JP4781599B2 - Epitaxial substrate and multilayer structure - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
INDUSTRIAL APPLICABILITY The present invention is a group III nitride film that can be suitably used as a base film constituting a semiconductor light emitting element such as a light emitting diode element or a semiconductor element such as a high-speed IC chip, and an epitaxial substrate and multilayer structure using the same. About.
[0002]
[Prior art]
Group III nitride films are used as semiconductor films constituting semiconductor elements such as photonic devices and electronic devices. In recent years, group III nitride films are also attracting attention as semiconductor films constituting high-speed IC chips used for mobile phones and the like. Have been bathed. In particular, a group III nitride film containing Al is attracting attention as an application material for field emitters.
[0003]
As a substrate on which such a group III nitride film is formed, there is a so-called epitaxial substrate including an underlayer formed by epitaxial growth on a predetermined base material made of sapphire single crystal or the like. In particular, the underlayer is preferably composed of a group III nitride containing Al in order to facilitate the epitaxial growth of a group III nitride film containing Al. Furthermore, since Al-containing nitrides have a large band gap and a large thermal conductivity, light absorption can be achieved by inserting a base layer made of such a material between the group III nitride film and the substrate. From the viewpoint of suppression or thermal diffusion, the efficiency of the semiconductor element can be improved.
[0004]
After the epitaxial substrate is placed on a susceptor provided in the reaction tube, the epitaxial substrate is heated to a predetermined temperature by a heating mechanism inside and outside the susceptor. Next, a group III element feedstock and a nitrogen feedstock, and, if necessary, a feedstock of other elements are introduced into the reaction tube together with a carrier gas and supplied onto the epitaxial substrate, and a group III nitridation is performed according to the MOCVD method. A material film is formed.
[0005]
[Problems to be solved by the invention]
However, in the above-described epitaxial substrate, misfit dislocations occur due to a difference in lattice constant between the base material and the underlayer, and the misfit dislocations penetrate through the underlayer as threading dislocations, and the surface thereof. That is, it reaches the surface of the epitaxial substrate. For this reason, a large amount of dislocations due to the misfit dislocations are also generated on the group III nitride film formed on the underlayer, that is, on the epitaxial substrate.
[0006]
The same phenomenon was observed not only when forming an underlayer made of Al-containing group III nitride on the substrate, but also when forming an underlayer made of Ga-containing and other group III nitrides. As a result, when a semiconductor light emitting device or the like is formed from these group III nitride films, for example, the light emission efficiency does not deteriorate, and a semiconductor light emitting device having desired characteristics cannot be obtained.
[0007]
An object of the present invention is to form a nitride film with low dislocations and excellent crystallinity, particularly an Al-containing nitride film.
[0008]
[Means for Solving the Problems]
In order to achieve the above object, the present invention provides a predetermined base material, a group III nitride film containing at least Al formed on the main surface of the base material, the base material, and the group III nitride film. A group III nitride base film provided between the Al group and the group III nitride film, wherein the Al content in the group III nitride film is 50 atomic% or more with respect to all group III elements, The present invention relates to an epitaxial substrate in which edge dislocations are bent and propagated so as to be substantially parallel to the main surface after rising from the main surface of the base material .
[0009]
The inventors of the present invention have intensively studied to achieve the above object. As a result, misfit dislocations generated between the base material and the underlayer as described above propagate in the underlayer as edge dislocations and / or screw dislocations, and in particular, the edge dislocation penetrates the underlayer. And found that there is a strong tendency to reach the surface. It has been found that such threading edge dislocations cause an increase in dislocation density in various group III nitride films formed above. Therefore, the present inventors have intensively studied to prevent edge dislocations from penetrating through the film, and have come to invent the above-described group III nitride film of the present invention.
[0010]
The group III nitride film of the present invention is formed by sequentially setting different first film formation conditions and second film formation conditions in the CVD method, and continuously forming these two film formation conditions. Obtainable.
[0011]
Specifically, the amount of nitrogen source gas in the first film formation condition is made larger than the nitrogen source gas flow rate in the second film formation condition, or the base material is heated in the first film formation condition. It can be formed by making the temperature lower than the heating temperature of the base material in the second film formation condition. Furthermore, it can also be formed by making the atmospheric pressure under the first film forming condition larger than the atmospheric pressure under the second film forming condition.
[0012]
The propagation behavior of edge dislocations in the group III nitride film of the present invention can be considered as follows. FIG. 1 is a diagram for explaining a conventional epitaxial growth process of a group III nitride film, and FIG. 2 is a diagram for explaining an epitaxial growth process of a group III nitride film of the present invention.
