JP5005266B2 - AlN crystal fabrication method and AlN thick film - Google Patents

Description

  The present invention relates to a technique for producing a thick film of AlN group III nitride.

  Group III nitride crystals are used as a material constituting semiconductor elements such as photonic devices and electronic devices.

  A thick film of a group III nitride crystal (group III nitride thick film) is generally manufactured by epitaxially forming a group III nitride crystal on a predetermined substrate. There is an embodiment in which a single crystal base material such as sapphire or SiC is used as the substrate, but the group III nitride crystal is not more than about 10 μm on such a single crystal base material (the occurrence of warpage due to the difference in thermal expansion coefficient). A so-called epitaxial substrate (also referred to as a template substrate) that is epitaxially formed (to some extent) may be used.

  The epitaxial substrate is usually a group III nitridation that is the same or different from the thick film formed on the thin film forming method such as MOCVD (metal organic chemical vapor deposition) or MBE (molecular beam epitaxy). It is obtained by epitaxially forming a physical crystal.

  A technique for forming a Group III nitride thick film on such an epitaxial substrate using a growth method such as LPE (Liquid Phase Epitaxy), HVPE (Hydride Vapor Phase Epitaxy), and sublimation is already known ( For example, see Patent Literature 1 and Patent Literature 2). Since these methods have a feature that the film forming speed is high, it can be said that these methods are suitable for forming a group III nitride thick film.

  On the other hand, a light emitting device is produced by forming an AlN buffer layer having a thickness of about 100 nm on sapphire or SiC and then forming a functional layer of GaN and Ga-rich group III nitride having a thickness of about several μm. In the method, a technique is known in which annealing is performed at 1350 ° C. to 1500 ° C. after the formation of the buffer layer for the purpose of preventing cracking of the functional layer (see, for example, Patent Document 3).

JP-A-2005-223126 JP 2005-225248 A JP-A-9-64477

  When an epitaxial substrate is used for forming the group III nitride thick film, the crystal quality of the group III nitride thick film depends on the crystal quality of the group III nitride layer (template layer) formed epitaxially on the substrate. For example, as disclosed in Patent Document 2, it is known that dislocations generated in the template layer penetrate through a thick group III nitride film formed thereon.

  Patent Document 2 discloses an apparatus for performing vapor phase growth by an HVPE method in a quartz reaction chamber, which is provided with means for independently heating a source gas and a substrate. When forming an AlN thick film on the sapphire substrate, Si substrate, or template layer formed of AlN on these substrates using the apparatus, the substrate temperature is set to 1300 ° C. It is disclosed that the direction growth property can be enhanced, and thereby threading dislocations in the thick film can be reduced.

During the growth of the AlN thick film disclosed in Patent Document 2, the temperature of the reaction gas is set to 750 ° C. or lower so that AlCl ₃ is preferentially generated over AlCl having a corrosive ability to quartz. However, a considerable amount of AlCl is generated depending on the temperature setting. On the other hand, the substrate is heated to 1300 ° C. or higher, and in some cases to 1700 ° C. Further, since the heating is maintained while the thick AlN film is being grown, the heating time becomes longer as the thickness of the AlN film is increased. Even if such heating to the substrate does not directly heat the quartz chamber, when the substrate is heated for a long time at a high heating temperature, the possibility that the ambient temperature around the substrate becomes higher than expected is denied. In such a case, the possibility of corrosion by AlCl is considered high.

  The present invention has been made in view of the above problems, and an object of the present invention is to provide a method by which a low dislocation AlN-based group III nitride thick film can be obtained by a relatively simple technique.

The invention according to claim 1 is a first forming step of obtaining a growth substrate by forming a base layer made of a group III nitride having a ratio of Al in all group III elements of 80 mol% or more on a predetermined base material. And a second forming step of forming a crystal layer made of AlN having a thickness of 10 μm or more on the growth substrate, the method for producing an AlN crystal , wherein the underlayer in the first forming step A heating step of heating the growth substrate using a temperature higher than the formation temperature and 1500 ° C. or higher as a heating temperature, and using the growth substrate that has undergone the heating step, the second formation step. It is characterized by performing.

The invention according to claim 2 is the method for producing an AlN crystal according to claim 1, wherein the formation thickness of the underlayer in the first formation step is the formation of the crystal layer in the second formation step. It is smaller than the film thickness.

The invention according to claim 3 is the method for producing an AlN crystal according to claim 1 or 2, wherein the formation rate of the underlayer in the first formation step is the crystal in the second formation step. It is characterized by being smaller than the layer formation rate.

The invention of claim 4 is the method for producing an AlN crystal according to any one of claims 1 to 3, wherein the first forming step is performed by MOCVD, and the second forming step is performed by an HVPE method. It is characterized by that.

The invention of claim 5 is the method for producing an AlN crystal according to any one of claims 1 to 4, wherein, in the first forming step, the underlayer made of AlN is epitaxially formed. It is characterized by that.

The invention of claim 6 is the method for producing an AlN crystal according to any one of claims 1 to 5, wherein the formation temperature of the crystal layer in the second formation step is equal to or lower than the heating temperature. It is characterized by that.

The invention according to claim 7 is the method for producing an AlN crystal according to claim 6, wherein the formation temperature of the crystal layer in the second formation step is 1400 ° C. or more.

The invention of claim 8 is the method for producing an AlN crystal according to any one of claims 1 to 7, wherein the formation temperature of the crystal layer in the first formation step is 1300 ° C. or less. It is characterized by that.

The invention according to claim 9 is the method for producing an AlN crystal according to any one of claims 1 to 8, wherein the heating step is used for the first forming step and the second forming step. Is performed using different heating means.
The invention according to claim 10 is the method for producing an AlN crystal according to any one of claims 1 to 9, wherein the heating step is performed in a nitrogen gas atmosphere.

