SiC epitaxial wafer

Abstract

To provide a SiC epitaxial wafer with high surface uniformity of doping concentration in a high concentration layer.SOLUTION: A SiC epitaxial wafer 100 includes a SiC single crystal substrate 10 and a high concentration layer 21 (20) provided on the SiC single crystal substrate, having an average value of n-type doping concentration of 1.03×1018 / cm3 or more and 1×1019 / cm3 or less, and having a doping concentration uniformity on the surface of 30% or less. Nitrogen is doped in the high concentration layer 21 (20). The uniformity within the surface is the absolute value of (the maximum value of the doping concentration within the surface-the minimum value of the doping concentration within the surface) / the average value of the doping concentration within the surface.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a SiC epitaxial wafer.

Background Art

[0002] Silicon carbide (SiC) has a breakdown electric field one order of magnitude larger, a bandgap three times larger, and a thermal conductivity about three times higher than that of silicon (Si). Therefore, silicon carbide (SiC) is expected to be applied to power devices, high-frequency devices, high-temperature operation devices, etc.

[0003] The promotion of the practical application of SiC devices requires the establishment of high-quality and low-cost SiC epitaxial wafers and epitaxial growth technologies.

[0004] SiC devices are formed on a SiC epitaxial wafer including a SiC substrate and an epitaxial layer laminated on the substrate. The SiC substrate is obtained by processing a bulk single crystal of SiC grown by a sublimation recrystallization method or the like. The epitaxial layer is formed by a chemical vapor deposition (CVD) method or the like and serves as a breakdown voltage maintaining region of the device.

[0005] More specifically, the epitaxial layer is formed on the SiC substrate with a plane having an off-angle in the <11-20> direction from the (0001) plane as a growth plane. The epitaxial layer grows by step-flow growth (lateral growth from atomic steps) on the SiC substrate and becomes 4H-SiC.

[0006] In SiC epitaxial wafers, basal plane dislocations (BPDs) are known as device killer defects that can cause fatal defects in SiC devices. For example, when a forward current is passed through a bipolar device, the recombination energy of the flowing carriers causes partial dislocations of basal plane dislocations transferred from the SiC substrate to the epitaxial layer to move and expand, forming high-resistance stacking faults. When high-resistance regions occur within the device, it leads to a decrease in device reliability (forward degradation). Therefore, efforts have been made to reduce the amount of basal plane dislocations transferred to the epitaxial layer.

[0007] Many basal plane dislocations in SiC substrates can be converted into threading edge dislocations (TED) that do not cause defect expansion when the epitaxial layer is formed (Patent Document 1). However, recent studies have shown that when a large forward current is applied, basal plane dislocations converted to through-edge dislocations at the interface between the SiC substrate and the epitaxial layer also expand into stacking faults (SF) within the epitaxial layer. Therefore, for high-current power devices, which are expected to see market expansion in the future, simply converting basal plane dislocations to through-edge dislocations is insufficient to adequately suppress the formation of stacking faults, and concerns about deterioration of device reliability always remain.

[0008] Patent Document 2 discloses that by forming an epitaxial layer with an even higher impurity concentration in addition to the normal epitaxial layer within a SiC epitaxial wafer, the conversion efficiency from basal plane dislocations to through-edge dislocations at the interface between the SiC single crystal substrate and the epitaxial layer can be increased. By increasing the conversion efficiency to basal plane dislocations, the elongation and expansion of basal plane dislocations can be suppressed. Elongation and expansion of basal plane dislocations are a cause of forward degradation of devices. Therefore, the formation of an epitaxial layer with a high impurity concentration is considered a promising solution for suppressing forward degradation of SiC devices using SiC epitaxial wafers.

[0009] Patent document 3 discloses a manufacturing method for improving the in-plane uniformity of the doping concentration in the low-concentration layer. However, it does not mention the in-plane uniformity of the doping concentration in the high-concentration layer. [Prior art documents] [Patent Documents]

[0010] [Patent Document 1] Japanese Patent Publication No. 2009-88223 [Patent Document 2] International Publication No. 2017 / 094764 [Patent Document 3] Patent No. 6386706 [Overview of the Initiative] [Problems that the invention aims to solve]

[0011] The inventors of the present invention have found a problem in the fabrication of a SiC epitaxial wafer comprising a SiC single crystal substrate, a normal epitaxial layer, and an n-type high-doping concentration epitaxial layer between them: the in-plane uniformity of the n-type doping concentration in the n-type high-doping concentration epitaxial layer (high-concentration layer) deteriorates (wherein-plane uniformity of doping concentration in this specification means "the absolute value of the difference between the maximum and minimum doping concentrations" / the average value of the doping concentrations). As a result of diligent research, the inventors have arrived at the present invention, which solves this problem.

[0012] This invention has been made in view of the above circumstances, and aims to provide a SiC epitaxial wafer with high in-plane uniformity of n-type doping concentration in the high-concentration layer. [Means for solving the problem]

[0013] To solve the above problems, the present invention provides the following means.

[0014] (1) Embodiment 1 of the present invention is a SiC single crystal substrate and an n-type doping on the SiC single crystal substrate with an average concentration of 2 × 1018 / cm 3 Above, 1×10 19 / cm 3 Below, and including a high-concentration layer with an in-plane uniformity of the doping concentration of 30% or less, and the high-concentration layer is doped with nitrogen, a SiC epitaxial wafer; here, the in-plane uniformity is defined as the absolute value of (the maximum value of the doping concentration in the plane - the minimum value of the doping concentration in the plane) / the average value of the doping concentration in the plane.

