JP2012174705A - Epitaxial wafer for nitride semiconductor device and manufacturing method of the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an epitaxial wafer for a nitride semiconductor device without including a V-shaped flaw on a top face caused by a surface unevenness of an AlN ground layer by forming an AlGaN layer of a high Al composition ratio having a smooth top face masking the unevenness on a top face of the AlN ground layer on a Si substrate.SOLUTION: An epitaxial wafer (10) for a nitride semiconductor device comprises an AlN ground layer (2), a morphology improved layer (3) of AlGaN(0.6≤x<1) and an AlGaN buffer layer (4) sequentially laminated on a Si substrate (1) by MOCVD. The morphology improved layer (3) has a top face smoothed by utilizing thermal diffusion.

Description

  The present invention relates to an epitaxial wafer for a nitride semiconductor device including a GaN epitaxial layer on a Si substrate and an improvement of the manufacturing method thereof.

  In recent years, technological development for epitaxial growth of a GaN layer using a Si substrate capable of increasing the diameter has been actively attempted from the viewpoint of cost reduction. However, it is not easy to grow a high quality GaN crystal layer on a Si substrate. The cause is that crystal defects, wafer warpage, cracks, and the like are likely to occur due to differences in thermal expansion coefficient and lattice constant between the Si substrate and the GaN layer.

  Although the crystal defects near the surface can be reduced by increasing the thickness of the epitaxial growth layer, the warpage of the wafer tends to increase as the layer thickness increases. The warpage of the wafer becomes more prominent as the substrate becomes larger in diameter.

Japanese Patent Application Laid-Open No. 2000-277441 discloses that an AlN layer and an Al _0.25 Ga _0.75 N layer are sequentially stacked on a (111) surface of an n-type Si substrate, and a GaN layer is grown thereon. As a result, the surface of the GaN layer becomes mirror-like and no cracks or pits are observed. Patent Document 1 also _discloses an AlN layer, an Al _0.80 Ga _0.20 N layer, an Al _0.60 Ga _0.40 N layer, an Al _0.40 Ga _{0. By} laminating a ₆₀ N layer and an Al _0.25 Ga _0.75 N layer in this order and growing a GaN layer thereon, the top surface of the GaN layer becomes mirror-like and no cracks or pits are observed. Says.

  Japanese Patent Laid-Open No. 2007-250721 discloses a technique in which an AlN buffer layer and an AlGaN intermediate layer are sequentially stacked on a (111) surface of a Si substrate, and two types of AlGaN layers having different Al composition ratios are formed thereon. It is stated that by alternately laminating until the number reaches 21 or more and less than 70 and further growing a GaN layer thereon, stress in the GaN layer is suppressed and crystallinity and flatness are improved.

JP 2000-277441 A JP 2007-250721 A

  In order to epitaxially grow a high-quality GaN layer using a Si substrate, the Si substrate and the GaN layer are likely to react with each other. It is necessary to form a stratum.

  However, since the AlN crystal has a characteristic of easily growing in a granular form, when the AlN layer is deposited, irregularities tend to be easily formed on the surface of the AlN layer. As a result, when an AlGaN layer is grown on the AlN underlayer, the smoothness of the upper surface of the AlGaN layer may be deteriorated due to surface irregularities of the AlN underlayer.

FIG. 2 shows a case in which an Al _0.7 Ga _{0.. 0} layer is formed by sequentially laminating an AlN underlayer and an Al _0.7 Ga _0.3 N layer on a Si substrate using a MOCVD (metal organic chemical vapor deposition) apparatus _{. 3} shows an SEM (scanning electron microscope) photograph of the surface of the N layer. The scale of the white line segment shown at the bottom of this SEM photograph shows a length of 1 μm. As observed in FIG. 2, when an Al _0.7 Ga _0.3 N layer is grown on the AlN underlayer, many depressions are formed on the surface of the Al _0.7 Ga _0.3 N layer. Can be confirmed.

