Group-iii nitride substrate and method for manufacturing the same

Abstract

To provide a group-III nitride substrate having good surface flatness while satisfying high carrier concentration, low resistance and low dislocation density.SOLUTION: A group-III nitride substrate comprises a GaN epitaxial layer formed on a GaN substrate, contains oxygen as an impurity element, and includes: a first region showing a first impurity concentration in a surface of a c-surface of the group-III nitride substrate; and a second region showing a second impurity concentration lower than the first impurity concentration. A first dislocation density in the first region is lower than a second dislocation density in the second region. A square average surface roughness RMS value is 10 nm or lower in an optional range of 0.2 mm square in the surface of the group-III nitride substrate.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a group III nitride substrate and a method for manufacturing the same. [Background technology]

[0002] Vertical GaN power devices require GaN substrates with low resistance and low dislocation density. For example, when fabricating n-type low-resistivity GaN substrates, crystal growth has been performed while forming pits to increase the carrier concentration and reduce the dislocation density (see, for example, Patent Document 1). [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2021-50107 Summary of the Invention [Problem to be solved by the invention]

[0004] The surfaces that form the pits (growth pits) that appear during crystal growth are composed of m-plane and a-plane components, and each surface has different impurity concentrations, such as oxygen, and carrier concentrations. Therefore, during the polishing process used to smooth the surface, there are differences in the polishing rate within the surface due to differences in impurity and carrier concentrations on each growth surface. The difference in polishing rate creates surface irregularities. The surface irregularities cause deterioration of surface morphology and the appearance of pits when device layers are subsequently formed on the substrate surface using methods such as MOVPE.

[0005] An object of the present disclosure is to provide a group III nitride substrate that has good surface flatness while satisfying the requirements of high carrier concentration, low resistivity, and low dislocation density. [Means for solving the problem]

[0006] A III-nitride substrate according to the present disclosure is a III-nitride substrate consisting of a GaN epitaxial layer formed on a GaN substrate, the III-nitride substrate containing oxygen as an impurity element, the III-nitride substrate having, within a polished c-plane thereof, a first region exhibiting a first impurity concentration and a second region exhibiting a second impurity concentration lower than the first impurity concentration, the first dislocation density of the first region being lower than the second dislocation density of the second region, and the III-nitride substrate having a root-mean-square roughness (RMS) value of 10 nm or less within any 0.2 mm square range within the surface thereof.

[0007] A method for manufacturing a Group III nitride substrate according to the present disclosure includes the steps of preparing a seed substrate, supplying a Group III element oxide gas and a nitrogen-containing gas to grow a Group III nitride crystal on the seed substrate, and polishing the Group III nitride crystal to a root-mean-square roughness (RMS) value of 10 nm or less within any 0.2 mm square range within the plane of the Group III nitride crystal. [Effects of the Invention]

[0008] The Group III nitride substrate according to the present disclosure has a high carrier concentration, low resistance, low dislocation density, and excellent surface flatness, and can suppress the generation of pits when forming a device layer on the surface of the substrate, thereby improving the withstand voltage when driving diodes, transistors, etc. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 shows an AFM image of the surface of a GaN substrate after surface CMP produced by OVPE. [Figure 2] (a) Optical microscope image of the surface and (b) SIMS mapping image of the same location on a GaN substrate after surface CMP, fabricated by OVPE. [Figure 3] FIG. 1A is a flowchart showing a time-series manufacturing method for a Group III nitride crystal according to an embodiment of the present disclosure, and FIG. 1B is a flowchart showing, as steps, each functional unit from upstream to downstream in a manufacturing apparatus used in the manufacturing method. [Figure 4]FIG. 1 is a schematic diagram of a manufacturing apparatus used to manufacture Group III nitride crystals according to an embodiment of the present disclosure. [Figure 5] 1A and 1B are conceptual diagrams of a group III nitride crystal grown on a seed substrate before and after slicing. [Figure 6] FIG. 1 is a diagram showing the relationship between the double average surface roughness RMS of an OVPE-produced GaN substrate before growth by MOVPE and the pit density of a GaN growth layer grown by MOVPE. [Figure 7] FIG. 1 is a graph showing the relationship between CMP time and double mean surface roughness RMS of an OVPE GaN substrate surface (Ga face). [Figure 8] This is a photoluminescence image (PL image) of the surface (Ga face) of an OVPE GaN substrate when the CMP time was 60 minutes. DETAILED DESCRIPTION OF THE INVENTION

[0010] A III-nitride substrate according to a first aspect is a III-nitride substrate consisting of a GaN epitaxial layer formed on a GaN substrate, the III-nitride substrate containing oxygen as an impurity element, the III-nitride substrate having, within a polished c-plane thereof, a first region exhibiting a first impurity concentration and a second region exhibiting a second impurity concentration lower than the first impurity concentration, the first dislocation density of the first region being lower than the second dislocation density of the second region, and the III-nitride substrate having a root-mean-square roughness (RMS) value of 10 nm or less within any 0.2 mm square range within the surface thereof.