[0015] (2) Aspect 2 of the present invention is a SiC single-crystal substrate, and on the SiC single-crystal substrate, a high-concentration layer with an average value of the n-type doping concentration of 3×10 18 / cm 3 Above, 1×10 19 / cm 3 Below, and including a high-concentration layer with an in-plane uniformity of the doping concentration of 30% or less, and the high-concentration layer is doped with nitrogen, a SiC epitaxial wafer; here, the in-plane uniformity is defined as the absolute value of (the maximum value of the doping concentration in the plane - the minimum value of the doping concentration in the plane) / the average value of the doping concentration in the plane.

[0016] (3) Aspect 3 of the present invention is a SiC single-crystal substrate, and on the SiC single-crystal substrate, a high-concentration layer with an average value of the n-type doping concentration of 5×10 18 / cm 3 Above, 1×10 19 / cm 3 Below, and including a high-concentration layer with an in-plane uniformity of the doping concentration of 30% or less, and the high-concentration layer is doped with nitrogen, a SiC epitaxial wafer; here, the in-plane uniformity is defined as the absolute value of (the maximum value of the doping concentration in the plane - the minimum value of the doping concentration in the plane) / the average value of the doping concentration in the plane.

[0017] (4) Aspect 4 of the present invention is a SiC single-crystal substrate, and on the SiC single-crystal substrate, a high-concentration layer with an average value of the n-type doping concentration of 1×10 18 / cm 3 Above, 1×10 19 / cm 3The SiC epitaxial wafer comprises the following: a high-concentration layer having an in-plane uniformity of doping concentration of 30% or less; and a buffer layer between the SiC single crystal substrate and the high-concentration layer that converts basal plane dislocations into through-edge dislocations, wherein the high-concentration layer is doped with nitrogen; here, the in-plane uniformity is the absolute value of (maximum in-plane doping concentration - minimum in-plane doping concentration) / the average value of the in-plane doping concentration.

[0018] (5) Embodiment 5 of the present invention is a SiC epitaxial wafer in any one of Embodiments 1 to 4 in which the high-concentration layer is free of basal plane dislocations.

[0019] (6) Aspect 6 of the present invention is a SiC epitaxial wafer in any one of aspects 1 to 5, wherein the high-concentration layer is a buffer layer, and the buffer layer is provided with a drift layer having an average doping concentration lower than the average doping concentration of the buffer layer.

[0020] (7) Embodiment 7 of the present invention is a SiC epitaxial wafer in any one of Embodiments 1 to 6 in which the in-plane uniformity is 20% or less.

[0021] (8) Embodiment 8 of the present invention is a SiC epitaxial wafer in any one of Embodiments 1 to 6 in which the in-plane uniformity is 10% or less.

[0022] (9) Embodiment 9 of the present invention is a SiC epitaxial wafer in any one of Embodiments 1 to 8, wherein the diameter is 150 mm or more. [Effects of the Invention]

[0023] The SiC epitaxial wafer of the present invention provides a SiC epitaxial wafer with high in-plane uniformity of the n-type doping concentration in the high-concentration layer. [Brief explanation of the drawing]

[0024] [Figure 1]This is a schematic cross-sectional view showing a SiC epitaxial wafer according to one embodiment of the present invention. [Figure 2] This is a schematic cross-sectional view showing a SiC epitaxial wafer according to another embodiment of the present invention. [Figure 3] This is a schematic cross-sectional diagram showing an example of a configuration in which the in-plane distribution of C-based gas supply and Si-based gas supply can be controlled independently. [Figure 4] Table 1 shows a graph illustrating the relationship between the input nitrogen flow rate and the obtained doping concentration (average value) for three samples with a C / Si ratio of 1.15. [Figure 5] This is the result of an investigation into the relationship between the average doping concentration and growth rate. [Figure 6] (a) is an illustrative diagram of nitrogen (N) doping, (b) is an illustrative diagram of the case where the C / Si ratio is low compared to (a), (c) is an illustrative diagram of the case where the C / Si ratio is high, and (d) is an illustrative diagram of the case where the doping flow rate is high. [Figure 7] This is a conceptual diagram illustrating how to estimate the amount of C-series gas input that is effectively insufficient, based on the calibration curve and the decrease in growth rate. [Figure 8] This graph shows the relationship between the average doping concentration, the in-plane uniformity of the doping concentration, and the C / Si ratio. [Modes for carrying out the invention]

[0025] Embodiments of the present invention will be described below with reference to the drawings. In the following embodiments, parts that are identical or equivalent to each other may be denoted by the same reference numerals in the drawings. Also, in the drawings used in the following description, characteristic parts may be enlarged for convenience in order to make the features easier to understand, and the dimensional ratios of each component may not be the same as in reality. Furthermore, the materials, dimensions, etc. exemplified in the following description are examples, and the present invention is not limited to them, and can be implemented with appropriate modifications within the scope of achieving the effects of the present invention. The configuration shown in one embodiment can also be applied to other embodiments.

[0026] (SiC Epitaxial Wafer) FIG. 1 is a schematic cross-sectional view showing a SiC epitaxial wafer according to an embodiment of the present invention, and FIG. 2 is a schematic cross-sectional view showing a SiC epitaxial wafer according to another embodiment.

[0027] The SiC epitaxial wafer 100 shown in FIG. 1 includes a SiC single crystal substrate 10 and a SiC epitaxial layer 20 formed on the main surface 10a of the SiC single crystal substrate 10. The SiC epitaxial layer 20 included in the SiC epitaxial wafer 100 has an average doping concentration of 1×10 18 / cm 3 or more and 1×10 19 / cm 3 or less, and is composed of a high-concentration layer 21 having an in-plane uniformity of doping concentration of 30% or less.

