Gallium nitride single crystal substrate and method for manufacturing the same

Abstract

In a gallium nitride single crystal substrate, high thermal conductivity is maintained even in high-temperature environments while reducing resistivity. [Solution] A gallium nitride single crystal substrate in which the lowest index crystal plane closest to the main plane is the (0001) plane, and the resistivity at 200°C is 1 × 10 -2 This is a gallium nitride single crystal substrate with a thermal conductivity of Ω·cm or less and a thermal conductivity of 80 W / mk or more in the thickness direction at 200°C.

Description

[Technical Field]

[0001] The present invention relates to a gallium nitride single crystal substrate and a method for manufacturing the same. [Background technology]

[0002] Group III nitride semiconductors, exemplified by gallium nitride (GaN), are widely used as materials for semiconductor devices such as light-emitting devices and electronic devices. To improve the quality (semiconductor properties, etc.) of semiconductor devices composed of Group III nitride semiconductors, it is desirable to manufacture semiconductor laminates or nitride semiconductor freestanding substrates for semiconductor device manufacturing in a way that ensures good crystal quality.

[0003] GaN single-crystal substrates are doped with impurities to reduce their resistivity. In particular, for applications such as high-power laser diodes and power devices, a further reduction in resistivity is required. From the perspective of reducing resistivity, it is necessary to increase the concentration of impurities. Germanium (Ge) is being considered as an impurity because it does not significantly degrade the crystallinity of GaN even at high concentrations (see, for example, Patent Document 1). [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2021-195280 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] Incidentally, GaN single-crystal substrates are sometimes attached to heat sinks when they are fabricated into semiconductor devices. From the perspective of improving heat transfer to the heat sink, high thermal conductivity is required for GaN single-crystal substrates used in semiconductor devices. However, the thermal conductivity of GaN single-crystal substrates tends to decrease with the addition of germanium.

[0006] On the other hand, in the environments in which semiconductor devices are actually used, the temperature may rise due to the operation of the device. However, until now, thermal conductivity has been measured under room temperature conditions, and measurement under high-temperature conditions has not been considered. Therefore, the inventors investigated the change in thermal conductivity of gallium nitride with temperature and confirmed that even if the thermal conductivity of gallium nitride is high under room temperature conditions, it decreases as the temperature rises.

[0007] Thus, in GaN single-crystal substrates, even if the thermal conductivity at room temperature can be increased by adding germanium at high concentrations to reduce resistivity, it may not be possible to maintain a high thermal conductivity under high-temperature conditions.

[0008] The present invention aims to provide a technology that maintains high thermal conductivity even in high-temperature environments while reducing resistivity in a gallium nitride single crystal substrate. [Means for solving the problem]

[0009] According to one aspect of the present invention, A gallium nitride single crystal substrate in which the lowest index crystal plane closest to the principal plane is the (0001) plane, The resistivity at 200°C is 1 × 10⁻⁶ -2 It is less than or equal to Ω·cm, The thermal conductivity in the thickness direction at 200°C is 80 W / mk or higher. A gallium nitride single crystal substrate is provided.

[0010] According to another aspect of the present invention, (a) A step of preparing a substrate made of a gallium nitride single crystal in which the lowest index crystal plane closest to the main plane is the (0001) plane, (b) The step of epitaxially growing a gallium nitride single crystal on the main surface of the substrate, In (b) above, a raw material gas for forming the gallium nitride single crystal is supplied to the substrate under an ammonia-containing atmosphere with a growth pressure greater than 1 atm and 1.5 atm or less, while ammonia is supplied from around the support member that supports the substrate. A method for manufacturing a gallium nitride single crystal substrate is provided. [Effects of the Invention]

[0011] According to the present invention, it is possible to maintain high thermal conductivity even in high-temperature environments while reducing resistivity in a gallium nitride single crystal substrate. [Brief explanation of the drawing]

[0012] [Figure 1] Figures 1(a) to 1(c) are schematic cross-sectional views showing a part of the method for manufacturing a gallium nitride single crystal substrate according to one embodiment of the present invention. [Figure 2] Figure 2 is a flowchart showing the method for manufacturing a gallium nitride single crystal substrate according to this embodiment. [Figure 3] Figure 3 is a schematic diagram illustrating an example of an HVPE device. [Figure 4] Figure 4 shows the thermal conductivity R1 in the thickness direction at 25°C and 200°C for the example. [Figure 5] Figure 5 shows the in-plane thermal conductivity R2 at 25°C and 200°C for the example. [Modes for carrying out the invention]

[0013] <Knowledge gained by the inventors, etc.> First, we will explain the findings obtained by the inventors.

[0014] The inventors investigated the correlation between resistivity and thermal conductivity of GaN crystals doped with a high concentration of germanium. As a result, it was confirmed that germanium, compared to silicon, for example, can reduce the resistivity of GaN crystals while maintaining high thermal conductivity at room temperature. However, it was also confirmed that even GaN crystals exhibiting high thermal conductivity at room temperature may not achieve the desired heat dissipation when used in semiconductor devices, as their resistivity does not increase under high-temperature conditions, but their thermal conductivity decreases significantly.

[0015] The inventors further investigated the decrease in thermal conductivity with increasing temperature and found that this decrease is greatly influenced by point defects introduced into the GaN crystal. In GaN crystals, lattice mismatch can occur with the addition of impurities. When the crystal lattice is distorted, nitrogen atoms may detach from the lattice, forming nitrogen vacancies. These nitrogen vacancies can become point defects. The more point defects there are, the more easily the thermal conductivity tends to decrease with increasing temperature.

[0016] Based on this, the inventors investigated methods to reduce point defects in GaN crystals, particularly point defects formed by the desorption of nitrogen atoms. They found that, from the viewpoint of suppressing the desorption of nitrogen atoms, it is beneficial to grow GaN crystals in an ammonia-containing atmosphere, pressurize the growth pressure, and supply ammonia as a holder purge gas.

