JP2014222730A - Nitride semiconductor epitaxial wafer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve crystallinity in addition to inhibiting warpage of a wafer.SOLUTION: A nitride semiconductor epitaxial wafer comprises on a substrate (1) via an initial growth layer (2): Al composition inclination layers (6); three-layer structures in which AlGaN layers (8) each sandwiched by super lattice layers (7) in each of which low-Al-containing layers composed of AlGaN(0<a≤0.3) and high-Al-containing layers composed of AlGaN(0.5≤b≤1.0) are alternately laminated; and nitride semiconductors (9-12). A plurality of three-layer structures are formed between the Al composition inclination layers and the nitride semiconductors (9-12). The plurality of three-layer structures compose a crystallinity improvement layer for improving crystallinity of the nitride semiconductors (9-12) by reducing dislocation which occurs due to a stress caused by a difference between grating constants of the super lattice layer (7) and the AlGaN layer (8), at a boundary face between the substrate (1) and the initial growth layer (2) and extends upward. With this configuration, crystallinity is improved in addition to inhibiting warpage of a wafer.

Description

  The present invention relates to a nitride semiconductor epitaxial wafer in which an electronic device using a nitride semiconductor is epitaxially formed on a substrate.

  As an electronic device using a nitride semiconductor, a structure using a heterojunction made of AlGaN and GaN is generally used. In recent years, nitride semiconductors have been actively grown on inexpensive Si substrates in order to reduce the cost of electronic devices.

  A problem in growing the nitride semiconductor on the Si substrate is wafer warpage due to a difference in thermal expansion coefficient between the nitride semiconductor and Si. Since the thermal expansion coefficient of the nitride semiconductor is larger than the thermal expansion coefficient of Si, in the process of growing the nitride semiconductor on the Si substrate at a high temperature and then cooling to room temperature, the nitride semiconductor is warped in a convex shape. Become.

  As a method for suppressing the warp due to the difference in the thermal expansion coefficient, the gallium nitride material disclosed in JP-T-2004-524250 (Patent Document 1) or JP-A-2008-205117 (Patent Document 2) is disclosed. There is a semiconductor wafer.

  In the gallium nitride material disclosed in the above-mentioned Patent Document 1, a transition layer made of a gallium nitride alloy such as AlInGaN, AlGaN, InGaN or the like having a compositional gradient is provided between the silicon substrate and the gallium nitride material. Forming. Further, in the semiconductor wafer disclosed in Patent Document 2, a superlattice buffer layer formed by alternately laminating a plurality of AlGaN layers made of a high Al content layer and AlGaN layers made of a low Al content layer is a single layer. Layer structure has a structure separated by a buffer layer.

  With the structure as described above, warpage of the wafer due to the difference in thermal expansion coefficient between the nitride semiconductor and Si is suppressed. However, although the patent document 1 and the patent document 2 describe effects related to warpage, they do not describe crystallinity improvement.

Special table 2004-524250 gazette JP 2008-205117 A

  Accordingly, an object of the present invention is to provide a nitride semiconductor epitaxial wafer capable of improving crystallinity as well as suppressing warpage of the wafer.

In order to solve the above problems, the nitride semiconductor epitaxial wafer of the present invention is
A substrate,
An initial growth layer formed on the substrate;
An Al composition gradient layer which is formed on the initial growth layer and whose Al composition decreases continuously or stepwise;
A low Al-containing layer formed on the Al composition gradient layer and having a composition of Al _a Ga _1-a N (0 <a ≦ 0.3) and a composition of Al _b Ga _1-b N (0.5 A superlattice layer in which high Al-containing layers satisfying ≦ b ≦ 1.0) are alternately stacked, and a three-layer structure sandwiching Al _c Ga _1-c N layers;
A nitride semiconductor formed on the superlattice layer constituting the three-layer structure,
A plurality of the three-layer structures are formed between the Al composition gradient layer and the nitride semiconductor,
The plurality of three-layer structures are generated at the interface between the substrate and the initial growth layer due to stress caused by the difference in lattice constant between the superlattice layer and the Al _c Ga _1-c N layer. It is characterized in that a crystallinity improving layer for improving the crystallinity of the nitride semiconductor is formed by reducing dislocations extending toward the surface.

