Freestanding substrate for epitaxial crystal growth and functional element

Abstract

[Problem] To prevent, in a free-standing substrate for epitaxial crystal growth comprising a group 13 element nitride crystal selected from gallium nitride, aluminum nitride, indium nitride, and a mixed crystal thereof, the occurrence of cracks in the free-standing substrate or an epitaxial crystal during a step for growing the epitaxial crystal. [Solution] A free-standing substrate 21 for epitaxial crystal growth comprising a group 13 element nitride crystal selected from gallium nitride, aluminum nitride, indium nitride, and a mixed crystal thereof has a nitrogen polar surface 21a and a group 13 element polar surface 21b. The nitrogen polar surface 21a is curved in a convex shape, and a chamfer part 21c is provided to the outer peripheral part 21c of the nitrogen polar surface 21a.

Description

[Technical field]

[0001] The present invention relates to a free-standing substrate for epitaxial crystal growth and a functional element. [Background technology]

[0002] As light-emitting elements such as light-emitting diodes (LEDs) using single crystal substrates, those in which various gallium nitride (GaN) layers are formed on sapphire (α-alumina single crystal) are known. For example, those having a structure in which an n-type GaN layer, a multiple quantum well layer (MQW) in which a quantum well layer made of an InGaN layer and a barrier layer made of a GaN layer are alternately laminated on a sapphire substrate, and a p-type GaN layer are sequentially laminated on the sapphire substrate are being mass-produced. In addition, laminated substrates suitable for such applications have also been proposed.

[0003] It is known that a gallium nitride layer is formed on a base substrate such as a sapphire substrate, and the base substrate is then separated using LLO (laser lift-off) technology or the like to obtain a free-standing substrate made of gallium nitride. Light-emitting elements such as LEDs and power devices are manufactured by forming functional layers such as GaN, AlGaN, and InGaN on such free-standing substrates. Recently, there has been a demand for large-diameter base substrates of 4 inches or 6 inches for ultra-high brightness LEDs and power devices.

[0004] However, after the gallium nitride layer is peeled off from the base substrate, the gallium nitride layer may be significantly warped due to the stress distribution inside the gallium nitride crystal and the difference in dislocation density between the front and back of the gallium nitride layer. When this happens, the off-angle distribution on the surface of the gallium nitride layer becomes large, so that the function of the functional layer formed thereon varies depending on the location. For example, in the case of a light-emitting element, the variation in emission wavelength may become large depending on the location on the substrate surface.

[0005] Methods for solving these problems have been proposed in Patent Documents 1 to 3. That is, in Patent Document 1, an attempt is made to reduce the variation in emission wavelength by making the Group 13 element polar surface the film formation surface (as-grown surface) and thereby keeping the atomic step density of the growth interface substantially constant. In Patent Document 2, an attempt is made to reduce the variation in emission wavelength by processing the crystal growth surface in accordance with the variation in the crystal axis of the crystal growth surface. In Patent Document 3, a flat portion is provided in the center of the substrate on the crystal growth surface, and a curved portion is provided on the outer periphery thereof, and the off-angle distribution of the flat portion is made smaller, thereby reducing the off-angle distribution of the entire substrate. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Patent 4696935 [Patent Document 2] Patent Publication No. 2009-126727 [Patent Document 3] Patent Publication No. 2009-94230 Summary of the Invention [Problem to be solved by the invention]

[0007] The structures described in Patent Documents 1 to 3 can reduce the off-angle distribution in the crystal growth surface (group 13 element polar surface) of the free-standing substrate, thereby reducing the wavelength variation of the light-emitting device. However, in the process of forming a functional element structure such as a light-emitting element structure on the group 13 element polar surface of the free-standing substrate, cracks are likely to occur in the free-standing substrate and the functional element structure.

[0008] An object of the present invention is to prevent the occurrence of cracks in a free-standing substrate for growing epitaxial crystals made of a Group 13 element nitride crystal when the epitaxial crystal is grown. [Means for solving the problem]

[0009] The present invention provides a free-standing substrate for epitaxial crystal growth made of a Group 13 element nitride crystal selected from gallium nitride, aluminum nitride, indium nitride, or a mixed crystal thereof, comprising: the free-standing substrate has a nitrogen polarity face and a group 13 element polarity face, the Group 13 element polar face is a growth face of an epitaxial crystal, The nitrogen polarity surface is warped in a convex shape, and a chamfer is provided on the outer periphery of the nitrogen polarity surface. the absolute value of the radius of curvature of the warp of the Group 13 element polar surface is smaller than the absolute value of the radius of curvature of the warp of the nitrogen polar surface The present invention relates to a free-standing substrate for epitaxial crystal growth, characterized in that

[0010] The present invention also relates to a functional element comprising the above-mentioned freestanding substrate for epitaxial crystal growth, and a functional layer provided on the Group 13 element polar face of the freestanding substrate. Effect of the Invention

[0011] The present inventors have conducted various investigations into the reasons for the occurrence of cracks in the process of growing epitaxial crystals on a free-standing substrate made of a Group 13 element nitride crystal selected from gallium nitride, aluminum nitride, indium nitride, or a mixed crystal thereof, and have arrived at the following findings.

