Group iii nitride substrate, bonded substrate, semiconductor element, and method for producing group iii nitride substrate

Abstract

Provided is a group III nitride substrate with which a higher yield can be obtained. The group III nitride substrate includes a dopant and has a first main surface and a second main surface facing each other. The average value Cave1 of the dopant concentration in the thickness direction of the substrate is 1 × 1017 cm-3 or more and 1 × 1019 cm-3 or less. The absolute value of the difference between the dopant concentration in the thickness direction of the substrate and the average value Cave1 is 20% or less of the average value Cave1.

Description

Group III element nitride substrate, bonded substrate, semiconductor device, and method for manufacturing a group III element nitride substrate

[0001] The present invention relates to a Group III element nitride substrate, a bonded substrate, a semiconductor device, and a method for manufacturing a Group III element nitride substrate.

[0002] Group III element nitrides have a wide direct transition band gap, a high dielectric breakdown field, and a high saturated electron velocity, and therefore are being actively developed as semiconductor materials for, for example, high-frequency / high-power electronic devices.

[0003] For example, as described in Patent Document 1, depending on the application, it is desirable that the above-mentioned Group III element nitrides have high resistance.

[0004] Patent No. 5451085

[0005] Semiconductor devices obtained from the above-mentioned high-resistivity Group III element nitride substrates may not have sufficient uniformity in quality, and an improvement in yield is desired.

[0006] In view of the above, a main object of the present invention is to provide a Group III element nitride substrate that can improve yield.

[0007] 1. A Group III element nitride substrate according to an embodiment of the present invention is a Group III element nitride substrate containing a dopant and having a first main surface and a second main surface opposed to each other, wherein the average dopant concentration C in the thickness direction of the substrate is ａｖｅ１ is 1 x 10 １７ cm －３ 1x10 or more １９ cm －３ and the dopant concentration in the thickness direction of the substrate is equal to or less than the average value C ａｖｅ１ The absolute value of the difference between the average value C ａｖｅ１ 2. In the Group III element nitride substrate according to the above item 1, the dopant may be a carrier-compensating impurity. 3. The average value C of the dopant concentration at a plurality of locations within the plane of the Group III element nitride substrate according to the above item 1 or 2 ａｖｅ２ is 1 x 10 １７ cm －３ 1x10 or more１９ cm －３ or less, and the dopant concentration at each of a plurality of portions in the surface of the substrate and the average value C ａｖｅ２ The absolute value of the difference between the average value C ａｖｅ２ 4. In the Group III element nitride substrate according to any one of the above items 1 to 3, the dopant concentration in the thickness direction of the substrate and its average value C ａｖｅ１ 5. In the Group III nitride substrate according to any one of 1 to 4 above, the dopant concentration in the thickness direction of the substrate and its average value C ａｖｅ１ may be measured between a first portion of the first main surface and a second portion of the second main surface, the first portion being on the same line extending in the thickness direction of the substrate. ａｖｅ is 1 x 10 ７ Ω·cm or more, and the minimum value R of resistivity at a plurality of locations within the surface of the substrate ｍｉｎ is the average value R ａｖｅ may satisfy the relationship of the following formula (1): ｍｉｎ ≧R ａｖｅ ×0.1 (1). 7. In the Group III element nitride substrate according to any one of 1 to 6 above, the dopant may be manganese. 8. The Group III element nitride substrate according to any one of 1 to 7 above may contain gallium nitride. 9. A bonded substrate according to another embodiment of the present invention comprises the Group III element nitride substrate according to any one of 1 to 8 above and a support substrate. 10. A semiconductor device according to another embodiment of the present invention comprises the Group III element nitride substrate according to any one of 1 to 8 above, a stacked structure including a channel layer and a barrier layer in this order, and a source electrode, a drain electrode, and a gate electrode provided on the stacked structure.

[0008] 11. A method for producing a Group III nitride substrate according to another embodiment of the present invention is the method for producing a Group III nitride substrate as recited in any one of 1 to 8 above, comprising: preparing a growth substrate having opposing upper and lower surfaces; and growing a Group III nitride crystal on the growth substrate by a flux method, wherein a growth vessel containing the growth substrate and a source material is rotated for a predetermined period of time and then the rotation is stopped for a predetermined period of time, repeatedly, to grow the Group III nitride crystal. 12. In the method for producing a Group III nitride substrate as recited in 11 above, the rotation of the growth vessel may be stopped for 15 seconds or more.

[0009] According to an embodiment of the present invention, a Group III element nitride substrate capable of improving yield can be provided.

[0010] 1A is a schematic cross-sectional view showing the overall configuration of a group III element nitride substrate according to one embodiment of the present invention. FIG. 1B is a plan view of the group III element nitride substrate shown in FIG. 1A. FIG. 1C is a view for explaining a measurement range of dopant concentration. FIG. 1D is a view for explaining measurement locations of dopant concentration. FIG. 1E is a view showing an example of a manufacturing process for a group III element nitride substrate. FIG. 3A is a view following FIG. 3B. FIG. 3C is a view showing an example of a state in which a growth substrate is arranged in a growth container. FIG. 3D is a partial cross-sectional view showing, with color shading, an example of a distribution in which dopant concentration changes periodically. FIG. 3E is a cross-sectional view showing, with color shading, an example of a warpage that can occur in a laminated substrate. FIG. 3F is a cross-sectional view showing, with color shading, an example of a distribution in which dopant concentration changes periodically. FIG. 3G is a schematic cross-sectional view showing, with color shading, an example of a distribution in which dopant concentration changes periodically. FIG. 3H is a schematic cross-sectional view showing, with color shading, an example of a distribution in which dopant concentration changes periodically.

[0011] Hereinafter, embodiments of the present invention will be described with reference to the drawings, but the present invention is not limited to these embodiments. In order to clarify the description, the drawings may schematically show the width, thickness, shape, etc. of each part compared to the embodiments, but these are merely examples and do not limit the interpretation of the present invention. Furthermore, in the drawings, the same or equivalent elements are given the same reference numerals, and redundant explanations may be omitted.

[0012] A. Group III Element Nitride Substrate Figure 1A is a schematic cross-sectional view showing the general configuration of a Group III element nitride substrate according to one embodiment of the present invention, and Figure 1B is a plan view of the Group III element nitride substrate shown in Figure 1A. The Group III element nitride substrate 10 is plate-shaped and has a first main surface 11 and a second main surface 12 that face each other and are connected via a side surface 13.

