Method of manufacturing nitride semiconductor device and nitride semiconductor device

Abstract

To provide a method of manufacturing a nitride semiconductor device, with which variations in carrier concentration in a low concentration carrier region and variations in the breakdown voltage of a nitride semiconductor device resulting therefrom can be reduced and accordingly, a nitride semiconductor device having low on-resistance and high breakdown voltage can be manufactured, and a semiconductor device manufactured by the method.SOLUTION: There is provided a method of manufacturing a nitride semiconductor device, the nitride semiconductor device including a first nitride semiconductor layer of a first conductivity type, a second nitride semiconductor layer of the first conductivity type having a carrier concentration higher than that of the first nitride semiconductor layer, a third nitride semiconductor layer of a second conductivity type, a fourth nitride semiconductor layer of the first conductivity type, a first main electrode electrically connected to the first nitride semiconductor layer, a second main electrode electrically connected to the fourth nitride semiconductor layer, and a control electrode provided on the third nitride semiconductor layer via an insulating film. In the method, at any point from the start of the formation of the second nitride semiconductor layer to the end of the formation of the second nitride semiconductor layer, the second nitride semiconductor layer is formed with higher carbon concentration than the carbon concentration of the first nitride semiconductor layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for manufacturing a nitride semiconductor device and a nitride semiconductor device. [Background technology]

[0002] An example of a semiconductor device structure using a nitride-based compound semiconductor (GaN-based) is the structure shown in FIG. 4, which is disclosed in Patent Document 1 below.

[0003] A low-concentration carrier region 121 is provided on a substrate 110, a high-concentration carrier region 123 is provided on the low-concentration carrier region 121, a low-concentration carrier region 125 is provided on the high-concentration carrier region 123, a p-type semiconductor layer 130 that forms a channel is provided on the low-concentration carrier region 125, an n-type semiconductor layer 140 is provided on the p-type semiconductor layer 130, the n-type semiconductor layer 140 and a source electrode 174 are electrically connected, the substrate 110 and a drain electrode 178 are electrically connected, and a gate electrode 172 is formed via an insulating film 160 in a trench 152 that penetrates the p-type semiconductor layer 130 and reaches the low-concentration carrier region 125.

[0004] In such a structure, the high-concentration carrier region 123 is provided on the low-concentration carrier region 121 that functions as a drift region, and the high-concentration carrier region 123 disperses the current so that the current flowing through the channel generated along the sidewall of the trench 152 is dispersed more in the planar direction within the low-concentration carrier region 121. It is described that the carrier concentrations of the low-concentration carrier region 121 and the low-concentration carrier region 125 are made approximately the same (paragraph

[0030] of Patent Document 1).

[0005] Patent Document 1 also discloses that the donor element contained in the n-type semiconductor layer is not limited to silicon (Si) but may be germanium (Ge) and oxygen (O), etc., and that the acceptor element contained in the p-type semiconductor layer is not limited to magnesium (Mg) but may be zinc (Zn) and carbon (C), etc. (paragraphs

[0068] and

[0069] ). [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Laid-Open No. 2017-063174 Summary of the Invention [Problem to be solved by the invention]

[0007] In Patent Document 1, the p-type semiconductor layer 130 generates a channel, and therefore the MOVPE method, which allows precise film thickness and doping control, is used, and the low-concentration carrier region 121 is also formed by the MOVPE method (paragraph

[0031] ).

[0008] However, the low-concentration carrier region 121 must be formed relatively thick and with a relatively low donor concentration in order to ensure the breakdown voltage of the semiconductor device. Here, in the MOVPE method, carbon is doped into the nitride-based compound semiconductor layer, but the amount of carbon doped varies greatly depending on the growth temperature, etc. The low-concentration carrier region 121 is a layer formed relatively thick and with a relatively low donor concentration, and variations in the carrier concentration of the low-concentration carrier region 121 pose a problem leading to variations in the breakdown voltage of the semiconductor device.

[0009] The present invention has been made in consideration of the above-mentioned problems, and an object of the present invention is to provide a method for manufacturing a nitride semiconductor device, and a nitride semiconductor device, which reduce variations in carrier concentration in low-concentration carrier regions and the resulting variations in breakdown voltage of the nitride semiconductor device.

[0010] Another object of the present invention is to provide a method for manufacturing a nitride semiconductor device having a low on-resistance and a high breakdown voltage, and to provide the nitride semiconductor device. [Means for solving the problem]

[0011] In order to achieve the above object, the present invention provides a method for manufacturing a nitride semiconductor device comprising: a first nitride semiconductor layer of a first conductivity type; a second nitride semiconductor layer of a first conductivity type stacked on the first nitride semiconductor layer and having a higher carrier concentration than the first nitride semiconductor layer; a third nitride semiconductor layer of a second conductivity type stacked on the second nitride semiconductor layer; a fourth nitride semiconductor layer of a first conductivity type stacked on the third nitride semiconductor layer; a first main electrode electrically connected to the first nitride semiconductor layer; a second main electrode electrically connected to the fourth nitride semiconductor layer; and a control electrode provided on the third nitride semiconductor layer via an insulating film, wherein the first nitride semiconductor layer, the second nitride semiconductor layer, the third nitride semiconductor layer, and the fourth nitride semiconductor layer are deposited in this order, and the carbon concentration of the second nitride semiconductor layer is made higher than the carbon concentration of the first nitride semiconductor layer any time after the start of deposition of the second nitride semiconductor layer and before the end of deposition of the second nitride semiconductor layer.

