JP2006240895A - Method for producing aluminum-based nitride crystal and laminated substrate - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To appropriately remove a surface oxide layer existing on an aluminum nitride crystal substrate for growth and having a nitrogen polarity; and to epitaxially grow a good quality aluminum-based nitride crystal on the substrate by a vapor phase growth method such as an HVPE method. <P>SOLUTION: The oxide layer on the surface of the aluminum nitride crystal substrate is removed by an etching method of the aluminum nitride crystal substrate, comprising etching the surface of the aluminum nitride crystal substrate with a weak basic aqueous solution such as an ammonia aqueous solution. Then, the aluminum-based nitride crystal is epitaxially grown on the substrate. By this method, the quality of the aluminum-based nitride crystal in the growth layer side is improved by succeeding the crystallinity of the ground substrate. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention is based on a vapor phase epitaxial growth method of an aluminum nitride-based nitride crystal multilayer substrate and a semiconductor crystal film that can be used for the production of light-emitting diodes, laser diodes, high-power electronic devices, etc. operating in the ultraviolet region and the production of bulk crystal substrates It relates to a manufacturing method.

  Group III nitrides such as aluminum nitride and gallium nitride have a large band gap energy. The band gap energy of aluminum nitride is about 6.2 eV, and the band gap energy of gallium nitride is about 3.4 eV. These mixed crystals of aluminum gallium nitride take a band gap energy between the band gap energies of aluminum nitride and gallium nitride according to the ratio of both components. Therefore, by using these group III nitrides, it is possible to emit light in the ultraviolet region, which is impossible with other semiconductors, and for ultraviolet light emitting diodes for white light sources, ultraviolet light emitting diodes for sterilization, and high-density optical disk memories. Light emitting light sources such as lasers that can be used for reading and writing and lasers for communication can be manufactured. Furthermore, it is possible to manufacture an electronic device such as a high-speed electron transfer transistor by utilizing the high saturation drift velocity of electrons, and to apply it to a field emitter by utilizing negative electron affinity.

  In general, attempts have been made to form a thin film of several microns or less on a substrate by forming a portion that exhibits the functions of the light emitting light source and the electronic device as described above. This can be achieved by crystal growth methods such as the known molecular beam epitaxy (MBE) method, metalorganic vapor phase epitaxy (MOVPE) method, and hydride vapor phase epitaxy (HVPE) method. It is formed.

  As the substrate for forming the laminated structure exhibiting the light emitting function, a substrate made of the above group III nitride, particularly aluminum nitride crystal is preferable. This is because the influence of lattice mismatch at the interface and the stress generated by the temperature history during growth are minimized when forming a single layer or mixed crystal of group III nitride such as aluminum nitride or gallium nitride as the growth layer. This is because it is necessary to limit to the limit. As a result, it is considered that the dislocation density, defects, and cracks of the growth layer are reduced and the light emission efficiency is improved. In the case of growing the ultraviolet light emitting layer, by using aluminum nitride as the substrate, the band gap energy of the substrate portion becomes larger than the band gap energy of the light emitting layer, so the emitted ultraviolet light is not absorbed by the substrate, The light extraction efficiency is increased.

  With respect to the production of the above-described aluminum nitride crystal substrate, the present inventors have already proposed a method of producing by the HVPE method (Japanese Patent Laid-Open No. 2003-303774). According to this method, since a very high crystal growth rate can be obtained, it is possible to mass-produce a thick aluminum based III-V compound semiconductor crystal at a practical level. Therefore, by processing the thick film crystal obtained by this method into a wafer shape, it can be used as an aluminum-based III-V compound semiconductor crystal substrate. It is expected that a highly efficient light source can be obtained by forming a laminated structure for the purpose of light emission using a crystal growth method such as MOVPE, MBE, or HVPE on the most preferable substrate. . Note that the aluminum-based III-V compound semiconductor described in JP-A-2003-303774 includes the aluminum-based nitride crystal in the present invention.