[0013] In the illustrated example, the Group III element nitride substrate is disk-shaped (wafer), but is not limited thereto and may be any suitable shape. The size of the Group III element nitride substrate may be appropriately set depending on the purpose. The diameter of the disk-shaped Group III element nitride substrate is, for example, 50 mm or more and 200 mm or less.

[0014] The thickness of the Group III element nitride substrate is, for example, 250 μm or more and 800 μm or less, preferably 300 μm or more and 700 μm or less, and more preferably 300 μm or more and 600 μm or less.

[0015] The group III element nitride substrate is composed of group III element nitride crystals. Examples of group III elements that make up the group III element nitride include aluminum (Al), gallium (Ga), and indium (In). These may be used alone or in combination of two or more. Specific examples of group III element nitrides include aluminum nitride (typically AlN), gallium nitride (typically GaN), indium nitride (typically InN), and aluminum gallium nitride (Al). ｘ Ga ｙ N), gallium indium nitride (Ga ｙ In ｚ N), aluminum indium nitride (Al ｘ Inｚ N), aluminum gallium indium nitride (Al ｘ Ga ｙ In ｚ In each chemical formula in parentheses, typically, x+y+z=1.

[0016] The above-mentioned Group III element nitride is doped with an element other than Group III elements. Specifically, the Group III element nitride contains an element other than Group III elements as a dopant (impurity). By doping with an element other than Group III elements, for example, a Group III element nitride substrate (semi-insulating Group III element nitride substrate) that can satisfy a desired resistivity can be obtained. Examples of the dopant that can be used include transition metal elements such as iron (Fe), manganese (Mn), zinc (Zn), vanadium (V), chromium (Cr), cobalt (Co), and nickel (Ni). These transition metal elements can function as carrier-compensating impurities. Specifically, these transition metal elements can compensate for carriers that may be unintentionally introduced into the Group III element nitride as acceptors, thereby reducing the carrier concentration in the Group III element nitride and increasing the resistivity. These transition metal elements can be used alone or in combination of two or more. Among these, manganese is preferably used. By using manganese, a semi-insulating Group III element nitride substrate can be obtained satisfactorily.

[0017] The amount of dopant present in the Group III element nitride substrate (unit: atoms / cm ３ ) can be set to any appropriate value. The amount of dopant present in the Group III element nitride substrate (unit: atoms / cm ３ ) will be simply referred to as the dopant concentration (unit: cm －３ The dopant concentration is, for example, 1×10 １６ cm －３ 1x10 or more ２０ cm －３ The following is the result.

[0018] The dopant concentration can be measured by, for example, secondary ion mass spectrometry (SIMS).

[0019] The group III element nitride substrate may have a dopant concentration that varies in the thickness direction. Specifically, the group III element nitride substrate may have a dopant concentration distribution in the thickness direction. The average value C of the dopant concentration in the thickness direction of the group III element nitride substrate may be expressed as ａｖｅ１ is, for example, 1 × 10 １７ cm －３ or more, preferably 3×10 １７ cm －３ More preferably, 5×10 １７ cm －３ or more, and more preferably 7×10 １７ cm －３ More preferably, it is 1×10 １８ cm －３ The average value C of the dopant concentration in the thickness direction of the Group III element nitride substrate ａｖｅ１ is, for example, 1 × 10 １９ cm －３ The following is the result.

[0020] The dopant concentration in the thickness direction of the Group III element nitride substrate and its average value can be determined by measuring the dopant concentration at multiple locations within a predetermined thickness range. The dopant concentration in the thickness direction and its average value are preferably measured over a thickness range of 5 μm or more, and more preferably over a thickness range of 10 μm or more. For example, as shown in FIG. 2A , the average dopant concentration in the thickness direction of the Group III element nitride substrate may be calculated from the dopant concentrations at multiple locations within a predetermined thickness range A1 from the first main surface 11 of the Group III element nitride substrate 10 and the dopant concentrations at multiple locations within a predetermined thickness range A2 from the second main surface 12 of the Group III element nitride substrate 10.

[0021] The dopant concentration and its average value in the thickness direction of the Group III element nitride substrate are measured, for example, between a first portion 11a of the first main surface 11 and a second portion 12a of the second main surface 12, which are located on the same line extending in the thickness direction of the Group III element nitride substrate 10. Here, the first portion 11a and the second portion 12a do not need to be located strictly on the same line extending in the thickness direction, but only need to be located on substantially the same line extending in the thickness direction. For example, when the Group III element nitride substrate 10 is viewed from above, the distance between the first portion 11a and the second portion 12a may be 5 mm or less.

[0022] The dopant concentration in the thickness direction of the Group III element nitride substrate (specifically, the dopant concentration at any position in the thickness direction of the Group III element nitride substrate) and the average value C ａｖｅ１ The absolute value of the difference between ａｖｅ１ 25% or less of the average value C ａｖｅ１ More preferably, the average value C ａｖｅ１ It is less than 15% of the

[0023] The group III element nitride substrate may have different dopant concentrations in its plane (for example, the first main surface). Specifically, the group III element nitride substrate may have a dopant concentration distribution in its plane (for example, the first main surface). The average value C of the dopant concentration at multiple locations in the plane of the group III element nitride substrate may be expressed as ａｖｅ２ is, for example, 1 × 10 １７ cm －３ or more, preferably 3×10 １７ cm －３ More preferably, 5×10 １７ cm －３ or more, and more preferably 7×10 １７ cm －３ More preferably, it is 1×10 １８ cm －３ The average value C of the dopant concentration at multiple locations in the surface of the Group III element nitride substrate ａｖｅ２ is, for example, 1 × 10 １９ cm －３ The following is the result.

[0024] The average value of the dopant concentration at multiple locations within the plane of the Group III element nitride substrate may be, for example, the average value of the dopant concentrations at five points (left, top, center, right, and bottom) within the plane of the disk-shaped substrate 10 shown in Figure 2B. The distance L between the central measurement point and each of the upper, lower, left, and right measurement points may be, for example, 10 mm to 23 mm. Furthermore, the distance L between the central measurement point and each of the upper, lower, left, and right measurement points may be 0.7r to 0.9r, where r is the radius of the substrate.