[0012] With this method for manufacturing a nitride semiconductor device, the carbon concentration of the second nitride semiconductor layer, which has a higher carrier concentration than the first nitride semiconductor layer, is increased to be higher than that of the first nitride semiconductor layer at some point during the deposition of the second nitride semiconductor layer. That is, the first nitride semiconductor layer has a lower carrier concentration than the second nitride semiconductor layer, but also a lower carbon concentration. By suppressing the carbon concentration of the first nitride semiconductor layer, the carrier concentration of the first nitride semiconductor layer can be suppressed. Additionally, the second nitride semiconductor layer has a high carbon concentration, but also includes a high-concentration carrier region with a higher carrier concentration than the first nitride semiconductor layer. Therefore, even if the carbon concentration of the second nitride semiconductor layer is increased, the second nitride semiconductor layer also has a high carrier concentration. This suppresses the effect of variations in carbon concentration on the carrier concentration, and thus suppresses variations in carrier concentration due to variations in carbon concentration. As a result, a nitride semiconductor device with low on-resistance and high breakdown voltage can be manufactured. In this case, the region in the second nitride semiconductor layer where the carbon concentration is increased may be the entire thickness region or only a portion of the thickness region, i.e., the upper thickness region.

[0013] It is also preferable that the first nitride semiconductor layer is deposited by the HVPE method, and that the deposition of the second nitride semiconductor layer is performed by switching from the HVPE method to the MOVPE method after the start of deposition of the second nitride semiconductor layer and before the end of deposition of the second nitride semiconductor layer.

[0014] When used as a method for manufacturing such a nitride semiconductor device, the HVPE method has a faster growth rate and less carbon contamination than the MOVPE method. By depositing the first nitride semiconductor layer using the HVPE method, the carbon concentration of the first nitride semiconductor layer can be reduced, the effect of fluctuations in carbon concentration on the carrier concentration can be suppressed, and variations in carrier concentration can be reduced.

[0015] It is also preferable to form a fifth nitride semiconductor layer of the first conductivity type between the second nitride semiconductor layer and the third nitride semiconductor layer, the fifth nitride semiconductor layer having a carrier concentration lower than that of the second nitride semiconductor layer, and to make the carrier concentration in the fifth nitride semiconductor layer higher than the carrier concentration in the first nitride semiconductor layer.

[0016] In this method for manufacturing a nitride semiconductor device, by making the carrier concentration of the fifth nitride semiconductor layer lower than that of the second nitride semiconductor layer and higher than that of the first nitride semiconductor layer, the effect of variations in carbon concentration on the carrier concentration can be suppressed more than in the first nitride semiconductor layer, and variations in carrier concentration can be suppressed. Furthermore, since a depletion layer extends from the interface between the fifth nitride semiconductor layer and the third nitride semiconductor layer of the second conductivity type, suppressing the effect of variations in carbon concentration on the carrier concentration can reduce variations in the breakdown voltage of the nitride semiconductor device caused by this.

[0017] It is also preferable that the peak oxygen concentration of the second nitride semiconductor layer be higher than the peak oxygen concentration of the first nitride semiconductor layer.

[0018] According to such a method for manufacturing a nitride semiconductor device, the donor concentration can be increased in the second nitride semiconductor layer, which has a higher carrier concentration than the first nitride semiconductor layer.

[0019] Furthermore, it is preferable that the carbon concentration of the second nitride semiconductor layer be made higher than the carbon concentration of the first nitride semiconductor layer in the latter half of the deposition of the second nitride semiconductor layer.

[0020] Such a method for manufacturing a nitride semiconductor device can suppress the influence of fluctuations in the carbon concentration in the first nitride semiconductor layer, and can suppress variations in the carrier concentration.

[0021] The present invention also provides a nitride semiconductor device comprising: a first nitride semiconductor layer of a first conductivity type; a second nitride semiconductor layer of a first conductivity type stacked on the first nitride semiconductor layer and having a higher carrier concentration than the first nitride semiconductor layer; a third nitride semiconductor layer of a second conductivity type stacked on the second nitride semiconductor layer; a fourth nitride semiconductor layer of a first conductivity type stacked on the third nitride semiconductor layer; a first main electrode electrically connected to the first nitride semiconductor layer; a second main electrode electrically connected to the fourth nitride semiconductor layer; and a control electrode provided on the third nitride semiconductor layer via an insulating film, wherein the second nitride semiconductor layer has a higher carbon concentration at least in an upper part thereof than the carbon concentration of the first nitride semiconductor layer.

[0022] In such a nitride semiconductor device, the first nitride semiconductor layer is a low-concentration carrier region with a lower carrier concentration than the second nitride semiconductor layer, but at the same time, the carbon concentration itself is low, thereby suppressing the effect of variations in carbon concentration on the carrier concentration and reducing variations in carrier concentration. Therefore, variations in carrier concentration in the low-concentration carrier region and the resulting variations in the breakdown voltage of the nitride semiconductor device are reduced. Additionally, the second nitride semiconductor layer has a high carbon concentration at least in the upper part, but also has a high-concentration carrier region with a higher carrier concentration than the first nitride semiconductor layer, thereby suppressing the effect of variations in carbon concentration on the carrier concentration and reducing variations in carrier concentration. This results in a nitride semiconductor device with low on-resistance and high breakdown voltage.

[0023] It is also preferable that a fifth nitride semiconductor layer of the first conductivity type having a lower carrier concentration than the second nitride semiconductor layer is included between the second nitride semiconductor layer and the third nitride semiconductor layer, and that the carrier concentration in the fifth nitride semiconductor layer is higher than the carrier concentration in the first nitride semiconductor layer.