[0011] A III-nitride substrate according to a second aspect may be the above-mentioned first aspect, wherein the first region is arranged so as to surround the periphery of the second region with the second region as the center.

[0012] A III-nitride substrate according to a third aspect is the above-mentioned second aspect, further comprising six third regions arranged alternately with the first regions in the circumferential direction so as to surround the second region with the second region at the center, and the third regions may be recessed relative to the first regions.

[0013] The group-III nitride substrate according to the fourth aspect may have less than one polishing scratch within an arbitrary range of 0.2 mm square on the surface of the group-III nitride substrate in any of the first to third aspects.

[0014] The group-III nitride substrate according to the fifth aspect may have at least one impurity contained in the first region selected from the group consisting of oxygen and silicon in any of the first to fourth aspects.

[0015] The group-III nitride substrate according to the sixth aspect may have a first impurity concentration such that the oxygen concentration is 1×10 20 / cm 3 or more in any of the first to fifth aspects.

[0016] The group-III nitride substrate according to the seventh aspect may have the first region composed of an m-plane component and the second region composed of an a-plane component in any of the first to sixth aspects.

[0017] The method for manufacturing a group-III nitride crystal according to the eighth aspect includes a step of preparing a seed substrate, a step of supplying a group-III element oxide gas and a nitrogen element-containing gas to grow a group-III nitride crystal on the seed substrate, and a step of polishing the group-III nitride crystal so that the root mean square (RMS) value of the surface roughness within an arbitrary range of 0.2 mm square in the plane of the group-III nitride crystal is 10 nm or less.

[0018] The method for manufacturing a group-III nitride substrate according to the ninth aspect includes, in the eighth aspect, polishing including chemical mechanical polishing (CMP), and the time of chemical mechanical polishing may be in the range of 120 minutes to 700 minutes.

[0019] Hereinafter, the group-III nitride substrate according to the embodiment will be described with reference to the accompanying drawings. In the drawings, substantially the same members are denoted by the same reference numerals.

[0020] (Embodiment 1) <Group-III Nitride Substrate> The III-nitride substrate according to the first embodiment is a III-nitride substrate comprising a GaN epitaxial layer formed on a GaN substrate. The III-nitride substrate contains oxygen as an impurity element, and has, within a polished c-plane of the III-nitride substrate, a first region exhibiting a first impurity concentration and a second region exhibiting a second impurity concentration lower than the first impurity concentration. The first region has a lower first dislocation density than the second region. The root-mean-square roughness (Rq value: JIS B0601-2001, ISO25178) is 10 nm or less within any 0.2 mm square area within the surface of the III-nitride substrate. In this way, by making the root mean square surface roughness (RMS) value 10 nm or less, the pit density of the growth layer formed by crystal growth on the III-nitride substrate can be reduced to 20 / cm. 2 Furthermore, by making the root mean square roughness (RMS) value 8.4 nm or less, the pit density of the growth layer can be made less than 1 / cm 2 , so the pit density is essentially 0 pcs / cm 2 In other words, by using a group III nitride substrate with a double average surface roughness RMS value of 10 nm or less, it is possible to suppress the occurrence of pits in the device layer crystal-grown thereon, thereby achieving the effects of improving the withstand voltage and reducing leakage current when driving diodes, transistors, etc.

[0021] FIG. 1 is an AFM image of the surface of a group III nitride substrate fabricated by the OVPE method according to the first embodiment. As shown in FIG. 1, the wafer surface (substrate surface) is composed of regions α and β that can be clearly distinguished by an AFM image. Here, let α be the first region and β be the third region. Also, six of the first regions and six of the third regions are alternately arranged in the circumferential direction surrounding the second region. Let the second region be γ. Further, the first region and the third region are reduced in diameter toward the second region. In other words, the first region and the third region extend radially around the second region. Also, there are dislocations at the center of the second region. The difference in the unevenness between α and β is due to the fact that the surfaces exposed during the crystal growth process are surfaces with different plane indices of {30-34} and {11-22}, respectively. Comparing α and β, it can be seen that the surface depression is larger in plane β. Therefore, a step occurs at the boundary between α and β.

