WO2016166949A1 - Semiconductor wafer and semiconductor device - Google Patents

Abstract

A semiconductor wafer is provided with a substrate and a group III nitride semiconductor layer provided on the substrate with a multilayer buffer layer interposed therebetween. The multilayer buffer layer includes at least a first buffer layer and a second buffer layer. The first buffer layer is in contact with the group III nitride semiconductor layer. The second buffer layer is in contact with the first buffer layer, and has a different composition from the first buffer layer. The average lattice constant of the multilayer buffer layer is lower than that of the group III nitride semiconductor layer, and the interface lattice distortion of the group III nitride semiconductor layer at the interface between the group III nitride semiconductor layer and the first buffer layer is lower than the interface lattice distortion of the first buffer layer at the interface between the first buffer layer and the second buffer layer.

Description

Semiconductor wafer and semiconductor device Cross-reference of related applications

This application is based on Japanese Patent Application No. 2015-84187 filed on April 16, 2015, the contents of which are incorporated herein by reference.

The present disclosure relates to a semiconductor wafer and a semiconductor device.

Research is being conducted on techniques for producing semiconductor wafers by vapor-phase growth of group III nitride semiconductor layers on a substrate. Patent Document 1 discloses a technique for growing a multilayer buffer layer on a silicon substrate and growing a group III nitride semiconductor layer (functional layer) on the multilayer buffer layer. Patent Document 1 discloses a composition modulation layer in which a first unit layer and a second unit layer having different compositions are repeatedly laminated, a termination layer grown on the composition modulation layer, and an intermediate layer (strain strengthening) grown on the termination layer. A multilayer buffer layer having a unit laminated structure and having a plurality of unit laminated structures. When a material with a smaller coefficient of thermal expansion than the group III nitride semiconductor is used as the substrate for growing the group III nitride semiconductor layer, the group III nitride semiconductor is cooled from a high temperature (during vapor phase growth) to a low temperature (typically room temperature). When it is done, tensile strain occurs inside. The multilayer buffer layer has a compressive strain. In Patent Document 1, an intermediate layer (strain strengthening layer) is provided at a portion in contact with the group III nitride semiconductor layer, thereby strengthening the compressive strain of the multilayer buffer layer and canceling the tensile strain of the group III nitride semiconductor layer. .

However, when the compressive strain of the multilayer buffer layer is strong at the portion in contact with the group III nitride semiconductor layer, the group III nitride semiconductor layer grows while maintaining the compressive strain. As a result, strain-induced step bunching is likely to occur in the group III nitride semiconductor layer, and the surface of the group III nitride semiconductor layer cannot be smoothed.

Special table 2011-155498 gazette

An object of the present disclosure is to provide a semiconductor wafer and a semiconductor device in which the surface of the group III nitride semiconductor layer is smooth.

In the first aspect of the present disclosure, a semiconductor wafer includes a substrate and a group III nitride semiconductor layer provided on the substrate via a multilayer buffer layer. The multilayer buffer layer includes at least a first buffer layer and a second buffer layer. The first buffer layer is in contact with the group III nitride semiconductor layer. The second buffer layer is in contact with the first buffer layer and has a composition different from that of the first buffer layer. The multilayer buffer layer has an average lattice constant smaller than that of the group III nitride semiconductor layer. Interfacial lattice distortion of the group III nitride semiconductor layer at the interface between the group III nitride semiconductor layer and the first buffer layer is caused by the first buffer layer at the interface between the first buffer layer and the second buffer layer. Less than interfacial lattice distortion.

In the semiconductor wafer, the interfacial lattice strain of the group III nitride semiconductor layer at the interface between the group III nitride semiconductor layer and the first buffer layer (hereinafter referred to as the interfacial lattice strain of the group III nitride semiconductor layer) is the first buffer. It is smaller than the interface lattice strain of the first buffer layer at the interface between the layer and the second buffer layer (hereinafter referred to as the interface lattice strain of the first buffer layer). Therefore, the group III nitride semiconductor layer is suppressed from growing while having the compressive strain of the first buffer layer. By making the interface lattice strain of the group III nitride semiconductor layer smaller than the interface lattice strain of the first buffer layer, it is possible to suppress the occurrence of strain-induced step bunching in the group III nitride semiconductor layer. The semiconductor wafer described above can smooth the surface of the group III nitride semiconductor layer.

In a second aspect of the present disclosure, a semiconductor device includes a substrate and a group III nitride semiconductor layer provided on the substrate via a multilayer buffer layer. A semiconductor element is formed on the surface side of the group III nitride semiconductor layer. The multilayer buffer layer includes at least a first buffer layer and a second buffer layer. The first buffer layer is in contact with the group III nitride semiconductor layer. The second buffer layer is in contact with the first buffer layer and has a composition different from that of the first buffer layer. The multilayer buffer layer has an average lattice constant smaller than that of the group III nitride semiconductor layer. Interfacial lattice distortion of the group III nitride semiconductor layer at the interface between the group III nitride semiconductor layer and the first buffer layer is caused by the first buffer layer at the interface between the first buffer layer and the second buffer layer. Less than interfacial lattice distortion.

In the semiconductor device, the interfacial lattice strain of the group III nitride semiconductor layer (hereinafter referred to as the interfacial lattice strain of the group III nitride semiconductor layer) at the interface between the group III nitride semiconductor layer and the first buffer layer is the first buffer. It is smaller than the interface lattice strain of the first buffer layer at the interface between the layer and the second buffer layer (hereinafter referred to as the interface lattice strain of the first buffer layer). Therefore, the group III nitride semiconductor layer is suppressed from growing while having the compressive strain of the first buffer layer. By making the interface lattice strain of the group III nitride semiconductor layer smaller than the interface lattice strain of the first buffer layer, it is possible to suppress the occurrence of strain-induced step bunching in the group III nitride semiconductor layer. The semiconductor wafer described above can smooth the surface of the group III nitride semiconductor layer.