  As an aluminum nitride crystal substrate, there is a so-called template substrate in which an aluminum nitride crystal is preliminarily grown on a base material substrate made of sapphire, silicon carbide, silicon or the like. Since the pre-grown crystal layer is present on the surface of the template substrate, it can be grown in a state where the lattice constant misfit is relaxed when the aluminum-based nitride crystal is grown on the substrate. At this time, the crystal quality on the epitaxial growth layer side is strongly influenced by the crystal quality of the pre-growth layer on the template substrate which is the base. That is, if a pre-growth layer with good crystal quality is obtained, an aluminum-based nitride crystal with good crystal quality is epitaxially grown. The template substrate is manufactured by a method such as a liquid phase method or a surface nitridation of a sapphire substrate in addition to those manufactured by a vapor phase growth method such as MOVPE method, MBE method, and HVPE method. On such a template substrate, a laminated structure composed of an aluminum nitride crystal capable of exhibiting a function such as light emission, and an aluminum nitride crystal, particularly an aluminum nitride crystal, are more frequently grown on the substrate. Is called.

  As described above, an aluminum nitride crystal substrate grows by satisfying the material properties required for the substrate at a high level when the substrate is manufactured with a laminated structure having functions such as light emission and electron transfer and further thickening. It makes it possible to finish the layer properties with higher performance and higher quality. Therefore, providing an aluminum nitride crystal substrate with good quality is recognized as one of the most desired techniques in the field of group III nitride crystals.

JP 2003-303774 A Physica Status Solidi (c) 0, No.7 2498-2501 (2003) MRS Internet Journal Nitride Semiconductor Research 7, 4, 1-6 (2002)

  However, there has been a big problem in considering the formation of a growth layer on an aluminum nitride crystal substrate. That is, the manufactured aluminum nitride crystal substrate is transported between different crystal growth apparatuses to form an epitaxial growth layer on the substrate, but when exposed to air during that time, the surface of the aluminum nitride crystal substrate is in the air. It reacts with oxygen and moisture, and the vicinity of the surface is oxidized.

FIG. 1 shows the results of measuring the element profile in the depth direction of an aluminum nitride crystal substrate (template substrate) manufactured by the MOVPE method by X-ray photoelectron spectroscopy (XPS). In FIG. 1, Al _2p , O _1s , N _1s, etc. indicate the core electron levels measured by X-ray photoelectron spectroscopy. The vertical axis represents the concentration of elements contained in the aluminum nitride crystal substrate in atomic%, and the horizontal axis represents the depth from the substrate surface. From this result, oxygen exceeding 60 atomic% was present in the vicinity of the surface, and oxygen was detected to a depth exceeding 50 nm from the surface. That is, it was confirmed that a surface oxide layer (region) exists in the aluminum nitride crystal substrate.

  The presence of this surface oxide layer deteriorates the crystal quality of the aluminum nitride crystal substrate surface, thereby inhibiting aluminum nitride crystal epitaxial growth on the substrate, and as a result, the original crystal quality of the aluminum nitride crystal substrate is inherited by the growth layer. Therefore, it was considered that the contribution to the improvement of the crystal quality of the epitaxial growth layer was reduced. Therefore, the present inventors considered that a pretreatment for removing the surface oxide layer of the aluminum nitride crystal substrate, that is, an etching treatment is necessary immediately before forming the epitaxial growth layer.

  Therefore, the present inventors diligently studied an etching method for the aluminum nitride crystal substrate. As a result, the surface of the aluminum nitride crystal substrate is etched with a weakly basic aqueous solution, and then an aluminum-based nitride crystal is epitaxially grown on the substrate, thereby taking over the crystallinity of the underlying substrate and the aluminum-based nitride on the growth layer side. The inventors have found that the quality of product crystals is improved, and have completed the present invention. Furthermore, paying attention to the crystal polarity of the aluminum nitride crystal substrate, it was confirmed that the effect of the method of the present invention is better exhibited in the aluminum nitride crystal substrate having nitrogen polarity.

  That is, the present invention is characterized by removing an oxide layer present on the surface of an aluminum nitride crystal substrate having nitrogen polarity by etching with a weakly basic aqueous solution, and then epitaxially growing an aluminum-based nitride crystal on the substrate. This is a method for producing an aluminum-based nitride crystal.

