JP5327778B2 - Semiconductor device and manufacturing method thereof - Google Patents

Description

  The present invention relates to a semiconductor device including a semiconductor layer crystal-grown on a substrate and a method for manufacturing the same.

  When manufacturing a semiconductor device using a nitride-based semiconductor such as gallium nitride, in addition to a method of growing a nitride-based semiconductor layer on the nitride-based semiconductor substrate, a sapphire substrate, a SiC substrate, ZnO There is a method of growing a nitride-based semiconductor layer on a different substrate such as a substrate. When different types of substrates are used, there is a problem that crystal defects are likely to occur in the grown nitride semiconductor layer because the crystal lattice between the substrate and the nitride semiconductor is mismatched. In order to improve the lifetime of the element, it is necessary to improve the crystallinity.

  Various techniques have been proposed to improve the crystallinity of the nitride-based semiconductor on the substrate. For example, in Patent Document 1, an island-like crystal region is grown on a sapphire substrate at a slow crystal growth rate, and then the island-like crystal region is further grown at a high crystal growth rate to bend the direction of crystal dislocation. A method for reducing the dislocation density is disclosed.

  On the other hand, Patent Document 2 discloses a method in which a multilayer structure buffer layer in which two or more types of nitride-based semiconductor layers having different compositions are sequentially stacked on a substrate is disposed, and a nitride-based semiconductor layer is formed thereon. It is disclosed. Thus, it is described that a nitride semiconductor layer having excellent flatness as well as crystallinity can be obtained, and the characteristics of the field effect transistor can be improved.

JP 2002-313733 A JP 2004-296717 A

  In the case of manufacturing a light emitting element, it is important for improving the light emission efficiency to grow a semiconductor layer that has not only improved crystallinity but also excellent flatness with less unevenness. However, the technique of Patent Document 1 aims at obtaining a crystal having a low dislocation density, and does not consider improvement of flatness. Although the technique of Patent Document 2 describes the improvement of the characteristics of a field effect transistor by using a multi-layered buffer layer having a plurality of types of compositions, the effectiveness and optimization in the case where the semiconductor element is a light emitting element are described. Is unknown. In addition, since a plurality of kinds of compositions are used for the buffer layer, the structure becomes complicated.

  An object of the present invention is to provide an optical semiconductor device including a semiconductor layer having excellent crystallinity and flatness.

  In order to achieve the above object, according to the present invention, the following semiconductor device is provided. That is, a semiconductor element having a substrate, a buffer layer disposed on the substrate, and a semiconductor layer crystal-grown on the buffer layer, the buffer layer being a first layer crystal-grown at a predetermined growth rate And a second layer obtained by crystal growth at a faster growth rate than the first layer, and a semiconductor element having a structure in which two or more sets are stacked. According to the inventors, by stacking two or more pairs of a first layer having a slow growth rate and a second layer having a fast growth rate, the unevenness on the upper surface is significantly reduced while maintaining high crystallinity. be able to.

  The first layer and the second layer can have the same composition. Thereby, crystallinity and flatness can be improved while having a simple structure in which the composition is not changed.

  It is preferable to set a configuration in which the second layer is disposed on the first layer.

  The first layer formed at a low growth rate has a predetermined crystallinity and a predetermined flatness of the upper surface, and the second layer formed at a higher growth rate has a lower crystallinity than the first layer, but the upper surface. Excellent flatness. A buffer having both crystallinity and flatness can be obtained by stacking two or more sets of the first layer and the second layer.

  The growth rate of the second layer is set to, for example, not less than 2 times and not more than 5 times the growth rate of the first layer. For example, the thickness of the first layer is set to about 20 nm, and the thickness of the second layer is set to about 80 nm.

The first layer, the second layer, and the semiconductor layer are all represented by, for example, Al _x In _y Ga _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1). It is possible to use a different type of substrate as the substrate. In the present invention, a semiconductor layer having excellent crystallinity and flatness can be grown by using a multilayer buffer layer.

  According to a third aspect of the present invention, there is provided a semiconductor device having a substrate, a buffer layer disposed on the substrate, and a semiconductor layer crystal-grown on the buffer layer, A set of a first layer having crystallinity and flatness on a predetermined upper surface and a second layer having lower crystallinity than the first layer but excellent in flatness on the upper surface, and two or more sets thereof A semiconductor element having a stacked configuration is provided.