[0012] That is, functional elements such as light-emitting elements are deposited on the group 13 element polar surface of a free-standing substrate. For this reason, during deposition, the nitrogen polar surface of the free-standing substrate is placed on a susceptor, and the free-standing substrate is transported and held. Here, in order to make the thickness of the deposited film of the functional element uniform, it is necessary to make the temperature distribution of the group 13 element polar surface on the free-standing substrate uniform. For this reason, it is technically common sense to make the nitrogen polar surface of the free-standing substrate flat.

[0013] However, the inventors considered that the cracks were generated because the nitrogen polarity surface of the free-standing substrate was easily damaged by gripping and transporting the substrate with tweezers or a robot arm when the nitrogen polarity surface of the free-standing substrate was made flat, and therefore came up with the idea of ​​chamfering the outer periphery of the nitrogen polarity surface of the free-standing substrate. However, even when the nitrogen-polarity surface on the outer periphery of the free-standing substrate was chamfered, minute chipping or cracks could occur because the Group 13 element nitride crystal was hard, making it difficult to completely suppress cracks during the manufacturing process.

[0014] For this reason, the inventors attempted to chamfer the outer periphery of the nitrogen polarity surface of the free-standing substrate, and further to warp the nitrogen polarity surface into a convex shape. This was previously difficult to conceive from the viewpoint of uniform heat transfer from the susceptor to the free-standing substrate. However, as a result of this, it was found that when an epitaxial crystal was formed on the free-standing substrate, cracks in the free-standing substrate and the epitaxial crystal were significantly reduced. In other words, although it would seem that cracks would be less likely to occur in the substrate if the nitrogen polarity surface of the free-standing substrate was concave, the inventors found that the opposite was true in reality.

[0015] Furthermore, by reducing chipping and chipping on the edge of the substrate and microscopic scratches on the back surface, the present invention is expected to have an effect of reducing substrate cracks not only in the process of epitaxially growing a functional layer on a freestanding substrate, but also in the device formation process after the functional layer is formed, the thinning process by grinding, lapping, and polishing, the transportation between each process, and the robot transportation in equipment equipped with an automatic transportation mechanism. [Brief description of the drawings]

[0016] [Figure 1] FIG. 1A is a schematic diagram showing a free-standing substrate 21 according to an embodiment of the present invention, and FIG. 1B is a schematic diagram showing a free-standing substrate 24 of a reference example. [Diagram 2] 2 is a bottom view of the free-standing substrate 21. FIG. [Diagram 3] 1A is a schematic diagram showing the outer periphery of a free-standing substrate 21 according to an embodiment of the present invention, and FIG. 1B is a schematic diagram showing the outer periphery of a free-standing substrate 21G. [Figure 4] 1(a), (b), and (c) are diagrams showing the outer periphery of free-standing substrates 21A, 21B, and 21C, respectively. [Diagram 5]13(a), (b), and (c) are diagrams showing the outer periphery of free-standing substrates 21D, 21E, and 21F, respectively. [Figure 6] 1(a) and 1(b) are schematic diagrams showing free-standing substrates 30A and 30B of comparative examples, respectively. [Figure 7] (a) shows a seed crystal film 2 and a gallium nitride layer 3 provided on a base substrate 1, (b) shows the gallium nitride layer 3 separated from the base substrate, and (c) shows a functional layer provided on the gallium nitride layer 3A. [Figure 8] 1(a) shows the crystal axes of the base substrate 1, the seed crystal film 2 and the gallium nitride layer 3, and FIG. 1(b) shows the state in which laser light A is irradiated from the base substrate side. [Figure 9] (a) shows the state in which the gallium nitride layer 13 and seed crystal film 12 have been separated from the base substrate, (b) shows the surface of the gallium nitride layer 13 after front and back surface processing, and (c) shows a free-standing substrate 14 after front and back surface processing. [Figure 10] FIG. 2 is a front view showing a schematic diagram of a free-standing substrate according to an embodiment of the present invention, in which the warp of a group 13 element polar surface 17a is different from the warp of a nitrogen polar surface 17b. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] Each element of the present invention is further described below. The present invention is a free-standing substrate for epitaxial crystal growth, which is made of a Group 13 element nitride crystal selected from gallium nitride, aluminum nitride, indium nitride, or a mixed crystal thereof. In the present invention, the term "free-standing substrate" refers to a substrate that does not deform or break under its own weight when handled and can be handled as a solid object. The free-standing substrate of the present invention can be used as a substrate for various semiconductor devices such as light-emitting elements.

[0018] The nitrides constituting the group 13 element nitride crystal layer are gallium nitride, aluminum nitride, indium nitride, or mixed crystals thereof. Specifically, GaN, AlN, InN, Ga x Al 1-x N(1>x>0), Gax In 1-x N(1>x>0), Al x In 1-x N(1>x>0), Ga x Al y In z N (1>x>0, 1>y>0, x+y+z=1), which may be doped with various n-type or p-type dopants. Preferred examples of p-type dopants include one or more selected from the group consisting of beryllium (Be), magnesium (Mg), strontium (Sr), and cadmium (Cd). Preferred examples of n-type dopants include one or more selected from the group consisting of silicon (Si), germanium (Ge), tin (Sn), and oxygen (O).