[0024] In such a nitride semiconductor device, the carrier concentration of the fifth nitride semiconductor layer is lower than that of the second nitride semiconductor layer but higher than that of the first nitride semiconductor layer, so the effect of variations in carbon concentration on the carrier concentration is suppressed more than in the first nitride semiconductor layer, resulting in reduced variations in carrier concentration. Furthermore, since a depletion layer extends from the interface between the fifth nitride semiconductor layer and the second conductivity type third nitride semiconductor layer, suppressing the effect of variations in carbon concentration on the carrier concentration can reduce variations in the breakdown voltage of the nitride semiconductor device caused by this.

[0025] Preferably, the peak oxygen concentration of the second nitride semiconductor layer is higher than the peak oxygen concentration of the first nitride semiconductor layer.

[0026] In such a nitride semiconductor device, the donor concentration can be increased in the second nitride semiconductor layer, which has a higher carrier concentration than the first nitride semiconductor layer.

[0027] Preferably, the second nitride semiconductor layer has a higher carbon concentration in the upper portion than in the lower portion.

[0028] In such a nitride semiconductor device, the influence of fluctuations in the carbon concentration in the first nitride semiconductor layer is suppressed, and variations in the carrier concentration are suppressed. [Effects of the Invention]

[0029] As described above, the method for manufacturing a nitride semiconductor device of the present invention increases the carbon concentration of the second nitride semiconductor layer, which has a higher carrier concentration than the first nitride semiconductor layer, at some point during the formation of the second nitride semiconductor layer. This means that the first nitride semiconductor layer has a low-carrier concentration region with a lower carrier concentration than the second nitride semiconductor layer, but at the same time, the carbon concentration itself is low, thereby suppressing the effect of variations in carbon concentration on the carrier concentration and reducing carrier concentration variations. This reduces variations in the carrier concentration of the low-carrier region and the resulting variations in the breakdown voltage of the nitride semiconductor device. Additionally, the second nitride semiconductor layer has a higher carbon concentration, but also includes a high-carrier region with a higher carrier concentration than the first nitride semiconductor layer. This reduces the effect of variations in carbon concentration on the carrier concentration and reduces carrier concentration variations. As a result, a nitride semiconductor device with low on-resistance and high breakdown voltage can be manufactured.

[0030] Furthermore, in the nitride semiconductor device of the present invention, the second nitride semiconductor layer has a higher carrier concentration than the first nitride semiconductor layer, and at least the upper portion thereof has a higher carbon concentration than the first nitride semiconductor layer. That is, the first nitride semiconductor layer has a low-concentration carrier region with a lower carrier concentration than the second nitride semiconductor layer, but at the same time, the carbon concentration itself is low. This suppresses the effect of variations in carbon concentration on the carrier concentration, thereby suppressing variations in carrier concentration. Therefore, variations in carrier concentration in the low-concentration carrier region and the resulting variations in breakdown voltage of the nitride semiconductor device are reduced. Additionally, the second nitride semiconductor layer has a high-concentration carrier region with a higher carrier concentration than the first nitride semiconductor layer, while at least the upper portion thereof has a high-concentration carrier region. This suppresses the effect of variations in carbon concentration on the carrier concentration, thereby suppressing variations in carrier concentration. This results in a nitride semiconductor device with low on-resistance and high breakdown voltage. [Brief explanation of the drawings]

[0031] [Figure 1] 1 is a cross-sectional view showing an embodiment of a nitride semiconductor device and a manufacturing method of the present invention. [Figure 2] 10 is a graph showing changes in each carrier concentration in each nitride semiconductor layer. [Figure 3] 10 is a graph showing the change in donor concentration in each nitride semiconductor layer. [Figure 4] FIG. 1 is a cross-sectional view of a conventional nitride semiconductor device. DETAILED DESCRIPTION OF THE INVENTION

[0032] As described above, there has been a demand for a method for manufacturing a nitride semiconductor device and a nitride semiconductor device that reduce variations in carrier concentration in the low-concentration carrier region and the resulting variations in breakdown voltage of the nitride semiconductor device.

[0033] Therefore, the inventors conducted extensive research and found that by making the carbon concentration higher than the carbon concentration of the first nitride semiconductor layer during film formation of the second nitride semiconductor layer, which has a higher carrier concentration than the first nitride semiconductor layer, the effect of fluctuations in carbon concentration on the carrier concentration can be suppressed, and the variation in carrier concentration can be suppressed, thereby completing the present invention.

[0034] That is, the present invention is a method for manufacturing a nitride semiconductor device comprising: a first nitride semiconductor layer of a first conductivity type; a second nitride semiconductor layer of a first conductivity type stacked on the first nitride semiconductor layer and having a higher carrier concentration than the first nitride semiconductor layer; a third nitride semiconductor layer of a second conductivity type stacked on the second nitride semiconductor layer; a fourth nitride semiconductor layer of a first conductivity type stacked on the third nitride semiconductor layer; a first main electrode electrically connected to the first nitride semiconductor layer; a second main electrode electrically connected to the fourth nitride semiconductor layer; and a control electrode provided on the third nitride semiconductor layer via an insulating film, wherein the first nitride semiconductor layer, the second nitride semiconductor layer, the third nitride semiconductor layer, and the fourth nitride semiconductor layer are formed in this order, and the carbon concentration of the second nitride semiconductor layer is made higher than the carbon concentration of the first nitride semiconductor layer any time after the start of formation of the second nitride semiconductor layer and before the end of formation of the second nitride semiconductor layer.

