GaN EPITAXIAL WAFER AND METHOD FOR MANUFACTURING SAME

Abstract

A plurality of HEMTs (2) are formed in a matrix form on a GaN epitaxial wafer (1). A buffer layer (4) which is formed of a group III-V compound semiconductor is formed on a wafer-like substrate (3). A barrier layer (5) which is formed of a group III-V compound semiconductor containing Al is formed on the buffer layer (4). The higher the carbon concentration of the buffer layer (4) in a region in the wafer surface, the higher the Al composition ratio of the barrier layer (5).

Description

GaN epiwafer and manufacturing method thereof

[0001] The present disclosure relates to GaN epi-wafers and methods for manufacturing the same.

[0002] In GaN-HEMTs, a leakage current (buffer leakage) occurs that bypasses the buffer layer under the channel during off-state operation. Adding carbon to the buffer layer has been proposed to suppress the buffer leakage and increase the breakdown voltage (see, for example, Non-Patent Document 1). However, even if carbon is not intentionally added, carbon from the Group III source gas naturally becomes mixed into the epitaxial layer during crystal growth.

[0003] S. Gustafsson et al., IEEE Trans. Electron Devices 62, 2162-2169 (2015)

[0004] However, due to the structure of the crystal growth equipment, the carbon concentration varies across the wafer surface. The higher the carbon concentration in the buffer layer, the higher the resistance value of the HEMT. Because multiple HEMTs are formed in a matrix on the wafer, variations in resistance across the wafer surface lead to variations in the characteristics of each HEMT.

[0005] The present disclosure has been made to solve the above-mentioned problems, and its object is to provide a GaN epitaxial wafer and a method for manufacturing the same that can improve the uniformity of resistance values ​​across the wafer surface.

[0006] The GaN epitaxial wafer according to the present disclosure is a GaN epitaxial wafer having a plurality of HEMTs formed in a matrix, and includes a wafer-shaped substrate, a buffer layer formed on the substrate and made of a III-V compound semiconductor, and a barrier layer formed on the buffer layer and made of a III-V compound semiconductor containing Al, wherein the carbon concentration in the buffer layer increases from the wafer center to the wafer periphery, and the Al composition ratio in the barrier layer increases from the wafer center to the wafer periphery.

[0007] In the present disclosure, the Al composition ratio of the barrier layer is increased in regions of the wafer where the carbon concentration of the buffer layer is higher, and the change in resistance value due to the Al composition ratio of the barrier layer offsets the change in resistance value due to the carbon concentration of the buffer layer, thereby improving the uniformity of the resistance value within the wafer.

[0008] FIG. 1 is a plan view showing a GaN epitaxial wafer; FIG. 2 is a cross-sectional view showing each HEMT of the GaN epitaxial wafer; FIG. 3 is a diagram showing a crystal growth apparatus; FIG. 4 is a diagram showing the relationship between substrate surface temperature and carbon concentration within a wafer surface; FIG. 5 is a diagram showing wafer in-plane distribution of a GaN epitaxial wafer according to a comparative example; FIG. 6 is a diagram showing wafer in-plane distribution of a GaN epitaxial wafer according to a first embodiment; and FIG. 7 is a diagram showing wafer in-plane distribution of a GaN epitaxial wafer according to a second embodiment.

[0009] A GaN epitaxial wafer and a method for manufacturing the same according to an embodiment will be described with reference to the drawings. The same or corresponding components are designated by the same reference numerals, and repeated description may be omitted.

[0010] 1 is a plan view showing a GaN epitaxial wafer. A plurality of GaN-HEMTs 2 are formed in a matrix on the GaN epitaxial wafer 1. The GaN-HEMTs 2 are high electron mobility transistors made of GaN-based materials.

