Group iii nitride laminate, and method for manufacturing group iii nitride laminate

Abstract

To reduce surface defects of a group III nitride laminate.SOLUTION: A group III nitride laminate comprises: a ground substrate; a first layer provided on the ground substrate and containing aluminum nitride; and a second layer provided on the first layer and containing gallium nitride. Thickness of the first layer is 11.0 nm or more, and an AlN abundance ratio which is a ratio of an amount of aluminum nitride to a total amount of aluminum nitride and gallium nitride in a region corresponding to the thickness of the first layer, is less than 78%.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a group III nitride laminate and a method for manufacturing the group III nitride laminate.

Background Art

[0002] Group III nitride-based high electron mobility transistors (HEMTs) are widely used as power amplifiers for base stations of mobile phones (for example, Patent Document 1). In group III nitride-based HEMTs, the power that can be input per element can be significantly increased compared to conventionally used Si-based devices. As a result, the base station can be miniaturized, and the installation cost can be significantly reduced.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Semiconductor elements such as HEMTs are manufactured from group III nitride laminates. According to the studies by the present inventors, surface defects may exist in the group III nitride laminate, and desired device characteristics may not be obtained in the semiconductor element due to the surface defects.

[0005] An object of the present invention is to provide a technique for reducing surface defects in a group III nitride laminate.

Means for Solving the Problems

[0006] According to one aspect of the present invention, a lower base plate, a first layer provided on the lower base plate and containing aluminum nitride, A second layer containing gallium nitride, provided on the first layer, is provided, the thickness of the first layer is 11.0 nm or more, an AlN abundance ratio, which is the ratio of the amount of aluminum nitride to the total amount of aluminum nitride and gallium nitride in the region corresponding to the thickness of the first layer, is less than 78%, a group III nitride laminate is provided.

[0007] According to another aspect of the present invention, a step of forming a first layer containing aluminum nitride on a lower base plate, a step of forming a second layer containing gallium nitride on the first layer, is provided, in the step of forming the first layer, while setting the thickness of the first layer to 11.0 nm or more, the first layer is formed so that an AlN abundance ratio, which is the ratio of aluminum nitride to the total of aluminum nitride and gallium nitride in the region corresponding to the thickness of the first layer, is less than 78%. a method for manufacturing a group III nitride laminate is provided.

Advantages of the Invention

[0008] According to the present invention, surface defects in a group III nitride laminate can be reduced.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

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Figure 5

Figure 6

Figure 7

Embodiments for Carrying Out the Invention

[0010] <Findings Obtained by the Inventors> First, the findings obtained by the inventors will be described.

[0011] A group III nitride laminate (hereinafter, also simply referred to as a laminate) for manufacturing a group III nitride semiconductor device is composed of, for example, a predetermined base substrate, a first layer containing aluminum nitride (AlN) and functioning as a nucleation layer, a second layer containing gallium nitride (GaN) and functioning as a channel layer, and a third layer containing a barrier layer and a cap layer, which are laminated.

[0012] In the laminate, surface defects may occur due to complex factors such as the surface state of the base substrate and crystal growth conditions. Surface defects are fine irregularities (growth pits) that become apparent on the surface of the laminate due to complex factors. For example, during crystal growth, surface defects may occur on the surface at locations where there are dislocations with large strains or locally concentrated dislocations. Also, for example, surface defects may occur on the crystal surface due to foreign substances attached to the base surface or surface roughness, etc., which are caused by the surface state of the base. Also, for example, surface defects may occur on the crystal surface when impurities are mixed into the crystal during growth.

[0013] In order to reduce surface defects caused by such complex factors, the inventors focused on the first layer that serves as a nucleation layer and is the base for crystal growth. The nucleation layer is formed by three-dimensional growth, and its surface may be formed with irregularities. The irregular structure of the nucleation layer varies, for example, in the density, shape, roughness, etc. of the irregularities depending on the growth conditions. When the inventors formed the nucleation layer by adjusting the growth conditions, they found that there is a high correlation between the occurrence of surface defects and the AlN abundance ratio, which is an index related to the irregular structure and thickness described later, and the thickness of the nucleation layer.

[0014] And they found that by adjusting the AlN abundance ratio and the thickness of the nucleation layer to respective predetermined ranges, the surface defects in the laminate can be significantly reduced.

[0015] The present invention is based on the above findings discovered by the inventors.

[0016] [Details of Embodiments of the Present Invention] Next, an embodiment of the present invention will be described below with reference to the drawings. It should be noted that the present invention is not limited to these examples, and is defined by the claims, and is intended to include all modifications within the meaning and scope equivalent to the claims.

[0017] [First Embodiment of the Present Invention]< Hereinafter, a first embodiment of the present invention will be described with reference to the drawings.

[0018] (1) Group III Nitride Laminate The group III nitride laminate according to this embodiment will be described with reference to FIG. 1. FIG. 1 is a schematic cross-sectional view showing the group III nitride laminate according to this embodiment.

[0019] In the following, in a crystal such as a group III nitride semiconductor having a wurtzite structure, the <0001> axis is referred to as the "c-axis", and the (0001) plane is referred to as the "c-plane".

[0020] As shown in FIG. 1, the group-III nitride laminate 1 (laminate 1) of the present embodiment includes, for example, a lower base substrate 10, a nucleation layer 20 as a first layer containing aluminum nitride (AlN), a channel layer 30 as a second layer containing gallium nitride (GaN), and In x Al y Ga (1-x-y) a third layer 40 containing a group-III nitride represented by the composition formula of N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1). Here, a case where the third layer 40 is composed of a barrier layer 41 and a cap layer 42 will be described as an example. Each layer will be described below.

[0021] (Lower base substrate) The lower base substrate 10 is made of, for example, silicon carbide (SiC) or sapphire (Al2O3). Here, the lower base substrate 10 is, for example, a SiC substrate. The polytype of the SiC substrate as the lower base substrate 10 is not limited, but is, for example, 4H, 6H, or 3C. Also, the SiC substrate as the lower base substrate 10 is preferably semi-insulating.

[0022] The lower base substrate 10 has a main surface that serves as the lower surface. In the present embodiment, the low-index crystal plane closest to the lower surface is, for example, the c-plane ((0001) plane, Si plane).