  In the present invention, the surface oxide layer present in the growth aluminum nitride crystal substrate having nitrogen polarity is appropriately removed, and an aluminum nitride crystal is epitaxially grown on the substrate by using a vapor phase growth method such as HVPE method. Therefore, the crystal quality of the aluminum-based nitride crystal on the growth layer side is improved as compared with the case without etching.

  In the present invention, an aluminum-based nitride crystal is epitaxially grown on an aluminum nitride crystal substrate. As this aluminum nitride crystal substrate, as shown in FIG. 2, a preliminary aluminum nitride crystal is previously formed on a substrate made of a single aluminum nitride single crystal or a crystalline base material substrate other than an aluminum nitride crystal such as sapphire, silicon or silicon carbide. A template substrate on which a growth layer is formed is employed. These aluminum nitride crystal substrates have an aluminum nitride crystal having a nitrogen polarity (hereinafter also referred to as a nitrogen polar aluminum nitride crystal) as a growth surface.

  Polarity indicates the directionality of atomic arrangement. Group III nitride crystals such as aluminum nitride have a hexagonal wurtzite structure as shown in FIG. In this crystal structure, there is no symmetry plane with respect to the c-axis direction, and the c-plane grown epitaxial film has two sides. When attention is paid to Al atoms, a crystal in which N atoms are arranged vertically above the Al atoms is called aluminum polarity (Al polarity). On the contrary, a crystal in which Al atoms are arranged vertically above N atoms is called nitrogen polarity (N polarity).

  The influence on the physical properties of polar aluminum nitride crystals is manifested in chemical durability. Aluminum nitride crystals having Al polarity are excellent in chemical durability, while aluminum nitride crystals having N polarity are inferior in chemical durability. For example, as described in Non-Patent Document 2, an aluminum nitride crystal having a nitrogen polarity is dissolved by immersing the aluminum nitride crystal in a potassium hydroxide aqueous solution heated to 50 ° C. In aluminum nitride having Al polarity, when there is a dislocation on the surface, it is etched from the dislocation to form pits, but is hardly etched on the other surface. Conversely, since dislocations correspond to pits, the dislocation density (threading dislocation density) existing on the crystal surface can be calculated by measuring the pits, which is applied to the evaluation of crystal quality. Further, as described above, it is applied to the polarity determination of the aluminum nitride crystal from the difference in etching resistance depending on the polarity.

  Since the above basic aqueous solution is strongly basic and has a very high etching rate, it has been used exclusively for estimating the threading dislocation density of aluminum nitride crystals and determining the polarity. However, the present inventors paid attention to an aqueous solution of a weak base material that is considered to have a slow etching rate, and considered and studied to appropriately remove the surface oxidation region of the aluminum nitride crystal under such conditions.

  In the present invention, it is necessary to use an aqueous solution of a weak base substance (weak basic aqueous solution). The weak base substance is a substance having a small ionization degree when made into an aqueous solution, and examples thereof include ammonia. When this is made into an aqueous solution, it becomes ammonium hydroxide, and a part thereof is ionized into ammonium ions and hydroxide ions to show basicity. Only a small part of the ammonium hydroxide is ionized and the rest is present in water as ammonium hydroxide. Therefore, since the effective hydroxide ions are less than when a strong base material is used, the etching rate is relatively slow.

  On the other hand, a strong base substance has an ionization degree close to 1 when it is made into an aqueous solution. For example, potassium hydroxide and sodium hydroxide are strong base substances having an ionization degree of 1, and when made into an aqueous solution, they are completely ionized into potassium ions (sodium ions) and hydroxide ions. When such a solution is used for etching, the etching rate is increased and control is difficult. Moreover, since there exists a fault that a metal ion remains as an impurity, it is not preferable.

  The weakly basic solution used in the present invention is most preferably an aqueous ammonia solution from the viewpoint of prevention of contamination by metal ions and ease of control of the etching treatment. Further, organic ammonium hydroxides such as tetramethylammonium hydroxide and tetraethylammonium hydroxide are also weak base materials, and these can be used for etching treatment. Such a weak base substance is preferable because it is easy to obtain a high-purity product with very few metal ions.