[0017] Holder purging refers to the process of supplying a predetermined gas around a holder during crystal growth in an HVPE apparatus, for example, to remove crystals deposited on the holder that holds the substrate, and to prevent the raw material gas from seeping to the back of the holder. In holder purging, for example, gas is supplied and flowed from the side of the holder opposite to the substrate-holding surface, i.e., from the back of the holder, towards the periphery of the holder. This creates a locally positive pressure around the holder.

[0018] From the viewpoint of the above-mentioned objectives, nitrogen gas or hydrogen gas is usually used as the gas for holder purging. However, according to the inventors' research, it has been found that by supplying ammonia instead of nitrogen gas, the desorption of nitrogen atoms in GaN crystals can be suppressed and point defects can be reduced.

[0019] This invention is based on the above-mentioned findings discovered by the inventors.

[0020] [Details of the Embodiments of the Invention] Next, one embodiment of the present invention will be described below with reference to the drawings. However, the present invention is not limited to these examples and is intended to include all modifications within the meaning and scope equivalent to the claims as shown in the claims.

[0021] <First Embodiment of the Invention> Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.

[0022] (1) Gallium nitride single crystal substrate The gallium nitride single crystal substrate (hereinafter also simply referred to as GaN substrate) of this embodiment is configured as a disc-shaped substrate used in the manufacture of semiconductor laminates or semiconductor devices. The GaN substrate is a self-supporting substrate made of a single crystal of gallium nitride (GaN).

[0023] As shown in Figure 1(c), the GaN substrate 1 has a main surface 1s which serves as the epitaxial growth surface. The main surface 1s of the GaN substrate 1 is a so-called epiready surface. The closest low-index crystal plane to the main surface 1s of the GaN substrate 1 is, for example, the (0001) plane (+c plane, Ga polarity plane). Hereinafter, the (0001) plane will also be referred to as the c plane. The main surface 1s of the GaN substrate 1 is, for example, mirror-finished, and the mean square roughness RMS of the main surface 1s of the GaN substrate 1 is preferably less than 1 nm.

[0024] Furthermore, while there are no particular restrictions on the diameter of the GaN substrate 1, it is preferably 50 mm or more. If the diameter of the GaN substrate 1 is less than 50 mm, the productivity of the semiconductor device tends to decrease. For this reason, it is preferable that the diameter of the GaN substrate 1 be 50 mm or more. Also, the thickness of the GaN substrate 1 is preferably 300 μm or more and 1 mm or less. If the thickness of the GaN substrate 1 is less than 300 μm, the mechanical strength of the GaN substrate 1 decreases, and it may become difficult to maintain a self-supporting state. For this reason, it is preferable that the thickness of the GaN substrate 1 be 300 μm or more.

[0025] The GaN substrate 1 is configured to have low resistivity even in high-temperature environments, which are expected to generate heat from the device. Specifically, the resistivity of the GaN substrate 1 at 200°C is 1 × 10⁻⁶. -2 It is less than or equal to Ω·cm, preferably 1 × 10⁻⁶ -3 It is less than Ω·cm. Generally, resistivity tends to decrease with increasing temperature, but in the GaN substrate 1 of this embodiment, it is possible to make the resistivity even lower. Semiconductor devices fabricated from such a GaN substrate 1 can suppress power loss and maintain a high switching speed even in high-temperature environments. Note that the lower limit of resistivity at 200°C is not particularly limited, for example, 1.0 × 10⁻⁶ -4 It should be greater than or equal to Ωcm.

[0026] Furthermore, the GaN substrate 1 is configured to have a low resistivity in low-temperature environments. Specifically, the resistivity of the GaN substrate 1 at 25°C is 1.0 × 10⁻⁶. -4 Ωcm or greater: 1.0 × 10 -2 It is preferable that the coefficient be Ωcm or less, and 1.0 × 10 -4 Ωcm or greater: 1.0 × 10 -3 It is more preferable that the resistivity is less than Ωcm. With a GaN substrate 1 having such resistivity, the resistivity can be kept low in both low and high temperature environments.

[0027] The GaN substrate 1 is preferably configured to contain impurities including germanium (Ge) from the viewpoint of reducing the specific resistance. The impurities may include at least Ge, and may also include other n-type impurities such as silicon (Si). According to Ge, the crystallinity of the GaN crystal can be maintained high while being added at a high concentration compared with other n-type impurities.

[0028] The Ge concentration in the GaN substrate 1 is not particularly limited as long as the specific resistance at 200 °C and 25 °C satisfies the above range. The Ge concentration is preferably 1.0×10 18 cm -3 or more, more preferably 5.0×10 18 cm -3 or more, still more preferably 1.8×10 19 cm -3 or more, and even more preferably 1.0×10 20 cm -3 or more. On the other hand, the upper limit value of the Ge concentration is not particularly limited, but is preferably 1.0×10 21 cm -3 or less, more preferably 5.0×10 20 cm -3 or less. Although the specific resistance can be reduced as the Ge concentration increases, point defects are likely to occur and the thermal conductivity tends to decrease in a high-temperature environment. However, in this embodiment, crystal growth is performed under conditions that suppress point defects as described later, so that the thermal conductivity can be maintained high.

[0029] In addition to impurities such as Ge, oxygen (O) and hydrogen (H) may inevitably be mixed into the GaN substrate 1. O may reduce the thermal conductivity in a room temperature (normal temperature) environment or deteriorate the decrease in thermal conductivity due to temperature rise. Therefore, the O concentration is preferably so low as to be negligible with respect to the Ge concentration. For example, it is preferably 1 / 10 or less of the Ge concentration, and more preferably 1 / 100 or less. Specifically, the O concentration is preferably 6×10 16 cm -3 or less. Also, the H concentration is 1.0×10 17 cm-3 The following is preferable:

[0030] The GaN substrate 1 of this embodiment is formed under conditions that suppress the desorption of nitrogen atoms, as described later, resulting in fewer point defects and maintaining high thermal conductivity even in high-temperature environments. Specifically, when R1 is the thermal conductivity along the thickness direction of the main surface 1s of the GaN substrate 1, R1(H) at 200°C is 80 W / mk or more and 120 W / mk or less. Therefore, even in a high-temperature environment of 200°C, the GaN substrate 1 can easily dissipate heat in the thickness direction, achieving high heat dissipation performance. Furthermore, the GaN substrate 1 is configured to have high thermal conductivity not only in high-temperature environments but also in room-temperature environments. Specifically, it is preferable that the thermal conductivity R1(L) in the thickness direction at 25°C is 120 W / mk or higher. There is no particular upper limit, but for example, it is preferable to have it at 230 W / mk or lower.

