Composite substrate and substrate for growing group 13 element nitride epitaxially

Abstract

[Problem] In a composite substrate having a Group 13 element nitride semiconductor substrate and a support substrate bonded to the Group 13 element nitride semiconductor substrate, when an epitaxial film is grown on the Group 13 element nitride semiconductor substrate, the warpage of the composite substrate is suppressed, and the in-plane sheet resistance distribution of a HEMT structure formed on the composite substrate through epitaxial growth is suppressed. [Solution] This composite substrate 3 comprises: a Group 13 element nitride semiconductor substrate 2 having a first principal surface 2a and a second principal surface 2b; and a support substrate 1 having a bonding surface 1a bonded to the first principal surface 2a of the Group 13 element nitride semiconductor substrate 2. The bonding region of the support substrate 1 is composed of: silicon carbide having an average micropipe density of 10 cm-2 to 100 cm-2 in the bonding surface 1a of the support substrate 1; or synthetic diamond having an atomic number ratio of nitrogen atoms to carbon atoms of 500-2,000 ppm.

Description

Technical Field

[0001] The present invention relates to a composite substrate including a group 13 element nitride semiconductor substrate and a substrate for epitaxial growth of a group 13 element nitride.

Background Art

[0002] Group 13 element nitrides have a direct-transition wide bandgap, a high breakdown electric field, and a high saturated electron velocity, and thus are actively developed as semiconductor materials for high-frequency / high-power electronic devices. A typical electronic device using a group 13 element nitride is a high electron mobility transistor (HEMT). In recent years, development of HEMT devices using a group 13 element nitride substrate such as gallium nitride as a substrate for epitaxial growth, which can be expected to have high performance and high reliability due to epitaxial growth of a channel layer with little lattice strain, has also been promoted. The group 13 element nitride substrate serving as such a substrate for epitaxial growth is manufactured by a vapor phase method or a liquid phase method.

[0003] The group 13 element nitride substrate applied to epitaxial growth of HEMT devices preferably has a sufficiently high resistivity. And it is known that group 13 element nitrides doped with iron or manganese can obtain a relatively high resistivity (Patent Document 1).

[0004] As in Patent Document 1, when a transition element such as iron or manganese is doped into a group 13 element nitride, a group 13 element nitride substrate having a high resistivity can be obtained. When, for example, a gallium nitride layer is epitaxially grown on the epitaxial growth surface of this substrate, the dislocation density becomes lower than that of a gallium nitride layer grown using a high-resistance silicon carbide substrate or a sapphire substrate as a seed substrate. Therefore, especially when an HEMT structure is formed, the number of dislocations contained in the channel layer (gallium nitride layer) is reduced, and current collapse phenomena caused by the inside of the gallium nitride crystal are less likely to occur, and high reliability is expected to be obtained by suppressing the decrease in current gain.

[0005] In Patent Document 2, consideration has been given to bonding a support substrate made of a material having a higher thermal conductivity than the Group 13 element nitride semiconductor to the main surface of the Group 13 element nitride semiconductor substrate to form a composite substrate, and growing an epitaxial layer on this composite substrate. As the material of the support substrate, materials having a higher thermal conductivity than the material of the Group 13 element nitride semiconductor substrate and a small difference in thermal expansion coefficient have been studied. Specifically, a composite substrate in which a thermally conductive support substrate made of silicon carbide or diamond having a high thermal conductivity is bonded to a Group 13 element nitride semiconductor substrate is described (Patent Document 2).

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0007] On the other hand, as described in Patent Document 2, even when the material of the support substrate bonded to the Group 13 element nitride semiconductor substrate is silicon carbide or diamond, when an epitaxial film is grown on the Group 13 element nitride semiconductor substrate, warping of the composite substrate easily occurs, and when a HEMT structure is formed by epitaxial growth on the composite substrate, a sheet resistance distribution in the plane may occur. As a result, for example, in-plane variations occur in the characteristics of HEMT elements.

[0008] An object of the present invention is to suppress warping of a composite substrate and suppress the in-plane sheet resistance distribution of a HEMT structure formed by epitaxial growth on the composite substrate when an epitaxial film is grown on a Group 13 element nitride semiconductor substrate in a composite substrate having a Group 13 element nitride semiconductor substrate and a support substrate bonded to the Group 13 element nitride semiconductor substrate.