In the nitride semiconductor epitaxial wafer of one embodiment,
The Al composition c in the Al _c Ga _1-c N layer constituting the three-layer structure is:
When the thickness of the Al _a Ga _1-a N in the superlattice layer is d1 and the thickness of the Al _b Ga _1-b N is d2,
c <(d1 * a + d2 * b) / (d1 + d2)
Have a relationship.

In the nitride semiconductor epitaxial wafer of one embodiment,
The film thickness of the Al _c Ga _1-c N layer constituting the three-layer structure is 10 nm or more and 300 nm or less.

  As is clear from the above, the nitride semiconductor epitaxial wafer of the present invention includes a plurality of three-layer structures in which an Al composition gradient layer is formed on a substrate via an initial growth layer, and an AlGaN layer is sandwiched between superlattice layers. A nitride semiconductor is formed on the three-layer structure. Therefore, a convex warp under the wafer by forming the initial growth layer on the substrate is turned into a convex warp by forming the Al composition gradient layer, thereby forming a plurality of the above three-layer structures. By doing so, the amount of upward warping can be increased.

  As a result, by forming the nitride semiconductor on the three-layer structure, the wafer can be greatly warped upwardly convex before the wafer is greatly warped downwardly convex, Warpage of the nitride semiconductor epitaxial wafer can be reduced.

  Furthermore, since the plurality of three-layer structures function as crystallinity improving layers, dislocations generated at the interface between the substrate and the initial growth layer and extending upward are reduced. Can improve sex.

  Therefore, it is possible not only to suppress the warpage of the nitride semiconductor epitaxial wafer but also to improve the crystallinity.

It is sectional drawing in the nitride semiconductor epitaxial wafer of this invention. It is a figure which shows the relationship between the film thickness of the nitride semiconductor formed on the board | substrate, and the amount of curvature.

  Hereinafter, the present invention will be described in detail with reference to the illustrated embodiments.

First Embodiment FIG. 1 is a cross-sectional view of a transistor as a nitride semiconductor epitaxial wafer according to the present embodiment.

In FIG. 1, an AlN layer 2 having a thickness of 100 nm, an Al _0.7 Ga _0.3 N3 having a thickness of 200 nm, an Al _0.4 Ga _0.6 N4 having a thickness of 400 nm, and an Al _0.1 Ga _0.9 N5 having a thickness of 400 nm are formed on the Si substrate 1. A composition gradient buffer layer 6 is formed. Here, the number of layers, the composition, and the film thickness of the composition gradient buffer layer 6 are not limited to the present embodiment, and any structure can be used as long as the warp can be formed in a convex shape.

Further, Al _a Ga _1-a N (0 <a ≦ 0.3) low Al content layer and Al _b Ga _1-b N (0.5 ≦ b ≦ 1.0) high Al content layer are alternated. The superlattice layer 7 stacked on the substrate 3 is composed of a three-layer structure sandwiching the Al _c Ga _1-c N layer 8 as one set, and several sets are formed. Here, the Al composition “c” of the Al _c Ga _1-c N layer 8 is such that the film thickness of the Al _a Ga _1-a N of the superlattice layer 7 is d1, and the Al _b Ga _1-b N When the film thickness is d2,
c <(d1 * a + d2 * b) / (d1 + d2)
Have the relationship.

In the example shown in FIG. 1, the above set comprising the Al _0.1 Ga _0.9 N layer 8 (100 nm) sandwiched between the superlattice layer 7 formed by growing Al _0.1 Ga _0.9 N (23 nm) / AlN (2 nm) for 25 periods, Two sets are formed.

  If the thicknesses d1 and d2 of the layers constituting the superlattice layer 7 are in the range of 3 nm or more and 25 nm or less, the effect of reducing the warpage of the wafer and reducing the vertical leakage current can be obtained. it can.