[0022] FIGS. 2(a) and (b) are SIMS mapping images showing the oxygen concentration distribution and silicon concentration distribution on the surface of a group-III nitride substrate according to Embodiment 1. As can be seen from FIG. 2(a), comparing α and β, it can be seen that the oxygen concentration is higher in plane α. In the oxygen concentration distribution of FIG. 2(a), the average oxygen concentration in the high-oxygen-concentration region is 5.1×10 20 atoms / cm 3 and the average oxygen concentration in the low-oxygen-concentration region is 3.2×10 20 atoms / cm 3 . Oxygen in a group-III nitride crystal becomes an n-type dopant. Therefore, a group-III nitride crystal with oxygen added at a high concentration has a high carrier concentration and shows a low value of its electrical resistance. Also, it can be seen that the oxygen concentration distribution in FIG. 2(a) has the same pattern as the AFM image shown in FIG. 1. Therefore, from the results of FIGS. 1 and 2(a), it can be seen that the surface unevenness and the surface oxygen concentration distribution correspond to each other. Also, in the region of the high-oxygen-concentration distribution, the surface protrusion is larger compared to the region of the low-oxygen-concentration distribution. Also, from FIG. 2(b), it can be seen that the silicon concentration is not as high as the oxygen concentration, but there is a distribution within the plane.

[0023] <Outline of the method for manufacturing a group-III nitride crystal> An outline of the method for manufacturing a Group III nitride substrate according to the first embodiment will be described with reference to the flowchart of Fig. 3 and Fig. 4. Fig. 3(a) shows a chronological flowchart of the manufacturing method. Fig. 3(b) shows, as steps, each functional unit from upstream to downstream in a manufacturing apparatus for a Group III nitride substrate used in this manufacturing method. The method for manufacturing a Group III nitride substrate according to the first embodiment includes the steps of preparing a seed substrate, supplying a Group III element oxide gas and a nitrogen-containing gas to grow a Group III nitride crystal on the seed substrate, and polishing the extracted Group III nitride crystal.

[0024] (1) In the seed substrate preparation step of preparing a seed substrate 116, the seed substrate 116 is placed on a substrate susceptor 117. (2) In the temperature increasing step, the temperature of the growth chamber 111 is increased to 100° C. or higher and lower than 500° C. in an inert gas atmosphere. (3) In the decomposition protection temperature increasing step 1, the temperature of the growth chamber 111 is increased to 500° C. or more and less than 1100° C. in an NH 3 gas atmosphere. (4) In the decomposition protection temperature increasing step 2, the growth chamber 111 is heated to 1100° C. or higher and lower than 1500° C. in an atmosphere of Ga 2 O gas and NH 3 gas. (5) In the growth step of growing a Group III nitride crystal on the seed substrate 116, a Group III element oxide gas is generated in the source chamber 100 and supplied to the growth chamber 111, and a nitrogen-containing gas is also supplied to the growth chamber 111, thereby growing a Group III nitride crystal on the seed substrate 116.

[0025] The growth step includes a reactive gas supply step, a Group III element oxide gas generation step, a Group III element oxide gas supply step, a nitrogen-containing gas supply step, a Group III nitride crystal generation step, and a residual gas exhaust step. Note that each step included in the growth step may be performed simultaneously within a Group III nitride crystal manufacturing apparatus.

[0026] (5-1) In the reactive gas supply step, a reactive gas is supplied to the raw material reaction chamber. (5-2) In the Group III element oxide gas generation step, the starting Group III element source is reacted with a reactive gas (a reducing gas if the starting Group III element source is an oxide, and an oxidizing gas if the starting Group III element source is a metal) to generate a Group III element oxide gas. (5-3) In the Group III element oxide gas supplying step, the Group III element oxide gas produced in the Group III element oxide gas producing step is supplied to the growth chamber. (5-4) In the nitrogen-containing gas supplying step, a nitrogen-containing gas is supplied to the growth chamber. (5-5) In the Group III nitride crystal generating step, the Group III element oxide gas supplied into the growth chamber in the Group III element oxide gas supplying step and the nitrogen-containing gas supplied into the growth chamber in the nitrogen-containing gas supplying step are reacted with each other to grow a Group III nitride crystal on the seed substrate. (5-6) In the residual gas exhaust step, unreacted gases that do not contribute to the formation of Group III nitride crystals are exhausted to the outside of the chamber.