The invention of claim 11 is characterized in that the AlN thick film is produced by using the method for producing an AlN crystal according to any one of claims 1 to 9.

According to the first to eleventh aspects of the invention, the dislocation reduction of the crystal layer made of AlN can be realized only by performing a relatively simple process such as heat treatment prior to the thick film growth. This also contributes to the disappearance of the coalescence due to the growth of threading dislocations from the underlayer inclined with respect to the growth direction of the crystal layer. In addition, even if tensile stress occurs in the in-plane direction of the crystal layer due to the thermal stress between the base material and the underlayer, the tensile stress can be reduced by heat treatment, Generation of cracks in the crystal layer can also be suppressed.

In particular, according to the invention of claim 9, by performing the heat treatment using a general-purpose heat treatment apparatus without using the treatment apparatus for performing crystal growth by the MOCVD method or the HVPE method for the heat treatment, It is possible to realize a low dislocation in the crystal layer made of AlN . Therefore, the processing apparatus used for the MOCVD method or the HVPE method does not need to have a special configuration associated with the realization of the heat treatment, and the processing conditions in the second forming process using the HVPE method are not particularly limited. .

<Formation of growth underlayer>
FIG. 1 is a schematic diagram for explaining the formation of an AlN-based group III nitride thick film according to an embodiment of the present invention. In FIG. 1, a thick film layer 2 which is a crystal layer made of AlN group III nitride is formed on an AlN group III nitride thick film forming substrate (hereinafter simply referred to as “substrate”) 1. The laminated structure 3 is shown as a cross-sectional view.

The substrate 1 is composed of a base material 1a and a growth foundation layer 1b formed thereon. The growth underlayer 1b is a crystal layer made of an AlN group III nitride which is an Al-rich group III nitride. Note that the group III nitride in the present embodiment, is represented by a composition of _{B x Al y Ga z In 1} -xyz N (x, y, z ≧ 0), but generally indicate what wurtzite A substance having a zinc blende structure may be used. The growth foundation layer 1 b is a surface layer for the substrate 1, but plays a role as a foundation layer during single crystal growth using the substrate 1. The growth underlayer 1b is formed by growing an AlN group III nitride on the substrate 1a. That is, the substrate 1 is a so-called template substrate. During the thick film growth, the thick film layer 2 is formed immediately above the growth base layer 1b. The substrate 1 according to the present embodiment is provided as a substrate suitable for obtaining a high crystalline quality AlN-based group III nitride thick film by suitably forming the growth foundation layer 1b. .

The base material 1a is appropriately selected according to the composition and structure of the growth base layer 1b formed thereon or the thick film layer 2 formed thereon. For example, a substrate such as SiC (silicon carbide) or sapphire is used. Alternatively, various oxide materials such as ZnO, LiAlO ₂ , LiGaO ₂ , MgAl ₂ O ₄ , (LaSr) (AlTa) O ₃ , NdGaO ₃ , and MgO, various IV group single crystals such as Si, Ge, and diamond, and various IVs such as SiGe Various III-V compounds such as -IV group compounds, GaAs, AlN, GaN and AlGaN, and various borides such as ZrB ₂ may be appropriately selected and used. Of these, in the case where the growth base layer 1b is made of a group III nitride having a (0001) plane as a main surface, for example, a (0001) plane SiC or a (11-20) plane and a (0001) plane sapphire are used as a base material. It can be used as 1a. Further, when the growth foundation layer 1b is constituted by a group III nitride having a (11-20) plane as a main surface, for example, (11-20) plane SiC or (10-12) plane sapphire is used as the base material 1a. Can be used. The thickness of the substrate 1a is not particularly limited in terms of material, but a thickness of several hundreds μm to several mm is preferable for convenience of handling.

  In the case of an optical device application in the ultraviolet region, it is desirable to use a material transparent to the light of the operating wavelength as the substrate 1a, and in view of compatibility with the crystal structure of the group III nitride crystal, sapphire is the most Is preferred. In addition, when a high-power optical device or an electronic device that requires heat dissipation is used, SiC having high thermal conductivity is most suitable as the base material 1a.

  The growth underlayer 1b is preferably an epitaxial film made of AlN group III nitride formed by various thin film forming methods. As a film forming method, MOCVD method, MBE method, sputtering method, HVPE method and the like are assumed. When the MOCVD method is applied, a PALE method (pulsed atomic layer epitaxy), a plasma assist method, a laser assist method, or the like can be used in combination. The MOCVD method is suitable for growing high-quality crystals because the manufacturing conditions can be controlled with high accuracy. From the viewpoint of forming the thick film layer 2 having excellent crystal quality on the growth base layer 1b, at least the growth base layer 1b is formed so that the formation speed is lower than the formation speed of the thick film layer 2. It is preferable to select a method for forming the base layer 1b and the thick film layer 2. A method for forming the thick film layer 2 will be described later.

As the growth base layer 1b, an amorphous film or the like is assumed in addition to the epitaxial film. Even when the film forming methods are the same, it is possible to make both of them separately by setting different growth formation conditions (formation temperature, etc.). For example, in the case of MOCVD, an amorphous film is formed if the formation temperature is low, and an epitaxial film is formed if the formation temperature is high. The case where the growth base layer 1b is formed as an epitaxial film rather than an amorphous film is preferable in that a film having a lower dislocation density can be easily obtained after the heat treatment described later. When formed as an epitaxial film, the growth underlayer 1b generally contains dislocations of about 1 × 10 ⁹ / cm ² or more. Further, edge dislocations mainly exist in the growth underlayer 1b.