[0023] The lower base substrate 10 preferably has a large area, for example, to improve productivity when manufacturing a semiconductor device. Specifically, the diameter of the lower base substrate 10 is, for example, 2 inches (50 mm) or more, preferably 4 inches (100 mm) or more, and more preferably 6 inches (150 mm) or more.

[0024] The thickness of the lower base plate 10 is not particularly limited, but depends on the diameter of the lower base plate 10. Specifically, for the lower base plate 10 with a diameter of 2 inches, the thickness is, for example, 300 μm or more and 500 μm or less (typically 430 μm); for the lower base plate 10 with a diameter of 4 inches, the thickness is, for example, 400 μm or more and 1000 μm or less (typically 500 μm); and for the lower base plate 10 with a diameter of 6 inches, the thickness is, for example, 400 μm or more and 1500 μm or less (typically 500 μm).

[0025] (First layer) On the lower base plate 10, a nucleation layer 20 is provided as the first layer. The nucleation layer 20 contains AlN and is configured to generate crystal nuclei for growing, for example, a channel layer 30 as the second layer containing GaN described later. The nucleation layer 20 is formed, for example, by heteroepitaxially growing a single crystal of AlN on the lower surface of the lower base plate 10.

[0026] As shown in FIG. 2, the nucleation layer 20 is composed of a stacked initial growth layer 21 provided on the lower base plate 10 and a three-dimensional structure layer 22 provided thereon. FIG. 2 is a partially enlarged cross-sectional view of the group III nitride laminate according to the present embodiment. An uneven structure derived from the three-dimensional structure layer 22 is formed on the surface of the nucleation layer 20.

[0027] In the nucleation layer 20, the initial growth layer 21 serves as a base for growing the three-dimensional structure layer 22. The initial growth layer 21 is formed by growing AlN crystals at a first growth rate. As the first growth rate, it is preferable to set conditions such that the wettability of the AlN crystals with respect to the lower base plate 10 is high. By setting such conditions, the AlN crystals can be grown so as to continuously cover the surface of the lower base plate 10. As a result, the initial growth layer 21 can be formed to be thin while having few exposed portions of the lower base plate 10, and preferably, to continuously cover the entire surface of the lower base plate 10 without any exposed portions.

[0028] The three-dimensional structure layer 22 is formed by three-dimensionally growing AlN crystals on the initial growth layer 21 at a second growth rate. As the second growth rate, conditions that promote the three-dimensional growth of AlN crystals are preferably used. The three-dimensional structure layer 22 is preferably formed uniformly on the main surface of the initial growth layer 21. Further, as will be described in detail later, the three-dimensional structure layer 22 is preferably configured such that the top upper surface of the uneven structure is the C plane.

[0029] (Second layer) A channel layer 30 is provided as a second layer on the nucleation layer 20. The channel layer 30 is configured to allow electrons to travel during the operation of a semiconductor device such as a HEMT, for example. The channel layer 30 is formed, for example, by heteroepitaxially growing a single crystal of GaN on the main surface of the nucleation layer 20. The low-index crystal plane closest to the main surface of the channel layer 30 is, for example, the c plane ((0001) plane, Ga plane).

[0030] The thickness of the channel layer 30 is not particularly limited, but is preferably, for example, 100 nm or more and less than 1 μm. Note that the channel layer 30 may be provided directly on the nucleation layer 20, or may be provided on the nucleation layer 20 via a known buffer layer provided on the nucleation layer 20, for example.

[0031] (Mixed layer) In this embodiment, by forming the channel layer 30 on the concavo-convex structure of the nucleation layer 20, the region corresponding to the thickness of the nucleation layer 20 becomes a mixed layer 23 formed by embedding the channel layer 30 in the concave portions on the surface of the nucleation layer 20. In the mixed layer 23, for example, the amount occupied by the channel layer 30 varies depending on the density, shape, height of the convex portions in the concavo-convex structure, and the thickness of the nucleation layer 20. Therefore, in the mixed layer 23, the ratio of AlN and the ratio of GaN increase or decrease due to the differences in the concavo-convex structure and the thickness of the nucleation layer 20. For example, when the thickness of the nucleation layer 20 is constant, if the density of the convex portions in the concavo-convex structure decreases or the cross-sectional shape of the convex portions becomes triangular, the ratio of GaN increases and the ratio of AlN tends to decrease. Conversely, if the density of the convex portions increases or the concavo-convex structure becomes flat, the ratio of AlN increases and the ratio of GaN tends to decrease. Also, for example, when the concavo-convex structure of the nucleation layer 20 is the same, the thicker the nucleation layer 20, the higher the ratio of AlN in the mixed layer 23 tends to be. That is, in the mixed layer 23, the quantitative ratio (composition ratio) of AlN and GaN constituting the mixed layer 23 varies depending on the thickness and concavo-convex structure of the nucleation layer 20. Therefore, the AlN abundance ratio, which is the ratio of the amount of AlN to the total amount of AlN and GaN in the mixed layer 23, serves as an index related to the concavo-convex structure and thickness of the nucleation layer 20.

[0032] And as described above, according to the studies by the present inventors, there is a high correlation between the AlN abundance ratio in the mixed layer 23 and the thickness of the nucleation layer 20 and the generation of surface defects in the laminate 1, and it has been found that surface defects can be significantly reduced by setting these within predetermined ranges. This will be described below.

[0033] First, the AlN abundance ratio will be described. The AlN abundance ratio can be calculated by measuring the amounts of AlN and GaN in the mixed layer 23. As methods for measuring these amounts, methods using cross-sectional TEM capable of analyzing fine structures, methods using an atomic force microscope (AFM), methods using spectroscopic ellipsometry, etc. can be considered. In this regard, according to the studies by the present inventors, it has been found that the method using spectroscopic ellipsometry is good. In the method using cross-sectional TEM, not only is it necessary to destroy the sample to obtain a cross-section of the mixed layer 23, but also information in the depth direction of the mixed layer 23 cannot be obtained, and only local information can be obtained. In the method using AFM, not only can only local information be obtained, but it is also necessary to observe the nucleation layer 20 before the formation of the channel layer 30, and there are cases where it cannot be measured as the laminate 1. On the other hand, according to spectroscopic ellipsometry, since the mixed layer 23 can be modeled by the effective medium approximation and the reflection polarization spectrum can be analyzed, the composition ratio can be obtained more accurately. Also, according to spectroscopic ellipsometry, the laminate 1 can be detected non-destructively and with high throughput.

