JP2003089596A - Boride single crystal and substrate for forming semiconductor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain a good quality ZrB2 or TiB2 diboride single crystal which is free from inclusions or surface defects and which is useful as a substrate for growing a thin film semiconductor layer of gallium nitride and aluminum nitride. SOLUTION: When a single crystal of ZrB2 or TiB2 is produced by a zone melting method, a single crystal substantially free from inclusions is produced by blending a hexaboride of an alkaline earth metal with a diboride raw material and subjecting the resulting mixture to zone melting. The substrate is cut out from the single crystal, and the inclusions or surface defects such as pits being the empty shells of the inclusions are not present in the main face of the substrate. The substrate can be used for growing the nitrides semiconductor layer on its surface.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to TiB ₂ or Zr.
The present invention relates to a boride single crystal of B _{2 and} a method for producing the same. The present invention also relates to a substrate for semiconductor formation using these boride single crystals.

[0002]

2. Description of the Related Art In recent years, gallium nitride-based semiconductors have been put to practical use in light-emitting diodes and the like, and gallium nitride-based semiconductors include aluminum nitride (AlN), gallium nitride (GaN), and indium nitride (InN). also includes an indium gallium aluminum nitride _in x _Ga y Al _z N a mixed crystal thereof (x + y ≦
1, z = 1-xy). Conventionally, such a gallium nitride-based semiconductor has been conventionally epitaxially grown on a sapphire substrate to form a multi-layer semiconductor layer.

TiB ₂ and ZrB represented by the chemical formula XB ₂
The crystal of No. ₂ is known, and for example, (Reference 1) S. Otani and Y. Ishizawa: "Preparation o.
f ZrB ₂ single crystals by the floating zone metho
d ", J. Crystal Growth 165 (1996) 319-322, (Reference 2) S. Otani, MM Korskova and T. Mitshhas.
hi: "Preparation of HfB ₂ and ZrB ₂ single crystals
by the floating zone method ", J. CrystalGrowth 168
6 (1998) 582-586, and (Reference 3) S. Otani and Y. Ishizawa: "Preparation o
f TiB ₂ single crystals by the floating zone metho
d ", J. Crystal Growth 140 (1994) 451-453.

Since the above-mentioned TiB ₂ and ZrB ₂ crystals have a very high melting point of about 3000 ° C., a floating zone method (FZ method), which is one of zone melting methods, is used to form a single crystal. And the flux method. It has been known that the FZ method is advantageous for the growth method, particularly for the production of a large single crystal, and the crystal can be obtained most stably in the past.

Regarding the production of ZrB ₂ crystals,
Japanese Patent Application Laid-Open No. 10-
Although disclosed in 95699, this manufacturing method sets the atomic ratio (B / Zr ratio) of the contents of boron B and zirconium Zr in the melt zone to about 1.5 to 2.8 and 3 to 5 atm. When the single crystal is grown by the floating zone method in the atmosphere of He gas as described above, inclusion of inclusions inside the crystal is suppressed, and it becomes possible to grow the crystal at a high growth rate of 3 to 10 cm / hr. Is listed.

[0006]

The above-mentioned sapphire is
It has a large lattice mismatch with the gallium nitride based semiconductor, and a crystal lattice defect was introduced into the epitaxial growth layer due to the lattice mismatch, and a gallium nitride based semiconductor layer with excellent crystallinity could not be obtained. Also, the sapphire substrate is
Since it is an insulator, it was not possible to take out the electrode from the sapphire substrate surface side in the structure such as a light emitting diode. Therefore, conventionally, both electrodes of the positive electrode and the negative electrode are formed only on the surface on which the gallium nitride based semiconductor is formed. For this reason, the manufacturing process of the light emitting diode or the like becomes complicated, and it is necessary to increase the element area compared to the light emitting area.