The above and other objects, features, and advantages of the present disclosure will become more apparent from the following detailed description with reference to the accompanying drawings. The drawing
FIG. 1 schematically shows a cross-sectional view of the semiconductor wafer of the first embodiment, FIG. 2 shows the characteristics of the semiconductor wafer of the first embodiment. FIG. 3 schematically shows a cross-sectional view of the semiconductor wafer of the second embodiment, FIG. 4 shows the characteristics of the semiconductor wafer of the second embodiment. FIG. 5 shows the characteristics of the semiconductor wafer of the third embodiment. FIG. 6 shows the characteristics of the semiconductor wafer of Comparative Example 1. FIG. 7 shows the characteristics of the semiconductor wafer of Comparative Example 2, FIG. 8 shows the results of the surface roughness of the semiconductor wafer for the examples and comparative examples.

In the semiconductor wafer disclosed in this specification, a group III nitride semiconductor layer is provided on a substrate via a multilayer buffer layer. The multilayer buffer layer has an average lattice constant smaller than that of the group III nitride semiconductor layer. The multilayer buffer layer includes at least a first buffer layer and a second buffer layer. The first buffer layer is in contact with the group III nitride semiconductor layer. The second buffer layer is in contact with the first buffer layer. The first buffer layer and the second buffer layer have different compositions. In the semiconductor wafer disclosed in the present specification, the interfacial lattice strain of the group III nitride semiconductor layer at the interface between the group III nitride semiconductor layer and the first buffer layer is the first at the interface between the first buffer layer and the second buffer layer. It is smaller than the interfacial lattice distortion of one buffer layer.

In the semiconductor wafer, the interfacial lattice strain of the group III nitride semiconductor layer at the interface between the group III nitride semiconductor layer and the first buffer layer (hereinafter referred to as the interfacial lattice strain of the group III nitride semiconductor layer) is the first buffer. It is smaller than the interface lattice strain of the first buffer layer at the interface between the layer and the second buffer layer (hereinafter referred to as the interface lattice strain of the first buffer layer). Therefore, the group III nitride semiconductor layer is suppressed from growing while having the compressive strain of the first buffer layer. By making the interface lattice strain of the group III nitride semiconductor layer smaller than the interface lattice strain of the first buffer layer, it is possible to suppress the occurrence of strain-induced step bunching in the group III nitride semiconductor layer. The semiconductor wafer described above can smooth the surface of the group III nitride semiconductor layer.

As used herein, the term “multilayer buffer layer” does not mean a so-called “low temperature buffer layer” for reducing the lattice constant difference between the substrate and the semiconductor element layer, but a group III nitride semiconductor layer. In order to relieve the thermal strain generated in the group III nitride semiconductor layer when the group III nitride semiconductor layer changes from a high temperature to a low temperature after film formation. “Strain-induced step bunching” is a mechanism of step bunching occurrence discovered and proposed by Tersoff et al. And disclosed in “Physical Review Letters Volume 75 2730 (1995)”. The mechanism is that when a strain is generated on a semiconductor surface formed by periodic step terraces, an attractive force between steps is proportional to the square of the strain and inversely proportional to the distance between steps. The attractive force between steps forms a bundle of steps (step bunching) one after another, and a step is formed on the surface of the semiconductor wafer, so that the surface smoothness of the semiconductor wafer is lowered. Therefore, generation of step bunching can be suppressed by reducing the distortion of the semiconductor surface.

Further, the strain-induced step bunching becomes more prominent in a wafer (so-called wafer having an off angle) whose main axis is slightly inclined with respect to the normal of the growth surface. For example, in the case of a group III nitride semiconductor, the c-axis is generally the main axis. Therefore, in the case of a group III nitride semiconductor, it can be said that the state where the c-axis is inclined with respect to the normal of the growth surface has an off-angle. Typically, by providing an off-angle, the growth surface has a periodic structure in which step terraces are regularly repeated, and the flatness of the wafer surface can be improved. In addition, by providing an off-angle, it is possible to obtain advantages such as realizing a steep heterojunction and improving the interface characteristics of a semiconductor element formed on a wafer. However, the distance between steps on the wafer surface having an off angle is small under a situation where a strain-induced step attractive force is applied. Therefore, in the wafer having an off-angle, the inter-step attractive force cannot be ignored. For this reason, step bunching is likely to occur on the surface of the semiconductor wafer under a situation in which a strain-induced attraction between steps works.

Further, the strain-induced step bunching in the nitride semiconductor layer is more remarkable in the group III nitride semiconductor whose main component is gallium as the group III element. Step bunching occurs in the process where group III atomic species on the terrace are diffused into the step edge. Therefore, step bunching is likely to occur in an environment that satisfies the condition of “average distance between steps × (1/2) ≦ surface diffusion length of group III atomic species”. In group III atomic species, gallium is known to have a longer surface diffusion length than aluminum and indium. Therefore, strain-induced step bunching is likely to occur in a group III nitride semiconductor mainly containing gallium, particularly gallium nitride. As used herein, “gallium nitride” includes gallium nitride containing less than 1% impurities (B, Al, In, C, Si, Ge, Mg, S, Fe, As, Sb, etc.). Is also included.

In addition, “the multilayer buffer layer has a smaller lattice constant than the group III nitride semiconductor layer” means that when comparing the entire multilayer buffer layer and the entire group III nitride semiconductor layer, the lattice constant of the multilayer buffer layer ( Mean lattice constant) is smaller than the lattice constant (average lattice constant) of the group III nitride semiconductor layer. Therefore, when a portion of the multilayer buffer layer is compared with a portion of the group III nitride semiconductor layer, the lattice constant of a portion of the multilayer buffer layer may be larger than the lattice constant of a portion of the group III nitride semiconductor layer. .

In the present specification, the terms “average lattice strain” and “interface lattice strain” are used. Here, a case where the first layer is laminated on the surface of the second layer will be described with respect to “average lattice strain” and “interface lattice strain”. The ideal lattice constant (theoretical lattice constant) in the plane direction of the first layer (direction perpendicular to the crystal growth direction) is a ₁ , the actual lattice constant is a ₁ ′, and the ideal lattice in the plane direction of the second layer The constant is a ₂ , and the actual lattice constant is a ₂ ′. Average lattice strain ε ₁ of the first layer, average lattice strain ε _{2 of} the second layer, interface lattice strain ε _If1 of the first layer at the interface between the first layer and the second layer, strain relaxation of the first layer with respect to the second layer The rate R ₁ is expressed by the following equation.