[0034] Subsequently, the correlation between the AlN abundance ratio and the number of surface defects will be described with reference to FIG. 3. FIG. 3 is a diagram for explaining the correlation between the AlN abundance ratio and the number of surface defects. FIG. 3 shows the number of surface defects on the surface of the laminate 1 when the thickness of the nucleation layer 20 was adjusted in the range of 12 nm to 14 nm, the growth conditions were appropriately changed, and the AlN abundance ratio was varied. In FIG. 3, the horizontal axis represents the AlN abundance ratio [%], and the vertical axis represents the number of surface defects [pieces / waf] in the laminate 1. Note that the number of surface defects indicates the total number of surface defects per one laminate 1.

[0035] As shown in FIG. 3, when the AlN abundance ratio is 100%, that is, when no unevenness is formed on the surface of the nucleation layer 20 and it becomes flat, it can be confirmed that many surface defects occur in the laminate 1. On the other hand, as the AlN abundance ratio decreases and an uneven structure is introduced on the surface of the nucleation layer 20, the surface defects tend to decrease. In particular, when the AlN abundance ratio is less than 78%, it can be confirmed that the surface defects are significantly reduced. In this case, it is presumed that the density, shape, etc. of the convex portions in the uneven structure can be configured so as to reduce the surface defects when the laminate 1 is manufactured. From the viewpoint of more reliably reducing the surface defects, the AlN abundance ratio is preferably 75% or less, and more preferably 70% or less.

[0036] The lower limit value of the AlN abundance ratio is not particularly limited, but it is preferably 47% or more. If the AlN abundance ratio becomes excessively low, the nucleation layer 20 may not be continuously formed on the underlying substrate 10. For example, the initial growth layer 21 may not be formed so as to cover the underlying substrate 10. As a result, the underlying substrate 10 may be exposed. When GaN is epitaxially grown on the exposed underlying substrate 10, the crystallinity of GaN itself may not be maintained. In this regard, by forming the nucleation layer 20 such that the AlN abundance ratio is 47% or more, the initial growth layer 21 can be continuously formed on the underlying substrate 10, and the three-dimensional structure layer 22 can be formed thereon, and a nucleation layer 20 having an appropriate uneven structure and thickness capable of reducing surface defects can be obtained. From the viewpoint of more stably forming the nucleation layer 20 having an appropriate uneven structure and thickness, the AlN presence is preferably 56% or more.

[0037] Next, the correlation between the thickness of the nucleation layer 20 and the number of surface defects will be described with reference to FIG. 4. FIG. 4 is a diagram for explaining the correlation between the thickness of the nucleation layer 20 containing aluminum nitride and the number of surface defects. FIG. 4 shows the measurement of the number of surface defects on the surface of the laminate 1 when the thickness of the nucleation layer 20 is changed with the AlN abundance ratio in the mixed layer 23 being within the range of 65% to 75%. In FIG. 4, the horizontal axis shows the thickness [nm] of the nucleation layer 20, and the vertical axis shows the number of surface defects [pieces / waf] in the laminate 1. The thickness of the nucleation layer 20 is the thickness corresponding to the mixed layer 23, and indicates the maximum thickness of the nucleation layer 20.

[0038] As shown in FIG. 4, the thinner the nucleation layer 20, the more surface defects tend to occur. When the nucleation layer 20 is formed thin, for example, the initial growth layer 21 cannot be formed to cover the base substrate 10 continuously, and the base substrate 10 may be exposed. In addition, for example, the three-dimensional growth of the three-dimensional structure layer 22 may be insufficient, and the convex portions may be low or the density of the convex portions may be low, making it impossible to form a predetermined uneven structure. As a result, it is presumed that when the channel layer 30 or the third layer 40 described later is grown, the influence of the factors that cause surface defects becomes stronger, and the surface defects increase. In this regard, the thicker the nucleation layer 20, the more likely it is that a moderate uneven structure that can reduce surface defects can be formed while the nucleation layer 20 is formed continuously on the base substrate 10, and when the thickness of the nucleation layer 20 is set to 11 nm or more, the surface defects can be reduced more significantly.

[0039] The upper limit of the thickness of the nucleation layer 20 is not particularly limited. Since the thicker the nucleation layer 20, the higher the AlN abundance ratio tends to be, it is advisable to appropriately adjust the thickness within a range in which the AlN abundance ratio is less than 78%. If the nucleation layer 20 is excessively thick, not only the productivity of the laminate 1 decreases, but also the high frequency characteristics and electrical characteristics of the semiconductor element tend to deteriorate. From the viewpoint of maintaining high high frequency characteristics and electrical characteristics while obtaining high productivity, the thickness of the nucleation layer 20 is preferably 30 nm or less, more preferably 20 nm or less, and even more preferably 15 nm or less.

[0040] As described above, by setting the AlN abundance ratio in the mixed layer 23 and the thickness of the nucleation layer 20 within predetermined ranges respectively, surface defects in the laminate 1 can be reduced.

[0041] From the viewpoint of stably adjusting the AlN abundance ratio in the mixed layer 23 within a predetermined range, it is preferable to configure the three-dimensional structure layer 22 of the nucleation layer 20 such that the top upper surface of its concavo-convex structure is the C-plane. According to the three-dimensional structure layer 22 having the C-plane, as shown in FIG. 2, the cross-sectional shape of the convex portion can be trapezoidal. Also, the surface area of the surface on the channel layer 30 side can be made smaller than the surface area of the surface on the underlying substrate 10 side. By introducing the C-plane into the three-dimensional structure layer 22, the embedding amount of the channel layer 30 in the mixed layer 23 can be reduced as compared with the case where the convex portion has an acute angle and does not have the C-plane. That is, the AlN abundance ratio can be increased. Moreover, the AlN abundance ratio can be adjusted by forming the area of the C-plane wide or narrow. Thereby, even when the nucleation layer 20 is formed thinly, the AlN abundance ratio can be stably adjusted within the above range.