[0019] In the present invention, as shown in Fig. 1(a), for example, a free-standing substrate 21 has a group 13 element polar surface 21b and a nitrogen polar surface 21a. In addition, a side surface 21e is provided on the free-standing substrate 21. An epitaxial crystal is grown on the group 13 element polar surface 21b, and the nitrogen polar surface 21a can be held and transferred by a susceptor or the like.

[0020] Here, the nitrogen polar surface 21a of the free-standing substrate 21 is warped in a convex shape. This means that when the free-standing substrate is viewed from the nitrogen polar surface side, the nitrogen polar surface is warped so as to protrude from the free-standing substrate. A chamfered portion 21c is provided on the outer periphery 22 of the nitrogen polar surface 21a. In this example, a chamfered portion 21d is provided on the outer periphery 23 of the group 13 element polar surface 21b. Furthermore, the group 13 element polar surface 21b is warped in a direction in which the group 13 element polar surface is recessed when viewed from the free-standing substrate.

[0021] From the viewpoint of the present invention, the radius of curvature of the warp of the nitrogen polarity face is preferably +5 m or more and +65 m or less, and more preferably +7 m or more and +32 m or less, provided that when the nitrogen polarity face is convex, the value of the radius of curvature is a positive value (+), and when the nitrogen polarity face is concave, the value of the radius of curvature is a negative value (-).

[0022] On the other hand, the group 13 element polar surface may be warped in a convex shape, or may be warped in a concave shape, or may be flat when viewed from the free-standing substrate. However, from the viewpoint of suppressing the variation in the off-angle in the group 13 element polar surface, it is preferable that the group 13 element polar surface is warped in the opposite direction to the unevenness of the warp of the nitrogen polar surface, that is, in a concave shape when viewed from the free-standing substrate. This can reduce the variation in the off-angle in the group 13 element polar surface. In this case, the radius of curvature of the warp of the group 13 element polar surface is preferably -3 m or less, and from the viewpoint of ease of polishing, it is more preferable that it is -7 m or less. However, when the group 13 element polar surface is convex, the value of the radius of curvature is a positive value (+), and when the group 13 element polar surface is concave, the value of the radius of curvature is a negative value (-).

[0023] In a preferred embodiment, the absolute value of the radius of curvature of the warp of the group 13 element polar surface is smaller than the absolute value of the radius of curvature of the warp of the nitrogen polar surface. This means that the warp of the group 13 element polar surface is steeper than that of the nitrogen polar surface. This can reduce the variation in the off-angle of the nitrogen polar surface. In this embodiment, (absolute value of the radius of curvature of the warp of the group 13 element polar surface) / (absolute value of the radius of curvature of the warp of the nitrogen polar surface) is more preferably 0.7 or less, and even more preferably 0.5 or less. In addition, (absolute value of the radius of curvature of the warp of the group 13 element polar surface) / (absolute value of the radius of curvature of the warp of the nitrogen polar surface) can generally be 0.01 or more.

[0024] For example, the free-standing substrate 24 of the reference example shown in FIG. 1(b) has a group 13 element polar surface 24b and a nitrogen polar surface 24a. A chamfered portion 24d is provided on the outer periphery 23 of the group 13 element polar surface 24b, and a chamfered portion 24c is provided on the outer periphery 22 of the nitrogen polar surface 24a. The group 13 element polar surface is warped in a convex shape from the free-standing substrate, and the value of the radius of curvature is positive. The nitrogen polar surface 24a is warped in a concave shape from the free-standing substrate, and the value of the radius of curvature is negative. 24e is a side surface.

[0025] In a preferred embodiment, the group 13 element polar surface is a mirror-finished surface. A mirror-finished surface refers to a surface in a state where the roughness and waviness of the substrate surface are reduced to a degree that light is reflected and an object is visible on the processed surface after the substrate surface processing. In other words, the roughness and waviness of the substrate surface after processing are reduced to a degree that can be sufficiently ignored with respect to the wavelength of visible light. If the substrate is mirror-finished, epitaxial crystal growth is sufficiently possible on the substrate.

[0026] In a preferred embodiment, the off-angle distribution of the group 13 element polar plane is 0.25° or less, which can suppress the variation in the crystallinity of the epitaxial crystal and the variation in the characteristics of the functional layer made of the epitaxial crystal. It is more preferable that the off-angle distribution of the group 13 element polar plane is 0.1° or less. In addition, the off-angle distribution of the group 13 element polar plane is often 0.02° or more.

[0027] The warpage of the group 13 element polar surface and the nitrogen polar surface is measured, and the radius of curvature is calculated from the warpage. Each warpage is measured as follows. First, the warping of group 13 element polar surfaces and nitrogen polar surfaces can be measured by a laser displacement meter. A laser displacement meter is a device that measures the displacement of each surface by irradiating each surface with laser light. The laser wavelength is 655 nm, and the measurement method can be the confocal method, the triangulation method, or the optical interference method depending on the surface roughness.

[0028] Here, for the nitrogen polarity surface, the waveform is obtained excluding a range of 3 mm from the edge of the substrate. Next, an approximation curve for this waveform is obtained by the least squares method using a quadratic function, and the difference between the maximum and minimum values ​​of this approximation curve is measured on two axes perpendicular to each other on the substrate surface, and the average of the two values ​​is taken as the warpage. The radius of curvature R is calculated from this warpage value using the following formula.