  Further, a GaN channel layer 9, an AlN intermediate layer 10, an AlGaN barrier layer 11, and a GaN cap layer 12 having a thickness of 1.5 μm are sequentially formed on the uppermost superlattice layer 7 ″. The AlN intermediate layer 10 and the GaN cap layer 12 can be eliminated if necessary.

  The growth of each layer is an example, but is performed by the following growth method.

  Prior to the growth of the AlN layer 2, the oxide film on the surface of the Si substrate 1 is removed with a hydrofluoric acid-based etchant, and then the Si substrate 1 is set in a metal organic chemical vapor deposition (MOCVD) apparatus. Then, the temperature of the Si substrate 1 is set to 1100 ° C., and the substrate surface is cleaned at a chamber pressure of 13.3 kPa.

Next, the surface of the Si substrate 1 is nitrided by supplying ammonia NH ₃ (12.5 slm) while keeping the substrate temperature and the chamber pressure constant. Subsequently, AlN is grown to 100 nm (TMA (trimethylaluminum) flow rate = 117 μmol / min, NH ₃ flow rate = 12.5 slm) to form the AlN layer 2. Next, Al _0.7 Ga _0.3 N3 is grown at a substrate temperature of 1150 ° C. by 200 nm (TMG (trimethylgallium) flow rate = 57 μmol / min, TMA flow rate = 97 μmol / min, NH ₃ flow rate = 12.5 slm). Next, Al _0.4 Ga _0.6 N4 is grown to 400 nm (TMG flow rate = 99 μmol / min, TMA flow rate = 55 μmol / min, NH ₃ flow rate = 12.5 slm). Next, Al _0.1 Ga _0.9 N5 is grown to 400 nm (TMG flow rate = 137 μmol / min, TMA flow rate = 18 μmol / min, NH ₃ flow rate = 12.5 slm). Thus, the composition gradient buffer layer 6 composed of Al _0.7 Ga _0.3 N3, Al _0.4 Ga _0.6 N4, and Al _0.1 Ga _0.9 N5 is formed.

Next, 23 nm of Al _0.1 Ga _0.9 N (TMG flow rate = 720 μmol / min, TMA flow rate = 80 μmol / min, NH ₃ flow rate = 12.5 slm) and 2 nm of AlN (TMA flow rate = 102 μmol / min, NH ₃ flow rate = And 12.5 slm) are alternately grown for 25 periods to form the superlattice layer 7. Further, the Al _0.1 Ga _0.9 N layer 8 is formed to a thickness of 100 nm. In this case, the growth conditions of the Al _0.1 Ga _0.9 N layer 8 are the same as those of the Al _0.1 Ga _0.9 N5. Then, two sets are formed in which the Al _0.1 Ga _0.9 N layer 8 is sandwiched between the two superlattice layers 7.

Thereafter, a GaN channel layer 9 (TMG flow rate = 50 μmol / min, NH ₃ flow rate = 12.5 slm) is grown at a thickness of 1.5 μm. Further, an AlN intermediate layer 10 (TMA flow rate = 51 μmol / min, NH ₃ flow rate = 12.5 slm) is grown. Subsequently, an AlGaN barrier layer 11 (TMG flow rate = 46 μmol / min, TMA flow rate = 7 μmol / min, NH ₃ flow rate = 12.5 slm) is grown. Further, a GaN cap layer 12 (TMG flow rate = 58 μmol / min, NH ₃ flow rate = 12.5 slm) is grown.

  Hereinafter, although a detailed description is omitted, the nitride semiconductor layer formed on the Si substrate 1 as described above is processed according to the desired electronic device to form the electronic device.

  For example, when a transistor is formed as the electronic device, a source electrode (not shown) that forms an ohmic contact with a two-dimensional electron gas layer formed at the interface between the AlN intermediate layer 10 and the GaN channel layer 9. A drain electrode (not shown) is formed, and a gate electrode (not shown) is formed between the source electrode and the drain electrode.