[0042] Also, from the viewpoint of stably adjusting the AlN abundance ratio in the mixed layer 23 within a predetermined range, it is preferable that the thickness of the initial growth layer 21 in the nucleation layer 20 is smaller than the thickness of the three-dimensional structure layer 22. Thereby, while reducing the thickness of the nucleation layer 20, the AlN abundance ratio can be stably adjusted within a predetermined range. Note that the initial growth layer 21 preferably has a thickness that uniformly covers the entire surface so that at least the underlying substrate 10 is not exposed. Also, the thickness of the three-dimensional structure layer 22 may be appropriately adjusted according to the thickness of the initial growth layer 21 so that the AlN abundance ratio falls within a predetermined range.

[0043] (Third layer) On the channel layer 30, a barrier layer 41 and a cap layer 42 are provided as the third layer formed from a group-III nitride crystal represented by the composition formula of In x Al y Ga (1-x-y) N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1).

[0044] The barrier layer 41 is provided on the channel layer 30. The barrier layer 41 is configured to generate a two-dimensional electron gas (2DEG) in the channel layer 30 and spatially confine the 2DEG within the channel layer 30. The barrier layer 41 is formed, for example, by heteroepitaxially growing a group III nitride crystal on the main surface of the channel layer 30. For example, the barrier layer 41 is composed of a group III nitride having a smaller electron affinity than the group III nitride crystal constituting the channel layer 30, such as AlGaN containing aluminum (Al) and gallium (Ga). The thickness of the barrier layer 41 is preferably, for example, 1 nm or more and 50 nm or less.

[0045] The cap layer 42 is provided on the barrier layer 50. The cap layer 42 is interposed between the barrier layer 41 and the electrode provided thereon in order to improve the device characteristics (such as controllability of the threshold voltage) of a semiconductor device such as a HEMT. The cap layer 42 is composed of, for example, GaN. The cap layer 42 is formed as necessary and may be omitted.

[0046] (Surface defects) In the laminate 1 of the present embodiment, the nucleation layer 20 is configured to have a predetermined AlN abundance ratio and thickness, and the channel layer 30, the barrier layer 41, etc. are sequentially formed on the nucleation layer 20. Therefore, the laminate 1 is configured such that the generation of surface defects is suppressed and the density thereof is reduced.

[0047] As described above, surface defects are fine irregularities (growth pits) that occur due to complex factors such as crystal growth conditions and the surface state of the underlying substrate, and are manifested on the surface of the laminate 1. When the surface of the laminate 1 is irradiated with laser light and scanned, the presence of surface defects can be confirmed and detected from the reflection and scattering conditions of the irradiated light. The surface defects preferably have a size of 0.165 μm or more and 2.0 μm or less when viewed in plan view. Such surface defects indicate those generated by crystal growth and substantially do not contain coarse foreign substances (e.g., external particles) attached to the crystal surface. Note that the surface defects in the laminate 1 refer to those formed on the surface of the uppermost layer of the laminate 1. In the present embodiment, since the uppermost layer of the laminate 1 is the cap layer 42, the surface defects in the laminate 1 refer to those formed on the surface of the cap layer 42.

[0048] The average density of the surface defects in the laminate 1 is preferably 500 pieces / cm 2 or less, more preferably 100 pieces / cm 2 or less, and even more preferably 25 pieces / cm 2 or less. Here, the calculation of the average density of the surface defects will be described with reference to FIG. 5. FIG. 5 is a diagram for explaining the average density of the surface defects and is a schematic view when the surface of the laminate 1 is viewed from above. As shown in FIG. 5, the average density of the surface defects is the total number of surface defects present in the internal region 1B (the region surrounded by the broken line in the figure) excluding a width of 5 mm from the outer edge on the surface 1A of the laminate 1 divided by the area of the internal region 1B, and indicates the number of surface defects per unit area.

[0049] (3) Method for manufacturing a group III nitride laminate Next, with reference to FIG. 6, the method for manufacturing a group III nitride laminate according to the present embodiment will be described. FIG. 6 is a flowchart showing the method for manufacturing a group III nitride laminate according to the present embodiment.

[0050] The method for manufacturing a group-III nitride laminate according to this embodiment includes, for example, a lower substrate preparation step S10, a first layer formation step S20, a second layer formation step S30, and a third layer formation step S40.

[0051] (S10: Lower substrate preparation step) First, a lower substrate 10 is prepared. As the lower substrate 10, for example, a polytype 6H semi-insulating SiC substrate is prepared.

[0052] (S20: First layer formation step) Next, a single crystal of AlN is heteroepitaxially grown on the lower surface of the lower substrate 10 to form a nucleation layer 20 as the first layer. The growth of the nucleation layer 20 may be performed, for example, by a metalorganic vapor phase epitaxy (MOVPE) method.

[0053] As the group-III (Al) source gas, for example, trimethylaluminum (TMA) gas is used. As the N source gas, for example, NH3 gas is used. These source gases may be mixed with a carrier gas using hydrogen (H2) gas, nitrogen (N2) gas, or a mixed gas thereof and supplied.

[0054] In this embodiment, in the first layer formation step S20, first, an initial growth layer 21 is formed, and then a three-dimensional structure layer 22 is formed. This will be specifically described below.

[0055] (Initial growth layer formation step S21) First, an AlN crystal is grown at a first growth rate to form an initial growth layer 21. Here, the first growth rate is preferably set such that the AlN crystal covers the lower substrate 10 with high wettability. Thereby, the initial growth layer 21 can be formed so as to continuously cover the entire lower surface of the lower substrate 10. As a result, even when the initial growth layer 21 is formed thinly, the lower substrate 10 can be formed so as not to be exposed.

[0056] The first growth rate is not particularly limited, but from the viewpoint of growing AlN crystals with high wettability with respect to the underlying substrate 10, it is preferably 50 nm / h or more and 500 nm / h or less. The first growth rate can be changed, for example, by appropriately adjusting the supply amount of group III source gas such as TMA.

[0057] Regarding the growth conditions for forming the initial growth layer 21, conditions other than the growth rate, such as the growth temperature, V / III ratio, and growth pressure, may be appropriately adjusted according to the growth rate. Here, the "V / III ratio" refers to the ratio of the supply amount (partial pressure) of group V (N) source gas to the supply amount (partial pressure) of group III (Al) source gas.