[0028] In the SiC epitaxial wafer 200 shown in FIG. 2, the SiC epitaxial layer 20 has a high-concentration layer 21 as a buffer layer, and a drift layer 22 having an average doping concentration lower than the average doping concentration of the buffer layer is provided on the buffer layer.

[0029] <SiC Single Crystal Substrate> As the SiC single crystal substrate 10, one obtained by slicing a SiC ingot obtained by a sublimation method or the like can be used. In this specification, the SiC epitaxial wafer means a wafer after forming an epitaxial layer, and the SiC single crystal substrate means a wafer before forming an epitaxial layer.

[0030] The SiC single crystal substrate 10 is not limited in size, but is preferably 100 mm, and more preferably 150 mm or more.

[0031] The SiC single crystal substrate 10 can be one having a plane with an offset angle in the direction from (0001) to <11-20> as a growth plane.

[0032] In the SiC single crystal substrate 10, basal plane dislocations exist along the (0001) plane (c plane). While it is preferable to have a small number of basal plane dislocations exposed on the growth surface of the SiC single crystal substrate, this number is not particularly limited.

[0033] If the SiC single crystal substrate 10 has a growth plane with an offset angle from (0001) in the <11-20> direction, then the basal plane dislocations exist at an angle with respect to the growth plane.

[0034] The SiC single crystal substrate 10 is doped with, for example, nitrogen. The doping concentration of the SiC single crystal substrate 10 is not particularly limited and can be the same as that of a typical SiC substrate for power semiconductors.

[0035] <High concentration layer> In the high-concentration layer 21, the average value of the n-type doping concentration is 1 × 10⁻⁶. 18 / cm 3 The above is 1 x 10 19 / cm 3 The following conditions must be met, and the in-plane uniformity of the n-type doping concentration must be 30% or less. The in-plane uniformity of the n-type doping concentration in the high-concentration layer 21 is preferably 20% or less, and more preferably 10% or less. While a lower value for the in-plane uniformity of the n-type doping concentration is better for the quality of the SiC epitaxial wafer, it can be set to 1% as an example of a lower limit from the viewpoint of yield.

[0036] At the interface between the epitaxial layer and the SiC single crystal substrate, positive carriers (holes) and negative carriers (electrons) recombine at the basal plane dislocations of the SiC single crystal substrate, causing the basal plane dislocations to expand in the epitaxial layer. The high-concentration layer 21, which has a high concentration of carriers, suppresses the carriers in the epitaxial layer from reaching the SiC single crystal substrate.

[0037] When the high-concentration layer 21 is a buffer layer and a drift layer 22 is provided on top of it, the high-concentration buffer layer 21 and the drift layer 22 can be clearly distinguished by the difference in doping concentration.

[0038] When the inventor fabricated a SiC epitaxial wafer on a SiC single crystal substrate, which included a high-concentration buffer layer and a drift layer, the in-plane uniformity of the doping concentration of the high-concentration buffer layer was 50% or more. By using the manufacturing method described later, it is possible to produce a high-concentration layer with an in-plane uniformity of doping concentration of 30% or less. Poor in-plane uniformity of doping concentration can lead to problems such as regions with lower concentrations than the target being present on the wafer surface, resulting in reduced carrier recombination effects, or regions with higher concentrations being present, causing defects due to high concentrations. These problems can be prevented by achieving good in-plane uniformity of 30% or less.

[0039] The high-concentration layer 21 is n-type, and nitrogen is used as the impurity for doping.

[0040] The average value of the n-type doping concentration in the high-concentration layer 21 is 1 × 10⁻⁶. 18 / cm 3 The above is 1 x 10 19 / cm 3 The following applies:

[0041] There are no particular limitations on the thickness of the high-concentration layer 21, but it can be, for example, about 1 μm to 10 μm. If it is too thin, the effect of suppressing carriers from reaching the SiC single crystal substrate will be reduced, and if it is too thick, the cost will be high.

[0042] The film thickness distribution of the high-concentration layer 21 is preferably 10% or less. When the film thickness distribution of the high-concentration layer 21 is 10% or less, it can be said that at least one of the in-plane distribution of the C-based gas supply on the substrate surface and the in-plane distribution of the Si-based gas on the substrate surface is 10% or less, and the surface of the high-concentration layer 21 is likely to become mirror-like.

[0043] <Drift layer> The high-concentration layer 21 is a buffer layer, and a drift layer 22 can be provided on top of it. The drift layer 22 is a layer through which drift current flows, functioning as a device. Drift current is the current generated by the flow of carriers when a voltage is applied to a semiconductor. The doping concentration of the drift layer 22 is, for example, 1 × 10⁻⁶. 14 cm -3 The above is 1 x 10 17 cm -3 The following applies, and is typically 1 × 10 16 cm -3 It is to that extent.

[0044] <Method for measuring doping concentration> The concentration of n-type doping in the high-concentration layer can be measured using the mercury probe (Hg-CV) method or secondary ion mass spectrometry (SIMS). In the Hg-CV method, the n-type doping concentration is N d -N a N is measured here. d This is the donor concentration, N a This is the acceptor concentration. d Compared to N a If it can be confirmed that is sufficiently small, then N d -N a ≒N d It is reasonable to think so. Using secondary ion mass spectrometry (SIMS), the doping concentration of a high-concentration layer can be measured in a SiC epitaxial wafer with a high-concentration layer by performing measurements while simultaneously removing the high-concentration layer in the depth direction. The same method can be used for SiC epitaxial wafers with a drift layer on top of the high-concentration layer.

