Semiconductor Device

Abstract

This semiconductor device is provided with: a substrate (101); a buffer layer (102) that is formed on the substrate (101); a first semiconductor layer (103) that is formed on the buffer layer (102); a second semiconductor layer (104) that is formed on the first semiconductor layer (103); and a channel layer (105) and a barrier layer (106), which are formed on the second semiconductor layer. The substrate (101) is configured from a nitride semiconductor that has been doped with an impurity so as to have semi-insulating properties or a high resistance; the buffer layer (102) is configured from GaN; the first semiconductor layer (103) is configured from GaN that has been doped with an acceptor; and the second semiconductor layer (104) is configured from AlGaN.

Description

[Technical field]

[0001] The present invention relates to a semiconductor device using a nitride semiconductor. [Background technology]

[0002] A heterojunction field effect transistor (HFET) or high electron mobility transistor (HEMT) is a transistor that switches on and off by changing the carrier density in the channel layer using an electric field generated by a gate voltage.

[0003] When a nitride semiconductor is used, for example, an AlGaN layer and a GaN layer are stacked to compensate for the difference in polarization between them, and a two-dimensional electron gas (2DEG) formed by electrons gathering at the interface is used as the channel. In a typical HEMT made of a nitride semiconductor with Ga polarity, a gate electrode is formed on an AlGaN layer of several to several tens of nm in thickness to control the 2DEG concentration at the interface between the AlGaN and GaN.

[0004] When considering high-frequency applications of nitride semiconductor HEMTs, it is important to confine carriers in the thin region at the interface between AlGaN and GaN and to eliminate other leakage paths. By using such a structure, it is possible to speed up the response to the voltage applied to the gate electrode and achieve stable operation.

[0005] As a substrate used for forming elements such as transistors using nitride semiconductors as described above, GaN substrates are superior in terms of epitaxial growth, etc. However, GaN substrates that are not intentionally doped exhibit n-type conductivity, so impurities must be doped to make the GaN substrate semi-insulating or highly resistive. In general, Fe and Zn are used as impurities to make the GaN substrate semi-insulating or highly resistive.

[0006] The advantage of semi-insulating (or high-resistivity) GaN substrates in high-frequency devices is that they can reduce leakage paths. By stacking devices on a thin buffer using a semi-insulating (or high-resistivity) GaN substrate, leakage to the substrate and buffer leakage can be reduced. [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] T. Ishiguro et al., "New Model of Fe Diffusion in Highly Resistive Fe-Doped Buffer Layer for GaN High-Electron-Mobility Transistor", Japanese Journal of Applied Physics, vol. 52, 08JB17, 2013. [Non-Patent Document 2] Y. Ni et al., "Low-leakage current and high-breakdown voltage GaN-on-Si(111) System with an AlGaN impurity blocking layer", Journal of Materials Science: Materials in Electronics, vol. 27, pp. 5158-5163, 2016. Summary of the Invention [Problem to be solved by the invention]

[0008] However, semi-insulating or high-resistance GaN substrates have a problem in that impurities contained in the substrate diffuse into the nitride semiconductor layer epitaxially grown on the substrate, adversely affecting the device. For example, Non-Patent Document 1 shows that Fe doped in GaN on an Fe-doped layer diffuses over several hundred nm. The Zn and Fe impurities in the substrate act as electron traps in the layer epitaxially grown on the substrate, so when the impurities diffuse to the vicinity of the device structure, they capture carriers in the channel and limit the high-speed operation of the device.

[0009] One method to prevent impurities from diffusing into the device layer is to grow a thick GaN buffer. The concentration of impurities diffused from the substrate in the epitaxial growth layer peaks at the interface between the substrate and the epitaxial growth layer, and decreases as the epitaxial growth layer becomes thicker. Therefore, if the buffer layer is thick enough, the impurity concentration in the epitaxial growth layer that will become the device structure can be kept low. However, since the impurity concentration remains in the GaN buffer layer for several hundred nm, the above-mentioned method requires the GaN buffer to be thick, which is uneconomical. In addition, this method does not take advantage of the effect of reducing buffer leakage, which is an advantage of using a semi-insulating or highly resistive GaN substrate.

[0010] Another method for preventing the diffusion of impurities into the device layer is to insert an AlGaN layer as an impurity diffusion prevention layer. According to Non-Patent Document 2, the diffusion of Si is suppressed by the presence of an AlGaN layer. Based on the same principle, it is considered that the AlGaN layer can also function as a diffusion prevention layer for Fe and Zn. In this case, the thickness of the AlGaN layer may be about several tens to several hundred nm, and compared to the method of making the buffer layer thick, it is possible to suppress the diffusion of impurities from the substrate with a thin thickness. However, in this method, 2DEG is formed at the interface between the inserted AlGaN layer and the underlying GaN buffer, and this 2DEG becomes a problem as a new leak path.