FIG. 3 shows a TEM (transmission) showing a cross section of an AlN underlayer, an Al _0.7 Ga _0.3 N layer, and an Al _0.4 Ga _0.6 N layer sequentially stacked on a Si substrate. It is a type | mold electron microscope) photograph. The total length of the 10-scale scale shown at the bottom of the TEM photograph represents a length of 2 μm. As observed in FIG. 3, it can be seen that large V-shaped defects (dents) are formed on the surface of the uppermost Al _0.4 Ga _0.6 N layer.

4 is an enlarged TEM photograph showing the V-shaped defect shown in FIG. The total length of the scale of 10 scales shown at the bottom of this TEM photograph represents a length of 300 nm. As observed in FIG. 4, the surface depression of the Al _0.7 Ga _0.3 N film is filled by the growth of the Al _0.4 Ga _0.6 N layer thereon, but the depression is taken over. It can be seen that large V-shaped defects are generated on the surface of the Al _0.4 Ga _0.6 N layer. That is, it can be seen that the depression on the upper surface of the lower layer causes the formation of the upper surface V-shaped defect.

FIG. 5 shows an AlN underlayer, an Al _0.7 Ga _0.3 N layer, an Al _0.4 Ga _0.6 N layer, an Al _0.1 Ga _0.9 N layer, and a GaN layer in this order on a Si substrate. It is a SEM photograph which shows the fracture section of the lamination when laminating. The scale of the white line segment shown at the bottom of this SEM photograph shows a length of 100 nm. As can be seen in FIG. 5, it can be seen that large V-shaped defects (dents) are formed on the surface of the GaN layer that is the uppermost layer. As described in connection with FIGS. 3 and 4, V-shaped defects generated in the GaN layer for the same reason that large V-shaped defects occurred in the Al _0.4 Ga _0.6 N layer. Is considered to be caused by the upper surface depression of the lower layer.

  From the above results, it can be said that the upper surface depression of the AlGaN layer formed when the AlGaN layer is grown on the AlN underlayer is the cause of V-shaped defect formation on the surface of the GaN layer. These V-shaped defects become serious defects in the epitaxial wafer for nitride semiconductor devices.

  By the way, in general, an AlGaN layer is less likely to grow laterally than a GaN layer. In particular, the AlGaN layer is less likely to grow laterally as the Al composition ratio increases. Therefore, it is considered that the surface depression of the AlN underlayer is easier to fill as the Al composition ratio of the AlGaN layer is smaller, and conversely, the surface depression of the AlN underlayer is more difficult to fill as the Al composition ratio of the AlGaN layer is larger.

Therefore, in Patent Document 1, when an Al _0.8 Ga _0.2 N layer is grown on the AlN underlayer, in particular, an Al _0.7 Ga _0.3 N layer is grown on the AlN underlayer. Similar to the observation result of FIG. 2, it is considered that the surface roughness due to the depression of the AlN underlayer is generated on the surface of the Al _0.8 Ga _0.2 N layer. As a result, it is considered that a large V-shaped defect is generated on the upper surface of the GaN layer formed on the AlGaN layer including those surface depressions.

In Patent Document 2, an Al _0.26 Ga _0.74 N layer is formed on an AlN underlayer. Although the patent document 2 itself is not described, the Al _0.26 Ga _0.74 N layer having a small Al composition ratio that easily grows in the lateral direction can be laminated to mask the surface unevenness of the AlN underlayer and planarize. It is thought that.

On the other hand, from the viewpoint of reducing the warpage of the epitaxial wafer caused by the lattice constant of the AlN underlayer being smaller than that of the Si substrate, the Al _x Ga _1-x N layer (0 <x <) formed on the AlN underlayer. The Al composition ratio x of 1) is preferably larger. This is because the larger the difference in lattice constant from the Al _y Ga _1-y N layer (0 ≦ y <x) stacked on the Al _x Ga _1-x N layer (that is, the Al composition ratio difference xy is smaller). This is because the warpage of the wafer due to the difference in lattice constant can be reduced as a whole.

In other words, when the Al composition ratio x of the Al _x Ga _1-x N layer (0 <x <1) formed on the AlN underlayer is small, the Al _x Ga _1-x N layer (0 <x < 1) Stress due to the difference in lattice constant (Al composition ratio difference xy) with the Al _y Ga _1-y N layer (0 ≦ y <x) stacked on the surface is reduced, and the stress that reduces the warpage of the wafer is reduced. This is not preferable.