[0029]

number

[0030] For the Group 13 element polarity surface, the waveform is obtained excluding a 3 mm wide range from the edge of the substrate. Next, an approximation curve for this waveform is obtained by the least squares method using a quadratic function, and the difference between the maximum and minimum values ​​of this approximation curve is measured on two orthogonal axes on the substrate surface, and the average of the two values ​​is taken as the warpage. The radius of curvature R is calculated from this warpage value in the same manner as for the nitrogen polarity surface. However, in the case of the nitrogen polarity surface and the group 13 element polarity surface, if the least squares approximation waveform using a quadratic function is convex, the value of the radius of curvature is a positive value (+), and if it is concave, the value of the radius of curvature is a negative value (-).

[0031] In the present invention, a chamfered portion is provided on the outer periphery of the nitrogen polarity surface. Preferably, as shown in FIG. 2, for example, a chamfered portion 21c is provided on the entire outer periphery 22 of the nitrogen polarity surface 21a of the free-standing substrate 21. However, the chamfered portion 21c does not need to be provided on the entire outer periphery of the nitrogen polarity surface, and may be provided on a portion of the free-standing substrate that comes into contact with a carrier (such as tweezers or a robot hand). For example, in FIG. 2, when a carrier comes into contact with region H of the outer periphery 22 of the free-standing substrate, the chamfered portion 21c may be provided on the outer periphery 22 in these regions H. The chamfered portion may be provided over the entire width of the outer periphery of the nitrogen polarity face, or may be provided over only a part of the outer periphery. The width of the chamfered portion may be larger than the width of the outer periphery.

[0032] In addition, when a chamfered portion is provided on the outer periphery of the group 13 element polar surface, it is preferable to provide the chamfered portion 21d over the entire circumference of the outer periphery 23 of the group 13 element polar surface 21b, similarly to the nitrogen polar surface. However, the chamfered portion 21d does not need to be provided over the entire circumference of the outer periphery 23 of the group 13 element polar surface, but it is preferable to provide the chamfered portion 21d over 50% or more of the outer periphery 23 of the group 13 element polar surface. The chamfered portion may be provided over the entire width of the outer periphery of the Group 13 element polar surface, or may be provided over only a portion of the outer periphery. The width of the chamfered portion may be greater than the width of the outer periphery.

[0033] The outer periphery of the group 13 element polar surface refers to a 1 mm wide band region including the outer edge of the group 13 element polar surface, and the outer periphery of the nitrogen polar surface refers to a 1 mm wide band region including the outer edge of the nitrogen polar surface.

[0034] (Explanation of chamfer shape) 3 and 4 further illustrate examples of the shape of the chamfered portion on each outer periphery.

[0035] 3(a), a chamfered portion 21c is provided on the outer periphery 22 of the nitrogen polarity surface 21a, and a chamfered portion 21d is provided on the outer periphery 23 of the group 13 element polarity surface 21b. In this example, each chamfered portion is a flat surface, and each flat surface is inclined with respect to the group 13 element polarity surface, the nitrogen polarity surface, and the side surface 21e. The side surface 21e is also a flat surface.

[0036] 3(b), a chamfered portion 21c is provided on the outer periphery 22 of the nitrogen polarity surface 21a, and a chamfered portion 21d is provided on the outer periphery 23 of the group 13 element polarity surface 21b. In this example, each chamfered portion has a protruding curved surface, and the curved surfaces meet at the outer periphery edge, with no flat side remaining.

[0037] In the free-standing substrate 21A shown in FIG. 4(a) of the reference example, no chamfer is provided on the outer periphery 22 of the nitrogen polar surface 21a and on the outer periphery 23 of the group 13 element polar surface 21b, and therefore the side surface 21e is generally flat.

[0038] 4(b), a chamfered portion 21c is provided on the outer periphery 22 of the nitrogen polarity surface 21a, and a chamfered portion 21d is provided on the outer periphery 23 of the group 13 element polarity surface 21b. The chamfered portions 21c and 21d are inclined with respect to the group 13 element polarity surface, the nitrogen polarity surface, and the side surface 21e. The side surface 21e is a flat surface.

[0039] In the free-standing substrate 21C shown in Fig. 4(c), a chamfered portion 21c is provided on the outer periphery 22 of the nitrogen polarity surface 21a, and a chamfered portion 21d is provided on the outer periphery 23 of the group 13 element polarity surface 21b. In this example, each chamfered portion is a curved surface (so-called R chamfer) that protrudes outward. The side surface 21e is a flat surface.

[0040] In the free-standing substrate 21D shown in Fig. 5(a), a chamfered portion 21c is provided on the outer periphery 22 of the nitrogen polarity surface 21a, and a chamfered portion 21d is provided on the outer periphery 23 of the group 13 element polarity surface 21b. In this example, the chamfered portion 21d is a flat surface that is inclined with respect to the group 13 element polarity surface 21b. The chamfered portion 21c is a curved surface that protrudes outward (so-called R chamfer). The side surface 21e is a flat surface.

[0041] 5(b), a chamfered portion 21c is provided on the outer periphery 22 of the nitrogen polarity surface 21a, and a chamfered portion 21d is provided on the outer periphery 23 of the group 13 element polarity surface 21b. In this example, the chamfered portions 21c and 21d are flat surfaces. In addition, the side surface 21e is a curved surface that protrudes outward.