Means for Solving the Problems

[0009] The present invention relates to a group 13 element nitride semiconductor substrate having a first main surface and a second main surface, and a composite substrate having a support substrate having a bonding surface bonded to the first main surface of the group 13 element nitride semiconductor substrate, wherein the bonding region of the support substrate is made of silicon carbide having an average micropipe density of 10 cm -2 or more and 100 cm -2 or less, or synthetic diamond having an atomic number ratio of nitrogen atoms to carbon atoms of 500 ppm or more and 2000 ppm or less.

[0010] The present invention also relates to a substrate for group 13 element nitride epitaxial growth, which comprises the composite substrate and has the second main surface as an epitaxial growth surface of a group 13 element nitride.

Advantages of the Invention

[0011] The inventor studied the phenomenon that when an epitaxial film was grown on a group 13 element nitride semiconductor substrate, warping of the composite substrate was likely to occur, and an in-plane sheet resistance distribution occurred in a HEMT structure formed by epitaxial growth on the composite substrate. As a result, the following findings were obtained.

[0012] That is, for example, in order to grow a Group 13 element nitride by the MOCVD method, heat treatment at 900 to 1100 °C is required. In this case, the material of the support substrate bonded to the Group 13 element nitride semiconductor substrate is diamond or silicon carbide, and even when the thermal expansion coefficients are matched as much as possible, after epitaxial growth, it has been found that due to the stress of the formed epitaxial film, the composite substrate warps at room temperature. When the composite substrate warps, in the exposure process for forming an electrode on the epitaxial film, in-plane focus variations occur, making it difficult to form an electrode with a desired shape and dimensions. Therefore, it is desired that the warpage amount be 20 μm or less. Furthermore, it has been found that in accordance with the warpage of the composite substrate after growth of the epitaxial film, in-plane property distributions occur in the epitaxial film, and in particular, in-plane sheet resistance distributions occur.

[0013] For this reason, the inventor further examined the material of the support substrate bonded to the Group 13 element nitride semiconductor substrate. Here, ordinary diamond or silicon carbide with high density and good crystallinity was used as the material of the support substrate, but this was changed to silicon carbide with an average micropipe density of 10 cm -2 or more at the bonding surface of the support substrate, or to synthetic diamond with a high impurity concentration where the atomic number ratio of nitrogen atoms to carbon atoms is 500 ppm or more. As a result, it was found that when an epitaxial film was grown on the Group 13 element nitride semiconductor substrate, the warpage of the composite substrate was suppressed, and the in-plane sheet resistance distribution of the HEMT structure formed by epitaxial growth on the composite substrate was suppressed, leading to the present invention.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0015] Fig. 1(a) is a schematic diagram of a composite substrate 3 according to an embodiment of the present invention, and Fig. 1(b) is a schematic diagram showing a HEMT element 10. The group-13 element nitride semiconductor substrate 2 has a first main surface 2a and a second main surface 2b facing the opposite side of the first main surface 2a. The support substrate 1 consists of a base substrate 11 and a bonding region 12, and the bonding surface 1a of the support substrate 1 is bonded to the first main surface 2a of the group-13 element nitride semiconductor substrate 2. The second main surface 2b of the group-13 element nitride semiconductor substrate 2 is selected as an epitaxial growth surface, and an epitaxial film is formed on the second main surface 2b. Specifically, in this example, a buffer layer 4 is formed on the second main surface 2b of the group-13 element nitride semiconductor substrate 2, a channel layer 5 is formed on the buffer layer 4, and a barrier layer 6 is formed on the channel layer 5. A predetermined electrode can be provided on the surface 6a of the barrier layer 6. In this example, a source electrode 9, a gate electrode 8, and a drain electrode 7 are formed.

[0016] By using the group-13 element nitride semiconductor substrate of the present invention as an epitaxial growth template substrate, a HEMT element capable of high-power operation can be realized. By using such a HEMT element, a power amplifier that operates with high power, high frequency, and high efficiency required for a base station for next-generation wireless communication can be realized.

[0017] (Group-13 element nitride semiconductor substrate) The group-13 element nitride semiconductor substrate is composed of a group-13 element nitride semiconductor. The group-13 element is a group-13 element defined by IUPAC, and is particularly preferably gallium, aluminum, and / or indium. Further, as the group-13 element nitride semiconductor, a group-13 element nitride semiconductor selected from gallium nitride, aluminum nitride, indium nitride, or a mixed crystal thereof is preferable. More specifically, GaN, AlN, InN, Ga x Al 1-x N(0 < x < 1), Ga x In 1-x N(0 < x < 1), Al x In 1-x N(0 < x < 1), Gax Al y In z N satisfies 0 < x < 1, 0 < y < 1, and x + y + z = 1.