[0025] The dopant concentration at each of a plurality of locations within the surface of the Group III element nitride substrate and the average value C ａｖｅ２ The absolute value of the difference between ａｖｅ２ 25% or less of the average value C ａｖｅ２ More preferably, the average value C ａｖｅ２ The dopant concentration is 15% or less of the total dopant concentration. By satisfying such a dopant concentration distribution, for example, a high yield can be achieved in obtaining semiconductor devices. Such a dopant concentration distribution can be favorably achieved by having the above-mentioned predetermined dopant concentration distribution in the thickness direction.

[0026] In one embodiment, the dopant concentration is a manganese concentration.

[0027] In one embodiment, the resistivity of the group III nitride substrate is, for example, 1×10 ５ Ω・cm or more 1×10 １４ Ω cm or less, preferably 1×10 ６ Ω cm or more, more preferably 1×10 ７ The resistivity is Ω cm or more. Such a semi-insulating Group III element nitride substrate can be suitably used, for example, as a substrate for a high electron mobility transistor (HEMT) device. Specifically, a channel layer and a barrier layer can be formed on the Group III element nitride substrate, and the resulting substrate can be used as a HEMT device.

[0028] The resistivity of the Group III element nitride substrate can be measured by any appropriate method. Examples of resistivity measurement methods include the capacitance method, the two-terminal method, and the double-ring electrode method. Among these, the capacitance method is preferably used. The capacitance method allows the resistivity to be determined without destroying the Group III element nitride substrate. Specifically, the Group III element nitride substrate is inserted into a capacitor consisting of a probe and a stage, a pulse voltage is applied, the change in the amount of charge on the Group III element nitride substrate over time is measured, and the resistivity is calculated from the measured value. Since the probe does not contact the Group III element nitride substrate, the resistivity can be determined without forming an ohmic contact electrode. The spatial resolution of the probe can be approximately 1 mm to 10 mm. The method for determining resistivity is described, for example, in the non-patent document "R. Stibal et al., "Contactless evaluation of semi-insulating GaAs wafer resistivity using the time-dependent charge measurement," Semiconductor Science and Technology, 6, p. 995 (1991)."

[0029] The Group III element nitride substrate may have different resistivities in its plane (for example, the first main surface). Specifically, the Group III element nitride substrate may have a distribution of resistivities in its plane (for example, the first main surface). The average value R of resistivities at multiple locations in the plane of the Group III element nitride substrate may be expressed as ａｖｅ is, for example, 1 × 10 ６ Ω cm or more, preferably 1×10 ７ On the other hand, the average value R of the resistivity at multiple locations in the surface of the Group III element nitride substrate is ａｖｅ is, for example, 1 × 10 １３ Ω cm or less, preferably 1×10 １２ It is Ω·cm or less.

[0030] The minimum resistivity R at multiple locations within the surface of the Group III element nitride substrate ｍｉｎ is the average value R ａｖｅIt is preferable that the relationship of the following formula (1) is satisfied for R ｍｉｎ ≧R ａｖｅ ×0.1 (1) Such a group III element nitride substrate can provide excellent yields in producing semiconductor devices, for example.

[0031] The maximum value R of the resistivity at multiple locations within the surface of the Group III element nitride substrate ｍａｘ is the average value R ａｖｅ It is preferable that the relationship of the following formula (2) is satisfied for R ｍａｘ ≦R ａｖｅ × 10 ... (2)

[0032] In the above-described Group III element nitride crystal, typically, the <0001> direction is the c-axis direction, the <1-100> direction is the m-axis direction, and the <11-20> direction is the a-axis direction. The crystal plane perpendicular to the c-axis is the c-plane, the crystal plane perpendicular to the m-axis is the m-plane, and the crystal plane perpendicular to the a-axis is the a-plane. In one embodiment, the thickness direction of the Group III element nitride substrate is parallel or approximately parallel to the c-axis, the first main surface 11 is a Group III element polar plane on the (0001) plane side, and the second main surface 12 is a nitrogen polar plane on the (000-1) plane side. The first main surface 11 may be parallel to the (0001) plane or may be inclined with respect to the (0001) plane. The inclination angle of the first main surface 11 with respect to the (0001) plane is, for example, 10° or less, 5° or less, 2° or less, or 1° or less. The second main surface 12 may be parallel to the (000-1) plane or may be inclined with respect to the (000-1) plane. The inclination angle of the second main surface 12 with respect to the (000-1) plane is, for example, 10° or less, 5° or less, 2° or less, or 1° or less.

[0033] B. Manufacturing Method A method for manufacturing a Group III element nitride substrate according to one embodiment of the present invention includes preparing a growth substrate and growing a Group III element nitride crystal on the growth substrate. The growth substrate can be, for example, a seed crystal substrate having a base substrate and a seed crystal film.

[0034] 3A to 3C are diagrams showing an example of a manufacturing process for a Group III element nitride substrate, in which a seed crystal film 22 is formed on an upper surface 21 a of a base substrate 21 having opposing upper and lower surfaces 21 a and 21 b, thereby completing a seed crystal substrate (growth substrate) 20.

[0035] The base substrate may be, for example, a substrate having a shape and size that allows a Group III nitride substrate having the desired shape and size to be produced. Typically, the base substrate is disk-shaped with a diameter of 50 mm to 200 mm. The thickness of the base substrate is, for example, 200 μm to 800 μm.

[0036] Any suitable substrate can be used as the base substrate. The base substrate is typically made of a single crystal. Examples of materials that can be used for the base substrate include sapphire, crystal-oriented alumina, silicon, gallium oxide, aluminum gallium nitride, gallium arsenide, and silicon carbide (SiC).

[0037] The thickness of the seed crystal film is, for example, 0.2 μm or more. From the viewpoint of preventing meltback or disappearance during growth of the Group III nitride crystal, the thickness of the seed crystal film is preferably 1 μm or more, more preferably 2 μm or more. On the other hand, from the viewpoint of productivity, the thickness of the seed crystal film is preferably 10 μm or less, more preferably 5 μm or less.

[0038] Any appropriate material can be used as the material for the seed crystal film. A typical material for the seed crystal film is a group III element nitride. Details of the group III element nitride are as described above.

