JP6318290B2 - Method for manufacturing nitride semiconductor crystal - Google Patents

Description

The present invention relates to a method for manufacturing a nitride semiconductor crystal, a nitride semiconductor epitaxial wafer, and a nitride semiconductor free-standing substrate.

For the growth of GaN-based materials, vapor phase growth methods such as metal organic chemical vapor deposition (MOVPE) and hydride vapor deposition (HVPE) are mainly used. In these growths, a seed crystal substrate such as a sapphire substrate, a SiC substrate, or a nitride semiconductor substrate is placed on a holder, a tray, or the like, and a source gas is supplied to the seed crystal substrate to grow a nitride semiconductor. Specifically, in the case of a metal organic vapor phase growth method, a nitride semiconductor is grown on a seed crystal substrate by supplying an organic metal gas such as trimethyl gallium (TMG) or trimethyl aluminum (TMA) and ammonia. . In the case of the hydride vapor phase growth method, gallium chloride (GaCl) gas or aluminum chloride (AlCl, A) is applied to a seed crystal substrate placed on a holder, a tray or the like.
By supplying a group III source gas such as lCl ₃ ) and ammonia, a nitride semiconductor is grown on the seed crystal substrate.
As a seed crystal substrate, a heterogeneous substrate made of sapphire, SiC, Si, or the like, GaN or Al
A nitride semiconductor free-standing substrate made of N or the like is used. Crystal growth is usually performed on these seed crystal substrates so that a group III polar C-plane such as a Ga-plane is the surface, and this plane is used for device formation.

When the nitride semiconductor growth layer becomes thicker, it becomes a problem in the crystal growth of the nitride semiconductor.
The point which becomes easy to produce a crack in a growth layer is mentioned.
This is a problem particularly in the case of thin film growth on a heterogeneous substrate, when the thermal expansion coefficients of the seed crystal substrate and the nitride semiconductor layer grown thereon are greatly different. For example, Ga on a sapphire substrate
During the growth of the N layer, cracks are likely to occur when the thickness of the GaN layer exceeds 5 to 6 μm. In the growth on the SiC substrate or on the Si substrate, a crack is generated even in a thinner GaN layer of about 2 to 3 μm unless a complicated stress relaxation layer is incorporated. Thus Ga
The limited thickness of the N layer that can be grown limits the types of devices that can be applied,
Various inconveniences occur when improvement of device characteristics is hindered.

The generation mechanism of cracks in the growth of thin films on different types of substrates is usually explained as follows. Even if a GaN layer is grown on a heterogeneous substrate at a growth temperature (˜1000 ° C.), no large stress is generated during the growth. However, after the growth, when the temperature of the nitride semiconductor epitaxial wafer in which the GaN layer is formed on the different substrate is returned to room temperature, stress due to the difference in thermal expansion coefficient is generated, and the nitride semiconductor epitaxial wafer is caused by the bimetal effect. Warp. When the thickness of the GaN layer on the sapphire substrate exceeds 5 to 6 μm, the stress exceeds the critical value and cracks are introduced into the GaN layer. Here, the critical value (critical film thickness) of the GaN layer thickness on the sapphire substrate is set to 5 to 6 μm. Actually, this critical value varies depending on the growth apparatus used, growth conditions, etc. With regard to whether cracks occur, it is difficult to make a clear prediction at present.

Similar to the cracks generated during the growth of the thin film having a thickness of about several μm as described above, several hundred μm to several mm.
In the growth of a thick nitride semiconductor free-standing substrate, the generation of cracks is a serious problem.
As in the case of the above thin film, there are two types of manufacture of a nitride semiconductor free-standing substrate: a case where a heterogeneous substrate is used as a seed crystal and a case where a nitride semiconductor free-standing substrate is used as a seed crystal.

In the production of a self-supporting substrate on a different substrate, for example, when the void formation peeling method (VAS method) described in Patent Document 3 is used, the stress between the different substrate and the nitride semiconductor layer grown thereon Can be relaxed by the void layer. For this reason, generation of cracks due to stress between the heterogeneous substrate and the nitride semiconductor layer as in the case of thin film growth described above is suppressed, and a nitride semiconductor layer having a thickness of about 100 μm can be grown without cracks. However, in this case as well, the situation at the start of growth is the same as in the case of thin film growth, and growth on the surface inclined from the C-plane occurs at the outer peripheral edge from the initial stage of growth, which causes stress generation, The thickness of the nitride semiconductor layer is 1
If it exceeds 00 μm, cracks are likely to occur during growth or cooling. Since the thickness of a general semiconductor wafer is required to be about 400 μm to 1 mm from the viewpoint of easy handling, a very thick nitride semiconductor layer of this level is grown even when growing a nitride semiconductor free-standing substrate. There is a need. For this reason, the occurrence of cracks due to the presence of stress at the outer peripheral edge is a serious problem that extremely reduces the yield in the manufacture of a nitride semiconductor free-standing substrate.

In Patent Document 1, cooling is performed by introducing an etching gas into the reactor after growth in order to remove the nitride semiconductor polycrystal adhering to the inner wall of the reactor or the vicinity of the susceptor on which the base substrate is placed. Describes a method for suppressing the occurrence of cracks in the nitride semiconductor substrate and damage to the inner wall of the reactor. In the method of Patent Document 1, when the nitride semiconductor layer is grown thick, the generation of cracks during the growth cannot be reduced, the yield of the nitride semiconductor crystal cannot be increased, and an unnecessary number of nitride semiconductors in the vicinity of the susceptor can be increased. Since the number of crystals increases, the area of the nitride semiconductor substrate that can be obtained is reduced. Patent Document 2 describes a method in which HCl (hydrogen chloride gas) is supplied as an etching gas into the reactor during the growth of GaN to reduce GaN adhering to the inner wall of the reactor. Further, in Patent Document 4, in order to reduce the warpage of the stacked body in which the GaN layer is grown on the sapphire substrate, the surface of the sapphire substrate is applied to the surface of the sapphire substrate by performing nitriding treatment and etching treatment with hydrogen chloride gas. A method is described in which an uneven structure of aluminum nitride is formed and GaN is grown on a sapphire substrate having the uneven structure of aluminum nitride.

JP 2007-320811 A US Pat. No. 6,632,725 JP 2004-039810 A JP 2007-106667 A

As described above, one of the reasons why it is difficult to predict the critical film thickness of the GaN thin film on the heterogeneous substrate is the effect of the end face.
Usually, a Ga-polar C-plane is used for device fabrication. As shown in FIG. 1, the surface of an epitaxial wafer in which a nitride semiconductor is formed on a seed crystal substrate 1 which is a heterogeneous substrate is almost the entire surface. Is covered with the nitride semiconductor crystal 2a grown on the Ga-polar C-plane f1. However, the situation is different from the inner peripheral portion (flat portion for C-plane growth) at the outer peripheral end portion of the epitaxial wafer, and a nitride semiconductor crystal 2b having a surface f2 inclined from the C plane f1 as a surface grows. FIG.
Then, the nitride semiconductor crystal 2 which is a GaN crystal starts to grow from the seed crystal substrate 1 which is a heterogeneous substrate (for example, a sapphire substrate), and schematically shows a state in which the crystal growth proceeds as shown by a dotted line. ing. That is, the surface (growth surface) f ₁ of the C-plane grown nitride semiconductor crystal 2a grown on the main surface (eg, C surface) of the seed crystal substrate 1 is f _1-1 , f _1-2 , f _{1−. 3} sequentially grown, also, the surface of the nitride semiconductor crystal 2b of the outer peripheral edge to grow in the plane f ₂ inclined from the C-plane f ₁ (growth surface)
f _2, similarly, f _2-1, f _2-2, sequentially grown with f _2-3.
Different crystal planes generally have different dangling bond densities on the surface and the method of surface reconstruction, so that the impurity uptake efficiency generally differs greatly. For this reason, in the growth of a nitride semiconductor epitaxial wafer, a situation occurs in which the impurity concentration of the nitride semiconductor crystal 2a at the inner peripheral portion of the wafer and the nitride semiconductor crystal 2b at the outer peripheral end portion are greatly different. Differences in impurity concentration affect the elastic and plastic properties, thermal properties, lattice constants, and the like of crystals. Therefore, when crystals with different impurity concentrations are adjacent to each other, stress is generated between the crystals. That is, in a wafer having a growth surface different from the C-plane at the outer peripheral portion, a region of crystal 2b having an impurity concentration different from that of the C-plane grown crystal 2a is generated at the outer peripheral end portion, and a large stress is included in the outer peripheral end portion. This will cause cracks. It is difficult to predict the critical film thickness of a GaN thin film on a heterogeneous substrate because the size of the region having a different impurity concentration at the outer peripheral edge and the value of the impurity concentration at the outer peripheral edge vary depending on the device configuration and growth conditions. It is.

Also, when the nitride semiconductor free-standing substrate itself is used as a seed crystal substrate, crystal growth on a surface other than the C-plane occurs at the outer peripheral edge of the nitride semiconductor crystal, which causes stress and causes cracks. That is, as shown in FIG. 2, a nitride semiconductor free-standing seed crystal substrate 1 is placed on the mounting surface 4 of the tray 3, and the nitride semiconductor crystal 2 is grown on the seed crystal substrate 1. As a result, stress is generated at the outer peripheral end of the C-plane grown nitride semiconductor crystal 2a that grows on the main surface (for example, C-plane) of the seed crystal substrate 1 and grows on the plane f2 inclined from the C-plane f1. Nitride semiconductor crystal 2b
Will grow.
In addition, when a nitride semiconductor free-standing substrate is used as a seed crystal substrate, in addition to the stress at the outer peripheral edge, the free-standing substrate itself, which is a seed crystal, is originally distorted by including stress, Nitride semiconductor free-standing substrate (seed crystal substrate) and nitride semiconductor layer grown thereon due to different growth conditions of the nitride semiconductor of the free-standing substrate that is a seed crystal and the nitride semiconductor layer grown thereon In many cases, stress is generated between the (growth layer) and these often cause cracks during growth or cooling.

As described above, in any case of the growth of a nitride semiconductor thin film on a seed crystal substrate which is a heterogeneous substrate, or the growth of a nitride semiconductor crystal on a seed crystal substrate which is a nitride semiconductor free-standing substrate,
When the intended nitride semiconductor crystal is intentionally grown on the main surface of the seed crystal substrate or the like, a nitride semiconductor crystal other than the nitride semiconductor crystal that has been intentionally grown is, for example, a nitride semiconductor crystal As a nitride semiconductor free-standing substrate, which is an outer peripheral end portion of the semiconductor substrate, or a seed crystal substrate, these cause stress generation and cause generation of cracks in the nitride semiconductor crystal.

An object of the present invention is to provide a nitride semiconductor crystal manufacturing method capable of suppressing the occurrence of cracks in the nitride semiconductor crystal and improving the yield of the nitride semiconductor crystal, and a nitride semiconductor epitaxial wafer realized by this method It is to provide a nitride semiconductor free-standing substrate.

According to a first aspect of the present invention, there is provided a nitride semiconductor crystal manufacturing method for growing a nitride semiconductor crystal on a seed crystal substrate, wherein an outer peripheral edge portion of the seed crystal substrate is grown during the growth of the nitride semiconductor crystal. This is a method for producing a nitride semiconductor crystal, in which the nitride semiconductor crystal is grown while an etching action is applied thereto.
According to a second aspect of the present invention, in the method for producing a nitride semiconductor crystal according to the first aspect,
The seed crystal substrate is placed in a container having a side wall that surrounds the outside of the seed crystal substrate, and an environment in the vicinity of a portion of the inner surface that does not come into contact with the seed crystal substrate at the start of growth of the inner surface of the container,
By providing an environment in which an etching action is applied during the growth of the nitride semiconductor crystal, the nitride semiconductor crystal does not come into contact with the inner surface portion of the container throughout the entire period of crystal growth and has a cross-sectional shape inside the container. This is a method for manufacturing a nitride semiconductor crystal in which the nitride semiconductor crystal grows with a similar cross-sectional shape.

According to a third aspect of the present invention, in the method for producing a nitride semiconductor crystal according to the second aspect, the inner surface of the container that does not contact the nitride semiconductor crystal includes the side surface of the side wall. It is.

According to a fourth aspect of the present invention, in the method for producing a nitride semiconductor crystal of the second or third aspect,
In the method of manufacturing a nitride semiconductor crystal, the inner surface of the container that does not contact the nitride semiconductor crystal includes a surface on the side where the seed crystal substrate is placed.

