Epitaxial wafer and method for manufacturing the same

Abstract

To provide a method for manufacturing a β-Ga2O3 based epitaxial wafer, capable of effectively revitalizing donor impurities in an inactivated substrate by using O2 gas as the oxygen material gas of an epitaxial film in the HVPE method, and an epitaxial wafer manufactured thereby.SOLUTION: A method for manufacturing an epitaxial wafer by the HVPE method comprises the steps of: exposing a substrate 10 consisting of a β-Ga2O3 based single crystal and including donor impurities to GaCl gas and O2 gas to grow an epitaxial film 12 consisting of a β-Ga2O3 based single crystal on the main surface 11 of the substrate 10 and form an epitaxial wafer 1; and subjecting the epitaxial wafer 1 to annealing at a temperature of 1200°C or more in a nitrogen atmosphere.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to epitaxial wafers and methods for manufacturing the same. [Background technology]

[0002] Conventionally, a technique for growing a β-Ga2O3 single crystal film on the main surface of a β-Ga2O3 single crystal substrate using the HVPE (Halide Vapor Phase Epitaxy) method is known (see Patent Document 1). In the technique described in Patent Document 1, a β-Ga2O3 single crystal substrate is exposed to a gallium chloride-based gas as a source gas for Ga and an oxygen-containing gas as a source gas for oxygen, and a β-Ga2O3 single crystal film is epitaxially grown on the main surface of the β-Ga2O3 single crystal substrate.

[0003] Patent Document 1 discloses that if hydrogen is present in the atmosphere when growing a β-Ga2O3-based single crystal film, the flatness of the surface of the β-Ga2O3-based single crystal film and the crystal growth driving force decrease, therefore, it is preferable to use hydrogen-free O2 gas as the oxygen source gas. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Publication No. 2015-91740 [Overview of the Initiative] [Problems that the invention aims to solve]

[0005] However, according to the technology described in Patent Document 1, when O2 gas is used as the oxygen source gas, the β-Ga2O3 single crystal substrate is exposed to O2 gas during the epitaxial growth of the β-Ga2O3 single crystal film, which inactivates the donor impurities in the β-Ga2O3 single crystal substrate. Therefore, after the formation of the β-Ga2O3 single crystal film, it is necessary to reactivate the donor impurities in the β-Ga2O3 single crystal substrate by performing an annealing treatment under a nitrogen atmosphere.

[0006] The object of the present invention is to provide a method for manufacturing an epitaxial wafer in which an epitaxial film made of a β-Ga2O3-based single crystal is formed on a substrate made of a β-Ga2O3-based single crystal, wherein O2 gas is used as the oxygen source gas for the epitaxial film in the HVPE method, thereby enabling more effective reactivation of donor impurities in the inactivated substrate, and to provide an epitaxial wafer manufactured by this method. [Means for solving the problem]

[0007] One aspect of the present invention provides the following method for manufacturing an epitaxial wafer and an epitaxial wafer in order to achieve the above objective.

[0008] [1] The process of forming an epitaxial wafer by exposing a substrate made of β-Ga2O3 single crystal and containing donor impurities to GaCl gas and O2 gas using the HVPE method, growing an epitaxial film made of β-Ga2O3 single crystal on the main surface of the substrate, and heating the epitaxial wafer at 1200°C or higher in a nitrogen atmosphere. , below 1400℃ A method for manufacturing an epitaxial wafer, comprising the step of performing an annealing treatment at a temperature. [ 2 The device comprises a substrate made of a β-Ga2O3 single crystal containing donor impurities, and an epitaxial film made of a β-Ga2O3 single crystal on the substrate, wherein the donor concentration of the substrate is 80% or more of the concentration of the donor impurities, and the Cl concentration of the epitaxial film is 1 × 10⁻¹⁰ 14 cm -3 The above is true, and the H concentration of the substrate and the epitaxial film is 3 × 10 17 cm -3 The following is an epitaxial wafer. [ 3 The variation in the donor concentration within the plane of the substrate, relative to the midpoint between the maximum and minimum values ​​of the donor concentration within the plane of the substrate, is 10% or less. 2 Epitaxial wafers as described in [ ]. [Effects of the Invention]

[0009] According to the present invention, there is provided a method for manufacturing an epitaxial wafer in which an epitaxial film made of a β-Ga2O3 single crystal is formed on a substrate made of a β-Ga2O3 single crystal, and a method capable of more effectively reactivating donor impurities in the substrate inactivated by using O2 gas as a source gas for oxygen of the epitaxial film in the HVPE method, and an epitaxial wafer manufactured by the method.