  According to the second aspect of the present invention, the first layer is crystal-grown on the substrate at a predetermined growth rate, and the second layer is grown on the first layer at a growth rate faster than that of the first layer. And a method of forming a multilayer buffer layer by repeatedly laminating two or more layers, and a method of crystal growth of a predetermined semiconductor layer on the multilayer buffer layer. Is provided.

  In the above manufacturing method, the growth rate of the second layer is set to, for example, not less than 2 times and not more than 5 times the growth rate of the first layer.

  According to the present inventor, by stacking two or more pairs of a first layer having a slow growth rate and a second layer having a fast growth rate, the unevenness on the upper surface is significantly reduced while maintaining high crystallinity. be able to. By using this as a buffer layer, a semiconductor layer having high crystallinity and flatness can be grown thereon. Therefore, when applied to a light-emitting element, light emission intensity can be improved.

  In addition, since the first layer and the second layer can have the same composition, crystallinity and flatness can be improved while having a simple structure that does not change the composition.

  An embodiment of the present invention will be described with reference to the drawings for an optical semiconductor element.

First, the structure of the optical semiconductor element of this embodiment will be described with reference to FIG. This semiconductor light emitting device includes a low-temperature buffer layer 2, a multilayer buffer layer 3, an n-type semiconductor layer 4, an active layer 5, a p-type cladding layer 6, and a p-type, all made of a nitride compound semiconductor on a substrate 1. In this structure, semiconductor layers 7 are stacked. In the present embodiment, the nitride-based compound semiconductor is a composition represented by In _x Al _y Ga _1-xy N (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1). A semiconductor.

  A transparent electrode 8 is disposed on the p-type semiconductor layer 7, and a p-electrode pad 10 is disposed thereon. The p-type semiconductor layer 7, the p-type cladding layer 6, and the active layer 5 are partially cut away to expose a part of the n-type semiconductor layer 4. An n− electrode 9 is disposed on the exposed n-type semiconductor layer 4.

As the substrate 1, for example, a sapphire substrate, a SiC substrate, a ZnO substrate, a GaN substrate, or a Ga ₂ O ₃ substrate can be used.

  The low-temperature buffer layer 2 is a discontinuous film made of crystal grains (grains) of a nitride-based compound semiconductor that is formed at a low temperature and heat-treated at a high temperature. The thickness is a thin film of about several tens of nm. For example, the low temperature buffer layer 2 is formed of GaN. The low-temperature buffer layer 2 functions as a nucleus for crystal growth of the multilayer buffer layer 3 formed thereon.

  As shown in FIG. 2, the multilayer buffer layer 3 includes a nitride-based compound semiconductor layer 31 grown at a slow growth rate (A) and a faster crystal rate (kA, where k> 1). A nitride-based compound semiconductor layer 32 that has been crystal-grown is taken as one set, and two or more sets (repetition number B = 2) are stacked. That is, a total of four or more layers 31 formed at a slow growth rate and layers 32 formed at a high growth rate are alternately stacked. More preferably, three sets (repetition number B = 3, total of 6 layers) or more are laminated. The order of stacking is preferably such that a layer 31 having a slow growth rate is first formed, a layer 32 having a high growth rate is stacked thereon, and this is repeated as a set.

  When two or more sets of the layer 31 formed at such a slow growth rate and the layer 32 formed at a high growth rate are repeatedly stacked, the layer 31 and the layer 32 are simply stacked as shown in FIG. The inventors have found that the flatness of the surface of the layer can be greatly improved as compared with the number of repetitions B = 1 (Comparative Example 2)). Regarding crystallinity, by laminating two or more pairs of the layer 31 and the layer 32, a layer having excellent crystallinity can be obtained as in the case of laminating one set. Therefore, the buffer layer 3 having both flatness and crystallinity can be obtained.

  In general, nitride-based compound semiconductor layers formed at a slow growth rate have good crystallinity but large irregularities on the surface, and nitride-based compound semiconductor layers formed at a high growth rate have poor crystallinity but a flat surface. There is a tendency. In the case of a two-layer structure in which a layer 32 formed at a high growth rate is stacked on a layer 31 formed at a low growth rate, the crystallinity can be improved, but the irregularities on the layer surface are shown in FIG. As described above, it was difficult to improve the flatness without much difference from the unevenness of the layer formed at a constant growth rate (repetition number B = 0 (Comparative Example 1)). For this reason, in order to improve the flatness with only two layers of the layer 31 and the layer 32, it is necessary to form the entire layer thicker and reduce the unevenness due to the effect of the thickness. It was necessary to make it thicker.