[0018] In a preferred embodiment, the resistivity of the Group 13 element nitride semiconductor substrate at room temperature is 1×10 6 Ω·cm or more. That is, the Group 13 element nitride semiconductor substrate is semi-insulating. From this perspective, the resistivity of the Group 13 element nitride semiconductor substrate at room temperature is preferably 1×10 7 Ω·cm or more, and more preferably 1×10 9 Ω·cm or more. Also, the resistivity of the Group 13 element nitride semiconductor substrate at room temperature is often 1×10 13 Ω·cm or less.

[0019] Also, in a preferred embodiment, the dislocation density of the second main surface of the Group 13 element nitride semiconductor substrate is 10 6 cm -2 or less. This dislocation density is preferably 10 5 cm -2 or less. Also, in practice, this dislocation density is often 10 5 cm -2 or more.

[0020] The second main surface (epitaxial growth surface) of the Group 13 element nitride semiconductor substrate may be a Group 13 element polar surface or a nitrogen polar surface.

[0021] In a preferred embodiment, the Group 13 element nitride semiconductor substrate is doped with one or more elements selected from the group consisting of manganese, iron, and zinc. This can improve the resistivity of the Group 13 element nitride semiconductor substrate.

[0022] In a preferred embodiment, the manganese concentration in the Group 13 element nitride semiconductor is preferably 1×10 18 atoms / cm 3 ~1×10 19 atoms / cm 3 and more preferably 2×1018 atoms / cm 3 ~5×10 18 atoms / cm 3 It is more preferably that. In a preferred embodiment, the iron concentration in the group 13 element nitride semiconductor is 8×10 16 atoms / cm 3 ~5×10 19 atoms / cm 3 It is preferably that, and 5×10 17 atoms / cm 3 ~1×10 19 atoms / cm 3 It is more preferably that. Also, in a preferred embodiment, the zinc concentration in the group 13 element nitride semiconductor is 1×10 17 atoms / cm 3 ~3×10 18 atoms / cm 3 It is preferably that, and 2×10 17 atoms / cm 3 ~1×10 18 atoms / cm 3 It is more preferably that. Note that the manganese concentration, iron concentration, and zinc concentration in the group 13 element nitride semiconductor are measured by SIMS (Secondary Ion Mass Spectrometry).

[0023] Note that the group 13 element nitride semiconductor may contain elements other than zinc, iron, and manganese. Examples of the elements include hydrogen (H), oxygen (O), silicon (Si), carbon (C), and the like.

[0024] (Manufacture of Group 13 Element Nitride Semiconductor Substrate) Examples of the manufacturing method of the group 13 element nitride semiconductor substrate include vapor phase methods such as metalorganic chemical vapor deposition (MOCVD) method, hydride vapor phase epitaxy (HVPE) method, pulse excitation deposition (PXD) method, molecular beam epitaxy (MBE) method, sublimation method, and liquid phase methods such as ammonothermal method and flux method. Particularly preferably, the group 13 element nitride semiconductor substrate is manufactured by the flux method.

[0025] In the case of the flux method, it is preferable to obtain a Group 13 element nitride semiconductor substrate by immersing a seed substrate in a flux containing manganese, iron, and / or zinc and growing a Group 13 element nitride on the seed substrate in a high-temperature and high-pressure atmosphere. Particularly preferably, a seed crystal film is provided on the surface of a support substrate made of sapphire, a Group 13 element nitride single crystal, etc. to form a seed substrate, and a Group 13 element nitride semiconductor is grown on the seed crystal film.

[0026] As a material of the seed crystal film, AlxGa1-xN (0 ≤ x ≤ 1) or InxGa1-xN (0 ≤ x ≤ 1) can be exemplified as suitable examples, and gallium nitride is particularly preferable. As a method for forming the seed crystal film, a vapor phase growth method is preferable, and an organometallic chemical vapor deposition (MOCVD) method, a hydride vapor phase growth (HVPE) method, a pulse excitation deposition (PXD) method, an MBE method, and a sublimation method can be exemplified. The organometallic chemical vapor deposition method is particularly preferable. Also, the growth temperature is preferably 950 to 1100°C.

[0027] When a Group 13 element nitride semiconductor is grown by the flux method, the type of the flux is not particularly limited as long as it can grow a Group 13 element nitride semiconductor. In a preferred embodiment, it is a flux containing at least one of an alkali metal and an alkaline earth metal, and a flux containing sodium metal is particularly preferable. A metal raw material substance is mixed and used in the flux. As the metal raw material substance, a simple metal, an alloy, and a metal compound can be applied, but a simple metal is preferable from the viewpoint of handling.