[0011] As described above, in conventional techniques, there was a problem in that it was not easy to suppress the diffusion of impurities introduced into a nitride semiconductor substrate to make the substrate semi-insulating or highly resistive in a nitride semiconductor device formed on the substrate without affecting the characteristics of the device formed on the substrate.

[0012] The present invention has been made to solve the above-mentioned problems, and has an object to suppress the diffusion of impurities introduced to make a nitride semiconductor substrate semi-insulating or highly resistive, without affecting the characteristics of devices formed on the substrate. [Means for solving the problem]

[0013] The semiconductor device according to the present invention comprises a substrate made of a nitride semiconductor doped with impurities to make it semi-insulating or highly resistive, a buffer layer made of GaN formed on the substrate, a first semiconductor layer made of GaN doped with an acceptor formed on the buffer layer, a second semiconductor layer made of AlGaN formed on the first semiconductor layer, and a channel layer and a barrier layer made of a nitride semiconductor formed on the second semiconductor layer. Effect of the Invention

[0014] As described above, according to the present invention, a first semiconductor layer made of GaN doped with acceptors is provided on a buffer layer made of GaN, so that diffusion of impurities introduced to make a nitride semiconductor substrate semi-insulating or highly resistive can be suppressed without affecting the characteristics of devices formed on the substrate. [Brief description of the drawings]

[0015] [Figure 1A] FIG. 1A is a diagram showing a partial configuration of a semiconductor device according to an embodiment of the present invention. [Figure 1B] FIG. 1B is a band diagram showing a calculated band structure in a structure excluding first semiconductor layer 103 of the semiconductor device according to the embodiment of the present invention. [Figure 1C] FIG. 1C is a band diagram showing a calculated band structure in the structure of a semiconductor device according to an embodiment of the present invention. [Diagram 2] FIG. 2 is a band diagram showing a calculated band structure in the structure of the semiconductor device according to the embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] A semiconductor device according to an embodiment of the present invention will be described below with reference to Fig. 1A. This semiconductor device includes a substrate 101, a buffer layer 102 formed on the substrate 101, a first semiconductor layer 103 formed on the buffer layer 102, a second semiconductor layer 104 formed on the first semiconductor layer 103, and a channel layer 105 and a barrier layer 106 formed on the second semiconductor layer. This semiconductor device is a field effect transistor (HEMT) that includes the channel layer 105 and the barrier layer 106 made of a nitride semiconductor as part of the device structure, and further includes, for example, a gate electrode and a source / drain electrode (not shown) on the barrier layer 106, and uses a 2DEG formed near the interface between these electrodes as a channel.

[0017] The substrate 101 is made of a nitride semiconductor doped with impurities to make it semi-insulating or highly resistive. The substrate 101 can be made of, for example, GaN doped with Zn or Fe. This is a commonly used semi-insulating GaN substrate or highly resistive GaN substrate. Since GaN becomes n-type when not doped with impurities, the above-mentioned impurities are doped to make it semi-insulating (or highly resistive). The buffer layer 102 is made of GaN and is formed on, for example, the substrate 101. The buffer layer 102 can have a thickness of 300 nm or less.

[0018] The first semiconductor layer 103 is composed of GaN doped with an acceptor, and is formed, for example, in contact with the buffer layer 102. The acceptor of the first semiconductor layer 103 can be at least one of Mg, Fe, and Zn. Also, in this case, the concentration of the acceptor can be 1×10 17 cm -3 ~1×10 19 cm -3 Moreover, in this case, the first semiconductor layer 103 can have a thickness of 50 nm or less.

[0019] Alternatively, the acceptor of the first semiconductor layer 103 can be C, and the concentration of the acceptor can be 1×10 18 cm -3 or more. In this case, the first semiconductor layer 103 can have a thickness of 10 nm or less.

[0020] The second semiconductor layer 104 is composed of AlGaN and is formed, for example, in contact with the first semiconductor layer 103. The second semiconductor layer 104 can be composed of, for example, Al x Ga 1-x N (0 < x ≤ 0.1) and can have a thickness of 200 nm or less.

[0021] The semiconductor device according to the embodiment has a structure in which a first semiconductor layer 103 made of GaN doped with an acceptor and a second semiconductor layer 104 made of AlGaN are inserted between a buffer layer 102, which is an initial growth layer on the substrate 101, and a channel layer 105 having a device structure. Thereby, the following effects can be obtained.

[0022] First, while thinning the entire layer of each nitride semiconductor epitaxially grown on the substrate 101, it is possible to effectively suppress the diffusion of impurities from the substrate 101. The presence of the second semiconductor layer 104 makes it possible to reduce the concentration of impurities diffusing into the channel layer 105 without increasing the thickness of the buffer layer 102.