  The thickness of the growth foundation layer 1b is not particularly limited, and an optimum film thickness is selected for the device structure or usage pattern to be finally used. For example, a film thickness of about several nm to several mm is assumed. However, as will be described later, when a thick film layer 2 having a thickness of 10 μm or more is formed as a crystal layer on the growth foundation layer 1b, the growth foundation layer is suppressed from the viewpoint of suppressing the generation of cracks in the thick film layer 2. The thickness of 1b is preferably smaller than the thickness of the thick film layer 2. In addition, the composition of the growth base layer 1b shows an average composition, and it is not always necessary that the composition is uniform. For example, a gradient composition or a stress relaxation layer having a different composition can be inserted. is there.

  In addition, impurities such as H, C, O, Si, and transition metals that are inevitably included when the growth base layer 1b is formed may exist in the growth base layer 1b, and conductivity control may be performed. It is also possible to include impurities such as Si, Ge, Be, Mg, Zn and Cd, which are intentionally introduced for the purpose.

  In particular, when an AlN epitaxial film having a (0001) plane as the main surface is used as the growth foundation layer 1b, the half-width of the (0002) plane by X-ray rocking curve measurement (ω scan) for the growth foundation layer 1b before the heat treatment. Is preferably 200 seconds or shorter, more preferably 100 seconds or shorter. In addition, the lower limit value of the half width of the (0002) plane by X-ray rocking curve measurement (ω scan) is not particularly defined, but is less than the theoretical value (-10 seconds) calculated from the material and crystal structure. Absent. The realization of such a half width means that a state in which the growth orientation is less fluctuated, the C-plane is aligned, and the dislocation of the helical component is less is realized on the surface of the growth base layer 1b. This is because it is more suitable for forming the thick film layer 2 having good crystal quality on the growth base layer 1b. In order to realize the half width of the X-ray rocking curve as described above, it is not desirable to insert a so-called low-temperature buffer layer on the substrate 1a, but a thin low-temperature buffer layer that does not deteriorate the crystal quality is inserted. It is possible to do.

When an AlN epitaxial film having a (0001) plane as a main surface is used as the growth underlayer 1b, the edge dislocation density is 5 × 10 ¹⁰ / cm ² or less, which is a low value for an AlN epitaxial film. Is desirable. In this embodiment, the dislocation density is evaluated using a planar TEM. When a nitride layer is formed on the surface of the substrate 1a by nitriding, the dislocation density of AlN can be kept low as described above. Depending on the condition setting, the dislocation density of the growth underlayer 1b can be reduced to about 1 × 10 ⁹ / cm ² . However, in the present embodiment, since the effect of reducing dislocation is obtained by the heat treatment described later, the nitriding treatment is not necessarily applied, or the dislocation density is larger than 5 × 10 ¹⁰ / cm ^2. Is not excluded.

  In the case where an AlN epitaxial film having such a crystal quality as a principal surface and having a (0001) plane as the growth underlayer 1b is formed by the MOCVD method, the formation rate is at most several μm / hr, and the substrate It is desirable that the temperature of itself is 1100 ° C. or higher and 1300 ° C. or lower. For example, a preferable example is that the formation speed is 1.5 μm / hr and the substrate temperature is 1200 ° C. By keeping the formation rate low and increasing the temperature of the substrate itself, it is possible to bring it closer to an equilibrium state. In such a case, the film thickness is 10 μm or less, preferably 3 μm or less in view of the efficiency of the formation time and the like. In the case of AlN group III nitride, which is an Al-rich group III nitride, in particular, in the case of AlN, it is assumed that the formation temperature by the MOCVD method is raised to 1250 ° C. or higher. Note that an AlN epitaxial film having an equivalent crystal quality can be obtained by setting the same conditions in the HVPE method. However, since it is difficult to finely adjust the amount of raw material supply compared to the MOCVD method, the MOCVD method is suitable for controlling the film thickness stably.

  In such a case, the forming pressure is a reduced pressure atmosphere of 1 Torr or more, preferably 100 Torr or less, more preferably 20 Torr or less, and the supply ratio of the Al source gas such as trimethylaluminum to ammonia is 1: 500 or less, more preferably. Is preferably 1: 200 or less. This is because the reaction of the raw material in the gas phase can be efficiently suppressed. Moreover, in order to suppress the deterioration of the crystal quality due to the shortage of nitrogen raw material, it is desirable that the ratio is 1: 1 or more.

<Heat treatment>
In the present embodiment, prior to the formation of the thick film layer 2, a heat treatment (heating treatment) is performed in which the substrate 1 is heated by a predetermined processing apparatus.

  The heat treatment is intended to improve the crystal quality of the group III nitride constituting the growth underlayer 1b. This is particularly effective for reducing dislocations in the growth base layer 1b and eliminating pits on the surface. For example, the dislocation density decreases to approximately 1/10 or less. In particular, edge dislocations can be effectively eliminated. Note that as the dislocation density before the heat treatment is smaller, the improvement of the crystal quality by the heat treatment can be realized in a shorter time and more effectively.

  In the heat treatment, heating is performed so that the heating temperature is at least higher than the heating temperature at the time of forming the growth base layer 1b and further reaches 1500 ° C. or higher.

  The reason why the heating is performed so as to reach at least a temperature higher than the heating temperature at the time of forming the growth base layer 1b is that at least the effect of reducing dislocations can be obtained by performing such heating. Taking the case of using the most suitable MOCVD method for forming the growth underlayer 1b as an example, the MOCVD method is generally a method of forming a film by a non-equilibrium reaction. It is thought that more crystal defects (dislocations, etc.) than the number existing in the thermal equilibrium state exist in a frozen state. Is estimated to be reduced. Further, the heating temperature of a general substrate when performing crystal growth by the MOCVD method is 1250 ° C. or less, and even when AlN is 1500 ° C. or less at most, the formation of the growth base layer 1b is performed within the temperature range concerned. If it is, the effect of reduction of dislocation can be obtained by performing the heat treatment so as to reach 1500 ° C. or higher. Of course, even when the growth base layer 1b is formed at a substrate temperature of 1500 ° C. or higher, the effect of reducing dislocations can be obtained by performing the heat treatment at a temperature higher than the substrate temperature. Further, when the growth foundation layer 1b is formed by another film formation method, the same dislocation reduction effect can be obtained by heating to 1500 ° C. or higher.