  In the present embodiment, the multi-layer buffer layer 3 is formed as a thin total film by forming the multi-layer buffer layer 3 in which two or more sets of layers 31 formed at a slow growth rate and layers 32 formed at a high growth rate are repeatedly stacked. With high thickness, high crystallinity and high flatness can be realized. Thereby, the n-type semiconductor layer 4, the active layer 5, and the p-type semiconductor layer 8 formed on the multilayer buffer layer 3 can be crystal-grown with high crystallinity and flatness, and the light emission intensity can be improved. it can.

  The growth rate (A) of the layer 31 grown at a slow growth rate is preferably set to 5 nm / min or more and 20 nm / min or less. The growth rate (kA) of the layer 32 may be faster than the growth rate of the layer 31, but is preferably twice or more. Moreover, it is preferable that it is 5 times or less from a viewpoint of growing a single crystal.

  The layer 31 preferably has a thickness of 5 nm to 50 nm, and the layer 32 preferably has a thickness of 20 nm to 200 nm.

  The composition of the nitride-based compound semiconductor layers 31 and 32 may be the same. For example, non-doped GaN can be used.

  The n-type semiconductor layer 4, the active layer 5, the p-type cladding layer 6, and the p-type semiconductor layer 7 formed on the multilayer buffer layer 3 have the same structure as a light-emitting element using a known nitride-based compound semiconductor. This is the structure. For example, the n-type semiconductor layer 4 can be formed of n-type GaN doped with Si using impurities. The active layer 5 may have a quantum well structure having a multilayer structure of GaN / InGaN. The p-type cladding layer 6 can be formed of p-type AlGaN doped with Mg as an impurity. The p-type semiconductor layer 7 can be formed of p-type GaN doped with Mg as an impurity.

  The transparent electrode 8, the n-electrode 9, and the p-electrode pad 10 can be formed of the same material as that of a known light-emitting element.

  Next, a method for manufacturing the optical semiconductor element of this embodiment will be described.

  First, as the substrate 1, a sapphire substrate or the like having a predetermined plane (for example, C plane) as a main plane is prepared and cleaned by heat treatment at a predetermined temperature. Thereafter, the substrate 1 is heated at a predetermined low temperature to form an amorphous nitride-based compound semiconductor layer with a thin film thickness. As a film forming method, a known method can be used, for example, MOCVD method can be used. When the MOCVD method is used, trimethylgallium and ammonia can be used as a reaction gas to be supplied. Thereafter, the amorphous nitride-based compound semiconductor layer is crystallized by heat treatment at a predetermined temperature. Thus, the discontinuous film low-temperature buffer layer 2 made of nitride compound semiconductor crystal grains is formed.

  Next, while heating the substrate 1 at a predetermined temperature, a nitride-based compound semiconductor is crystal-grown at a predetermined thickness on the low-temperature buffer layer 2 at a predetermined growth rate, thereby forming a layer 31. On the layer 31, a nitride compound semiconductor is crystal-grown with a predetermined film thickness at a predetermined growth rate faster than that of the layer 31 to form the layer 32. A total of four or more multilayer buffer layers 3 are formed by repeating two or more sets of the layers 31 and 32 as a set. Thereby, the multilayer buffer layer 3 having both high crystallinity and flatness can be formed with a thin film thickness.

  As a crystal growth method of the layer 31 and the layer 32, a known method can be used, for example, MOCVD method can be used. As the reaction gas, the same gas as the low-temperature buffer layer 2 can be used.

  Next, while heating the substrate 1 at a predetermined temperature, a nitride-based compound semiconductor doped with an n-type impurity at a predetermined concentration is grown on the multilayer buffer layer 3 with a predetermined film thickness to form an n-type semiconductor layer. 4 is formed. A nitride compound semiconductor is grown on the n-type semiconductor layer 4 to form an active layer 5 having a predetermined structure. Further, a nitride compound semiconductor doped with a p-type impurity at a predetermined concentration is grown on the active layer 5 to form a p-type cladding layer 6 and a p-type semiconductor layer 7 respectively. Any of them can be formed by a known crystal growth method such as MOCVD.