[0013]
As shown in FIG. 1, in the conventional epitaxial growth process of a group III nitride film, when a film forming process is performed on the main surface 10A on the substrate 10 shown in FIG. As shown in FIG. 1B, the initial nucleus 12 of the group III nitride film is generated on the main surface 10 </ b> A of the base material 10. Furthermore, by performing the film forming process, nucleus growth occurs from the initial nucleus 12, and an island-like structure 13 as shown in FIG. 1C is formed. Further, when the film forming process is continued, epitaxial growth is promoted centering on the island-like structure 13, and a uniform group III nitride film 20 as shown in FIG. 1D is formed.
[0014]
In the epitaxial growth process shown in FIG. 1, a film forming process under the same film forming conditions is performed throughout the entire process. In this case,
Most of the screw dislocations rising from the main surface of the base material 10 disappear with the expansion and coalescence of the island crystals 13, but most of the edge dislocations penetrate the upper surface 13A of the island structure 13 and Propagate upward as shown by. Therefore, in the conventional group III nitride film, the ratio of threading edge dislocations has increased.
[0015]
On the other hand, in the epitaxial growth process of the group III nitride film of the present invention, the first film formation conditions in which various conditions are set on the main surface 10A of the base material 10 shown in FIG. Then, a film forming process is performed by the MOCVD method or the like. Then, unlike the conventional epitaxial growth process, an initial nucleus 12 having a needle shape as shown in FIG. 2B is formed, and as shown in FIG. 2C, the initial nucleus 12 is stochastically centered. A part of the structure grows large and further grows, whereby a needle-like structure 15 as shown in FIG. 2D is formed. In this case, a trapezoidal island-like structure may be formed by combining the needle-shaped tips.
[0016]
Next, the film formation condition under the MOCVD method is changed from the first film formation condition to the second film formation condition. Then, the epitaxial growth is promoted in the lateral direction with the needle-like structure 15 as the center, and finally a uniform group III nitride film 20 as shown in FIG. 2E is formed. The timing for changing from the first film forming condition to the second film forming condition is preferably performed when the height of the needle-like structure 15 is 0.05 μm or more, and further 0.1 μm or more. .
[0017]
In this case, as shown by B in the drawing, most of the edge dislocations rising from the main surface 10A of the base material 10 are bent laterally at the side surface 15B of the needle-like structure 15 and propagate substantially parallel to the main surface 10A. To do. Therefore, the ratio of edge dislocations propagating upward is significantly reduced, and the ratio of threading edge dislocations is significantly reduced.
[0018]
In a preferred embodiment of the present invention, a surface nitrided layer is formed on the main surface of the substrate. In this case, it is possible to suppress the propagation of the screw dislocation caused by the misfit dislocation into the group III nitride film. Therefore, the ratio of threading screw dislocations can be reduced, and the dislocation density in various group III nitride films formed above can be further reduced.
[0019]
Similarly, a film formation process may be performed on the main surface of the base material to form a predetermined group III nitride underlayer instead of the surface nitride layer. Even in this case, the same effect as described above can be obtained.
[0020]
The group III nitride film of the present invention described above can be preferably used as an underlayer constituting an epitaxial substrate. In this case, when a group III nitride layer group is formed on the underlayer, that is, the epitaxial substrate, the ratio of threading edge dislocations and / or threading screw dislocations in the underlayer is reduced. The edge dislocations and / or screw dislocations propagating in the group III nitride layer group can be reduced. As a result, the dislocation density in the group III nitride layer group is decreased, and the crystal quality is improved. .
[0021]
The “Group III nitride layer group” may be composed of a single Group III nitride layer or a plurality of Group III nitride layers stacked depending on the type of semiconductor element to be manufactured. It can also be configured as a multilayer film structure. Furthermore, it can be configured as a periodic multilayer film structure in which a plurality of group III nitride layers are alternately and periodically stacked.
[0022]
Other features and details of the present invention will be described in the following embodiments of the present invention.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail based on embodiments of the invention.
[0024]
As described above, in the group III nitride film of the present invention, different first film formation conditions and second film formation conditions in the CVD method are sequentially set, and the two film formation conditions are continuous. Can be obtained. Specifically, the following three aspects can be mentioned.
[0025]
First, the nitrogen source gas flow rate in the second film forming condition and the first film forming condition is changed. Specifically, as described above, the nitrogen source gas flow rate under the first film formation condition is set larger than the nitrogen source gas flow rate under the second film formation condition. For example, when ammonia gas (NH ₃ ) is used as the nitrogen source gas, the ratio (V / III ratio) between the ammonia gas flow rate and the group III source gas flow rate in the first film formation condition is 1000 to 10,000. The V / III ratio in the second film formation condition is set to 50 to 1000.