  For this reason, in order to reduce the warpage of the wafer due to the difference in lattice constant between the Si substrate and the AlN underlayer, it is desirable to form an AlGaN layer having a high Al composition ratio in contact with the AlN underlayer.

  However, as described above, since an AlGaN layer having a high Al composition ratio is difficult to grow in the lateral direction, it is difficult to flatten it by masking the unevenness formed on the surface of the AlN underlayer.

  Therefore, the main object of the present invention is to form an AlGaN layer having a high Al composition ratio having a smooth upper surface so as to mask the unevenness on the upper surface of the AlN underlayer on the Si substrate, resulting from the upper surface unevenness of the AlN underlayer. An epitaxial wafer for a nitride semiconductor device that does not include a V-shaped defect on its upper surface is provided.

According to one aspect of the present invention, an epitaxial wafer for a nitride semiconductor device includes an AlN underlayer, Al _x Ga _1-x N (0.6 ≦ x <1), which is sequentially stacked on a Si substrate by MOCVD. It includes a morphology improving layer and an AlGaN buffer layer, and the morphology improving layer has an upper surface smoothed by utilizing thermal diffusion.

According to another aspect of the present invention, a method of manufacturing an epitaxial wafer for a nitride semiconductor device includes a morphology of an AlN underlayer and Al _x Ga _1-x N (0.6 ≦ x <1) on a Si substrate. A step of laminating an improvement layer and an AlGaN buffer layer in sequence by MOCVD, wherein the morphology improvement layer is deposited by supplying a group III element source gas and a group V element source gas at a first substrate temperature, and thereafter Then, the supply of the group III element source gas is stopped at the second substrate temperature higher than the first substrate temperature, and only the group V element source gas is supplied to cause thermal diffusion, whereby the top surface of the morphology improving layer Is smoothed.

The AlGaN buffer layer is a first sub-buffer layer of Al _x Ga _1-x N (0.6 ≦ x <1) and a second layer of Al _y Ga _1-y N (0 <y <x), which are sequentially stacked. A multilayer buffer layer including a sub-buffer layer and a third sub _- buffer layer of Al _z Ga _1-z N (0 <z <y) is preferable.

  According to the present invention, since the top surface of the morphology improving layer is smoothed, the buffer layer thereabove also has a smooth top surface, and an epitaxial wafer having no V-shaped defect on the top surface is used for a nitride semiconductor device. Can be provided. Further, when the buffer layer is formed of a multilayer buffer layer, it is possible to provide a flat epitaxial wafer having a particularly reduced warpage as a whole.

1 is a schematic cross-sectional view showing a laminated structure of an epitaxial wafer for nitride semiconductor devices according to an embodiment of the present invention. On a Si substrate by conventional techniques is a SEM photograph showing the top surface of the wafer was grown AlN underlayer and _Al 0.7 _{Ga 0.3} N layer in this order. AlN underlayer on the Si substrate by conventional _techniques, Al 0.7 _{Ga 0.3} N layer, and an _Al 0.4 _{Ga 0.6} N layer is a TEM photograph showing a laminated cross-section of the stacked wafer in order. It is a TEM photograph which expands and shows the V-shaped defect shown by FIG. An AlN underlayer, an Al _0.7 Ga _0.3 N layer, an Al _0.4 Ga _0.6 N layer, an Al _0.1 Ga _0.9 N layer, and a GaN layer are sequentially stacked on a Si substrate by a conventional technique. It is a SEM photograph which shows the fracture | rupture cross section of the done wafer. The AlN underlayer and _Al 0.7 _{Ga 0.3} N layer on a Si substrate by the present invention is a SEM photograph showing a surface on the wafer grown in this order.

<Structure>
FIG. 1 is a schematic cross-sectional view showing a laminated structure of an epitaxial wafer for nitride semiconductor devices according to an embodiment of the present invention. In this schematic diagram, the thickness of each layer is appropriately changed for clarity and simplification of the drawings, and does not represent an actual dimensional relationship.