The present invention intends to utilize a single crystal of TiB ₂ or ZrB ₂ as a substrate for growing a gallium nitride-based semiconductor. The hexagonal plane index (0001) plane of these crystals is used. The lattice constant is substantially close to that of a nitride semiconductor such as GaN or AlN, and has a higher lattice matching than that of sapphire.

Further, the single crystal of TiB ₂ and ZrB ₂ is
The coefficient of thermal expansion is almost equal to that of the nitride semiconductor, and XB
_{Since the 2} single crystal has a relatively high thermal conductivity and is an electrically good conductor, it is superior to sapphire as a substrate crystal for epitaxially growing a gallium nitride based semiconductor layer.

However, in the conventional method for producing a ZrB ₂ single crystal by S. Otani et al., When a single crystal is grown by zone melting using a raw material having a ZrB ₂ composition as it is, B and Zr in the crystal are The atomic ratio B / Zr of is reduced to 1.8 or less. Furthermore, when a (0001) plane is cut out from such a single crystal and subjected to polishing, 0.0
It is observed that a large number of inclusion particles and depressions having a size of 1 to 0.1 mm are exposed. The dents are formed by dropping inclusion particles from the polished surface during polishing, and have a diameter of 10 μm to 100 μm on the polished crystal surface.
The depth was about several μm and the depth was several tens of μm.

If such surface defects are present on the substrate surface, lattice defects such as high-density dislocations are generated in the growth layer of the gallium nitride based semiconductor grown on the surface defects, and the electrical performance of the semiconductor layer is deteriorated. Let

In order to prevent the generation of surface defects on the polished surface of the above XB ₂ single crystal, the present invention uses a floating zone method to clean XB ₂ single crystal without inclusions and a method for producing the same. Is provided. A further object of the present invention is to provide a substrate for growing a nitride semiconductor layer by using an XB ₂ single crystal having no surface defects such as inclusions and depressions on the growth crystal plane.

[0012]

The present invention provides a compound of formula XB ₂
(X is Ti or Zr) prevents the formation of inclusions when the diboride represented by the formula (1) is single-crystallized by the zone melting method. For this reason, the XB ₂ compound and the alkaline earth element R The XB ₂ single crystal is formed from the raw material rod containing the boride RB ₆ of Example ₁ by the zone melting method. here,
As the alkaline earth element R, at least one of Ca, Ba and Sr is used. The XB ₂ boride single crystal produced in this manner contains 1 to 100 pp of alkaline earth element R.
The content of m is included, but inclusions are not substantially included.

Since the formation of inclusions in the XB ₂ boride single crystal is due to the stoichiometric deficiency of XB ₂ due to the evaporation of B in the melting zone, the present invention provides RB ₆
Is used to prevent the evaporation of B. That is, RB ₆
Has a high melting point, the R component evaporates during melting, B dissolves in the melt zone, and B / X in the melt zone is increased to prevent decomposition of the XB ₂ boride.

Preferably, in the raw material rod in which the XB ₂ compound and the boride RB ₆ of the alkaline earth element R are mixed, the compounding amount of the boride RB ₆ is 0.5 to 20 mol%, and the zone melting method is used. To grow into a single crystal.

By this manufacturing method, an XB ₂ boride single crystal containing no inclusions can be manufactured, and a single crystal containing substantially no inclusions or depressions (hereinafter referred to as pits) is obtained on the polished surface. In such a single crystal, the polished surface can be used as an epitaxial growth surface of the nitride semiconductor layer to form a gallium nitride based semiconductor layer with extremely few lattice defects.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a boride crystal represented by XB ₂ (X contains at least one of Ti and Zr) is produced by a zone melting method. The atomic ratio of the content of B to the content of X (hereinafter referred to as the B / X ratio) is set in the range of 1.95 to 2.2 to prevent the formation of inclusions. In this method, the adjustment of B is not carried out by blending boron as a simple substance, but the present invention is carried out by blending the fluoride RB ₆ of alkaline earth element in the compound of XB ₂ as a raw material.