[0058] Specifically, the crystal growth conditions other than the growth rate may be set as follows, for example. Growth temperature: 1200 °C or more and 1390 °C or less V / III ratio: 5000 or more and 25000 or less Growth pressure: 0.059 atm or more and 0.197 atm or less

[0059] (Three-dimensional structure layer formation step S22) Subsequently, AlN crystals are grown on the main surface of the initial growth layer 21 at the second growth rate to form a three-dimensional structure layer 22. Thereby, a nucleation layer 20 is formed.

[0060] The second growth rate is preferably set to conditions that promote the three-dimensional growth of AlN crystals. Also, the three-dimensional structure layer 22 is preferably formed uniformly on the main surface of the initial growth layer 21. Thereby, when the channel layer 30 is formed on the nucleation layer 20, the AlN abundance ratio in the mixed layer 23 can be adjusted within a predetermined range. Also, the three-dimensional structure layer 22 is preferably formed under conditions such that the top upper surface of its uneven structure is the C plane, that is, the cross-sectional shape of the convex portion is trapezoidal. Thereby, the AlN abundance ratio can be reliably adjusted within a predetermined range.

[0061] The second growth rate can be changed by appropriately adjusting the supply amount of the group-III raw material gas, similar to the first growth rate. The second growth rate is not particularly limited, but from the perspective of three-dimensionally growing AlN crystals, it is preferably 80 nm / h or more and 700 nm / h or less. The second growth rate can be changed by appropriately adjusting the supply amount of, for example, TMA, similar to the first growth rate.

[0062] The first growth rate and the second growth rate may each be appropriately adjusted from the above ranges. When forming the channel layer 30 on the nucleation layer 20 in the second layer formation step S30 described later, from the perspective of adjusting the AlN abundance ratio in the mixed layer 23 to a predetermined range, the second growth rate is preferably the same as the first growth rate, or the second growth rate is preferably greater than the first growth rate. That is, the second growth rate is preferably the first growth rate or more. Thereby, while growing AlN crystals with high wettability in the initial growth layer formation step S21, in the three-dimensional structure layer formation step S22, AlN crystals can be grown more three-dimensionally to form a nucleation layer 20 having a desired thickness and uneven structure, and the AlN abundance ratio can be adjusted to a predetermined range.

[0063] The second growth rate may be constant or continuously increased during the formation of the three-dimensional structure layer 22. When the second growth rate is continuously increased, the three-dimensional growth of AlN crystals can be further promoted, a three-dimensional structure layer 22 having a desired uneven structure can be formed, and the AlN abundance ratio can be adjusted to a predetermined range. The increase rate of the second growth rate is preferably set such that, for example, when the growth rate at the start point of growth of the three-dimensional structure layer 22 is v1 and the growth rate at the end point of growth is v2, v2 / v1 is greater than 1 times and 2 times or less. By increasing the second growth rate at such an increase rate, a desired uneven structure can be stably formed in the three-dimensional structure layer 22.

[0064] Regarding the crystal growth conditions for forming the three-dimensional structure layer 22, conditions other than the growth rate, such as the growth temperature, V / III ratio, and growth pressure, may be appropriately adjusted according to the growth rate.

[0065] Specifically, the crystal growth conditions other than the growth rate may be set as follows, for example. Growth temperature: 1200 °C or higher and 1390 °C or lower V / III ratio: 5000 or higher and 25000 or lower Growth pressure: 0.059 atm or higher and 0.197 atm or lower

[0066] (S30: Second layer formation step) Next, a channel layer 30 made of a single crystal of GaN is heteroepitaxially grown on the main surface of the nucleation layer 20. The growth of the channel layer 30 may be performed, for example, by MOVPE in the same manner as the formation of the nucleation layer 20.

[0067] As the group III (Ga) source gas, for example, trimethylgallium (Ga(CH3)3, TMG) gas is used. As the N source gas, for example, NH3 gas is used. These source gases may be mixed with a carrier gas using hydrogen (H2) gas, nitrogen (N2) gas, or a mixed gas thereof and supplied.

[0068] As the crystal growth conditions of the channel layer 30, at least one of the growth temperature, growth rate, and growth pressure may be appropriately controlled so that the GaN crystal can grow three-dimensionally.

[0069] Specifically, the crystal growth conditions may be set as follows, for example. Growth temperature: 1100 °C or higher and 1300 °C or lower Growth rate: 1000 nm / h or higher and 3000 nm / h or lower V / III ratio: 1000 or higher and 3000 or lower Growth pressure: 0.098 atm or higher and 0.296 atm or lower

[0070] (S40: Third layer formation step) Next, on the main surface of the channel layer 30, In x Al y Ga (1-x-y)A single crystal of a group III nitride represented by the composition formula N(0≦x≦1, 0≦y≦1, x + y≦1) is heteroepitaxially grown to form a third layer 40 including a barrier layer 31 and a cap layer 42. In this embodiment, first, for example, a single crystal of AlN, AlGaN, InAlN, or AlInGaN is grown to form a barrier layer 41. Subsequently, a single crystal of, for example, GaN is grown on the barrier layer 41 to form a cap layer 42.

[0071] The growth of the barrier layer 41 and the cap layer 42 may be performed, for example, by the MOVPE method in the same manner as the formation of the nucleation layer 20 and the like. As the Al source gas, for example, trimethylaluminum (Al(CH3)3, TMA) gas is used. As the In source gas, for example, trimethylindium (In(CH3)3, TMI) gas is used. For other gases, the same gases as those in the second layer formation step S30 are used.

[0072] Also, the thickness of the barrier layer 41 is preferably, for example, 5 nm or more and 50 nm or less. The thickness of the cap layer 42 is preferably, for example, 1 nm or more and 10 nm or less.

[0073] Thus, the laminate 1 of this embodiment is manufactured.

[0074] (4) Effects Obtained by this Embodiment According to this embodiment, one or more of the following effects can be obtained.