[0035] The present invention also provides a nitride semiconductor device comprising: a first nitride semiconductor layer of a first conductivity type; a second nitride semiconductor layer of a first conductivity type stacked on the first nitride semiconductor layer and having a higher carrier concentration than the first nitride semiconductor layer; a third nitride semiconductor layer of a second conductivity type stacked on the second nitride semiconductor layer; a fourth nitride semiconductor layer of a first conductivity type stacked on the third nitride semiconductor layer; a first main electrode electrically connected to the first nitride semiconductor layer; a second main electrode electrically connected to the fourth nitride semiconductor layer; and a control electrode provided on the third nitride semiconductor layer via an insulating film, wherein the second nitride semiconductor layer has a carbon concentration higher than that of the first nitride semiconductor layer at least in an upper portion thereof.

[0036] The present invention will be described in detail below with reference to the drawings, but the present invention is not limited thereto. In this invention, the terms "upper" and "lower" do not refer to thickness, but merely to relative positional relationships. The thickness of the "upper" and the "lower" are not limited to being the same. The present invention also includes cases where the "upper" is thicker than the "lower" or where the "upper" is thinner than the "lower." Furthermore, "on" includes not only cases where the "upper" is formed in contact with the object, but also cases where the "upper" is formed via another layer. Even if "on" is provided on the side, it falls within the scope of the present invention as long as it is substantially the same as the constituent elements of the present invention. Furthermore, in this invention, "connection" is not limited to direct connection; even if a connection is made via something such as a resistor, it falls within the scope of the present invention as long as it is substantially the same as the constituent elements of the present invention.

[0037] FIG. 1 is a diagram showing an embodiment of a nitride semiconductor device and a method for manufacturing the same according to the present invention.

[0038] Figure 1(a) shows the first half of the nitride semiconductor device formed using the HVPE method, Figure 1(b) shows the second half of the nitride semiconductor device formed using the MOVPE method, and Figure 1(c) shows a cross section of the nitride semiconductor device.

[0039] First, by HVPE, a first conductivity type (e.g., N-type) first nitride semiconductor layer (low-concentration carrier region) 11 and a front half portion 12a of a first conductivity type (N-type) second nitride semiconductor layer (high-concentration carrier region) 12 having a higher carrier concentration than the first nitride semiconductor layer 11 are formed on a substrate (low-resistivity GaN substrate) 10.

[0040] After the start of deposition of second nitride semiconductor layer 12 and before the end of deposition of second nitride semiconductor layer 12, the method is switched to MOVPE to deposit a second half portion 12b of second nitride semiconductor layer 12, a fifth nitride semiconductor layer (low-concentration carrier region) 15 of a first conductivity type (N-type) having a carrier concentration lower than that of second nitride semiconductor layer 12 and higher than that of first nitride semiconductor layer 11, a third nitride semiconductor layer (p-type semiconductor layer) 13 of a second conductivity type (P-type), and a fourth nitride semiconductor layer (n-type semiconductor layer) 14 of the first conductivity type (N-type). Note that the thickness of first half portion 12a of second nitride semiconductor layer 12 and the thickness of second half portion 12b of second nitride semiconductor layer (high-concentration carrier region) 12 may be approximately equal, or the thickness of first half portion 12a may be thicker than that of second half portion 12b, or the thickness of first half portion 12a may be thinner than that of second half portion 12b.

[0041] The semiconductor device may also be configured to have a first main electrode (drain electrode) 23 electrically connected to the substrate 10, a second main electrode (source electrode) 22 electrically connected to the fourth nitride semiconductor layer 14, and a control electrode (gate electrode) 21 provided via an insulating film 20 in a trench 19 that penetrates the third nitride semiconductor layer 13 and reaches the fifth nitride semiconductor layer 15.

[0042] 4 can be utilized in many parts. However, in the past, the low-concentration carrier region 121 (corresponding to the first nitride semiconductor layer 11 in this embodiment) to the high-concentration carrier region 123 (corresponding to the second nitride semiconductor layer 12 in this embodiment) were formed by the same method (MOVPE method), and the high-concentration carrier region 123 was only one layer. In contrast, in this embodiment, the film formation method is switched (from HVPE method to MOVPE method) between the first half 12a and the second half 12b of the second nitride semiconductor layer 12 in the high-concentration carrier region.

[0043] Here, in the method for manufacturing a nitride semiconductor device of the present invention, it is necessary to make the carbon concentration of second nitride semiconductor layer 12 higher than the carbon concentration of first nitride semiconductor layer 11 at any time after the start of deposition of second nitride semiconductor layer 12 until the end of deposition of second nitride semiconductor layer 12.

[0044] It is preferable to make the carbon concentration of the second nitride semiconductor layer higher than that of the first nitride semiconductor layer in the latter half of the deposition of the second nitride semiconductor layer. Of course, the carbon concentration may be made higher from the first half.

[0045] Furthermore, in the nitride semiconductor device of the present invention, the carbon concentration of at least the upper part of second nitride semiconductor layer 12 must be higher than the carbon concentration of first nitride semiconductor layer 11 .

[0046] It is more preferable that the carbon concentration in the upper part of the second nitride semiconductor layer 12 is higher than that in the lower part.