[0039] The seed crystal film can be formed by any appropriate method. A typical method for forming the seed crystal film is a vapor phase growth method. Specific examples of the vapor phase growth method include metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), pulsed excited deposition (PXD), molecular beam epitaxy (MBE), evaporation, and sublimation. Among these, the MOCVD method is preferably used.

[0040] The formation of the seed crystal film by the MOCVD method includes, for example, a first formation step and a second formation step, in this order. Specifically, in the first formation step, a first layer (low-temperature grown buffer layer) (not shown) is formed on a base substrate at a temperature T1 (e.g., 450°C to 550°C), and in the second formation step, a second layer (not shown) is formed at a temperature T2 (e.g., 1000°C to 1200°C) higher than temperature T1. The thickness of the first layer is, for example, 20 nm to 50 nm. The thickness of the second layer is, for example, 1 μm to 5 μm.

[0041] Next, a group III element nitride crystal is grown on the seed crystal film 22 of the seed crystal substrate 20 to form a group III element nitride crystal layer 16, thereby obtaining a layered substrate 30 as shown in FIG. 3B . The degree of growth of the group III element nitride crystal (the thickness of the group III element nitride crystal layer 16) can be adjusted depending on the desired thickness of the group III element nitride substrate. Any appropriate direction can be selected as the growth direction of the group III element nitride crystal depending on the application, purpose, etc. Specific examples include the normal directions to the c-plane, a-plane, and m-plane, and the normal directions to planes inclined relative to the c-plane, a-plane, and m-plane.

[0042] Group III element nitride crystals can be grown by any appropriate method. The method for growing Group III element nitride crystals is not particularly limited, as long as it is a method that can achieve a crystal orientation that roughly follows the crystal orientation of the seed crystal film. Specific examples of methods for growing Group III element nitride crystals include vapor phase growth methods such as metal organic chemical vapor deposition (MOCVD), hydride vapor phase epitaxy (HVPE), pulsed excited vapor deposition (PXD), molecular beam epitaxy (MBE), and sublimation, and liquid phase growth methods such as flux deposition, ammonothermal deposition, hydrothermal deposition, and sol-gel deposition. These methods can be used alone or in combination of two or more.

[0043] Preferably, a flux method (e.g., a Na flux method) is employed as a method for growing Group III element nitride crystals. Details of such growth methods are described, for example, in Japanese Patent No. 5451085, and growth may be performed by adjusting various conditions of the described growth method as appropriate. Specifically, Group III element nitride crystals can be grown by adjusting various conditions using a crystal manufacturing apparatus that includes a pressure-resistant vessel capable of supplying high-pressure nitrogen gas, a turntable that can rotate within the pressure-resistant vessel, and an outer vessel placed on the turntable.

[0044] Growth of Group III nitride crystals by the flux method is typically performed using a crucible as a growth vessel. Specifically, the growth substrate is placed at a predetermined position within the crucible, and raw materials are then filled into the crucible. FIG. 3D is a diagram showing an example of the state in which the growth substrate is placed within the growth vessel. During growth of Group III nitride crystals, the growth substrate 20 is placed within the growth vessel (crucible) 70 so that the entire substrate is immersed in the melt composition 18. In the illustrated example, one end of the growth substrate 20 is in contact with a support pedestal 72, and the other end is in contact with the bottom surface of the growth vessel 70. The crucible containing the growth substrate is typically placed with a lid on, in a nitrogen-containing atmosphere, at a predetermined pressure and temperature, and subjected to the growth process.

[0045] The raw material is, for example, a melt composition containing a flux, a Group III element, and a dopant. The flux preferably contains at least one of an alkali metal and an alkaline earth metal, and more preferably metallic sodium. Typically, the flux and a metal source material are mixed together. As the metal source material, an elemental metal, an alloy, a metal compound, etc. can be used, but from the viewpoint of handling, an elemental metal is preferably used.

[0046] The crucible (including the lid) can be made of any suitable material that can be used in the flux method. Examples of the crucible material include alumina, yttria, and YAG (yttrium aluminum garnet). The crucible material may be a single crystal or a polycrystal (ceramic). The ceramic may have a high relative density, such as by HIP treatment, to give it translucency.

[0047] As described above, the growth can be carried out in a nitrogen-containing atmosphere. The growth atmosphere can contain other gases in addition to nitrogen. The other gases are preferably inert gases such as argon, helium, and neon.

[0048] The pressure of the atmosphere during growth can be set to any appropriate pressure. For example, from the viewpoint of preventing evaporation of the flux, the pressure of the atmosphere during growth is, for example, 1 MPa or more, or may be 2 MPa or more, or may be 3 MPa or more. On the other hand, for example, from the viewpoint of preventing the crystal growth apparatus from becoming large-scale, the pressure of the atmosphere during growth can be, for example, 50 MPa or less, or may be 10 MPa or less.

[0049] The temperature of the atmosphere during growth can be set to any appropriate temperature, preferably 700°C to 1000°C, more preferably 800°C to 900°C.

[0050] The growth is preferably carried out while rotating the crucible (growth substrate). For example, the covered crucible is placed in the outer container of the crystal production apparatus and placed on a turntable, and the turntable is rotated (e.g., on its axis) in this state to rotate the crucible as shown by the arrow in Figure 3D. The rotation can promote dissolution of high-pressure nitrogen gas into the melt composition, allowing for good growth of Group III element nitride crystals. The rotation speed is, for example, 5 rpm to 40 rpm.

[0051] In one embodiment, the crucible is rotated intermittently. Specifically, the crucible is rotated for a predetermined time, and then the rotation is stopped for a predetermined time, and this operation is repeated. In this operation, by appropriately setting the rotation stop time, a Group III element nitride substrate having a satisfactory dopant concentration distribution in the thickness direction can be produced. This is thought to be because the flow of the melt composition in the crucible continues even after the rotation is stopped. By providing a predetermined stop time, the generation of turbulence can be suppressed, and abrupt changes in the crystal growth rate and the amount of dopant incorporated can be suppressed. If the stop time is short (which may include a case where the rotation is not stopped), the flow of the melt composition becomes turbulent, and the degree of stirring of the Group III element and dopant in the depth direction is thought to be increased. Furthermore, the amount of dopant incorporated into the growing crystal may vary periodically, and the dopant concentration in the thickness direction of the resulting Group III element nitride substrate may vary periodically. FIG. 4 is a cross-sectional view schematically illustrating an example of a distribution in which the dopant concentration changes periodically, using shades of color. In the example shown in FIG. 4, low-concentration regions with low dopant concentrations and high-concentration regions with high dopant concentrations are alternately formed. The arrows in FIG. 4 indicate the crystal growth direction, and the thickness t from when a low-concentration region begins to form to when the next low-concentration region begins to form is, for example, about 10 μm. In contrast, when turbulence in the melt composition in the crucible is suppressed (e.g., when the flow of the melt composition is laminar or when there is essentially no flow of the melt composition), the degree of stirring of the Group III element and the dopant in the depth direction is small, and it is assumed that the composition of the Group III element and the dopant near the gas-liquid interface and near the substrate surface is not significantly different. It is therefore believed that the dopant concentration in the resulting Group III element nitride substrate can be uniform across the thickness.