[0027] (6) In the decomposition protection temperature-lowering step, the temperatures of the source chamber 100 and the growth chamber 111 are lowered to 500° C. while supplying NH 3 gas in order to suppress decomposition of the group III nitride crystal grown on the seed substrate 116. (7) In the temperature lowering step, the temperatures of the source chamber 100 and the growth chamber 111 are lowered to below 100° C. in an inert gas atmosphere. (8) In the unloading step, the seed substrate 116 on which the group III nitride crystal has grown is unloaded from the growth chamber 111.

[0028] (9) In the slicing step, the produced group III nitride crystal and seed substrate are sliced to obtain a plurality of group III nitride substrates, as shown in Fig. 5. This allows the seed substrate to be reused. (10) In the polishing step, the front and back surfaces of the prepared Group III nitride substrate are polished and smoothed. Note that the slicing step may not be performed, and the seed substrate may be removed in the polishing step to produce a Group III nitride substrate composed of grown Group III nitride crystals.

[0029] <Overview of the manufacturing apparatus for group III nitride crystals> The overview of the manufacturing apparatus for group III nitride crystals used in the manufacturing method of group III nitride crystals according to Embodiment 1 will be described with reference to FIG. 4. In FIG. 4, the sizes, ratios, etc. of each component member may be different from the actual ones. In the manufacturing apparatus for group III nitride crystals, a raw material reaction chamber 101 is disposed in a raw material chamber 100, and a raw material boat 104 on which a starting group III element source 105 is placed is disposed in the raw material reaction chamber 101. A reactive gas supply pipe 103 for supplying a gas that reacts with the starting group III element source 105 is connected to the raw material reaction chamber 101. The raw material reaction chamber 101 has a group III element oxide gas discharge port 107 for discharging the generated group III element oxide gas. When the starting group III source is an oxide, a reducing gas is used as the reactive gas. When the starting group III source is a metal, an oxidizing gas is used as the reactive gas. Further, a first carrier gas supply port 102 for supplying a first carrier gas is connected to the raw material chamber 100, and the first carrier gas supplied from the first carrier gas supply port 102 and the group III element oxide gas discharged from the group III element oxide gas discharge port 107 flow through a connection pipe 109 from a gas discharge port 108 to a growth chamber 111 and are supplied into the growth chamber 111 from a gas supply port 118 connected to the growth chamber 111. The growth chamber 111 has a gas supply port 118, a third carrier gas supply port 112, a nitrogen element-containing gas supply port 113, a second carrier gas supply port 114, and an exhaust port 119. The growth chamber 111 includes a substrate susceptor 117 on which a seed substrate 116 is installed.

[0030] <Details of the manufacturing method and manufacturing apparatus for group III nitride crystals> Referring to FIGS. 3 and 4, the manufacturing method of group III nitride crystals according to the present embodiment will be described in detail. In the present embodiment, metallic Ga is used as the starting group III element source 105, but it is not limited thereto. For example, Al or In may be used.

[0031] <Seed substrate preparation step> First, prepare the seed substrate 116. For example, gallium nitride, gallium arsenide, silicon, sapphire, silicon carbide, zinc oxide, gallium oxide, or ScAlMgO4 can be used as the seed substrate 116. In this embodiment, gallium nitride is used as the seed substrate 116.

[0032] <Heating process> In the temperature-raising step, the growth chamber is heated in an inert gas atmosphere to a temperature that does not cause decomposition of the seed substrate 116. In producing Group III nitride crystals by OVPE, heating is performed in an inert gas (e.g., N2 gas) atmosphere to approximately 500°C.

[0033] <Decomposition protection temperature increase step 1> In the decomposition protection temperature increase step 1, the temperature is increased in a nitrogen-element-containing gas atmosphere while suppressing decomposition of the seed substrate 116. In the production of Group III nitride crystals by OVPE, heating is performed in a state where an inert gas and a nitrogen-element-containing gas, NH3 gas, are mixed up to a temperature of 500°C or higher but lower than 1100°C. The reason for mixing NH3 is to prevent decomposition of the seed substrate 116 due to the detachment of N atoms. Heating may also be performed in a state where H2 gas is further mixed in.

[0034] <Decomposition protection temperature increase step 2> In the decomposition protection temperature rise step 2, the temperature is raised in an atmosphere of Group III oxide gas and nitrogen-containing gas while suppressing decomposition of the seed substrate 116. In the production of Group III nitride crystals by OVPE, heating is performed up to 1100°C or higher but lower than 1500°C in a state where H2 gas, inert gas, Group III oxide gas, and nitrogen-containing gas NH3 gas are mixed. The reason for mixing the Group III oxide gas is that decomposition cannot be suppressed with the nitrogen-containing gas alone. By applying a driving force for the growth of the Group III nitride crystal, it is possible to suppress decomposition.