ε ₁ = (a ₁ '−a ₁ ) / a ₁
ε ₂ = (a ₂ '-a ₂ ) / a ₂
ε _If1 = (a ₂ '-a ₁ ) / a ₁
R ₁ = (a ₁ ′ −a ₂ ′) / (a ₁ −a ₂ ′)
In the semiconductor device disclosed in this specification, a group III nitride semiconductor layer is provided over a substrate via a multilayer buffer layer, and a semiconductor element is formed on the surface side of the group III nitride semiconductor layer. The multilayer buffer layer includes at least a first buffer layer and a second buffer layer. The first buffer layer is in contact with the group III nitride semiconductor layer. The second buffer layer is in contact with the first buffer layer. The first buffer layer and the second buffer layer have different compositions. The multilayer buffer layer has an average lattice constant smaller than that of the group III nitride semiconductor layer. The interface lattice strain of the group III nitride semiconductor layer at the interface between the group III nitride semiconductor layer and the first buffer layer is smaller than the interface lattice strain of the first buffer layer at the interface between the first buffer layer and the second buffer layer.

The following summarizes the features of the technology disclosed in this specification. The items described below have technical usefulness independently.

The semiconductor wafer includes a substrate, a multilayer buffer layer provided on the substrate, and a group III nitride semiconductor layer provided on the multilayer buffer layer. The substrate has a smaller coefficient of thermal expansion than the group III nitride semiconductor layer. Specifically, in the substrate, the temperature integral value of the linear expansion coefficient of 300 to 1300K is smaller than the temperature integral value of the linear expansion coefficient of 300 to 1300K in the a-axis direction of the material constituting the group III nitride semiconductor layer. More specifically, the material of the substrate is silicon (Si) or silicon carbide (SiC). The thickness of the substrate is 0.1 to 2 mm. Preferably, an interface layer and an aluminum nitride (AlN) layer are provided between the substrate and the multilayer buffer layer. As the material for the interface layer, Al ₂ O ₃ , AlON, or the like can be used. By providing the interface layer, the orientation of the aluminum nitride layer is improved, and the crystallinity of the multilayer buffer layer and the group III nitride semiconductor layer can be improved. The interface layer can be omitted. The thickness of the aluminum nitride layer is 10 to 500 nm. When the substrate is silicon, the aluminum nitride layer preferably has a thickness of 50 to 500 nm. When the substrate is silicon carbide, the thickness of the aluminum nitride layer is preferably 10 to 100 nm. Aluminum nitride has the smallest lattice constant among group III nitride semiconductors. Therefore, by providing the aluminum nitride layer, a necessary amount of compressive strain can be applied to the multilayer buffer layer described later. In addition, as described above, the interface layer and the aluminum nitride layer can be omitted, and the multilayer buffer layer can be formed directly on the surface of the substrate.

The multilayer buffer layer includes at least a first buffer layer in contact with the group III nitride semiconductor layer and a second buffer layer in contact with the first buffer layer. The multilayer buffer layer may include a third buffer layer in contact with the second buffer layer, a fourth buffer layer in contact with the third buffer layer, and the like. That is, the multilayer buffer layer has a multilayer structure including two or more layers. The first buffer layer is a layer located on the most surface side (group III nitride semiconductor layer side) among the layers constituting the multilayer buffer layer. The first buffer layer and the second buffer layer have different compositions. Also, the first buffer layer is not coherent with respect to the second buffer layer. Specifically, when the average lattice constant of the first buffer layer and the average lattice constant of the second buffer layer are compared to 3 digits after the decimal point in angstrom units, they are different. Since the first buffer layer is not coherent with the second buffer layer, strain relaxation occurs in the first buffer layer. Therefore, it is possible to prevent the distortion of the second buffer layer from being directly transferred to the group III nitride semiconductor layer. That is, since the first buffer layer is not coherent with respect to the second buffer layer, the strain of the group III nitride semiconductor layer can be reduced and the strain-induced step bunching can be suppressed. Preferably, the strain relaxation rate (R ₁ ) of the _first buffer layer with respect to the second buffer layer is 0.8 or more. By setting the strain relaxation rate (R ₁ ) to 0.8 or more, sufficient strain relaxation occurs in the first buffer layer, and the strain of the group III nitride semiconductor layer can be effectively reduced.

By setting the thickness of the first buffer layer to 0.05 μm or more, the first buffer layer can be prevented from growing coherently with respect to the second buffer layer. More preferably, the thickness of the first buffer layer is 0.2 μm or more. By setting the thickness of the first buffer layer to 0.2 μm or more, the first buffer layer is more reliably connected to the second buffer regardless of the size of the interfacial lattice strain of the first buffer layer and the growth conditions of the first buffer layer. It is possible to prevent coherent growth with respect to the layer.

When comparing the entire multilayer buffer layer and the entire group III nitride semiconductor layer, the average lattice constant of the multilayer buffer layer is smaller than the average lattice constant of the group III nitride semiconductor layer. Further, when the entire multilayer buffer layer and aluminum nitride are compared, the average lattice constant of the multilayer buffer layer is larger than the lattice constant of aluminum nitride. The multilayer buffer layer preferably has a compressive strain as a whole. Due to the compressive strain of the multilayer buffer layer, it occurs in the group III nitride semiconductor layer when the temperature of the group III nitride semiconductor layer changes from the growth temperature (high temperature) to room temperature (low temperature) after film formation. Tensile strain can be offset. It is preferable that no tensile strain remains in the first buffer layer. In other words, in the first buffer layer, no strain remains or compressive strain remains. More preferably, the first buffer layer has a compressive strain remaining. Note that “the compressive strain remains in the first buffer layer” corresponds to the average lattice strain of the first buffer layer being a negative value. In the semiconductor wafer disclosed in this specification, the layer having the largest absolute value of the average lattice strain among the layers of the multilayer structure constituting the multilayer buffer layer is a layer other than the first buffer layer. In other words, the absolute value of the average lattice strain of the first buffer layer is not the maximum among the layers of the multilayer structure constituting the multilayer buffer layer.