[0052] The time for which the rotation of the crucible is maintained is, for example, 10 to 1000 seconds, preferably 30 to 600 seconds. The time for which the rotation of the crucible is stopped is preferably 15 seconds or more, more preferably 20 seconds or more, even more preferably 25 seconds or more, and particularly preferably 30 seconds or more. On the other hand, the time for which the rotation of the crucible is stopped is, for example, 600 seconds or less.

[0053] The rotation direction of the crucible can be set to any appropriate direction. Specifically, the rotation direction before and after stopping may be the same direction or different directions. For example, during growth, the crucible may rotate clockwise, then stop, and then rotate counterclockwise. Also, during growth, the crucible may rotate clockwise, then stop, and then rotate clockwise again.

[0054] After growth of the group III element nitride crystal, in the example shown in FIG. 3C , the base substrate 21 is removed from the group III element nitride crystal (group III element nitride crystal layer 16) to obtain a free-standing substrate 32. Typically, as shown in the figure, the free-standing substrate 32 may include the group III element nitride crystal layer 16 and a seed crystal film 22. For example, the free-standing substrate 32 is obtained by separating the group III element nitride crystal layer 16 from the base substrate 21. The group III element nitride crystal may be separated from the base substrate by any appropriate method. Examples of methods for separating the group III element nitride crystal include a method of spontaneously separating the group III element nitride crystal from the base substrate by utilizing the difference in thermal contraction between the group III element nitride crystal and the base substrate during a cooling step after growth of the group III element nitride crystal, a separation method using chemical etching, and a laser lift-off method using laser light irradiation. Alternatively, the free-standing substrate may be obtained by, for example, grinding and removing the base substrate 21, slicing the base substrate using a wire saw, or the like.

[0055] The laminated substrate 30 and the free-standing substrate 32 may each have warpage. Fig. 5A is a cross-sectional view showing an example of warpage that may occur in the laminated substrate, and Fig. 5B is a cross-sectional view showing an example of warpage that may occur in the free-standing substrate. Note that in Figs. 5A and 5B, hatching is omitted from the cross sections of the laminated substrate and the free-standing substrate to make the drawings easier to see. Also, for convenience, the seed crystal film is omitted from the illustration.

[0056] In the example shown in FIG. 5A , the laminated substrate 30 has a convex warp toward the group III element nitride crystal layer 16 side. The base substrate 21 may be made of a material with a different composition (chemical composition) from that of the group III element nitride crystal layer 16. When a group III element nitride crystal is heteroepitaxially grown on such a base substrate 21, the resulting laminated substrate 30 tends to warp. Possible causes of warp include stress caused by a mismatch in lattice constants between the base substrate 21 and the group III element nitride crystal to be grown, or differences in thermal expansion coefficients. For example, warp can occur when the laminated substrate 30 is cooled after the group III element nitride crystal is grown at a high temperature. If the thermal expansion coefficient of the base substrate 21 is larger than that of the group III element nitride crystal to be grown (for example, when a sapphire substrate is used as the base substrate), a convex warp can occur toward the group III element nitride crystal layer 16 side, as shown in FIG. 5A . On the other hand, if the thermal expansion coefficient of the base substrate 21 is smaller than the thermal expansion coefficient of the Group III element nitride crystal to be grown (for example, if a silicon substrate or SiC substrate is used as the base substrate), convex warping may occur on the base substrate 21 side, contrary to the example shown in the figure.

[0057] The warpage of the laminated substrate can cause warpage in the resulting free-standing substrate 32 (Group III element nitride substrate). In the free-standing substrate 32 obtained by separating the base substrate 21 from the laminated substrate 30 shown in Fig. 5A, the direction of warpage can be reversed, resulting in convex warpage on the lower surface 33 side where the base substrate 21 was located, as shown in Fig. 5B.

[0058] The free-standing substrate 32 can be used as the above-mentioned Group III element nitride substrate as it is, but typically, the free-standing substrate 32 is subjected to any appropriate processing to obtain the above-mentioned Group III element nitride substrate.

[0059] One example of processing performed on the free-standing substrate is grinding of the peripheral portion (e.g., grinding using a diamond grinding wheel). Typically, the free-standing substrate is ground to have the desired shape and size (e.g., a disk shape having a desired diameter).

[0060] Other examples of processing performed on the freestanding substrate include grinding and polishing (e.g., lapping and chemical mechanical polishing (CMP)) of the main surfaces (upper and lower surfaces). Typically, the substrate is thinned and flattened to a desired thickness by grinding and polishing. In one embodiment, the seed crystal film 22 is removed by processing the main surface, leaving only the group III element nitride crystal layer 16 (only a single crystal growth layer).

[0061] 4 is used as the polished surface due to the processing of the main surface. If the dopant concentration varies significantly in the growth direction, the dopant concentration on the polished surface will vary significantly. Furthermore, as shown in Fig. 5C , if the freestanding substrate 32 is warped and the dopant concentration varies significantly in the growth direction (the direction of the arrow in Fig. 5C ), the dopant concentration on the polished surface shown by the dashed line in Fig. 5C will vary significantly. Here, the polished surface may correspond to the main surface of the resulting Group III nitride substrate.

[0062] Furthermore, for example, processing performed on the freestanding substrate includes chamfering the outer peripheral edge, removing a process-affected layer formed on the surface by grinding or polishing, and removing residual stress that may be caused by the process-affected layer.

[0063] C. Applications The above-described group III element nitride substrate can be typically applied to any appropriate semiconductor device. Specifically, any appropriate upper structural layer can be formed on the group III element nitride substrate. Note that a device substrate may be obtained that includes the group III element nitride substrate and at least a portion of the layers that constitute the upper structural layer.