[0042] 5(c), a chamfered portion 21c is provided on the outer periphery 22 of the nitrogen polarity surface 21a, and a chamfered portion 21d is provided on the outer periphery 23 of the group 13 element polarity surface 21b. In this example, the chamfered portions 21c and 21d are curved surfaces (so-called R surfaces) that protrude outward. In addition, the side surface 21e is also a curved surface that protrudes outward.

[0043] In a preferred embodiment, as described above, the nitrogen polarity plane is warped in a convex shape, and the group 13 element polarity plane is warped in a concave shape. This can further suppress the variation in the off-angle in the group 13 element polarity plane. The advantages of this will be described in more detail.

[0044] Typically, as shown in Fig. 7(a), a seed crystal film 2 is formed on the front surface 1a of a base substrate 1, and a group 13 element nitride layer 3 is formed on the group 13 element polar surface 2a of the seed crystal film 2. Next, the group 13 element nitride layer is separated from the base substrate 1 to obtain a free-standing substrate 3. A laser beam is irradiated from the rear surface 1b side of the base substrate 1 as indicated by arrow A, and the group 13 element nitride layer is separated by a laser lift-off method as shown in Fig. 7(b) to obtain a free-standing substrate, or the free-standing substrate 3 may be obtained by a spontaneous separation method utilizing the difference in thermal contraction during cooling after the formation of the group 13 element nitride layer, or by slicing the group 13 element nitride using a wire saw or the like. Next, after grinding to adjust the shape to a circle, the nitrogen polarity surface of the free-standing substrate is attached to a processing table. When attaching, the load applied to the free-standing substrate is adjusted to change the wax thickness, or a jig is placed between the free-standing substrate and the processing table to change the surface shape of the free-standing substrate. Next, the group 13 element polarity surface 3a is removed by grinding, lapping, polishing, etc. to thin the free-standing substrate to the desired thickness and flatten the surface, forming the free-standing substrate.

[0045] Next, the polar surface of the group 13 element of the free-standing substrate is attached to a processing platen. When attaching, the surface shape of the free-standing substrate is changed by adjusting the load applied to the free-standing substrate to change the wax thickness or by inserting a jig between the free-standing substrate and the processing platen. Next, the nitrogen polarity surface was removed by grinding, lapping, polishing, etc. to make a thin plate of the desired thickness and flatten the surface, thereby obtaining a free-standing substrate. Here, the nitrogen polarity surface was finished after the Group 13 element polarity surface, but the order may be reversed. Next, the outer peripheral edge of the free-standing substrate is chamfered by grinding, and the final free-standing substrate 3A can be obtained. Next, a crystal is epitaxially grown on the Group 13 element polarity surface 3a of the free-standing substrate 3A, and the functional layer 4 is formed as shown in Figure 7(c) and Figure 2, to obtain the functional element 5. 3b is a nitrogen polarity surface.

[0046] Here, the problems associated with the warping of the free-standing substrate will be further described. In general, a base substrate is used in which the crystal axis of the seed crystal film surface has an off-angle inclined from the a-axis, m-axis, and c-axis of the wurtzite structure. However, for ease of understanding, a base substrate with an off-angle of 0° (just substrate) is used. As shown in FIG. 8(a), the crystal axis in the base substrate 1 is assumed to be oriented in a certain direction as indicated by arrow B. The crystal axis B is typically the a-axis, m-axis, and c-axis of the wurtzite structure. In the seed crystal film 2, the crystal axis grows following the crystal orientation of the base substrate as indicated by arrow C. Then, when the group 13 element nitride layer 3 is formed, the crystal axis grows following the crystal orientation in the seed crystal film as indicated by arrow D.

[0047] In this state, laser light is irradiated as indicated by arrow A, and the group 13 element nitride layer is peeled off from the base substrate (FIG. 8(b)). Then, as shown in FIG. 9(a), the group 13 element nitride layer 13 and the seed crystal film 12 are warped. This warping is caused by, for example, internal stress due to a difference in defect density or a difference in crystal growth mode between the group 13 element polar surface 13a and the nitrogen polar surface 13b of the gallium nitride layer. In response to this warping, the direction of the crystal axis E in the group 13 element nitride layer 13 also changes overall.

[0048] For example, as shown in FIG. 9(b), the pressure applied to the group 13 element nitride layer is adjusted, and the gallium nitride layer is attached to a processing table while leaving the warped shape of the gallium nitride layer to some extent, and the gallium nitride layer is ground and polished to make a thin plate so that a pair of planes X are the front and back surfaces of the substrate. In this case, a free-standing substrate 14 as shown in FIG. 9(c) is obtained. However, on the group 13 element polar surface 14a of this free-standing substrate 14, the off-angle of the crystal axis E varies greatly depending on the location. However, the off-angle on the group 13 element polar surface means the inclination angle of the crystal axis E with respect to the normal line L perpendicular to the group 13 element polar surface. If the distribution of the off-angle on the group 13 element polar surface becomes large, the composition and crystallinity of the functional layer epitaxially grown thereon are affected, and the performance varies depending on the location. For example, when a light-emitting element is formed on the group 13 element polar surface, the emission wavelength varies depending on the location, which causes a decrease in yield. Note that 14b is a nitrogen polar surface.