[0045] The measurement points can be any points on the wafer surface as long as they reflect the distribution, but the wafer center and the measurement point furthest from the wafer center, specifically the point located 5 mm from the wafer edge, are included, while measurement points located less than 5 mm from the edge are not included. A specific procedure for measuring the n-type doping concentration in a high-concentration layer is as follows: For example, in the case of a 6-inch wafer, the n-type doping concentration is measured at multiple points, for example 21 points, in a cross shape with the center of the wafer as the origin, and from the center outwards. The average value of the n-type doping concentration is calculated using the n-type doping concentration obtained at each point, and in-plane uniformity can be obtained by dividing the absolute value of the difference between the maximum and minimum n-type doping concentrations by the calculated average value of the n-type doping concentration. One of the directions of the cross can be parallel to the orientation flag.

[0046] <Other layers> The SiC epitaxial wafer of the present invention may include other layers, to the extent that it achieves the effects of the present invention. For example, another buffer layer (hereinafter referred to as the first buffer layer) of n-type or p-type semiconductor with a lower impurity concentration (doping concentration) than the SiC single crystal substrate 10 may be provided between the SiC single crystal substrate 10 and the high-concentration layer 21. The first buffer layer may be provided to convert basal plane dislocations into through-edge dislocations. In this view, the first buffer layer is a BPD conversion layer. The impurity concentration of the first buffer layer is preferably lower than that of the SiC single crystal substrate 10, and is also preferably less than or equal to the impurity concentration of the high-concentration layer 21. The value of the impurity concentration of the first buffer layer is 1 × 10⁻⁶. 17 cm -3 Preferably, the above is true. The value of the impurity concentration in the first buffer layer is 1 × 10⁻⁶. 19 cm -3 The following is preferable: The impurity concentration of the first buffer layer can be set to be intermediate between the impurity concentrations of the SiC single crystal substrate 10 and the high-concentration layer 21 in order to mitigate lattice mismatch between the two.

[0047] (Method for manufacturing SiC epitaxial wafers) The manufacturing method for the SiC epitaxial wafer 100 or SiC epitaxial wafer 200 according to this embodiment involves, for example, growing an epitaxial layer 20 on a SiC single crystal substrate 10 whose main surface has an off-angle of 0.4° to 5° with respect to the (0001) plane.

[0048] First, prepare the SiC single crystal substrate 10. The method of preparing the SiC single crystal substrate 10 is not particularly important. For example, it can be obtained by slicing a SiC ingot obtained by sublimation or the like.

[0049] Next, a SiC epitaxial layer 20 is epitaxially grown on a SiC single crystal substrate 10 to produce a SiC epitaxial wafer 100. The SiC epitaxial layer 20 can be formed on the growth surface 10a of the SiC single crystal substrate 10 by step flow growth (lateral growth from atomic steps) using chemical vapor deposition (CVD).

[0050] The process of forming the SiC epitaxial layer 20 is carried out by flowing a source gas and a dopant gas over a SiC single crystal substrate that is kept at a high temperature.

[0051] Source gases are the gases used as raw materials when forming a SiC epitaxial layer. Generally, they are divided into Si-based source gases, which contain Si in their molecules, and C-based source gases, which contain C in their molecules.

[0052] Known silicon-based raw material gases can be used, such as silane (SiH4). In addition, chlorine-based silicon raw material gases containing chlorine with etching properties, such as dichlorosilane (SiH2Cl2), trichlorosilane (SiHCl3), and tetrachlorosilane (SiCl4), can also be used. As carbon-based raw material gases, for example, propane (C3H8) and ethylene (C2H4) can be used.

[0053] A dopant gas is a gas that contains elements that act as either donors or acceptors (carriers). Nitrogen and ammonia are used as dopant gases to grow n-type epitaxial layers, while trimethylaluminum (TMA) and triethylaluminum (TEA) are used to grow p-type epitaxial layers.

[0054] In addition, other gases may be used simultaneously to transport these gases into the reactor. For example, hydrogen, which is inert to SiC, may be used.

[0055] When manufacturing a SiC epitaxial wafer 100, the process of forming the SiC epitaxial layer 20 is a high-concentration layer process in which a high-concentration layer 21 is formed on a SiC single crystal substrate 10. When manufacturing a SiC epitaxial wafer 200, the process of forming the SiC epitaxial layer 20 is divided into a high-concentration layer process in which a high-concentration layer 21 is formed on a SiC single crystal substrate 10, and a drift layer process in which a drift layer is formed on the high-concentration layer 21.

[0056] <High concentration layer process> The growth temperature can be, for example, 1400 to 1800°C, and more preferably 1500 to 1700°C. If the temperature is too low, polytypes other than 4H are likely to occur, and if the temperature is too high, surface roughness is likely to occur.

[0057] As will be discussed later, a higher C / Si ratio in the source gas improves the in-plane uniformity of the doping concentration, but increasing the doping concentration lowers the effective C / Si ratio. Here, the C / Si ratio is the molar ratio of carbon atoms in the carbon-based source gas to the silicon atoms in the silicon-based source gas. Therefore, the C / Si ratio of the input source gas must be further increased to compensate for this. In other words, the target average value of the doping concentration is 1 × 10⁻⁶. 18 cm -3 ~5×10 18 cm -3 In this case, the C / Si ratio should be 1.1 or more and 1.5 or less, more preferably 1.2 or more and 1.4 or less. Also, if the target concentration is 5 × 10 18 cm -3 ~1 × 10 19 cm -3 In that case, it is preferable to have a value of 1.3 or more and 1.7 or less, more preferably 1.4 or more and 1.6 or less.

