxed due to the melting point decrease due to the addition of Mo or W.

The manufacturing method of this embodiment can be applied to heating methods other than high frequency induction heating, for example, FZ method by laser heating. Also, MoB ₂ or WB ₂
The method of improving the crystallinity by adding a can be used not only for crystal growth by the FZ method, but also for manufacturing a single crystal by another melting method.

[Example 2] A substrate was cut out from a ZrB ₂ single crystal ingot having a diameter of 13 mm of sample No. 4 manufactured in Example 1. The crystal orientation of the single crystal ingot was determined by the X-ray diffraction method, and a plate having a (0001) plane as a main surface and a thickness of 0.6 mm was cut into a plate shape using a band saw. Since the obtained plate has an irregular outer shape, the outer peripheral portion was ground to a 12.7 mm (0.5 inch) square, and the thickness direction was ground using a diamond grindstone to a thickness of 0. It was a 4 mm crystal plate. One side of this plate was chemically polished with colloidal silica and washed.

In the conventional FZ method, the diameter is 1
For crystals with a diameter of more than 0 mm, grain boundaries are visually observed after chemical polishing, and (0
When the (001) plane was measured, a plurality of diffraction peaks were observed, and there was a functional problem as a substrate.

However, after polishing the crystal plate of this example, the main surface was observed, but no grain boundaries and subgrain boundaries were observed, and the single crystal X-ray diffraction peak was also single. Furthermore, it was found that grain boundaries did not appear even when the polished substrate was chemically etched with hydrochloric acid / nitric acid, and that it was a homogeneous single crystal.

[Example 3] The crystal plate prepared in Example 2 was used as a substrate, and an organometallic compound growth method (hereinafter MOC) was used.
The growth of the gallium nitride based semiconductor layer from the vapor phase was tested by the VD method). The substrate was undoped by heating the susceptor in a reaction furnace while flowing hydrogen gas to 1050 ° C. and holding it for 10 minutes, and then at 950 ° C. and flowing trimethylaluminum (TMA) and ammonia NH _3. The AlN layer was grown to 1 μm. This A
The 1N layer is not a so-called amorphous low temperature buffer layer,
It is a crystalline layer. 100 with ammonia flowing
The substrate is heated to 0 ° C. and then trimethylgallium (T
MG) was flowed to grow an undoped GaN layer to a thickness of 2 μm. The electrical characteristics of this GaN epitaxial crystal were measured by the Hall effect method, and it was n-type and had a carrier concentration of 1
It was found that a crystal having good characteristics with few lattice defects could be grown at about × 10 ¹⁶ cm ⁻³ .

[0037]

Effects of the Invention boride single crystal of the present invention, XB 2 _(X
Is at least one of Ti and Zr) as a main component, and contains 0.05 to 3% by weight of at least one of MoB ₂ or WB ₂ .
A large single crystal having high crystallinity without subgrain boundaries can be obtained.

The method for producing a boride single crystal according to the present invention comprises:
B_Two(X includes at least one of Ti and Zr)
As a main component, 0.05 to 3% by weight of MoB_TwoYoung
Is WB _TwoUsing a raw material bar containing at least one of
Since crystals are grown by the tinging zone method, grain boundaries and
High-quality large Ti and Zr with high crystallinity that does not contain grain boundaries
Can be obtained.

The diboride single crystal produced by the production method of the present invention can be used as a substrate for semiconductor layer growth. In particular, such a Ti and Zr diboride single crystal substrate is excellent as a substrate for growing a nitride semiconductor layer of aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), or the like. .

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view of a high-frequency induction heating floating zone device that can be used in the manufacturing method of the present invention.

FIG. 2 is an external view of a sample including a crystal rod formed by a floating zone method.

FIG. 3 (A) is a schematic diagram showing a polished surface of a longitudinal section of a crystal ingot obtained by an example of the production method of the present invention, (B).
FIG. 3 is a schematic view showing a polished surface of a longitudinal section of a crystal rod manufactured by a conventional manufacturing method.

[Explanation of symbols]

1: Upper shaft drive 2: Upper axis 3: Holder 4: High frequency coil 5: Seed crystal 6: Crystal 7: Zone 8: Raw material bar 9: Holder 10: Lower axis 11: Lower shaft drive unit 12: Grain boundary 13 subgrain boundaries 60 single crystal 61 Beginning part 62 Terminal

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Hiroyuki Kinoshita             6 at 1166 Haseno, Jamizo-cho, Yokaichi-shi, Shiga               Kyocera Corporation Shiga Factory F-term (reference) 4G077 AA02 BE06 CE03 EB01 EC05                       FE17 HA12

Claims (7)

[Claims] 1. A crystal containing XB ₂ (X is at least one of Ti and Zr) as a main component, wherein the crystal is 0.0
A boride single crystal containing 5 to 3% by weight of at least one of MoB _{2 and} WB _2. 2. The boride single crystal according to claim 1, wherein the single crystal is substantially free of subgrain boundaries. 3. 0.05 to 3% by weight of XB ₂ (X contains at least one of Ti and Zr) as a main component.
1. A method for producing a boride single crystal, which comprises growing a raw material rod containing at least one of MoB _{2 and} WB ₂ into a crystal rod by a floating zone method. 4. The starting point of melting of the raw material bar is MoB ₂ and WB _2.
4. The manufacturing method according to claim 3, wherein neither of 5. The crystal rod is further heated to perform strain relief annealing.
The manufacturing method described in. 6. A substrate for semiconductor formation, comprising the single crystal according to claim 1 or 2, wherein the substrate has a main surface for growing and forming a semiconductor layer. 7. A substrate cut out from a crystal rod manufactured by the manufacturing method according to claim 3, wherein the substrate has a main surface for growing and forming a semiconductor layer. Substrate for semiconductor formation.