[0049] The inventors have succeeded in making the performance of the functional layer formed on the group 13 element polar surface more uniform by making the warpage amount of the group 13 element polar surface side and the warpage amount of the nitrogen polar surface of the freestanding substrate different, and by making the off-angle distribution on the group 13 element polar surface side small even if the nitrogen polar surface is warped. That is, as shown in FIG. 10, for example, by making the warpage WU of the group 13 element polar surface 17a of the freestanding substrate 17 relatively large, the off-angle distribution on the group 13 element polar surface 17a can be made small. In addition, the warpage of the group 13 element polar surface 17a of the freestanding substrate 1 is different from the warpage of the nitrogen polar surface 17b. By making the warpage WB of the nitrogen polar surface 17b relatively small, the temperature distribution on the group 13 element polar surface is made small, thereby suppressing the fluctuation in the composition and crystallinity of the functional layer, and further suppressing the occurrence of microscopic scratches on the nitrogen polar surface surface that may occur when the substrate is held or transported by tweezers or a robot arm. 17c and 17d are chamfered portions.

[0050] If the warping of the group 13 element polar surface differs from the warping of the nitrogen polar surface, a total thickness variation (TTV) occurs in the free-standing substrate. However, even if a thickness variation occurs in the free-standing substrate, no significant thickness variation occurs in the group 13 element polar surface when the functional layer is epitaxially grown on the free-standing substrate. This is because the high thermal conductivity of the gallium nitride crystal that constitutes the free-standing substrate is utilized. As a result, no morphological abnormalities occur when the functional layer is formed, and emission wavelength shifts are sufficiently suppressed.

[0051] The material of the base substrate is not particularly limited, but suitable examples include sapphire, crystal-oriented alumina, gallium oxide, AlxGa1-xN (0≦x≦1), and SiC.

[0052] Suitable examples of the material for the seed crystal film include AlxGa1-xN (0≦x≦1) and InxGa1-xN (0≦x≦1), and gallium nitride is particularly preferred. The material for the seed crystal film is particularly preferably gallium nitride, which exhibits a yellow emission effect when observed under a fluorescent microscope. The yellow emission is a peak that appears in the range of 2.2 to 2.5 eV (yellow emission (YL) or yellow band (YB)) in addition to band-to-band exciton transition (UV).

[0053] The seed crystal film is preferably formed by vapor phase growth, but examples of the method include metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), pulsed excitation deposition (PXD), MBE, and sublimation. Metal organic chemical vapor deposition is particularly preferred. The growth temperature is preferably 950 to 1200°C.

[0054] The growth direction of the gallium nitride crystal is not particularly limited, but may be the normal direction to the c-plane of the wurtzite structure, or the normal directions to each of the a-plane and m-plane, or may be a plane inclined from the c-plane, a-plane, or m-plane. The method for producing the gallium nitride layer is not particularly limited, but examples include gas phase methods such as metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), pulsed photoexcitation deposition (PXD), MBE, and sublimation, and liquid phase methods such as ammonothermal and flux methods.

[0055] In a preferred embodiment, the thickness of the free-standing substrate after polishing is preferably 300 μm or more and 1000 μm or less.

[0056] The size of the free-standing substrate is not particularly limited, but is preferably 2 inches, 4 inches, or 6 inches, and may be 8 inches or more.

[0057] In the nitrogen polarity plane and the Group 13 element polarity plane, the width W of the chamfer (see FIGS. 3 to 5) is not particularly limited, but from the viewpoint of the present invention, the width W is preferably 1 / 20 to 3 times the thickness of the free-standing substrate, and more preferably 1 / 10 to 1 time. In the nitrogen polarity face and the group 13 element polarity face, the height D of the chamfer (see Figs. 3 to 5) is not particularly limited, but from the viewpoint of the present invention, when the side surface 21e is a flat surface, the length is preferably 1 / 20 to 1 / 2 the free-standing substrate thickness, more preferably 1 / 10 to 2 / 5 the free-standing substrate thickness, and when the side surface 21e is a curved surface, the length is preferably 1 / 20 to 1 / 2 the free-standing substrate thickness. However, the height D of the chamfer is the dimension in the free-standing substrate thickness direction from the starting point of the chamfer to the end of the free-standing substrate. From the viewpoint of the present invention, in the nitrogen polarity plane and the group 13 element polarity plane, the opening width H of the free-standing substrate end shown in Fig. 3 is preferably 70 µm or more, more preferably 100 µm or more. The opening width H is preferably half or less, more preferably two-fifths or less of the free-standing substrate thickness. The opening width H means the height of each chamfered portion when viewed from the center O of each polarity plane 21a, 21b in the thickness direction of the free-standing substrate.