[0031] The GaN substrate 1 is configured to have high thermal conductivity in both the thickness direction and the in-plane direction. Specifically, when the thermal conductivity of the main surface 1s in the in-plane direction is R2, it is preferable that the R2(H) at 200°C is 80 W / mk or more and 110 W / mk or less. It also has similarly high thermal conductivity at room temperature, and it is preferable that the in-plane thermal conductivity R2(L) at 25°C is 150 W / mk or more. There is no particular upper limit, but for example, it is preferable to have 250 W / mk or less. With such a configuration, heat can be easily diffused in the in-plane direction in the GaN substrate 1.

[0032] Furthermore, since the GaN substrate 1 has suppressed nitrogen atom desorption and the resulting generation of point defects, the decrease in thermal conductivity due to temperature rise is suppressed. In other words, the rate of change of the thermal conductivity R1 in the thickness direction of the GaN substrate 1 when the temperature is raised from low to high is small. Specifically, it is preferable that the rate of change of the thermal conductivity R1 in the thickness direction of the GaN substrate 1 when the temperature is raised from 25°C to 200°C is small, and that the rate of change x is 0.5 or less. Here, the rate of change x of the thermal conductivity R1 in the thickness direction due to temperature rise is given by x = (R1(L) - R1(H)) / R1(L), using R1(L) at 25°C and R1(H) at 200°C, and it is preferable that x ≤ 0.5 is satisfied. The lower x is, the less the decrease in thermal conductivity due to temperature rise, and the higher the thermal conductivity that can be maintained even at high temperatures.

[0033] Furthermore, the in-plane thermal conductivity R2 of the GaN substrate 1, like the thermal conductivity R1 in the thickness direction, preferably has a small rate of change when the temperature is raised from 25°C to 200°C, and its rate of change y is preferably 0.6 or less. Here, the rate of change y of the in-plane thermal conductivity R2 due to temperature rise is given by y = (R2(L) - R2(H)) / R2(L), using R2(L) at 25°C and R2(H) at 200°C, and it is preferable that y ≤ 0.6 is satisfied. A lower y indicates less decrease in thermal conductivity due to temperature rise, and that high thermal conductivity can be maintained even at high temperatures.

[0034] When fabricating semiconductor devices, it is preferable for the GaN substrate 1 to have a high carrier concentration in order to improve the operating speed and reduce power loss. The carrier concentration is determined by the concentration of impurities contained in the GaN crystal, for example, the germanium concentration, and is approximately equal to the germanium concentration. Specifically, the carrier concentration is 1.0 × 10⁻⁶. 18 cm -3 Preferably, it is 5.0 × 10 18 cm -3 It is more preferable that the above be the case, 1.8 × 10 19 cm -3 It is even more preferable that the amount be greater than or equal to 1.0 × 10 20 cm -3It is even more preferable that the above conditions are met. The upper limit of the carrier concentration is not particularly limited, but is 1.0 × 10⁻⁶. 21 cm -3 Preferably, it is 5.0 × 10 20 cm -3 The following is more preferable. Furthermore, from the viewpoint of achieving a high level of both improved operating speed and reduced power loss, the carrier concentration should be 3 × 10⁻⁶. 19 cm -3 It is preferable to do so.

[0035] As described above, in the GaN substrate 1, the resistivity and thermal conductivity are determined by the Ge concentration. For example, as the Ge concentration increases, the carrier concentration increases, the resistivity decreases, and the thermal conductivity decreases. Therefore, in the GaN substrate 1, it is advisable to appropriately change the Ge concentration according to the required carrier concentration, resistivity, and thermal conductivity. For example, in GaN substrate 1, the resistivity at 25°C is 1 × 10⁻⁶ -3 From the perspective of keeping the thermal conductivity R1(L) in the thickness direction at 25°C at 140 W / mk or higher while keeping it below Ω·cm, the impurity concentration (e.g., Ge concentration) should be 7.0 × 10 19 cm -3 It is preferable to have a value of 7.0 × 10 19 cm -3 The above 1.0 × 10 21 cm -3 The following is more preferable. For example, in GaN substrate 1, the carrier concentration is set to 3 × 10⁻⁶. 19 cm -3 From the perspective of achieving a greater thermal conductivity R1(L) in the thickness direction at 25°C of 140 W / mk or higher, the impurity concentration (e.g., Ge concentration) should be 3.0 × 10 19 cm -3 It is preferable to have a value of 3.0 × 10 19 cm -3 The above 1.0 × 10 21 cm -3 The following is more preferable.

[0036] (2) Method for manufacturing a gallium nitride single crystal substrate Next, the method for manufacturing the gallium nitride single crystal substrate described above will be explained with reference to Figures 1 and 2. Figures 1(a) to 1(c) are schematic cross-sectional views showing a part of the method for manufacturing a gallium nitride single crystal substrate according to one embodiment of the present invention. Figure 2 is a flowchart showing the method for manufacturing a gallium nitride single crystal substrate according to this embodiment. As shown in Figure 2, the method for manufacturing a gallium nitride single crystal substrate according to this embodiment includes, for example, a substrate preparation step S100, a growth step S110, and a slicing and processing step S120.

[0037] (S100: Base board preparation process) First, in the substrate preparation step S100, as shown in Figure 1(a), a substrate 10 made of a gallium nitride single crystal in which the lowest index crystal plane closest to the main surface 10s is the (0001) plane (c plane) is prepared. Specifically, a commercially available gallium nitride single crystal substrate (a self-supporting substrate, not a template) may be used as the substrate 10, or a substrate 10 made of a gallium nitride single crystal may be fabricated by the conventionally known VAS (Void-Assisted Separation) method. The substrate 10 can be one with an n-type dopant such as Ge or Si added, or an undoped substrate. Preferably, the substrate 10 should be constructed with the same dopant added, depending on the type of dopant to be added to the growth layer 11, which will be described later. This reduces the difference in dopant concentration between the substrate 10 and the growth layer 11.