  In addition, when the thickness of the growth base layer 1b is large to some extent, for example, about 0.5 μm or more, the elimination of pits existing on the surface can be realized by heating. In addition, the tensile stress generated in the in-plane direction of the thick film layer 2 described later can be effectively reduced. The effect of reducing the tensile stress can be confirmed by heat treatment at 1500 ° C. or lower, but when a crystal layer having a thickness of 10 μm or more is to be grown as the thick film layer 2, heating in such a temperature range reduces the stress. The effect is insufficient, and there is a problem that it becomes difficult to suppress the occurrence of cracks in the thick film layer 2 during the growth process of the thick film layer 2. It has been confirmed that the effect of improving the surface flatness can be obtained when heating at 1500 ° C. or higher.

  On the other hand, when the thickness of the growth base layer 1b is about 0.005 μm or more and 0.5 μm or less, for example, about 0.2 μm, both pit elimination and surface roughness improvement are realized. In order to do so, heat treatment at 1600 ° C. or higher is necessary. Thus, when the film thickness is small, when forming an epitaxial film made of a group III nitride crystal as the growth underlayer 1b, the surface accompanying the formation of the three-dimensional nucleus due to the lattice mismatch between the growth underlayer 1b and the substrate 1a. Since the flatness is greatly deteriorated, it is considered that the effect of mass transfer needs to be further promoted by increasing the temperature of the heat treatment to 1600 ° C. or higher. Of course, this effect also exists in the growth base layer 1b having a film thickness of 0.5 μm or more, and pits can be more effectively eliminated. In some cases, heat treatment at 1600 ° C. or higher is not excluded.

  The dislocation reduction and other improvements in the crystal quality of the growth underlayer 1b by such heat treatment are such that the proportion of Al in all group III elements of the AlN group III nitride constituting the growth underlayer 1b is 80 mol% or more. It is particularly effective in the case of AlN, especially in the case of AlN. In the case of AlN, there is no problem of variations such as composition fluctuations, so this is the most desirable for quality control. However, if the proportion of Al in all group III elements is 80 mol% or more, it is almost the same as the case of forming with AlN. The growth underlayer 1b having the same crystal quality can be obtained by the MOCVD method, and further, the same temperature improvement effect as in the case of AlN is confirmed by performing the heat treatment at the same temperature as in the case of AlN. . When the proportion of Al in all Group III elements is less than 80%, when heat treatment is performed at the same temperature as in AlN, the occurrence of pits and point defects due to evaporation of other Group III elements, such as Ga components, becomes a problem. The surface flatness may be impaired.

  In addition, Al-rich group III nitride can improve the crystal quality more effectively, and the effect is most remarkable in the case of AlN. Group III nitrides containing a large amount of Al have a higher melting point than GaN and InN, which are also Group III nitrides, and are less susceptible to deterioration of crystal quality due to thermal decomposition. The reason is that it can be used most effectively. Note that since BN also has a high melting point, the present method can be applied even when it contains a large amount of B. However, since BN itself is not a crystal structure in which the wurtzite structure is in a stable state, it is prominent when it contains a large amount of B. It is difficult to obtain a sufficient effect, and it is necessary to make the B concentration below a level that can realize a wurtzite structure as a single phase.

  By the way, when the formation of the growth foundation layer 1b by the epitaxial film of the group III nitride crystal itself is performed at a temperature as high as that of the heat treatment according to the present embodiment, when crystal defects are to be suppressed, Since the crystal defects are suppressed while suitably maintaining the conditions, it is generally difficult to set the conditions and control the film formation. On the other hand, in the present embodiment, a Group III nitride crystal epitaxial film is once produced by some method and then heated to a temperature higher than the production temperature (film formation temperature). There is an advantage that a high-quality group III nitride crystal can be obtained without any particular restriction on the condition setting and control of itself.

  The atmosphere during the heat treatment is preferably an atmosphere containing nitrogen element in order to prevent the decomposition of the group III nitride. For example, an atmosphere containing nitrogen gas or ammonia gas can be used, and a nitrogen gas atmosphere is most preferred because it does not have reducing properties. This is because when a reducing gas is used, the group III nitride crystal is etched, the film is reduced, and the surface is roughened. In view of this, with respect to the processing apparatus used for the heat treatment, if the substrate 1 can be heated to the intended heating temperature and can further form such a nitrogen element-containing atmosphere, There is no limitation. Therefore, a known processing apparatus can be used. Regarding the pressure condition during the heat treatment, it has been confirmed that the crystal quality is improved regardless of the pressure from reduced pressure to increased pressure.

  In the heat treatment, the crystal quality of the growth base layer 1b formed thereon is improved by utilizing the regularity of the crystal arrangement of the substrate 1a that is a single crystal. Therefore, the material used as the substrate 1a does not decompose or melt in the temperature range of the heat treatment performed for improving the crystal quality, or does not strongly react with the group III nitride crystal forming the growth underlayer 1b. desirable. This is because it is necessary to avoid the disorder of the crystal arrangement of the substrate 1a during the heat treatment. Therefore, it is desirable that the reaction product of the two is not significantly formed at the interface between the substrate 1a and the growth base layer 1b during the heat treatment. Specifically, the fact that the reaction product is not formed remarkably means that there is no reaction product at the interface between the two after the heat treatment, or even if it exists, the thickness of the growth base layer 1b is at most. It means 1/10 or less. This is because if the film thickness is exceeded, the surface flatness of the growth base layer 1b may be impaired due to the presence of the reaction product. Therefore, it is not excluded from the present invention that an ultrathin reaction product is generated entirely or locally at the interface between the substrate 1a and the growth underlayer 1b by the heat treatment. In some cases, the presence of such an ultrathin reaction product is preferable, such as acting as a buffer layer for reducing dislocations. From such a viewpoint, sapphire, MgO, and SiC having a high melting point are desirable as the material of the substrate 1a.