[0047] In this case, the carbon concentration of the first half portion 12a located at the bottom of the second nitride semiconductor layer 12 is equal to the carbon concentration of the first nitride semiconductor layer 11, and the carbon concentration of the second half portion 12b located at the top of the second nitride semiconductor layer 12 is higher than the carbon concentration of the first nitride semiconductor layer 11.

[0048] During deposition of the second nitride semiconductor layer 12, which has a higher carrier concentration than the first nitride semiconductor layer 11, the carbon concentration of the upper rear half portion 12b is made higher than that of the first nitride semiconductor layer 11. This means that the first nitride semiconductor layer 11 becomes a low-concentration carrier region with a lower carrier concentration than the second nitride semiconductor layer 12, but at the same time, its carbon concentration is low. This suppresses the effect of variations in carbon concentration on the carrier concentration, thereby suppressing variations in carrier concentration. This reduces variations in carrier concentration in the low-concentration carrier region and the resulting variations in the breakdown voltage of the nitride semiconductor device. Additionally, the upper rear half portion 12b of the second nitride semiconductor layer 12 has a higher carbon concentration, but at the same time, it is a high-concentration carrier region with a higher carrier concentration than the first nitride semiconductor layer 11. This suppresses the effect of variations in carbon concentration on the carrier concentration, thereby suppressing variations in carrier concentration. As a result, a nitride semiconductor device with low on-resistance and high breakdown voltage can be manufactured.

[0049] Although not particularly limited, it is preferable to form first nitride semiconductor layer 11 by HVPE, and then switch from HVPE to MOVPE after the start of deposition of second nitride semiconductor layer 12 and before the end of deposition of second nitride semiconductor layer 12. This is because HVPE is a method that introduces little carbon, while MOVPE is a method that introduces some carbon, and by switching from HVPE to MOVPE, the carbon concentration can be easily increased midway through deposition.

[0050] It is more preferable to make the oxygen peak concentration of second nitride semiconductor layer 12 higher than the oxygen peak concentration of first nitride semiconductor layer 11. Therefore, in this embodiment, oxygen-containing region 16 is formed from first half portion 12a located in the lower part of second nitride semiconductor layer 12 to second half portion 12b located in the upper part. The method for forming oxygen-containing region 16 is not particularly limited, but since oxygen tends to be easily mixed into the surface of the nitride semiconductor layer when the reactor is changed from HVPE to MOVPE, it is easy to form oxygen-containing region 16 by taking advantage of this tendency.

[0051] As a result, in the second nitride semiconductor layer 12, which has a higher carrier concentration than the first nitride semiconductor layer 11, both the carbon concentration and the oxygen peak concentration are made higher than those in the first nitride semiconductor layer 11, so that the concentrations of both carbon, which can be a p-type acceptor element, and oxygen, which can be an n-type donor element, become higher, and the carrier concentrations cancel each other out, making it possible to suppress fluctuations in the carrier concentration.

[0052] The nitride semiconductor device thus fabricated has a low-carbon region 17 and a carbon-containing region 18 thereon. Although not particularly limited, the thickness of the low-carbon region 17 is preferably 10 μm or more to ensure the breakdown voltage of the nitride semiconductor device. Of course, the thickness can be set depending on the breakdown voltage required for the nitride semiconductor device.

[0053] While it has been stated above that the conventional configuration of Fig. 4 can be used in many parts, the present invention is not limited to the trench gate type as shown in Fig. 4. The bottom of the trench may be in the deeper low-concentration carrier region 121 (corresponding to the first nitride semiconductor layer 11 of this embodiment) rather than the low-concentration carrier region 125 of Fig. 4 (corresponding to the fifth nitride semiconductor layer 15 of this embodiment). Furthermore, the low-concentration carrier region 125 (corresponding to the fifth nitride semiconductor layer 15 of this embodiment) does not necessarily have to be formed.

[0054] [Method of manufacturing a nitride semiconductor device] In the manufacturing method of this embodiment, although not particularly limited, the first nitride semiconductor layer 11, which is a low-concentration carrier region on the substrate, has a donor concentration of, for example, n×10 15 [cm -3 ], and it is preferable to precisely control the carbon concentration. Furthermore, although not particularly limited, it is preferable to make first nitride semiconductor layer 11, which is a low-carrier-concentration region, relatively thick in order to ensure a sufficient breakdown voltage. For this reason, it is preferable to form it using the HVPE method, which has a fast growth rate and little carbon contamination.

[0055] Then, after the start of deposition of the second nitride semiconductor layer (high concentration carrier region) 12 on the first nitride semiconductor layer (low concentration carrier region) 11, the method is switched from the HVPE method to the MOVPE method before the completion of deposition of the second nitride semiconductor layer (high concentration carrier region) 12. The method may be switched to the MOVPE method either at the beginning of deposition of the second nitride semiconductor layer (high concentration carrier region) 12 or during deposition of the second nitride semiconductor layer (high concentration carrier region) 12.

[0056] In this embodiment, the carbon concentration of the second nitride semiconductor layer (high-concentration carrier region) 12 is high; specifically, the carbon concentration of the second nitride semiconductor layer (high-concentration carrier region) 12 is higher than the carbon concentration of the first nitride semiconductor layer (low-concentration carrier region) 11. On the other hand, the n-type carrier concentration of, for example, silicon (silicon (Si)) or the like in the second nitride semiconductor layer (high-concentration carrier region) 12 is doped to be relatively higher (for example, about one order of magnitude higher) than the carbon concentration. Therefore, even if the carbon concentration fluctuates slightly due to the influence of the growth temperature, it is unlikely to lead to variations in the carrier concentration, and the effect on the breakdown voltage of the nitride semiconductor device is small.