[0058] When depositing a high-concentration layer, it is necessary to dope it with a large amount of impurities to achieve the high concentration. In this case, the C / Si ratio is usually lowered. If the C / Si ratio is kept the same and doping is to achieve a high concentration, it will be necessary to introduce a large amount of doping gas. By lowering the C / Si ratio, it is possible to prevent the amount of doping gas introduced from becoming excessive. In response to this, the inventors conducted thorough research and discovered that lowering the C / Si ratio during the deposition of the high-concentration layer leads to a decrease in the in-plane uniformity of the n-type doping concentration. They found that by using a higher-than-usual C / Si ratio during the deposition of the high-concentration layer, the decrease in the in-plane uniformity of the n-type doping concentration can be suppressed. Furthermore, they found that it is important to have good supply of C-based gas and good in-plane distribution of Si-based gas on the substrate surface before deposition of the high-concentration layer.

[0059] <<Confirmation of C-based gas supply and in-plane distribution of Si-based gas on the substrate surface>> As described later, when depositing a high-concentration layer, a high-concentration layer with a mirror-like surface is formed on a SiC single-crystal substrate using a predetermined, higher-than-normal C / Si ratio (SiC epitaxial layer formation). However, a non-mirror-like region sometimes occurs in a part of it. This non-mirror-like region was often in the range of 5% to 50% in size. Through diligent investigation of the phenomenon of non-mirror-like region occurrence, it was determined that the cause was poor in-plane distribution of C-based gas and Si-based gas on the substrate surface. To improve this, it was found that it is effective to configure the gas supply to the deposition apparatus so that the in-plane distribution of C-based gas and Si-based gas can be controlled independently.

[0060] Figure 3 shows an example of a configuration that allows independent control of the in-plane distribution of C-based gas supply and Si-based gas supply. In the vertical film deposition apparatus 30 shown in Figure 3, there are gas supply sections 32a, 32b, and 32c that supply gas from the top to the bottom of the apparatus toward the substrate 10. The gas introduction section includes a supply section 32a for C-based gas only and a supply section 32b for Si-based gas only, and the supply sections 32a and 32b are structured to allow independent position adjustment in the horizontal direction (in-plane direction of the substrate). For example, a carrier gas is supplied to the gas supply section 32c. In the example shown in Figure 3, the C-based gas supply section 32a is equipped with gas supply pipes 32aa, 32ab, and 32ac, and the Si-based gas supply section 32b is equipped with gas supply pipes 32ba and 32bb.

[0061] Here, the in-plane distribution of C-based gas supply on the substrate surface can be measured by measuring the film thickness distribution (growth rate distribution) of an epitaxial wafer deposited under C-supply rate-limiting conditions. In this specification, the in-plane distribution of gas supply on the substrate surface refers to "the absolute value of (maximum film thickness of epitaxial wafer - minimum film thickness of epitaxial wafer) / average value of epitaxial wafer film thickness". The film thickness of a SiC epitaxial wafer can be measured by a known method, for example, FT-IR (Fourier Transform Infrared Spectroscopy). The C-supply rate-limiting condition is a state in which the supply of C-based gas is insufficient compared to the supply of Si-based gas, and preferably C / Si is in the range of 0.6 to 0.9. However, in order that the amount of N-based gas supplied does not change the effective C / Si, the carrier concentration of the epitaxial wafer is 1 × 10⁻⁶. 17 cm -3 The film is deposited under conditions where the value is less than [amount missing]. On the other hand, the in-plane distribution of Si-based gas supply on the substrate surface can be measured by measuring the film thickness distribution (growth rate distribution) of an epitaxial wafer deposited under Si supply rate-limiting conditions. Si supply rate-limiting conditions refer to a state in which the supply of Si-based gas is insufficient compared to C-based gas, preferably with a C / Si ratio in the range of 1.1 to 1.2. However, to ensure that the amount of N-based gas supplied does not change the effective C / Si ratio, the carrier concentration of the epitaxial wafer should be 1 × 10⁻⁶. 17 cm -3 The film is deposited under conditions where the value is less than [amount missing].

[0062] In this way, the in-plane distribution of C-based gas supply (film thickness distribution of an epitaxial wafer deposited under C-supply rate-limited conditions) and the in-plane distribution of Si-based gas supply (film thickness distribution of an epitaxial wafer deposited under Si-supply rate-limited conditions) are measured. If the in-plane distribution of C-based gas supply is 10% or less, and the in-plane distribution of Si-based gas is not 10% or less, the supply positions of the C-based gas supply unit and the Si-based gas supply unit are adjusted. For example, in a vertical film deposition apparatus, if the supply of C-based gas is low in the center and high on the outer edge, the position of the C-based gas supply unit at the gas introduction section is moved to the center. When the in-plane distribution of C-based gas on the substrate surface was 10% or less, and the in-plane distribution of Si-based gas on the substrate surface was also 10% or less, no non-mirror-finish regions were generated.

[0063] Before preparing the samples described below, the supply positions of the C-based gas supply section and the Si-based gas supply section were adjusted, and the in-plane growth rate distributions of C-rate-limited and Si-rate-limited growth were measured. (i)C rate limiting With C / Si = 0.8, the doping concentration is 8 × 10⁻⁶ 15 cm -3 The amount of nitrogen was adjusted to achieve this, and the film was deposited. Then, by adjusting the position of the supply section for C-based gas only, the in-plane distribution of C-based gas supply was set to 5.6%. (ii) Si rate limiting factor With C / Si = 1.1, the doping concentration is 1.3 × 10⁻⁶. 16 cm -3 The amount of nitrogen was adjusted to achieve this, and the film was deposited. Then, by adjusting the position of the supply section for Si-based gas only, the in-plane distribution of Si-based gas supply was set to 3.4%. The in-plane distribution of C-based gas supply and Si-based gas supply was measured at 21 points on the wafer, similar to the in-plane uniformity of n-type doping concentration described below.