[0075] (a) The laminate 1 of this embodiment has a nucleation layer 20 which is the first layer containing AlN, a channel layer 30 which is the second layer containing GaN, and a third layer 40 laminated on the lower base substrate 10. The AlN abundance ratio in the mixed layer 23 corresponding to the thickness of the nucleation layer 20 is less than 78%, and the thickness of the nucleation layer 20 is configured to be 11 nm or more. By having the AlN abundance ratio less than 78%, the uneven structure of the nucleation layer 20 has convex portions of a predetermined height at an appropriate density, and since the nucleation layer 20 has a predetermined thickness, when a nitride semiconductor crystal is grown on the lower base substrate 10, surface defects that appear on the crystal surface can be significantly reduced. This is presumably because the predetermined nucleation layer 20 can reduce the influence of factors that cause surface defects, such as dislocations in the crystal, the surface state of the lower base substrate 10, or impurities mixed into the crystal. Actually, as shown in FIG. 3, by setting the AlN abundance ratio to less than 78%, surface defects are significantly reduced, and as shown in FIG. 4, by setting the thickness of the nucleation layer 20 to 11 nm or more, it has been confirmed that surface defects can be significantly reduced. As described above, the laminate 1 with suppressed surface defects can be obtained.

[0076] (b) On the surface of the third layer 40 of the laminate 1, the average density of surface defects having a size of 0.165 μm or more and 2.0 μm or less is preferably 500 pieces / cm 2 or less. According to such a laminate 1, for example, when manufacturing a semiconductor device such as a HEMT, high device characteristics can be realized.

[0077] (c) The nucleation layer 20 includes an initial growth layer 21 provided on the lower base substrate 10 and a three-dimensional structure layer 22 provided on the initial growth layer 21. The three-dimensional structure layer 22 is preferably configured such that the top upper surface of the uneven structure is the C plane. By introducing the C plane into the three-dimensional structure layer 22, the cross-sectional shape of the convex portion can be made trapezoidal. As a result, while forming the nucleation layer 20 thinly, the AlN abundance ratio can be increased, and the AlN abundance ratio can be stably adjusted within a predetermined range. That is, the effect of (a) described above can be obtained more stably.

[0078] (d) The initial growth layer 21 is preferably provided so as to continuously cover the entire surface of the underlying substrate 10. Thereby, exposure of the underlying substrate 10 can be suppressed, so that a GaN single crystal can be grown on the nucleation layer 20 and the channel layer 30 can be formed.

[0079] (e) The thickness of the nucleation layer 20 is preferably 30 nm or less. By forming the nucleation layer 20 thinly with a predetermined thickness, the productivity of the laminate 1 can be improved, and high-frequency characteristics and electrical characteristics can be maintained at a high level in the semiconductor device fabricated from the laminate 1.

[0080] (f) In the mixed layer 23, the AlN abundance ratio is preferably 47% or more. Thereby, the effect of (d) described above can be obtained more stably.

[0081] <Other Embodiments> As described above, the embodiments of the present invention have been specifically described. However, the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the gist thereof.

[0082] In the above-described embodiment, the case where the third layer 40 including the barrier layer 41 and the cap layer 42 is formed on the channel layer 30 has been described, but the present invention is not limited thereto. For example, the third layer 40 may be configured to include the barrier layer 41 and not include the cap layer 42. Further, for example, the third layer 40 may be configured such that an AlN layer is interposed between the barrier layer 41 and the channel layer 30. Further, for example, the third layer 40 may be configured such that an InGaN layer or AlGaN is interposed in the channel layer 30.

[0083] In the above-described embodiment, the growth of the nucleation layer 20 and the channel layer 30 is described by using the MOVPE method, but the growth of either one or both of them may be performed by, for example, the hydride vapor phase epitaxy (HVPE) method.

Examples

[0084] Next, examples according to the present disclosure will be described. These examples are an example of the present disclosure, and the present disclosure is not limited by these examples.

[0085] In this example, a group III nitride laminate was fabricated, and the average density of surface defects revealed on its surface was evaluated. Specific details are described below.

[0086] (1) Fabrication of group III nitride laminate First, a SiC substrate was prepared as a base substrate. Subsequently, the SiC substrate was introduced into the processing chamber of a MOVPE apparatus. In the processing chamber, AlN was grown epitaxially on the SiC substrate to form a nucleation layer as a first layer including an initial growth layer and a three-dimensional structure layer. Subsequently, GaN was grown epitaxially on the nucleation layer to form a channel layer as a second layer. Subsequently, a third layer including a barrier layer and a cap layer was formed on the channel layer to fabricate a laminate. In this example, among the growth conditions of the first layer, the growth rate and the heating temperature were appropriately changed as shown in Table 1 below, and laminates of Samples 1 to 8 were fabricated. Note that the first growth rate and the second growth rate were adjusted to the numerical values shown in Table 1 by appropriately changing the supply amount of the group III raw material gas and the like according to the heating temperature.

[0087] [Table 1]

[0088] Note that the details of the SiC substrate and the growth conditions of each layer are as follows.

[0089] (Base substrate) Material: SiC (semi-insulating) Diameter: 6 inches Thickness: 500 μm Low-index crystal plane closest to the base surface: c-plane (no pattern processing of the base surface) Polytype: 6H

[0090] (Growth conditions of the initial growth layer) Material: AlN Growth method: MOVPE method V / III ratio: 5000 - 25000 Growth pressure: 0.059 atm - 0.098 atm

[0091] (Growth conditions of the three - dimensional structure layer) Material: AlN Growth method: MOVPE method V / III ratio: 5000 - 25000 Growth pressure: 0.059 atm - 0.098 atm

[0092] (Growth conditions of the channel layer) Material: GaN Growth method: MOVPE method Growth temperature: 1100°C - 1200°C V / III ratio: 1000 - 3000 Growth pressure: 0.098 atm - 0.197 atm Thickness: 400 nm

[0093] (Growth conditions of the barrier layer) Material: AlGaN Growth method: MOVPE method Growth temperature: 1100°C - 1200°C Growth pressure: 0.098 atm - 0.197 atm Thickness: 20 nm

[0094] (Cap layer) Material: GaN Growth method: MOVPE method Thickness: 2 nm

[0095] (2) Evaluation For the fabricated Samples 1 - 8, the thickness of the nucleation layer and the AlN abundance ratio in the region (mixed layer) corresponding to the thickness of the nucleation layer were measured by spectroscopic ellipsometry. Also, surface defects in the laminate were detected and their average density was calculated. Specifically, it will be described below.