  Since these four layers 4, 5, 6, and 7 are formed on the multilayer buffer layer 3 excellent in crystallinity and flatness, they become layers excellent in crystallinity and flatness.

  Next, the p-type semiconductor layer 7 is subjected to an activation process by heat treatment. After the transparent electrode 8 is formed on the p-type semiconductor layer 7, the substrate 1 and each layer are divided, and the p-type semiconductor layer 7, the p-type cladding layer 6 and the active layer 5 are cut into a predetermined shape. An n-electrode 9 is disposed on a part of the exposed n-type semiconductor layer 4, and a p-electrode pad 10 is disposed on the transparent electrode 8. As described above, the optical semiconductor element of this embodiment can be manufactured.

  In this optical semiconductor element, when an electric current is supplied from the n-electrode 9 and the p-electrode pad 10, the active layer 5 emits light and emits light to the outside.

  Since the optical semiconductor element of the present embodiment uses the multilayer buffer layer 3 having both high crystallinity and flatness, the layers 4, 5, 6, and 7 having a light emitting element structure excellent in crystallinity and flatness are formed. be able to. Thereby, the light emission intensity can be improved.

  Note that although a semiconductor element that emits light has been described in this embodiment mode, the present invention is not limited to the light-emitting element, and can be applied to a light-receiving element and an electronic element such as a field-effect transistor. Further, the layer structure of the light emitting element is not limited to the structure of the above-described layers 4, 5, 6, and 7, and a known structure can be used.

Examples of the present invention will be described.
(Examples 1 to 11)
As Examples 1 to 11, optical semiconductor elements having the structure shown in FIG. 2 of the above-described embodiment were manufactured. At this time, as the multilayer buffer layer 3, a low growth rate layer 31 and a fast growth rate layer 32 are made into one set, and these are set to 2 sets (repetition number B = 2, a total of 4 layers) to 12 sets (repetition number B = 12, a total of 24 layers) Samples of repeated optical semiconductor elements were prepared and designated as Examples 1-11. The film forming conditions and device forming steps of the other layers excluding the multilayer buffer layer 3 were performed under the same conditions in Examples 1 to 11.

  The specific manufacturing method of Examples 1-11 is as follows. First, a sapphire substrate having a main plane (C plane) was prepared as the substrate 1 and cleaned by heat treatment at 1200 ° C. in a hydrogen atmosphere.

  Thereafter, 10 μmol / min of trimethylgallium and 3.3 L / min of ammonia are supplied at a temperature of the substrate 1 of 500 ° C., and amorphous GaN is formed to a thickness of 20 to 50 nm by MOCVD in a mixed atmosphere of hydrogen and nitrogen. As a result, the low-temperature buffer layer 2 was formed. Thereafter, the low-temperature buffer layer 2 was heated at 1000 ° C. for 4 minutes to form a discontinuous crystal grain layer.

  Next, 23 μmol / min of trimethyl gallium and 2.2 L / min of ammonia are supplied at a temperature of 1000 ° C. of the substrate 1, and GaN is grown at a growth rate of 20 nm / min by MOCVD in a mixed atmosphere of hydrogen and nitrogen. As a result, a layer 31 having a thickness of about 20 nm was formed. The layer 31 is supplied with 45 μmol / min trimethylgallium and 4.4 L / min ammonia, and is grown by crystal growth at a growth rate of 40 nm / min by MOCVD in a mixed atmosphere of hydrogen and nitrogen. An 80 nm layer 32 was formed. The layer 31 and the layer 32 are set as one set. In the first embodiment, two sets (repetition number B = 2, four layers in total) and in the second to eleventh embodiments, three sets (repetition number B = 3, six layers in total) The multilayer buffer layer 3 was formed by repeating ˜12 groups (repetition number B = 12, 24 layers in total).

Next, Si doping is performed at a concentration of about 1 × 10 ¹⁸ atms / cm ³ by MOCVD in a mixed atmosphere of hydrogen and nitrogen by supplying a substrate 1 temperature of 1000 ° C., trimethylgallium 40 μmol / min, and ammonia 4 L / min. The n-GaN layer having been subjected to the above was grown to form an n-type semiconductor layer 4 having a thickness of about 3 to 5 μm.