[0026]
Second, the heating temperature of the base material under the second film forming condition and the first film forming condition is changed. Specifically, as described above, the heating temperature of the base material under the first film forming condition is set lower than the heating temperature of the base material under the second film forming condition. For example, the heating temperature of the base material in the first film forming condition is set to 800 ° C. to 1100 ° C., and the heating temperature of the base material in the second film forming condition is set to 1100 ° C. to 1500 ° C.
[0027]
Third, the atmospheric pressure in the second film formation condition and the first film formation condition is changed. Specifically, as described above, the atmospheric pressure under the first film formation condition is set larger than the atmospheric pressure under the second film formation condition. For example, the atmospheric pressure in the first film forming condition is set to 1 Torr to 30 Torr, and the atmospheric pressure in the second film forming condition is set to 30 Torr to 200 Torr.
[0028]
These three modes can be used alone or in combination of two or more thereof.
[0029]
It should be noted that the specific numerical values described above are only given as an example, and depending on the form and size of the CVD apparatus used, it may be necessary to select and set a numerical value different from the above numerical values.
[0030]
The group III nitride film of the present invention needs to contain at least Al. Accordingly, the edge dislocations are substantially parallel to the main surface of the substrate according to the mechanism shown in FIG. 2 only by using a CVD method employing different first film formation conditions and second film formation conditions. Propagates. Such a tendency becomes more prominent as the Al content in the group III nitride film increases, and the Al content is preferably 50 atomic% or more with respect to all group III elements, and more preferably the group III The nitride film is preferably made of AlN.
[0031]
In addition to Al, the group III nitride film of the present invention can also contain group III elements such as Ga and In, and additive elements such as B, Si, Ge, Zn, Be, and Mg. Furthermore, it is possible to include not only elements added intentionally but also trace elements that are inevitably taken in depending on the film forming conditions and the like, as well as trace impurities contained in the raw materials and reaction tube materials.
[0032]
Further, as described above, in the present invention, in the case where the group III nitride film is to be formed and the base material is composed of a predetermined single crystal substrate, a surface nitride layer is formed on the main surface of the base material. Is preferably formed. Thereby, the upward propagation of the screw dislocation from the base material can be effectively suppressed. Therefore, not only the threading edge dislocation but also the ratio of threading screw dislocation can be significantly reduced.
[0033]
The surface nitrided layer can be formed as follows. The base material on which the group III nitride film is to be formed was filled with a nitriding gas such as ammonia (NH ₃ ) gas, hydrazine (N ₂ H ₄ ) gas, and methyl hydrazine (N ₂ H ₃ CH ₃ ) gas. Place in a container and heat to 500-1200 ° C. The nitriding gas generates reducing nitrogen gas by contacting the main surface of the heated base material. Since this reducing nitrogen gas is extremely reactive, it causes a reduction reaction with the main surface of the heated base material to nitride the main surface, and as a result, a surface nitride layer as described above is formed. The
[0034]
The surface nitride layer is preferably formed to a thickness of 0.2 nm to 10 nm. This thickness is obtained by rough estimation from the nitrogen concentration distribution of the etching profile in the thickness direction by ESCA.
[0035]
Further, as described above, instead of subjecting the main surface of the base material to surface nitriding treatment, a group III nitride base film can be formed on the main surface. Even in this case, the same effect as described above can be realized. The base film preferably has a low threading screw dislocation density and contains at least Al. Further, the Al content is preferably 50 atomic% or more with respect to all group III elements, more preferably all the group III elements are Al and are composed of AlN.
[0036]
FIG. 3 schematically shows a multilayer film structure in which a group III nitride layer group is formed on an epitaxial substrate using the group III nitride film of the present invention as an underlayer. As is apparent from FIG. 3, the base layer 20 made of the group III nitride film of the present invention is formed on the base material 10 to constitute the epitaxial substrate 40. A predetermined group III nitride layer group 30 is formed on the epitaxial substrate 40 to constitute a multilayer structure 50.
[0037]
When a semiconductor light emitting device or the like is configured from the multilayer structure 50, the group III nitride layer group 30 is configured from an n-type conductive layer, a light emitting layer, a p-type conductive layer, and a clad layer as necessary. When a Schottky device or the like is configured from the multilayer film structure 50, the group III nitride layer group 30 is configured from a conductive layer or the like.