As the material of the multilayer buffer layer, a nitride semiconductor represented by Al _x Ga _1-x N (0 <x <1) can be used. The first buffer layer is preferably a nitride semiconductor represented by Al _x Ga _1-x N (0 <x <0.5). More preferably, the first buffer layer is a nitride semiconductor represented by Al _x Ga _1-x N (0 <x <0.2). Since the average lattice constant of the first buffer layer is closer to GaN (nitride semiconductor) than to AlN, the difference in average lattice constant between the group III nitride semiconductor layer and the first buffer layer is reduced, and the group III nitride semiconductor layer is reduced. It is possible to reduce the interfacial lattice distortion.

When the multilayer buffer layer is a nitride semiconductor represented by Al _x Ga _1-x N (0 <x <1), each layer of the multilayer buffer layer may have a different “x” value. In addition, it is preferable that the value of “x” becomes smaller toward the surface (the group III nitride semiconductor layer side). That is, the multilayer buffer layer preferably has a multilayer structure in which the Al composition becomes smaller toward the surface. Alternatively, the multilayer buffer layer may have a structure in which different materials are repeatedly stacked. For example, the multilayer buffer layer can have a structure in which a laminated structure of AlN and GaN is repeatedly provided. Alternatively, the multilayer buffer layer may have a structure in which a laminated structure of AlN and AlGaN is repeatedly provided. The thickness of the multilayer buffer layer can be 0.5 to 10 μm.

The group III nitride semiconductor layer may be a single layer or multiple layers. Note that the single-layer group III nitride semiconductor layer means that the composition from the front surface to the back surface of the group III nitride semiconductor layer is the same. In addition, the multiple group III nitride semiconductor layer means that the group III nitride semiconductor layer includes a plurality of layers having different compositions of group III elements. In the case of a single-layer group III nitride semiconductor layer, the group III nitride semiconductor layer is preferably a group III nitride semiconductor mainly composed of gallium (Ga). Further, in the case of a multi-layer group III nitride semiconductor layer, it is preferable that at least a layer material in contact with the first buffer layer among the plurality of layers is a group III nitride semiconductor mainly composed of gallium. The “group III nitride semiconductor mainly composed of gallium” typically means gallium nitride (GaN), and B, Al, In on the order of atomic percent of less than 1% with respect to GaN as impurities. Further, gallium nitride containing elements such as C, Si, Ge, Mg, S, Fe, As, and Sb is also included.

The group III nitride semiconductor layer preferably contains carbon. When the group III nitride semiconductor layer contains carbon, leakage current can be prevented from flowing when the semiconductor device is in the off state. The group III nitride semiconductor layer can be used as a functional layer (a layer constituting a semiconductor element). A single group III nitride semiconductor layer can also be used as a functional layer. In addition, among the multiple group III nitride semiconductor layers, a group III nitride semiconductor layer provided on the surface side of the layer in contact with the first buffer layer can also be used as a functional layer. In the semiconductor wafer disclosed in this specification, the interface lattice strain of the nitride semiconductor element (interface lattice strain of the nitride semiconductor element) at the interface between the group III nitride semiconductor layer and the first buffer layer is less than that of the first buffer layer. It is smaller than the interface lattice strain of the first buffer layer at the interface of the second buffer layer (interface lattice strain of the first buffer layer). Since the interfacial lattice strain of the nitride semiconductor element is smaller than the interfacial lattice strain of the first buffer layer, it is possible to suppress the occurrence of strain-induced step bunching in the group III nitride semiconductor layer.
(First embodiment)
The semiconductor wafer 1 will be described with reference to FIG. The semiconductor wafer 1 includes a silicon substrate 2, an interface layer 4 formed on the silicon substrate 2, an AlN layer 6 formed on the interface layer 4, a multilayer buffer layer 8 formed on the AlN layer 6, A group III nitride semiconductor layer 10 formed on the multilayer buffer layer 8 and a nitride semiconductor functional layer 12 formed on the group III nitride semiconductor layer 10 are provided. The thickness T2 of the silicon substrate 2 is 675 μm. The material of the interface layer 4 is Al ₂ O ₃ . The thickness T4 of the interface layer 4 is adjusted to be less than 3 nm. The thickness T6 of the AlN layer 6 is adjusted to 0.3 μm. The material of the multilayer buffer layer 8 is Al _x Ga _1-x N. The thickness T8 of the multilayer buffer layer 8 is adjusted to 2.5 μm.

The multilayer buffer layer 8 has a four-layer structure including a first buffer layer 8a, a second buffer layer 8b, a third buffer layer 8c, and a fourth buffer layer 8d. The first buffer layer 8 a is in contact with the group III nitride semiconductor layer 10, and the fourth buffer layer 8 d is in contact with the AlN layer 6. Note that the value of “x” of Al _x Ga _1-x N increases from the first buffer layer 8a toward the fourth buffer layer 8d. The thickness of the first buffer layer 8a is adjusted to 0.8 μm, the thickness of the second buffer layer 8b is adjusted to 0.2 μm, and the thickness of the third buffer layer 8c is adjusted to 0.5 μm. The thickness of the fourth buffer layer 8d is adjusted to 1.0 μm. The interface layer 4 is formed by using an atomic layer deposition method (ALD method). As the raw material of the interface layer 4, trimethylaluminum is used as the Al raw material, and ozone is used as the O raw material. The AlN layer 6 and the multilayer buffer layer 8 are formed using a metal organic chemical vapor deposition method (MOCVD method). As raw materials for the AlN layer 6 and the multilayer buffer layer 8, trimethylaluminum is used as the Al raw material, trimethylgallium is used as the Ga raw material, and ammonia is used as the N raw material. The growth temperature is about 1000 ° C.

The material of the group III nitride semiconductor layer 10 is gallium nitride (GaN). The thickness of group III nitride semiconductor layer 10 is adjusted to 0.6 μm, and the thickness of nitride semiconductor functional layer 12 is adjusted to 0.4 μm. The group III nitride semiconductor layer 10 contains 2 × 10 ¹⁸ cm ^{−3 of} carbon. The carbon introduced into the group III nitride semiconductor layer 10 prevents leakage current from flowing between the semiconductor element formed in the nitride semiconductor functional layer 12 and the silicon substrate 2. The nitride semiconductor functional layer 12 is i-type that does not contain impurities (it may contain unavoidable carbon). A semiconductor element can be formed using the nitride semiconductor functional layer 12.