[0011] 2 is a cross-sectional view showing each HEMT on a GaN epitaxial wafer. A buffer layer 4 (electron transit layer) made of GaN or AlGaInN is formed on a SiC substrate 3. A barrier layer 5 (electron supply layer) made of AlGaInN is formed on the buffer layer 4. That is, the barrier layer 5 is made of a III-V compound semiconductor containing Al. The source electrode, drain electrode, and gate electrode of the HEMT may be formed on the barrier layer 5. A Si substrate or a GaN substrate may be used instead of the SiC substrate 3.

[0012] FIG. 3 is a diagram showing a crystal growth apparatus. A stage 7 is disposed in a chamber 6. A heating device 8 heats the stage 7. TMGa or TEGa, which is a Ga source gas, is supplied to the chamber 6 from gas supply sources 9a1 and 9a2, and the flow rates thereof are controlled by mass flow controllers 10a1 and 10a2, respectively. TMAl, which is an Al source gas, is supplied to the chamber 6 from gas supply sources 9b1 and 9b2, and the flow rates thereof are controlled by mass flow controllers 10b1 and 10b2, respectively. TMIn, which is an In source gas, is supplied to the chamber 6 from gas supply sources 9c1 and 9c2, and the flow rates thereof are controlled by mass flow controllers 10c1 and 10c2, respectively. NH, which is an N source gas, is supplied to the chamber 6 from gas supply sources 9c1 and 9c2, and the flow rates thereof are controlled by mass flow controllers 10c1 and 10c2, respectively. ３ is supplied to the chamber 6 from a gas supply source 9d, and its flow rate is controlled by a mass flow controller 10d. An exhaust device 11 exhausts the chamber 6. As shown in FIG. 3, there are two Group III source gas supply lines, an upper system and an lower system. The lower system supplies Group III source gas from gas supply sources 9a1, 9b1, and 9c1 to a region close to the gas inlet. Because the wafer is rotating, the outer periphery of the wafer is the region close to the gas inlet. The upper system supplies Group III source gas from gas supply sources 9a2, 9b2, and 9c2 to the center of the wafer. Therefore, the Al composition ratio within the wafer surface can be changed by adjusting the source gas supply rates of the two systems.

[0013] When crystal growth is performed, first, a wafer-shaped SiC substrate 3 is placed on a stage 7. Source gases are supplied into the chamber 6 while the stage 7 and the SiC substrate 3 are heated by a heating device 8. In this way, a buffer layer 4 and a barrier layer 5 are formed in order on the SiC substrate 3. Since there are multiple supply systems for the Group III source gases, the Al content of the buffer layer 4 and the barrier layer 5 can be controlled by controlling the flow rate of each source gas. ｘ In ｙ Ga ( １－ｘ－ｙ ) The N mixed crystal composition can be adjusted.

[0014] During crystal growth, carbon from the methyl or ethyl groups in the Group III source gas is mixed into the buffer layer 4 and the barrier layer 5. However, the concentration of the mixed carbon varies depending on the substrate surface temperature. FIG. 4 is a diagram showing the relationship between the substrate surface temperature and the carbon concentration within the wafer surface. Because the temperature at the wafer center of the heated SiC substrate 3 increases, the carbon concentration at the wafer center decreases. In other words, because the substrate surface temperature decreases from the wafer center toward the wafer periphery, the carbon concentration in the buffer layer 4 increases from the wafer center toward the wafer periphery.

[0015] In this embodiment, the Al content of the barrier layer 5 is increased by adjusting the source gas supply system during the growth of the barrier layer 5. ｘ In ｙ Ga ( １－ｘ－ｙ ) The Al composition ratio of N increases from the wafer center toward the wafer periphery, while the Al composition ratio of the buffer layer 4 is uniform within the wafer surface.

[0016] Next, the effects of this embodiment will be explained in comparison with a comparative example. Figure 5 shows the wafer surface distribution of a GaN epitaxial wafer according to the comparative example. In the comparative example, the Al composition ratio of the buffer layer 4 and the barrier layer 5 is uniform across the wafer surface. However, because the carbon concentration is low at the wafer center, the resistance value of the HEMT varies across the wafer surface. For example, the sheet resistance differs by 50 Ω / □ between the wafer center and the wafer periphery.