[0058] Examples of epitaxial crystals grown on a free-standing substrate include gallium nitride, aluminum nitride, indium nitride, and mixed crystals thereof. Specifically, GaN, AlN, InN, GaxAl1-xN (1>x>0), GaxIn1-xN (1>x>0), Al x In 1-x N(1>x>0), GaxAlyInzN(1>x>0, 1>y>0, x+y+z=1). In addition, examples of the functional layer provided on the free-standing substrate include a rectifier layer, a switching element, and a power semiconductor layer in addition to the light-emitting layer. In addition, after providing a functional layer on the group 13 element polar surface of the free-standing substrate, the nitrogen polar surface can be processed, for example, by grinding or polishing, to reduce the thickness and thickness distribution of the free-standing substrate. EXAMPLES

[0059] Example 1 A free-standing substrate 21 shown in FIG. 1(a) and FIG. 3(a) was fabricated as a prototype. Specifically, a seed crystal film made of gallium nitride was provided on a base substrate made of sapphire to prepare a GaN template (substrate diameter 2.5 inches). The seed crystal film had a thickness of 2 μm. Next, a gallium nitride layer having a thickness of 1 mm was formed by the Na flux method. The gallium nitride layer was then peeled off by the laser lift-off method to obtain a gallium nitride substrate having a thickness of 1 mm. At this time, warping was observed in the gallium nitride substrate.

[0060] Next, the outer periphery of the gallium nitride substrate was ground to form a circle with a diameter of 2 inches, which was then ground, lapped, and polished to obtain a free-standing substrate having a thickness of 400 μm and having a mirror-finished Group 13 element polar surface and nitrogen polar surface. At this time, the warping of the gallium polar surface and nitrogen polar surface was adjusted to the range shown in Table 1 by changing the attachment load to the processing table and the amount of wax used for attachment. However, ten samples were produced at a time, and each warping was shown as a numerical range between maximum and minimum values. Furthermore, chamfers were formed on the outer periphery of each of the nitrogen polarity surface and the gallium polarity surface. The width W of the chamfer was 200 μm, the opening width H of the chamfer was 100 μm, and the shape of the chamfer was a flat surface as shown in Figure 4(b). The height D of the chamfer is shown in Table 1 as a numerical range of maximum and minimum values.

[0061] The off-angle distribution on the gallium polarity surface of the obtained free-standing substrate was evaluated. Next, an LED element structure was grown as an epitaxial film on the gallium polarity surface of the free-standing substrate by MOCVD, and the performance of the LED element structure (surface morphology, emission wavelength) was evaluated. The evaluation results are shown in Table 1.

[0062] The evaluation method is as follows. (warp) The warpage of the gallium polar surface and the nitrogen polar surface of the free-standing substrate was measured as described above, and the radius of curvature was calculated.

[0063] (Method of measuring off-angle distribution) Five points on the substrate surface were measured using an energy dispersive X-ray diffractometer (D2 CRYSO manufactured by Bruker AXS). The five points were the center of the substrate and four points 5 mm inward from the edge of the substrate on the X and Y axes when the center of the substrate was the origin. The "maximum value - minimum value" was taken as the off-angle distribution. For example, for a free-standing substrate with a diameter of 50.8 mm, measurements are taken at five points: (0,0), (45.8,0), (-45.8,0), (0,45.8), and (0,-45.8).

[0064] (Morphology of epitaxial film on wafer surface) In addition, the surface morphology of the resulting epitaxial film was observed to confirm the presence or absence of abnormalities. (Method of measuring LED emission wavelength distribution) The PL (photoluminescence) measurement was performed to measure the emission peak wavelength at the same five points as the off angle. The emission wavelength distribution was calculated by subtracting the maximum wavelength from the minimum wavelength. (Cracks or not) Epitaxial films were formed on the 10 free-standing substrates in the manner described above, and the free-standing substrates and the epitaxial films were observed for the presence or absence of cracks, and the crack occurrence rate was measured.

[0065] [Table 1]

[0066] As shown in Table 1, in Example 1, chamfers were provided on the outer periphery of each of the gallium polar surface and the nitrogen polar surface, the radius of curvature of the nitrogen polar surface was in the range of +24.8 to 46.1 m, and the radius of curvature of the gallium polar surface was in the range of -46.1 to -24.8 m. As a result, no morphological abnormalities were observed in the epitaxial film, the LED emission wavelength distribution was 12 nm, and the yield of free-standing substrates without cracks was 100%.

[0067] (Examples 2 to 4) A free-standing substrate and an LED structure were fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 1. That is, in Examples 2 to 4, chamfers are provided on the outer periphery of each of the gallium polar surface and the nitrogen polar surface. The radii of curvature of the nitrogen polar surface and the radii of curvature of the gallium polar surface are shown in Table 1, and the nitrogen polar surface is warped in a convex shape, while the gallium polar surface is warped in a concave shape. As a result, no morphological abnormalities were observed in the epitaxial film, the LED emission wavelength distribution was relatively small, and the yield of freestanding substrates without cracks was 100%.

[0068] (Comparative Examples 1 to 3) The free-standing substrate and LED structure of each example were fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 2. That is, in Comparative Examples 1 to 3, the gallium polar surface and the nitrogen polar surface were not chamfered at their outer peripheries, and the nitrogen polar surface was warped in a convex shape. As a result, no morphological abnormalities were observed in the epitaxial film, and the LED emission wavelength distribution was relatively small, but the yield of freestanding substrates without cracks was significantly reduced to 70%.

[0069] (Comparative Examples 4 to 5) The free-standing substrate and LED structure of each example were fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 2. That is, in Comparative Examples 4 and 5, chamfers were provided on the outer periphery of the gallium polar surface and the nitrogen polar surface, but the nitrogen polar surface was warped in a concave shape. As a result, no morphological abnormalities were observed in the epitaxial film, and the LED emission wavelength distribution was relatively small, but the yield of freestanding substrates without cracks was significantly reduced to 50%.