FIG. 2 shows the characteristics of the AlN layer 6, the multilayer buffer layer 8, and the group III nitride semiconductor layer 10 for the semiconductor wafer 1. The Al composition (x) corresponds to the value of “x” when each layer is represented by Al _x Ga _1-x N. Hereinafter, a method for calculating the characteristics of each layer will be described. In the following calculation formula, the group III nitride semiconductor layer 10 is denoted by GaN, the first buffer layer 8a is denoted by TOP, and the “i” th buffer layer from the first buffer layer 8a is denoted by i. For example, the second buffer layer 8b is “i = 1”, and the third buffer layer 8c is “i = 2”.

The average lattice constant of each layer was measured by X-ray reciprocal lattice mapping. Specifically, the peak of the reciprocal lattice mapping 114 diffraction and 004 diffraction, GaN, TOP, was determined 114 side of the plane spacing _{d 114j} and 004 face the surface interval _{d 004J} "i" (j = GaN, TOP , I). The average lattice constant a ′ _{j in the} a-axis direction and the average lattice constant c ′ _j in the c-axis direction were calculated from the following formulas (1) and (2) using the face spacings d _114j and d _004j . FIG. 2 shows an average lattice constant a ′ _j calculated by the following equation (1).
Expression (1): a ′ _j = 2 × d _110j = 2 × {(1 / d _114j ² ) − (1 / d _004j ² )} − ^0.5
Formula (2): c ′ _j = 4 × d _004j
Next, the Al composition (x) will be described. The Al composition (x) is obtained by using the interplanar spacings d _114j and d _004j obtained by the X-ray reciprocal mapping and the results obtained by the above formulas (1) and (2), using “x” in the following formula (3). ”Was calculated. Note that a _AlN is the ideal lattice constant of the a-axis of AlN, and c _AlN is the ideal lattice constant of the c-axis of AlN. The ideal lattice constants a _AlN and c _AlN of _AlN are 3.112Å (= 0.3112 nm) and 4.980Å (= 0.4980 nm), respectively. The ideal lattice constants a _GaN and c _GaN of _GaN are 3.189 で (= 0.0.3189 nm) and 5.185Å (= 0.5185 nm), respectively. Note that σ = 0.38 was used as the Poisson's ratio. The Poisson's ratio is disclosed in “T. Detchprohm et.al., Jpn.J.Appl.phys.31, L454 (1992)”.
Formula (3): (1 + σ) × (a _GaN −a _AlN ) × (c _GaN −c _AlN ) × x ² − {[(1 + σ) × c _GaN ] −c ′ _j } × (a _GaN −a _AlN ) + [(1 + σ) × a _GaN −σ × a ′ _j ] × (c _GaN −c _AlN )} × x + σ × c _GaN × (a _GaN −a ′ _j ) + a _GaN × (c _GaN −c ′ _j ) = 0
Next, a method for calculating the ideal lattice constant of the a axis of the buffer layer will be described. As described above, the ideal lattice constant a _GaN of _GaN is 3.189 Å (= 0.3189 nm), and the ideal lattice constant a _AlN of _AlN is 3.112 Å (= 0.3112 nm). The ideal lattice constant a _j of each layer (TOP, i) of the buffer layer was calculated from the following formula (4) using the value of “x” of each layer as a variable when each layer was represented by Al _x Ga _1-x N.
Formula (4): a _j = 3.112 × x + 3.189 × (1−x)
Then, the average lattice strain _{ε j (j = GaN, TOP} , i) and the interface lattice distortion epsilon _{If_GaN} group III nitride semiconductor layer, and the interface lattice distortion epsilon _{If_TOP} the first buffer layer, the strain relaxation rate _{R j} (J = GaN, TOP, i) will be described. ε _j , ε _{If_GaN} , ε _{If_TOP} , and R _j were calculated from the following equations (5) to (8) using the results obtained from the above equations (1) to (4). The average lattice strain and the interface lattice strain are positive strains when they are positive values, and compressive strains when they are negative values. Therefore, the magnitude relation of the lattice distortion does not compare actual numerical values but compares absolute values. In the following formula (8), when j = GaN, j + 1 corresponds to TOP. Similarly, when j = TOP, j + 1 corresponds to i = 1.
Expression (5): ε _j (j = GaN, TOP, i) = (a ′ _j −a _j ) / a _j
Formula (6): ε _If — _GaN = (a ′ _TOP −a _GaN ) / a _GaN
Expression (7): ε _If — _TOP = (a ′ ₁ −a _TOP ) / a _TOP
Formula (8): R _j (j = GaN, TOP, i) = (a _j '−a _{j + 1} ′) / (a _j −a _{j + 1} ′)
As shown in FIG. 2, the semiconductor wafer 1, the interface lattice distortion of the group III nitride semiconductor layer 10 (ε _{If_GaN)} is the interface lattice distortion (ε _{If_TOP)} of the first buffer layer 8a smaller. The average lattice strain epsilon _'1 of the second buffer layer 8b is the maximum among the average lattice strain of the buffer layer 8a ~ 8d. That is, the average lattice strain (ε _TOP ) of the first buffer layer 8 a is not the maximum among the average lattice strains (ε _{1 to 3} ) of the layers constituting the multilayer buffer layer 8. The average lattice constant of the first buffer layer 8a is 3.1815３ (= 0.31815 nm), and the average lattice constant of the second buffer layer 8b is 3.1378Å (= 0.31378 nm). Therefore, it can be said that the first buffer layer 8a is not coherent with respect to the second buffer layer 8b. Note that due to an error in the X-ray reciprocal lattice mapping measurement (accuracy of the X-ray reciprocal lattice mapping measurement), even if the first buffer layer and the second buffer layer are in a coherent state, the average lattice constants of both may be different. . However, even if the accuracy of the X-ray reciprocal lattice mapping measurement is taken into consideration, the average lattice constant of the first buffer layer and the second buffer layer is compared when the average lattice constant is compared to the third decimal place in units of angstroms (Å). If there is a difference, it can be said that the first buffer layer and the second buffer layer are not coherent.
(Second embodiment)
The semiconductor wafer 11 will be described with reference to FIG. The semiconductor wafer 11 is a modification of the semiconductor wafer 1. The semiconductor wafer 11 has a larger number of layers constituting the multilayer buffer layer 18 than the semiconductor wafer 1. About the semiconductor wafer 11, about the substantially same structure as the semiconductor wafer 1, description may be abbreviate | omitted by attaching | subjecting the same reference number as the semiconductor wafer 1. FIG.