That is, in the method for producing a boride crystal of XB ₂ by the zone melting method, 0.5 to 20 mol% of the alkaline earth element R together with the XB ₂ compound as a main component in the raw material before melting in the zone melting method. Boride RB ₆ (R is Cr, S
(including at least one of r and Ba). From the mixture thus prepared and prepared, a sintered rod can be formed, the sintered rod can be band-melted to grow a single crystal, and the boride single crystal substantially free of inclusions can be obtained.

In the process of zone melting, B / X becomes lower than 2 because B of the raw material XB ₂ compound evaporates from the zone of melting.
A large amount of inclusions are formed during the crystallization process. When a mixed raw material of an XB ₂ compound and an alkaline earth fluoride RB ₆ is used, if RB ₆ in the raw material is less than 0.5 mol%, inclusions are still contained in the single crystal after band melting. The other is
If the content of RB ₆ exceeds 20 mol% in the blended raw material, polycrystallization occurs due to crystallization by zone melting and a single crystal cannot be obtained. The addition amount of RB ₆ is preferably 0.5 to 20 mo.
1%, more preferably in the range of 1 to 10 mol%, and particularly in the range of 4 to 10 mol%, a perfect single crystal and a clean crystal containing no inclusions can be obtained.

XB_TwoOn the formation of inclusions in single crystals
Is generally in the form of X or its boron compound XB.
It B in the melting zone evaporates, and in the course of solidification and crystallization, B
XB by foot_TwoThe phase decomposes and the metal X phase or its
Elementary compound XB phase precipitates, XB _TwoIn the phase matrix
Disperse as inclusions. TiB_TwoIn crystals, inclusions are T
i phase or TiB phase, ZrB_TwoIn crystals, Zr phase
Alternatively, both ZrB phases are fine particles. B in the zone
Feet are exclusively melted during the melting process, usually XB_TwoMelting point of
Since it is heated to the temperature directly above 3000 ° C, XB_Twoset
It is generated by preferentially evaporating B from the melt.

In order to prevent the generation of inclusions, B in the melt zone
It is possible to maintain the content in the stoichiometric composition of the XB ₂ phase or in a composition in which the B content is in excess of that of the XB ₂ phase. The vaporization of B from the raw material bar causes the vaporization of B, which cannot be made sufficiently high.

In the present invention, in order to adjust the B content in the melt band, alkaline earth boride RB ₆ is blended with the raw material XB ₂ compound in place of the simple substance B. _Although R gradually evaporates due to the decomposition of ₆ , the evaporation of B is relatively small, which increases the B content in the melt zone to prevent the formation of the metal X phase or its monoboron compound XB phase. By doing so, inclusions are substantially eliminated.

The XB ₂ boride single crystal produced was
It does not contain dispersed particles of r, Ti, ZrB, TiB, etc., and therefore does not substantially contain pits.
R is included in about 0 ppm.

The alkaline earth metal element R is Ca, Sr,
Although it is Ba, they have substantially the same effect, and the same degree of residual is observed in the crystals, and there is no significant difference between them.

By using the single crystal obtained by this method, even if the crystal orientation is determined so that the main surface is the (0001) plane and the plate is cut out and subjected to polishing, the pits as shown in FIG. It is possible to manufacture a semiconductor-forming substrate that does not exist at all.

The method for producing a single crystal of the present invention can be widely applied to the zone melting method, but preferably, the float zone melting is performed.
zone melting) method (FZ method), in which Ti or Zr XB _{2 is used} as a local heating method for melting the zone.
The high frequency heating method can be used by utilizing the fact that the compound has an appropriate electric conductivity. Furthermore, heating methods such as plasma heating, laser irradiation, and high intensity lamp irradiation can also be used.

The use of an alkaline earth metal boride as a boron additive in the production of a single crystal is applicable not only to the zone melting method such as the FZ method but also to other crystallization such as the skull melting method. can do.