  Therefore, the heat treatment is performed within a temperature range that does not exceed the melting point of the substrate 1a, or a temperature range in which the formation of a reaction product between the substrate 1a and the growth base layer 1b does not occur remarkably, that is, growth due to excessive reaction. It is desirable to carry out in a temperature range in which the crystal quality of the formation 1b does not deteriorate. In particular, when sapphire is used as the substrate 1a and the growth base layer 1b is formed of AlN group III nitride, the heat treatment may be performed in a temperature range in which γ-ALON is not significantly formed at the interface between the two. preferable. This is because if the γ-ALON is formed remarkably, the surface roughness of the growth foundation layer 1b increases.

  Note that the reduction of dislocations by heat treatment is realized by setting the thermal equilibrium state as a target, and therefore, a longer heat treatment time is desirable. However, in order to avoid the deterioration of surface flatness caused by excessive heat treatment, the heat treatment time needs to be set appropriately according to the thickness of the growth base layer 1b.

  The heat treatment does not need to be performed using an MOCVD apparatus or an HVPE apparatus which are apparatuses for performing a film formation process, and can be performed using another processing apparatus such as a so-called heat treatment furnace. Of course, the heat treatment is performed in the same MOCVD apparatus continuously with the formation of the growth base layer 1b, or the HVPE apparatus used for forming the thick film layer 2 by the HVPE method described below is used. Similar effects can be obtained by performing heat treatment prior to formation. However, when the film formation process and the heat treatment are performed in different apparatuses, the process can be performed using an apparatus suitable for each process, so that there are restrictions on the apparatus configuration or restrictions on setting the processing conditions. There is a merit that there is little.

  Further, when the growth foundation layer 1b is formed as a group III nitride epitaxial film having a (0001) plane as a main surface, the effect of reducing dislocation is remarkably obtained and the surface of the substrate 1 after the heat treatment is obtained. It is also possible to realize flatness that allows atomic steps to be observed. In order to form the growth foundation layer 1b as an epitaxial film having such a main surface, it is preferable to use (0001) plane sapphire, (11-20) plane sapphire, and (0001) plane SiC as the substrate 1a. is there. In this case, a substrate slightly tilted from the setting surface can be used.

  In addition, as described above, AlN having a wurtzite structure constituting the growth underlayer 1b has a crystal structure that does not have a target center, and the direction of the crystal is reversed when Al atoms and nitrogen atoms are interchanged. become. That is, it can be said that the crystal has polarity according to the atomic arrangement. In view of this, if an inversion region that is a region having different polarities coexists on the surface of the growth base layer 1b, the inversion region boundary (inversion boundary) becomes a kind of surface defect. End up. In this case, even after the heat treatment, defects due to the surface defects may occur, which is not preferable. Therefore, it is preferable that the growth base layer 1b has the same surface polarity.

  In particular, when AlN is used as the epitaxial film of the growth underlayer 1b, the above-described dislocation reduction effect is not found only in the surface portion, but is about 0.01 μm in the vicinity of the interface between the substrate 1a and the group III nitride epitaxial film. Also in the range, it is characteristic that it is found to the same extent as the surface portion. This is because a plurality of edge dislocations have disappeared due to heat treatment even in the vicinity of the interface with the substrate. This is in contrast to the case where the dislocation of the AlN epitaxial film, which is the growth base layer 1b, gradually decreases as the film thickness increases when heat treatment is not performed.

  In view of such a reduced state of dislocation, in order to bring out not only the improvement of the surface flatness but also the reduction of the dislocation density in the substrate 1 using the AlN epitaxial film as the growth underlayer 1b, the growth underlayer 1b The thickness needs to be 5 nm or more, which is a film thickness at which the disappearance of the edge dislocations is almost finished. Preferably, it is 0.05 μm or more. This takes into account the reduction in film thickness due to the etching of the AlN epitaxial film during heat treatment.

  A member for controlling impurities in the gas such as a hydrogen component, an oxygen component, and a carbon component may be disposed inside the treatment apparatus used for the heat treatment. Moreover, this function can be given to a jig for fixing the substrate 1.

  Further, during the heat treatment, for the purpose of suppressing etching on the surface of the growth base layer 1b, adhesion of impurities, or suppression of surface roughness due to excessive heat treatment, a protection made of, for example, silicon nitride is formed on the surface of the growth base layer 1b. Layers can also be provided. However, in particular, when an AlN epitaxial film is used as the growth underlayer 1b, the heat treatment effect can be obtained stably without using such a protective layer because of its chemical stability.

<Thick film growth>
The thick film layer 2 is formed by epitaxially growing an AlN group III nitride on the substrate 1 by a CVD method such as HVPE method or MOCVD method, a sublimation method or a flux method. From the viewpoint of raw material cost and formation rate, the HVPE method is preferable. In the case of the HVPE method, a group III metal such as Al and a hydride gas such as HCl gas are directly reacted at a temperature of about 500 ° C. to 700 ° C. to generate a group III source gas (for example, AlCl _x gas). And NH ₃ gas are reacted to produce an AlN-based group III nitride, which is epitaxially grown on the substrate 1 heated to about 1100 ° C. to 1700 ° C., for example. Since high-concentration Al raw material can be supplied, the HVPE method has a rate higher than the rate of formation by other film forming methods (at most, several μm / hr for MOCVD method), for example, several tens to several hundreds μm. In principle, it is possible to grow a single crystal film at a formation rate of / hr.