In the semiconductor wafer 11, the multilayer buffer layer 18 has a five-layer structure including a first buffer layer 18a, a second buffer layer 18b, a third buffer layer 18c, a fourth buffer layer 18d, and a fifth buffer layer 18e. The first buffer layer 18a is a composition gradient layer having an Al composition (x) of 0.08 to 0.03. The average Al composition (x) of the first buffer layer 18a is 0.033. In FIG. 4, a value obtained by rounding off the third decimal place (ie, x = 0.03) is shown. The multilayer buffer layer 18 has a thickness T18 of 2.43 μm. The thickness T18a of the first buffer layer 18a is adjusted to 0.63 μm. The thicknesses T18b to T18e of the second to fifth buffer layers 18b to 18e are adjusted to 0.1, 0.2, 0.5, and 1.0 μm, respectively.

FIG. 4 shows the characteristics of the AlN layer 6, the multilayer buffer layer 18, and the group III nitride semiconductor layer 10 for the semiconductor wafer 11. As shown in FIG. 4, the interface lattice distortion of the group III nitride semiconductor layer 10 (ε _{If_GaN)} is, interface lattice distortion of the first buffer layer 18a (ε _{If_TOP)} smaller. The average lattice strain epsilon _'1 of the second buffer layer 18b is the largest among the average lattice strain of the buffer layer 18a ~ 18e (absolute value). That is, the average lattice strain (ε _TOP ) of the first buffer layer 18 a is not the maximum among the average lattice strains (ε _{1 to 4} ) of the layers constituting the multilayer buffer layer 18. The average lattice constant of the first buffer layer 18a is 3.1804３ (= 0.31804 nm), and the average lattice constant of the second buffer layer 18b is 3.1421Å (= 0.42121 nm). Therefore, it can be said that the first buffer layer 18a is not coherent with respect to the second buffer layer 18b.

In the semiconductor wafer 11, the group III nitride semiconductor layer 10 has a thickness adjusted to 0.75 μm and contains 4 × 10 ¹⁹ cm ^{−3 of} carbon. The nitride semiconductor functional layer 12 is adjusted to a thickness of 0.48 μm and is i-type. As an example, in the semiconductor wafer 11, an AlGaN barrier layer 20 nm and a GaN cap layer 5 nm are formed on the surface of the nitride semiconductor functional layer 12. Thereby, a semiconductor device having a HEMT structure that operates as a heterojunction two-dimensional electron gas and a channel can be manufactured.

Various semiconductor devices can be manufactured using the semiconductor wafers 1 and 11 described above. As an example, a field effect transistor can be manufactured by forming a gate electrode, a source electrode, and a drain electrode on the surfaces of the semiconductor wafers 1 and 11. In this case, the gate electrode is opposed to the nitride semiconductor functional layer 12 through the insulating film. The source electrode and the drain electrode are in ohmic contact with the nitride semiconductor functional layer 12. The nitride semiconductor functional layer 12 functions as a channel layer of the field effect transistor. As another example, a Schottky electrode and an ohmic electrode can be formed on the surfaces of the semiconductor wafers 1 and 11 to manufacture a Schottky diode. In this case, the nitride semiconductor functional layer 12 functions as a current-carrying layer for the Schottky diode.
(Third embodiment)
A semiconductor wafer according to a third embodiment will be described. The structure of the semiconductor wafer of this embodiment is substantially the same as that of the semiconductor wafer 1 of the first embodiment. However, the semiconductor wafer of this example has different characteristics of each layer. Hereinafter, differences from the semiconductor wafer 1 will be described with reference to FIG.

In the semiconductor wafer of this embodiment, the average lattice strain ε ′ _TOP of the first buffer layer 8a is the largest among the average lattice strains (absolute values) of the buffer layers 8a to 8d. Different from wafer 1. That is, the portion of the buffer layer in contact with the group III nitride semiconductor layer 10 has the highest compressive strain. This difference can be caused by adjusting the manufacturing conditions (buffer layer growth conditions). Also in the semiconductor wafer of the present embodiment, the interface lattice distortion of the group III nitride semiconductor layer 10 (ε _{If_GaN)} is in that the interface lattice distortion (ε _{If_TOP)} is smaller than the first buffer layer 8a, the first embodiment The semiconductor wafer 1 has common characteristics.
(Experimental example)
The surface roughness of the semiconductor wafers of Examples 1 to 3 was measured. The results are shown in FIG. The surface roughness is a root mean square roughness (RMS) of an atomic force microscope in an area of 50 μm square. In addition, the surface roughness of the semiconductor wafers of Comparative Examples 1 and 2 was also measured. First, the characteristics of the semiconductor wafers of Comparative Examples 1 and 2 will be described.

The characteristics of the semiconductor wafer of Comparative Example 1 are shown in FIG. The semiconductor wafer of Comparative Example 1 includes four buffer layers having different Al compositions (x). The thickness of the first buffer layer is 0.5 μm. The group III nitride semiconductor layer contains 4 × 10 ¹⁸ cm ^{−3 of} carbon and has a thickness of 0.4 μm. In Comparative Example 1, an i-type nitride semiconductor functional layer is laminated on the surface of the group III nitride semiconductor layer. The nitride semiconductor functional layer has a thickness of 0.6 μm. In the semiconductor wafer of Comparative Example 1, the average lattice strain ε ′ _TOP of the first buffer layer is the largest among the average lattice strains (absolute values) of the first to fourth buffer layers. Further, the interfacial lattice strain (ε _{If_GaN} ) of the group III nitride semiconductor layer is larger than the interfacial lattice strain (ε _{If_TOP} ) of the first buffer layer.