[0064] 6 is a schematic cross-sectional view showing the general configuration of a semiconductor device according to one embodiment of the present invention, taking a HEMT device as an example. HEMT device 40 includes a group III element nitride substrate 10, a stacked structure 43 including a channel layer 41 and a barrier layer 42, in that order, and a source electrode 44, a drain electrode 45, and a gate electrode 46 provided on stacked structure 43. These electrodes may be metal electrodes each having a thickness of about 10-15 nm.

[0065] Each layer of the stacked structure 43 can be obtained by heterogrowth. For example, the channel layer 41 and the barrier layer 42 can be formed by epitaxial growth on the Group III element nitride substrate 10. This stacked structure may be referred to as an epitaxial substrate. The thickness of the channel layer 41 is, for example, 50 nm to 5 μm. The thickness of the barrier layer 42 is, for example, 2 nm to 40 nm.

[0066] The channel layer 41 and the barrier layer 42 may each be made of a Group III element nitride crystal. Examples of Group III elements constituting the Group III element nitride include Ga (gallium), Al (aluminum), and In (indium). These may be used alone or in combination. In one embodiment, the Group III element nitride substrate 10 may be made of gallium nitride doped with an element other than Ga. In this case, the channel layer 41 is preferably made of gallium nitride. The barrier layer 42 is preferably made of at least one selected from aluminum gallium nitride, aluminum indium nitride, and aluminum indium gallium nitride.

[0067] The channel layer 41 and the barrier layer 42 may each be formed by any appropriate method. In one embodiment, the channel layer 41 and the barrier layer 42 may each be formed by MOCVD. When the channel layer 41 and the barrier layer 42 are formed by MOCVD, a metal-organic (MO) precursor gas may be used as a Group III element source. For example, when a gallium nitride layer is formed as the channel layer 41 and an aluminum gallium nitride layer is formed as the barrier layer 42 by MOCVD, trimethylgallium (TMG) and trimethylaluminum (TMA) may be used as the Ga source and the Al source, respectively. Ammonia gas may be used as the nitrogen source. At least one of hydrogen gas and nitrogen gas may be used as the carrier gas.

[0068] Although not shown, a buffer layer may be disposed between the group III element nitride substrate 10 and the channel layer 41. For example, the buffer layer may be formed during the deposition of the channel layer, and may contain the material that constitutes the channel layer.

[0069] In one embodiment, the group III element nitride substrate can be bonded to a support substrate. Fig. 7 is a schematic cross-sectional view showing an outline of the configuration of a bonded substrate according to one embodiment of the present invention. The bonded substrate 60 includes a group III element nitride substrate 10 and a support substrate 62 disposed on the second main surface (e.g., nitrogen-polar surface) 12 side of the group III element nitride substrate 10. Although not shown, any appropriate epitaxial film can be formed on the group III element nitride substrate 10 of the bonded substrate 60.

[0070] The thickness of the support substrate 62 is, for example, 100 μm to 1000 μm. Any appropriate substrate can be used as the support substrate. Specifically, the support substrate may be made of a single crystal or a polycrystalline material.

[0071] Although not shown, the laminated substrate may further include any appropriate layer, the type, function, number, combination, arrangement, etc. of such layers may be appropriately determined depending on the purpose.

[0072] In one embodiment, the bonded substrate can be obtained by directly bonding the Group III element nitride substrate and the support substrate. For example, the bonding surface of the support substrate and / or the bonding surface of the Group III element nitride substrate can be surface-activated, and then the Group III element nitride substrate and the support substrate can be bonded to each other to obtain a bonded substrate.

[0073] The bonded substrate according to the embodiment of the present invention may have, for example, a bonding layer (not shown) disposed between the Group III element nitride substrate and the support substrate. In this case, the bonded substrate can be obtained by, for example, forming a bonding layer on the support substrate and / or the Group III element nitride substrate, and then bonding the Group III element nitride substrate and the support substrate via the bonding layer. During bonding, the bonding surfaces may be surface-activated.

[0074] From the viewpoint of obtaining excellent bonding strength, the bonding layer preferably contains at least one material selected from the group consisting of tantalum oxide, alumina, aluminum nitride, silicon carbide, sialon, and silicon oxide.

[0075] Typically, the sialon is a ceramic obtained by sintering a mixture of silicon nitride and alumina, and for example, Si ６－ａ Al ａ O ａ N ８－ａ Specifically, sialon has a composition in which alumina is mixed in silicon nitride, and a in the formula indicates the mixing ratio of alumina. a is preferably 0.5 or more and 4.0 or less. The silicon oxide is typically Si （１－ｂ） O ｂ (wherein 0.008≦b≦0.408).

[0076] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0077] Experimental Example 1 (Preparation of Seed Crystal Substrate) A c-plane sapphire substrate having a diameter of 50 mm was prepared, and a gallium nitride film having a thickness of 2 μm was formed on the sapphire substrate by MOCVD to prepare a seed crystal substrate.

[0078] (Growth of Gallium Nitride Crystals) Gallium nitride crystals were grown using a crystal manufacturing apparatus equipped with a pressure-resistant container capable of supplying high-pressure nitrogen gas, a turntable rotatable within the pressure-resistant container, an outer container placed on the turntable, and a crystal growth furnace for placing the outer container under a desired temperature environment. The obtained seed crystal substrate was placed in an alumina crucible in a nitrogen atmosphere glove box. Next, 40 g of metallic gallium, 80 g of metallic sodium, and 0.1 g of manganese as a doping element were filled into the crucible in the glove box, and the seed crystal substrate was immersed in the flux melt and covered with an alumina plate. In this state, the crucible was placed in a stainless steel inner container, which was then placed in a stainless steel outer container capable of accommodating the inner container, and the outer container was closed with a lid equipped with a nitrogen inlet pipe. In this state, the outer container was placed on a turntable located within the crystal manufacturing apparatus, and the pressure-resistant container of the crystal manufacturing apparatus was sealed with a lid. Next, the pressure vessel was evacuated to 0.1 Pa or less using a vacuum pump. Subsequently, the heater unit was operated to heat the crystal growth furnace in the crystal manufacturing apparatus to a uniform temperature of 850 °C, while nitrogen gas was introduced from a nitrogen gas cylinder into the pressure vessel until the pressure reached 4 MPa, and the outer vessel was rotated horizontally. Specifically, the outer vessel was rotated around the central axis at a speed of 20 rpm in a constant cycle of clockwise and counterclockwise rotation. Here, when the rotation direction was changed from clockwise to counterclockwise and from counterclockwise to clockwise, the rotation was temporarily stopped. The rotation stop time was 120 seconds. The rotation hold time was 300 seconds. This state was maintained for 35 hours, allowing gallium nitride crystal to grow. After that, the vessel was naturally cooled to room temperature and depressurized to atmospheric pressure. When the lid of the alumina crucible was opened, the grown gallium nitride crystal and the sapphire substrate were naturally peeled off. In this way, a gallium nitride crystal with a diameter of 50 mm and a thickness of 1 mm was obtained.