[0057] Furthermore, when the nitride semiconductor device is off, a depletion layer spreads from the interface between the third nitride semiconductor layer 13, which is a p-type semiconductor layer, and the n-type semiconductor layer (the low-concentration carrier region (fifth nitride semiconductor layer 15), or the high-concentration carrier region (second nitride semiconductor layer 12) in the case of a nitride semiconductor device without the fifth nitride semiconductor layer 15) that is in contact with the third nitride semiconductor layer 13. Since it is desired that the depletion layer spread quickly to the low-concentration carrier region (first nitride semiconductor layer 11) below the high-concentration carrier region (second nitride semiconductor layer 12), the high-concentration carrier region (second nitride semiconductor layer 12) is formed relatively thin. For this reason, even if the carbon concentration becomes high in the high-concentration carrier region (second nitride semiconductor layer 12), the impact on the breakdown voltage of the nitride semiconductor device is small.

[0058] It is also possible to use the oxygen at the interface that is mixed in when switching from HVPE to MOVPE as a donor when forming the high-concentration carrier region (second nitride semiconductor layer 12), and to use the carbon that is mixed in after switching to MOVPE as a high-resistance material (N-type suppressor). In this case, the concentrations are, for example, as shown in the following example. [Example]

[0059] Figure 2 is a graph showing the change in each carrier concentration, and Figure 3 is a graph showing the change in donor concentration. In the figures, the low-concentration carrier region [1] corresponds to the first nitride semiconductor layer 11 in Figure 1, the high-concentration carrier region [1] corresponds to the first half 12a of the second nitride semiconductor layer, the high-concentration carrier region [2] corresponds to the second half 12b of the second nitride semiconductor layer, and the low-concentration carrier region [2] corresponds to the fifth nitride semiconductor layer 15, and all figures show the case of n-type semiconductor layers.

[0060] <Low-concentration carrier region[1]> First, referring to Figure 2, in the low-concentration carrier region [1] formed by HVPE, the concentration of the donor Si (silicon) is low, but the concentration of the acceptor carbon is much lower.

[0061] The low carbon concentration itself can suppress the influence of variations in carbon concentration on the carrier concentration (here, the donor concentration), thereby suppressing variations in carrier concentration, thereby reducing variations in carrier concentration in the low-carrier-concentration region and the resulting variations in breakdown voltage of the nitride semiconductor device.

[0062] <High-concentration carrier region[1]> The next region, the high carrier concentration region [1], is deposited using the HVPE method, and has a high concentration of Si (silicon) while the carbon concentration remains low.

[0063] Therefore, the difference between the Si concentration and the carbon concentration increases, and the influence of fluctuations in the carbon concentration can be further suppressed.

[0064] <High-concentration carrier region[2]> Next, when the MOVPE method is used, the silicon concentration remains high, but the carbon concentration increases. At this time, the concentration of oxygen, which acts as a donor, also increases, resulting in a peak in the oxygen concentration.

[0065] Here, it might seem that the effect of fluctuations in carbon concentration would increase as the carbon concentration increases, but since the concentration of Si, which acts as a donor, is high to begin with, and the concentration of oxygen, which also acts as a donor, also increases, the ratio of carbon concentration to donor concentration does not increase, and the effect of fluctuations in carbon concentration can continue to be suppressed.

[0066] <Low-concentration carrier region[2]> Next, in the low-concentration carrier region [2], the concentration of Si (silicon) decreases but is still higher than in the low-concentration carrier region [1], and the concentration of carbon remains high.

[0067] By not lowering the Si (silicon) concentration in the low-concentration carrier region [2] too much to the same level as the Si (silicon) concentration in the low-concentration carrier region [1], the effect of fluctuations in carbon concentration can be suppressed to some extent.

[0068] This can be expressed as a change in donor concentration (the difference between donors and acceptors), as shown in Figure 3. The donor concentration is lowest in the low-concentration carrier region [1], and does not become lower thereafter.

[0069] In this example, oxygen doping is performed when switching from HVPE to MOVPE, and a peak occurs in the high-concentration carrier region [1]-[2] due to oxygen doping. If the Si doping amount is not changed in the high-concentration carrier region, the region above the switching point is doped with carbon, which acts as an acceptor, resulting in a relative decrease in the carrier concentration (which acts as a donor). The low-concentration carrier region [2] above the high-concentration carrier region [2] suppresses the effect of variations in the carbon doping amount and suppresses variations in the breakdown voltage of the nitride semiconductor device, so the carrier concentration of the low-concentration carrier region [2] is higher than that of the low-concentration carrier region [1] (the Si doping amount is higher as shown in Figure 2).

[0070] The depletion layer spreads from the interface of the n-type semiconductor layer in contact with the p-type semiconductor layer. As shown in Figure 3, the carrier concentration is low in the upper part of the high-concentration carrier region (high-concentration carrier region [2]), which makes it easier for the depletion layer to spread within the high-concentration carrier region, thereby increasing the breakdown voltage of the semiconductor device.

[0071] Although there are no particular limitations on the timing for switching from HVPE to MOVPE, it is preferable to switch before reaching halfway through the final thickness of the high-concentration carrier region. This reduces the suppression of the depletion layer expansion caused by the large carrier increase above the high-concentration carrier region due to the oxygen doping introduced when switching from HVPE to MOVPE.

[0072] [Nitride semiconductor devices] The above mainly describes a method for manufacturing a nitride semiconductor device. Here, we will provide additional information about nitride semiconductor devices.