[0038]
In addition, the base material 10 is an oxide single crystal such as sapphire single crystal, ZnO single crystal, LiAlO ₂ single crystal, LiGaO ₂ single crystal, MgAl ₂ O ₄ single crystal, MgO single crystal, Si single crystal, SiC single crystal, or the like. Group IV or IV-IV single crystals, GaAs single crystals, AlN single crystals, GaN single crystals, III-V single crystals such as AlGaN single crystals, and boride single crystals such as ZrB ₂ are known substrates. It can consist of materials. In particular, when the substrate 10 is made of a sapphire single crystal, the effects of the present invention can be exhibited more effectively.
[0039]
【Example】
Hereinafter, the present invention will be described specifically by way of examples.
Example 1
A base material was formed from a C-plane sapphire single crystal, and this was placed on a susceptor installed in a quartz reaction tube, and then fixed by suction. Next, the substrate was heated with a heater in the susceptor until the surface temperature reached 1200 ° C. Next, ammonia gas (NH ₃ ) was introduced into the reaction tube and held for 5 minutes to form a surface nitride layer with a thickness of 1 nm on the main surface of the substrate.
[0040]
Next, trimethylaluminum (TMA) is used as the Al feed gas, NH ₃ is used as the nitrogen feed gas, and these feed gases are introduced into the reaction tube together with the hydrogen carrier gas so that NH ₃ / TMA = 500 sccm / 1 sccm. At the same time, the substrate was supplied onto the substrate and epitaxial growth was performed for 60 minutes to form an AlN film having a uniform surface. The surface roughness Ra of 5 μm square of this AlN film was 1.5 mm, and the half width of the X-ray rocking curve on the (002) plane was 60 seconds.
[0041]
Further, using the AlN film as a base film, the film forming process was continued. Initially, NH ₃ / TMA = 5000 sccm / 1 sccm was set (first film formation conditions), and an epitaxial process was performed for 20 minutes to form an AlN needle-like structure with a needle height of 0.1 μm. Next, NH ₃ / TMA = 500 sccm / 1 sccm was set (second film formation condition), and epitaxial growth was performed for 120 minutes to form a uniform AlN film. In the epitaxial growth process, the pressure in the reaction tube was kept constant at 20 Torr.
[0042]
The state of dislocation propagation in the AlN film was examined by TEM observation. As a result, it was observed that most of the dislocations rising from the base material were bent inside the AlN film and propagated substantially parallel to the main surface of the base material.
[0043]
Next, TMA and trimethylgallium (TMG) and NH ₃ were supplied as a Ga supply source gas on the AlN film to form an Al _0.5 Ga _0.5 N film having a thickness of 2 μm. When the dislocation density in this Al _0.5 Ga _0.5 N film was measured, it was 5 × 10 ⁸ / cm ² .
[0044]
(Example 2)
A base material was formed from a C-plane sapphire single crystal, and an AlN film having a uniform surface was formed in the same manner as in Example 1. Next, TMA and NH ₃ are introduced into the reaction tube so that NH ₃ / TMA = 500 sccm / 1 sccm, and the substrate temperature is set to 900 ° C. (first film formation condition), and epitaxial growth is performed for 20 minutes. Then, an AlN needle-like structure having a needle height of 0.1 μm is formed, and then the substrate temperature is set to 1150 ° C. (second film formation condition), and epitaxial growth is performed for 120 minutes to obtain a uniform AlN film. Formed. In the epitaxial growth process, the pressure in the reaction tube was kept constant at 20 Torr.
[0045]
The state of dislocation propagation in the AlN film was examined by TEM observation. As a result, it was observed that most of the dislocations rising from the base material were bent inside the AlN film and propagated substantially parallel to the main surface of the base material.
[0046]
Next, an Al _0.5 Ga _0.5 N film having a thickness of 2 μm was formed on the AlN film in the same manner as in Example 1. When the dislocation density in this Al _0.5 Ga _0.5 N film was measured, it was 5 × 10 ⁸ / cm ² .
[0047]
(Example 3)
A base material was formed from a C-plane sapphire single crystal, and an AlN film having a uniform surface was formed in the same manner as in Example 1. Next, the substrate temperature was set to 1150 ° C., and TMA and NH ₃ were introduced into the reaction tube so that NH ₃ / TMA = 500 sccm / 1 sccm, and the atmospheric pressure was set to 80 Torr (first film formation condition) ) Was grown for 20 minutes to form a needle-like structure with a height of 0.2 μm. Next, the atmospheric pressure was changed to 20 Torr (second film formation condition), and epitaxial growth was performed for 120 minutes to form a uniform AlN film.
[0048]
The state of dislocation propagation in the AlN film was examined by TEM observation. As a result, it was observed that most of the dislocations rising from the base material were bent inside the AlN film and propagated substantially parallel to the main surface of the base material.