[0035] <Growth process> In the growth process, a group III element oxide gas is generated in the raw material chamber 100 and supplied to the growth chamber 111, and a nitrogen element-containing gas is supplied to the growth chamber 111 to generate a group III nitride crystal on the seed substrate 116. Specifically, the growth process includes a reactive gas supply process, a group III element oxide gas generation process, a group III element oxide gas supply process, a nitrogen element-containing gas supply process, a group III nitride crystal generation process, and a residual gas discharge process.

[0036] <Reactive Gas Supply Process> In the reactive gas supply process, the reactive gas is supplied from the reactive gas supply pipe 103 to the raw material reaction chamber 101 in the raw material chamber 100. As described above, the reactive gas can use a reducing gas or an oxidizing gas as needed. In the first embodiment, since metallic Ga is used as the starting group III element source 105, H2O gas is used as the reactive gas.

[0037] <Group III Element Oxide Gas Generation Process> In the Group III element oxide gas generation process, the reactive gas supplied to the raw material reaction chamber 101 in the reactive gas supply process reacts with Ga, which is the starting Group III element source 105, to generate GaO gas, which is a Group III element oxide gas. The generated GaO gas is discharged from the raw material reaction chamber 101 to the raw material chamber 100 via the Group III element oxide gas outlet 107. The discharged GaO gas is mixed with the first carrier gas supplied to the raw material chamber from the first carrier gas supply port 102, and then supplied to the gas outlet 108. In the first embodiment, the raw material chamber 100 is heated by the first heater 106. When the raw material chamber 100 is heated, the temperature of the raw material chamber 100 is preferably 800°C or higher in view of the boiling point of the GaO gas. Furthermore, the temperature of the raw material chamber 100 is preferably set lower than that of the growth chamber 111. When the growth chamber is heated by the second heater 115 as described below, the temperature of the raw material chamber 100 is preferably set to less than 1800°C, for example. The starting group III element source 105 is placed in a source boat 104 arranged in the source reaction chamber 101. The source boat 104 preferably has a shape that can increase the contact area between the reactive gas and the starting group III element source. For example, the source boat 104 preferably has a multi-stage dish shape to prevent the starting group III element source 105 and the reactive gas from passing through the source reaction chamber 101 without contacting each other.

[0038] Methods for generating a Group III element oxide gas can be broadly divided into methods for reducing the starting Group III element source 105 and methods for oxidizing the starting Group III element source 105. For example, the reduction method uses an oxide (e.g., GaO) as the starting Group III element source 105 and a reducing gas (e.g., H, CO, CH, CH, HS, or SO) as the reactive gas. On the other hand, the oxidation method uses a non-oxide (e.g., liquid Ga) as the starting Group III element source 105 and an oxidizing gas (e.g., HO, O, CO, CO, NO, NO, or NO) as the reactive gas. In addition to the starting group III element source 105, an In source and an Al source can be adopted as the starting group III elements. As the first carrier gas, an inert gas, H2 gas, etc. can be used.

[0039] <Group III Element Oxide Gas Supply Step> In the group III element oxide gas supply step, the Ga2O gas generated in the group III element oxide gas generation step is supplied to the growth chamber 111 via the gas outlet 108, the connecting pipe 109, and the gas supply port 118. When the temperature of the connecting pipe 109 connecting the raw material chamber 100 and the growth chamber 111 drops below the temperature of the raw material chamber 100, a reverse reaction of the reaction to generate the group III element oxide gas occurs, and the starting group III element source 105 precipitates in the connecting pipe 109. Therefore, it is preferable that the connecting pipe 109 is heated to a higher temperature than the first heater 106 by the third heater 110 so as not to drop below the temperature of the raw material chamber 100.

[0040] <Nitrogen Element-Containing Gas Supply Step> In the nitrogen element-containing gas supply step, the nitrogen element-containing gas is supplied from the nitrogen element-containing gas supply port 113 to the growth chamber 111. Examples of the nitrogen element-containing gas include, for example, NH3 gas, NO gas, NO2 gas, N2O gas, N2H2 gas, and N2H4 gas.