  Next, a substrate 1 having a temperature of 700 ° C., trimethylgallium 3.6 μmol / min, ammonia 4.4 L / min is supplied, and GaN is crystal-grown by MOCVD in a nitrogen atmosphere. A layer was formed. Next, a substrate 1 having a temperature of 700 ° C., trimethylgallium 3.6 μmol / min, trimethylindium 10 μmol / min, and ammonia 4 L / min are supplied, and InGaN is grown by MOCVD in a nitrogen atmosphere to obtain a thickness of 10 nm. The well layer was formed. The barrier layer and the well layer were repeatedly grown in 3 to 10 pairs to form the active layer 5 having a quantum well structure of GaN / InGaN.

Next, the temperature of the substrate 1 is 870 ° C., trimethylgallium 8.1 μmol / min, trimethylaluminum 7.56 μmol / min, and ammonia 4.4 L / min are supplied, and 1 × by MOCVD in a mixed atmosphere of hydrogen and nitrogen. A p-type cladding layer 6 having a thickness of about 40 nm was formed by crystal growth of Mg doped p-AlGaN at a concentration of about 10 ²⁰ atms / cm ³ .

Next, the temperature of the substrate 1 is 870 ° C., trimethylgallium 18 μmol / min, ammonia 4.4 L / min is supplied, and the concentration is about 2 × 10 ²⁰ atms / cm ³ by MOCVD in a mixed atmosphere of hydrogen and nitrogen. A p-type semiconductor layer 7 having a thickness of about 100 nm was formed by crystal growth of Mg-doped p-GaN.

  Thereafter, heat treatment was performed at 850 ° C. for 1 minute in a nitrogen atmosphere, and the p-type semiconductor layer 7 was activated.

  An indium tin oxide film was formed as the transparent electrode 8 on the p-type semiconductor layer 7. Each of the transparent electrodes 8, the p-type semiconductor layer 7, the p-type cladding layer 6 and the active layer 5 was cut out as shown in FIG. A p-electrode pad 10 was formed on the transparent electrode 8 with an Au 1.0 μm / Ti 25 nm film. On the n-type semiconductor layer 4 exposed by the notch, an n-electrode 9 was formed by an Al 1.0 μm / Ti 25 nm film.

  The optical semiconductor element of Examples 1-11 was manufactured by the above.

(Comparative Example 1)
As Comparative Example 1, instead of the multilayer buffer layer 3 of Examples 1 to 11, an optical semiconductor element including a single-layer buffer layer having a thickness of 0.8 μm obtained by crystal growth of GaN at a fast growth rate of 40 nm / min was manufactured. .

  A single buffer layer having a high growth rate was formed by MOCVD in a mixed atmosphere of hydrogen and nitrogen by supplying trimethylgallium 45 μmol / min and ammonia 4.4 L / min. Other layer configurations and film forming methods than the single-layer buffer layer with a fast growth rate were the same as those in Examples 1 to 11.

(Comparative Example 2)
As Comparative Example 2, instead of the multilayer buffer layer 3 of Examples 1 to 11, a pair of a slow growth rate (20 nm / min) layer and a fast growth rate (40 nm / min) layer (repetition number B = 1, An optical semiconductor element provided only with a total of two layers) was manufactured.

  As with the layer 31 of Examples 1 to 11, the slow growth rate layer was supplied with trimethylgallium 23 μmol / min and ammonia 2.2 L / min at the substrate 1 temperature of 1000 ° C., and in a mixed atmosphere of hydrogen and nitrogen. The GaN crystal was grown by MOCVD at a growth rate of 20 nm / min. The thickness was about 20 nm. Further, similarly to the layer 32 of Examples 1 to 11, 45 μmol / min of trimethylgallium and 4.4 L / min of ammonia were supplied, and the growth rate was 40 nm / min by MOCVD in a mixed atmosphere of hydrogen and nitrogen. A layer having a high growth rate was formed by crystal growth of GaN. The thickness was about 80 nm.

  Other layer configurations and film forming methods other than the buffer layer were the same as those in Examples 1 to 11.