[0023] Secondly, it is possible to suppress the formation of 2DEG at the interface when the second semiconductor layer 104 and the buffer layer 102 are formed in contact with each other. By doping the acceptor only in a relatively thin region near the interface to form the first semiconductor layer 103, the band is raised and the formation of 2DEG is prevented. The first semiconductor layer 103 formed by doping the acceptor has a thickness of 50 nm or less and can achieve the above-mentioned effects, so that the leak path can be efficiently eliminated without significantly increasing the overall thickness.

[0024] A more detailed explanation will be given below. Substrate 101 made of GaN will be n-type if a manufacturing method is used that does not dope impurities, so it is doped with impurities to make it semi-insulating (or highly resistive). The doping impurity may be, for example, Zn or Fe. In the following, an example will be described in which substrate 101 is made of Zn-doped semi-insulating GaN.

[0025] It is known that impurities doped into the substrate 101 will diffuse into the epitaxially grown layers when crystals are grown on the substrate 101, and if the impurities diffuse to the vicinity of the device layer, they will act as a trap for carriers and affect the device characteristics. Since the diffusion of impurities from the substrate 101 reaches a depth of 200 to 300 nm, in order to block the effect of impurities on the device by thickening the buffer layer 102, it is necessary to grow the buffer layer to a thickness of 300 nm or more, which is uneconomical. Furthermore, although thickening the buffer layer 102 can prevent impurities, it can also lead to new factors that inhibit device performance, such as buffer leaks.

[0026] In response to this, the second semiconductor layer 104 made of AlGaN is inserted between the buffer layer 102 and the channel layer 105, thereby preventing the diffusion of impurities while keeping the buffer layer 102 thin at 300 nm or less. Note that the second semiconductor layer 104 also functions as a back barrier layer in terms of the device structure above it.

[0027] The second semiconductor layer 104 is required to have a thickness of about several tens to several hundreds of nm to prevent the diffusion of impurities, and the Al composition cannot be large considering the critical film thickness associated with the lattice mismatch with the underlying buffer layer 102. In order to grow an AlGaN layer of several tens to several hundreds of nm on a GaN layer while avoiding the occurrence of cracks, it is desirable to keep the Al composition at 0.1 or less. For this reason, as an example, the second semiconductor layer 104 has an Al composition of 0.05 and a thickness of 200 nm.

[0028] 1B shows the band structure calculated under these conditions for a structure excluding first semiconductor layer 103. Barrier layer 106 has an Al composition of 0.4 and a thickness of 10 nm. Channel layer 105 has a thickness of 30 nm. Note that the Al composition and thickness of barrier layer 106 and the thickness of channel layer 105 are not limited to these values, and roughly similar results can be obtained under conditions for barrier layers and channel layers used in general HEMTs.

[0029] As shown in FIG. 1B, the conduction band E c is the Fermi level E F This is below the threshold voltage, and carriers are generated that are approximately a few percent of the carrier density at the interface between the barrier layer 106 and the channel layer 105. High-concentration carriers generated in layers other than the channel layer 105 are undesirable because they can become leak paths during HEMT operation.

[0030] In contrast to this, in the present invention, as described above, the first semiconductor layer 103 doped with an impurity is inserted between the second semiconductor layer 104 and the buffer layer 102 so as to prevent carrier generation. In order to prevent the generation of electrons, the first semiconductor layer 103 may be doped with an impurity that acts as an acceptor, such as Mg, Fe, Zn, or C. For example, Mg may be used as the acceptor. When Mg is used as the acceptor, high concentration doping may cause new problems such as the introduction of defects and polarity inversion of GaN. For this reason, when Mg is used as the acceptor, it is necessary to dope the first semiconductor layer 103 with an impurity that acts as an acceptor, such as Mg, Fe, Zn, or C. 20 cm -3It is difficult to dope at such a high concentration.

[0031] In addition, when the Mg concentration is increased, holes exceeding the concentration of the 2DEG formed under the second semiconductor layer 104 are formed in the first semiconductor layer 103 having a thickness of several nm are formed, which becomes a new leak path. 18 cm -3 A Mg concentration of 0.5 is desirable.

[0032] On the other hand, if the Mg concentration is reduced, the thin first semiconductor layer 103 cannot obtain a sufficient 2DEG suppression effect, which is not in keeping with the purpose (objective) of thinning the buffer layer 102. 17 cm -3 For example, the doping amount of Mg in the first semiconductor layer 103 is 1×10 18 cm -3 With this doping amount, the first semiconductor layer 103 is set to a thickness of, for example, 30 nm, as shown in FIG. 1C.