[0064] Table 1 shows the results of investigating the in-plane uniformity of the n-type doping concentration in the high-concentration epitaxial layer of a 4H-SiC single crystal substrate with a diameter of 150 mm and a main plane with an off-angle of 4°. A high-concentration epitaxial layer was formed on the Si surface using nitrogen as an n-type dopant in the prepared sample. Specifically, the C / Si ratio of the source gas was set to 1.05, 1.15, or 1.35, and measurements were taken at 21 points in the wafer plane. The doping gas was introduced to target a predetermined value as the average value of the doping concentration, and the in-plane uniformity of the obtained n-type doping concentration was measured. Samples were prepared under the same conditions except for the C / Si ratio and the flow rate of the doping gas. The measurement points were defined as follows, with the wafer center as (0,0) and the orientation flat in the Y direction, using millimeters as the unit: (X,Y) = (0,70), (0,60), (0,45), (0,30), (0,15), (0,0), (0,-15), (0,-30), (0,-45), (0,-60), (0,-67), (-70,0), (-60,0), (-45,0), (-30,0), (-15,0), (15,0), (30,0), (45,0), (60,0), (70,0), a total of 21 points.

[0065] Furthermore, higher doping concentrations (average) can be achieved by increasing the nitrogen flow rate. The relationship between the nitrogen flow rate and the obtained doping concentration (average) is shown in Table 1 for three samples with a C / Si ratio of 1.15. In Figure 4, the horizontal axis represents the doping concentration (average) when it is 1.03 × 10⁻⁶. 18 cm -3 The graph shows the relative value of the nitrogen input flow rate when the input nitrogen flow rate at a given time is set to 1, and the vertical axis represents the obtained doping concentration (average value).

[0066] [Table 1]

[0067] As shown in Table 1, when using the commonly used C / Si ratio of 1.05, the average value of the n-type doping concentration is 2 × 10⁻⁶. 18 cm -3Even at this level, the in-plane uniformity of n-type doping concentrations exceeds 30%, and it can be seen that the in-plane uniformity of n-type doping concentrations deteriorates further when the average value of n-type doping concentrations is increased. On the other hand, when using a C / Si ratio of 1.15, which is higher than the normally used ratio, the average n-type doping concentration is 1 × 10⁻⁶. 18 cm -3 A high in-plane uniformity of n-type doping concentration of 12.2% was achieved, and the average value of the n-type doping concentration was 2.6 × 10⁻⁶. 18 cm -3 When the concentration is increased to a certain level, the in-plane uniformity of the n-type doping concentration decreases slightly, but it remains good at 17.8%. Furthermore, when using a C / Si ratio of 1.35, which is much higher than the commonly used ratio, the average n-type doping concentration was 2.2 × 10⁻⁶. 18 cm -3 An extremely high in-plane uniformity of n-type doping concentration of 6.1% was achieved. From these results, it was found that using a C / Si ratio higher than the commonly used 1.05 improves the in-plane uniformity of the n-type doping concentration, and that the higher the C / Si ratio, the better the in-plane uniformity of the n-type doping concentration. Furthermore, it was found that increasing the average value of the n-type doping concentration worsens the in-plane uniformity of the n-type doping concentration regardless of the C / Si ratio used.

[0068] Figure 5 shows the results of investigating the relationship between the average n-type doping concentration and the growth rate for a 150 mm diameter SiC epitaxial wafer sample. The horizontal axis represents the average n-type doping concentration, and the vertical axis represents the average n-type doping concentration of 1 × 10⁻⁶. 16 cm -3 This is the standard value for growth rate when the growth rate is normalized to 1 when epitaxial growth is performed in such a manner.

[0069] Figure 5 shows that the growth rate tends to decrease as the average n-type doping concentration increases. This trend is presumed to be due to nitrogen (N) inhibiting SiC growth (site-competition effect). This point will be explained below.

[0070] Figures 6(a) to (d) are illustrative diagrams of nitrogen (N) doping. Compared to (a), (b) shows the case when C / Si is low, (c) shows the case when C / Si is high, and (d) shows the case when the doping gas flow rate is high (aiming for a high average doping concentration). Here, Figures 6(a) to (c) have been known as site-competition effects, but Figure 6(d) represents a new finding discovered by the inventors.

[0071] As shown in Figure 6(a), Si is incorporated into the Si site, C into the C site, and N into the C site. In contrast, in the case of a low C / Si ratio, as shown in Figure 6(b), the ratio of N to C increases, thus increasing the probability that N will enter the C site. Furthermore, in the case of a high C / Si ratio, as shown in Figure 6(c), the ratio of N to C decreases, and the probability of N entering the C site decreases. Furthermore, when the nitrogen concentration is as high as that of the raw material gas, the ratio of nitrogen to carbon increases, as shown in Figure 6(d). As a result, carbon is less likely to be incorporated into the carbon site, which leads to a decrease in the effective carbon / silicon ratio and a reduction in the growth rate.