Brief Description of the Drawings

[0010] [Figure 1] FIG. 1 is a vertical cross-sectional view of an epitaxial wafer according to an embodiment of the present invention. [Figure 2] FIG. 2 is a vertical cross-sectional view of a vapor phase growth apparatus according to an embodiment of the present invention. [Figure 3] FIG. 3 is a graph showing the temperature change in an annealing furnace when an annealing treatment is performed on an epitaxial wafer. [Figure 4] FIG. 4 is a graph showing the activation rate of donor impurities in the substrate before and after forming the epitaxial film and after annealing treatment at 1150 to 1400°C. [Figure 5] FIG. 5 is a graph showing the impurity concentration in an epitaxial wafer measured by secondary ion mass spectrometry (SIMS).

Embodiments for Carrying Out the Invention

[0011] (Configuration of Epitaxial Wafer) FIG. 1 is a vertical cross-sectional view of an epitaxial wafer 1 according to an embodiment of the present invention. The epitaxial wafer 1 is made of a β-Ga2O3 single crystal, and includes a substrate 10 containing donor impurities and an epitaxial film 12 made of a β-Ga2O3 single crystal formed by epitaxial crystal growth on a main surface 11 of the substrate 10.

[0012] Here, the β-Ga2O3 single crystal refers to a β-Ga2O3 single crystal or a β-Ga2O3 single crystal doped with elements such as Al and In. For example, it is a β-type (Ga x Al y In (1-x-y) )2O3 (0 < x ≤ 1, 0 ≤ y ≤ 1, 0 < x + y ≤ 1) single crystal. When Al is added, the bandgap widens, and when In is added, the bandgap narrows.

[0013] The substrate 10 is formed by slicing a bulk crystal of a β-Ga2O3 single crystal grown by a melt growth method such as the FZ (Floating Zone) method or the EFG (Edge-Defined Film-Fed Growth) method and polishing the surface. The substrate 10 contains donor impurities such as Sn, Si, and Ge. The donor concentration of the substrate 10 is 80% or more of the concentration of the donor impurities contained in the substrate 10.

[0014] The epitaxial film 12 is formed on the substrate 10 by the HVPE (Halide Vapor Phase Epitaxy) method. The epitaxial film 12 may contain donor impurities such as Sn, Si, and Ge and acceptor impurities such as Mg. Since the epitaxial film 12 is formed by the HVPE method using a gas containing Cl as a source gas, it contains Cl at a concentration of 1×10 14 cm -3 or higher.

[0015] In addition, the epitaxial film 12 is formed using an O2 gas not containing H as a source gas for oxygen. Therefore, the concentration of H contained in the substrate 10 and the epitaxial film 12 is low, 3×10 17 cm -3 or less.

[0016] (Structure of the vapor deposition apparatus) Hereinafter, an example of the structure of a vapor deposition apparatus used for growing the epitaxial film 12 according to the present embodiment of the present invention will be described.

[0017] Figure 2 is a vertical cross-sectional view of a vapor phase growth apparatus 2 according to an embodiment of the present invention. The vapor phase growth apparatus 2 is a vapor phase growth apparatus for the HVPE method and includes a reactor 20 having a first gas introduction port 21, a second gas introduction port 22, a third gas introduction port 23, and an exhaust port 24, and a heating means 26 installed around the reactor 20 to heat the inside of the reactor 20.

[0018] The reactor 20 has a reaction region R1 in which a reaction vessel 25 containing Ga raw materials is placed and gallium raw material gas is generated, and a crystal growth region R2 in which the substrate 10 is placed and the epitaxial film 12 is grown. The reactor 20 is made of, for example, quartz glass.

[0019] The reaction vessel 25 is, for example, made of quartz glass, and the Ga raw material contained in the reaction vessel 25 is metallic gallium.