[0028] The growth temperature and the holding time during growth of the Group 13 element nitride semiconductor in the flux method are not particularly limited and can be appropriately changed according to the composition of the flux. In one example, when growing gallium nitride using a sodium- or lithium-containing flux, the growth temperature is preferably 800 to 950°C, and more preferably 850 to 900°C.

[0029] In the flux method, a group 13 element nitride semiconductor is grown in an atmosphere containing a gas containing nitrogen atoms. This atmosphere is preferably nitrogen gas, but ammonia may also be used. The pressure of the atmosphere is not particularly limited, but from the viewpoint of preventing evaporation of the flux, a pressure of 10 atmospheres or more is preferable, and 30 atmospheres or more is more preferable. However, since the apparatus becomes large when the pressure is high, the total pressure of the atmosphere is preferably 2000 atmospheres or less, and more preferably 500 atmospheres or less. The gas other than the gas containing nitrogen atoms in the atmosphere is not limited, but an inert gas is preferable, and argon, helium, and neon are particularly preferable.

[0030] In a particularly preferred embodiment, a seed crystal film made of gallium nitride is grown on a sapphire substrate by the MOCVD method to obtain a seed substrate. This seed substrate is placed in a crucible, and subsequently, the crucible is filled with 10 to 50 mol% of metallic Ga, 50 to 90 parts by mass of metallic Na, and 0.0001 to 1 mol% of metallic Mn, metallic Fe, and metallic Zn. By appropriately controlling the addition amounts of metallic Mn, metallic Fe, and metallic Zn within the above-described ranges, it is possible to control the respective concentrations in the group 13 element nitride semiconductor. This crucible is placed in a heating furnace, the temperature inside the furnace is set to 800°C to 950°C, the pressure inside the furnace is set to 3 MPa to 5 MPa, and it is heated for about 20 hours to 400 hours, and then cooled to room temperature. After cooling is completed, the crucible is taken out of the furnace. The gallium nitride thus obtained is polished using diamond abrasive grains to flatten its surface.

[0031] (Support substrate) The support substrate is bonded to the first main surface of the group 13 element nitride semiconductor substrate. Here, the bonding region of the support substrate is silicon carbide in which the average micropipe density on the bonding surface of the support substrate is 10 cm -2 or more and 100 cm -2 or less, or synthetic diamond in which the atomic number ratio of nitrogen atoms to carbon atoms is 500 ppm or more and 2000 ppm or less.

[0032] As the silicon carbide which is the material of the bonding region of the support substrate, by using silicon carbide of poor quality, so-called dummy grade, which has many dislocations, defects and micropipes, when an epitaxial film is grown on a group-13 element nitride semiconductor substrate, the warpage of the composite substrate is suppressed, and the in-plane sheet resistance distribution of the HEMT structure formed by epitaxial growth on the composite substrate is suppressed.

[0033] Specifically, the bonding region of the support substrate is made of silicon carbide in which the average micropipe density on the bonding surface is 10 cm -2 or more and 100 cm -2 or less. By setting the average micropipe density of this silicon carbide to 10 cm -2 or more, the warpage of the composite substrate after forming the HEMT structure can be reduced, and the variation in the in-plane sheet resistance of the HEMT structure can also be reduced. From this viewpoint, it is more preferable that the average micropipe density of silicon carbide on the bonding surface is 30 cm -2 or more. Further, when the average micropipe density of silicon carbide exceeds 100 cm -2 , it has been found that the warpage of the composite substrate after forming the HEMT structure can be reduced, but the variation in the in-plane sheet resistance of the HEMT structure rather becomes large. For this reason, the average micropipe density of silicon carbide is set to 100 cm -2 or less, and more preferably 70 cm -2 or less.

[0034] Examples of the manufacturing method of the silicon carbide include the sublimation method and the high-temperature chemical vapor deposition (CVD) method. There are various crystal polymorphs (polytypes) of silicon carbide, and any polytype can be applied. However, from the viewpoints of thermal conductivity and availability, 4H and 6H are preferable. Further, this silicon carbide may be either single crystal or polycrystal, but it is desirable that the bonding surface is smooth, and from this viewpoint, single crystal is desirable.

[0035] In addition, when the bonding region of the support substrate is formed of the aforementioned synthetic diamond, point defects associated with the inclusion of nitrogen atoms are generated inside the bonding region. As a result, when an epitaxial film is grown on a Group 13 element nitride semiconductor substrate, the warpage of the composite substrate is suppressed, and the in-plane sheet resistance distribution of the HEMT structure on the composite substrate is also suppressed.