  The thick film layer 2 is a suitable example formed to a thickness of 10 μm or more, for example, a thickness of about several tens to several hundreds of μm. However, it is not excluded to form the film thinner or thicker than the case. The thick film layer 2 may contain elements other than those constituting the group III nitride as impurities, regardless of whether they are intentional.

  Forming the thick film layer 2 made of AlN group III nitride on the growth base layer 1b also made of AlN group III nitride occurs when the layer is formed directly on the substrate 1a. This is preferable in that deterioration of crystal quality due to lattice mismatch or crystal structure difference accompanying heterogeneous interface bonding can be prevented. In particular, since it is difficult to accurately control conditions at the interface in the HVPE method, when the thick film layer 2 is directly grown on the substrate 1a by the HVPE method without using the growth base layer 1b, its crystal quality However, such a situation can be avoided by forming it on the growth base layer 1b.

  Furthermore, in the present embodiment, the substrate 1 that has been subjected to heat treatment as described above to reduce dislocations and improve other crystal quality, specifically, the dislocation compared to before the treatment by heat treatment. The thick film layer 2 is formed by using the substrate 1 whose density is reduced to about 1/10. Thereby, formation of the thick film layer 2 with low dislocation is realized as compared with the case where heat treatment is not performed.

  In particular, threading dislocations penetrating from the growth base layer 1b to the thick film layer 2 are grown rather than penetrating in a direction substantially perpendicular to the main surface (growth surface) of the substrate 1 and maintaining the same direction. It is confirmed that those that penetrate and grow in an oblique direction with respect to the direction are dominant, and that threading dislocations almost disappear together inside the thick film layer 2 with such growth. That is, it can be said that the above-described heat treatment prior to the formation of the thick film layer 2 is effective in suppressing the growth of dislocations in the thickness direction (height direction) of the thick film layer 2. The thickness range in which threading dislocations remain (that is, the thickness range until almost all threading dislocations have disappeared) is approximately 50 μm from the interface with the growth base layer 1b. By forming the thick thick film layer 2, it is possible to realize the thick film layer 2 having a sufficiently low dislocation region as compared with the case where the heat treatment is not performed. In addition, if it forms in the thickness of at least 10 micrometers or more, the thick film layer 2 which has the area | region where the threading dislocation was substantially reduced can be obtained. On the other hand, when the thickness of the thick film layer 2 is 10 μm or less, reduction of threading dislocations may not always be sufficiently realized. That is, it can be said that the heat treatment according to the present embodiment is a more suitable method when it is desired to obtain the thick film layer 2 of 10 μm or more.

  When tensile stress occurs in the in-plane direction in the vicinity of the interface between the thick film layer 2 and the growth foundation layer 1b due to the presence of thermal stress between the substrate 1a and the growth foundation layer 1b. Cracks may occur in the thick film layer 2. In order to avoid this, it is preferable to perform crystal growth so that the in-plane lattice constant of the thick film layer 2 is equal to or greater than the in-plane lattice constant of the growth base layer 1b.

  On the other hand, when compressive stress is generated in the plane of the thick film layer 2 in the vicinity of the interface with the growth base layer 1b, the effect of suppressing the dislocation density is further promoted by advancing the dislocations described above in an oblique direction. The Therefore, it is more preferable to perform crystal growth so that the main-plane lattice constant of the thick film layer 2 is larger than the main-plane lattice constant of the growth base layer 1b. Specifically, the crystal growth is realized by increasing the composition of Ga and In in the thick film layer 2, increasing the composition of Al and B in the underlayer 1b, or a combination thereof. .

  In the present embodiment, the dislocation of the growth base layer 1b is reduced in advance by heat treatment, and then the thick film layer 2 is formed by the HVPE method or the like. Therefore, the thick film by the HVPE method or the like is used. During the crystal growth of the layer 2, it is not necessary to heat the substrate 1 for the purpose of reducing dislocations. Therefore, it is not necessary to set the heating temperature of the substrate with the intention of realizing the reduction of dislocations, and it is only necessary to set a suitable temperature condition for the growth of the thick film layer 2. By doing so, it can be said that there are few restrictions on setting the processing conditions, compared to a technique for reducing dislocations. For example, the heating temperature of the substrate 1 when forming the thick film layer 2 may be lower than the heating temperature in the heat treatment. In particular, when the formation of the thick film layer 2 satisfying such a temperature relationship is performed using the substrate 1 having a larger thermal expansion coefficient of the base material 1 than the thermal expansion coefficient of the forming material of the growth base layer 1b, between the two Since a large compressive stress is generated in the in-plane direction of the thick film layer 2 due to the thermal stress generated in, the occurrence of cracks in the thick film layer 2 can be further reduced.

  Furthermore, it is possible to form the thick film layer 2 with good surface flatness by appropriately maintaining the relationship between the substrate temperature and the formation speed. In that case, if the formation temperature is 1400 ° C. or higher, it has been confirmed that the degree of freedom in formation speed is relatively high. The formation rate can be controlled by controlling the supply amount of the Al source gas. This is because there is a positive correlation between the supply amount and the formation speed of the thick film layer 2. However, the specific supply amount range is appropriately determined depending on the configuration of each manufacturing apparatus. In addition to or in addition to the supply amount of the Al source gas, any of the pressure during formation in the reaction tube of the production apparatus, the flow rate of NH3 gas, the gas flow rate of the Al source gas, or a part thereof Or the aspect which controls formation speed by performing control which combined all may be sufficient.

  When the thick film layer 2 is formed by the HVPE method, the flatness of the surface of the thick film layer 2 can be further improved by using nitrogen gas as a main component.

Further, when the AlCl _x gas and the NH ₃ gas are reacted by the HVPE method, it is desirable that the forming pressure is 5 Torr or more and 50 Torr or less. When such conditions are selected, the reaction in the gas phase is suppressed, and the uptake rate of the source gas into the thick film layer 2 is increased, so that there is an advantage that the utilization efficiency of the source gas is increased.