According to a fifth aspect of the present invention, in the method for producing a nitride semiconductor crystal according to any one of the second to fourth aspects, the environment near the inner surface portion of the container that does not contact the nitride semiconductor crystal is This is a method for producing a nitride semiconductor crystal in an environment where the etching action is weakened with the distance from the side surface of the side wall.

According to a sixth aspect of the present invention, in the method for producing a nitride semiconductor crystal according to any one of the second to fifth aspects, the nitride semiconductor crystal is grown in an environment in which growth and etching coexist, and the nitridation is performed. By diluting the growth raw material in the vicinity of the inner surface portion of the vessel that does not come into contact with the semiconductor crystal
This is a method for producing a nitride semiconductor crystal in which the etching action is enhanced.

According to a seventh aspect of the present invention, in the method for producing a nitride semiconductor crystal according to the sixth aspect, the growth material is diluted by supplying an inert gas containing nitrogen, argon or helium. It is a manufacturing method of a crystal.

According to an eighth aspect of the present invention, in the method for producing a nitride semiconductor crystal according to any one of the second to fifth aspects, an etching action is provided near a portion of the inner surface of the container that does not contact the nitride semiconductor crystal. This is a method for manufacturing a nitride semiconductor crystal in which the etching action is applied by supplying a gas or a liquid.

A ninth aspect of the present invention is the method for producing a nitride semiconductor crystal according to the eighth aspect, wherein the gas having an etching action contains at least one of hydrogen, chlorine and hydrogen chloride. It is a manufacturing method.

According to a tenth aspect of the present invention, in the method for producing a nitride semiconductor crystal according to any one of the second to fifth aspects, a substance that generates an etching species by the action of a catalyst is supplied to the growth of the nitride semiconductor crystal. However, it is a method for producing a nitride semiconductor crystal that exhibits the etching action by using at least a part of the inner surface of the container that does not come into contact with the nitride semiconductor crystal as a catalytic substance having the action of the catalyst. .

An eleventh aspect of the present invention is the method for producing a nitride semiconductor crystal according to the tenth aspect, wherein the substance that generates etching species by the action of the catalyst is hydrogen gas.

According to a twelfth aspect of the present invention, in the method for producing a nitride semiconductor crystal according to the tenth or eleventh aspect, the production of a nitride semiconductor crystal in which the catalyst substance having the catalytic action is a metal or a metal nitride. Is the method.

A thirteenth aspect of the present invention is the nitride semiconductor crystal manufacturing method according to the twelfth aspect, wherein the metal is any one of Ti, Zr, Nb, Ta, Cr, W, Mo, and Ni. It is a manufacturing method of a crystal.

According to a fourteenth aspect of the present invention, in the method for producing a nitride semiconductor crystal according to any one of the second to thirteenth aspects, the inner surface of the vessel that does not contact the nitride semiconductor crystal and causing an etching action and the nitride semiconductor This is a method for producing a nitride semiconductor crystal in which the distance from the crystal is in the range of 1 to 10 mm during the period from the start to the end of crystal growth.

According to a fifteenth aspect of the present invention, in the nitride semiconductor crystal manufacturing method according to any one of the second to fourteenth aspects, the side surface of the side wall and the mounting surface of the container on which the seed crystal substrate is mounted Is formed in a range in which the cross section inside the container is enlarged toward the opening side, and the nitride semiconductor crystal has a nitrogen surface as a growth surface and the diameter thereof. This is a method for producing a nitride semiconductor crystal that grows while increasing the thickness.

According to a sixteenth aspect of the present invention, there is provided a nitride semiconductor epitaxial wafer obtained by growing a nitride semiconductor crystal on a plate-like seed crystal substrate, wherein the nitride semiconductor crystal is oriented in a main surface direction of the seed crystal substrate. The grown nitride semiconductor crystal and the nitride semiconductor crystal grown in the surface direction inclined from the main surface and grown in the main surface direction at the outer peripheral end of the nitride semiconductor crystal grown in the main surface direction The nitride semiconductor crystal having a higher impurity concentration than the nitride semiconductor crystal having a higher impurity concentration or having a growth thickness of the nitride semiconductor crystal having a higher impurity concentration grown in the main surface direction. This is a nitride semiconductor epitaxial wafer having a growth thickness of less than 1/10.

A seventeenth aspect of the present invention is the nitride semiconductor epitaxial wafer according to the sixteenth aspect of the method for producing a nitride semiconductor crystal, wherein the seed crystal substrate is a sapphire substrate and the nitride semiconductor crystal is a GaN layer. The radius of curvature of the nitride semiconductor epitaxial wafer is R (m), the thickness of the GaN layer is t (μm), the thickness of the sapphire substrate is Y (μm),
When the coefficient is A, the following equations (1) and (2)
R = A / t Equation (1)
A> 0.000024 × Y ^1.58483 ...... Formula (2)
It is a nitride semiconductor epitaxial wafer that satisfies the above.

According to an eighteenth aspect of the present invention, there is provided a plate-like nitride semiconductor free-standing substrate having a higher impurity concentration than that of a nitride semiconductor crystal grown in the main surface direction of the nitride semiconductor free-standing substrate at an outer peripheral end portion thereof. The nitride semiconductor crystal grown in the main surface direction has a growth thickness of the nitride semiconductor crystal having a high impurity concentration at the outer peripheral edge portion even if it does not have or has a nitride semiconductor crystal. It is a nitride semiconductor free-standing substrate that is less than one-tenth of the thickness.

According to the present invention, the occurrence of cracks in a nitride semiconductor crystal grown on a seed crystal substrate can be suppressed, and the yield can be improved.

It is a cross-sectional schematic diagram of the vicinity of the outer peripheral edge part of the nitride semiconductor layer grown on the seed crystal substrate which is a different kind | species substrate by the conventional method. It is a cross-sectional schematic diagram of the vicinity of the outer periphery edge part of the nitride semiconductor layer grown on the seed crystal substrate which is a nitride semiconductor self-supporting substrate by the conventional method. BRIEF DESCRIPTION OF THE DRAWINGS The principal part structure of the manufacturing method of the nitride semiconductor crystal which concerns on one Embodiment of this invention is shown, Comprising: (a) is sectional drawing, (b) is a top view. It is sectional drawing which shows the principal part structure of the manufacturing method of the nitride semiconductor crystal which concerns on other embodiment of this invention. It is sectional drawing which shows the principal part structure of the manufacturing method of the nitride semiconductor crystal which concerns on other embodiment of this invention. It is sectional drawing which shows the principal part structure of the manufacturing method of the nitride semiconductor crystal which concerns on other embodiment of this invention. It is the schematic of the conventional HVPE apparatus used in the comparative example 1. In Example 1 and Comparative Example 1 of the present invention, the characteristics of a GaN layer formed on a sapphire substrate are shown. (A) is the relationship between the thickness of the GaN layer and the yield, and (b) is the thickness of the GaN layer. (C) is a graph showing the relationship between the thickness of the GaN layer and the (10-12) diffraction half width. 6 is a diagram showing an image obtained by observing a cross section of a GaN crystal formed on a seed crystal substrate in Comparative Example 1 with a fluorescence microscope. FIG. It is the schematic of the HVPE apparatus which enforces the manufacturing method of the nitride semiconductor crystal of this invention used in Example 1. FIG. FIG. 6 shows the result of measuring the radius of curvature R of an epitaxial wafer obtained when growing GaN layers of various thicknesses on sapphire substrates of various thicknesses using the methods of Example 1 and Comparative Example 1. (A) shows a case where a GaN layer is grown on a sapphire substrate with a diameter of 50 mm and a thickness of 350 μm, (b) shows a case where a GaN layer is grown on a sapphire substrate with a diameter of 100 mm and a thickness of 900 μm, and (c) shows It is a graph which shows the result at the time of growing a GaN layer on the sapphire substrate 150 mm in diameter and 1500 micrometers in thickness. FIG. 11 is a schematic view of a cross section in the vicinity of an outer peripheral end portion of a GaN layer grown on a seed crystal substrate that is a different substrate using the HVPE apparatus of FIG. 10 in Example 1. It is process drawing which shows the manufacturing method of the GaN self-supporting substrate by VAS method used in Example 10 and Comparative Example 2. It is a graph which shows the relationship between the thickness of the GaN crystal grown on the dissimilar substrate, and the yield in Example 10 and Comparative Example 2 of the present invention. It is a graph which shows the relationship between the thickness of the GaN growth layer grown on the GaN substrate, and the yield in Example 11 and Comparative Example 3 of the present invention. It is a cross-sectional schematic diagram of the vicinity of the outer periphery edge part of the GaN crystal grown using the GaN self-supporting substrate in Example 11 of this invention as a seed crystal. It is a graph which shows the relationship between the thickness of the GaN growth layer grown on the GaN substrate in Example 12 of this invention, and a yield. It is sectional drawing which shows the growth apparatus of the horizontal flow system in the modification 9 of this invention.

(Knowledge)
The present inventor has intensively studied to solve the defect that cracks are likely to occur during the growth of a nitride semiconductor. As a result, as described above, when growing on a seed crystal substrate made of a heterogeneous substrate or a nitride semiconductor free-standing substrate, the growth surface is intended to grow (for example, on the main surface of the seed crystal substrate). A crystal having a property different from that of the crystal to be grown exists, for example, on the outer peripheral edge of the nitride semiconductor crystal that is intentionally grown or as the seed crystal substrate itself, and the growth surface on which these crystals are intended It has been found that stress is generated between the upper nitride semiconductor crystal and this causes cracks during growth.
Based on this knowledge, the inventor applied an etching action to a portion (particularly the outer peripheral end portion) of the nitride semiconductor crystal growing on a surface other than the intended growth surface, or the outer peripheral end portion of the seed crystal substrate. Cracks during the growth of nitride semiconductor crystals can be obtained by gradually etching the nitride semiconductor free-standing substrate as a seed crystal substrate, which suppresses the growth of the outer peripheral edge that generates stress, during growth. The inventors have conceived a method for producing a nitride semiconductor crystal of the present invention that suppresses the above-described phenomenon.

A nitride semiconductor crystal manufacturing method, a nitride semiconductor epitaxial wafer, and a nitride semiconductor free-standing substrate according to embodiments of the present invention will be described below.

A method for producing a nitride semiconductor crystal according to an embodiment of the present invention, for example, as shown in FIG.
The seed crystal substrate 1 is placed on a bottom wall 5b in a crucible-shaped or shallow cup-shaped container 5 having a side wall 5a that surrounds the outside of the seed crystal substrate 1, and the seed crystal substrate 1 is grown on the growth surface (seed crystal substrate 1 The source gas G is supplied to the main surface of the substrate to grow a nitride semiconductor crystal 2 on the seed crystal substrate 1. Container 5
The environment / atmosphere in the vicinity of the inner surface portion of the inner surface of the substrate that was not in contact with the seed crystal substrate 1 at the start of growth (in FIG. 3, the portion of the side surface 6 of the side wall 5a and the bottom surface 7 of the bottom wall 5b near the side surface 6). The environment / atmosphere is such that an etching action is applied to the growing nitride semiconductor crystal 2. This
The nitride semiconductor crystal 2 has a cross-sectional shape similar to the cross-sectional shape inside the container 5 (circular in FIG. 3) without contacting the inner surface of the container 5 throughout the crystal growth period. grow up.

The inner surface of the container that does not come into contact with the nitride semiconductor crystal preferably includes the side surface of the side wall of the container. In addition, when a nitride semiconductor crystal is grown on a seed crystal substrate that is a nitride semiconductor free-standing substrate, the inner surface of the container that does not contact the nitride semiconductor crystal is nitrided in addition to the side surface of the side wall of the container. It is preferable to include a surface on which the physical semiconductor free-standing substrate is installed.

The etching environment / etching atmosphere in the vicinity of the inner surface portion of the container that does not come into contact with the nitride semiconductor preferably has an etching action that decreases with the distance from the inner surface of the container. If the etching action does not weaken with the distance from the inner surface, many of the nitride semiconductor crystals that grow during the growth period may be etched.
Even when the etching action attenuates with the distance from the inner surface, if the etching action is too strong even at a position away from the inner surface, the growth layer growing on the main surface of the nitride semiconductor crystal is lost. Also, if the etching action is too weak, the growth layer of the nitride semiconductor crystal grows on an unintended surface (for example, an end face), stress is generated in the growing nitride semiconductor crystal, and the nitride semiconductor crystal is cracked. It will occur. From these, the strength of the etching action,
The degree of weakening (attenuation) of the etching action due to the distance from the inner surface of the container must be appropriately selected so that excessive stress is not generated or is removed.