[0096] (Thickness and AlN abundance ratio of the nucleation layer) The thickness of the nuclear generation layer and the AlN abundance were measured using a rotating compensator type spectroscopic ellipsometer ("M2000UI" manufactured by J.A. Woollam). Specifically, first, light (spot diameter of approximately 300 μm) was irradiated onto the surface of the laminate at an incident angle of 70°, and the reflected light from the surface of the laminate was measured. Subsequently, from the measurement data of the reflected light, the amount of change in polarization between the incident light and the reflected light was obtained for each wavelength, and a polarization spectrum in the wavelength range of 245 nm to 1690 nm was obtained. Here, as the amount of change in polarization, the spectroscopic phase difference and the spectroscopic reflection amplitude ratio angle between s-polarized light and p-polarized light were obtained. Subsequently, curve fitting was performed on the polarization spectrum by adopting predetermined fitting parameters based on a predetermined analysis model. Here, as the analysis model, a laminated structure of SiC, a mixed layer (mixture of AlN and GaN), GaN, AlGaN, and GaN was adopted. For the mixed layer, an optical model of effective medium approximation based on Bruggeman's equation was adopted. Then, curve fitting was performed so that the mean square error (MSE) between the measured polarization spectrum and the theoretical value in the model was minimized. At this time, as the fitting parameters, the thickness of each layer constituting the laminate, the AlN abundance in the mixed layer, the Al composition ratio in the AlGaN layer, and the roughness were selected. Thereby, the thickness and composition of each layer constituting the laminate were determined. Note that the composition of the mixed layer indicates the mixing ratio of AlN and GaN and corresponds to the AlN abundance.

[0097] (Average density of surface defects) For the fabricated Samples 1 to 8, surface defects in the laminate were detected and their average density was evaluated. Specifically, using a wafer surface inspection apparatus ("YPI-MX-θ" manufactured by Yamanashi Technical Workshop Co., Ltd.), the entire surface of the laminate was irradiated with laser light, and surface defects were detected from the state of reflection and scattering of the laser light. In this example, as surface defects, those having a size of 0.165 μm or more and 2.0 μm or less when visually observed on the surface were detected. At this time, when detecting surface defects, the wavelength of the laser light was set to 375 nm, and a photomultiplier was adopted as the detector. The average density of surface defects was calculated by obtaining the total number of surface defects in the internal region excluding the range of 5 mm in width from the outer shape on the surface of the laminate and dividing it by the area of the internal region. Note that the surface defects were detected in an environment where the number of external particles was less than 1.0 piece / cm 3 under the following conditions.

[0098] (3) Evaluation Results The results of the above-described evaluation are summarized in Table 1.

[0099] When the cross-section of Sample 1 was observed by TEM, it was confirmed that the state was as shown in FIG. 7. FIG. 7 is a cross-sectional image when the cross-section of Sample 1 was observed by TEM. In FIG. 7, an AlN layer as a nucleation layer and a GaN layer as a channel layer were formed on a SiC substrate as a lower base substrate. It was confirmed that an uneven structure was formed on the surface of the AlN layer, and the region corresponding to the thickness of the AlN layer was a mixed layer in which AlN and GaN were mixed. Further, it was confirmed that the cross-sectional shape of the convex portion of the uneven structure was trapezoidal and its top was the C plane. Further, it was confirmed that the initial growth layer constituting the AlN layer was formed so as to continuously cover the entire main surface of the lower base substrate, and its thickness was thinner than that of the three-dimensional structure layer. For Samples 2 to 5, it was also confirmed that the same uneven structure as that of Sample 1 was formed.

[0100] On the one hand, as shown in Table 1, in Sample 1, it was confirmed that the thickness of the nucleation layer (AlN layer) was 12.5 nm, which was 11 nm or more, and the AlN abundance ratio in the mixed layer was 47.1%, which was less than 78%. Also, the average density of surface defects on the surface of Sample 1 was 18.4 pieces / cm 2 and it was confirmed that this was sufficiently small.

[0101] The reason why surface defects could be reduced in Sample 1 is considered as follows. As shown in Fig. 7, since the AlN abundance ratio in the mixed layer of the AlN layer of Sample 1 was less than 78%, it was confirmed that the AlN layer of Sample 1 had an uneven structure in which convex portions of a predetermined height were distributed at an appropriate density and was configured to have a reduced thickness. According to such an AlN layer, when a nitride semiconductor crystal is grown thereon, it is presumed that the influence of factors causing surface defects, such as dislocations, the surface state of the underlying substrate, and the incorporation of impurities, can be suppressed, and surface defects that appear on the crystal surface can be reduced.

[0102] Also, as shown in Fig. 7, by introducing a C plane onto the top surface of the convex portion structure of the AlN layer, it was confirmed that the cross-sectional shape of the convex portion could be made trapezoidal, and the AlN abundance ratio could be adjusted to be high while forming the AlN layer thinly.

[0103] In Samples 2 to 5, it was confirmed that, similar to Sample 1, by setting the thickness of the AlN layer to 11 nm or more and the AlN abundance ratio to less than 78%, surface defects could be reduced.

[0104] On the other hand, in Sample 6, although the AlN layer was formed so that the AlN abundance ratio was less than 78%, the thickness of the AlN layer was less than 11 nm, and the average density of surface defects exceeded 500 pieces / cm 2 This is presumably because the AlN layer was formed to be excessively thin, the initial growth layer could not be formed continuously, or convex portions could not be formed at an appropriate height and appropriate density in the three-dimensional structure layer, so the influence of factors causing surface defects could not be reduced.

[0105] In Sample 7, the AlN layer was formed to have a thickness of 11 nm or more. Since the AlN abundance ratio was 80.6%, which is 78% or more, the average density was confirmed to exceed 500 pieces / cm 2 2 . As a reason for the high average density, it is presumed that since the AlN abundance ratio is high and the amount of GaN embedded in the mixed layer is small, an uneven structure that can suppress surface defects, such as an excessive density of protrusions or low protrusions, cannot be formed.

[0106] In Sample 8, since the thickness of the AlN layer was less than 11 nm and the AlN abundance ratio was 78% or more, the average density was confirmed to exceed 500 pieces / cm 2 2 . As a reason for the high average density, it is presumed that there were locally thin portions in the AlN layer or a predetermined uneven structure could not be formed.

[0107] <Preferred Embodiment of the Present Invention> Hereinafter, preferred embodiments of the present invention will be appended.