  As described above, according to the present embodiment, on the epitaxial substrate formed with the growth base layer made of the AlN group III nitride formed epitaxially on the predetermined base material by MOCVD or the like. In addition, when an AlN group III nitride thick film is obtained by epitaxially forming a thick film layer made of AlN group III nitride by the HVPE method or the like, the temperature is higher than the heating temperature when forming the growth underlayer Moreover, the dislocation of the thick film layer can be reduced by forming the thick film layer after the epitaxial substrate is heated at a temperature of 1500 ° C. or higher. In particular, when the thick film layer is formed so as to have a thickness of 10 μm or more, the effect of lowering the dislocation can be suitably obtained.

  That is, prior to thick film growth using the HVPE method or the like, only a relatively simple process such as heat treatment is performed, and an apparatus having a special configuration is used for the growth of the thick film layer. Even without limitation, a thick film layer made of low dislocation AlN group III nitride can be formed.

<Modification>
In the above-described embodiment, the heat treatment and the thick film growth are mainly described in the case of using a template substrate provided with a growth base layer as an epitaxial film, but instead of this, a predetermined single crystal substrate is used. Even when crystal growth is performed using a base substrate in which an amorphous or polycrystalline so-called low-temperature buffer layer is formed on a material with a thickness of about 20 nm, the heat treatment for the base substrate is excellent in crystal quality. It is effective in forming a crystalline layer on the substrate. For example, the base substrate is subjected to a heat treatment at a heating temperature of 1300 ° C. or higher, and then a group III nitride layer is formed at a formation temperature lower than the heating temperature, so that the crystal quality is excellent. Further, a group III nitride single crystal layer can be obtained.

Example 1
As the substrate 1a, a 2-inch diameter (0001) plane sapphire single crystal having a thickness of 400 μm was used, and this was placed on a susceptor in a reaction vessel of a known MOCVD apparatus. After setting the pressure in the reaction vessel to 20 Torr or less, the flow rate was set so that the average flow rate of all gases was 1 m / sec or more, and the temperature of the substrate 1a itself was raised to 1200 ° C.

Thereafter, NH ₃ gas was supplied to nitride the surface of the substrate 1a, and then TMA and NH ₃ were supplied to epitaxially form an AlN layer having a thickness of 1 μm as the growth base layer 1b. Thereby, the substrate 1 was obtained. At this time, the supply amounts of TMA and NH ₃ were set so that the formation rate was 1.5 μm / hr. When the X-ray rocking curve of the (0002) plane of AlN was measured, the half width was 200 seconds or less, the dislocation density was 8 × 10 ⁹ / cm ² , and the atomic step was measured by AFM (Atomic Force Microscope). Observed clearly. At this time, in order to realize the growth of the group III atomic plane, the supply molar ratio of the group III material and the group V material during the growth of the AlN layer is set to 1: 100. After the growth of the AlN layer was completed, the substrate 1 was taken out from the reaction vessel.

Next, the substrate 1 was placed at a predetermined position in the furnace chamber of a known heat treatment furnace, N ₂ gas was supplied while maintaining the pressure in the furnace chamber at 1 atm, and heat treatment was performed at 1650 ° C. for 2 hours. .

When the crystal quality of the AlN layer was evaluated after the above heat treatment, the half width of the (0002) plane of the X-ray rocking curve was 100 seconds and the half width of the (10-12) plane was 500 seconds. . The dislocation density was 2 × 10 ⁹ / cm ² in all cases. After completion of the heat treatment, the substrate 1 was taken out from the furnace chamber.

  Next, this board | substrate 1 was installed in the susceptor of the reaction tube of a well-known HVPE apparatus. Further, the boat in the reaction tube retained the metal Al raw material, that is, only Al was used as the group III element.

The pressure in the reaction tube was set to 30 Torr, and the temperature was increased so that the temperature of the gas reaction region in the reaction tube was 550 ° C. and the substrate temperature was 1450 ° C. During the temperature increase, only NH ₃ and N ₂ gas are supplied.

When the pressure and temperature in the reaction tube become stable, NH ₃ gas, H ₂ gas and HCl gas (H ₂ diluted, containing 20% HCl gas) are supplied from each gas supply source through a predetermined gas introduction tube. Then, NH ₃ gas and H ₂ gas were supplied to the vicinity of the substrate 1 held by the susceptor via different gas introduction pipes. Thereby, an AlN thick film layer as a thick film layer 2 made of AlN was epitaxially formed on the substrate 1 to a thickness of 100 μm. In forming the AlN thick film layer, the supply rate of HCl is adjusted to control the AlN growth rate to be 30 μm / hr, and the pressure in the reaction tube is set to 30 Torr.

The dislocation density on the surface of the obtained AlN thick film layer was 5 × 10 ⁷ / cm ² . When the dislocation state inside the AlN thick film layer was confirmed by TEM, it was observed that the dislocation progress direction was greatly inclined with respect to the growth direction, and the coalescence disappearance between dislocations was remarkably generated. There were no cracks. In this example, the same results were confirmed when the combination of the heat treatment temperature and time was 1550 ° C./20 hours and 1750 ° C./10 minutes.

(Example 2)
The AlN thick film layer is formed in the same manner as in Example 1 except that the heat treatment of the substrate 1 is performed following the formation of the AlN layer in the MOCVD apparatus used to form the AlN layer as the growth base layer 1b. Went up. As in Example 1, the heat treatment in the MOCVD apparatus was performed at 1650 ° C. for 2 hours while supplying N ₂ gas while maintaining the pressure in the reaction tube at 1 atm.

  The dislocation state of the resulting AlN thick film layer was the same as in Example 1, and no cracks were present.