[0020] The heating means 26 can heat the raw material reaction region R1 and the crystal growth region R2 of the reactor 20. The heating means 26 is, for example, a resistance heating type or a radiant heating type heating device.

[0021] The first gas introduction port 21 is for introducing a Cl-containing gas, such as Cl2 gas or HCl gas, into the raw material reaction region R1 of the reactor 20 using an inert carrier gas (N2 gas, Ar gas, or He gas). The second gas introduction port 22 is for introducing O2 gas, which is the raw material gas for oxygen, into the crystal growth region R2 of the reactor 20 using an inert carrier gas (N2 gas, Ar gas, or He gas). The third gas introduction port 23 is for introducing a chloride-based gas (e.g., silicon tetrachloride) for adding a dopant such as Si to the epitaxial film 12 into the crystal growth region R2 of the reactor 20 using an inert carrier gas (N2 gas, Ar gas, or He gas).

[0022] (Epitaxial film growth) An example of the growth process for the epitaxial film 12 according to this embodiment will be described below.

[0023] First, using the heating means 26, the ambient temperature of the raw material reaction region R1 of the reactor 20 is maintained at a predetermined temperature, for example, 500 to 900°C. Then, a Cl-containing gas is introduced from the first gas introduction port 21 using a carrier gas. In the raw material reaction region R1, the metallic gallium in the reaction vessel 25 reacts with the Cl-containing gas at the above ambient temperature to produce GaCl gas.

[0024] Furthermore, if hydrogen is present in the atmosphere used to grow the epitaxial film 12, the flatness of the surface of the epitaxial film 12 and the crystal growth driving force will decrease. Therefore, it is preferable to use hydrogen-free Cl2 gas as the Cl-containing gas.

[0025] Furthermore, the reaction between metallic gallium and Cl-containing gases also produces gallium chloride-based gases other than GaCl gas, such as GaCl2, GaCl3, and (GaCl3)2. However, among these gallium chloride-based gases, the partial pressure of GaCl gas is overwhelmingly high, so the gases other than GaCl gas hardly contribute to the growth of Ga2O3-based single crystals.

[0026] Next, using the heating means 26, the ambient temperature of the crystal growth region R2 of the reactor 20 is maintained at a predetermined temperature, for example, 800 to 1100°C. In the crystal growth region R2, the GaCl gas generated in the raw material reaction region R1 and the O2 gas introduced from the second gas introduction port 22 are mixed, and the substrate 10 is exposed to the mixed gas to epitaxially grow an epitaxial film 12 on the main surface 11 of the substrate 10. At this time, the pressure in the crystal growth region R2 inside the furnace housing the reactor 20 is maintained at, for example, 1 atm.

[0027] In this case, when forming an epitaxial film 12 containing additive elements such as Si and Al, the raw material gas for the additive elements (for example, a chloride-based gas such as silicon tetrachloride (SiCl4)) is also introduced into the crystal growth region R2 from the third gas introduction port 23, along with the GaCl gas and O2 gas.

[0028] By using O2 gas as the oxygen source gas, it is possible to suppress the reduction in surface flatness and crystal growth driving force of the epitaxial film 12 caused by hydrogen in the atmosphere during growth of the epitaxial film 12, compared to the case where a hydrogen-containing gas such as H2O gas is used.

[0029] Next, the epitaxial wafer 1 is transferred from the vapor phase growth apparatus 2 to the annealing furnace, and in order to reactivate the donor impurities in the substrate 10 that have been inactivated by exposure to O2 gas during the deposition of the epitaxial film 12, the wafer is subjected to an annealing treatment at a temperature of 1200°C or higher under a nitrogen atmosphere.

[0030] Figure 3 is a graph showing the temperature change inside the annealing furnace when performing the annealing process on the epitaxial wafer 1. As shown in Figure 3, the temperature changes from room temperature to the annealing temperature T. a Raise the temperature to T a It is held at this temperature for time t, and then allowed to cool down to room temperature. Here, the annealing temperature T a For example, it is within the range of 1200°C or higher and 1400°C or lower. Also, temperature T a The time t for which the data is retained is, for example, within the range of 1 hour or more and 10 hours or less.