[0108] (Appendix 1) A lower base plate, A first layer provided on the lower base plate and containing aluminum nitride, A second layer provided on the first layer and containing gallium nitride, Comprising The thickness of the first layer is 11.0 nm or more, The AlN abundance ratio, which is the ratio of the amount of aluminum nitride to the total amount of aluminum nitride and gallium nitride in the region corresponding to the thickness of the first layer, is less than 78%, A group III nitride laminate.

[0109] (Appendix 2) The average density of surface defects having a size of 0.165 μm or more and 2.0 μm or less on the surface of the second layer is 500 pieces / cm 2 Or less, The group III nitride laminate according to Appendix 1.

[0110] (Appendix 3) provided on the second layer, In x Al y Ga (1-x-y) a third layer containing a group-III nitride represented by the composition formula of N(0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1), and the average density of surface defects having a size of 0.165 μm or more and 2.0 μm or less on the surface of the third layer is 500 pieces / cm 2 or less, The group-III nitride laminate according to Appendix 1 or 2.

[0111] (Appendix 4) The first layer is an initial growth layer provided so as to continuously cover the surface of the underlying substrate, and a three-dimensional structure layer provided on the initial growth layer and having an uneven structure on the surface, and the upper surface of the top of the uneven structure is the C plane, The group-III nitride laminate according to any one of Appendices 1 to 3.

[0112] (Appendix 5) The thickness of the first layer is 30 nm or less, The group-III nitride laminate according to any one of Appendices 1 to 4.

[0113] (Appendix 6) The AlN abundance ratio is 47% or more, The group-III nitride laminate according to any one of Appendices 1 to 5.

[0114] (Appendix 7) The thickness of the initial growth layer is smaller than the thickness of the three-dimensional structure layer, The group-III nitride laminate according to any one of Appendices 1 to 6.

[0115] (Appendix 8) a step of forming a first layer containing aluminum nitride on the underlying substrate, and a step of forming a second layer containing gallium nitride on the first layer, and having, In the step of forming the first layer, while setting the thickness of the first layer to 11.0 nm or more, the first layer is formed such that the AlN abundance ratio, which is the ratio of aluminum nitride to the total of aluminum nitride and gallium nitride in the region corresponding to the thickness of the first layer, is less than 78%. Method for manufacturing a group III nitride laminate.

[0116] (Appendix 9) The step of forming the first layer is a step of growing a crystal containing aluminum nitride so as to continuously cover the surface of the underlying substrate to form an initial growth layer, and a step of three-dimensionally growing a crystal containing aluminum nitride on the initial growth layer to form a three-dimensional structure layer having an uneven structure on the surface. In the step of forming the three-dimensional structure layer, growth is performed under conditions such that a C-plane is formed on the top surface of the uneven structure. Group III nitride laminate according to Appendix 8.

[0117] (Appendix 10) In the step of forming the three-dimensional structure layer, the crystal containing aluminum nitride is grown while continuously increasing the second growth rate. Method for manufacturing a group III nitride laminate according to Appendix 9.

[0118] (Appendix 11) In the step of forming the three-dimensional structure layer, when the growth rate at the start of growth is v1 and the growth rate at the end of growth is v2, the second growth rate is continuously increased such that v2 / v1 is greater than 1 times and less than or equal to 2 times. Method for manufacturing a group III nitride laminate according to Appendix 9 or 10.

[0119] (Appendix 12) In the step of forming the first layer, the first growth rate is 50 nm / h or more and 500 nm / h or less, and the second growth rate is 80 nm / h or more and 700 nm / h or less. The method for manufacturing a group-III nitride laminate according to any one of Appendices 9 to 11.

Explanation of symbols

[0120] 1 Group-III nitride laminate (laminate) 2 Semiconductor element 10 Substrate 20 Nucleation layer 21 Initial growth layer 22 Three-dimensional structure layer 23 Mixed layer 30 Channel layer 40 Third layer 41 Barrier layer 42 Cap layer

Claims

1. A lower base plate, a first layer provided on the lower base plate and containing aluminum nitride, a second layer provided on the first layer and containing gallium nitride, comprising: wherein the thickness of the first layer is 11.0 nm or more, and the AlN abundance ratio, which is the ratio of the amount of aluminum nitride to the total amount of aluminum nitride and gallium nitride in the region corresponding to the thickness of the first layer, is less than 78%, a group-III nitride laminate.

2. The average density of surface defects having a size of 0.165 μm or more and 2.0 μm or less on the surface of the second layer is 500 pieces / cm ２ or less, The group-III nitride laminate according to Claim 1.

3. provided on the second layer, In ｘ Al ｙ Ga （１－ｘ－ｙ） a third layer containing a group III nitride represented by a composition formula of N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1), and The average density of surface defects with a size of 0.165 μm or more and 2.0 μm or less on the surface of the third layer is 500 pieces / cm ２ or less, The group-III nitride laminate according to Claim 1.

4. The first layer comprises an initial growth layer provided so as to continuously cover the surface of the lower base plate, and a three-dimensional structure layer provided on the initial growth layer and having an uneven structure on the surface, wherein the upper surface of the top of the uneven structure is a C-plane, The group-III nitride laminate according to Claim 1 or Claim 2.

5. wherein the thickness of the first layer is 30 nm or less, The group-III nitride laminate according to Claim 1 or Claim 2.

6. wherein the AlN abundance ratio is 47% or more, The group-III nitride laminate according to Claim 1 or Claim 2.

7. A step of forming a first layer containing aluminum nitride on a lower base plate, and a step of forming a second layer containing gallium nitride on the first layer, having: In the step of forming the first layer, while making the thickness of the first layer 11.0 nm or more, the first layer is formed such that the AlN abundance ratio, which is the ratio of aluminum nitride to the total of aluminum nitride and gallium nitride in the region corresponding to the thickness of the first layer, is less than 78%, A method for manufacturing a group-III nitride laminate.

8. The step of forming the first layer includes a step of growing a crystal containing aluminum nitride so as to continuously cover the surface of the lower base plate to form an initial growth layer, and a step of three-dimensionally growing a crystal containing aluminum nitride on the initial growth layer to form a three-dimensional structure layer having an uneven structure on the surface, wherein in the step of forming the three-dimensional structure layer, growth is performed under conditions such that a C-plane is formed on the upper surface of the top of the uneven structure, The method for manufacturing a group-III nitride laminate according to Claim 7.

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