[0038] (S110: Growth process) Next, in the growth process S110, as shown in Figure 1(b), a single crystal of GaN is epitaxially grown on the main surface 10s of the substrate 10 prepared in the substrate preparation process S100, for example, with the c-plane as the growth surface. Specifically, for example, by supplying a raw material gas to the heated substrate 10 using the HVPE method, epitaxial growth is directly carried out on the main surface 10s of the substrate 10, and the growth layer 11 is grown.

[0039] Here, the HVPE device used in this embodiment will be specifically described with reference to Figure 3. Figure 3 is a schematic diagram illustrating an example of an HVPE device.

[0040] The HVPE apparatus 200 is made of a heat-resistant material such as quartz and includes an airtight container 203 with a film deposition chamber 201 inside. A support member 208 (hereinafter also referred to as a holder 208) for holding the substrate 10 is provided inside the film deposition chamber 201. A heater 207 is provided on the outer circumference of the airtight container 203 for heating the substrate 10 held inside the gas generator 233 or on the holder 208 to a desired temperature. The holder 208 is connected to a rotating shaft 215 having a rotation mechanism (not shown) and is configured to be rotatable. Gas supply pipes 232a to 232c are connected to one end of the airtight container 203. These gas supply pipes 232a to 232c are each made of a heat-resistant material such as quartz and are arranged to supply a predetermined gas to the substrate 10 in the film deposition chamber 201. An exhaust pipe (not shown) for exhausting the film deposition chamber 201 is provided at the other end of the airtight container 203. In this embodiment, a gas supply pipe 232d is also provided to supply gas around the holder 208.

[0041] A carrier gas is supplied to the gas supply pipe 232a along with a chlorine-containing gas (e.g., HCl, Cl2, etc.) containing a chlorine-based gas. Downstream of the gas supply pipe 232a, a gas generator 233 is provided to contain the Ga melt as a raw material. In the gas generator 233, gallium chloride (GaCl) gas is produced by the reaction of the chlorine-containing gas and the Ga melt. The produced gallium chloride gas is supplied onto the substrate 10. It is preferable that the supply port of the gas supply pipe 232a, which supplies the GaCl gas as the Ga raw material gas, is located further from the holder 208 than the gas supply pipes 232b and 232c that supply the other raw material gases. With this configuration, unintended reactions between the GaCl gas discharged from the supply port and the ammonia gas supplied by the holder purge can be suppressed. As a result, the desorption of nitrogen atoms by the holder purge can be suppressed more reliably.

[0042] The gas supply pipe 232b is supplied with a carrier gas along with a doping material-containing gas, which includes the doping material gas. As the doping material gas, at least tetrachlorogermane (GeCl4) gas, which is a germanium source, can be used. The doping material gas is supplied onto the substrate 10.

[0043] The gas supply pipe 232c is supplied with a carrier gas along with an NH3-containing gas, which includes NH3 gas (ammonia gas). The NH3-containing gas is supplied from the gas supply pipe 232c onto the substrate 10.

[0044] In this embodiment, the gas supply pipe 232d is positioned to supply a predetermined gas around the holder 208. In this embodiment, ammonia gas is supplied from the viewpoint of suppressing the desorption of nitrogen atoms during the process of forming the growth layer 11. The ammonia gas is supplied from the gas supply pipe 232d toward the back side of the holder 208 and flows from the back side of the holder 208 toward the side where the base substrate 10 is held. Therefore, during the crystal growth of the growth layer 11, ammonia gas can be continuously supplied around the outer edge of the base substrate 10 and the growth interface (outer edge) of the growth layer 11. This makes it possible to locally create positive pressure around the holder 208 with ammonia gas. As a result, the GaN crystal growth environment can be made N-rich, and even if nitrogen atoms are desorbed, they can be replenished.

[0045] Each component of the HVPE device 200 is connected to a controller (not shown) configured as a computer, and the processing procedures and conditions described later are controlled by a program executed on the controller.

[0046] Specifically, in the growth process S110, the HVPE apparatus 200 holds the substrate 10 to be processed on the holder 208. While heating and exhausting are performed in the deposition chamber 201, NH3 gas is supplied into the deposition chamber 201. For example, NH3 gas is supplied to the deposition chamber 201 from gas supply pipes 232c and 232d. When the deposition chamber 201 reaches the desired deposition temperature and deposition pressure, and the atmosphere inside the deposition chamber 201 becomes an ammonia-containing atmosphere, gas is supplied from gas supply pipes 232a to 232c to supply GaCl gas, doping material-containing gas (GeCl4), and NH3 gas to the substrate 10 as raw material gases. Here, the Ge concentration is 5.0 × 10⁻⁶. 18 cm -3 To epitaxially grow the GaN crystals described above, a doping material-containing gas is supplied. By reacting these gases, a growth layer 11 containing a high concentration of Ge can be formed on the substrate 10. The ammonia-containing atmosphere may also contain gases other than ammonia, such as hydrogen.

[0047] In this embodiment, in order to suppress the desorption of nitrogen atoms from the GaN crystal and the resulting generation of point defects, crystal growth of the growth layer 11 is performed in a pressurized ammonia-containing atmosphere inside the deposition chamber 201. Here, the growth condition of growth step S110, being in a pressurized state, means that crystal growth is performed at a pressure higher than atmospheric pressure. Specifically, the growth pressure is set to be greater than 1 atm and 1.5 atm or less. By using such growth conditions, the desorption of nitrogen atoms during crystal growth can be suppressed. Moreover, ammonia gas is supplied around the substrate 10, and ammonia is constantly supplied around the growth interface during the crystal growth process. This further suppresses the desorption of nitrogen atoms.