[0049]
Next, an Al _0.5 Ga _0.5 N film having a thickness of 2 μm was formed on the AlN film in the same manner as in Example 1. When the dislocation density in this Al _0.5 Ga _0.5 N film was measured, it was 5 × 10 ⁸ / cm ² .
[0050]
(Comparative example)
In the same manner as in the example, while forming a base material from a C-plane sapphire single crystal, an AlN film having a uniform surface was formed on the base material. Next, the substrate temperature was set to 1200 ° C., and TMA and NH ₃ were introduced into the reaction tube so that NH ₃ / TMA = 500 sccm / 1 sccm, and supplied onto the substrate for 120 minutes. Epitaxial growth was performed to form a uniform AlN film. In the epitaxial growth process, the pressure in the reaction tube was kept constant at 20 Torr.
[0051]
The state of dislocation propagation in the AlN film was examined by TEM observation. As a result, it was found that a considerable portion of dislocations rising from the base material propagated upward in the AlN film to form threading dislocations.
[0052]
Next, an Al _0.5 Ga _0.5 N film having a thickness of 2 μm was formed on the AlN film in the same manner as in Example 1. When the dislocation density in this Al _0.5 Ga _0.5 N film was measured, it was 1 × 10 ¹⁰ / cm ² .
[0053]
As can be seen from the examples and comparative examples, in the AlN film according to the present invention, it can be seen that most of the dislocations from the substrate are bent in the lateral direction in the film and do not propagate upward. Therefore, it can be seen that the threading dislocation ratio decreases and the dislocation density in the GaN film formed on the AlN film is also reduced.
[0054]
The present invention has been described in detail based on the embodiments of the invention with specific examples. However, the present invention is not limited to the embodiments of the invention, and does not depart from the scope of the invention. All changes and modifications are possible. For example, a multilayer laminated film such as a buffer layer or a strained superlattice can be inserted between the epitaxial substrate and the group III nitride layer group to further improve the crystal quality of the group III nitride layer group.
[0055]
The processing from the formation of the group III nitride underlayer to the formation of the needle-like structure and the formation of the group III nitride film can be performed using the same MOCVD apparatus or performed using different MOCVD apparatuses. can do.
[0056]
【The invention's effect】
As described above, according to the present invention, a group III nitride film with reduced threading dislocations can be obtained. Therefore, by producing an epitaxial substrate using the group III nitride film as an underlayer, a group III nitride formed on the epitaxial substrate is formed of a single group III nitride film or a multilayer structure of group III nitride films. The dislocation density in the nitride layer group can be sufficiently reduced.
[Brief description of the drawings]
FIG. 1 is a view for explaining a conventional epitaxial growth process of a group III nitride film.
FIG. 2 is a diagram for explaining an epitaxial growth process of a group III nitride film of the present invention.
FIG. 3 is a configuration diagram schematically showing a multilayer film structure of the present invention.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 10 Base material, 12 Initial nucleus, 13 Island-like structure, 15 Needle-like structure, 20 III group nitride film, 30 III group nitride layer group, 40 Epitaxial substrate, 50 Multilayer film structure

Claims (7)

A predetermined base material, a group III nitride film containing at least Al formed on the main surface of the base material, and a group III nitride provided between the base material and the group III nitride film An Al content in the group III nitride film is 50 atomic% or more with respect to all group III elements , and edge dislocations in the group III nitride film are formed on the base material. An epitaxial substrate characterized in that after rising from the main surface, the substrate is bent and propagated so as to be substantially parallel to the main surface. The group III nitride film is characterized in that the first film formation condition and the second film formation condition which are different from each other in the CVD method are sequentially set, and are continuously formed under the two film formation conditions. The epitaxial substrate according to claim 1 . Wherein the nitrogen source gas amount in the first film forming conditions, characterized by being larger than the nitrogen source gas flow rate in the second deposition condition, the epitaxial substrate according to claim 2. Wherein the heating temperature of the substrate in the first film forming conditions, characterized by being smaller than the heating temperature of the substrate in the second film formation conditions, an epitaxial substrate according to claim 2 or 3 . The pressure of the atmosphere in the first film formation conditions, than said ambient pressure in the second deposition condition characterized by being larger, the epitaxial substrate according to any one of claims 2-4. The epitaxial substrate according to claim 1 , wherein the group III nitride film is made of AlN. A multilayer film structure comprising: the epitaxial substrate according to any one of claims 1 to 6 ; and a group III nitride layer group formed on the epitaxial substrate.

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