Epitaxial wafer and manufacturing method thereof

Abstract

Provided is a method for producing an epitaxial wafer, comprising: a step for forming an epitaxial wafer (1) by exposing a substrate (10) containing a beta-Ga2O3-based single crystal and containing a donor impurity to GaCl gas and O2 gas by an HVPE method, and growing an epitaxial film (12) containing a beta-Ga2O3-based single crystal on a main surface (11) of the substrate (10); and a step for annealing the epitaxial wafer (1) in a nitrogen atmosphere at a temperature of 1200 DEG C or higher.

Description

Technical Field

[0001] The present invention relates to an epitaxial wafer and a method for manufacturing the same. Background Art

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

[0003] Patent document 1 discloses that if hydrogen is contained in the atmosphere when growing a β-Ga2O3 single crystal film, the flatness of the surface of the β-Ga2O3 single crystal film and the driving force for crystal growth will decrease. Therefore, it is preferred to use hydrogen-free O2 gas as the raw material gas for oxygen.

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Publication No. 2015-91740 Summary of the invention

[0007] Problem that the invention aims to solve

[0008] However, according to the technology of Patent Document 1, when O2 gas is used as the raw material gas of oxygen, the donor impurities in the β-Ga2O3 system single crystal substrate are deactivated because the β-Ga2O3 system single crystal substrate is exposed to O2 gas during the epitaxial growth of the β-Ga2O3 system single crystal film. Therefore, after the β-Ga2O3 system single crystal film is formed, it is necessary to perform annealing treatment in a nitrogen atmosphere to reactivate the donor impurities in the β-Ga2O3 system single crystal substrate.

[0009] An object of the present invention is to provide a method for manufacturing an epitaxial wafer having an epitaxial film comprising a β-Ga2O3 system single crystal formed on a substrate comprising a β-Ga2O3 system single crystal, and an epitaxial wafer manufactured by the manufacturing method, wherein the manufacturing method can more effectively reactivate donor impurities in the substrate that have been deactivated due to the use of O2 gas as a raw material gas for oxygen of the epitaxial film in the HVPE method.

[0010] Solutions for solving problems

[0011] In order to achieve the above-mentioned object, one aspect of the present invention provides the following method for manufacturing an epitaxial wafer and an epitaxial wafer.

[0012] [1] A method for manufacturing an epitaxial wafer, comprising: exposing a substrate including a β-Ga2O3 system single crystal and containing donor impurities to GaCl gas and O2 gas by the HVPE method, growing an epitaxial film including the β-Ga2O3 system single crystal on the main surface of the substrate to form an epitaxial wafer; and annealing the epitaxial wafer at a temperature of 1200°C or above in a nitrogen atmosphere.

[0013] [2] The method for manufacturing an epitaxial wafer according to [1] above, wherein the temperature of the annealing treatment is 1400° C. or less.

[0014] [3] An epitaxial wafer comprising: a substrate comprising a β-Ga2O3 system single crystal and containing a donor impurity; and an epitaxial film comprising a β-Ga2O3 system single crystal on the substrate, wherein the donor concentration of the substrate is 80% or more of the concentration of the donor impurity, and the Cl concentration of the epitaxial film is 1×10 14 cm -3 The H concentration of the substrate and the epitaxial film is 3×10 17 cm -3 the following.

[0015] [4] The epitaxial wafer according to [3] above, wherein the deviation of the donor concentration within the surface of the substrate relative to the center value of the maximum value and the minimum value of the donor concentration within the surface of the substrate is 10% or less.

[0016] Effects of the Invention

[0017] According to the present invention, there can be provided a method for manufacturing an epitaxial wafer having an epitaxial film including a β-Ga2O3 system single crystal formed on a substrate including a β-Ga2O3 system single crystal, and an epitaxial wafer manufactured by the manufacturing method, wherein the manufacturing method can more effectively reactivate donor impurities in the substrate that have been deactivated due to the use of O2 gas as a raw material gas for oxygen of the epitaxial film in the HVPE method. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a vertical cross-sectional view of an epitaxial wafer according to an embodiment of the present invention.