If this etching action exists in the vicinity of the side surface 6 of the crucible-like container 5 as shown in FIG. 3, for example, the growth of the outer peripheral end (end surface) 8 of the nitride semiconductor crystal 2 growing on the seed crystal substrate 1 occurs. It is suppressed. That is, the growth of the crystal at the outer peripheral end 8 having a different impurity concentration from that of the crystal on the intentionally growing surface (for example, the C plane) 9 is suppressed, and a crack is generated due to the stress caused by the crystal at the outer peripheral end 8. Can be suppressed. In this situation, a nitride semiconductor crystal having a cross-sectional shape approximately similar to the cross-sectional shape (circular in FIG. 3) of the inner surface of the container grew, and the inner surface of the container was not in contact with the seed crystal substrate at the start of growth. A situation is realized where the inner part is grown without contact throughout the growth period. Since the outer peripheral portion of the seed crystal substrate 1 is not opened and the side wall 5a of the container 5 is provided so as to surround the outer side of the seed crystal substrate 1, the outer peripheral end portion 8 of the outer peripheral portion of the seed crystal substrate 1 is provided. It is possible to form and maintain an etching environment / etching atmosphere in which the nitride semiconductor crystal can be selectively etched.
In this case, it is preferable that the growth rate at which the crystal at the outer peripheral edge grows is 0 under the above-described etching action. This situation is a state where the etching action and growth are balanced. That is, since the outer peripheral end portion is an environment / atmosphere in which an etching action is applied, growth of the outer peripheral end portion can be suppressed. However, as the growth rate in the normal direction of the growth surface at the outer peripheral edge, the growth direction in the normal direction of the intended growth surface (for example, the growth surface growing in the normal direction of the main surface (for example, C surface) of the seed crystal substrate) If it is less than 1/10 of the growth rate, the effect of preventing cracks can be obtained. In the case of 1/10 or more, excessive stress is generated and the probability of occurrence of cracks is increased.

When etching is performed on the outer peripheral edge, the etching rate in the normal direction of the etched surface is preferably equal to or lower than the intended growth rate. This is because even if the etching rate at the outer peripheral edge is higher than the intended growth rate of the growth surface, the crack prevention effect can be obtained, but the size of the finally obtained crystal becomes extremely small.

Further, when a nitride semiconductor is grown on a nitride semiconductor free-standing substrate that is a seed crystal substrate, if this etching action exists on the surface on the side where the nitride semiconductor free-standing substrate is installed, the back surface of the seed crystal substrate during growth Is gradually etched, so that the stress due to the seed crystal substrate is removed and reduced at the stage where a new nitride semiconductor growth layer is grown thick on the surface side of the seed crystal substrate, thereby suppressing the generation of cracks during growth. Can do.
In this case, good results can be obtained when the etching rate of the back surface of the seed crystal substrate is 1/100 or more of the growth rate on the intended growth surface. In addition, when the etching rate of the back surface of the seed crystal substrate is high, it may be assumed that the entire thickness of the nitride semiconductor becomes smaller than the initial nitride semiconductor free-standing substrate after growth. In order to avoid such a situation, it is preferable that the etching rate of the back surface of the seed crystal substrate is not more than half of the intended growth rate on the growth surface.

Further, in order to maintain the stress balance during growth, it is preferable to add the etching action while maintaining symmetry with respect to a certain point, line or plane in the growing crystal. In the case of a crystal substrate, for example, a disc-shaped seed crystal substrate, where one surface is the intended growth surface, it is preferable to use a container having a circular cross section and apply an etching action evenly to the end surface of the crystal periphery. In addition, in the case of a quadrangular, hexagonal or other polygonal plate-like seed crystal substrate, where one surface is the main growth surface, a container having a cross-sectional shape similar to that of the seed crystal substrate is used. It is preferable to apply an etching action evenly to the outer peripheral end face of the nitride semiconductor crystal. Furthermore, the seed crystal substrate of the present invention is not limited to a plate-like substrate, and includes various seed crystals. For example, in the case of a seed crystal substrate having a cylindrical shape or a polygonal pyramid shape, when the circumferential surface or the polygonal surface is a main growth surface, it is preferable to apply an etching action evenly to the side surface with respect to the main growth surface. .

In addition, when an etching action is applied to the back surface of the seed crystal substrate, it is preferable to apply the etching action uniformly to the entire surface, or to apply the etching action symmetrically with respect to in-plane points or lines. However, in order to apply an etching action to the back surface of the seed crystal substrate facing the inner surface of the container, the back surface of the seed crystal substrate needs to be separated from the inner surface of the container by, for example, a block smaller than the seed crystal substrate. Therefore, it is necessary to devise a method of installing the seed crystal substrate in the container.
In this case, a plurality of sufficiently small blocks are used, and the area of the surface hidden in contact with the block is preferably 1/10 or less of the entire area of the rear surface of the seed crystal substrate. If this ratio is too high, the effect of stress relaxation by etching the seed crystal substrate cannot be obtained sufficiently.
Specifically, for example, as shown in FIG. 6, the seed crystal substrate 1 is a container 10 having a side wall 10a.
A gas g for introducing an etching action on the back surface of the seed crystal substrate 1 is introduced on the bottom wall 10b of the seed crystal via a plurality of blocks 17. In FIG. 6, a supply pipe 18 that supplies a gas g is connected to the center of the bottom wall 10 b of the container 10, and the gas g introduced from the supply pipe 18 is supplied to the bottom wall 10 b of the container 10 and the seed crystal substrate. 1 and the back surface 16 of the seed crystal substrate 1 is etched by the gas g. Further, the gas g flowing along the back surface 16 is released from between the side wall 10 a of the container 10 and the outer periphery of the seed crystal substrate 1, and grows on the seed crystal substrate 1.
Of these, the nitride semiconductor crystal 2 at the outer peripheral end 8 is mainly etched.

The above etching action is preferably applied constantly, but may be added intermittently. For example, in order to achieve the object of the present invention, it is one of the preferable methods to constantly flow a gas having an etching action to the outer peripheral portion of the wafer. It is necessary to install a discharge port. On the other hand, a gas discharge port having an etching action is provided only at a part of the outer periphery of the wafer, or a gas having an etching action is blown from one direction to the outer periphery of the wafer, and the wafer is rotated to rotate the entire outer periphery of the wafer. Alternatively, the entire back surface of the wafer can be etched intermittently. If the etching action is sufficient, this method has an advantage that the apparatus configuration is simplified.

There are several methods for developing the above-mentioned etching action. One preferable method is to perform nitride semiconductor growth in an environment in which growth and etching coexist, and to grow near the surface to which the above-mentioned etching action is applied. There is a method in which the etching action is strengthened by diluting the raw material. The environment where growth and etching coexist corresponds to the case where hydrogen, chlorine, hydrogen chloride, or the like is added to the growth atmosphere in vapor phase growth such as MOVPE growth or HVPE growth. In this case, the growth raw material is preferably diluted by supplying an inert gas such as nitrogen, argon or helium. For example, in FIG. 3, a gas having an etching action such as hydrogen, chlorine, hydrogen chloride or the like is added to the raw material gas G in addition to the growth raw material (group III raw material and group V raw material). Then, an inert gas such as argon or helium is supplied to dilute the growth raw material in the raw material gas G.
A desirable value of the addition amount of these dilution gases (inert gas) for obtaining an appropriate etching action varies depending on the growth conditions, but is in the range of 1/10 to 10 times the supply amount of the group III raw material, as described above. It is preferable to set so as to realize the growth rate and the etching rate.
The present invention can also be applied to growth using a solution performed in a closed system such as a Na flux method or a low temperature synthesis method. In these cases, the etching action is manifested by forcibly introducing a solution having a lower raw material solubility than the solution in the vicinity of the intentionally growing face into the vicinity of the face to which the etching action is applied.

The etching action may be expressed by supplying a gas or liquid (solution) having an etching action to the vicinity of the surface to which the etching action is applied.
The gas having an etching action preferably contains at least one of hydrogen, chlorine and hydrogen chloride. In order to obtain an appropriate etching action, the desired value of the addition amount of these etching gases varies depending on the growth conditions, but the growth rate described above is in the range of 1/10 to 10 times the supply amount of the group III raw material. It is preferable that the etching rate is set to be realized.
In the Na flux method and the low-temperature synthesis method, as described above, a solution having a growth raw material solubility lower than the solution in the vicinity of the surface on which the surface is intentionally grown becomes a liquid (solution) having an etching action.

FIG. 4 shows an embodiment of the above method for supplying a gas having an etching action in the vicinity of the surface to which the etching action is applied. The container in this embodiment is mainly composed of a cup-shaped container 10 having a side wall 10a and a tray 3 on which the seed crystal substrate 1 is placed. The mounting surface (installation surface) 4 of the tray 3 and the side surface of the side wall 10a constitute the inner surface of the shallow cup-shaped container,
A gas g having an etching action was introduced from the outer periphery of the bottom surface of the inner surface of the container into the outer periphery of the container.
A supply pipe 11 that supplies an etching gas g to the center of the bottom wall 10b of the container 10
Are connected, and a support shaft 13 that supports the tray 3 through the supply pipe 11 is provided.
The tray 3 on the support shaft 13 is disposed with a predetermined gap from the bottom wall 10 b of the container 10. The gas g supplied from the supply pipe 11 to the central portion of the bottom wall 10b of the container 10 flows radially between the tray 3 and the bottom wall 10b with the support shaft 13 as a center, and the outer surface of the tray 3 and the side wall 10a. The gas flows out from an annular gas discharge port (gas outlet) 19 formed between the inner peripheral surface and the inner peripheral surface. On the other hand, on the growth surface of the seed crystal substrate 1 placed on the mounting surface 4 of the tray 3 (on the main surface of the seed crystal substrate), a source gas G containing a growth material is supplied. The nitride semiconductor crystal 2 grows on the surface. The outer peripheral end portion 8 of the nitride semiconductor crystal 2 is subjected to predetermined etching by a gas g having an etching action released from the gas discharge port 19.

Further, the nitride semiconductor crystal is grown while supplying a substance that generates etching species by the action of a catalyst, and at least a part of the inner surface of the container that does not come into contact with the nitride semiconductor crystal is used as a catalyst substance. An etching action may be developed. Since this method does not involve the introduction of a local gas / liquid, there is an advantage that it can be carried out with a simpler apparatus than the above two methods.
Hydrogen gas is preferable as the substance that generates etching species by the action of the catalyst, and the catalyst substance is preferably a metal or a metal nitride.
Hydrogen gas comes into contact with a catalyst material made of a metal or metal nitride in a high temperature state, and atomic hydrogen having a strong etching action is generated, and the generated atomic hydrogen diffuses to reach the nitride semiconductor crystal. This is presumed to be etched. Atomic hydrogen is unstable and reacts in a short time and disappears. Therefore, the etching action by atomic hydrogen is rapidly attenuated as the distance from the inner surface of the container increases, and only the nitride semiconductor crystal located at a certain distance from the inner surface of the container is etched. That is, the vicinity of the inner surface of the container becomes an atmosphere in which the etching action becomes weaker with the distance from the inner surface of the container. Appropriate etching can be performed without causing cracks in the semiconductor crystal.

Further, as the metal as the catalyst substance, Ti (titanium), Zr (zirconium), N
b (niobium), Ta (tantalum), Cr (chromium), W (tungsten), Mo (molybdenum), and Ni (nickel) are preferable. For example, nitriding the inner surface of a container made of the above-described solid metal material in a nitride semiconductor crystal growth apparatus before the growth of the nitride semiconductor and then performing the crystal growth is one preferred embodiment of the present invention. One. The metal or metal nitride film may be formed on the inner surface of the container that is not in contact with the nitride semiconductor crystal.
In this method, for example, in FIG. 3, the hydrogen gas of the source gas G is added, and the side surface 6 of the container 5 may be formed of the metal or metal nitride. Further, for example, in FIGS. 4 and 6, hydrogen gas is included in the gas g, and the inner surface of the side wall 10a of the container 10 or the bottom wall 10b of the container 10 is formed of the above metal or metal nitride. It only has to be done.

The distance between the side wall of the container that is not in contact with the nitride semiconductor crystal and the nitride semiconductor crystal is preferably in the range of 1 to 10 mm during the period from the start of growth to the end of growth. This distance is 1m
If it is closer than m, the nitride semiconductor crystal contacts the inner surface of the container due to slight fluctuations in growth conditions,
The container and the nitride semiconductor crystal are fixed to each other, and stress is generated by the fixation, thereby generating a crack in the nitride semiconductor crystal. On the other hand, if this distance is longer than 10 mm, the steepness of the attenuation of the etching action is lost, and the growth of only the outer peripheral edge cannot be selectively inhibited by etching. The distance between the inner surface of the container and the nitride semiconductor crystal is, for example, a distance d shown in FIG.