  Specifically, an example of a 25% aqueous ammonia solution will be described. A 25% aqueous ammonia solution is heated on a hot plate set at 50 ° C., and an aluminum nitride crystal substrate having nitrogen polarity is put therein and etched within 25 minutes to remove the surface oxide layer. The treatment temperature and treatment time depend on the concentration of the ammonia water to be used and the thickness of the surface oxide layer to be removed, and are appropriately selected in consideration of these factors. Usually, the processing temperature is selected from the range of room temperature to 100 ° C. Further, degreasing and cleaning with an organic solvent may be performed in order to remove organic substances adhering to the surface before the etching treatment with the weak basic solution.

  However, if the etching of the aluminum nitride crystal substrate is continued even after the surface oxide layer is removed, the crystal quality of the aluminum-based nitride crystal that is epitaxially grown is deteriorated. As long as the nitrogen-polar aluminum nitride crystal is in contact with a basic aqueous solution, dissolution of the surface proceeds. For this reason, the subsequent epitaxial growth is adversely affected by a phenomenon such as a rough morphology of the substrate surface. Accordingly, the etching process may be performed to remove the surface oxide layer.

  After the etching process is completed, the aluminum nitride crystal substrate is rinsed in ultrapure water to remove the weakly basic aqueous solution. In order to neutralize and remove remaining basic ions, it is also effective to rinse in an aqueous acid solution such as dilute hydrochloric acid and then rinse with ultrapure water. Next, high purity nitrogen gas is blown to dry the aluminum nitride crystal substrate, and the substrate is immediately transferred to a crystal growth apparatus.

  In the present invention, the aluminum-based nitride crystal is a crystal made of aluminum nitride alone or a mixed crystal containing at least aluminum nitride and the balance being gallium nitride and / or indium nitride.

  As a method for epitaxially growing an aluminum-based nitride crystal on an aluminum nitride crystal substrate, a vapor phase growth method such as an existing MOVPE method, MBE method, or HVPE method is employed.

  Hereinafter, the details of the epitaxial crystal growth will be described using the HVPE method as an example, but the present invention is not limited to this. FIG. 4 is a plan view conceptually showing a merged portion of the source gas of nitrogen nitride crystal and the nitrogen source gas of the vapor phase growth apparatus of the present invention. Reference numeral 41 denotes a tube wall of the reaction tube. Quartz glass is preferably used as the material for the reaction tube used here. A carrier gas always flows in the reaction tube in order to flow the gas in one direction. As the kind of carrier gas, hydrogen, nitrogen, helium, argon simple gas, or a mixed gas thereof can be used. It is preferable to remove impure gas components such as oxygen, water vapor, carbon monoxide or carbon dioxide in advance using a purifier.

  A heating device 42 is disposed in the reaction tube 41. The method of the present invention is also effective for a cold wall type heating method (high frequency induction heating method, substrate heating method using light, etc.). FIG. 4 shows a case where a hot wall heating device is used, and a heating device 42 is arranged so as to surround the reaction tube 41. A known resistance heating device or radiation heating device may be used as the hot wall heating device. A susceptor 4 3 is installed at a predetermined position of the reaction tube to hold the substrate, and an aluminum nitride crystal substrate 44 having been etched is installed on the susceptor.

  Although the present invention can be carried out at atmospheric pressure or below atmospheric pressure, it is usually carried out at atmospheric pressure. After the aluminum nitride crystal substrate is placed on the susceptor, it is heated by the above heating device. Generally, it is performed in a temperature range of 800 ° C. or higher.

  Next, an aluminum halide gas and a nitrogen source gas are supplied, mixed and reacted on the substrate to epitaxially grow an aluminum nitride crystal. The aluminum halide gas is supplied from the halide gas supply nozzle 15. Examples of the aluminum halide gas include aluminum halides such as aluminum trichloride, gallium halides such as gallium trichloride, and indium halides such as indium trichloride, depending on the composition of the target aluminum nitride crystal. Are appropriately mixed and supplied to the halide gas supply nozzle 15.