The characteristics of the semiconductor wafer of Comparative Example 2 are shown in FIG. The semiconductor wafer of Comparative Example 1 includes four buffer layers having different Al compositions (x). The thickness of the first buffer layer is 0.44 μm. The group III nitride semiconductor layer contains 1.5 × 10 ¹⁸ cm ^{−3 of} carbon and has a thickness of 0.66 μm. In Comparative Example 2, an i-type nitride semiconductor functional layer is laminated on the surface of the group III nitride semiconductor layer. The nitride semiconductor functional layer has a thickness of 0.33 μm. In Comparative Example 2, an AlGaN barrier layer of 20 nm and a GaN cap layer of 5 nm are formed on the surface of the i-type nitride semiconductor functional layer. In the semiconductor wafer of Comparative Example 2, the average lattice strain ε ′ _TOP of the first buffer layer is the largest among the average lattice strains (absolute values) of the first to fourth buffer layers. Further, the interfacial lattice strain (ε _{If_GaN} ) of the group III nitride semiconductor layer is larger than the interfacial lattice strain (ε _{If_TOP} ) of the first buffer layer.

For Examples 1 to 3 and Comparative Examples 1 and 2, the inclination angle of the group III nitride semiconductor layer with respect to the c-axis growth surface was measured by X-ray diffraction. The c-axes of the group III nitride semiconductor layers of Examples 1 to 3 were inclined 0.22 degrees, 0.31 degrees, and 0.30 degrees in the m-axis direction with respect to the normal to the growth surface, respectively. Further, the c-axes of the group III nitride semiconductor layers of Comparative Examples 1 and 2 were inclined by 0.26 degrees and 0.33 degrees in the m-axis direction with respect to the normal to the growth surface, respectively.

In Figure 8, the interface lattice distortion of the group III nitride semiconductor layer (epsilon _{If_GaN)} is marked with ○ in experimental examples that satisfy the interface lattice distortion of the first buffer layer a (ε _{If_TOP)} condition that is less than (condition 1) In the experiment example which is not satisfied, x is marked. In addition, in the experimental example that satisfies the condition (Condition 2) that the average lattice strain (absolute value, | ε _TOP |) of the first buffer layer is not the maximum among the layers constituting the multilayer buffer layer, ○ is marked and not satisfied X is described in the experimental example. In addition, “◎” is marked for an experimental example having an RMS value of 5.0 or less, “◯” is marked for an experimental example of 10.0 or less, and “x” is marked for an experimental example of 10.0 or more.

As shown in FIG. 8, when the condition 1 is satisfied, it can be confirmed that the RMS value becomes small and a semiconductor wafer having a smooth surface can be obtained (Examples 1 to 3). In particular, when both condition 1 and condition 2 are satisfied, the RMS value is 5.0 or less, and it can be confirmed that the surface of the semiconductor wafer is extremely smooth. In the semiconductor wafer of the example, the average lattice strain (absolute value) of the group III nitride semiconductor layer is 3.97 × 10 ⁻⁴ to 1.03 × 10 ⁻³ , and the group III nitride semiconductor of the comparative example It is less than half of the average lattice strain (2.44 × 10 ⁻³ , 2.04 × 10 ⁻³ ) of the layer. By smaller than interface lattice distortion of the group III nitride semiconductor layer (ε _{If_GaN)} interface lattice distortion of the first buffer layer a (ε _{If_TOP),} distortion of the group III nitride semiconductor layer is reduced, distortion inducement step bunching Is suppressed. In the semiconductor wafers of Examples 1 to 3, the strain relaxation rates (R _TOP ) of the first buffer layer with respect to the second buffer layer are 0.974, 0.863, and 0.837, respectively (FIGS. 2 and 4). And 5). That is, the semiconductor wafers of Examples 1 to 3 have a strain relaxation rate (R _TOP ) of 0.8 or more. In contrast, the semiconductor wafers of Comparative Examples 1 and 2 have strain relaxation rates (R _TOP ) of 0.464 and 0.431, respectively, which are smaller than those of the semiconductor wafers of Examples 1 to 3 (see FIG. 6 and 7).

In addition, the average lattice strain (3.97 × 10 ⁻⁴ , 4.96 × 10 ⁻⁴ ) of the group III nitride semiconductor layers of Examples 1 and 2 satisfies both Condition 1 and Condition 2, It is one quarter or less of the average lattice strain of the Group III nitride semiconductor layer of the comparative example. This means that by reducing the average lattice strain of the first buffer layer, the group III nitride semiconductor layer can be prevented from growing with inherent strain, and the occurrence of strain-induced step bunching can be suppressed. Show. That is, when the average lattice strain of the first buffer layer is the largest among the multilayer buffer layers, the group III nitride semiconductor layer is likely to grow in a state where the strain is inherited.

As described above, Examples 1 and 3 have the same structure in the upper layer than the group III nitride semiconductor layer. Example 1 and 3 differ in whether condition 2 is satisfied. When the X-ray half widths of Examples 1 and 3 were compared, the 004 diffraction half widths related to the helical component dislocation were 390 sec (Example 1) and 470 sec (Example 3), respectively. In addition, the 114 diffraction half widths including information on the edge component dislocations were 540 sec (Example 1) and 740 sec (Example 3), respectively. When both conditions 1 and 2 are satisfied, the X-ray half width is narrow and the crystallinity is improved. In particular, the edge dislocation density was suppressed to about ½ to ３ of Example 3 in Example 1. This is because, when both conditions 1 and 2 are satisfied, a first buffer layer having a large interface lattice strain but a small average lattice strain is formed, and a large strain relaxation occurs in the first buffer layer. This is probably because the edge dislocations having the Burgers vector of opposite polarity disappeared. It was also shown that there is an effect of reducing the dislocation density by satisfying both conditions 1 and 2. When dislocation pair annihilation having a reverse polarity Burgers vector occurs, the first buffer layer is compressive strain at the growth temperature, and the strain remaining at room temperature is either compression or no strain. That is, at least a tensile strain does not remain in the first buffer layer at room temperature.