[0031] Figure 4 is a graph showing the activation rate of donor impurities in the substrate 10 before and after the deposition of the epitaxial film 12 (indicated as before deposition and after deposition) and after annealing at 1150-1400°C.

[0032] The activation rate of donor impurities is the proportion of donor impurities that actually function as donors out of all donor impurities, and is equal to the ratio of the donor concentration to the donor impurity concentration. The donor concentration of the epitaxial film 12 before deposition is approximately equal to the donor impurity concentration, and the activation rate of donor impurities is approximately 100%.

[0033] The activation rate of donor impurities in substrate 10, shown in Figure 4, was determined as the ratio of the donor concentration to the donor concentration before the deposition of the epitaxial film 12. The donor concentration of substrate 10 was obtained by performing electrochemical capacitance voltage (ECV) measurement on the substrate 10 after polishing the substrate 10 side of the epitaxial wafer 1 by chemical mechanical polishing (CMP).

[0034] The substrate 10 shown in Figure 4 is made of a β-Ga2O3 single crystal containing Sn and Si (Si was unintentionally included in the raw materials of the substrate 10) as donor impurities. However, the same results as in Figure 4 can be obtained regardless of the type of β-Ga2O3 single crystal or donor impurities that make up the substrate 10.

[0035] As shown in Figure 4, the activation rate of donor impurities decreases to 12% when the substrate 10 is exposed to O2 gas during the deposition of the epitaxial film 12, and the activation rate is restored by the subsequent annealing treatment.

[0036] Furthermore, as shown in Figure 4, when the annealing temperature is 1200°C or higher, the activation rate of donor impurities in the substrate 10 becomes 80% or higher, meaning the donor concentration becomes 80% or higher of the donor impurity concentration. Also, the higher the annealing temperature, the greater the reactivation rate. In Figure 4, the activation rates when the annealing temperatures are 1150°C, 1200°C, 1300°C, and 1400°C are 45%, 84.8%, 95.2%, and 100%, respectively.

[0037] When the annealing temperature exceeds 1400°C, the amount of thermal decomposition of the β-Ga2O3-based single crystal increases; therefore, it is preferable that the annealing temperature be 1400°C or lower. When the annealing temperature is 1400°C or lower, the amount of thermal decomposition (reduction in film thickness due to thermal decomposition) of the epitaxial film 12 made of β-Ga2O3-based single crystal can be kept to 1 μm or less.

[0038] Table 1 shows the change in the thickness of the epitaxial film 12 before and after annealing at an annealing temperature of 1400°C. Table 1 shows the change in thickness at five different locations on the epitaxial film 12, as measured by a Fourier transform infrared spectrophotometer (FTIR).

[0039] JPEG0007870539000001.jpg37170

[0040] Table 1 shows that the change in film thickness at all locations is 1.0 μm or less, indicating that the thermal decomposition of the epitaxial film 12 is suppressed. Note that the epitaxial wafer 1 is colorless and transparent before annealing, but after annealing at a temperature of approximately 1200°C or higher, it changes to a light blue color.

[0041] To perform annealing at temperatures above 1200°C, for example, an electric annealing furnace made of alumina is used. In general-purpose annealing furnaces made of quartz, it is difficult to perform annealing at temperatures above 1200°C due to the heat resistance temperature of quartz. Typically, the maximum temperature that can be achieved with a general-purpose quartz annealing furnace is around 1150°C, and the activation rate in this case is approximately 45%, as shown in Figure 4.

[0042] Furthermore, in order to suppress contamination of the epitaxial wafer 1, it is preferable that the annealing furnace used for the annealing process be made of a material that does not contain Si or C, such as alumina.

[0043] When annealing was performed at temperatures between 1200°C and 1400°C, the variation in donor concentration within the surface of the substrate 10, relative to the midpoint between the maximum and minimum donor concentrations within the surface of the substrate 10, was 10% or less.