[0036] Specifically, the bonding region of the support substrate is made of synthetic diamond in which the atomic ratio of nitrogen atoms to carbon atoms is 500 ppm or more and 2,000 ppm or less. By setting the atomic ratio of nitrogen atoms to carbon atoms to 500 ppm or more, the warpage of the composite substrate after forming the epitaxial film can be reduced, and the variation in the in-plane sheet resistance of the HEMT structure can also be reduced. From this perspective, it is more preferable to set the atomic ratio of nitrogen atoms to carbon atoms to 800 ppm or more. Also, when the atomic ratio of nitrogen atoms to carbon atoms exceeds 2,000 ppm, although the warpage of the composite substrate after forming the HEMT structure can be reduced, it has been found that the variation in the in-plane sheet resistance of the HEMT structure rather increases. For this reason, the atomic ratio of nitrogen atoms to carbon atoms is set to 2,000 ppm or less, preferably 1,500 ppm or less. Further, in order to cause stress relaxation evenly in the plane, it is desirable that nitrogen atoms are uniformly dispersed in the synthetic diamond as isolated substitutional impurities.

[0037] Examples of the method for producing the synthetic diamond used in the present invention include the HPHT method and the CVD method, and the CVD method is more desirable. Also, the synthetic diamond may be either single crystal or polycrystal, but it is desirable that the bonding surface is smooth. From this perspective, a single crystal that can obtain a flat surface by polishing is desirable.

[0038] For example, the entire support substrate 1 as shown in Fig. 1(a) may be made of the silicon carbide or the synthetic diamond. That is, the entire underlying substrate 11 and the bonding region 12 may be made of the silicon carbide or the synthetic diamond. However, it is not necessary for the entire support substrate 1 to be made of the silicon carbide or the synthetic diamond, and it is sufficient if the bonding region 12 including the bonding surface of the support substrate is made of at least the silicon carbide or the synthetic diamond. This "bonding region" indicates a range with a thickness of 100 μm as viewed from the bonding surface of the support substrate. When the materials of the bonding region 12 and the underlying substrate 11 are different, the underlying substrate 11 may be made of a single crystal silicon, polycrystalline silicon, single crystal sapphire, polycrystalline alumina, single crystal and sintered body made of aluminum nitride.

[0039] The bonding surface of the support substrate is preferably made into a flat surface by performing planarization by polishing such as CMP. Alternatively, a flat surface can also be formed by forming a thin film made of the silicon carbide or the synthetic diamond on the bonding surface of the support substrate by CVD method. Further, the arithmetic mean roughness Ra of the bonding surface of the support substrate is preferably 5 nm or less, and more preferably 0.5 nm or less.

[0040] When an element produced by epitaxial growth is formed on silicon carbide with a high micropipe density or synthetic diamond with a high nitrogen content using such materials, generally the characteristics deteriorate, so it is considered unfavorable. However, in the structure of the present invention, since an epitaxial film that operates as a functional layer of the element is formed on a group-13 element nitride semiconductor substrate with a low dislocation density bonded to the support substrate, the influence due to the poor material of the support substrate does not directly affect the epitaxial film.

[0041] From the viewpoint of suppressing a decrease in operating efficiency due to a temperature rise during the operation of an epitaxial film formed on the epitaxial growth surface of a Group 13 element nitride semiconductor substrate, for example, a HEMT device, it is preferable to shorten the distance between the support substrate and the epitaxial film. From such a viewpoint, the thickness of the Group 13 element nitride semiconductor substrate is preferably 150 μm or less, and more preferably 50 μm or less. Further, since the Group 13 element nitride semiconductor substrate is bonded to the support substrate, even after the Group 13 element nitride semiconductor substrate is polished thinly to 150 μm or less, there is no concern of breakage and it becomes easy to handle. Therefore, it is preferable to polish the Group 13 element nitride semiconductor substrate after bonding.