(Flatness evaluation)
Irregularities on the upper surface of the multilayer buffer layer 3 of Examples 1 to 11 and the upper surface of the buffer layers of Comparative Examples 1 and 2 were measured. The result is shown in FIG. As is apparent from FIG. 3, the unevenness of the upper surface of the single-layer buffer layer (repetition number B = 0) formed at a fast growth rate in Comparative Example 1 has a height difference of 30 nm. Further, the unevenness on the upper surface of the buffer layer of Comparative Example 2 having only one set of slow growth rate layers and fast growth rate layers (repetition number B = 1, 2 layers in total) is 28 nm, which is hardly improved. . In contrast, the buffer layers of Examples 1 to 11 in which 2 to 12 pairs of slow growth rate layers 31 and fast growth rate layers 32 were laminated were 23 nm in Example 1 in which 2 pairs were laminated, and 3 or more pairs were laminated. Each of Examples 2 to 11 had a thickness of 20 nm, and the flatness could be greatly improved. The flatness of the multilayer buffer layer 3 of Examples 2 to 11 was improved by 30% compared to the flatness of Comparative Example 1.

(Crystallinity evaluation)
For the measurement of crystallinity, the rocking curve was measured by the X-ray diffraction method for the multilayer buffer layer 3 of Example 3 in which the buffer layer of Comparative Example 1 and four pairs of layers 31 and 32 (repetition number B = 4) were laminated. Went. The peaks of (002) and (102) of the buffer layer of Comparative Example 1 were 240 arcsec and 340 arcsec, respectively, whereas the peaks of (002) and (102) of the multilayer buffer layer 3 of Example 3 were respectively 238 arcsec and 334 arcsec. In Example 3, it was confirmed that the crystallinity was maintained by stacking with a layer having a slow growth rate even when the film thickness was increased by including a plurality of layers having a fast growth rate.

(Emission characteristic evaluation)
In order to confirm the light emission characteristics, the intensity of photoluminescence was measured for the optical semiconductor element of Comparative Example 1 and the optical semiconductor element of Example 3 in which four sets of layers 31 and 32 were laminated. However, all measured the photoluminescence by the laminated structure from the board | substrate 1 to the active layer 5. FIG. The result is shown in FIG.

  As is clear from FIG. 4, the device of Example 3 was improved in emission intensity by 1.3 times that of the device of Comparative Example 1.

  This is presumably because the flatness and crystallinity of the active layer 5 and the like formed on the buffer layer 3 were improved by using the multilayer buffer layer 3 having both high flatness and crystallinity.

Sectional drawing which shows the structure of the optical semiconductor element of this embodiment. Sectional drawing which shows the structure of the multilayer buffer layer 4 of the optical semiconductor element of FIG. The graph which shows the magnitude | size of the unevenness | corrugation of the upper surface of the buffer layer of the optical semiconductor element of the Examples 1-11 and Comparative Examples 1 and 2. The graph which shows the photoluminescence intensity | strength of the optical semiconductor element of the present Example 3 and the comparative example 1. FIG.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Substrate, 2 ... Low-temperature buffer layer, 3 ... Multi-layer buffer layer, 4 ... n-type semiconductor layer, 5 ... Active layer, 6 ... p-type cladding layer, 7 ... p-type semiconductor layer, 8 ... Transparent electrode, 9 ... n -Electrode, 10 ... p-electrode pad, 31 ... layer formed at a slow growth rate, 32 ... layer formed at a fast growth rate.

Claims (4)

A first layer is crystal-grown on the substrate at a predetermined growth rate and a ratio between a predetermined ammonia flow rate and a trimethyl gallium flow rate (ammonia flow rate / trimethyl gallium flow rate), and the first layer is formed on the first layer. The second layer is crystal-grown at a growth rate higher than that of the first layer and the ratio of the ammonia flow rate and the trimethyl gallium flow rate same as those of the first layer, and two or more sets of these layers are repeatedly laminated to form a multilayer buffer layer. Forming, and
And a step of crystal-growing a predetermined semiconductor layer on the multilayer buffer layer.   The method of manufacturing a semiconductor device according to claim 1, wherein the first layer and the second layer have the same composition.   The growth rate of the first layer is not less than 5 nm / min and not more than 20 nm / min, and the growth rate of the second layer is not less than 2 times and not more than 5 times the first growth rate. A method for manufacturing a semiconductor device according to claim 1.   4. The film thickness of the first layer is 5 nm to 50 nm, and the film thickness of the second layer is 20 nm to 200 nm. 5. A method for manufacturing a semiconductor device.

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