  The structure of the nitride semiconductor epitaxial wafer in the present embodiment is that a composition gradient buffer layer 6 composed of an Al composition gradient AlGaN layer and a buffer layer formed by sandwiching an AlGaN single layer 8 between superlattice layers 7 on an Si substrate 1. It is composed of a combination. With this structure, the warpage of the nitride semiconductor epitaxial wafer is suppressed. The reason is as follows.

FIG. 2 shows the relationship between the thickness of the nitride semiconductor formed on the Si substrate 1 and the amount of warpage. As shown in FIG. 2, when the AlN layer 2 is grown on the Si substrate 1, as shown by a broken line (A), the warpage of the nitride semiconductor epitaxial wafer increases in proportion to the square of the film thickness. Further, when the first AlGaN layer (Al _0.7 Ga _0.3 N3) constituting the composition gradient buffer layer 6 is grown thereon, the increase in warpage is proportional to the square of the film thickness as shown by the broken line (B). However, the rate of increase decreases. Further, when the second AlGaN layer (Al _0.4 Ga _0.6 N4) is grown, as shown by the broken line (C), a minimum value of the warp amount appears with respect to the film thickness, and further, the third AlGaN layer (Al _0.1 Ga _0.9). When N5) is grown, the direction of warpage changes from “convex downward” to “convex upward” as indicated by a broken line (D).

  In the present embodiment, the composition gradient buffer layer 6 is described as being composed of three AlGaN layers 3 to 5. However, the Al composition and film thickness of the AlGaN layer constituting the composition gradient buffer layer 6 are selected. By doing so, the number of layers can be changed, such as four layers or five layers.

  Thereafter, by growing a superlattice layer 7 configured by combining an appropriate film thickness and an appropriate composition, the warpage of the wafer increases linearly as the film thickness increases, as indicated by a straight line (E). The AlGaN layer 8 formed on the superlattice layer 7 and forming the set with the superlattice layer 7 changes the warp in the shape of a quadratic curve having a minimum value as shown by the broken line (F). Accordingly, by appropriately selecting the film thickness of the AlGaN layer 8, the amount of warp upward can be increased. Further, by forming a plurality of sets in which the AlGaN layer 8 is sandwiched by the superlattice layer 7, the amount of upward warping can be further increased.

  As a result, when the GaN channel layer 9 of the target thick film (1.5 μm) for the electronic device is grown, as shown by the broken line (G), the warp of the wafer is in the “convex downward” direction ( That is, it returns to the direction of the warping amount “0”.

  Thus, according to the present embodiment, a nitride semiconductor is obtained by combining two types of buffer layers, the composition gradient buffer layer 6 and the buffer layer in which the AlGaN single layer 8 is sandwiched between the superlattice layer 7. An effect can be produced in reducing the warpage of the epitaxial wafer.

  Next, another effect in the present embodiment will be described.

Table 1 shows the number of sets formed by sandwiching the Al _0.1 Ga _0.9 N layer 8 with the superlattice layer 7 in the nitride semiconductor epitaxial wafer structure in which the nitride semiconductor layer is formed on the Si substrate 1 in the present embodiment. And the full width at half maximum (FWHM) in X-ray diffraction of the GaN channel layer 9 is shown.

As can be seen from Table 1, as the number of sets increases, the full width at half maximum of the (004) plane related to the screw dislocation and the full width at half maximum of the (100) plane related to the edge dislocation decrease, It shows that the crystallinity is improved.
Table 1

From Table 1, it can be seen that the crystallinity is improved as the number of sets in which the Al _0.1 Ga _0.9 N layer 8 is sandwiched between the superlattice layer 7 is increased. However, the mechanism of crystallinity improvement by repeating the superlattice layer 7 and the AlGaN layer 8 has not been clearly elucidated. However, the reason can be estimated as follows.