[0017] The two-dimensional electron gas density varies depending on the mixed crystal composition of the barrier layer 5 and the buffer layer 4. The resistance value of the HEMT is inversely proportional to the two-dimensional electron gas density. Specifically, the greater the Al composition ratio of the barrier layer 5, the smaller the resistance value of the HEMT. Therefore, in this embodiment, when forming the barrier layer 5, the source gas supply system is adjusted so that the Al composition ratio of the barrier layer 5 becomes higher in regions of the buffer layer 4 with higher carbon concentrations within the wafer surface.

[0018] 6 is a diagram showing the distribution of carbon concentration within the wafer surface of the GaN epitaxial wafer according to the first embodiment. From the center of the wafer to the periphery of the wafer, the carbon concentration of the buffer layer 4 increases, and the Al composition ratio of the barrier layer 5 also increases. For example, if the difference in carbon concentration within the wafer surface is 5×10 １７ cm －３When the difference in carbon concentration within the wafer surface is 5×10, the Al composition ratio is changed by 40%. １６ cm －３ In this case, the Al composition ratio is changed by 5% within the wafer surface. The Al composition ratio of the barrier layer 5 is increased in regions within the wafer surface where the carbon concentration of the buffer layer 4 is higher. This allows the change in resistance value due to the Al composition ratio of the barrier layer 5 to offset the change in resistance value due to the carbon concentration of the buffer layer 4, thereby improving the uniformity of the resistance value of the HEMT within the wafer surface. It is preferable to adjust the Al composition ratio of the barrier layer 5 so that the variation in the resistance value of a plurality of GaN-HEMTs 2 within the wafer surface is less than 2%.

[0019] Second Embodiment Fig. 7 is a diagram showing the in-plane distribution of carbon in a GaN epitaxial wafer according to a second embodiment. As in the first embodiment, the carbon concentration in the buffer layer 4 increases from the center of the wafer toward the periphery of the wafer. In the first embodiment, the Al composition ratio of the barrier layer 5 is changed, but in this embodiment, the Al composition ratio of the buffer layer 4 is changed. That is, the buffer layer 4 is made of a III-V compound semiconductor containing Al. By adjusting the source supply gas system during the growth of the buffer layer 4, the Al content of the buffer layer 4 can be controlled. ｘ In ｙ Ga ( １－ｘ－ｙ ) The Al composition ratio of N decreases from the wafer center toward the wafer periphery, while the Al composition ratio of the barrier layer 5 is uniform within the wafer surface.

[0020] The higher the carbon concentration of the buffer layer 4, the higher the resistance value of the HEMT, whereas the higher the Al composition ratio of the buffer layer 4, the higher the resistance value of the HEMT. Therefore, in this embodiment, when forming the buffer layer 4, the source gas supply system is adjusted so that the Al composition ratio of the buffer layer 4 decreases from the wafer center toward the wafer periphery. Therefore, the higher the carbon concentration of the buffer layer 4 within the wafer surface, the lower the Al composition ratio of the buffer layer 4. This change in resistance due to the Al composition ratio of the buffer layer 4 offsets the change in resistance due to the carbon concentration of the buffer layer 4, thereby improving the uniformity of the HEMT resistance within the wafer surface. It is preferable to adjust the Al composition ratio of the buffer layer 4 so that the variation in the resistance of multiple GaN-HEMTs 2 within the wafer surface is less than 2%.

[0021] Embodiment 3. As in Embodiment 1, the carbon concentration of the buffer layer 4 increases from the wafer center toward the wafer periphery. In this embodiment, the Al composition ratio of the buffer layer 4 decreases from the wafer center toward the wafer periphery as in Embodiment 2, while the Al composition ratio of the barrier layer 5 increases from the wafer center toward the wafer periphery as in Embodiment 1. This change in resistance value due to the Al composition ratio of the buffer layer 4 and the barrier layer 5 offsets the change in resistance value due to the carbon concentration of the buffer layer 4, thereby improving the uniformity of the resistance value of the HEMT across the wafer. It is preferable to adjust the Al composition ratio of the buffer layer 4 and the barrier layer 5 so that the variation in the resistance value of a plurality of GaN-HEMTs 2 across the wafer is less than 2%.