[0073] In the nitride semiconductor device of this embodiment, the carbon concentration of the high-concentration carrier region is higher than the carbon concentration of the low-concentration carrier region [1]. As a result, the carbon concentration is likely to vary depending on manufacturing temperature conditions, etc. However, by lowering the carbon concentration of the low-concentration carrier region [1], the influence of carbon doping due to manufacturing temperature conditions, etc. can be reduced even if the carrier concentration of the low-concentration carrier region [1] is lowered, and a semiconductor device with small variations in breakdown voltage and relatively small on-resistance and high breakdown voltage can be provided.

[0074] Furthermore, it is desirable that the carbon concentration in the upper part of the high-concentration carrier region (high-concentration carrier region [2]) is higher than the carbon concentration in the lower part of the high-concentration carrier region (high-concentration carrier region [1]). For example, when the Si donor concentration in the high-concentration carrier regions ([1]-[2]) is constant in the thickness direction as shown in FIG. 2, the carrier concentration (donor concentration) in the upper part of the high-concentration carrier region (high-concentration carrier region [2]) is lower than the carrier concentration (donor concentration) in the lower part of the high-concentration carrier region (high-concentration carrier region [1]) (see FIG. 3). This makes it easier for a depletion layer to expand in the upper part of the high-concentration carrier region and for current to be dispersed in the lower part. As a result, it is possible to provide a nitride semiconductor device with a relatively low on-resistance and a high breakdown voltage.

[0075] Furthermore, by doping the high-concentration carrier region with oxygen, the carrier concentration (donor concentration) of the high-concentration carrier region can be increased.

[0076] In another embodiment, the low-concentration carrier region [1] may be formed without doping silicon, since oxygen in the device is doped into the semiconductor surface at the initial stage of its formation. This improves the crystallinity of the low-concentration carrier region [1].

[0077] The present invention includes the following aspects. [1]: a first conductivity type first nitride semiconductor layer; a second nitride semiconductor layer of a first conductivity type stacked on the first nitride semiconductor layer and having a higher carrier concentration than the first nitride semiconductor layer; a third nitride semiconductor layer of a second conductivity type stacked on the second nitride semiconductor layer; a fourth nitride semiconductor layer of the first conductivity type stacked on the third nitride semiconductor layer; a first main electrode electrically connected to the first nitride semiconductor layer; a second main electrode electrically connected to the fourth nitride semiconductor layer; a control electrode provided on the third nitride semiconductor layer via an insulating film; A method for manufacturing a nitride semiconductor device comprising: depositing the first nitride semiconductor layer, the second nitride semiconductor layer, the third nitride semiconductor layer, and the fourth nitride semiconductor layer in this order; a carbon concentration of the second nitride semiconductor layer being made higher than the carbon concentration of the first nitride semiconductor layer at any time from the start of deposition of the second nitride semiconductor layer to the end of deposition of the second nitride semiconductor layer. [2]: The first nitride semiconductor layer is formed by an HVPE method; The method for manufacturing a nitride semiconductor device according to the above-mentioned [1], wherein the deposition of the second nitride semiconductor layer is performed by switching from the HVPE method to the MOVPE method after the start of deposition of the second nitride semiconductor layer and before the end of deposition of the second nitride semiconductor layer. [3]: forming a fifth nitride semiconductor layer of a first conductivity type between the second nitride semiconductor layer and the third nitride semiconductor layer, the fifth nitride semiconductor layer having a carrier concentration lower than that of the second nitride semiconductor layer; The method for manufacturing a nitride semiconductor device according to the above [1] or [2], wherein the carrier concentration in the fifth nitride semiconductor layer is made higher than the carrier concentration in the first nitride semiconductor layer. [4]: The method for manufacturing a nitride semiconductor device according to any one of [1] to [3] above, wherein the oxygen peak concentration of the second nitride semiconductor layer is made higher than the oxygen peak concentration of the first nitride semiconductor layer. [5]: The method for manufacturing a nitride semiconductor device according to any one of [1] to [4] above, wherein the carbon concentration of the second nitride semiconductor layer is made higher than the carbon concentration of the first nitride semiconductor layer in the latter half of the formation of the second nitride semiconductor layer. [6]: a first conductivity type first nitride semiconductor layer; a second nitride semiconductor layer of a first conductivity type stacked on the first nitride semiconductor layer and having a higher carrier concentration than the first nitride semiconductor layer; a third nitride semiconductor layer of a second conductivity type stacked on the second nitride semiconductor layer; a fourth nitride semiconductor layer of the first conductivity type stacked on the third nitride semiconductor layer; a first main electrode electrically connected to the first nitride semiconductor layer; a second main electrode electrically connected to the fourth nitride semiconductor layer; a control electrode provided on the third nitride semiconductor layer via an insulating film; A nitride semiconductor device comprising: The nitride semiconductor device is characterized in that the carbon concentration of at least an upper portion of the second nitride semiconductor layer is higher than the carbon concentration of the first nitride semiconductor layer. [7]: a fifth nitride semiconductor layer of a first conductivity type between the second nitride semiconductor layer and the third nitride semiconductor layer, the fifth nitride semiconductor layer having a carrier concentration lower than that of the second nitride semiconductor layer; The nitride semiconductor device according to [6] above, wherein the carrier concentration in the fifth nitride semiconductor layer is higher than the carrier concentration in the first nitride semiconductor layer. [8]: The nitride semiconductor device according to the above [6] or [7], wherein the second nitride semiconductor layer has a higher oxygen peak concentration than the first nitride semiconductor layer. [9]: The nitride semiconductor device according to any one of [6] to [8] above, wherein the second nitride semiconductor layer has a higher carbon concentration in an upper portion than in a lower portion.