  During the growth of the nitride semiconductor, in the process in which dislocations generated at the interface between the Si substrate 1 and the AlN layer 2 extend upward, the superlattice layer 7 sandwiches the single layer 8 of AlGaN and the super layer in the buffer layer. Dislocations are reduced by the stress at the interface between the lattice layer 7 and the AlGaN layer 8. Then, by forming a plurality of sets in which the AlGaN layer 8 is sandwiched by the superlattice layer 7 and forming a plurality of interfaces between the superlattice layer 7 and the AlGaN layer 8, the above-described dislocation reduction effect is repeated. In this way, it is presumed that dislocations generated at the interface between the Si substrate 1 and the AlN layer 2 are reduced and the crystallinity is improved.

Based on the above assumption, in order to obtain the effect of reducing dislocation, the superlattice layer 7 is composed of Al _a Ga _1-a N having a film thickness d1 and Al _b Ga _1-b N having a film thickness d2. In this case, the Al composition “c” of the Al _c Ga _1-c N layer which is the AlGaN layer 8 is the average composition of the superlattice layer 7 given by the following equation:
(d1 * a + d2 * b) / (d1 + d2)
It is desirable to be smaller.

  The reason is that the effect of reducing dislocations due to stress due to the difference in lattice constant between the superlattice layer 7 and the AlGaN layer 8 appears.

As described above, in the present embodiment, the composition gradient buffer layer 6 composed of the Al composition gradient AlGaN layer in which the Al composition decreases stepwise is provided on the Si substrate 1 via the AlN layer 2 as the initial growth layer. Form. Further, a low Al content layer of Al _a Ga _1-a N (0 <a ≦ 0.3) and Al _b Ga _1-b N (0.5 ≦ b ≦ 1.0) are formed on the composition gradient buffer layer 6. The superlattice layer 7 formed by alternately laminating the high Al-containing layers is composed of a three-layer structure sandwiching the Al _c Ga _1-c N layer 8 as one set, and several sets of this set are grown. Further, a GaN channel layer 9 as a nitride semiconductor is formed on the superlattice layer 7 in the final set.

  Therefore, by growing the AlN layer 2 on the Si substrate 1, the nitride semiconductor epitaxial wafer that is warped downward by forming the composition gradient buffer layer 6 in which the Al composition is reduced is formed. As the rate of increase in the amount of warpage decreases, a minimum value appears in the change in the amount of warpage with respect to the film thickness, and the direction of warpage eventually changes from “convex downward” to “convex upward”.

Further, the Al _c Ga _1-c N layer 8 that changes the warp in the shape of a quadratic curve having a minimum value is formed by alternately laminating the low Al content layer and the high Al content layer, and the warpage of the wafer. By forming the superlattice layer 7 to increase linearly with respect to the increase in film thickness, the portion of decrease in the warp amount on the thick film side is smaller than the minimum value of the warp amount change by the Al _c Ga _1-c N layer 8. Is replaced with a linear increase by the superlattice layer 7 to increase the amount of warpage upward. Then, by forming a plurality of sets in which the Al _c Ga _1-c N layer 8 is sandwiched by the superlattice layer 7, the amount of upward convex warpage is further increased.

  Therefore, when the thick GaN channel layer 9 is formed on the superlattice layer 7 in the final set, the thick GaN channel layer 9 is warped in a convex shape downward. The warpage of the nitride semiconductor epitaxial wafer can be reduced by greatly curving the convex shape.

Further, in the present embodiment, the Al composition “c” in the Al _c Ga _1-c N layer 8 is set to Al _a Ga _1-a N having a film thickness d1 and Al _b Ga _1-b N having a film thickness d2. In the superlattice layer 7 composed of
(d1 * a + d2 * b) / (d1 + d2)
Is set smaller.

Accordingly, dislocations that are generated at the interface between the Si substrate 1 and the nitride semiconductor and extend upward due to the stress caused by the difference in lattice constant between the superlattice layer 7 and the Al _c Ga _1-c N layer 8. Reduced. Then, by forming a plurality of interfaces between the superlattice layer 7 and the Al _c Ga _1-c N layer 8, dislocations are repeatedly reduced and the crystallinity can be improved.