[0079] Thereafter, the surface of the gallium nitride crystal that had been separated from the sapphire substrate and the opposite surface were polished and flattened using diamond abrasive grains to obtain a Mn-doped gallium nitride substrate having a diameter of 50 mm and a thickness of 500 μm.

[0080] Experimental Example 2 A Mn-doped gallium nitride substrate was obtained in the same manner as in Experimental Example 1, except that the rotation stop time during gallium nitride crystal growth was changed from 120 seconds to 60 seconds.

[0081] Experimental Example 3 A Mn-doped gallium nitride substrate was obtained in the same manner as in Experimental Example 1, except that the rotation stop time during gallium nitride crystal growth was changed from 120 seconds to 30 seconds.

[0082] Experimental Example 4 A Mn-doped gallium nitride substrate was obtained in the same manner as in Experimental Example 1, except that the rotation stop time during gallium nitride crystal growth was changed from 120 seconds to 10 seconds.

[0083] Experimental Example 5 A Mn-doped gallium nitride substrate was obtained in the same manner as in Experimental Example 1, except that the rotation stop time during gallium nitride crystal growth was changed from 120 seconds to 5 seconds.

[0084] Experimental Example 6 A Mn-doped gallium nitride substrate was obtained in the same manner as in Experimental Example 1, except that the rotation stop time during gallium nitride crystal growth was changed from 120 seconds to 2 seconds.

[0085] Experimental Example 7 A Mn-doped gallium nitride substrate was obtained in the same manner as in Experimental Example 1, except that the rotation stop time during gallium nitride crystal growth was changed from 120 seconds to 1 second.

[0086] <Evaluation> The following evaluations were carried out on the gallium nitride substrates obtained in Experimental Examples 1 to 7. The evaluation results are summarized in Table 1. Furthermore, the resistivity distributions of the gallium nitride substrates of Experimental Examples 1 and 4 are shown in Figures 8 and 9 as representative examples.

[0087] 1. Distribution of Manganese Concentration in the Thickness Direction The manganese concentration was measured at the center position in a planar view of the substrate obtained in each experimental example. Specifically, a measurement sample measuring 20 mm x 20 mm was cut out so as to include the center position in a planar view of the obtained substrate, and the manganese concentration was measured from both the upper and lower surfaces of the measurement sample by secondary ion mass spectrometry (SIMS). The measurement conditions were as follows, and the maximum, minimum, and average values ​​of the manganese concentration were determined. The average value was calculated from 70 data points obtained at each of the upper and lower ends of the measurement sample. (Measurement Conditions) Type of secondary ion mass spectrometer: double focusing type Type of primary ion: O ２ ＋ Measurement range: from the surface (top and bottom surfaces of the obtained substrate) to a depth of 10 μm. Detection limit: 4 × 10 １４ cm －３

[0088] 2. Distribution of Manganese Concentration on the Main Surface The manganese concentration on the main surface (upper surface) of the substrates obtained in Experimental Examples 1, 3, 4, and 7 was measured. Specifically, the manganese concentration was measured at each of the five points (left, top, center, right, and bottom) shown in Figure 2B, and the manganese concentration was measured by secondary ion mass spectrometry (SIMS). The measurement conditions were as follows, and the maximum, minimum, and average values ​​of the manganese concentration were determined. Note that L in Figure 2B was set to 20 mm. (Measurement Conditions) Type of secondary ion mass spectrometer: double focusing type Type of primary ion: O ２ ＋ Measurement site: surface (upper surface of the obtained substrate) Detection limit: 4 x 10 １４ cm －３

[0089] 3. Resistivity Distribution on the Main Surface The resistivity of the main surface (top surface) of the substrate obtained in each experimental example was measured. Specifically, a 50 mm diameter substrate was divided into 4 mm grids, and the resistivity was measured at 121 points on the main surface of the substrate. The resistivity was measured using the capacitance method (a non-contact method using the change in charge amount over time). Specifically, the substrate was placed on a stage, and a probe was brought close to the substrate (with a gap of 1 mm to 2 mm) to form a capacitor. A pulse voltage with a pulse width of 100 ns was applied, and the change in the charge amount on the substrate over time was measured for 1 second at room temperature (25°C) to calculate the resistivity (Ω cm). The maximum, minimum, and average resistivity values ​​were then determined.

[0090] 4. Device Breakdown Voltage (Yield) (Fabrication of Epitaxial Substrate) A 1000 nm thick gallium nitride layer (channel layer) and a 25 nm thick aluminum gallium nitride layer (barrier layer) were epitaxially grown in this order on the main surface of the gallium nitride substrate obtained in each experimental example by MOCVD to fabricate an epitaxial substrate. Specifically, the obtained gallium nitride substrate was placed on a susceptor installed in the reactor of an MOCVD furnace, and each layer was formed under the following conditions. Before film formation, a mixed flow of hydrogen gas and nitrogen gas was introduced into the reactor of the MOCVD furnace, and the temperature was raised to 1050°C. After film formation, the substrate temperature was lowered to room temperature and restored to atmospheric pressure, and the epitaxial substrate was then removed from the MOCVD furnace. The film formation temperature below refers to the susceptor heating temperature, and the group 15 / group 13 gas ratio refers to the ratio (molar ratio) of the supply amount of ammonia, which is the group 15 (N) source, to the total supply amount of TMG (trimethylgallium), TMA (trimethylaluminum), and TMI (trimethylindium), which are group 13 (Ga, Al, In) source gases. The Al source gas / group 13 source gas ratio when forming an aluminum gallium nitride layer refers to the ratio (molar ratio) of the supply amount of Al source to the total supply amount of the group 13 (Ga, Al) sources, and can be determined depending on the desired aluminum gallium nitride composition. [Gallium nitride layer (channel layer)] Film formation temperature: 1050° C. Pressure inside reactor: 100 kPa Group 15 / Group 13 gas ratio = 2000 [Aluminum gallium nitride layer (barrier layer)] Film formation temperature: 1050° C. Pressure inside reactor: 5 kPa Group 15 / Group 13 gas ratio: 12000 Al source gas / Group 13 source gas ratio: 0.25