A wafer 10 in FIG. 1 includes an AlN underlayer 2 sequentially laminated on a Si substrate 1 using MOCVD, a morphology improving layer 3 made of Al _x Ga _1-x N (0.6 ≦ x <1), a multilayer A buffer layer 4, a nitride semiconductor channel layer 5, and a nitride semiconductor electron supply layer 6 are included.

  In order to grow a good crystalline GaN layer above the Si substrate 1, it is desirable to use the Si (111) surface as the substrate surface. Further, the base layer 2 is required to play a role of a barrier so that the Si substrate and the GaN layer are not in direct contact with each other, and it is desirable to form an AlN base layer that does not contain Ga as described above.

The morphology improving layer 3 plays a role of improving the smoothness of the upper surface of the wafer by filling the concaves and convexes formed on the upper surface of the underlayer 2. Specifically, Al _x Ga _1-x N (0.6 ≦ x <1) is used as the morphology improving layer 3 as described above. The morphology improving layer 3 is a layer whose surface is smoothed using thermal diffusion without layer growth after the layer is grown.

In the case of FIG. 1, in the multilayer buffer layer 4, the first AlGaN sub-buffer layer 41 having the same Al composition ratio x as the Al composition ratio x of the Al _x Ga _1-x N morphology improvement layer 3 is represented by the Al composition ratio y. A second AlGaN subbuffer layer 42 having an Al composition ratio z and a third subbuffer layer 43 having an Al composition ratio z are sequentially stacked. The Al composition ratios x, y, and z of the first, second, and third sub-buffer layers are set so as to satisfy the relationship x>y> z. That is, the multilayer buffer layer 4 has a smaller Al composition ratio as the upper sub-buffer layer.

The Al composition ratio x of the Al _x Ga _1-x N morphology improving layer 3 and the first Al _x Ga _1-x N subbuffer layer 41 is preferably larger. This is because, as described above, the first Al _x Ga _1-x N sub-buffer layer and the second Al _y Ga _1-y N sub-buffer layer (0 ≦ y <x) stacked on the first Al _x Ga _1-x N sub-buffer layer. This is because the larger the lattice constant difference, the greater the stress that reduces the warpage of the wafer due to the lattice constant difference. That is, the warp of the wafer decreases as the difference (xy) in the Al composition ratio between the first Al _x Ga _1-x N sub-buffer layer and the second Al _y Ga _1-y N sub-buffer layer increases. Can be done.

  The epitaxial wafer for a nitride semiconductor device in FIG. 1 includes a channel layer 5 and an electron supply layer 6 on a buffer layer 4, and this wafer is assumed to be used for manufacturing a heterojunction FET (field effect transistor). As the material of the channel layer 5, for example, GaN, AlGaN, InGaN, AlGaInN, or the like can be used.

A nitride compound semiconductor can also be used as the material of the electron supply layer 6. Note that the lattice constant _{a 2} of the lattice constants _{a 1} and the electron supply layer 6 of the channel layer 5 _a 1> satisfy the relation of _{a 2,} the band gap Eg ₂ of the band gap Eg ₁ and the electron supply layer 6 of the channel layer 5 Satisfies the relationship of Eg ₁ <Eg ₂ . If the relationship between the lattice constant and the band gap is satisfied, the material of the electron supply layer 6 is not particularly limited. For example, when GaN is used for the channel layer 5, AlGaN, AlN, or the like can be used for the electron supply layer 6. When InGaN is used for the channel layer 5, GaN or the like can be used for the electron supply layer, and when InN is used for the channel layer 5, GaN, InGaN or the like is used for the electron supply layer 6. can do.

<Manufacturing method>
The gallium nitride-based epitaxial wafer 10 having the cross-sectional structure shown in FIG. 1 can be manufactured as follows using the MOCVD method. First, an AlN underlayer 2 is formed on the Si substrate 1. Subsequently, a morphology improving layer 3 made of Al _x Ga _1-x N (0.6 ≦ x <1) is formed on the AlN underlayer 2 using MOCVD and thermal diffusion.