In other words, in the present embodiment, a plurality of three-layer structures each having the Al _c Ga _1-c N layer 8 sandwiched between the superlattice layer 7 are stacked to form a GaN channel layer 9, an AlN intermediate layer 10, and an AlGaN barrier. The crystallinity improving layer for improving the crystallinity of the nitride semiconductor composed of the layer 11 and the GaN cap layer 12 is formed.

  As described above, according to the present embodiment, in the nitride semiconductor epitaxial wafer in which the nitride semiconductor is grown on the Si substrate 1, not only the warpage of the wafer can be suppressed but also the crystallinity can be improved. is there.

Second Embodiment In this embodiment, in the first embodiment, the film thickness of the Al _0.1 Ga _0.9 N layer 8 that is sandwiched between the superlattice layers 7 and forms the set affects the warpage of the wafer. It is about influence.

The structure of the nitride semiconductor epitaxial wafer in the above embodiment is basically the same as that in the first embodiment. However, the number of sets of the super lattice layer 7 with Al _0.1 Ga formed by interposing a _0.9 N layer 8 three-layer structure being fixed to "2", 100 nm the thickness of the Al _0.1 Ga _0.9 N layer 8 (the first In this case, the amount of warpage of the nitride semiconductor epitaxial wafer is measured.

Table 2 shows the measurement results. Here, the amount of warpage is represented by the amount of warpage (μm) at the maximum warpage position in a 6-inch nitride semiconductor epitaxial wafer. Furthermore, minus represents a convex warp upward, and positive represents a convex warp downward.
Table 2

As shown in Table 2, the warpage of the wafer can be suppressed until the film thickness of the Al _0.1 Ga _0.9 N layer 8 is up to 300 nm. However, when the film thickness is 500 nm, the warpage of the wafer increases rapidly.

This is because the Al _0.1 Ga _0.9 N layer 8 has a film thickness exceeding 300 nm, so that a quadratic curve as shown by a broken line (F) in the relationship between the film thickness and the warp amount shown in FIG. The broken line (H) in which the boundary position between the Al _0.1 Ga _0.9 N layer 8 exhibiting the following change shape and the superlattice layer 7 'formed thereon exceeds the minimum value of the quadratic curve shown by the broken line (F) For example, it is the position of the black circle (I) above.

As a result, the warpage of the wafer moves to the “convex downward” side as compared with the case where the boundary position is the minimum value of the broken line (F). Therefore, when this is repeated by the number of sets in which the Al _0.1 Ga _0.9 N layer 8 is sandwiched by the superlattice layer 7, the warp position moves further to the “convex downward” side, and finally the thick GaN This is because when the channel layer 9 is formed, the amount of warpage of the wafer exceeds “0” and is greatly displaced to the “convex downward” side.

When the Al _0.1 Ga _0.9 N layer 8 has a thickness of less than 10 nm, the boundary position between the Al _0.1 Ga _0.9 N layer 8 and the superlattice layer 7 ′ is less than the above minimum value in the broken line (F). In the same way as when the thickness exceeds 300 nm, the warpage of the wafer moves to the “convex downward” side, and when the thick GaN channel layer 9 is formed, The amount of warpage exceeds “0” and is greatly displaced to the “convex downward” side.

From the above, the film thickness of the Al _0.1 Ga _0.9 N layer 8 sandwiched between the superlattice layers 7 is preferably 10 nm or more and 300 nm or less in order to exert the effect of reducing the warp of the nitride semiconductor epitaxial wafer. It can be said.

  In each of the above embodiments, the composition gradient buffer layer 6 is composed of an Al composition gradient AlGaN layer in which the Al composition decreases stepwise. However, the present invention is not limited in steps, and it may be composed of an Al composition gradient AlGaN layer in which the Al composition continuously decreases.

  In each of the above embodiments, the composition gradient buffer layer 6 is composed of an AlGaN layer. However, the present invention is not limited to this, and may be composed of an Al composition graded nitride semiconductor layer having another composition.