[0091] (Fabrication of Transistor Element) Next, a transistor element was fabricated using the epitaxial substrate. Specifically, photolithography and reactive ion etching (RIE) were used to etch away the aluminum gallium nitride layer (barrier layer) and the gallium nitride layer (channel layer) to a depth of approximately 100 nm at the boundary portions of each resulting transistor element. The boundary spacing was 1 mm. Next, photoresist was applied to the aluminum gallium nitride layer (barrier layer), and openings were formed by photolithography in the regions where the source and drain electrodes would be formed. Metal films of Ti, Al, Ni, and Au were sequentially deposited by vacuum deposition to thicknesses of 25 nm, 75 nm, 15 nm, and 100 nm, respectively, to form a multilayer structure. The substrate was then immersed in an organic solvent or a stripping solution, and the photoresist film was removed by lift-off to obtain source and drain electrodes. Next, to improve the ohmic properties of the source and drain electrodes, the substrate was subjected to heat treatment at 800°C for 30 seconds in a nitrogen gas atmosphere. Next, similar to the formation of the source and drain electrodes, Ni and Au metal films were sequentially formed to thicknesses of 6 nm and 12 nm, respectively, using photolithography and vacuum deposition to form gate electrodes, thereby fabricating a transistor element having electrodes with a gate length of 1 μm, a gate width of 100 μm, a source-to-gate spacing of 1 μm, and a gate-to-drain spacing of 10 μm.

[0092] Of the transistor elements fabricated as described above, 36 samples were randomly selected from the epitaxial substrate and the breakdown voltage was evaluated. Specifically, when the drain voltage Vd was gradually increased from 0 V while the gate voltage Vg = -10 V was applied, the drain current Id was 1 × 10 －５ A (normalized to a gate width of 100 μm, 1×10 －４ The drain voltage Vdb at which the drain voltage exceeded 300 V (A / mm) for the first time was determined. The larger the Vdb, the more desirable it is, and the evaluation criteria are as follows. Table 1 shows the yield (%) calculated from the number of samples out of 36 samples that had a Vdb of 300 V or more. (Evaluation criteria) Good: Vdb of 300 V or more Poor: Vdb of less than 300 V

[0093]

[0094] Group III nitride substrates according to embodiments of the present invention can be used, for example, as substrates for various semiconductor devices.

[0095] 10 Group III element nitride substrate, 11 First main surface, 12 Second main surface, 13 Side surface, 16 Group III element nitride crystal layer, 20 Seed crystal substrate (growth substrate), 21 Base substrate, 21a Upper surface, 21b Lower surface, 22 Seed crystal film, 30 Laminated substrate, 32 Freestanding substrate, 40 HEMT element, 41 Channel layer, 42 Barrier layer, 43 Laminated structure, 44 Source electrode, 45 Drain electrode, 46 Gate electrode, 60 Bonded substrate, 62 Support substrate, 70 Growth container.

Claims

1. A Group III element nitride substrate containing a dopant and having a first main surface and a second main surface facing each other, wherein the average dopant concentration C in the thickness direction of the substrate ａｖｅ１ is 1 x 10 １７ cm －３ 1x10 or more １９ cm －３ the dopant concentration in the thickness direction of the substrate and the average value C ａｖｅ１ The absolute value of the difference between the average value C ａｖｅ１ 20% or less of the above.

2. The Group III element nitride substrate according to claim 1, wherein the dopant is a carrier-compensating impurity.

3. The average value C of the dopant concentration at multiple locations within the surface of the substrate ａｖｅ２ is 1 x 10 １７ cm －３ 1x10 or more １９ cm －３ the dopant concentration at each of the plurality of portions in the surface of the substrate and the average value C ａｖｅ２ The absolute value of the difference between the average value C ａｖｅ２ The group III element nitride substrate according to claim 1 , wherein the surface roughness is 20% or less of the surface roughness.

4. The dopant concentration in the thickness direction of the substrate and its average value C ａｖｅ１ The Group III element nitride substrate according to claim 1 , wherein the thickness is measured in a range of 5 μm or more.

5. The dopant concentration in the thickness direction of the substrate and its average value C ａｖｅ１ is measured between a first portion of the first main surface and a second portion of the second main surface that are aligned on the same line extending in a thickness direction of the substrate.

6. The average value R of the resistivity at multiple locations on the surface of the substrate ａｖｅ is 1 x 10 ７ Ω·cm or more, and the minimum value R of resistivity at a plurality of locations within the surface of the substrate ｍｉｎ is the average value R ａｖｅ The group III element nitride substrate according to claim 1, which satisfies the following formula (1) with respect to R: ｍｉｎ ≧R ａｖｅ ×0.1...(1).

7. The III-nitride substrate of claim 1, wherein said dopant is manganese.

8. The Group III element nitride substrate of claim 1, comprising gallium nitride.

9. A bonded substrate comprising the Group III element nitride substrate according to any one of claims 1 to 8 and a support substrate.

10. A semiconductor device comprising: a layered structure including a Group III element nitride substrate according to any one of claims 1 to 8, a channel layer, and a barrier layer in this order; and a source electrode, a drain electrode, and a gate electrode provided on the layered structure.

11. A method for producing a Group III element nitride substrate according to any one of claims 1 to 8, comprising: preparing a growth substrate having upper and lower surfaces opposing each other; and growing a Group III element nitride crystal on said growth substrate by a flux method, wherein said growth vessel containing said growth substrate and raw materials is rotated for a predetermined period of time, and then said rotation is stopped for a predetermined period of time, and this is repeated to grow said Group III element nitride crystal.

12. The method for producing a Group III nitride substrate according to claim 11, wherein the rotation of the growth chamber is stopped for 15 seconds or more.

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