In a nitride semiconductor epitaxial wafer realized by the above-described method for producing a nitride semiconductor crystal and having a nitride semiconductor crystal grown on a plate-like seed crystal substrate, the nitride semiconductor crystal is the seed crystal. The nitride semiconductor crystal grown in the main surface direction of the substrate and the outer peripheral edge of the nitride semiconductor crystal grown in the main surface direction grew in the surface direction inclined from the main surface, and grew in the main surface direction. The nitride semiconductor crystal having a higher impurity concentration than that of the nitride semiconductor crystal is not present, or even if it has, the growth thickness of the nitride semiconductor crystal having a higher impurity concentration has grown in the main surface direction. A nitride semiconductor epitaxial wafer having a growth thickness of the nitride semiconductor crystal of less than 1/10 is obtained.
For this reason, during the growth and cooling, the generation of cracks in the nitride semiconductor crystal is dramatically suppressed, and a high yield can be realized. The high impurity concentration mentioned here means that the nitride semiconductor crystal grown in the principal plane direction which is a crystal on the intended plane has a certain in-plane distribution in the impurity concentration, but the maximum value of the impurity concentration of the distribution is It means that the impurity concentration is much higher (for example, 2 times or more), and the maximum value of the impurity concentration of the crystal on the main surface is slightly within the intended impurity concentration distribution range on the main surface. It does not mean an impurity concentration value exceeding that.

Even when a nitride semiconductor crystal having a high impurity concentration is grown on the outer peripheral edge of the nitride semiconductor epitaxial wafer, the growth thickness should be less than 1/10 of the growth thickness of the nitride semiconductor crystal grown in the main surface direction. For example, generation of cracks during growth and cooling is suppressed, but warpage of the nitride semiconductor epitaxial wafer can be further reduced.
In the above-described nitride semiconductor epitaxial wafer, in particular, an epitaxial wafer in which a GaN layer on a sapphire substrate is grown, an epitaxial wafer having a smaller warp than a conventional epitaxial wafer can be realized. Normally, when a GaN layer is grown on a sapphire substrate, the epitaxial wafer warps upward in a convex shape when the surface of the GaN layer faces upward. A nitride semiconductor crystal having a high impurity concentration is grown at one-tenth or more of the growth thickness of the nitride semiconductor crystal grown in the main surface direction on the outer peripheral edge of the nitride semiconductor epitaxial wafer manufactured by the conventional manufacturing method. The curvature radius of the nitride semiconductor epitaxial wafer is small,
The amount of warpage has increased. On the other hand, the nitride semiconductor epitaxial wafer manufactured by the manufacturing method according to the embodiment of the present invention has a large radius of curvature and a reduced amount of warpage.
FIG. 11 shows the radius of curvature R of the epitaxial wafer obtained when GaN layers with various thicknesses t are grown on sapphire substrates with various thicknesses Y using the growth method of the present invention and the conventional manufacturing method. The result of having measured is shown. 11A shows a case where a GaN layer is grown on a sapphire substrate having a diameter of 50 mm and a thickness of 350 μm. FIG. 11B shows a case where a GaN layer is grown on a sapphire substrate having a diameter of 100 mm and a thickness of 900 μm. (C) is 150 mm in diameter and 15 in thickness.
This is data when a GaN layer is grown on a 00 μm sapphire substrate.
When the relationship between the growth thickness of the nitride semiconductor crystal grown in the main surface direction of the nitride semiconductor epitaxial wafer and the warp of the nitride semiconductor epitaxial wafer is examined, the relationship is as shown in the following formula (1). I understood it. The radius of curvature of the epitaxial wafer is R (
m), when the thickness of the GaN layer is t (μm) and the coefficient is A,
R = A / t Equation (1)
Can be described.
From these results, when comparing the epitaxial wafer produced by the production method of the present invention and the epitaxial wafer produced by the conventional production method, the sapphire substrate thickness Y
It has been clarified that (μm) and the coefficient A have a correlation as shown in the following formula.
In the case of a GaN layer on sapphire grown by the conventional method, the thickness of the sapphire substrate is set to Y (μm)
The coefficient A is
A ≦ 0.00249 × Y ^1.58483 …… Formula (3)
However, by using the method for manufacturing a nitride semiconductor crystal according to the embodiment of the present invention,
A> 0.00249 × Y ^1.58483 ...... Formula (2)
Thus, it has become clear that the radius of curvature can be further increased.
An epitaxial wafer having a large radius of curvature, that is, a small amount of warpage is advantageous when a light emitting diode or a transistor structure is formed on the GaN layer and a photolithography process is applied thereto. If the epitaxial wafer is greatly warped in the photolithography process, the resolution of the element pattern transferred to the epitaxial wafer deteriorates, making it impossible to form fine elements, and reducing the yield of the photolithography process. This is because there is an adverse effect such as.

Further, even in the nitride semiconductor free-standing substrate realized by the above-described method for manufacturing a nitride semiconductor crystal, impurities higher than those of the nitride semiconductor crystal grown in the main surface direction of the nitride semiconductor free-standing substrate at the outer peripheral edge portion thereof Even if it has or does not have a nitride semiconductor crystal of the concentration, the growth thickness of the nitride semiconductor crystal having a high impurity concentration at the outer peripheral edge is the growth thickness of the nitride semiconductor crystal grown in the main surface direction. Thus, a nitride semiconductor free-standing substrate that is less than one tenth of the above can be obtained.
For this reason, during the growth and cooling, the generation of cracks in the nitride semiconductor crystal is dramatically suppressed, and a high yield can be realized. The meaning of the high impurity concentration here is the same as in the case of the nitride semiconductor epitaxial wafer.

In the above description, a GaN crystal having a Ga-polarity C-plane that has been intentionally grown has been described. However, in principle, the present invention can be applied to AlN, InN, BN, and mixed crystals of these Group III-polar C-planes. ,
This is applicable when all other surfaces are intended growth surfaces. For example, in the range of 0.1 to 2 degrees from the group III polarity C-plane, the surface inclined to the A-axis, the M-axis or the intermediate direction thereof, or the N-polar C-plane, A-plane, M-plane, R-plane, Other semipolar planes, or those N-polar planes and slightly inclined planes of the semipolar planes, can be used as growth planes.

The present invention is also applicable to the case where a nitride semiconductor crystal is grown in a form having an N-polar C-plane as a surface. In this case, the seed crystal substrate has an N-polar surface (surface intended to grow)
And crystal growth is performed thereon. In this case, the end face of the nitride semiconductor crystal has an inclination opposite to that shown in FIG. 2, and the N-polar C-plane expands as it grows. For this reason, the growth of a nitride semiconductor crystal by N-plane growth is a larger-diameter nitride semiconductor substrate (nitride semiconductor free-standing substrate).
This is a very effective method for realizing the above.
However, when the conventional HVPE apparatus is used, even in the N polar plane growth, III
Similar to the growth on the group polar face, stress was generated by the growth on the end face, and it was difficult to obtain a high yield.

However, by using the nitride semiconductor crystal manufacturing method of the present invention, a nitride semiconductor free-standing substrate can be grown with a high yield even in N-plane growth.
Specifically, as shown in FIG. 5, the angle θ formed by the side surface 14 of the side wall 12a of the container 12 and the mounting surface 15 of the bottom wall 12b of the container 12 on which the seed crystal substrate 1 is mounted is more than 90 degrees. The nitride semiconductor crystal 2 is grown while expanding its diameter with a nitrogen surface as a growth surface, in a shape that is in a large range of 135 degrees or less and the cross section inside the container 12 is enlarged toward the opening side. Good.
In particular, in N-plane growth, by setting the side wall so that the angle formed by the installation surface and the side surface of the container opens upward in a range greater than 90 degrees (side surface perpendicular to the installation surface) and not more than 135 degrees,
It is possible to form a nitride semiconductor crystal having an N-plane area larger than that of the seed crystal substrate while performing an etching action on the end portion of the nitride semiconductor crystal. In the case of this arrangement, most of the crystal planes that are likely to appear at the end of the nitride semiconductor crystal have an angle of 135 degrees or less, so when the angle formed by the side wall is larger than 135 degrees, along with the growth of the nitride semiconductor crystal, As the distance between the sidewall and the outer peripheral edge of the nitride semiconductor crystal increases and the growth proceeds, the etching action weakens and the growth of the nitride semiconductor crystal at the outer peripheral edge increases, and cracks are generated due to the stress at the outer peripheral edge. And only the same result as the conventional method is obtained.
When the angle θ is 135 degrees or less, the distance between the side wall 12a and the outer peripheral end of the nitride semiconductor crystal 2 is easily kept constant at a distance that balances growth and etching on the outer peripheral end. The growth rate of the part is kept at a low value, and the generation of cracks is suppressed. In particular, when the opening angle θ of the side wall 12a is 120 degrees or less, there are few stable nitride semiconductor crystal planes having an angle smaller than this, so that the growth rate at the outer peripheral edge can be suppressed to almost zero, High growth yield can be obtained.

As described above, by applying the nitride semiconductor crystal manufacturing method of the above embodiment to the growth of nitride semiconductor crystals, in the growth of thin films on different substrates, a nitride semiconductor layer thicker than before can be cracked. It becomes possible to grow and yields can be improved significantly compared with the conventional method. Further, by applying the nitride semiconductor crystal manufacturing method of the above-described embodiment to the manufacture of a nitride semiconductor free-standing substrate, it is possible to significantly reduce defects due to cracks in the nitride semiconductor crystal. Furthermore, since the nitride semiconductor epitaxial wafer and the nitride semiconductor free-standing substrate of the above embodiment have little residual strain, they are suitable for the production of light emitting diodes and laser diodes, and the production of high electron mobility transistors and heterotero bipolar transistors. .

Note that the nitride semiconductor crystal may be grown by appropriately combining the configurations of the containers and the like shown in FIGS. 3 to 6 used in the above embodiment. For example, a supply pipe for supplying a gas having an etching action as shown in FIG. 4 is provided on the bottom wall 12b of the container 12 shown in FIG. Above the seed crystal substrate 1 shown in FIG.
The nitride semiconductor crystal 2 expanded in the growth direction as shown in FIG.

The present invention will be described in more detail with reference to the following examples (including modifications), but the present invention is not limited to these examples.

[Example 1 and Comparative Example 1]
(Comparative Example 1)
In Comparative Example 1, the vertical arrangement HVPE apparatus shown in FIG. 7 is used to place 2 to 20 on the seed crystal substrate 1 which is a sapphire substrate through a low-temperature grown GaN buffer layer.
A μm GaN layer was grown. The HVPE apparatus is divided into an upper raw material portion 32 and a lower growth portion 33, and a raw material portion peak 30 is grown on the outer periphery of the raw material portion 32 of a reactor (growth furnace) 20 that performs crystal growth. Growing part peaks 31 are provided on the outer periphery of the part 33. The raw material part 32 in the reactor 20 is about 800 ° C. by the raw material part peak 30.
In addition, the growth part 33 in the reactor 20 is heated to 500 to 1200 ° C. by the growth part peak.
Group V line for supplying gas from the raw material part 32 to the growth part 33 (Group V gas supply piping)
23, three gas supply lines are installed: a group III line (group III gas supply pipe) 25, and an etching / dope line (etching gas / dope gas supply pipe) 24. From the group V line 23, hydrogen gas, nitrogen gas or a mixed gas thereof is supplied as a carrier gas together with NH ₃ (ammonia gas) which is a nitrogen source. From Group III line 25, HC
As well as l, hydrogen gas, nitrogen gas or a mixed gas thereof is supplied as a carrier gas. A Ga melt tank 26 for storing metal gallium 27 is installed in the middle of the group III line 25, where HCl gas and metal gallium react to generate GaCl gas, and GaC
l gas is sent to the growth section 33. From the etching / doping line 24, a mixed gas of hydrogen and nitrogen is used during ungrown and undoped GaN layer growth, and dichlorosilane (SiH ₂ Cl ₂ , concentration of 100 ppm by hydrogen dilution) is a Si source during n-type GaN layer growth. HC
l gas, hydrogen gas and nitrogen gas are introduced. Further, hydrogen chloride gas, hydrogen, and nitrogen are introduced from the etching / dope line 24 at the time of baking at a temperature of about 1100 ° C. for removing GaN adhering in the reactor 20 after growth.
The growth section 33 in the reactor 20 has a tray 3 that rotates at a rotational speed of about 3 to 100 rpm.
Is installed horizontally and the installation surface of the tray 3 facing the outlets of the gas supply lines 23 to 25 (
The seed crystal substrate 1 is placed on the (mounting surface) 4. The tray 3 is provided on a rotating shaft (support shaft) 13 disposed in the vertical direction, and the tray 3 is rotated by the rotation of the rotating shaft 13. The source gas is used for GaN growth on the seed crystal substrate 1 and then exhausted to the outside from the most downstream portion of the reactor 20. All growth in the reactor 20 was performed at normal pressure (1 atm) in Comparative Example 1.
The pipes 23, 24, 25 of each line, the Ga melt tank 26, and the rotating shaft 13 of the tray 3 are made of high-purity quartz, and the tray 3 is made of SiC-coated carbon. As a sapphire substrate, the surface has a surface inclined by 0.3 ° in the M-axis direction from the C plane, the thickness is 900 μm, and the diameter is 1
A 00 mm one was used.