[0019] Figure 2 It is a vertical cross-sectional view of a vapor phase growth apparatus according to an embodiment of the present invention.

[0020] Figure 3 It is a graph showing the temperature change in the annealing furnace when the epitaxial wafer is annealed.

[0021] Figure 4 It is a graph showing the activation rate of donor impurities in a substrate before and after formation of an epitaxial film and after annealing at 1150 to 1400° C.

[0022] Figure 5 It is a graph showing the impurity concentration in the epitaxial wafer measured by secondary ion mass spectrometry (SIMS). DETAILED DESCRIPTION

[0023] (Structure of epitaxial wafer)

[0024] Figure 1 1 is a vertical cross-sectional view of an epitaxial wafer 1 according to an embodiment of the present invention. The epitaxial wafer 1 comprises: a substrate 10 comprising a β-Ga2O3 system single crystal and containing donor impurities; and an epitaxial film 12 formed on a primary surface 11 of the substrate 10 by epitaxial crystal growth and comprising a β-Ga2O3 system single crystal.

[0025] Here, the β-Ga2O3 system single crystal refers to a β-Ga2O3 single crystal or a β-Ga2O3 single crystal to which elements such as Al and In are added. For example, a β-Ga2O3 single crystal to which Al and In are added 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 band gap becomes wider, and when In is added, the band gap becomes narrower.

[0026] The substrate 10 is formed by, for example, slicing a bulk crystal of a β-Ga2O3 single crystal grown by a melt growth method such as an FZ (Floating Zone) method or an EFG (Edge-Defined Film-Fed Growth) method, and grinding the surface. The substrate 10 contains donor impurities such as Sn, Si, and Ge. The donor concentration of the substrate 10 is more than 80% of the concentration of the donor impurities contained in the substrate 10.

[0027] The epitaxial film 12 is formed on the substrate 10 by the HVPE (Halide Vapor Phase Epitaxy) method. The epitaxial film 12 may also 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 raw material gas, it contains 1×10 14 cm -3 Above the concentration of Cl.

[0028] In addition, the epitaxial film 12 is formed using O2 gas that does not contain hydrogen (H) as the raw material gas of oxygen. Therefore, the concentration of H contained in the substrate 10 and the epitaxial film 12 is low, which is 3×10 17 cm -3 the following.

[0029] (Structure of Vapor Growth Apparatus)

[0030] Next, an example of the structure of a vapor phase growth apparatus used for growing the epitaxial film 12 according to the present embodiment of the present invention will be described.

[0031] Figure 2 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 HVPE method, and comprises: a reaction furnace 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 unit 26, which is arranged around the reaction furnace 20 and heats the interior of the reaction furnace 20.

[0032] The reactor 20 includes a raw material reaction region R1 in which a reaction container 25 containing Ga raw materials is placed to generate raw material gas of gallium, and a crystal growth region R2 in which a substrate 10 is placed to grow an epitaxial film 12. The reactor 20 is made of, for example, quartz glass.

[0033] The reaction container 25 is made of, for example, quartz glass, and the Ga raw material contained in the reaction container 25 is metal gallium.

[0034] The heating unit 26 can heat the raw material reaction region R1 and the crystal growth region R2 of the reaction furnace 20. The heating unit 26 is, for example, a heating device of a resistance heating type or a radiation heating type.

[0035] The first gas introduction port 21 is a port 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 a carrier gas (N2 gas, Ar gas, or He gas) as an inert gas. The second gas introduction port 22 is a port for introducing O2 gas, which is a raw material gas of oxygen, into the crystal growth region R2 of the reactor 20 using a carrier gas (N2 gas, Ar gas, or He gas) as an inert gas. The third gas introduction port 23 is a port for introducing a chloride-based gas (e.g., silicon tetrachloride, etc.) for adding a dopant such as Si to the epitaxial film 12 into the crystal growth region R2 of the reactor 20 using a carrier gas (N2 gas, Ar gas, or He gas) as an inert gas.