[0041] <Group III Nitride Crystal Generation Step> In the Group III nitride crystal growth process, the source gases supplied into the growth chamber through each supply process are reacted to grow a Group III nitride crystal on the seed substrate 116. The growth chamber 111 is preferably heated by the second heater 115 to a temperature at which the Group III element oxide gas and the nitrogen-containing gas react. At this time, the temperature of the growth chamber 111 is preferably controlled so that it does not drop below the temperature of the source chamber 100, in order to prevent a reverse reaction of the reaction that generates the Group III element oxide gas. The temperature of the growth chamber 111 heated by the second heater 115 is preferably 1000°C or higher and 1800°C or lower. Furthermore, in order to suppress temperature fluctuations in the growth chamber 111 due to the GaO gas generated in the source chamber 100 and the first carrier gas, it is desirable that the temperatures of the second heater 115 and the third heater 110 be the same.

[0042] By mixing the Group III element oxide gas supplied to the growth chamber 111 via the Group III element oxide supply step and the nitrogen element-containing gas supplied to the growth chamber 111 via the nitrogen element-containing gas supply step upstream of the seed substrate 116, it is possible to grow a Group III nitride crystal on the seed substrate 116.

[0043] The reactive gas supplying step, the Group III element oxide gas generating step, the Group III element oxide gas supplying step, the nitrogen-containing gas supplying step, the Group III nitride crystal generating step, and the residual gas discharging step, which are included in the growth step, may be performed simultaneously.

[0044] The second carrier gas may be an inert gas, H2 gas, etc. In the residual gas exhaust process, the unreacted Group III element oxide gas and nitrogen-containing gas, as well as the first carrier gas, second carrier gas, and third carrier gas are exhausted from the exhaust port 119.

[0045] <Decomposition protection temperature cooling process> In the decomposition protection temperature-reducing step, the temperature is reduced in a nitrogen-containing gas atmosphere while suppressing decomposition of the Group III nitride crystal. In the production of Group III nitride crystals by the OVPE method, the cooling is performed to 500°C or below in a state where an inert gas and a nitrogen-containing gas, NH3, are mixed.

[0046] <Temperature cooling process> In the temperature-lowering step, the temperature is lowered in an inert gas atmosphere to a temperature at which the Group III nitride crystal can be removed from the growth chamber.

[0047] In this embodiment, the seed substrate 116 on which the group III nitride crystal has grown is removed from the growth chamber 111 after the temperature lowering step.

[0048] <Slicing process> In the slicing step, the Group III nitride crystal grown on the seed substrate 116 is sliced using a wire or a laser. By slicing, the Group III nitride crystal is separated from the seed substrate 116. The grown Group III nitride crystal is also separated into a plurality of Group III nitride substrates. Note that the number of Group III nitride substrates obtained by slicing may be one or more.

[0049] <Polishing process> In the polishing process, the front and back surfaces of the group-III nitride substrate obtained by slicing are smoothed. From the perspective of forming a device layer on the polished group-III nitride substrate by MOVPE, it is important to improve flatness and remove polishing scratches. The polishing process in this disclosure includes mechanical polishing (MP) and chemical-mechanical polishing (CMP). Mechanical polishing (MP) primarily reduces the overall wafer thickness and thickness distribution. Polishing scratches may occur during this process. Polishing scratches are shown as streaks or lines in the photoluminescence image of FIG. 8 . Chemical-mechanical polishing (CMP) primarily removes damaged layers and polishing scratches. In particular, CMP is important for group-III nitride substrates fabricated using OVPE. While extending the polishing time by CMP removes polishing scratches introduced by MP, it also creates surface irregularities reflecting the α and β regions (growth history). Surface irregularities can cause pits when forming a device layer on the substrate surface by MOVPE. Therefore, CMP is performed to remove polishing scratches and minimize unevenness. Note that the pits are recesses in the shape of inverted polygonal pyramids, such as an inverted hexagonal pyramid or an inverted dodecagonal pyramid.

[0050] As a result, a group III nitride substrate can be obtained in which polishing scratches on the wafer surface are removed and unevenness is reduced.

[0051] (Summary of Examples and Comparative Examples) Group III nitride crystals were grown using the growth furnace shown in Figure 4. Here, GaN was grown as the group III nitride crystal. Liquid Ga was used as the starting group III element source. Ga was reacted with reactive gas H2O gas, and the resulting Ga2O gas was used as the group III element oxide gas. NH3 gas was used as the nitrogen-containing gas, and a mixture of H2 and N2 gases was used as the first and second carrier gases. The grown GaN crystals had a growth surface covered with numerous pits consisting of planes with indices {30-34} and {11-22}. Using this growth mode promotes the convergence of dislocations to the pit centers, resulting in dislocation coalescence and annihilation. This results in a thicker film and a reduced dislocation density.