[0042] For the bonding between the Group 13 element nitride semiconductor substrate and the support substrate, direct bonding is desirable, but indirect bonding using an inorganic material that can withstand high temperatures as an intermediate layer is also acceptable. Direct bonding is performed by obtaining a clean surface on the bonding surface by wet cleaning or the like, and then irradiating the bonding surface with a neutralized beam to activate it. As a beam source, a saddle field type high-speed atomic beam source is a suitable example. Further, the voltage during activation by beam irradiation is preferably 0.5 to 2.0 kV, and the current is preferably 50 to 200 mA. For indirect bonding, a SiOx-based material, which is an inorganic material, is preferable as the intermediate layer (x = 1 to 2). After the support substrate is plasma-treated, an amorphous SiOx-based glass film is formed by plasma CVD using a source gas containing a silicon compound, and then the Group 13 element nitride semiconductor substrate is bonded to the base substrate. Examples of the source gas containing a silicon compound include silane, disilane, hexamethyldisiloxane (HMDSO), tetramethyldisiloxane (TMDSO), methyltrimethoxysilane (MTMOS), methylsilane, dimethylsilane, trimethylsilane, diethylsilane, and the like.

[0043] (Growth of epitaxial film) As materials for the epitaxial film grown on the second main surface of the group 13 element nitride semiconductor substrate, gallium nitride, aluminum nitride, indium nitride, or a mixed crystal thereof can be exemplified. Specifically, GaN, AlN, InN, Ga x Al 1-x N (0 < x < 1), Ga x In 1-x N (0 < x < 1), Al x In 1-x N (0 < x < 1), Ga x Al y In z N (0 < x < 1, 0 < y < 1, x + y + z = 1) can be mentioned. Further, as functional layers provided on the group 13 element nitride semiconductor substrate, a channel layer, a buffer layer, a barrier layer, a light emitting layer, a rectifying element layer, and a switching element layer can be exemplified.

[0044] For example, as shown in FIG. 1(b), a buffer layer 4, a channel layer 5, and a barrier layer 6 are formed on the second main surface 2b of the group 13 element nitride semiconductor substrate 2. The formation of the buffer layer 4, the channel layer 5, and the barrier layer 6 can be realized, for example, by metalorganic chemical vapor deposition (MOCVD method). The layer formation by the MOCVD method supplies a metalorganic raw material gas (such as TMG (trimethylgallium), TMA (trimethylaluminum), TMI (trimethylindium), etc.) corresponding to the target composition, ammonia gas, hydrogen gas, and nitrogen gas into the reactor of the MOCVD furnace, and while heating the group 13 element nitride semiconductor substrate placed in the reactor to a predetermined temperature, group 13 element nitrides are sequentially generated by the gas phase reaction of the metalorganic raw material gas and ammonia gas corresponding to each layer.

Example

[0045] (Experiment 1) (Prototype of the support substrate 1 made of silicon carbide) Using the sublimation method (PVT method), the vanadium concentration is 1 × 10 17 ~1 × 10 18 cm -3A support substrate made of a 3-inch semi-insulating 4H-SiC single crystal with a thickness of 0.5 mm was prepared so as to achieve the following. On the bonding surface 1a of the support substrate 1 made of a 4H-SiC single crystal, nine points shown in FIG. 2 were observed with a polarized light microscope, the micropipes within a region of 3 mm × 4 mm were counted, and the average micropipe density was calculated. However, in FIG. 2, the approximate center of the bonding surface 1a was designated as P1, four points P2 on the circle C1 with a radius of 30 mm from the center P1, and four points P3 on the circle C2 with a radius of 60 mm from the center P1 were used as measurement points. The four points P2 on the circle C1 were each at a position 90° apart, and the four points P3 on the circle C2 were each at a position 90° apart. The average micropipe density on the bonding surface 1a of the support substrate 1 was changed as shown in Table 1. However, the average micropipe density was adjusted by changing the vanadium concentration. Incidentally, the presence or absence of micropipes can be confirmed by analyzing the birefringence image observed with a polarized light microscope.

[0046] (Prototype of gallium nitride substrate 2) Next, a gallium nitride substrate made of 3-inch Fe-doped gallium nitride was fabricated. Specifically, a seed crystal film made of gallium nitride with a thickness of 2 μm was formed on the surface of a 3-inch diameter c-plane sapphire substrate by the MOCVD method to obtain a seed substrate. On this seed substrate, a gallium nitride single crystal was formed using the Na flux method. Specifically, 50 g of metallic Ga, 100 g of metallic Na, and metallic Fe were each filled into an alumina crucible, and the crucible was covered with an alumina lid. The crucible was placed in a heating furnace, the temperature inside the furnace was set to 850 °C, the pressure inside the furnace was set to 4.0 MPa, and it was heated for 100 hours, and then cooled to room temperature. After the cooling was completed, when the alumina crucible was taken out of the furnace, a brown gallium nitride single crystal was deposited on the surface of the seed substrate with a thickness of about 1000 μm.