The HVPE growth was performed as follows. After setting the sapphire substrate 1 on the tray 3, pure nitrogen is flowed to expel the atmosphere in the reactor 20. Next, in a mixed gas of 3 slm hydrogen gas and 7 slm nitrogen gas, the substrate temperature of the growth portion 33 is set to 1100 ° C.
Hold for 10 minutes. Thereafter, the substrate temperature is set to 550 ° C., and the low temperature growth GaN buffer layer is set to 1
Growth was 20 nm at a growth rate of 200 nm / hour. As gases to be supplied at this time, HCl is 1 sccm from the group III line 25, hydrogen is 2 slm, nitrogen is 1 slm, ammonia is 1 slm and hydrogen is 2 slm from the group V line 23, and hydrogen is 3 s from the etching / dope line 24.
1 m each.
After the growth of the low-temperature grown GaN buffer layer, the substrate temperature was raised to 1050 ° C., and an undoped GaN layer of 2 to 20 mm was grown at a growth rate of 120 μm / hour. As gases to be supplied at this time, HCl is 100 sccm, hydrogen is 2 slm, nitrogen is 1 slm, V from the group III line 25.
Etching / doping line 2 for ammonia 2 slm and hydrogen 1 slm from group line 23
From 4 hydrogen was 3 slm.
After the growth, the substrate temperature was cooled to near room temperature while flowing 2 slm of ammonia and 8 slm of nitrogen. Thereafter, a nitrogen purge is performed for several tens of minutes, and the inside of the reactor 20 is made a nitrogen atmosphere.
The substrate was taken out.

As described above, a plurality of epitaxial wafers having different GaN layer thicknesses in the range of 2 to 20 μm were produced. Of 20 epitaxial wafers of each GaN layer thickness, yield, average value of half width of (0002) diffraction by X-ray diffraction (XRD) measurement of GaN layer, and (10-12) diffraction The average value of the full width at half maximum is indicated by a circle ○ in FIG. Here, the yield was calculated by considering that even one crack having a length of 5 mm or more occurred in the GaN layer was defective.
As shown in FIGS. 8A, 8B, and 8C, the yield is almost 100% until the thickness of the GaN layer is 4 μm, and the half width of the XRD decreases as the thickness of the GaN layer increases. . However, when the thickness of the GaN layer is 5 μm or more, cracks begin to occur and the yield decreases (FIG. 8A). Further, with the occurrence of cracks, the crystallinity of the GaN layer is deteriorated, and the XRD half-value width is increased (FIGS. 8B and 8C).

When the cross section of the GaN layer grown by the conventional HVPE apparatus was observed with a fluorescence microscope (observing light in the visible light region by applying ultraviolet light), regions of GaN crystals with different colors as shown in FIG. 9 were found. It was seen. In the lower part of FIG. 9, a diagram of the contour line of the crystal region in the observation image by the fluorescence microscope in the upper part of FIG. 9 is shown. The observation image by the fluorescence microscope shown in FIG. 9 shows an observation image when the GaN crystal is grown thick on the GaN free-standing substrate, but the main surface of the GaN crystal 2a (
The thickness of the GaN crystal 2b at the outer peripheral edge grown on the plane inclined from the C plane at a position 300 μm from the C plane) is 44 μm, and the thickness of the GaN crystal 2b is 10 times the thickness of the GaN crystal 2a. A large stress was generated at the outer peripheral edge of the GaN crystal, and cracks were observed.
The luminescence itself corresponding to the band gap of GaN cannot be observed with a fluorescence microscope because it is in the ultraviolet region. However, if the impurity concentration is different, the defect level concentration changes, and the difference in color is observed. That is, as described with reference to FIGS. 1 and 2, the two regions having different colors in FIG. 9 reflect the difference in impurity concentration, and are grown on different crystal planes. The difference in impurity concentration is caused by this difference.
Specifically, in the figure, the light gray portion of the GaN crystal 2a grown on the C plane f ₁ and the dark gray portion close to black is the GaN crystal 2b grown on the plane f ₂ inclined from the C plane f ₁ . It is an area. When the impurity concentration of each of these regions was examined by micro-Raman measurement, the GaN crystal 2a grown on the C plane f ₁ was n-type of about 0.5 × 10 ¹⁸ / cm ^{3 to} 5 × 10 ¹⁸ / cm ^3. The GaN crystal 2b grown on the plane f ₂ inclined from the C plane f ₁ is the same n-type, but Ga
The impurity concentration was as extremely high as 1 × 10 ¹⁹ / cm ^{3 to} 5 × 10 ¹⁹ / cm ^{3, which} is more than twice that of the N crystal 2a. Although no doping gas was allowed to flow during growth, it is considered that impurities released from the constituent members of the growth apparatus were taken into the crystal during growth. As a result of SIMS measurement for each of these regions, it was found that these n-type conductivities were due to incorporation of Si and oxygen. From this, the crystal 2b grown on the surface f ₂ inclined from the C plane of the outer peripheral portion has a higher impurity uptake efficiency and a very high impurity concentration than the crystal 2a on the C plane f ₁ . It is presumed that a stress is generated between the crystal 2a grown on the flat portion and this causes a crack to occur as the film thickness increases, resulting in a decrease in yield.

Example 1
In order to suppress the growth of the high impurity concentration GaN crystal 2b that grows on the outer peripheral edge of the GaN crystal in Comparative Example 1, the tray 3 and the rotating shaft that support and rotate the seed crystal substrate 1 in the HVPE apparatus of FIG. The structural portion including the support shaft 13 is remodeled into the same structure as that shown in FIG. 4 to obtain the HVPE apparatus shown in FIG. That is, the HVPE apparatus shown in FIG. 10 has a structure in which the purge gas g can be introduced to the entire outer peripheral portion of the installation surface (mounting surface) 4 of the seed crystal substrate 1. In the HVPE apparatus of FIG. 10, the supply shaft 11 for supplying the rotary shaft 13 and the purge gas g.
Are rotated together.
Above the gas discharge port 19 for the annular purge gas g formed between the outer peripheral surface of the tray 3 and the inner peripheral surface of the side wall 10a, the side wall 10a is provided from the installation surface 4 to a height of 3 mm. The installation surface 4 of the tray 3 and the side surface of the side wall 10a constitute the inner surface of a crucible-shaped or shallow cup-shaped container. The distance between the side surface of the side wall 10a and the outer peripheral end surface of the seed crystal substrate 1 was 5 mm.
Using the HVPE apparatus of Example 1, a GaN layer was grown on the sapphire seed crystal substrate 1 under the same conditions as in Comparative Example 1 above. The flow rate of each line during the growth of the low-temperature grown GaN buffer layer and the undoped GaN layer at 1050 ° C. was the same as in Comparative Example 1 above. However, it differs from Comparative Example 1 in that 3 slm nitrogen was introduced as the purge gas g around the seed crystal substrate 1.

The epitaxial wafers having various thicknesses of GaN layers grown in this manner are used for each G.
Based on yield and X-ray diffraction (XRD) measurements when 20 aN layer thicknesses were produced (
The average value of the half width of (0002) diffraction and the average value of the half width of (10-12) diffraction are indicated by crosses in FIG. In Comparative Example 1 using the HVPE apparatus of FIG. 7 to which the conventional method was applied, the yield dropped sharply when the GaN layer thickness exceeded 5 μm, but in the GaN layer growth by the HVPE apparatus of FIG. The yield was almost 100% until the thickness of the GaN layer was 8 μm. In Example 1, when the thickness of the GaN layer exceeded 8 μm, the yield gradually decreased, but the degree of the decrease was much slower than that of Comparative Example 1, and even at a thickness of 20 μm, the yield was still 15%. A distillate was obtained. In Comparative Example 1, the minimum XRD half-value width is 120 seconds for (0002) diffraction and 350 for (10-12) diffraction when the thickness of the GaN layer where the yield decreases is 5 to 6 μm.
Seconds were obtained. In the thickness exceeding this, the XRD half-width increased. On the other hand, in the GaN layer by the HVPE apparatus of Example 1, the full width at half maximum continues to decrease to a thickness of 15 μm, and the minimum half width is 60 seconds for (0002) diffraction and 150 seconds for (10-12) diffraction. Obtained.
That is, by using the HVPE apparatus shown in FIG. 10 of Example 1, a thicker GaN layer can be grown with a higher yield, and the thicker GaN layer thus grown has improved crystallinity over the conventional one. It was shown that

The result of having observed the cross section of the GaN layer by the HVPE apparatus of Example 1 with the fluorescence microscope is typically shown in FIG. As in the case of FIG. 9 of Comparative Example 1, the GaN crystal 2a and C on the C plane f ₁
There was also in the plane f ₁ from a plane inclined f ₂ on the GaN crystal 2b difference in color observed, the grown GaN crystal in the normal direction d ₂ of the surface f ₂ inclined from the C-plane Even then The thickness of 2b was very thin, and was at most less than 1/10 of the thickness of the GaN crystal 2a grown in the normal direction d ₁ of the C plane f ₁ . That is, as a result of performing nitrogen purge on the outer periphery of the seed crystal substrate 1, the growth raw material in the vicinity of the outer periphery of the seed crystal substrate 1 is diluted, and the etching action by hydrogen gas or HCl gas is strengthened. That is, the growth rate of the GaN crystal 2b on the surface inclined from the C plane is less than 1/10 of the growth rate of the GaN crystal 2a on the C plane.
As a result of micro-Raman measurement on the GaN crystal 2 of Example 1, the impurity concentration of each of the crystals 2a and 2b was the same as that of Comparative Example 1.
From the above results, by using the HVPE apparatus of FIG.
It is considered that the growth of the crystal 2b having a high impurity concentration on the surface inclined from the C-plane of the outer peripheral portion of the metal was suppressed, and cracks were suppressed. As a result, since the GaN layer 2 can be grown thick while preventing cracks, the crystallinity is improved more than before.

Further, when the method of Example 1 is used, the radius of curvature of the epitaxial wafer when a GaN layer is grown on a sapphire heterogeneous substrate and the epitaxial wafer is cooled to room temperature after growth is larger than that of Comparative Example 1. It turns out that you can.
When a GaN layer is grown on a sapphire substrate, when the GaN surface is directed upward, the epitaxial wafer is warped in an upwardly convex manner. For example, according to Comparative Example 1, the diameter is 2 inches and 350
When a GaN layer is grown on a sapphire substrate having a thickness of 8 μm, the amount of warpage (the difference in height between the center and end of the GaN surface) is about 120 μm, and the curvature radius of the wafer at that time is 2.
It is about 6m. When this is used in the method of Example 1, GaN is formed on the same sapphire substrate.
When the layer is grown by 8 μm, the amount of warpage is as small as 50 μm and the radius of curvature is as large as 6 m.

When GaN layers with various thicknesses were grown on sapphire substrates with various thicknesses and compared, the radius of curvature R (m) of the GaN layer on the sapphire substrate was determined by changing the thickness of the GaN layer to t (μm) Then, using the coefficient A,
R = A / t Equation (1)
It became clear that it can be described.

That is, the epitaxial wafer manufactured by the manufacturing method of the present invention is obtained by the above formula (2
), A radius of curvature is large, and an epitaxial wafer with a small amount of warpage is obtained. G above
This is advantageous when a light emitting diode or a transistor structure is formed on the aN layer and subjected to a photolithography process or the like. In the photolithography process, if the epitaxial wafer is greatly warped, the resolution of the element pattern transferred to the epitaxial wafer deteriorates, making it impossible to form fine elements, and the yield of the photolithography process is reduced. There are adverse effects such as.