[0078] The present invention is not limited to the above-described embodiments. The above-described embodiments are merely illustrative, and any configuration that is substantially identical to the technical concept described in the claims of the present invention and that provides similar effects is encompassed within the technical scope of the present invention. For example, a semiconductor device with a trench MIS structure in which a trench 19 is formed in the surface of a nitride semiconductor layer and a gate electrode 21 is provided via an insulating film 20 has been shown as an example, but the present invention may also be applied to a planar semiconductor device such as a DMOSFET in which a gate electrode is provided via an insulating film on the surface of a nitride semiconductor. [Explanation of symbols]

[0079] 10...Substrate (low-resistivity GaN substrate), 11...First nitride semiconductor layer (low-concentration carrier region), 12, 12a, 12b...Second nitride semiconductor layer (high-concentration carrier region), 13...Third nitride semiconductor layer (p-type semiconductor layer), 14...Fourth nitride semiconductor layer (n-type semiconductor layer), 15...Fifth nitride semiconductor layer (low-concentration carrier region), 16...Oxygen-containing region, 17...Low-carbon region, 18...Carbon-containing region, 19...Trench, 20...Insulating film, 21...Gate electrode, 22...Source electrode, 23...Drain electrode, 110...Substrate, 121, 125...Low-concentration carrier region, 123...High-concentration carrier region, 130...P-type semiconductor layer, 140...N-type semiconductor layer, 152...Trench, 160...Insulating film, 172...Gate electrode, 174...Source electrode 178...Drain electrode.

Claims

1. a first nitride semiconductor layer of a first conductivity type; a second nitride semiconductor layer of a first conductivity type stacked on the first nitride semiconductor layer and having a higher carrier concentration than the first nitride semiconductor layer; a third nitride semiconductor layer of a second conductivity type stacked on the second nitride semiconductor layer; a fourth nitride semiconductor layer of the first conductivity type stacked on the third nitride semiconductor layer; a first main electrode electrically connected to the first nitride semiconductor layer; a second main electrode electrically connected to the fourth nitride semiconductor layer; a control electrode provided on the third nitride semiconductor layer via an insulating film; A method for manufacturing a nitride semiconductor device comprising: the first nitride semiconductor layer, the second nitride semiconductor layer, the third nitride semiconductor layer, and the fourth nitride semiconductor layer are deposited in this order; a carbon concentration of the second nitride semiconductor layer being made higher than the carbon concentration of the first nitride semiconductor layer at any time from the start of deposition of the second nitride semiconductor layer to the end of deposition of the second nitride semiconductor layer.

2. The first nitride semiconductor layer is formed by an HVPE method; 2. The method for manufacturing a nitride semiconductor device according to claim 1, wherein the deposition of the second nitride semiconductor layer is performed by switching from the HVPE method to the MOVPE method after the start of deposition of the second nitride semiconductor layer and before the end of deposition of the second nitride semiconductor layer.

3. forming a fifth nitride semiconductor layer of a first conductivity type between the second nitride semiconductor layer and the third nitride semiconductor layer, the fifth nitride semiconductor layer having a carrier concentration lower than that of the second nitride semiconductor layer; 2. The method for manufacturing a nitride semiconductor device according to claim 1, wherein the carrier concentration in the fifth nitride semiconductor layer is made higher than the carrier concentration in the first nitride semiconductor layer.

4. 2. The method for manufacturing a nitride semiconductor device according to claim 1, wherein the peak oxygen concentration of the second nitride semiconductor layer is made higher than the peak oxygen concentration of the first nitride semiconductor layer.

5. 5. The method for manufacturing a nitride semiconductor device according to claim 1, wherein the carbon concentration of the second nitride semiconductor layer is made higher than the carbon concentration of the first nitride semiconductor layer in a latter half of the formation of the second nitride semiconductor layer.

6. a first nitride semiconductor layer of a first conductivity type; a second nitride semiconductor layer of a first conductivity type stacked on the first nitride semiconductor layer and having a higher carrier concentration than the first nitride semiconductor layer; a third nitride semiconductor layer of a second conductivity type stacked on the second nitride semiconductor layer; a fourth nitride semiconductor layer of the first conductivity type stacked on the third nitride semiconductor layer; a first main electrode electrically connected to the first nitride semiconductor layer; a second main electrode electrically connected to the fourth nitride semiconductor layer; a control electrode provided on the third nitride semiconductor layer via an insulating film; A nitride semiconductor device comprising: The nitride semiconductor device is characterized in that the carbon concentration of at least an upper portion of the second nitride semiconductor layer is higher than the carbon concentration of the first nitride semiconductor layer.

7. a fifth nitride semiconductor layer of a first conductivity type having a carrier concentration lower than that of the second nitride semiconductor layer is included between the second nitride semiconductor layer and the third nitride semiconductor layer; 7. The nitride semiconductor device according to claim 6, wherein the carrier concentration in the fifth nitride semiconductor layer is higher than the carrier concentration in the first nitride semiconductor layer.

8. 7. The nitride semiconductor device according to claim 6, wherein the second nitride semiconductor layer has a higher oxygen peak concentration than the first nitride semiconductor layer.

9. 9. The nitride semiconductor device according to claim 6, wherein the second nitride semiconductor layer has a higher carbon concentration in an upper portion than in a lower portion.

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