[0072] Here, the inventor investigated the relationship between the amount of C-based gas input and the growth rate in the CVD apparatus used to prepare the above-mentioned samples, and obtained a calibration curve showing a positive correlation in which the growth rate increases as the amount of C-based gas input increases. By using this calibration curve and the decrease in growth rate resulting from increasing the n-type doping concentration, we can estimate the amount of C-series gas that was effectively insufficient. This will be explained using the conceptual diagram in Figure 6. In Figure 7, the horizontal axis represents the amount of C-based gas injected, and the vertical axis represents the growth rate of the SiC epitaxial layer. Note that the amount of C-based gas injected corresponds to C / Si if the amount of Si-based gas injected is fixed. When producing a SiC epitaxial wafer with a higher average n-type doping concentration than the average n-type doping concentration of a SiC epitaxial wafer obtained under the P1 conditions on the calibration curve (C-based gas input amount C1, growth rate R1), if the growth rate was R3 when epitaxial growth was performed with a C-based gas input amount C1, then according to the calibration curve in Figure 7, the effective C-based gas input amount was C3. In this case, the ratio of N atoms to C atoms increased, making it more difficult for C atoms to be incorporated into the C sites, resulting in an effective C-based gas input amount of C3. By adding an amount of C-based gas to compensate for this deficiency, it becomes possible to perform epitaxial growth at a growth rate of R1.

[0073] As shown in Table 1, increasing the C / Si ratio to compensate for the C-based gas deficiency significantly improved the in-plane uniformity of the n-type doping concentration, reaching 6.1% at C / Si = 1.35. With C / Si = 1.35, this is not a high doping concentration, but a normal concentration (for example, 1 × 10⁻⁶). 16 / cm 3 When the following SiC epitaxial layer is formed, it becomes non-mirror-like; however, in this high-doping concentration SiC epitaxial layer, the entire surface was mirror-like. This is thought to support the idea that nitrogen (N) atoms inhibit SiC growth through the site compensation effect described above. In the case of the same C / Si ratio, a SiC epitaxial layer with a high doping concentration is thought to be more affected by the site compensation effect compared to a SiC epitaxial layer with a low doping concentration. This inhibits the uniform growth of SiC on the growth surface and impairs the in-plane uniformity of the doping concentration. Therefore, in a SiC epitaxial layer with a high doping concentration, it is thought that increasing the C / Si ratio further can reduce the effect of the site compensation effect, suppress the inhibition of uniform SiC growth on the growth surface, and prevent the deterioration of the in-plane uniformity of the doping concentration.

[0074] In this way, by estimating the amount of dopant inhibiting SiC epitaxial growth from a pre-obtained correlation between the amount of C-based gas input and the growth rate (e.g., a calibration curve), and by inputting the raw material gas at a C / Si ratio sufficient to compensate for that amount, it is possible to manufacture SiC epitaxial wafers with high in-plane uniformity.

[0075] Figure 8 is a graph showing the average n-type doping concentration, the in-plane uniformity of the n-type doping concentration, and the relationship between C / Si for a 150 mm diameter SiC epitaxial wafer sample, based on the experimental results and discussions described above, as shown in Table 1. Based on the relationship shown in Figure 8, the growth conditions for C / Si and doping gas flow rate were estimated, and the average n-type doping concentration was 1 × 10⁻⁶. 18 / cm 3 The above is 1 x 10 19 / cm 3 The following method allows for the production of SiC epitaxial wafers having a high-concentration layer with an in-plane uniformity of n-type doping concentration of 30% or less.

[0076] In Figure 8, the graph for the case where C / Si is 1.35 shows only one data point, but it was obtained based on the mechanism derived from the site compensation effect described above. This will now be explained. In Figure 8, the graphs for C / Si 1.05 and C / Si 1.15 were obtained from three data points. Both graphs have a positive slope, and the fact that the slope of the graph for C / Si 1.15 is lower than the slope of the graph for C / Si 1.05 indicates that the mechanism based on site compensation effect fits well. Furthermore, the concentration of 2 × 10⁻¹⁰, which is higher than the usual doping concentration, is also shown. 18 / cm 3 The fact that the in-plane uniformity of doping concentration in the adjacent SiC epitaxial layer is best when the C / Si ratio is 1.35, followed by 1.15, and then 1.05, also indicates that the mechanism based on site compensation effect fits well. Then, if C / Si is 1.35, 1 × 10 18 / cm 3 The above is 1 x 10 19 / cm 3 Within the following range of doping concentrations, it can be inferred that a mechanism based on site compensation effects holds true. If the mechanism based on site compensation effect fits well, the graph for C / Si = 1.35 will have a positive slope, and that slope will be lower than the slope of the graph for C / Si = 1.15. As described above, the graph for the case where C / Si is 1.35 was obtained based on a mechanism based on the site compensation effect. [Explanation of Symbols]

[0077] 10 SiC single crystal substrates 20 SiC epitaxial layer 21 High concentration layer 22 Drift Layers

Claims

1. SiC single crystal substrate and On the aforementioned SiC single crystal substrate, the average value of the n-type doping concentration is 1.03 × 10⁻¹⁰ １８ / cm ３ The above is 1 x 10 １９ / cm ３ The following is a high-concentration layer having a doping concentration uniformity of 30% or less in the plane: The aforementioned high-concentration layer is nitrogen-doped in a SiC epitaxial wafer; Here, the in-plane uniformity is defined as the absolute value of (maximum doping concentration in the plane - minimum doping concentration in the plane) divided by the average value of the doping concentration in the plane.

2. SiC single crystal substrate and On the aforementioned SiC single crystal substrate, the average value of the n-type doping concentration is 1.91 × 10⁻¹⁶. １８ / cm ３ The above is 1 x 10 １９ / cm ３ The following is a high-concentration layer having a doping concentration uniformity of 30% or less in the plane: The aforementioned high-concentration layer is nitrogen-doped in a SiC epitaxial wafer; Here, the in-plane uniformity is defined as the absolute value of (maximum doping concentration in the plane - minimum doping concentration in the plane) divided by the average value of the doping concentration in the plane.