[0052] GaN substrates with thicknesses of 0.3 to 0.5 mm were fabricated from the grown GaN crystals, and the front and back surfaces were planarized in a polishing process. The duration of chemical mechanical polishing (CMP) in the polishing process was varied to confirm the surface flatness and the presence of polishing scratches. CMP was performed for periods ranging from 200 to 780 minutes. Furthermore, a GaN film of approximately 5 to 10 μm was grown on the polished GaN substrates by MOVPE to confirm the correlation between the flatness of the GaN substrate before growth and the pit density of the growth layer formed by MOVPE. Pits smaller than 1 μm were not counted. MOVPE growth was performed at wafer surface temperatures ranging from 1070 to 1080°C. The surface flatness of the GaN substrates before MOVPE growth was evaluated using the root-mean-square roughness (RMS) value (Rq value: JIS B0601-2001, ISO 25178) within a 200- to 300-μm square area in white light interference microscope images. The RMS value is the Rq value in standards since 2001. The presence or absence of polishing scratches on the GaN substrate surface before growth was evaluated using surface photoluminescence images (PL images).

[0053] Example 1 Chemical mechanical polishing (CMP) was performed on the OVPE GaN substrate to produce a sample with a root-mean-square roughness (RMS) value of 8.42 nm. Photoluminescence (PL) observation of the GaN substrate before growth confirmed no polishing scratches on the surface. GaN was grown on the OVPE GaN substrate by MOVPE, and the pit density was 0 / cm. 2 It was.

[0054] Example 2 Chemical mechanical polishing (CMP) was performed on the OVPE GaN substrate to produce a sample with a root-mean-square roughness (RMS) value of 4.48 nm. Photoluminescence (PL) observation of the GaN substrate before growth confirmed no polishing scratches on the surface. GaN was grown on the OVPE GaN substrate by MOVPE, and the pit density was 0 / cm. 2 It was.

[0055] Example 3 Chemical mechanical polishing (CMP) was performed on the OVPE GaN substrate to produce a sample with a root-mean-square roughness (RMS) value of 2.44 nm. Photoluminescence (PL) observation of the GaN substrate before growth confirmed no polishing scratches on the surface. GaN was grown on the OVPE GaN substrate by MOVPE, and the pit density was 0 / cm. 2 It was.

[0056] Example 4 Chemical mechanical polishing (CMP) was performed on the OVPE GaN substrate to produce a sample with a root-mean-square roughness (RMS) value of 9.80 nm. Photoluminescence (PL) observation of the GaN substrate before growth confirmed no polishing scratches on the surface. GaN was grown on the OVPE GaN substrate by MOVPE, resulting in a pit density of 9.75 / cm. 2 It was.

[0057] Example 5 Chemical mechanical polishing (CMP) was performed on the OVPE GaN substrate to produce a sample with a root-mean-square roughness (RMS) value of 8.94 nm. Photoluminescence (PL) observation of the GaN substrate before growth confirmed no polishing scratches on the surface. GaN was grown on the OVPE GaN substrate by MOVPE, resulting in a pit density of 0.25 / cm. 2 It was.

[0058] (Comparative Example 1) Chemical mechanical polishing (CMP) was performed on an OVPE GaN substrate for approximately 780 minutes to produce a sample with a root-mean-square roughness (RMS) value of 10.7 nm. Photoluminescence (PL) observation of the GaN substrate before growth confirmed no polishing scratches on the surface. GaN was grown on the OVPE GaN substrate by MOVPE, resulting in a pit density of 73.5 / cm. 2 It was.

[0059] The above results are summarized in Figure 6. As can be seen, by keeping the root mean square roughness (RMS) value below 10 nm, the pit density of the MOVPE grown layer on the OVPE GaN substrate can be reduced to 20 / cm. 2 Furthermore, by keeping the root mean square roughness (RMS) value at 8.4 nm or less, the pit density of the MOVPE grown layer can be kept at less than 1 / cm. 2 , so the pit density is essentially 0 pcs / cm 2 In other words, by using an OVPE GaN substrate with a double average surface roughness RMS value of 10 nm or less, it is possible to suppress the occurrence of pits in the device layer crystal-grown on it, thereby improving the withstand voltage and reducing leakage current when driving diodes, transistors, etc.