[0044] Figure 5 is a graph showing the impurity concentration in epitaxial wafer 1, measured by secondary ion mass spectrometry (SIMS). The substrate 10 of epitaxial wafer 1 shown in Figure 5 is a β-Ga2O3 single crystal substrate containing Sn and Si (Si was unintentionally included in the raw materials of substrate 10) as donor impurities, and the epitaxial film 12 has a concentration of 3.0 × 10⁻¹⁶ 15 cm -3 This film is made of a β-Ga2O3 single crystal containing Si at a certain concentration as a donor impurity.

[0045] In Figure 5, the horizontal axis represents the depth (μm) from the surface 13 of the epitaxial film 12 on the epitaxial wafer 1, and the vertical axis represents the concentration (cm³) of each impurity. -3 This represents the interface between the substrate 10 and the epitaxial film 12 of the epitaxial wafer 1, where the interface depth is approximately 11.3 μm.

[0046] Figure 5 shows the concentrations of Si, Sn, and Cl in the epitaxial wafer 1. According to Figure 5, the concentration of Sn in the epitaxial film 12 is close to the detection limit of the SIMS analyzer (indicated as detection limit: Sn), confirming that no unintended contamination of the epitaxial film 12 with Sn has occurred. The concentrations of Si and Sn are higher in the region near the interface between the epitaxial film 12 and the substrate 10, but this is because Si and Sn contained in the substrate 10 have diffused into the epitaxial film 12, and is not a problem.

[0047] Furthermore, according to Figure 5, there are approximately 2.9 × 10¹³ units in the epitaxial film 12. 16 ~4.0×10 16 cm -3 It contains Cl at a concentration of 1 × 10⁻¹⁰. This is because the epitaxial film 12 is formed by the HVPE method using a Cl-containing gas. Normally, when a β-Ga2O3 single crystal film is formed by a method that does not use a Cl-containing gas or by a method that uses a Cl-containing gas other than the HVPE method (for example, the MOVPE method using SiCl4 gas), Cl is not contained in the β-Ga2O3 single crystal film, and is at least 1 × 10⁻¹⁰. 14 cm -3 The above Cl is not present.

[0048] (Effects of the embodiment) According to the above embodiment of the present invention, after the formation of the epitaxial film 12, by performing an annealing treatment on the epitaxial wafer 1 at a temperature of 1200°C or higher under a nitrogen atmosphere, donor impurities in the substrate 10 that have been inactivated by using O2 gas as the oxygen source gas for the epitaxial film 12 in the HVPE method can be effectively reactivated.

[0049] Although embodiments of the present invention have been described above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the invention. Furthermore, the components of the above embodiments can be arbitrarily combined without departing from the spirit of the invention. Moreover, the embodiments described above do not limit the invention as claimed. It should also be noted that not all combinations of features described in the embodiments are necessarily essential for solving the problem of the invention. [Explanation of symbols]

[0050] 1…Epitaxial wafer, 10…Substrate, 11…Main surface, 12…Epitaxial film, 2…Vapor phase growth apparatus, 20…Reaction furnace, 21…First gas introduction port, 22…Second gas introduction port, 23…Third gas introduction port, 24…Exhaust port, 25…Reaction vessel, 26…Heating means

Claims

1. By the HVPE method, β-Ga ２ O ３ It consists of a single crystal system, and the substrate containing donor impurities is treated with GaCl gas and O ２ Exposed to gas, β-Ga is applied to the main surface of the substrate. ２ O ３ A process of growing an epitaxial film made of a single crystal to form an epitaxial wafer, The epitaxial wafer is subjected to an annealing treatment at a temperature of 1200°C or higher and 1400°C or lower under a nitrogen atmosphere. including, A method for manufacturing epitaxial wafers.

2. β-Ga ２ O ３ It consists of a single crystal system and a substrate containing donor impurities, The β-Ga ２ O ３ epitaxial film made of a system single crystal, and Equipped with, The donor concentration of the substrate is 80% or more of the donor impurity concentration. The Cl concentration of the epitaxial film is 1 × 10 １４ cm －３ That's all. The H concentration of the substrate and the epitaxial film is 3 × 10 １７ cm －３ The following is: Epitaxial wafer.

3. The variation in donor concentration within the plane of the substrate, relative to the midpoint between the maximum and minimum values ​​of donor concentration within the plane of the substrate, is 10% or less. The epitaxial wafer according to claim 2.

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