[0070] [Table 2]

[0071] (Comparative Examples 6 to 7) The free-standing substrate and LED structure of each example were fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 3. In Comparative Examples 6 and 7, the gallium polar surface and the nitrogen polar surface were not chamfered at their outer peripheries, and the nitrogen polar surface was warped in a concave shape. As a result, no morphological abnormalities were observed in the epitaxial film, and the LED emission wavelength distribution was relatively small, but the yield of freestanding substrates without cracks was significantly reduced to 30%.

[0072] [Table 3]

[0073] (Comparative Examples 8 to 10) The free-standing substrate and LED structure of each example were fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 3. However, in Comparative Examples 8 and 10, chamfered portions were provided on the outer periphery of the gallium polar surface and the nitrogen polar surface, respectively, but the nitrogen polar surface was flat. As a result, no morphological abnormalities were observed in the epitaxial film, and the LED emission wavelength distribution was relatively small, but the yield of freestanding substrates without cracks was significantly reduced. In Comparative Example 9, no chamfers were provided on the periphery of the gallium polar surface and the nitrogen polar surface. In addition, the nitrogen polar surface was flat. As a result, no morphological abnormalities were observed in the epitaxial film, and the LED emission wavelength distribution was relatively small, but the yield of freestanding substrates without cracks was significantly reduced.

[0074] Comparative Example 11 A free-standing substrate and an LED structure were fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 3. However, a free-standing substrate 30A having the form shown in Fig. 6(a) was fabricated as a prototype. These are almost the same as the free-standing substrates described in Patent Document 3. That is, the nitrogen polar surface 30a of the free-standing substrate 30A in Fig. 6(a) is flat, and the gallium polar surface 30b is also flat, but the outer periphery 31 (width 5 mm) of the gallium polar surface 30b is provided with a concave curved surface 32. The side surface 30c of the free-standing substrate is also flat. No chamfered portion is provided. As a result, morphological abnormalities in the epitaxial film were observed mainly in the peripheral protrusion, the LED emission wavelength distribution was particularly large in the peripheral curved part, and the yield of free-standing substrates without cracks was significantly reduced.

[0075] Comparative Example 12 A free-standing substrate and an LED structure were fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 3. However, a free-standing substrate 30B having the form shown in Fig. 6(b) was fabricated as a free-standing substrate. That is, the side surface 30c of the free-standing substrate 30B was flat, and the gallium polar surface 30b was also flat, but the outer periphery 31 (width 3 mm) of the gallium polar surface 30b was provided with a concave curved surface 32. The nitrogen polar surface 30d was warped in a convex shape, and the radius of curvature was in the range of +24.8 to +46.1 m. No chamfered portion was provided. As a result, morphological abnormalities in the epitaxial film were observed mainly in the peripheral protrusion, the LED emission wavelength distribution was particularly large in the peripheral curved part, and the yield of free-standing substrates without cracks was significantly reduced.

Claims

1. A free-standing substrate for epitaxial crystal growth, comprising a Group 13 element nitride crystal selected from gallium nitride, aluminum nitride, indium nitride, or a mixed crystal thereof, the free-standing substrate for epitaxial crystal growth has a nitrogen polarity face and a group 13 element polarity face, the group 13 element polarity face is a growth face of an epitaxial crystal, the nitrogen polarity face is warped in a convex shape, a chamfer is provided on an outer periphery of the nitrogen polarity face, and an absolute value of a radius of curvature of the warp of the group 13 element polarity face is smaller than an absolute value of a radius of curvature of the warp of the nitrogen polarity face.

2. 2. The freestanding substrate for epitaxial crystal growth according to claim 1, wherein the radius of curvature of the warp of said nitrogen polarity face is +5 m or more and +65 m or less.

3. 3. The free-standing substrate for epitaxial crystal growth according to claim 1, wherein the chamfered portion is provided over the entire periphery of the outer periphery of the nitrogen polarity face.

4. 4. The freestanding substrate for epitaxial crystal growth according to claim 1, wherein a chamfer is provided on the outer periphery of the group 13 element polar face.

5. 5. The freestanding substrate for epitaxial crystal growth according to claim 1, wherein the Group 13 element polar face is warped in a concave shape.

6. 6. The freestanding substrate for epitaxial crystal growth according to claim 1, wherein the off-angle distribution in the Group 13 element polar plane is 0.25° or less.

7. 7. The freestanding substrate for epitaxial crystal growth according to claim 1, wherein the Group 13 element polar surface is a mirror-finished surface.

8. The free-standing substrate for epitaxial crystal growth according to any one of claims 1 to 7, characterized in that the opening width of the nitrogen polarity surface end of the free-standing substrate is 70 μm or more and half or less of the thickness of the free-standing substrate.

9. The free-standing substrate for epitaxial crystal growth according to any one of claims 1 to 8, characterized in that the height of the chamfered portion at the outer periphery of the nitrogen polarity surface of the free-standing substrate is 1 / 20 or more and 1 / 2 or less of the thickness of the free-standing substrate.

10. A free-standing substrate for epitaxial crystal growth according to any one of claims 1 to 9, and A functional element comprising a functional layer provided on the Group 13 element polar surface of the free-standing substrate.

11. 11. The functional element according to claim 10, having a light emitting function.

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