[0048] Furthermore, while GaN crystals may become contaminated with oxygen during their growth process, this embodiment can suppress such contamination. Specifically, oxygen tends to enter nitrogen sites as an N-type dopant, which can worsen thermal conductivity at room temperature and the decrease in thermal conductivity as temperature rises. In this embodiment, by growing GaN crystals under pressurized conditions in an ammonia-containing atmosphere and purging the holder with ammonia gas, the partial pressure of oxygen at the growth interface is increased, which is presumably suppressing the entry of oxygen into the nitrogen sites. As a result, the oxygen concentration in the GaN crystal can be reduced, making it possible to make the oxygen concentration negligible compared to the impurity concentration.

[0049] Other growth conditions besides growth pressure include, for example, the following: The preferred growth temperature is, for example, 980°C to 1200°C. It is preferable that the ratio of the supply rate of NH3 gas, a group V raw material gas, to the supply rate of GaCl gas, a group III raw material gas (hereinafter also referred to as the "V / III ratio") be, for example, 0.1 or more and 5.0 or less. It is preferable to set the partial pressure of the GaCl gas to, for example, 330 Pa or more and 1300 Pa or less. It is preferable to set the partial pressure of the doping material-containing gas to, for example, 0.05 Pa or more and 2 Pa or less.

[0050] (S120: Slicing and processing process) In the slicing and processing step S120, as shown in Figure 1(c), the growth layer 11 is sliced ​​using a wire saw or the like along a cutting plane substantially parallel to the main surface 11s (c-plane) of the growth layer 11. This yields at least one GaN substrate 1 having a diameter of 50 mm or more as an as-sliced ​​substrate. At this time, it is preferable to slice the GaN substrate 1 so that its thickness is, for example, 300 μm or more and 800 μm or less.

[0051] Once a GaN substrate 1 is obtained as an azu-sliced ​​substrate, both sides of the GaN substrate 1 may be polished using a polishing device, for example.

[0052] In this embodiment, the GaN substrate 1 obtained in the slicing and processing step S120 may be used as a new base substrate 10, and the growth step S110 and the slicing and processing step S120 may be repeated multiple times (for example, four or more times).

[0053] The GaN substrate 1 according to this embodiment is manufactured through the above process.

[0054] Furthermore, the obtained GaN substrate 1 may be used to fabricate a semiconductor stack by epitaxially growing a semiconductor functional layer made of a group III nitride semiconductor on its main surface 1s. After fabricating the semiconductor stack, electrodes and the like are further formed on the semiconductor stack, and the semiconductor stack is diced to cut out chips of a predetermined size. A semiconductor device may be fabricated in this way. The GaN substrate 1 has high thermal conductivity and excellent heat dissipation, so the device performance of the semiconductor device can be maintained at a high level for a long period of time.

[0055] (4) Effects obtained by the present invention According to this embodiment, one or more of the following effects can be obtained.

[0056] (a) The GaN substrate 1 of this embodiment has a resistivity of 1 × 10 at 200°C. -2The resistivity is configured to be Ω·cm or less. Furthermore, in the GaN substrate 1, it is necessary to add impurities to reduce resistivity, and the addition of impurities may lead to the formation of nitrogen vacancies and other point defects during the crystal growth process. In this embodiment, the crystal growth of the GaN substrate 1 is carried out under pressure in an ammonia-containing atmosphere. This suppresses the desorption of nitrogen atoms in the GaN substrate 1. Moreover, the crystal growth is carried out while purging the holder by locally supplying ammonia gas around the base substrate 10. This allows for continuous supply of ammonia gas around the growth interface of the GaN crystal, further suppressing the desorption of nitrogen atoms. As a result, during the GaN crystal growth process, even with a high concentration of impurities, the desorption of nitrogen atoms can be suppressed and the occurrence of point defects can be reduced. The resulting GaN substrate 1 can maintain a high thermal conductivity in the thickness direction even in high-temperature environments while keeping the resistivity low. Specifically, in the GaN substrate 1, the thermal conductivity R1(H) in the thickness direction at 200°C can be set to 80W / mk to 120W / mk. Because of the low resistivity of such a GaN substrate 1, when fabricating high-power laser diodes or power devices, for example, it is possible to improve switching characteristics and lower on-resistance even in high-temperature environments. Moreover, high heat dissipation can be achieved even in high-temperature environments, maintaining high device performance.

[0057] In comparison, when the substrate is grown under reduced pressure in an ammonia-containing atmosphere or when holder purging is performed with hydrogen or nitrogen, the deterioration of crystallinity due to high-concentration doping of impurities cannot be sufficiently suppressed, and point defects tend to increase. GaN substrates obtained under such conditions, even if they exhibit high thermal conductivity at room temperature, tend to decrease at high temperatures, with a decrease in thermal conductivity R1(H) in the thickness direction and thermal conductivity R2(H) in the in-plane direction. In this respect, the growth conditions of this embodiment suppress the decrease in thermal conductivity due to temperature increase, and high thermal conductivity can be maintained even at high temperatures.

[0058] (b) Preferably, the GaN substrate 1 has a thermal conductivity R1(L) in the thickness direction at 25°C of 120 W / mk or more and 230 W / mk or less. This makes it possible to increase the thermal conductivity in the thickness direction in a room temperature environment, which in turn promotes heat dissipation to the heat sink when a semiconductor device is fabricated.

[0059] (c) When the thermal conductivity in the thickness direction of the GaN substrate 1 is R1, it is preferable that the rate of change x of R1 when the temperature is raised from 25°C to 200°C is 0.5 or less. With such a GaN substrate 1, the decrease in thermal conductivity due to temperature rise is suppressed, and the thermal conductivity in the thickness direction R1(H) can be kept high even in high-temperature environments. Therefore, in semiconductor devices fabricated from the GaN substrate 1, heat dissipation to the heat sink can be maintained even when the temperature rises.

[0060] (d) Since point defects are suppressed in the GaN substrate 1, both the thermal conductivity R1 in the thickness direction and the thermal conductivity R2 along the in-plane direction can be maintained at a high level. Specifically, it is preferable that the thermal conductivity R2(L) along the in-plane direction at 25°C is 150 W / mk or higher, and the thermal conductivity R2(H) in the in-plane direction at 200°C is 80 W / mk to 110 W / mk. With such a GaN substrate 1, when a semiconductor device is fabricated, the heat generated inside the device is diffused and localized overheating is suppressed, thereby maintaining high device characteristics.