As the method for producing a single crystal according to the present invention, a zone melting method is preferably used, and an example of a production apparatus usable for this purpose 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 several atmospheres.

In this apparatus, when the raw material rod 8 is suspended from the upper part and lowered to the inside of the induction coil 4 in which high frequency current is flowing, an induced current is generated in the raw material rod 8 and Joule heat causes the raw material rod 8 to move. The tip part melts. The melted raw material is brought into contact with the seed crystal arranged below the raw material rod 8 or the sintered rod 5 for forming the initial melt zone to form the melt zone 7 in the central portion of the induction coil. The raw material rod 8 is continuously fed from above to the thus formed melt zone 7, and at the same time, the lower seed crystal 5 is pulled out to grow the single crystal 6 below the melt zone. .

In producing a single crystal, first, a sintered rod is produced. The raw material XB ₂ powder and RB ₆ powder were mixed well, a small amount of camphor was added as a binder, and the mixture was filled in a rubber container and sealed. At a high pressure in a hydrostatic press, for example, 1
A powder bar is manufactured by applying a pressure of ˜4 t / cm ² . Place the dust bar in vacuum or in an inert gas such as argon.
The raw material rod 8 which is heated and sintered at about 00 to 1800 ° C. is manufactured.

The sintered raw material rod 8 is set on the upper shaft 2 via the holder 3, and the seed crystal 5 is set on the lower shaft 10 by the holder 11.
Set through. Instead of the seed crystal 5, a sintered rod for forming an initial melt zone can be used. In this case, for example, a sintered body is used, in which a powder having the same composition as the raw material rod is pressed by the same procedure as the raw material rod, and the rod is vacuum-fired. next,
After filling the furnace with an inert gas of several atmospheres, the raw material rod 8 is fed to the center of the induction coil to melt the tip, form a fusion zone 7, and slowly move the upper shaft 2 and the lower shaft 10 downward. The single crystal 6 is grown.

At this time, since the raw material rod 8 has a lower density than the single crystal, the speed is set so as to guarantee the density difference, and the raw material rod 8 is fed at a speed faster than the speed of pulling out the single crystal. Further, as the atmosphere, in order to prevent discharge in the high frequency induction coil 4, an inert gas such as argon or helium at 1 to 10 atm is used.

To prepare a substrate for growing a semiconductor layer from a single crystal rod, the crystal orientation of the crystal rod is determined by using the X-ray diffraction method,
A thin plate with a thickness of about 0.8 to 1.2 mm is cut parallel to the (0001) plane using a multiband saw. After adjusting the outer shape of this crystal plate with a diamond grindstone,
Chemical polishing using strong alkali, thickness 0.3 ~
A 1.0 mm substrate is manufactured.

After that, the substrate is observed using an optical microscope. If the inclusions and pits are present, their shapes can be sufficiently confirmed at a magnification of 50 to 400 times.

[0034]

[Example] [Example 1] CaB ₆ was added to commercially available ZrB ₂ powder.
Add 5 levels of powder in the range of 0 to 20 mol%, mix well, add a small amount of camphor as a binder, and then add 12
It was filled in a rubber container of mm and subjected to a hydrostatic pressing of 2000 kg / cm ² to manufacture a dust bar. Each dust bar is heated in a vacuum atmosphere at 1400 ° C. and has a diameter of 10 mm and a length of 120.
A sintered rod of about mm was used. At this time, the relative density of the sintered rod was 55%.

A high frequency induction heating FZ shown in FIG.
It is fixed to the upper shaft 2 of the furnace through a holder, and the lower shaft 10 is Z-shaped.
The rB ₂ sintered rod was fixed. The FZ furnace was filled with 6 atm of argon, and the raw material rod 8 was melted by high frequency induction heating to form an initial melt zone, and a single crystal rod having a total length of 60 mm and a diameter of 9 mm was formed at a speed of 20 mm / hr for 3 hours. Was manufactured. Chemical analysis of the obtained crystal rod is carried out, Ca content, B / Zr
The ratio was measured. The (0001) plane was cut out from the obtained crystal, processed into a mirror surface, and as a result of microscopic observation, inclusions and pits were observed. The test results are summarized in Table 1.