[0022] Although the first to third embodiments have been described with reference to the case where the carbon concentration is low at the wafer center, the carbon concentration may be high at the wafer center. In this case, the Al composition ratio of the buffer layer 4 or the barrier layer 5 is changed in the opposite direction from the wafer center toward the wafer periphery.

[0023] 1 GaN epitaxial wafer, 2 GaN-HEMT, 3 SiC substrate, 4 buffer layer, 5 barrier layer, 7 stage

Claims

1. A GaN epiwafer on which a plurality of HEMTs are formed in a matrix, comprising: a wafer-shaped substrate; a buffer layer formed on the substrate and made of a III-V compound semiconductor; and a barrier layer formed on the buffer layer and made of an Al-containing III-V compound semiconductor, wherein the Al composition ratio of the barrier layer increases in regions of the wafer surface where the carbon concentration of the buffer layer is higher.

2. The GaN epitaxial wafer according to claim 1, wherein the carbon concentration of the buffer layer increases from the wafer center toward the wafer periphery, and the Al composition ratio of the barrier layer increases from the wafer center toward the wafer periphery.

3. The GaN epitaxial wafer according to claim 2, wherein the Al composition ratio of the buffer layer is uniform across the wafer surface.

4. A GaN epi-wafer on which a plurality of HEMTs are formed in a matrix, comprising: a wafer-shaped substrate; a buffer layer formed on the substrate and made of a III-V compound semiconductor containing Al; and a barrier layer formed on the buffer layer and made of a III-V compound semiconductor containing Al, wherein the Al composition ratio of the buffer layer decreases in regions of the wafer surface where the carbon concentration of the buffer layer is higher.

5. A GaN epitaxial wafer according to claim 4, wherein the carbon concentration of the buffer layer increases from the wafer center toward the wafer periphery, and the Al composition ratio of the buffer layer decreases from the wafer center toward the wafer periphery.

6. The GaN epitaxial wafer according to claim 5, wherein the Al composition ratio of the barrier layer is uniform across the wafer surface.

7. A GaN epitaxial wafer according to claim 5, wherein the Al composition ratio of said barrier layer increases from the center of the wafer toward the periphery of the wafer.

8. A GaN epitaxial wafer according to any one of claims 1 to 7, characterized in that the variation in resistance values ​​of the plurality of HEMTs within the wafer surface is less than 2%.

9. A method for manufacturing a GaN epi-wafer on which a plurality of HEMTs are formed in a matrix, comprising the steps of: placing a wafer-shaped substrate on a stage and heating it; and forming a buffer layer made of a III-V compound semiconductor on the substrate using a source gas containing a methyl group or an ethyl group; and forming a barrier layer made of an Al-containing III-V compound semiconductor on the buffer layer, wherein a source gas system is adjusted during the formation of the barrier layer so that the Al composition ratio of the barrier layer increases in regions of the wafer surface where the carbon concentration of the buffer layer is higher.

10. A method for manufacturing a GaN epi-wafer on which a plurality of HEMTs are formed in a matrix, comprising the steps of: placing a wafer-shaped substrate on a stage and heating it; and forming a buffer layer made of an Al-containing III-V compound semiconductor on the substrate using a source gas containing a methyl group or an ethyl group; and forming a barrier layer made of an Al-containing III-V compound semiconductor on the buffer layer, wherein, when forming the buffer layer, a source gas supply system is adjusted so that the Al composition ratio of the buffer layer becomes lower in regions of the wafer surface where the carbon concentration of the buffer layer is higher.

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