[0047] The thus obtained gallium nitride single crystal was polished using diamond abrasive grains to flatten its surface and to make the total thickness of the gallium nitride single crystal formed on the underlying substrate 700 μm. The seed substrate was separated from the gallium nitride single crystal by the laser lift-off method to obtain a gallium nitride substrate.

[0048] By polishing the first main surface and the second main surface of the gallium nitride substrate respectively, a gallium nitride substrate with a thickness of 400 μm was obtained. When measuring the in-plane resistivity of the bonding surface of the obtained gallium nitride substrate, a resistivity of 10 7 Ω·cm or more was obtained. Note that the resistivity of each gallium nitride substrate was measured by the capacitance method (COREMA-WT manufactured by SEMIMAP).

[0049] (Prototype of composite substrate) Next, the above gallium nitride substrate 2 and each support substrate 1 were bonded by a direct bonding method. Specifically, the first main surface (nitrogen-polar surface) 2a of the gallium nitride substrate 2 and the bonding surface (silicon-polar surface) 1a of the support substrate were surface-activated and directly bonded. The second main surface 2b of the gallium nitride substrate 2 was used as a gallium-polar surface and an epitaxial growth surface. Also, the warpage of the obtained composite substrate 3 was 5 μm or less in all cases.

[0050] (Prototype of HEMT device) Next, an HEMT structure as shown in Fig. 1(b) was epitaxially grown on the main surface 2b of the gallium nitride substrate of this composite substrate by the MOCVD method. The composition and film thickness of each layer are as follows. (Composition) (Film thickness: nm) Buffer layer 4: GaN 500 Channel layer 5: GaN 150 Barrier layer 6: AlGaN (Al ratio is 0.2): Film thickness is 20 nm

[0051] (Evaluation) After the growth of each epitaxial film, the obtained HEMT structure was taken out from the MOCVD apparatus, and the warpage of the composite substrate on which the HEMT structure was formed was measured. This measurement was performed using "FT-17" manufactured by NIDEK to measure the SORI value.

[0052] Also, the distribution of the sheet resistance value in the plane of the HEMT structure 10 was calculated. The sheet resistance was measured non - contact using the "NC - 80MAP" manufactured by Napson Corporation with a measurement probe having a diameter of 14 mm. However, the measurement points were set to 9 points shown in Fig. 2. In Fig. 2, taking the approximate center of the surface 6a of the barrier layer 6 as P1, four points P2 on the circle C1 with a radius of 30 mm from the center P1 and four points P3 on the circle C2 with a radius of 60 mm from the center P1 were used as the measurement points. The four points P2 on the circle C1 are each at a position 90° apart, and the four points P3 on the circle C2 are each at a position 90° apart. The calculation of the in - plane sheet resistance distribution used the following formula. In - plane sheet resistance distribution = (Maximum value of sheet resistance - Minimum value of sheet resistance) / (Average value of sheet resistance) The measurement results of the in - plane sheet resistance distribution and the warp are shown in Table 1.

[0053]

Table 1

[0054] When the average micropipe density on the bonding surface of silicon carbide, which is the material of the support substrate, is 10 cm -2 or more, the in - plane sheet resistance distribution becomes small and the warp also decreases. From this perspective, it is more preferable that the average micropipe density is 30 cm -2 or more. When the average micropipe density is low, the reason for the large in - plane distribution of the sheet resistance is considered to be that due to the large warp, an in - plane distribution occurred in the composition such as Al during the film formation of the epitaxial film.

[0055] Also, when the average micropipe density on the bonding surface of silicon carbide, which is the material of the support substrate, is 100 cm -2 or less, the in - plane sheet resistance distribution becomes small. From this perspective, it is more preferable that the average micropipe density is 70 cm -2 or less. lower From this perspective, it is more preferable that the average micropipe density is 70 cm -2When it exceeded, the obtained barrier layer surface was observed by an atomic force microscope (AFM). As a result, microcracks occurred on the barrier layer surface, and the distribution of the microcracks was biased in the plane. When the micro-pipe density was high, there was also a bias in the density of micro-pipes on the bonding surface of the support substrate made of silicon carbide. The stress relaxation between the bonding surface of the support substrate and the first main surface (bonding surface) of the gallium nitride substrate became non-uniform in the plane, resulting in microcracks. As a result, when the epitaxial film was formed, there were portions where the two-dimensional electron gas was suppressed low, and it was considered that the in-plane sheet resistance distribution increased. When this average micro-pipe density was 30 cm -2 or more and 70 cm -2 or less lower the in-plane sheet resistance distribution after the formation of the epitaxial film was less than 10%, and an SORI value of less than 10 μm was obtained.