[0061] (e) When the GaN substrate 1 is heated from 25°C to 200°C, the rate of change y of R2 in the in-plane direction is preferably 0.6 or less. With such a GaN substrate 1, the decrease in thermal conductivity due to temperature rise is suppressed, and the in-plane thermal conductivity R2 can be maintained at a high level even in high-temperature environments. Therefore, in semiconductor devices fabricated from the GaN substrate 1, the heat generated inside the device can be diffused in the in-plane direction, and localized overheating can be suppressed more reliably.

[0062] (f) The GaN substrate 1 contains impurities including Ge, and the Ge concentration is 1.0 × 10 18 cm -3 It is preferable that the Ge concentration is greater than or equal to the above. With such a Ge concentration, the resistivity of the GaN substrate 1 at 200°C can be made lower. On the other hand, the Ge concentration is 1.0 × 10 21 cm -3 The following is preferable. With such a Ge concentration, it is possible to maintain a high thermal conductivity R1(H) in the thickness direction at 200°C while keeping the resistivity of the GaN substrate 1 low.

[0063] (g) The GaN substrate 1 preferably contains oxygen as an impurity in addition to germanium, and the O concentration is 1 / 10 or less of the Ge concentration. This makes it possible to obtain the above effect (a) more reliably.

[0064] (h) The O concentration in GaN substrate 1 is 6 × 10 16 cm -3 The following is preferable. This makes it possible to obtain the above effect (g) more reliably.

[0065] (i) The carrier concentration of GaN substrate 1 is 1.0 × 10⁻⁶ 18 cm -3 The above is preferable. Such a GaN substrate 1 can improve the operating speed of semiconductor devices and reduce power loss.

[0066] (j) In this embodiment, the growth pressure in the growth atmosphere should be pressurized, preferably greater than 1 atm and 1.5 atm or less. By using such a growth pressure, the desorption of nitrogen atoms during the growth process of the GaN crystal can be suppressed more reliably.

[0067] (k) Furthermore, during the growth process of the GaN crystal, it is preferable to position the supply port of the gas supply pipe 232a that supplies the Ga raw material gas at the position furthest from the base substrate 10. This suppresses the reaction between the Ga raw material gas and the ammonia gas supplied by holder purging, and more reliably suppresses the desorption of nitrogen atoms.

[0068] <Other Embodiments> The embodiments of the present invention have been specifically described above. However, the present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention.

[0069] The above-described embodiment explains the case in which GaN crystals are grown on a substrate made of GaN crystals, but the present invention is not limited thereto. The present invention is not limited to homoepitaxial growth, and heteroepitaxial growth may also be performed. In this case, a conventionally known VAS method may be employed to grow GaN crystals on the substrate to form a growth layer, and then the growth layer may be peeled off.

[0070] In the above embodiment, the case in which the growth layer 11 is sliced ​​using a wire saw in the slicing and processing step S120 was described, but for example, an outer blade slicer, an inner blade slicer, an electrical discharge machine, etc. may also be used. [Examples]

[0071] Next, embodiments relating to this disclosure will be described. These embodiments are examples of this disclosure and the disclosure is not limited to these embodiments.

[0072] (1) Fabrication of gallium nitride single crystal substrates The gallium nitride single crystal substrates for samples 1-3 were fabricated as follows.

[0073] [Conditions for preparing Sample 1] (Substrate) Material: GaN Preparation method: VAS method Diameter: 2 inches Thickness: 400 μm The lowest index crystal plane closest to the principal plane: c-plane Main surface mean square roughness RMS: 0.5 nm

[0074] (growth layer) Material: GaN Growth method: HVPE method Growth temperature: 980℃ or higher and 1200℃ or lower Growth pressure: 1.2 atm to 1.5 atm Growth environment: ammonia, hydrogen V / III ratio: 0.1 or more and 5.0 or less Ge doping method: GeCl4 gas Partial pressure of GeCl4 gas: 0.15 Pa Thickness of the growth layer: 2000 μm (Holder purge conditions) Gas: Ammonia gas Supply amount: 1.0L / min

[0075] (Slicing and processing conditions) Thickness of gallium nitride single crystal substrate: 400 μm Carfloss: 200 μm

[0076] A GaN substrate was fabricated under the above manufacturing conditions. The Ge concentration of the GaN substrate was measured by secondary ion mass spectrometry, and the Ge concentration was found to be 7.05 × 10⁻⁶. 18 cm -3 Similarly, when the oxygen concentration was measured, it was found to be 1.00 × 10⁻⁶. 16 cm -3 ~6.00 x 10 16 cm -3 In other words, the O concentration was less than 1 / 100th of the Ge concentration.

[0077] [Conditions for preparing Sample 2] In Sample 2, the GaN substrate was fabricated under the same conditions as in Sample 1, except that the partial pressure of GeCl4 gas was set to 0.39 Pa to achieve a higher Ge concentration than in Sample 1. The Ge concentration of the GaN substrate was measured by secondary ion mass spectrometry, and the Ge concentration was found to be 1.99 × 10⁻⁶. 19 cm -3 Similarly, when the oxygen concentration was measured, it was found to be 1.00 × 10⁻⁶. 16 cm -3 ~6.00 x 10 16 cm -3 In other words, the O concentration was less than 1 / 100th of the Ge concentration.

[0078] [Conditions for preparing Sample 3] In Sample 3, the GaN substrate was fabricated under the same conditions as in Sample 1, except that the partial pressure of GeCl4 gas was set to 1.85 Pa to achieve a higher Ge concentration than in Sample 2. The Ge concentration of the GaN substrate was measured by secondary ion mass spectrometry, and the Ge concentration was found to be 1.03 × 10⁻⁶. 20 cm -3 Similarly, when the oxygen concentration was measured, it was found to be 1.00 × 10⁻⁶. 16 cm -3 ~6.00 x 10 16 cm -3 In other words, the O concentration was less than 1 / 100th of the Ge concentration.