3. SiC single crystal substrate and On the SiC single crystal substrate, a high concentration layer having an average value of the n-type doping concentration of 2.20×10 １８ / cm ３ or more and 1×10 １９ / cm ３ or less, and having an in-plane uniformity of the doping concentration of 30% or less is provided. The aforementioned high-concentration layer is nitrogen-doped in a SiC epitaxial wafer; Here, the in-plane uniformity is defined as the absolute value of (maximum doping concentration in the plane - minimum doping concentration in the plane) divided by the average value of the doping concentration in the plane.

4. SiC single crystal substrate and On the aforementioned SiC single crystal substrate, the average value of the n-type doping concentration is 2.59 × 10⁻¹⁶. １８ / cm ３ The above is 1 x 10 １９ / cm ３ The following is a high-concentration layer having a doping concentration uniformity of 30% or less in the plane: The aforementioned high-concentration layer is nitrogen-doped in a SiC epitaxial wafer; Here, the in-plane uniformity is defined as the absolute value of (maximum doping concentration in the plane - minimum doping concentration in the plane) divided by the average value of the doping concentration in the plane.

5. SiC single crystal substrate and On the aforementioned SiC single crystal substrate, the average value of the n-type doping concentration is 1.03 × 10⁻¹⁰ １８ / cm ３ The above is 2.59 x 10 １８ / cm ３ The following is a high-concentration layer having a doping concentration uniformity of 30% or less in the plane: The aforementioned high-concentration layer is nitrogen-doped in a SiC epitaxial wafer; Here, the in-plane uniformity is defined as the absolute value of (maximum doping concentration in the plane - minimum doping concentration in the plane) divided by the average value of the doping concentration in the plane.

6. SiC single crystal substrate and On the aforementioned SiC single crystal substrate, the average value of the n-type doping concentration is 1.91 × 10⁻¹⁶. １８ / cm ３ The above is 2.59 x 10 １８ / cm ３ The following is a high-concentration layer having a doping concentration uniformity of 30% or less in the plane: The aforementioned high-concentration layer is nitrogen-doped in a SiC epitaxial wafer; Here, the in-plane uniformity is defined as the absolute value of (maximum doping concentration in the plane - minimum doping concentration in the plane) divided by the average value of the doping concentration in the plane.

7. SiC single crystal substrate and On the aforementioned SiC single crystal substrate, the average value of the n-type doping concentration is 2.20 × 10⁻¹⁰. １８ / cm ３ The above is 2.59 x 10 １８ / cm ３ The following is a high-concentration layer having a doping concentration uniformity of 30% or less in the plane: The aforementioned high-concentration layer is nitrogen-doped in a SiC epitaxial wafer; Here, the in-plane uniformity is defined as the absolute value of (maximum doping concentration in the plane - minimum doping concentration in the plane) divided by the average value of the doping concentration in the plane.

8. SiC single crystal substrate and On the aforementioned SiC single crystal substrate, the average value of the n-type doping concentration is 1.03 × 10⁻¹⁰ １８ / cm ３ The above is 2.20 x 10 １８ / cm ３ The following is a high-concentration layer having a doping concentration uniformity of 30% or less in the plane: The aforementioned high-concentration layer is nitrogen-doped in a SiC epitaxial wafer; Here, the in-plane uniformity is defined as the absolute value of (maximum doping concentration in the plane - minimum doping concentration in the plane) divided by the average value of the doping concentration in the plane.

9. SiC single crystal substrate and On the aforementioned SiC single crystal substrate, the average value of the n-type doping concentration is 1.03 × 10⁻¹⁰ １８ / cm ３ The above is 1.91 × 10 １８ / cm ３ The following is a high-concentration layer having a doping concentration uniformity of 30% or less in the plane: The aforementioned high-concentration layer is nitrogen-doped in a SiC epitaxial wafer; Here, the in-plane uniformity is defined as the absolute value of (maximum doping concentration in the plane - minimum doping concentration in the plane) divided by the average value of the doping concentration in the plane.

10. SiC single crystal substrate and On the aforementioned SiC single crystal substrate, the average value of the n-type doping concentration is 1.91 × 10⁻¹⁶. １８ / cm ３ The above is 2.20 x 10 １８ / cm ３ The following is a high-concentration layer having a doping concentration uniformity of 30% or less in the plane: The aforementioned high-concentration layer is nitrogen-doped in a SiC epitaxial wafer; Here, the in-plane uniformity is defined as the absolute value of (maximum doping concentration in the plane - minimum doping concentration in the plane) divided by the average value of the doping concentration in the plane.

11. The SiC epitaxial wafer according to any one of claims 1 to 10, wherein the in-plane uniformity of the doping concentration is 20% or less.

12. The SiC epitaxial wafer according to any one of claims 1 to 10, wherein the in-plane uniformity of the doping concentration is 10% or less.

13. The SiC epitaxial wafer according to any one of claims 1 to 10, wherein the high-concentration layer is a buffer layer, and the buffer layer comprises a drift layer having an average doping concentration lower than the average doping concentration of the buffer layer.

14. The SiC epitaxial wafer according to any one of claims 1 to 10, comprising a buffer layer between the SiC single crystal substrate and the high-concentration layer that converts basal plane dislocations into through-edge dislocations.

15. A SiC epitaxial wafer according to any one of claims 1 to 10, wherein the diameter is 150 mm or more.

16. The SiC epitaxial wafer according to claim 11, wherein the diameter is 150 mm or more.

17. The SiC epitaxial wafer according to claim 12, wherein the diameter is 150 mm or more.

18. A SiC epitaxial wafer according to claim 13, wherein the diameter is 150 mm or more.

19. A SiC epitaxial wafer according to claim 14, wherein the diameter is 150 mm or more.

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