[0056] (Experiment 2) On the (100) surface of a 3-inch silicon single crystal substrate, a single crystal synthetic diamond layer was uniformly grown to a thickness of 0.1 mm while slightly adding nitrogen gas using the CVD method. Next, the bonding surface of this synthetic diamond layer and the first main surface (nitrogen polar surface) of the gallium nitride substrate were bonded by a direct bonding method. Next, the silicon single crystal substrate was removed by etching using hydrofluoric acid. As a result, a composite substrate 3 of a support substrate 1 made of synthetic diamond and a gallium nitride substrate was obtained.

[0057] Here, two support substrates were simultaneously fabricated under the same conditions. The nitrogen content in one of them was measured, and the remaining one was used for the fabrication of the composite substrate and the epitaxial growth process. The nitrogen content was measured by SIMS. At this time, measurements were performed to a depth of 30 μm at 9 points in the bonding plane shown in FIG. 2, and the average value was adopted.

[0058] Next, in the same manner as in Experiment 1, a buffer layer 4, a carrier layer 5, and a barrier layer 6 were formed to manufacture a HEMT structure 10. For the obtained HEMT structure 10, the SORI value and the in-plane sheet resistance distribution were measured, and the results are shown in Table 2.

[0059]

Table 2

[0060] As a result, by making the nitrogen content of the synthetic diamond constituting the support substrate much higher than that of ordinary products by nitrogen addition and setting it to 500 ppm or more, warpage was reduced and the in-plane sheet resistance distribution also became smaller. From this point of view, it is more preferable to set the nitrogen content of the synthetic diamond constituting the support substrate to 800 ppm or more. It is considered that increasing the nitrogen content of the synthetic diamond constituting the support substrate increased minute defects, resulting in a reduction in warpage and a narrowing of the in-plane sheet resistance distribution.

[0061] Also, when the nitrogen content of the synthetic diamond constituting the support substrate exceeded 2000 ppm, although the warpage was small, the in-plane sheet resistance distribution of the HEMT structure increased by more than 20%. When the obtained barrier layer surface was observed with an atomic force microscope (AFM), minute cracks were found on the barrier layer surface, and the distribution of the minute cracks was biased in the plane. Therefore, when the nitrogen content of the synthetic diamond is high, stress relaxation between the bonding surface of the support substrate and the first main surface (bonding surface) of the gallium nitride substrate becomes non-uniform in the plane, minute cracks occur, and as a result, there are places where the two-dimensional electron gas is suppressed low during the film formation of the epitaxial film, and it is considered that the in-plane sheet resistance distribution becomes large. When the material of the support substrate had a nitrogen content of 800 ppm or more and 1500 ppm or less, the in-plane sheet resistance distribution of the HEMT structure was less than 10%, and the SORI value was less than 20 μm.

Claims

1. A group-13 element nitride semiconductor substrate having a first major surface and a second major surface, and A composite substrate having a support substrate having a bonding surface bonded to the first major surface of the group-13 element nitride semiconductor substrate, The bonding region of the support substrate has an average micropipe density on the bonding surface of the support substrate of 10 cm －２ or more and 100 cm －２ or less, and is made of silicon carbide, or synthetic diamond having an atomic number ratio of nitrogen atoms to carbon atoms of 500 ppm or more and 2000 ppm or less. A composite substrate characterized by that.

2. The resistivity of the Group 13 element nitride semiconductor substrate at room temperature is 10 ６ Ω·cm or more, and the composite substrate according to claim 1, characterized in that.

3. The dislocation density of the second main surface of the Group 13 element nitride semiconductor substrate is 10 ６ cm －２ or less. The composite substrate according to claim 1 or 2, characterized in that.

4. The composite substrate according to claim 1 or 2, wherein the second major surface of the group-13 element nitride semiconductor substrate is a group-13 element polar surface or a nitrogen polar surface.

5. The composite substrate according to claim 1 or 2, wherein the group-13 element nitride semiconductor substrate is doped with an element selected from the group consisting of manganese, iron, and zinc.

6. The composite substrate according to claim 1 or 2, wherein the first major surface of the group-13 element nitride semiconductor substrate and the bonding surface of the support substrate are directly bonded.

7. The composite substrate according to claim 1 or 2, further comprising a bonding layer present between the first major surface of the group-13 element nitride semiconductor substrate and the bonding surface of the support substrate.

8. A substrate for group-13 element nitride epitaxial growth, comprising the composite substrate according to claim 1 or 2, wherein the second major surface is an epitaxial growth surface of a group-13 element nitride.

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