[0079] [Preparation conditions for Sample 4] In Sample 4, GaN crystals were grown in an ammonia-containing atmosphere, but the growth pressure was kept low at 0.6 atm. The partial pressure of GeCl4 gas was set to 0.39 Pa, adjusting the Ge concentration to be similar to that of Sample 2. Furthermore, during GaN crystal growth, holder purging was performed by supplying nitrogen gas instead of ammonia gas to the outer edge of the substrate. Except for the above, the GaN substrate was fabricated under the same conditions as Sample 1. The Ge concentration of the GaN substrate was measured by secondary ion mass spectrometry, and the Ge concentration was found to be 2.08 × 10⁻⁶. 19 cm -3 Similarly, when the O concentration was measured, it was found to be higher than that of samples 1-4, at 6.00 × 10⁻⁶. 16 cm -3 ~1.00×10 17 cm -3 That was the case.

[0080] (2) Evaluation For the fabricated GaN substrates, resistivity, carrier concentration, and thermal conductivity in the thickness direction and in-plane direction were measured at temperatures of 25°C and 200°C. The measurement methods are described below.

[0081] (specific resistance) The resistivity of the GaN substrate was measured at 25°C and 200°C, respectively. Specifically, the resistivity at 25°C was obtained by performing Hall effect measurement using the Van der Pauw method on the sample taken from the GaN substrate. Also, the resistivity at 200°C was obtained by heating the taken sample to 200°C and then performing the same operations as the measurement of the resistivity at 20°C.

[0082] (Carrier concentration) The carrier concentration was obtained by performing Hall effect measurement on the sample taken from the GaN substrate.

[0083] (Thermal conductivity in the thickness direction) The thermal conductivity R1(L) [W / m·K] in the thickness direction at 25°C was obtained by determining the density ρ [kg / m 3 , specific heat C P [J / kg·K], and thermal diffusivity α [m 2 / s] for the sample taken from the GaN substrate and calculating from the formula R1(L)=αρC P . The density was obtained by the Archimedes method. The specific heat was obtained by performing specific heat capacity measurement using the DSC method. The thermal diffusivity was obtained by performing thermal diffusivity measurement using the flash method. The thermal diffusivity was measured using the measuring device "LFA467HT" manufactured by NETZSCH.

[0084] The thermal conductivity R1(H) [W / m·K] in the thickness direction at 200°C was obtained by changing the temperature of the sample taken from the GaN substrate from 25°C to 200°C and determining and calculating the density, specific heat, and thermal diffusivity in the same manner as above. Note that since the thermal expansion due to temperature rise is negligible, the value at 25°C was adopted for the density.

[0085] (Thermal conductivity in the in-plane direction) The thermal conductivity R2(L) [W / m·K] in the in-plane direction at 25°C was obtained by determining the density ρ [kg / m 3 , specific heat C P [J / kg·K], and thermal diffusivity α [m 2 / s] for the sample taken from the GaN substrate and calculating from the formula R2(L)=αρC PIt was calculated from. The thermal diffusivity was determined by performing measurement of thermal diffusivity by the photoacoustic method. As the apparatus for measuring the thermal diffusivity, the thermal diffusivity measuring apparatus "LaserPIT" manufactured by ULVAC Technologies, Inc. was used. Note that the density and specific heat were measured in the same manner as described above.

[0086] The in-plane thermal conductivity R2(H) [W / m·K] at 200°C was obtained and calculated in the same manner as above by changing the temperature of the sample collected from the GaN substrate from 25°C to 200°C and determining the density, specific heat, and thermal diffusivity. Note that since the thermal expansion due to the temperature increase was negligible, the value at 25°C was adopted for the density.

[0087] (3) Evaluation results The evaluation results are summarized in Table 1 below.

[0088]

Table 1

[0089] As shown in Table 1, for Samples 1 to 3, since the Ge concentration was 1.0×10 18 cm -3 or more, it was confirmed that the resistivity at 25°C was 1×10 -2 Ω·cm or less. Also, the resistivity at 200°C was as low as at 25°C, and it was confirmed that it was 1×10 -2 Ω·cm or less. That is, for Samples 1 to 3, it was confirmed that the resistivity was low in both room temperature and high temperature environments. On the other hand, in GaN substrate 1, it was confirmed that the thermal conductivity R1(H) in the thickness direction at 200°C could be as high as 80 W / mk to 120 W / mk. Also, it was confirmed that the thermal conductivity R2(H) in the in-plane direction at 200°C was as high as 80 W / mk to 110 W / mk. Also, for Samples 1 to 3, it was confirmed that the thermal conductivities R1(L) and R2(L) in the thickness direction and in-plane direction at 25°C were both high. Specifically, the thermal conductivity R1(L) at 25°C was 120 W / mk or more, and the thermal conductivity R2(L) at 25°C was 150 W / mk or more. Thus, in samples 1 to 3, it was confirmed that even with high concentrations of Ge doping, the decrease in thermal conductivity due to the addition was suppressed, and high thermal conductivity could be maintained not only at room temperature but also at high temperatures.

[0090] Here, we will explain thermal conductivity in detail using Figures 4 and 5. Figure 4 shows the variation of thermal conductivity R1(L) and R1(H) in the thickness direction at 25°C and 200°C with respect to Ge concentration. Figure 5 shows the variation of thermal conductivity R2(L) and R2(H) in the in-plane direction at 25°C and 200°C with respect to Ge concentration. In both figures, the horizontal axis represents Ge concentration and the vertical axis represents thermal conductivity.

[0091] As shown in Figure 4, it was confirmed that the thermal conductivity R1 in the thickness direction tends to decrease as the Ge concentration increases. Furthermore, as shown in Figure 5, it was confirmed that the thermal conductivity R2 in the in-plane direction tends to decrease as the Ge concentration increases. Additionally, it was confirmed that the thermal conductivity in each direction tends to decrease with increasing temperature from 25°C to 200°C.

[0092] However, in samples 1 to 3, it was confirmed that the decrease in thermal conductivity due to temperature increase was small. Specifically, as shown in Table 1, when the temperature was increased from 25°C to 200°C, the rate of change x of the thermal conductivity R1 in the thickness direction was 0.5 or less, and the rate of change y of the thermal conductivity R2 in the in-plane direction was 0.6 or less.