Here, the method for forming the morphology improving layer 3 will be described in more detail. First, when the AlN underlayer 2 is formed on the Si substrate 1, the deposited AlN crystal has a characteristic of easily growing grains, so that irregularities are formed on the surface of the AlN underlayer 2. Therefore, if the Al _x Ga _1-x N (0.6 ≦ x <1) morphology improving layer 3 having a high Al composition ratio is grown on the AlN underlayer 2 having such surface irregularities by MOCVD, the AlN Due to the surface irregularities of the underlayer 2, the surface of the grown morphology improving layer contains many depressions as shown in FIG. 2.

Therefore, after growing the morphology improving layer, thermal diffusion is used to bury and smooth many depressions on the surface of the morphology improving layer. Specifically, after growing the morphology improving layer, nitrogen gas or hydrogen gas as a carrier gas and group V are not supplied without supplying TMG (trimethylgallium) and TMA (trimethylaluminum) as group III element sources. The substrate temperature is raised while ammonia (NH ₃ ) gas, which is an element source, is supplied.

In this way, Al _x Ga _1-x N (0.6 ≦ x <1) is once thermally decomposed on the surface of the morphology improving layer, and Ga atoms, Al atoms, or AlGaN molecules are diffused at or near the layer surface. In the process of moving, it is considered that the recess is filled again with the solid in the recess portion having a low surface energy, and thus the recess is embedded. As a result, it is considered that the surface of the morphology improving layer 3 is converted into a smooth surface that does not include a depression.

The conditions for controlling thermal decomposition, thermal diffusion, and recombination in the depressions for smoothing the surface of the morphology improving layer 3 include substrate temperature, atmospheric pressure, supply amount and supply of supply gas during heat treatment. Examples include ratios. That is, the smoothing speed of the morphology improving layer varies depending on the substrate temperature, the atmospheric pressure, the supply amount of the supply gas during the heat treatment, the supply ratio, and the like. For example, thermal decomposition and thermal diffusion are promoted as the substrate temperature increases, and decomposition is suppressed and recombination is promoted as the atmospheric pressure is higher and the ratio of NH ₃ gas is larger.

  On the morphology improving layer 3 whose surface is smoothed as described above, the multilayer buffer layer 4, the channel layer 5, and the electron supply layer 6 are sequentially formed by MOCVD as described above. Thus, the gallium nitride based epitaxial wafer 10 can be fabricated.

  Hereinafter, the fabrication of an epitaxial wafer for nitride semiconductor devices according to an embodiment of the present invention will be described more specifically with reference to FIG.

  On the (111) plane of the Si substrate 1, an AlN underlayer 2 having a thickness of 180 nm was grown by MOCVD at a substrate temperature of 1100 ° C. As raw materials, ammonia gas and TMA were used.

On the AlN underlayer 2, an Al _0.7 Ga _0.3 N morphology improving layer 3 having a thickness of 0.1 μm was grown using ammonia gas, TMG, and TMA as raw materials. During this growth, the substrate temperature was 1100 ° C. and the gas pressure was 13.3 kPa. Subsequently, the supply of TMG and TMA was stopped, only the carrier gas and ammonia were continuously supplied, the substrate temperature was raised to 1200 ° C., and the gas pressure was maintained at 13.3 kPa, and the morphology was changed by heating reaction for 10 minutes. The surface of the improvement layer 3 was smoothed.

FIG. 6 is an SEM photograph showing the smooth surface of the Al _0.7 Ga _0.3 N morphology improving layer 3 formed in this way. The white triangular mark in the SEM photograph is a mark for focusing the SEM photograph. The scale of the white line segment shown at the bottom of this photograph shows a length of 1 μm. As is apparent from a comparison between FIG. 6 and FIG. 2 described above, it can be seen that the surface of the morphology improving layer 3 according to this example is extremely smooth.

On the Al _0.7 Ga _0.3 N morphology improving layer 3, an Al _0.7 Ga _0.3 N first subbuffer layer 41 having a thickness of _0.3 μm and an Al _0.4 Ga layer having a thickness of 0.4 μm. _{A 0.6} N second sub-buffer layer 42 and a 0.7 μm thick Al _0.1 Ga _0.9 N third sub-buffer layer 43 are formed under conditions of a substrate temperature of 1150 ° C. and a gas pressure of 13.3 kPa. The multilayer buffer layer 4 was formed by stacking.