As described above, the nitride semiconductor epitaxial wafer of the present invention is
A substrate (1);
An initial growth layer (2) formed on the substrate (1);
An Al composition gradient layer (6) formed on the initial growth layer (2) and having an Al composition decreasing continuously or stepwise upward;
A low Al-containing layer having a composition Al _a Ga _1-a N (0 <a ≦ 0.3) and a composition Al _b Ga _1-b N () are formed on the Al composition gradient layer (6). A superlattice layer (7,7 ', 7'') alternately laminated with high Al content layers satisfying 0.5 ≦ b ≦ 1.0), and Al _c Ga _1-c N layers (8,8 ') And a three-layer structure
A nitride semiconductor (9-12) formed on the superlattice layer (7 ″) constituting the three-layer structure,
A plurality of the three-layer structures are formed between the Al composition gradient layer (6) and the nitride semiconductor (9 to 12).
The plurality of three-layer structures are caused by stress caused by a difference in lattice constant between the superlattice layer (7,7 ′, 7 ″) and the Al _c Ga _1-c N layer (8,8 ′). The crystallinity improvement which improves the crystallinity of the nitride semiconductor (9-12) by reducing dislocations generated at the interface between the substrate (1) and the initial growth layer (2) and extending upward. It is characterized by constituting a layer.

  According to the above configuration, the Al composition gradient layer (6) in which the Al composition decreases as the film thickness increases due to the convex warp under the wafer by forming the initial growth layer (2) on the substrate (1). By forming, the rate of increase in the amount of warpage with respect to the film thickness can be reduced, and the direction of warpage can be changed from “convex downward” to “convex upward”.

Further, the Al _c Ga _1-c N layer (8,8 ′) that changes the warpage of the wafer into a shape having a minimum value with respect to the increase in the film thickness is obtained by linearly changing the warpage with respect to the increase in the film thickness. By forming a plurality of three-layer structures sandwiched between the superlattice layers (7, 7 ′, 7 ″) to be increased, the amount of warp upward can be increased. Thus, when a thick nitride semiconductor (9 to 12) is formed on the superlattice layer (7 ″) and the wafer is greatly warped downwardly convex, it is greatly warped in advance to the convex shape. Accordingly, warpage due to the nitride semiconductor (9 to 12) can be suppressed, and warpage of the nitride semiconductor epitaxial wafer can be reduced.

Further, the plurality of three-layer structures function as crystallinity improving layers, whereby the superlattice layer (7,7 ′, 7 ″) and the Al _c Ga _1-c N layer (8,8 ′). The dislocations generated at the interface between the substrate (1) and the initial growth layer (2) due to the stress caused by the difference in the lattice constant from the above are reduced and the nitride semiconductor (9-12) ) Can be improved.

  Therefore, it is possible not only to suppress the warpage of the nitride semiconductor epitaxial wafer but also to improve the crystallinity.

In the nitride semiconductor epitaxial wafer of one embodiment,
The Al composition c in the Al _c Ga _1-c N layer (8,8 ′) constituting the three-layer structure is
When the thickness of the Al _a Ga _1-a N in the superlattice layer (7,7 ′, 7 ″) is d1 and the thickness of the Al _b Ga _1-b N is d2,
c <(d1 * a + d2 * b) / (d1 + d2)
Have a relationship.

According to this embodiment, the Al composition Ga in the Al _c Ga _1-c N layer (8,8 ′) is expressed as “(d1 * a + d2 * b) / (d1 + d2)”. Smaller than the average composition of (7,7 ′, 7 ″), the lattice constant of the Al _c Ga _1-c N layer (8,8 ′) and the superlattice layer (7,7 ′, 7 ″) There is a difference from the lattice constant of. Therefore, dislocations generated at the interface between the substrate (1) and the initial growth layer (2) due to the stress caused by the difference in lattice constant can be reduced.