[0036]

[Table 1]

When a single crystal of ZrB ₂ was produced in this example, when the content of CaB _{6 in} the raw material rod was increased, it was 1 m.
It was observed that the content in the single crystal tends to decrease at ol%, and 4
When blended in mol%, the B / Zr ratio in the melt zone becomes 1.95, and the inclusions (pits on the fracture surface) in the single crystal disappear. When the CaB ₆ content is further increased, pits caused by inclusions on the polished surface are substantially eliminated (100-300 / cm ² as pits). However, CaB ₆
However, when 10 mol% was added, the occurrence of two or more crystal grains was often observed in the crystal rod,
When the content is ol% or more, the obtained crystal becomes a polycrystal.

As described above, in the FZ method, when the B / Zr ratio in the melt zone is in the range of 1.95 to 2.2 in atomic ratio, the pits which are considered to be caused by inclusions are reduced. It becomes noticeable and almost unobservable. Further, preferably, the inclusion is not completely contained in the B / Zr ratio of 2.05 to 2.20.

In the example of the production of a single crystal of TiB ₂ , T
In the production of a single crystal of iB ₂ , when CaB ₆ was also mixed, a decrease tendency of inclusions was observed at a CaB ₆ content of 0.5 mol%, and inclusions disappeared at a 1 mol% composition. When the compounding amount was further increased, the occurrence of crystal grains was often observed at 5 mol%. In the case of TiB ₂ ,
Only when the B / Ti ratio of the melt zone was in the range of 2.15 or more, the pits decrease, which is thought to be due to the inclusions, became remarkable and could not be observed. Further, preferably, the content completely disappeared in the case of 2.2 or more.

In Table 1 of the above example, 0 mol% of CaB ₆
In the sample, the compounding ingredient has a B / Zr ratio of 2.0, but when a single crystal is produced by zone melting, as shown in FIG. 6, the content of 0.01 to 0.1 mm is contained in the crystal. In addition, a large number of pits where the inclusions were considered to have fallen off were confirmed. Looking at the distribution of the inclusions or pits, there was no evidence of dependence on a specific direction such as the crystal growth direction, and the distribution was almost uniform. From this distribution, these inclusions are not generated at the time of solidification of the crystal, that is, in the process from number 7 to 13 in the phase diagram shown in FIG. 2, but in the cooling process after solidification, that is, number 14 in FIG. It can be seen that in the process from No. 15 to No. 15, it occurred due to decomposition of the solid phase. As a result of studying the crystal thus produced as a semiconductor-forming substrate, a large number of inclusion particles are present in the crystal, so that the crystal surface after polishing has a diameter of several μm or more.
There are many pits with a depth of several tens of μm and a few hundreds of μm.
It turns out that it is difficult to use as a substrate for semiconductor formation.

As a comparative example, the above-mentioned Japanese Patent Laid-Open No. 10-9
Using the same method as 5699, elemental boron B was added to the theoretical ratio Z
The pits (inclusions) were confirmed in a state where a crystal was actually produced by mixing it with the rB ₂ raw material and processed as a semiconductor formation substrate. At this time, it was confirmed that the number of pits decreased as the amount of B added to the raw material rod was increased. But,
If the mixing concentration of elemental B in the raw material rod is increased to such an extent that the pits disappear, the B / Zr ratio of the raw material rod exceeds 2.8, the melting point of the raw material drops sharply, and the raw material should be in the melting region. It started to melt from the upper part and the crystal growth state became stable for a while, making the crystal growth difficult. Furthermore, since the melt is at a high temperature, volatilization of the simple substance of B is severe and the efficiency of supplying B into the melt is poor. Thus, it can be seen that although the simple substance B is expensive, a large amount of compound is required to reduce the number of pits.