On the multilayer buffer layer 4, a channel layer 5 (lattice constant a ₁ = 0.3189 nm, Eg ₁ = 3.42 eV) made of GaN was grown at a substrate temperature of 1100 ° C. to a thickness of 2 μm. . On top of that, an electron supply layer 6 (a ₂ = 0.3166 nm, Eg ₂ = 4.02 eV) made of Al _0.2 Ga _0.8 N at a substrate temperature of 1100 ° C. has a thickness of 20 nm. It was made to grow.

  As a result of including the morphology improving layer 3 as described above, V-shaped defects did not occur in the multilayer film including the multilayer buffer layer 4, the GaN channel layer 5, and the AlGaN electron supply layer.

Furthermore, the difference in Al composition ratio between the first sub-buffer layer 41 of Al _x Ga _1-x N and the second sub-buffer layer 42 of Al _y Ga _1-y N (0 ≦ y <x) (xy) ) Is large (that is, the lattice constant difference is large), the stress that reduces the warp caused by the lattice constant difference between the Si substrate 1 and the AlN underlayer 2 is increased, and as a result, the warp of the entire wafer is eliminated. I was able to.

  As described above, according to the present invention, an epitaxial wafer having a smooth surface can be provided as an application of a nitride semiconductor device by the effect of the morphology improving layer. Further, when the buffer layer is formed of a multilayer buffer layer, it is possible to provide an epitaxial wafer that is particularly reduced in warpage as a whole.

  1 Si substrate, 2 AlN underlayer, 3 morphology improvement layer, 4 multilayer buffer layer, 5 channel layer, 6 electron supply layer, 10 epitaxial wafer for nitride semiconductor device.

Claims (8)

An epitaxial wafer for a nitride semiconductor device,
An AlN underlayer sequentially laminated on a Si substrate by MOCVD, a morphology improvement layer of Al _x Ga _1-x N (0.6 ≦ x <1), and an AlGaN buffer layer, the morphology improvement layer using thermal diffusion A wafer having a smoothed upper surface. The AlGaN buffer layer includes a first sub buffer layer of Al _x Ga _1-x N (0.6 ≦ x <1) and a second sub buffer of Al _y Ga _1-y N (0 <y <x), which are sequentially stacked. The wafer according to claim 1, wherein the wafer is a multilayer buffer layer including a buffer layer and a third sub _- buffer layer of Al _z Ga _1-z N (0 <z <y).   The wafer according to claim 1, further comprising a nitride semiconductor channel layer and a nitride semiconductor electron supply layer that are sequentially stacked on the buffer layer by MOCVD.   The wafer according to claim 3, wherein the channel layer is made of GaN, and the electron supply layer is made of AlGaN. A method for manufacturing an epitaxial wafer for a nitride semiconductor device, comprising:
A step of laminating an AlN underlayer, an Al _x Ga _1-x N (0.6 ≦ x <1) morphology improving layer, and an AlGaN buffer layer on the Si substrate in order by MOCVD;
The morphology improving layer is deposited by supplying a group III element source gas and a group V element source gas at a first substrate temperature, and then a group III element at a second substrate temperature higher than the first substrate temperature. A process for producing thermal diffusion by stopping the supply of the elemental source gas and supplying only the group V elemental source gas, thereby smoothing the upper surface of the morphology improving layer. The AlGaN buffer layer is a multilayer buffer layer, and is a first sub-buffer layer of Al _x Ga _1-x N (0.6 ≦ x <1), Al _y Ga _1-y N (0 <y <x). The manufacturing method according to claim 5, wherein the second sub-buffer layer and the third sub-buffer layer of Al _z Ga _1-z N (0 <z <y) are sequentially stacked.   The manufacturing method according to claim 5, wherein a nitride semiconductor channel layer and a nitride semiconductor electron supply layer are further stacked in order on the buffer layer by MOCVD.   The manufacturing method according to claim 7, wherein the channel layer is made of GaN, and the electron supply layer is made of AlGaN.