(Example 2)
In Example 2, in the method of Example 1, the flow rate of the nitrogen gas that is the purge gas g to the outer periphery of the sapphire seed crystal substrate 1 was variously changed in the range of 2.0 slm to 10 slm, and Example 1 was performed. The same experiment was conducted.

When the flow rate of purge nitrogen gas is 2 slm or more, the yield of epitaxial wafers, Ga
Both the crystallinity of N and the radius of curvature of the epitaxial wafer were almost the same as those of Example 1. Further, when the purge nitrogen gas flow rate was between 2 and 5 slm, the growth rate of the surface inclined from the C plane was in the range of less than 1/10 to 0 of the growth rate on the C plane. For this reason, generation | occurrence | production of the stress in an outer peripheral part like a prior art example was suppressed, and generation | occurrence | production of a crack was suppressed.
Further, even when the purge nitrogen flow rate was 6 slm or more, a result almost the same as that of Example 1 was obtained. In this case, however, no growth on the plane inclined from the C-plane was observed even when the cross-section was observed with a fluorescence microscope. In this case, the width of the growth region on the sapphire substrate was smaller than that in the case of the purge nitrogen gas flow rate of 5 slm, and it was determined that etching was generated on the end face of the GaN growth layer instead of growth. The etching rate ranged from a growth rate equivalent to the growth rate on the C-plane to a growth rate of 1/10.
Even if this etching rate is higher, there is no problem from the viewpoint of yield and crystallinity. However, if the etching rate is too high, the size of the finally obtained crystal becomes extremely small. Therefore, it is desirable that the etching rate is practically equal to or lower than the growth rate on the intended growth surface (C plane). I think.

Here, it is considered that when the purge nitrogen gas flow rate is increased, the growth rate on the plane inclined from the C plane is decreased, and further etching occurs. In the growth of GaN in this embodiment, hydrogen is included in the growth atmosphere. GaN is known to be etched by hydrogen at a high temperature, and the growth of GaN is considered to be a result of the growth rate exceeding the etching rate. That is, in the situation of this example, the growth of GaN is performed in an environment where growth and etching coexist, and the outer peripheral portion of the epitaxial wafer is purged with nitrogen to dilute the source gas, so that the outer peripheral portion of the GaN crystal is diluted. The etching action is strengthened, and a decrease in the growth rate and etching are observed.
Since the above etching acts only on the GaN crystal at the outer peripheral edge of the epitaxial wafer, the etching action is rapid when the distance from the inner surface of the side wall of the crucible container is increased within the range of the flow rate of the purge nitrogen gas in this embodiment. It seems to be weakened.
(Reference example)
In the manufacturing method of Example 2, when the flow rate of the purge nitrogen gas was 1 slm or less, the yield of the epitaxial wafer, the crystallinity of GaN, and the radius of curvature of the epitaxial wafer were almost the same as those in Comparative Example 1. .
This is because the growth rate in the normal direction of the plane inclined from the C plane of the epitaxial wafer is 1/5 or more of the growth rate on the C plane because the flow rate of the purge gas g is small. The crystal portion having a high impurity concentration grown in the normal direction of the plane is 1/1 of the crystal growth thickness on the C plane.
Since it became 0 or more, it is considered that the stress was generated in the outer peripheral portion as in the conventional example. Therefore, it is understood that the purge gas g is preferably larger than 1 slm, and preferably 2 slm or more.

(Example 3)
In Example 3, the same experiment as in Example 2 was performed by changing the purge gas to the outer peripheral portion of the epitaxial wafer to argon and helium, and almost the same result as in Example 2 was obtained. From this result, it is considered that the effect of the present invention can be obtained by using an inert gas other than nitrogen, argon or helium as the purge gas.

Example 4
In Example 4, the same experiment as in Example 2 was performed using a gas for etching GaN such as hydrogen, chlorine, and hydrogen chloride as a purge gas to the outer peripheral portion of the epitaxial wafer. As a result, although the range of the purge gas flow rate is different from that of Example 2, the purge gas flow rate is appropriately adjusted so that the growth rate on the plane inclined from the C plane is less than 1/10 of the growth rate on the C plane. As a result, the same result as in Example 2 was obtained.
Moreover, the method of Example 4 uses the amount of hydrogen contained in the gas supplied as the raw material gas in Examples
There is an advantage that it can be reduced compared to the case of 3. For this reason, there is an advantage that the etching action on the growth surface intended as compared with Examples 1 to 3 is reduced and the raw material efficiency is improved.

(Example 5)
In Example 5, a cylindrical metal ring having a height of 3 mm is additionally installed on the tray 3 of the HVPE apparatus shown in FIG. 7, and the inner peripheral surface of the ring and the installation surface 4 of the tray 3 The inner surface of the crucible-shaped container was constructed. The material of the ring is Ti (titanium) and H before growth.
In the VPE apparatus, nitriding treatment was performed in 2 slm of ammonia and 8 slm of hydrogen for 2 hours, and the surface of the ring was changed to titanium nitride to perform crystal growth.
When an experiment similar to that in Comparative Example 1 was performed using the HVPE apparatus to which this ring was added, the same effects of yield improvement, crystallinity improvement, and curvature radius increase as in Example 1 were obtained. In this case, etching occurs on the surface inclined from the C plane, and the etching rate is about one third of the growth rate on the C plane.

In Example 5, the purge gas was not supplied to the outer peripheral portion of the epitaxial wafer, but the same effect as the purge gas supply was obtained because the metal nitride of the ring constituting the side wall of the container became a catalyst, This is considered to be because atomic hydrogen having a strong etching action was generated by decomposing hydrogen contained in.
For example, when the side wall is formed of a ring made of quartz or carbon, cracks were not suppressed. Even when a metal nitride ring was used, when the total flow rate of hydrogen in the source gas was set to 1 slm or less, the growth rate on the plane inclined from the C plane increased and cracks occurred. From the above results, it is shown that the presence of metal nitride and a certain amount of hydrogen are necessary, and the idea of the expression of etching action due to generation of atomic hydrogen by the catalytic effect of metal nitride is supported.
When the hydrogen flow rate in the source gas is changed in the range of 2 to 7 slm, the growth rate (etching rate) on the surface inclined from the C plane is the same as when the purge gas flow rate in Example 2 is changed.
However, the growth rate is less than one-tenth of the growth rate on the C plane, and the etching rate is less than or equal to the growth rate on the C plane. The effect of improving the yield and improving the crystallinity was observed.

(Example 6)
In Example 6, the same experiment as in Example 5 was performed, and the metal material of the ring constituting the container side wall was changed to Zr.
, Nb, Ta, Cr, W, Mo, or Ni. As a result, the same result as in Example 5 was obtained.

(Example 7)
In Example 7, the same experiment as in Examples 1 to 6 was performed, and dichlorosilane (SiH ₂ Cl ₂ ) was introduced from the etching / dope line 24 during the growth of 2 to 3 μm at the top of the GaN layer, and 0.5
An n-GaN layer having an impurity concentration of × 10 ¹⁸ / cm ^{3 to} 5 × 10 ¹⁸ / cm ³ was grown. The background impurity concentration differs depending on the growth conditions (undoped impurity concentration) and dichlorosilane doping is used in combination. In this case, the same results as in Examples 1 to 6 were obtained.

(Example 8)
In Example 8, experiments similar to those in Examples 1 to 7 were performed by changing the growth temperature, gas flow rate, growth rate, and growth pressure in various ways. Although the yield and XRD half-value width obtained are slightly different from those in the above examples, the growth rate is less than one-tenth of the growth rate of the C-plane. The same results as in Examples 1 to 7 were obtained, indicating that the yield was improved and the crystallinity was improved.
Throughout the experiments so far, the angle formed between the Ga-polar C-plane of the GaN growth layer and the plane inclined from the C-plane generated on the outer periphery is the M-plane at the end of the GaN crystal facing the M-axis direction. The angle formed between the C plane and the plane inclined from the C plane was about 110 degrees to 135 degrees, as shown in FIG.

Example 9
In Example 9, the same experiment as in Examples 1 to 8 was performed, and the distance between the GaN crystal and the side wall was changed to 0.5 to 20.
The change was made within the range of mm. If the above distance is less than 1 mm, during growth,
In some cases, the GaN crystal end face of the epitaxial wafer was in contact with the container side wall. in this case,
Due to the fact that the GaN crystal and the side wall of the end face of the epitaxial wafer were fixed, stress was generated during growth, and cracks were likely to occur. In addition, when the distance is larger than 10 mm, it becomes difficult to locally apply an etching action only to the edge portion of the epitaxial wafer. The reduction of was noticeable.
From the above, it was determined that 1 to 10 mm was appropriate as the distance between the GaN crystal and the side wall.

From the above examples, in order to grow a GaN layer with high yield and high crystallinity (a narrow XRD half width), a seed crystal substrate is placed in a crucible-shaped container having a side wall, and decreases with distance. The distance between the etching side wall and the outer periphery of the GaN crystal is maintained in the range of 1 to 10 mm, that is, the GaN crystal having a shape substantially following the inner shape of the crucible-shaped container is formed on the inner surface of the container at the start of growth. It can be concluded that it is important to grow without contact with the parts that were not in contact with the whole period of growth. In addition, G realized by the above embodiment
As a feature of the aN layer, the growth thickness of the crystal portion having a high impurity concentration in the outer peripheral portion of the crystal on the surface on which the growth is intentionally performed (mainly Ga-polar C-plane in the embodiment) is intentionally grown. Special mention is made of the fact that it does not have a thickness of 1/10 or more of the growth thickness of the crystal on the surface subjected to the above. Also,
The point that the GaN epitaxial wafer according to the above embodiment has a larger radius of curvature than the conventional method is also an important advantage in device application.

[Example 10 and Comparative Example 2]
In Example 10 and Comparative Example 2, the void formation peeling method (VAS method) described in Patent Document 3 above.
A GaN free-standing substrate was manufactured.

(Comparative Example 2)
FIG. 13 shows an outline of the VAS method. First, a void substrate 40 was prepared as a seed crystal substrate (FIG. 13A). The void substrate 40 is formed on the sapphire substrate 41 by metal organic chemical vapor deposition (MOV).
A GaN thin film having a thickness of about 300 nm is grown by a PE method, and a Ti film is deposited on the surface.
Obtained by heat treatment in hydrogen or ammonia. By the heat treatment, the Ti film is converted into a TiN layer 43 having a network structure, and a void-containing GaN layer 43 in which a large number of voids 44 are formed in the GaN thin film is obtained.
Next, a thick GaN layer 45 is grown on the void substrate 40 by the HVPE method (FIG. 13B
)) After that, the sapphire substrate 41 is peeled off from the void portion to form a GaN free-standing substrate.
An aN crystal (GaN single crystal) 46 was obtained (FIG. 13C).
The sapphire substrate 41 has a surface inclined in a range of 0.05 to 2 ° in the direction of the A-axis or M-axis from the C-plane or in the direction therebetween, the thickness is 300 to 1500 μm, and the diameter is 35
The one of ~ 200 mm was used. The thickness of Ti at the time of manufacturing the void substrate was set to 5 to 100 nm.

As conditions for the HVPE growth, for example, the substrate temperature is 800 to 1200 ° C., and the pressure is 10 kPa.
With a growth rate of 30 to 1000 μm / hour at ˜120 kPa, on the void substrate 40,
A GaN single crystal having a diameter of 35 to 200 mm and a thickness of 50 μm to 10 mm was manufactured. As the growth apparatus, the HVPE apparatus shown in FIG. 7 was used. The flow rate of each line was in the following range. From group III line 25, HCl was added to 25 to 1000 ccm and hydrogen was added to 2 slm, and nitrogen was adjusted to a total flow rate of 3 slm. 1 to 2 sl of ammonia from group V line 23
m and hydrogen were added to 1 slm, and nitrogen was adjusted to a flow rate at which the total flow rate of the group V line 23 was 3 slm. Further, 3 slm of hydrogen was passed from the etching / dope line 24.

The dislocation density of the GaN single crystal formed on the void substrate is determined by the thickness of Ti when the void substrate is manufactured. The thinner the Ti film, the higher the dislocation density because dislocations in the void-containing GaN layer 43 grown by MOVPE on the void substrate are more easily propagated to the thick GaN single crystal 45 formed thereon. The dislocation density of the GaN single crystal obtained when the Ti film thickness is 5 to 100 nm is 1
In the range of ^{× 10 4 / cm 2 ~1 ×} 10 8 / cm 2.
The obtained GaN crystals were mirror surfaces with almost no pits on the surface after the growth was completed. The electron concentration in the GaN crystal was adjusted to 1 × 10 ¹⁵ / cm ^{3 to} 5 × 10 ¹⁸ / cm ³ by adjusting the flow rate of undoped growth or dichlorosilane added during growth.