[0060] An example of the relationship between the chemical mechanical polishing (CMP) processing time for a GaN substrate and the double mean surface roughness RMS value is shown in Figure 7. As shown in Figure 7, it can be seen that the double mean surface roughness RMS value increases as the CMP processing time increases. In other words, from the relationship between Figures 6 and 7, it can be seen that in order to achieve a double mean surface roughness RMS value of 10 nm or less, the chemical mechanical polishing (CMP) processing time needs to be shortened from the conventional 800 minutes to approximately 700 minutes or less. Furthermore, it can be seen that in order to achieve a root mean square (RMS) roughness RMS value of 8.4 nm or less, the chemical mechanical polishing (CMP) processing time needs to be 600 minutes or less. No polishing scratches were observed on the surface of the sample plotted in Figure 7. Meanwhile, Figure 8 shows a surface photoluminescence (PL) image obtained when chemical mechanical polishing (CMP) was performed for 60 minutes. As indicated by the arrows in Figure 8, stripe-like polishing scratches 120 remain when the chemical mechanical polishing (CMP) time is too short. Polishing scratches are problematic because they can cause pits and dislocations during device layer formation. Specifically, to remove polishing scratches, it is necessary to ensure that there is less than one polishing scratch within any 0.2 mm square area on the surface of the III-nitride substrate. Therefore, it is clear that the chemical mechanical polishing (CMP) time must be longer than 60 minutes, for example, 120 minutes or more, or even 200 minutes or more. Therefore, the optimum conditions for chemical mechanical polishing (CMP) are those that remove polishing scratches and minimize the double mean surface roughness (RMS) value. Specifically, the time for chemical mechanical polishing (CMP) is, for example, 120 minutes or more and 700 minutes or less, and may further be 200 minutes or more and 600 minutes or less. [Industrial Applicability]

[0061] The group III nitride substrate according to the present disclosure has a high carrier concentration, low resistance, low dislocation density, and excellent surface flatness, and can suppress the generation of pits when forming a device layer on the surface of the substrate. [Explanation of symbols]

[0062] 100 raw material chamber 101 Raw material reaction chamber 102 First carrier gas supply port 103 Reactive gas supply pipe 104 Raw Material Boat 105 Starting Group III Element Sources 106 First heater 107 Group III element oxide gas outlet 108 Gas outlet 109 Connecting Pipe 110 Third heater 111 Growth Chamber 112 Third carrier gas supply port 113 Nitrogen-containing gas supply port 114 Second carrier gas supply port 115 Second heater 116 species substrate 117 Substrate susceptor 118 Gas supply port 119 Exhaust vent 120 Polishing scratches

Claims

1. A group III nitride substrate comprising a GaN epitaxial layer formed on a GaN substrate, Contains oxygen as an impurity element, a first region exhibiting a first impurity concentration within a surface polished to a c-plane of the Group III nitride substrate; a second region exhibiting a second impurity concentration lower than the first impurity concentration; and a first dislocation density in the first region is lower than a second dislocation density in the second region; A Group III nitride substrate, wherein the root mean square roughness (RMS) value within any 0.2 mm square area on the surface of the Group III nitride substrate is 10 nm or less.

2. The III-nitride substrate according to claim 1 , wherein the first region is disposed so as to surround the periphery of the second region with the second region as the center.

3. The third region is arranged alternately with the first region in a circumferential direction so as to surround the second region with the second region as a center, and six third regions are arranged in a circumferential direction so as to surround the second region, The III-nitride substrate of claim 2 , wherein the third region is recessed relative to the first region.

4. 4. The III-nitride substrate according to claim 1, wherein there is less than one polishing scratch in any 0.2 mm square area within the surface of the III-nitride substrate.

5. The Group III nitride substrate according to claim 1 , wherein the impurity contained in the first region is at least one selected from the group consisting of oxygen and silicon.

6. The first impurity concentration is an oxygen concentration of 1×10 ２０ / cm ３ The group III nitride substrate according to claim 1 , wherein:

7. The III-nitride substrate according to claim 1 , wherein the first region is composed of an m-plane component, and the second region is composed of an a-plane component.

8. providing a seed substrate; supplying a Group III element oxide gas and a nitrogen-containing gas to grow a Group III nitride crystal on the seed substrate; polishing the Group III nitride crystal to a root mean square roughness (RMS) value of 10 nm or less within any 0.2 mm square range within the plane of the Group III nitride crystal; A method for manufacturing a Group III nitride substrate, comprising:

9. 9. The method for producing a Group III nitride substrate according to claim 8, wherein the polishing includes chemical mechanical polishing (CMP), and the time for the chemical mechanical polishing is in the range of 120 minutes to 700 minutes.

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