(Example 3)
Example 1 except that the heat treatment of the substrate 1 was performed in the HVPE apparatus used to form the AlN thick film layer as the thick film layer 2 and subsequently the AlN thick film layer was formed in the apparatus. In the same manner as described above, the AlN thick film layer was formed. As in Example 1, the heat treatment in the HVPE apparatus was performed at 1650 ° C. for 2 hours while supplying N ₂ gas while maintaining the pressure in the reaction tube at 1 atm.

  The dislocation state of the resulting AlN thick film layer was the same as in Example 1, and no cracks were present.

Example 4
In addition to using a (0001) 6H-SiC single crystal having a thickness of 400 μm with a diameter of 2 inches as the substrate 1a and forming an AlN layer with a thickness of 0.5 μm as the growth base layer 1b, the examples 1 to 3 In the same manner as described above, an AlN thick film layer was formed.

  The dislocation state of the resulting AlN thick film layer was the same as in Examples 1 to 3, and no cracks were present.

(Example 5)
When forming the growth foundation layer 1b, TMA, TEB and NH ₃ are supplied to epitaxially form an Al _0.99 B _0.01 N layer having a thickness of 1 μm as the growth foundation layer 1b. Then, an AlN thick film layer was formed.

The dislocation density on the surface of the obtained AlN thick film layer was 3 × 10 ⁷ / cm ² . When the dislocation state inside the AlN thick film layer was confirmed by TEM, it was observed that the dislocation progress direction was greatly inclined with respect to the crystal growth direction, and the dislocation of dislocations was noticeably lost. There were no cracks.

(Example 6)
As the thick film layer 2, a thick film layer was formed in the same manner as in Examples 1 to 3, _except that an Al _0.98 Ga _0.02 N layer was formed using a metal Al raw material and a metal Ga raw material.

The dislocation density on the surface of the obtained Al _0.98 Ga _0.02 N thick film layer was 3 × 10 ⁷ / cm ² . When the dislocation state inside the Al _0.98 Ga _0.02 N thick film layer was confirmed by TEM, the dislocation progress direction was greatly inclined with respect to the crystal growth direction, and the dislocation disappearance between the dislocations occurred remarkably. It was observed.

(Comparative Example 1)
The same procedure as in Example 1 was performed except that the heat treatment in Example 1 was omitted. That is, after the formation of the AlN layer in the MOCVD apparatus, the AlN thick film layer was formed in the HVPE apparatus without performing heat treatment.

The dislocation density at the surface of the AlN thick film layer thus obtained was 8 × 10 ⁹ / cm ² . Further, when the state of dislocation inside the AlN thick film layer was confirmed, it was observed that the dislocation was proceeding substantially parallel to the crystal growth direction. The disappearance of coalescence occurred only in part, and the presence of cracks was confirmed.

(Comparative Example 2)
The AlN thick film layer was formed using the MOCVD apparatus used in Example 1. The thickness was 100 μm. The other fabrication conditions were the same as in the case where an AlN layer was formed as the growth foundation layer 1b.

The dislocation density on the surface of the AlN thick film layer thus obtained was 5 × 10 ⁹ / cm ² . When the state of dislocation inside the AlN thick film layer was confirmed, it was observed that the direction of dislocation progression was substantially parallel to the crystal growth direction. The disappearance of coalescence occurred only in part, and the presence of cracks was confirmed.

It is a mimetic diagram for explaining formation of an AlN system group III nitride thick film concerning an embodiment of the invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Substrate 1a Base material 1b Growth base layer 2 Thick film layer 3 Laminated structure

Claims (11)

A first forming step of obtaining a growth substrate by forming a base layer made of a group III nitride having a proportion of Al in all group III elements of 80 mol% or more on a predetermined base material;
A second forming step of forming a crystal layer made of AlN having a thickness of 10 μm or more on the growth substrate;
A method for producing an AlN crystal containing
A heating step of heating the growth substrate using a temperature higher than the formation temperature of the base layer in the first formation step and a temperature of 1500 ° C. or higher as a heating temperature;
Further comprising
Performing the second forming step using the growth substrate that has undergone the heating step;
A method for producing an AlN crystal characterized by the above. A method for producing an AlN crystal according to claim 1,
The formation thickness of the underlayer in the first formation step is smaller than the formation thickness of the crystal layer in the second formation step,
A method for producing an AlN crystal characterized by the above. A method for producing an AlN crystal according to claim 1 or claim 2,
The formation rate of the underlayer in the first formation step is smaller than the formation rate of the crystal layer in the second formation step;
A method for producing an AlN crystal characterized by the above. A method for producing an AlN crystal according to any one of claims 1 to 3,
The first forming step is performed by MOCVD method, and the second forming step is performed by HVPE method.
A method for producing an AlN crystal characterized by the above. A method for producing an AlN crystal according to any one of claims 1 to 4,
In the first forming step, the base layer made of AlN is epitaxially formed.
A method for producing an AlN crystal characterized by the above. A method for producing an AlN crystal according to any one of claims 1 to 5,
The formation temperature of the crystal layer in the second formation step is equal to or lower than the heating temperature.
A method for producing an AlN crystal characterized by the above. A method for producing an AlN crystal according to claim 6,
The formation temperature of the crystal layer in the second formation step is 1400 ° C. or higher.
A method for producing an AlN crystal characterized by the above. A method for producing an AlN crystal according to any one of claims 1 to 7,
The formation temperature of the crystal layer in the first formation step is 1300 ° C. or lower.
A method for producing an AlN crystal characterized by the above. A method for producing an AlN crystal according to any one of claims 1 to 8,
The heating step is performed using a heating means different from the heating means used in the first forming step and the second forming step.
A method for producing an AlN crystal characterized by the above. A method for producing an AlN crystal according to any one of claims 1 to 9 ,
Performing the heating step in a nitrogen gas atmosphere;
A method for producing an AlN crystal characterized by the above. An AlN thick film produced by using the method for producing an AlN crystal according to any one of claims 1 to 9.

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