The combination of these various conditions yields a yield of GaN crystals (GaN layer) with a thickness of 50 μm to 10 mm (a non-defective ratio when 20 pieces are grown under the same conditions), and a crack with a length of 5 mm or more is generated. The growth thickness dependence of the defect defined as defective is indicated by a circle ○ in FIG. The yield hardly depended on the carrier concentration or the dislocation density, but strongly depended only on the thickness of the GaN crystal. When the thickness of the GaN crystal was 100 μm or less, a yield of nearly 100% was obtained.
When the thickness of the N crystal exceeds 100 μm, the yield decreases rapidly, and the thickness of G exceeds 800 μm.
The yield of the aN free-standing substrate (GaN crystal) was less than 10%.

When cross sections of these GaN free-standing substrates (GaN crystals) were observed with a fluorescence microscope, regions of different colors similar to those of Comparative Example 1 were observed at the end portions of the GaN free-standing substrates as shown in FIG. The growth rate in the direction was 1/10 or more of the growth rate perpendicular to the main surface. When the GaN crystal in each of these parts was investigated by the micro-Raman method, the impurity concentration in the GaN crystal grown on the inclined surface of the free-standing substrate (GaN crystal) end and the GaN crystal grown on the Ga-polar C-plane However, the GaN crystal on the C plane had an n-type of about 0.5 × 10 ¹⁸ / cm ^{3 to} 5 × 10 ¹⁸ / cm ³ , but the same for the GaN crystal grown on the plane inclined from the C plane. although the n-type, has been a very high impurity concentration of ^{1 × 10 19 / cm 3 ~5} × 10 19 / c m 3 of more than twice.

(Example 10)
On the other hand, in Example 10, as in Example 5, an HVPE apparatus in which a metal nitride ring serving as a catalyst was installed on the tray 3 in FIG. 7 or an HVPE apparatus shown in FIG.
In the same manner as in Examples 1 to 9, the surface of the surface inclined from the C plane is obtained by the method of the present invention in which dilution gas, etching gas or hydrogen gas is introduced into the outer periphery of the void substrate 40 which is a seed crystal substrate. When the growth rate of the GaN crystal was set to be less than 1/10 of the growth rate of the C-plane GaN crystal, the yield was dramatically improved as shown by the crosses x in FIG. The thickness of the GaN layer is 1500μ
Up to m, a yield of almost 100% was obtained, and the yield of 10% was maintained even with the thickest 10 mm.

However, the above result is limited to the case where the surface of the GaN layer 45 is at a position lower than the height of the side wall of the container, and when the growth surface of the GaN layer 45 is higher than the height of the side wall,
Yield dropped sharply. This is because when the surface of the GaN layer 45 is higher than the side wall, the etching action at the outer peripheral portion of the GaN crystal is weakened, and stress is generated because growth occurs at the outer peripheral portion. In addition, when the distance between the GaN crystal 45 and the side wall is smaller than 1 mm, the side wall and the GaN crystal are fixed, cracks are generated, and the yield is deteriorated. When the distance between the GaN crystal 45 and the side wall is larger than 10 mm, the yield does not decrease, but it becomes difficult to localize the etching action only on the end face, and the growth region of the GaN crystal is greatly reduced. It was.
Further, as described in Example 8, the angle formed between the Ga-polar C-plane of the GaN growth layer and the plane inclined from the C-plane generated on the outer periphery is the end portion of the GaN crystal facing the M-axis direction. The M plane is formed and tends to be 90 degrees, and at the other end, as shown in FIG. 12, an inclined plane with an angle between the C plane and the plane inclined from the C plane of about 110 to 135 degrees is likely to occur. It was. However, this angle can be set to 90 degrees on the entire outer periphery by appropriately selecting the growth conditions.

In order to obtain a thick GaN free-standing substrate with good yield from Example 10 and Comparative Example 2, the seed crystal substrate is placed in a crucible-shaped container having a side wall, and the side wall and the outer periphery of the substrate having an etching action that weakens with distance. Maintaining the distance in the range of 1 to 10 mm, that is, the portion of the inner surface of the vessel that did not contact the seed crystal substrate at the start of growth of the GaN crystal having a shape generally following the inner shape of the crucible, and the entire growth period It is concluded that it is important to grow without contact through. Further, as a feature of the GaN free-standing substrate realized by the present embodiment, the growth thickness of the crystal portion having a higher impurity concentration than the crystal on the C plane on the outer periphery of the GaN free-standing substrate It is worth mentioning that it does not have a thickness of 1/10 or more.

[Example 11 and Comparative Example 3]
In general, a GaN free-standing substrate realized on a heterogeneous substrate by various methods including the VAS method has a thickness direction of 10 ⁸ / cm ² to 10 ⁵ / cm ² for a growing GaN crystal. A significant reduction in dislocation density occurs. Residual strain is introduced into the GaN crystal as the dislocation density is reduced. Therefore, in many cases, strain remains in a GaN free-standing substrate grown on such a heterogeneous substrate. Typically, in the case of a GaN free-standing substrate having a C-plane as a surface, the C-plane has a radius of curvature of 2 to 4 m immediately after growth.
As shown in FIG. 8 of Example 10 and the like, the presence of this strain causes the yield to decrease due to the generation of cracks when the GaN growth thickness exceeds 1500 μm even when the method of the present invention is used. ing.
Such residual strain can be greatly reduced by polishing and removing the back surface of the GaN free-standing substrate obtained by the VAS method or the like. For example, if the back surface is removed by 1000 μm after growing a 1500 μm-thick GaN free-standing substrate, the radius of curvature of the C-plane that was 2 to 4 m immediately after the growth increases to 10 m or more. If such a GaN free-standing substrate whose back surface is polished is used as a seed crystal, the residual stress becomes extremely small, and it can be expected that a GaN layer that is too thick to grow on a heterogeneous substrate is impossible.

(Comparative Example 3)
In Comparative Example 3, 150 grown according to the method of the present invention in Example 10 by the VAS method was used.
The back surface (N-polar C surface) of a 0 μm-thick GaN free-standing substrate is polished by 1000 μm, and the front surface side (
A 400 μm-thick GaN free-standing substrate with 100 μm polished (Ga-polar C-plane) to improve flatness was used as a seed crystal. Then, a GaN layer was grown on the Ga polar side surface of the GaN free-standing substrate by the HVPE method.
As a GaN substrate to be a seed crystal, the surface is Ga-polar C-plane to A-axis or M-axis direction,
Or it has a surface inclined in the range of 0.05 to 2 ° in the middle direction, and the thickness is 400m
m and a diameter of 35-200 mm were used. Typical dislocation density is 1 × 10 ⁶ / c
m ² .

As conditions for the HVPE growth, for example, the substrate temperature is 800 to 1200 ° C., and the pressure is 10 kPa.
A GaN single crystal having a thickness of 50 μm to 100 mm was manufactured at a growth rate of 30 to 1000 μm / hour at −120 kPa. As the growth apparatus, the HVPE apparatus shown in FIG. 7 was used. The flow rate of each line was in the following range. From group III line 25, HCl was added to 25 to 1000 ccm and hydrogen was added to 2 slm, and nitrogen was adjusted to a total flow rate of 3 slm. V
From the group line 23, ammonia was added to 1 to 2 slm and hydrogen was added to 1 slm, and nitrogen was adjusted to a flow rate at which the total flow rate of the group V line 23 was 3 slm. Further, 3 slm of hydrogen was passed from the etching / dope line 24.
The GaN single crystal (seed crystal) used here was a mirror surface with almost no pits on the surface after the growth by the VAS method. The electron concentration in the GaN crystal is 1 × 10 ¹⁵ / cm ³ by adjusting the flow rate of undoped growth or dichlorosilane added during growth.
The thing of -5 * 10 < ¹⁸ > / cm < ³ > was prepared.

By combining these various conditions, a GaN crystal having a thickness of 50 μm to 100 mm (GaN
The circle thickness (circle) in FIG. 15 shows the growth thickness dependence of the yield (defect when a crack with a length of 5 mm or more occurs) when the layer is grown. Also in this case, the yield hardly depends on the carrier concentration or the dislocation density, but strongly depends only on the thickness of the GaN crystal. When the thickness of the GaN crystal newly grown on the GaN crystal (seed crystal substrate) by the VAS method is 500 μm or less, a yield of nearly 100% was obtained, but the thickness of the growth layer of the GaN crystal exceeds 1 mm. Yield decreases rapidly, 10
The yield of a GaN free-standing substrate (GaN crystal) with a thickness exceeding mm was less than 10%.

When cross sections of these GaN free-standing substrates were observed with a fluorescence microscope, as schematically shown in FIG. 2, regions of different colors similar to those in FIG. 9 were observed at the ends, and growth in directions perpendicular to the respective surfaces was observed. The speed was almost the same. When these portions were investigated by the micro-Raman method, the impurity concentration was different between the crystal grown on the inclined surface of the wafer edge and the crystal grown on the Ga-polar C-plane, and 0 for the crystal on the C-plane. Although the n-type is about 5 × 10 ¹⁸ / cm ^{3 to} 5 × 10 ¹⁸ / cm ^3, the crystal grown on the plane inclined from the C-plane is the same n-type, but more than twice that 1 ×
The impurity concentration was extremely high, 10 ¹⁹ / cm ^{3 to} 5 × 10 ¹⁹ / cm ³ .

(Example 11)
On the other hand, in Example 11, as in Example 5, the HVPE apparatus in which a metal nitride ring serving as a catalyst was installed on the tray 3 in FIG. 7 or the HVPE apparatus shown in FIG.
Similarly to 9 and the like, a GaN crystal was grown by the method of the present invention in which a dilution gas, an etching gas or a hydrogen gas was introduced into the outer periphery of the GaN substrate which is a seed crystal substrate. When the growth rate of the plane inclined from the C plane is less than 1/10 of the growth rate of the intended growth plane in the same manner as in Examples 1 to 9, the yield is as shown by a cross mark x in FIG. Dramatically improved. A yield of almost 100% is maintained until the thickness of the newly grown GaN crystal on the GaN crystal (seed crystal substrate) by the VAS method is 5 mm. Even when the thickness of the growth layer of this GaN crystal is 100 mm, it is still % Yield was maintained.

FIG. 16 schematically shows a result of observing a cross section of an outer peripheral end portion of a GaN free-standing substrate grown using the GaN free-standing substrate as a seed crystal using Example 11 with a fluorescence microscope. Although there is a crystal 2b having a high impurity concentration on the end face, the amount of the crystal 2b is small, and the crystal 2b has a maximum growth rate of the crystal 2b on the C plane.
It was estimated to be less than 1/10 of the growth rate of a. In addition, when the growth thickness of the GaN crystal 2 is thin, a crystal plane f ₂ inclined from the C plane is generated at the end as the crystal grows, so that the area of the C plane f ₁ is gradually reduced. However, the C-plane f ₁ does not continue to shrink, and after the GaN crystal 2 is grown to a certain extent (about 2 mm at the maximum) and the distance between the end face and the side wall becomes a certain value,
Growth proceeded while maintaining the shape and area of the C surface constant, with the end surface forming a surface f ₃ perpendicular to the C surface at 90 degrees. This is a phenomenon that occurs when the distance between the end surface and the inner surface of the side wall of the container is increased, the etching action from the side wall is weakened, and the growth and etching at the end surface are balanced.

However, the above result is a story only when the surface of the GaN crystal 2a growing on the C plane is at a position lower than the height of the side wall of the container, and the growth surface of the GaN crystal 2a becomes higher than the height of the side wall. The yield dropped sharply. This is because when the surface of the GaN crystal 2a is higher than the side wall, the etching action at the outer peripheral portion of the GaN crystal 2 is weakened, and stress is generated because crystal growth occurs at the outer peripheral portion. In addition, even when the distance between the GaN crystal and the side wall was smaller than 1 mm, the side wall and the GaN crystal were fixed, cracks were generated, and the yield deteriorated. GaN
When the distance between the crystal and the side wall is larger than 10 mm, the yield does not decrease, but it becomes difficult to localize the etching action only on the end face, and the growth region of the GaN crystal is greatly reduced.