The warpage of the semiconductor wafer changes according to the product of the “strain” and the “film thickness” of the group III nitride semiconductor layer. That is, when the strain of the group III nitride semiconductor layer is large, the amount of warpage of the semiconductor wafer greatly changes due to a slight change in film thickness. Therefore, when the group III nitride semiconductor layer grows with inherent strain, it becomes difficult to control the amount of warpage of the semiconductor wafer. According to the technology disclosed in this specification, the group III nitride semiconductor layer is prevented from growing with inherent strain, so even if an error occurs in the film thickness, variation in the amount of warpage between batches is suppressed. be able to. The semiconductor wafer 1 can also improve the robustness of warpage control.

In the above example, the Al composition (x) of each layer of the multilayer buffer layer was calculated using Equations 2 and 3. However, for the Al composition (x), the cathodoluminescence measurement (CL measurement) is performed on the cross section of the buffer layer, and the band edge emission peak EGap is measured.   It can also be obtained optically from (eV). That is, the Al composition (x) can be obtained by measuring the band edge emission peak EGap (eV) and calculating “x” in the following formula (8). In addition, “b” in the following formula is called a bowing parameter, and for example, b = 0.82 eV is used. The bowing parameters are disclosed in (T. Onuma et.al., J. Appl. Phys. 95, 2495 (2004)).
Formula (8): EGap   (X) = 3.4 × (1−x) + 6.2 × x−b × xx × (1−x)
The average composition x when a multilayer periodic structure such as AlN / GaN is used for the multilayer buffer layer can be calculated by obtaining the AlN / GaN film thickness ratio by a cross-sectional TEM (Transmission Electron Microscope) or the like. _Further, AlN _{/ AlGaN,} the case of a multilayer periodic structure comprising _{Al x Ga 1-x N /} Al y Ga 1-y N, and calculates the respective layers of the Al composition (x) by CL measurement, the film thickness measurement by cross-sectional TEM By combining, it can be converted into an average Al composition (x).

In the above embodiment, a semiconductor wafer having four or five multilayer buffer layers has been described. However, the multilayer buffer layer may be present in at least two layers (a first buffer layer in contact with the group III nitride semiconductor layer and a second buffer layer in contact with the first buffer layer). Further, a structure having six or more multilayer buffer layers may be taken. In addition, when a multilayer periodic structure such as AlN / GaN is used for the multilayer buffer layer, and the layer period / film thickness ratio is different in the multilayer buffer layer, for each region where the period / film thickness ratio is constant The multilayer buffer layer is virtually divided, and the average lattice constant a ′ _j (j = TOP, i) in the virtually divided layer can be defined as each buffer layer j (j = TOP, i). Further, as described above, when a part of the buffer layer is a composition gradient layer, a ′ _j (j = TOP, i) can be determined by the average lattice constant in the composition gradient layer.

In the above embodiment, the example in which the i-type semiconductor functional layer is provided on the surface of the group III nitride semiconductor layer has been described. However, a layer containing an n-type or p-type impurity can also be provided. Further, when a heterojunction is formed, a structure including a heterojunction of a mixed crystal with InN can be taken.

Although the present disclosure has been described based on the embodiments, it is understood that the present disclosure is not limited to the embodiments and structures. The present disclosure includes various modifications and modifications within the equivalent range. In addition, various combinations and forms, as well as other combinations and forms including only one element, more or less, are within the scope and spirit of the present disclosure.

Claims (10)

1. A substrate,
   A group III nitride semiconductor layer provided on the substrate via a multilayer buffer layer,
   The multilayer buffer layer includes at least a first buffer layer and a second buffer layer;
   The first buffer layer is in contact with the group III nitride semiconductor layer;
   The second buffer layer is in contact with the first buffer layer and has a composition different from that of the first buffer layer.
   The multilayer lattice layer has an average lattice constant smaller than that of the group III nitride semiconductor layer,
   Interfacial lattice distortion of the group III nitride semiconductor layer at the interface between the group III nitride semiconductor layer and the first buffer layer is caused by the first buffer layer at the interface between the first buffer layer and the second buffer layer. A semiconductor wafer smaller than the interfacial lattice strain.
2. 2. The semiconductor wafer according to claim 1, wherein, in each of the multilayer buffer layers, the layer having the largest average lattice strain is a layer other than the first buffer layer.
3. The semiconductor wafer according to claim 1 or 2, wherein the first buffer layer is not coherent with the second buffer layer.
4. The semiconductor wafer according to claim 3, wherein a strain relaxation rate of the first buffer layer with respect to the second buffer layer is 0.8 or more.
5. 5. The semiconductor wafer according to claim 1, wherein the first buffer layer is a nitride semiconductor represented by Al _x Ga _1-x N (0 <x <0.2).
6. The semiconductor wafer according to claim 1, wherein a c-axis of the group III nitride semiconductor layer is inclined with respect to a normal to the growth surface.
7. The semiconductor wafer according to claim 1, wherein the group III nitride semiconductor layer is a single layer and the material thereof is gallium nitride.
8. The group III nitride semiconductor layer includes a plurality of layers having different compositions of group III elements,
   The semiconductor wafer according to any one of claims 1 to 6, wherein a material of a layer in contact with the first buffer layer among the plurality of layers is gallium nitride.
9. The semiconductor wafer according to any one of claims 1 to 8, wherein a material of the substrate is silicon.
10. A substrate,
    A group III nitride semiconductor layer provided on the substrate via a multilayer buffer layer,
    A semiconductor element is formed on the surface side of the group III nitride semiconductor layer,
    The multilayer buffer layer includes at least a first buffer layer and a second buffer layer;
    The first buffer layer is in contact with the group III nitride semiconductor layer;
    The second buffer layer is in contact with the first buffer layer and has a composition different from that of the first buffer layer.
    The multilayer lattice layer has an average lattice constant smaller than that of the group III nitride semiconductor layer,
    Interfacial lattice distortion of the group III nitride semiconductor layer at the interface between the group III nitride semiconductor layer and the first buffer layer is caused by the first buffer layer at the interface between the first buffer layer and the second buffer layer. A semiconductor device smaller than the interfacial lattice strain.
    

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