[0036] (Epitaxial Film Growth)

[0037] An example of a growth process of the epitaxial film 12 according to the present embodiment will be described below.

[0038] First, while the atmosphere temperature of the raw material reaction area R1 of the reactor 20 is maintained at a specified temperature, for example, 500 to 900°C, using the heating unit 26, a Cl-containing gas is introduced from the first gas inlet port 21 using a carrier gas, and in the raw material reaction area R1, the metal gallium in the reaction container 25 reacts with the Cl-containing gas at the above-mentioned atmosphere temperature to generate GaCl gas.

[0039] Furthermore, if hydrogen is contained in the atmosphere when the epitaxial film 12 is grown, the flatness of the surface of the epitaxial film 12 and the crystal growth driving force are reduced. Therefore, it is preferable to use Cl 2 gas that does not contain hydrogen as the Cl-containing gas.

[0040] In addition, the reaction between metallic gallium and Cl-containing gas will also generate GaCl2 gas, GaCl3 gas and (GaCl3)2 gas as gallium chloride-based gases other than GaCl gas, but the partial pressure of GaCl gas will become overwhelmingly high among these gallium chloride-based gases. Therefore, gases other than GaCl gas will hardly contribute to the growth of Ga2O3-based single crystals.

[0041] Next, while the atmosphere temperature of the crystal growth region R2 of the reactor 20 is maintained at a predetermined temperature, for example, 800 to 1100° C., by using the heating unit 26, the GaCl gas generated in the raw material reaction region R1 is mixed with the O2 gas introduced from the second gas introduction port 22 in the crystal growth region R2, and the substrate 10 is exposed to the mixed gas to epitaxially grow the epitaxial film 12 on the main surface 11 of the substrate 10. At this time, the pressure in the crystal growth region R2 in the furnace housing the reactor 20 is maintained at, for example, 1 atm.

[0042] Here, when forming an epitaxial film 12 containing additive elements such as Si and Al, the raw material gas of the additive element (for example, chloride-based gas such as silicon tetrachloride (SiCl4)) is also introduced into the crystal growth region R2 from the third gas inlet port 23 together with GaCl gas and O2 gas.

[0043] By using O2 gas as the raw material gas of oxygen, compared with the case of using a gas containing hydrogen such as H2O gas, it is possible to suppress the flatness of the surface of the epitaxial film 12 and the decrease in the crystal growth driving force caused by the hydrogen contained in the atmosphere when growing the epitaxial film 12.

[0044] Next, the epitaxial wafer 1 is transferred from the vapor growth apparatus 2 to an annealing furnace, and annealing treatment is performed at a temperature above 1200° C. in a nitrogen atmosphere in order to reactivate donor impurities in the substrate 10 that have been deactivated due to exposure of the substrate 10 to O 2 gas during the formation of the epitaxial film 12 .

[0045] Figure 3 Graph 1 is a graph showing the temperature change in the annealing furnace when the epitaxial wafer 1 is annealed. Figure 3 As shown, the temperature is raised from room temperature to the annealing temperature T a , at temperature T a The temperature is kept for a period of time t, and then dropped to room temperature. a For example, it is in the range of 1200°C to 1400°C. a The holding time t is, for example, in the range of 1 hour to 10 hours.

[0046] Figure 4 It is a graph showing the activation rate of the donor impurity in the substrate 10 before and after the formation of the epitaxial film 12 (represented as before film formation and after film formation) and after the annealing treatment at 1150 to 1400° C. is performed.

[0047] The activation rate of the donor impurity is the ratio of the donor impurity that actually functions as a donor among all the donor impurities, and is equal to the ratio of the donor concentration to the concentration of the donor impurity. The donor concentration of the epitaxial film 12 before film formation is approximately equal to the concentration of the donor impurity, and the activation rate of the donor impurity is approximately 100%.

[0048] Figure 4 The activation rate of the donor impurities of the substrate 10 shown is obtained as a ratio of the donor concentration to the donor concentration before the formation of the epitaxial film 12. The donor concentration of the substrate 10 is obtained by performing electrochemical capacitance voltage (ECV) measurement on the substrate 10 after the substrate 10 side of the epitaxial wafer 1 is polished by chemical mechanical polishing (CMP).