In order to obtain a thick GaN free-standing substrate with good yield from Example 11 and Comparative Example 3, even when a nitride semiconductor free-standing substrate is used as a seed crystal, the seed crystal substrate is placed in a crucible-shaped container having a side wall. It is installed, and the distance between the side wall having an etching action that weakens with distance and the outer periphery of the GaN crystal is maintained in the range of 1 to 10 mm, that is, G having a shape substantially following the inner surface shape of the container.
It is concluded that it is important to grow the aN crystal without contacting the portion of the inner surface of the vessel that was not in contact with the seed crystal substrate at the start of growth throughout the entire growth period. Further, as a feature of the GaN free-standing substrate realized by the present embodiment, the growth thickness of the crystal portion having a higher impurity concentration than the crystal on the C plane on the outer periphery of the GaN free-standing substrate It is worth mentioning that it does not have a thickness of 1/10 or more.

(Example 12)
In Example 11, the yield decreased when the GaN growth thickness exceeded 5 mm because a slight strain remained on the GaN free-standing substrate used as the seed crystal substrate. there,
In Example 12, not only the outer peripheral end face of the GaN crystal during the growth but also the back surface of the GaN free-standing substrate of the seed crystal substrate where the strain remained was added to try to further improve the yield.

The method used in Example 12 is the same method as in Example 11, except that the back surface of the seed crystal substrate is made of quartz, carbon, or metal nitride having a height of 1 to 2 mm (the figure of the above embodiment). The block 17) as shown in FIG. 6 is installed, the seed crystal substrate is floated from the installation surface (for example, the bottom surface of the container), and an etching action is also generated on the back surface of the seed crystal substrate. For example, a purge gas outlet is also provided at the center of the installation surface, and an etching gas such as hydrogen, chlorine, or hydrogen chloride is introduced from there, or the installation surface is made of metal nitride, and hydrogen is added to the source gas. The method of was used.

The etching rate of the back surface of the seed crystal substrate by these methods changed depending on the temperature, growth atmosphere, growth pressure, purge gas flow rate, etc., but the etching rate was 1/100 or more of the growth rate of Ga-polar C-plane In addition, a higher yield than Example 11 could be obtained. The yield increased as the back surface etching rate increased, but became constant when the back surface etching rate was 1/20 or more of the growth rate of the Ga-polar C-plane.
FIG. 17 shows the relationship between the yield and the thickness of the GaN growth layer when the back surface etching rate is 1/20 of the growth rate of the Ga-polar C-plane. Even when the GaN growth thickness is 100 mm, a high yield of 90% is still maintained.

(Example 13)
In Example 13, the same experiment as in Examples 11 and 12 was carried out to grow GaN using the N-polar C-plane as the growth surface instead of the Ga-polar C-plane to produce a GaN free-standing substrate.
In this case, when a conventional HVPE apparatus is used, the end face of the GaN crystal has an inclination opposite to that in FIG. 16, and the C plane expands as the growth proceeds. For this reason, the growth of a GaN free-standing substrate by N-plane growth is a very effective method for realizing a larger-diameter GaN substrate.
However, when the conventional HVPE apparatus is used, in the N-plane growth, stress is generated by the grown crystal on the end face as in the Ga-plane growth, and it is difficult to obtain a high yield.

However, by using the method of the present invention, it was confirmed that a GaN free-standing substrate can be produced with a high yield in the N-plane growth as in Examples 11 and 12. Furthermore, in this case
By balancing the N-plane expansion tendency and the etching action, it was possible to obtain a cylindrical free-standing substrate in which the GaN crystal was grown while the area of the N-plane following the cylindrical container shape was constant. Such a free-standing substrate is an industrially very useful form because a wafer having a certain diameter can be efficiently produced by slicing the substrate.

Particularly in the N-plane growth, for example, as in the above-described embodiment shown in FIG.
The angle θ formed by the side surface of the side wall 12a and the mounting surface 15 of the bottom wall 12b of the container 12 on which the seed crystal substrate 1 is mounted is in the range of 90 degrees to 135 degrees, and the side surface of the side wall 12a is the opening. By opening it toward, it is possible to enlarge the area of the N plane from the seed crystal substrate 1 while performing etching on the end of the GaN crystal 2.
When the angle θ formed between the side surface of the container and the mounting surface of the container is larger than 135 degrees, the crystal plane that tends to come out at the edge of the GaN crystal has an angle of 135 degrees or less. The distance between the outer peripheral edge of the GaN crystal and the GaN crystal grows at the outer peripheral edge of the GaN crystal,
This GaN growth makes it easy for cracks to occur, and only results equivalent to the conventional method can be obtained.
When this angle θ is 135 degrees or less, the distance between the container side surface and the outer peripheral edge of the GaN crystal is Ga
Since the distance between the growth to the N crystal edge and the etching is easily kept constant, the growth rate of the outer peripheral edge of the GaN crystal is kept almost zero, and the generation of cracks is suppressed. In particular, when the angle θ is 120 degrees or less, since there are few stable GaN crystal planes having an angle smaller than this, a higher growth yield can be obtained, and the above-described embodiments shown in FIGS. The result is almost the same as the result of.

Next, modifications of the present invention will be described below.
(Modification 1)
In Modification 1, the same experiment as in Examples 1 to 9 was performed, and the diameter of the sapphire substrate was 50 to 200 mm.
The surface (main surface) of the sapphire substrate is A axis, M within the range of 0.1 to 2 degrees from the Ga-polar C-plane.
Examples 1 to 9 were also performed in the case of a surface inclined in the direction of the axis or in the middle thereof, the A surface, the M surface, the R surface, other semipolar surfaces, or a slightly inclined surface of these surfaces. And almost the same result was obtained.

(Modification 2)
In Modification 2, the same experiment as in Modification 1 was performed by changing the sapphire substrate to an SiC substrate and an Si substrate, but the same effect as in Modification 1 was confirmed.

(Modification 3)
In Modification 3, the same experiment as in Examples 1 to 9 was performed by changing the buffer layer from the low temperature growth GaN buffer layer to the low temperature growth AlN buffer layer and the high temperature growth AlN buffer layer. The thickness of each buffer layer was between 10 nm and 2 μm. In any case, the same results as in Examples 1 to 9 were obtained.

(Modification 4)
In Modification 4, the same experiment as in Examples 1 to 9 was performed using a seed crystal substrate in which the top surface of the seed crystal substrate was subjected to uneven processing. As the shape of the concavo-convex processing, the height of the convex portion is 0.1 to 2 μm, the interval is 1 to 10 μm, the shape is a bowl shape, a conical shape, a triangular pyramid shape to a hexagonal pyramid shape, and the top thereof A shape having a flat portion was used. In addition, as the arrangement of the convex portions, the ones arranged at the positions of the meshes of a triangular lattice shape or a quadrangular lattice shape and the sides of the lattice face the A axis or the M axis were used. Advantages of forming a light-emitting element with a nitride semiconductor layer using a seed crystal substrate subjected to these irregularities as a base is higher in light extraction efficiency than a light-emitting element when using a flat seed crystal substrate There is.
In any case of Modification 4, when the method for producing a nitride semiconductor crystal of the present invention was applied, the same effects as in Examples 1 to 9 were obtained.

(Modification 5)
In the modified example 5, the same experiment as in the examples 10 to 13 was performed for the A plane, the M plane, the R plane, other semipolar planes, and their slightly inclined planes. Similar results to those of ˜13 were obtained.

(Modification 6)
The principle of the method for producing a nitride semiconductor crystal of the present invention is applicable not only to the HVPE method but also to the MOVPE method, the low temperature synthesis method, and the Na flux method as the growth method.

(Modification 7)
The method for producing a nitride semiconductor crystal according to the present invention includes a nitride semiconductor material other than GaN, for example, A
The present invention is also applicable to mixed crystals of these materials including lN, InN, BN, and GaN.

(Modification 8)
The principle of the method for producing a nitride semiconductor crystal of the present invention can be applied to a semiconductor other than a nitride semiconductor or a crystalline material other than a semiconductor.

(Modification 9)
The method of the present invention is a vertical growth crystal growth apparatus (HVPE apparatus, as shown in FIGS. 7 and 10).
MOVPE apparatus with a horizontal flow arrangement shown in FIG.
It can also be applied to PE equipment. That is, as shown in FIG. 18, a rectangular cylindrical reactor (growth furnace) 50 is horizontally arranged, and a container 51 having a side wall 51 a is provided at the opening of the bottom wall of the reactor 50. The tray 3 is set apart from the bottom wall 51 b of the container 51.
A supply pipe 53 for supplying a gas g having an etching action is connected to the bottom wall 51b, and a rotating shaft 52 is provided through the supply pipe 53. The tray 3 is rotatably supported on the rotating shaft 52, and a peak (not shown) is provided on the outer peripheral portion of the reactor 50. The source gas G flows horizontally in the reactor 50 from one end to the other, and crystals grow on the seed crystal substrate 1 installed on the tray 3. On the other hand, the gas g supplied from the supply pipe 52 into the container 51 is
It flows radially along the tray 3 between the tray 3 and the bottom wall 10b, and flows out from the gap between the outer peripheral surface of the tray 3 and the inner peripheral surface of the side wall 10a.
In the method of the present invention, for example, a plurality of seed crystal substrates are arranged along the same circumference on the susceptor, and the plurality of seed crystal substrates on the susceptor are rotated and revolved along the susceptor from the center of the susceptor. The present invention can also be applied to a crystal growth apparatus in which a source gas is allowed to flow radially on various crystal substrates, that is, a center blowing autorevolution type multi-charge type crystal growth apparatus.
Further, not only the face-up type arrangement in which the growth surface faces upward as shown in FIGS. 7, 10, and 18, but also the face-down type arrangement in which the growth surface faces downward, The present invention can also be applied to a crystal growth apparatus that faces an inclined direction by devising a method for holding the seed crystal substrate. However, when etching the back surface of the seed crystal substrate, if all the seed crystal substrate is etched, the crystal will be displaced or dropped from the position at the start of growth. It is necessary to keep it down.

1 Seed crystal substrate (dissimilar substrate or nitride semiconductor free-standing substrate)
2 Nitride semiconductor crystal (GaN crystal)
2a Nitride semiconductor crystal grown on C plane 2b Nitride semiconductor crystal grown on plane inclined from C plane 3 Tray 4 Placement surface (installation surface)
5, 10, 12 Container 5a, 10a, 12a Side wall 5b, 10b, 12b Bottom wall 6, 14 Side surface 7, 15 Bottom surface 8 Peripheral edge of nitride semiconductor crystal 9 Plane surface (C surface)
11, 18 Purge gas supply pipe 17 Block f ₁ C surface f ₂ Surface inclined from C surface G Raw material gas g Purge gas (gas having etching action)

Claims (8)

A nitride semiconductor crystal manufacturing method for growing a nitride semiconductor crystal on a seed crystal substrate, wherein the nitride semiconductor crystal is grown while an etching action is applied to an outer peripheral end portion of the seed crystal substrate during the growth of the nitride semiconductor crystal. To grow a semiconductor crystal ,
Placing the seed crystal substrate in a container having a side wall surrounding the outside of the seed crystal substrate;
The nitride semiconductor crystal is grown while supplying a material that generates etching species by the action of the catalyst to the source gas, and at least a part of the inner surface of the vessel not in contact with the nitride semiconductor crystal is formed on the catalyst. A method for producing a nitride semiconductor crystal, in which the etching action is locally exhibited on the wafer outer periphery of the nitride semiconductor crystal by using a catalytic substance having an action . The inner surface of the container not in contact with the nitride semiconductor crystal includes a side surface of the side wall.
The method for producing a nitride semiconductor crystal according to claim 1 . Environment around the portion of the inner surface of said container that is not in contact with the nitride semiconductor crystal, Ru environment near where the etching action weakens with distance from the side surface of the side wall
The method for producing a nitride semiconductor crystal according to claim 1 . The substance that generates etching species by the action of the catalyst is hydrogen gas.
The method for producing a nitride semiconductor crystal according to any one of claims 1 to 3 . Catalyst material having the action of said catalyst, Ru nitride der metals or metal
The method for producing a nitride semiconductor crystal according to claim 1 . The metal is any one of Ti, Zr, Nb, Ta, Cr, W, Mo, and Ni.
The method for producing a nitride semiconductor crystal according to claim 5 . The distance between the inner surface of the etching action of the container that is not in contact with the nitride semiconductor crystal is generated and the nitride semiconductor crystal, the period to the end from the crystal growth starting in the range of 1~10mm
The manufacturing method of the nitride semiconductor crystal of any one of Claim 1 to 6 . The angle formed between the side surface of the side wall and the placement surface of the container on which the seed crystal substrate is placed is in the range of greater than 90 degrees and less than or equal to 135 degrees, and the cross section inside the container is directed toward the opening side. Expanded shape,
The nitride semiconductor crystal is grown while expanding its diameter and nitrogen surface as a growth surface
The method for producing a nitride semiconductor crystal according to claim 2 .