For TiB ₂ as well, the method described in Document 3 was carried out using only TiB ₂ as a starting material.
Like ZrB _2, inclusions and pits were observed in the crystal. However, in the case of TiB ₂ , unlike the case of ZrB ₂ , the composition of the melt zone was excessive B and the B / Ti ratio was 2.1. This is because Ti vaporizes more than B vaporizes from the melt zone during crystal production. However, regarding the inclusion in the crystal, ZrB ₂
Similarly, it tends to decrease with the addition of B, and therefore Ti
It is assumed that the phase diagram between B and B has the form shown in FIG.

Example 2 Commercially available TiB ₂ powder was added to CaB.
_{The 6} powders were blended and mixed in an amount of 2 mol%, and hydrostatic pressure and vacuum firing were performed to manufacture a raw material rod 8 having a diameter of 10 mm and a length of 120 mm. The density was 55%. The raw material rod was set in the furnace, and the TiB ₂ sintered rod was fixed to the lower shaft side.

The FZ furnace was filled with 6 atm of helium,
The raw material rod 8 is melted by high frequency induction heating to form an initial melt zone, and the total length is 6 at a speed of 90 mm / hr for 0.6 hours.
A single crystal with a diameter of 0 mm and a diameter of 9 mm was produced. The Ca content in the obtained crystals was 20 ppm. Zone composition is B /
The Ti ratio was 3.00. From the crystals obtained (000
1) The surface was cut out, processed into a mirror surface and observed under a microscope. As a result, it was confirmed that inclusions and pits did not exist.

[Brief description of drawings]

FIG. 1 is a schematic sectional view of a single crystal manufacturing apparatus used in a manufacturing method according to an embodiment of the present invention.

FIG. 2 shows a part of a phase diagram of the Zr-B system.

FIG. 3 shows a part of a phase diagram of Ti—B system.

FIG. 4 shows a schematic view of a crystal ingot during the production of a single crystal by the FZ method.

FIG. 5 shows an enlarged view of the surface of a single crystal substrate manufactured by the manufacturing method of the present invention.

FIG. 6 shows an enlarged view of inclusions / pits existing on the substrate surface after polishing.

[Explanation of symbols]

1: Upper shaft drive 2: Upper axis 3: Holder 4: Induction coil 5: Seed crystal 6: Single crystal 7: Zone 8: Raw material bar 9: Lower shaft drive 10: Lower axis 11: Holder

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 EC08 HA12

Claims (7)

[Claims] 1. A diboride crystal represented by XB ₂ (X includes at least one of Ti and Zr), and containing 1 to 100 ppm of at least one of Ca, Sr and Ba in the crystal. A boride single crystal characterized by: _2. A boride crystal represented by XB ₂ (X includes at least one of Ti and Zr),
A boride single crystal which is substantially free of a metal X phase and X monoboride XB in the crystal. 3. A semiconductor-forming substrate made of the boride single crystal according to claim 1 or 2. 4. A method for producing a boride single crystal represented by XB ₂ (X includes at least one of Ti and Zr) by a zone melting method, wherein the boride crystal is produced by a zone melting method. In addition, the composition of the zone melt portion is set to an atomic ratio B / X of the content of boron B with respect to the content of X to 1.95 to 2.2, and a method for producing an XB ₂ single crystal. 5. The XB ₂ -containing raw material before melting in the zone melting method is alkaline earth boride RB ₆ (R is Cr, Sr, B
including at least one) manufacturing method of XB ₂ single crystal according to claim 4, characterized in that it contains 0.5～20Mol% of a. 6. A semiconductor-forming substrate using an XB ₂ single crystal manufactured by the manufacturing method according to claim 4. 7. The semiconductor forming substrate according to claim 3, wherein the semiconductor forming substrate is a substrate for growing a nitride semiconductor layer.