[0049] In addition, although Figure 4 The substrate 10 is a substrate including a β-Ga2O3 single crystal containing Sn and Si as donor impurities (Si is Si accidentally contained in the raw material of the substrate 10), but regardless of the type of β-Ga2O3 single crystal constituting the substrate 10 and the type of donor impurities, the same Figure 4 Same result.

[0050] according to Figure 4 Since the substrate 10 was exposed to O 2 gas during the formation of the epitaxial film 12 , the activation rate of the donor impurities dropped to 12%, and the activation rate was restored by the subsequent annealing process.

[0051] In addition, according to Figure 4When the annealing temperature is above 1200° C., the activation rate of the donor impurities in the substrate 10 becomes above 80%, that is, the donor concentration becomes above 80% of the concentration of the donor impurities. In addition, the higher the annealing temperature, the greater the reactivation rate. Figure 4 The activation rates when the annealing temperatures were 1150° C., 1200° C., 1300° C., and 1400° C. were 45%, 84.8%, 95.2%, and 100%, respectively.

[0052] If the annealing temperature exceeds 1400° C., the amount of thermal decomposition of the β-Ga2O3 single crystal will increase, so the annealing temperature is preferably below 1400° C. When the annealing temperature is below 1400° C., the amount of thermal decomposition (the amount of film thickness reduction caused by thermal decomposition) of the epitaxial film 12 including the β-Ga2O3 single crystal can be suppressed to below 1 μm.

[0053] Table 1 below shows the changes in film thickness of the epitaxial film 12 before and after annealing when the annealing temperature is 1400° C. Table 1 shows the changes in film thickness at five different locations on the epitaxial film 12 measured by Fourier transform infrared spectrophotometer (FTIR).

[0054] [Table 1]

[0055] Position 1 Position 2 Location 3 Position 4 Location 5 Before annealing 17.6 17.5 18.0 19.1 17.8 After annealing 17.4 16.9 17.5 18.6 17.4 Change 0.2 0.6 0.5 0.5 0.4

[0056] Table 1 shows that the film thickness variation at any position was less than 1.0 μm, and thermal decomposition of the epitaxial film 12 was suppressed. The epitaxial wafer 1 was colorless and transparent before annealing, but changed to light blue after annealing at a temperature of about 1200° C. or more.

[0057] In order to perform annealing at a temperature of 1200°C or higher, an electric annealing furnace made of alumina is used, for example. In a general-purpose annealing furnace made of quartz, it is difficult to perform annealing at a temperature of 1200°C or higher due to the heat resistance of quartz. Generally, the maximum annealing temperature that can be performed by a general-purpose annealing furnace made of quartz is about 1150°C. In this case, the activation rate is as follows: Figure 4 Shown is approximately 45%.

[0058] In order to suppress contamination of the epitaxial wafer 1, the annealing furnace used in the annealing treatment is preferably made of a material containing no Si or C, such as alumina.

[0059] When the annealing treatment is performed at a temperature of 1200° C. to 1400° C., the deviation of the donor concentration in the plane of the substrate 10 relative to the center value of the maximum value and the minimum value of the donor concentration in the plane of the substrate 10 is 10% or less.

[0060] Figure 5 It is a graph showing the impurity concentration in the epitaxial wafer 1 measured by secondary ion mass spectrometry (SIMS). Figure 5 The substrate 10 of the epitaxial wafer 1 is a substrate including a β-Ga2O3 single crystal containing Sn and Si (Si is Si accidentally contained in the raw material of the substrate 10) as donor impurities, and the epitaxial film 12 is a substrate containing 3.0×10 15 cm -3 A film of β-Ga2O3 single crystal including a concentration of Si as a donor impurity.

[0061] Figure 5 The horizontal axis represents the depth (μm) of the epitaxial film 12 of the epitaxial wafer 1 from the surface 13, and the vertical axis represents the concentration (cm -3 ). Here, the depth of the interface between the substrate 10 and the epitaxial film 12 of the epitaxial wafer 1 is about 11.3 μm.