In the nitride semiconductor epitaxial wafer of one embodiment,
The film thickness of the Al _c Ga _1-c N layer (8,8 ′) constituting the three-layer structure is 10 nm or more and 300 nm or less.

When the film thickness of the Al _c Ga _1-c N layer (8,8 ′) exceeds 300 nm, the Al _c Ga _1-c N layer changes in the shape of a quadratic curve as the film thickness increases. (8,8 ′) and the superlattice layer (7,7 ′, 7 ″) formed thereon are located on the Al _c Ga _1-c N layer (8,8 ′). It exceeds the minimum value of the quadratic curve, which is the shape of change of the warp amount, and the warp amount becomes “0”. Then, when this is repeated by the number of the three-layer structures, the warp amount further moves to the “0” side, and when the nitride semiconductor (9 to 12) is formed, the warp amount of the wafer is “ Exceeds “0” and is greatly displaced toward the “convex downward” side.

Further, when the thickness of the Al _c Ga _1-c N layer (8,8 ′) is less than 10 nm, the Al _c Ga _1-c N layer (8,8 ′) and the superlattice layer (7 , 7 ′, 7 ″), the boundary position of the Al _c Ga _1-c N layer (8, 8 ′) does not exceed the minimum value of the change shape of the warp amount and becomes the “0” side of the warp amount. . Therefore, as in the case of exceeding 300 nm, the amount of warpage of the wafer when the nitride semiconductor (9 to 12) is formed exceeds “0” and is greatly displaced to the “convex downward” side.

According to this embodiment, since the film thickness of the Al _c Ga _1-c N layer (8,8 ′) is not less than 10 nm and not more than 300 nm, the nitride semiconductor (9-12) is formed. The warpage of the nitride semiconductor epitaxial wafer in can be reduced.

1 ... Si substrate,
2 ... AlN layer,
3… Al _0.7 Ga _0.3 N,
4… Al _0.4 Ga _0.6 N,
5 ... Al _0.1 Ga _0.9 N,
6 ... Composition gradient buffer layer,
7,7 ', 7''... superlattice layer,
8, 8 '... Al _0.1 Ga _0.9 N layer (Al _c Ga _1-c N layer),
9 ... GaN channel layer,
10 ... AlN intermediate layer,
11 ... AlGaN barrier layer,
12 ... GaN cap layer.

Claims (3)

A substrate,
An initial growth layer formed on the substrate;
An Al composition gradient layer which is formed on the initial growth layer and whose Al composition decreases continuously or stepwise;
A low Al-containing layer formed on the Al composition gradient layer and having a composition of Al _a Ga _1-a N (0 <a ≦ 0.3) and a composition of Al _b Ga _1-b N (0.5 A superlattice layer in which high Al-containing layers satisfying ≦ b ≦ 1.0) are alternately stacked, and a three-layer structure sandwiching Al _c Ga _1-c N layers;
A nitride semiconductor formed on the superlattice layer constituting the three-layer structure,
A plurality of the three-layer structures are formed between the Al composition gradient layer and the nitride semiconductor,
The plurality of three-layer structures are generated at the interface between the substrate and the initial growth layer due to stress caused by the difference in lattice constant between the superlattice layer and the Al _c Ga _1-c N layer. A nitride semiconductor epitaxial wafer comprising a crystallinity-improving layer for improving the crystallinity of the nitride semiconductor by reducing dislocations extending toward the surface. The nitride semiconductor epitaxial wafer according to claim 1,
The Al composition c in the Al _c Ga _1-c N layer constituting the three-layer structure is:
When the thickness of the Al _a Ga _1-a N in the superlattice layer is d1 and the thickness of the Al _b Ga _1-b N is d2,
c <(d1 * a + d2 * b) / (d1 + d2)
A nitride semiconductor epitaxial wafer characterized by having the following relationship: In the nitride semiconductor epitaxial wafer according to claim 1 or 2,
The nitride semiconductor epitaxial wafer, wherein the Al _c Ga _1-c N layer constituting the three-layer structure has a thickness of 10 nm or more and 300 nm or less.

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