[0093] In samples 1-3, the reason why high thermal conductivity was maintained not only in room temperature environments but also in high-temperature environments is presumed to be because the growth conditions were designed to suppress the occurrence of point defects.

[0094] In contrast, in Sample 4, the resistivity at 25°C and 200°C was low, similar to Sample 1, and although the thermal conductivity R1(L) and R2(L) at 25°C were high, similar to Sample 1, the thermal conductivity R1(H) and R2(H) at 200°C were significantly lower. Furthermore, the rate of change of thermal conductivity x and y with increasing temperature were both high, confirming a large decrease in thermal conductivity with increasing temperature. These are presumed to be due to an increase in point defects in the GaN crystal due to nitrogen evaporation.

[0095] Based on the above, it was confirmed that by pressurizing the growth layer in an ammonia-containing atmosphere and purging the ammonia gas holder, it is possible to fabricate a GaN substrate that can maintain high thermal conductivity even in high-temperature environments while reducing resistivity. Such a GaN substrate exhibits high heat dissipation even in high-temperature environments, thus enabling the semiconductor device to maintain high performance over a long period of time.

[0096] <Preferred Embodiments of the Invention> Preferred embodiments of the present invention are described below.

[0097] (Note 1) A gallium nitride single crystal substrate in which the lowest index crystal plane closest to the principal plane is the (0001) plane, The resistivity at 200°C is 1 × 10⁻⁶ -2 It is less than or equal to Ω·cm, The thermal conductivity in the thickness direction at 200°C is 80 W / mk or higher. Gallium nitride single crystal substrate.

[0098] (Note 2) The thermal conductivity in the thickness direction at 25°C is between 120 W / mk and 230 W / mk. Gallium nitride single crystal substrate as described in Appendix 1.

[0099] (Note 3) When the thermal conductivity in the thickness direction at 200°C is R(H) and the thermal conductivity in the thickness direction at 25°C is R(L), R(H) and R(L) satisfy (R(L)-R(H)) / R(L)≦0.5. Gallium nitride single crystal substrate as described in Appendix 2.

[0100] (Note 4) It contains impurities including germanium, Germanium concentration is 1.0 × 10 18 cm -3 That's all. A gallium nitride single crystal substrate of any one of the following types (Notes 1-3).

[0101] (Note 5) The aforementioned impurities include oxygen, The oxygen concentration is 1 / 10 or less of the germanium concentration. Gallium nitride single crystal substrate as described in Appendix 4.

[0102] (Note 6) A gallium nitride single crystal substrate in which the lowest index crystal plane closest to the principal plane is the (0001) plane, The resistivity at 25°C is 1 × 10⁻⁶ -3 It is less than Ω·cm, The thermal conductivity in the thickness direction at 25°C is 140 W / mk or higher. Gallium nitride single crystal substrate.

[0103] (Note 7) It contains impurities including germanium, Germanium concentration is 7.0 × 10 19 cm -3 That's all. Gallium nitride single crystal substrate as described in Appendix 6.

[0104] (Note 8) A gallium nitride single crystal substrate in which the lowest index crystal plane closest to the principal plane is the (0001) plane, Carrier concentration is 3 × 10 19 cm -3 Larger, The thermal conductivity in the thickness direction at 25°C is 140 W / mk or higher. Gallium nitride single crystal substrate.

[0105] (Note 9) (a) A step of preparing a substrate made of a gallium nitride single crystal in which the lowest index crystal plane closest to the main plane is the (0001) plane, (b) The step of epitaxially growing a gallium nitride single crystal on the main surface of the substrate, In (b) above, a raw material gas for forming the gallium nitride single crystal is supplied to the substrate under an ammonia-containing atmosphere with a growth pressure greater than 1 atm and 1.5 atm or less, while ammonia is supplied from around the support member that supports the substrate. A method for manufacturing a gallium nitride single crystal substrate.

[0106] (Note 10) In (b) above, among the supply ports for supplying the raw material gas, the supply port for supplying the Ga-containing gas is positioned further from the support substrate than the supply ports for supplying the other raw material gases. The method for manufacturing a gallium nitride single crystal substrate as described in Appendix 9. [Explanation of Symbols]

[0107] 1. Gallium nitride single crystal substrate (GaN substrate) 10 Substrate 11 Growth layer

Claims

1. A gallium nitride single crystal substrate in which the lowest index crystal plane closest to the main plane is the (0001) plane, The resistivity at 200°C is 1 × 10⁻⁶ －２ It is less than or equal to Ω·cm, The thermal conductivity in the thickness direction at 200°C is 80 W / mk or higher. Gallium nitride single crystal substrate.

2. The thermal conductivity in the thickness direction at 25°C is between 120 W / mk and 230 W / mk. The gallium nitride single crystal substrate according to claim 1.

3. When R(H) is the thermal conductivity in the thickness direction at 200°C and R(L) is the thermal conductivity in the thickness direction at 25°C, R(H) and R(L) satisfy (R(L) - R(H)) / R(L) ≤ 0.

5. The gallium nitride single crystal substrate according to claim 2.

4. It contains impurities including germanium, Germanium concentration is 1.0 × 10 １８ cm －３ That's all. A gallium nitride single crystal substrate according to claim 1 or claim 2.

5. The aforementioned impurities include oxygen, The oxygen concentration is 1 / 10 or less of the germanium concentration. The gallium nitride single crystal substrate according to claim 4.

6. (a) A step of preparing a substrate made of a gallium nitride single crystal in which the lowest index crystal plane closest to the main plane is the (0001) plane, (b) The process includes the step of epitaxially growing a gallium nitride single crystal on the main surface of the substrate, In (b) above, a raw material gas for forming the gallium nitride single crystal is supplied to the substrate under an ammonia-containing atmosphere with a growth pressure greater than 1 atm and 1.5 atm or less, while ammonia is supplied from around the support member that supports the substrate. A method for manufacturing a gallium nitride single crystal substrate.

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