[0033] As shown in FIG. 1C, the first semiconductor layer 103 raises the band between the second semiconductor layer 104 and the buffer layer 102, and the conduction band E c is the Fermi level E F The carrier density between them is reduced to a level where the effect on device characteristics can be neglected. From this calculation result, it is possible to determine that the doping amount in the first semiconductor layer 103 described above is 1×10 18 cm -3 It can be seen that by setting the thickness to 30 nm, the thin first semiconductor layer 103 can effectively suppress the generation of 2DEG below the second semiconductor layer 104.

[0034] If the first semiconductor layer 103 using Mg as an acceptor is made thicker, it will not only suppress the 2DEG but also increase the hole density, creating a new leak path. Therefore, it is not desirable for the first semiconductor layer 103 to be thicker than 100 nm.

[0035] The effect of the first semiconductor layer 103 does not depend on the thickness of the underlying buffer layer 102. Furthermore, since the first semiconductor layer 103 and the buffer layer 102 are homoepitaxially grown (lattice matched) on the substrate 101, they can be thinned to about several tens of nm, unlike heteroepitaxial growth. Thinning the buffer layer 102 on the substrate 101 prevents the formation of unnecessary leak paths, and is expected to have the effect of reducing device costs.

[0036] Next, a case where the acceptor of the first semiconductor layer 103 is C will be described. When the acceptor of the first semiconductor layer 103 is C, it is possible to relax the restrictions on the doping amount considered when doping with Mg. Therefore, by doping the acceptor with C at a higher concentration, a similar effect can be obtained with a thin first semiconductor layer 103. The conditions for the layers other than the first semiconductor layer 103 were the same as described above, and the C concentration of the first semiconductor layer 103 was set to 5×10 18 cm -3 The calculation results for a thickness of 5 nm are shown in FIG.

[0037] Although the first semiconductor layer 103 is thinner, as shown in FIG. 2, the band between the second semiconductor layer 104 and the buffer layer 102 is raised, and the carrier density therebetween is reduced to a level where the effect on device characteristics is negligible.

[0038] In this way, when C (carbon) is used as the acceptor, unlike when the acceptor is Mg, Fe, Zn, etc., the generation of holes does not occur by thickening the first semiconductor layer 103, so there is no significant limit (upper limit) on the thickness. However, when homoepitaxial growth is assumed on the substrate 101, the effect of preventing the formation of unnecessary leak paths and reducing device costs can be expected by thinning the total thickness of the buffer layer 102 and the first semiconductor layer 103. Unlike the case of Mg, in the case of C doping, which has no limit on high concentration doping, the first semiconductor layer 103 can be made even thinner, and the effect of thinning the buffer layer 102 is greater.

[0039] As described above, according to the present invention, a first semiconductor layer made of GaN doped with acceptors is provided on a buffer layer made of GaN, so that the diffusion of impurities introduced to make a nitride semiconductor substrate semi-insulating or highly resistive can be suppressed without affecting the characteristics of devices formed on the substrate.

[0040] It should be noted that the present invention is not limited to the above-described embodiments, and it is apparent that many modifications and combinations can be made by a person having ordinary knowledge in the art within the technical concept of the present invention. [Explanation of symbols]

[0041] 101...substrate, 102...buffer layer, 103...first semiconductor layer, 104...second semiconductor layer, 105...channel layer, 106...barrier layer.

Claims

1. a substrate made of a nitride semiconductor doped with impurities to be semi-insulating or highly resistive; a buffer layer made of GaN formed on the substrate; a first semiconductor layer made of acceptor-doped GaN formed on the buffer layer; a second semiconductor layer made of AlGaN formed on the first semiconductor layer; a channel layer and a barrier layer made of a nitride semiconductor formed on the second semiconductor layer; A semiconductor device comprising:

2. 2. The semiconductor device according to claim 1, The semiconductor device is characterized in that the substrate is made of GaN doped with Zn or Fe.

3. 3. The semiconductor device according to claim 2, The buffer layer is formed to a thickness of 300 nm or less, The second semiconductor layer is Al x G 1-x N (0<x≦0.1) and has a thickness of 200 nm or less. A semiconductor device comprising:

4. 4. The semiconductor device according to claim 2, the acceptor is at least one of Mg, Fe, and Zn; The concentration of the acceptor is 1×10 17 cm -3 ~1×10 19 cm -3 It is said that, The first semiconductor layer has a thickness of 50 nm or less. A semiconductor device comprising:

5. 4. The semiconductor device according to claim 2, the acceptor is C, The concentration of the acceptor is 1×10 18 cm -3 That is all. The first semiconductor layer has a thickness of 10 nm or less. A semiconductor device comprising:

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