[0062] Figure 5 represents the concentration of Si, Sn, and Cl in the epitaxial wafer 1. Figure 5 It was confirmed that the concentration of Sn in the epitaxial film 12 was close to the detection limit of the SIMS analysis device (detection limit: denoted as Sn), and there was no accidental mixing of Sn into the epitaxial film 12. Although the concentration of Si and Sn in the area near the interface between the epitaxial film 12 and the substrate 10 was high, this was because the Si and Sn contained in the substrate 10 diffused to the epitaxial film 12 side, and there was no problem.

[0063] In addition, according to Figure 5 , the epitaxial film 12 contains about 2.9×10 16 ~4.0×10 16 cm -3 This is because the epitaxial film 12 is formed by the HVPE method using a Cl-containing gas. Generally, when a β-Ga2O3-based single crystal film is formed by a method that does not use a Cl-containing gas or a method that uses a Cl-containing gas other than the HVPE method (e.g., a MOVPE method using SiCl4 gas), the β-Ga2O3-based single crystal film does not contain Cl, at least not more than 1×10 14 cm -3 The above Cl.

[0064] (Effects of Embodiments)

[0065] According to the above-mentioned embodiment of the present invention, by performing an annealing treatment on the epitaxial wafer 1 at a temperature above 1200° C. in a nitrogen atmosphere after the formation of the epitaxial film 12, the donor impurities in the substrate 10 that have been deactivated due to the use of O2 gas as the raw material gas of oxygen for the epitaxial film 12 in the HVPE method can be effectively reactivated.

[0066] The above describes the embodiments of the present invention, but the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the main purpose of the invention. In addition, the constituent elements of the above embodiments can be arbitrarily combined without departing from the scope of the main purpose of the invention. In addition, the above-described embodiments do not limit the invention involved in the claims. In addition, it should be noted that all combinations of the features described in the embodiments are not necessarily necessary for the solutions to the problems of the invention.

[0067] Industrial Applicability

[0068] Provided are a method for manufacturing a β-Ga2O3-based epitaxial wafer and an epitaxial wafer manufactured by the manufacturing method, wherein the manufacturing method can more effectively reactivate donor impurities in a substrate that have been deactivated due to the use of O2 gas as a raw material gas for oxygen in an epitaxial film in an HVPE method.

[0069] Description of Reference Numerals

[0070] 1…epitaxial wafer, 10…substrate, 11…primary surface, 12…epitaxial film, 2…vapor phase growth device, 20…reaction furnace, 21…first gas introduction port, 22…second gas introduction port, 23…third gas introduction port, 24…exhaust port, 25…reaction container, 26…heating unit.

Claims

1. A method for manufacturing an epitaxial wafer, characterized in that: Include: A step of exposing a substrate including a β-Ga2O3 system single crystal and containing donor impurities to GaCl gas and O2 gas by HVPE method, and growing an epitaxial film including the β-Ga2O3 system single crystal on a main surface of the substrate to form an epitaxial wafer; and The epitaxial wafer is subjected to an annealing treatment at a temperature of 1200° C. or higher in a nitrogen atmosphere.

2. The method for manufacturing an epitaxial wafer according to claim 1, wherein: The temperature of the annealing treatment is below 1400°C.

3. An epitaxial wafer, characterized in that: have: A substrate comprising a β-Ga2O3 system single crystal and containing donor impurities; and The epitaxial film on the substrate includes a β-Ga2O3 system single crystal, The donor concentration of the substrate is greater than 80% of the concentration of the donor impurity, The Cl concentration of the epitaxial film is 1×10 14 cm -3 above, The H concentration of the substrate and the epitaxial film is 3×10 17 cm -3 the following.

4. The epitaxial wafer according to claim 3, wherein: The deviation of the donor concentration within the plane of the substrate is 10% or less with respect to a center value between a maximum value and a minimum value of the donor concentration within the plane of the substrate.

Images