  As a method for generating an aluminum halide gas, as described in Japanese Patent Publication No. 2003-303774, a reaction tube and a heating device are separately provided on the upstream side of the halide gas supply nozzle 15, and a group III metal such as aluminum, gallium, or indium is used. What is necessary is just to make hydrogen halide react.

  Alternatively, the halide itself such as aluminum halide, gallium halide, and indium halide may be heated and vaporized and introduced into the halide supply nozzle 15 using a carrier gas. In this case, the halide is preferably an anhydrous crystal and has few impurities. If impurities are mixed into the intended aluminum-based nitride crystal, it is not preferable because it causes uncertain physical and chemical characteristics such as crystal structure defects and unexpected electrical conduction.

  Nitrogen source gas is required for epitaxial growth by the HVPE method. The nitrogen source gas is a reactive gas for nitriding an aluminum halide gas to obtain an aluminum nitride crystal, and is usually diluted with a carrier gas and supplied. Although the reactive gas containing nitrogen is employ | adopted as the said nitrogen source gas, ammonia gas is preferable at the point of cost and the ease of handling. About this carrier gas and ammonia gas, the gas which should just push the whole reaction tube should just be supplied.

  Further, while supplying the aluminum halide gas and the nitrogen source gas, a barrier gas may be supplied into the reaction tube for the purpose of suppressing the reaction of both reaction gases at the nozzle tip. Both reaction gases are spatially separated at the tip of each supply nozzle, but are mixed by diffusion while flowing from the nozzle through the reaction zone to the substrate. The aluminum-based nitride crystal grows epitaxially by reacting on the substrate.

The supply amount of the aluminum halide gas is generally determined in consideration of the growth rate of the aluminum nitride crystal on the substrate. The ratio of the volume in the standard state of the aluminum halide gas to the total volume in the standard state of all gases (carrier gas, aluminum halide gas, nitrogen source gas, barrier gas) supplied onto the substrate is the aluminum halide gas. In general, the range of 1 × 10 ⁻⁴ atm to 5 × 10 ⁻² atm is selected. The supply amount of the nitrogen source gas is preferably selected as a supply amount of 1 to 200 times that of the generally supplied aluminum halide gas, but is not limited thereto.

  After growing for a certain time, the supply of aluminum halide gas is stopped, the growth is terminated, and the temperature of the heating device is lowered. When hydrogen is used as the carrier gas, it is desirable to circulate the nitrogen source gas until the temperature of the substrate decreases in order to prevent re-decomposition of the aluminum-based nitride crystals grown on the substrate.

  By the above procedure, an aluminum-based nitride crystal can be obtained. The film thickness of the growth crystal layer can be calculated from the weight change of the substrate before and after the growth, the surface area of the substrate, and the density of the growth layer. The film thickness can be controlled not only by the growth time but also by the supply amount of the aluminum halide gas to be supplied, the supply amount of the nitrogen source gas, and the like.

  The obtained aluminum nitride crystal is evaluated for its crystal quality by the above-mentioned X-ray rocking curve measurement. The rocking curve is diffraction obtained by changing the incident angle of the X-ray by fixing the detector at a position twice the angle at which the sample satisfies the Bragg diffraction condition. The rocking curve of the aluminum nitride crystal is measured in the (002) direction called Tilt (tilt) and the (100) direction called Twist. It can be said that the smaller the half width value, the better the crystal quality of the aluminum-based nitride crystal. When the defect is included, the half-width of the rocking curve becomes wide.

  Hereinafter, the contents of the present invention will be specifically described with reference to examples. In the present invention, the aluminum nitride crystal grown on the aluminum nitride crystal substrate by the HVPE method was evaluated by the half-value width of the Tilt component and the Twist component in the X-ray rocking curve.

  As the aluminum nitride crystal substrate, a template substrate in which an aluminum nitride pre-grown crystal film was previously grown on a c-plane sapphire substrate by the MOVPE method was used. The film thickness of the aluminum nitride pre-grown crystal film was 0.76 μm from cross-sectional observation with a field emission scanning electron microscope.

  The polarity of the aluminum nitride crystal substrate was determined by etching with potassium hydroxide. After etching, the cross section of the substrate was observed with a field emission scanning electron microscope, the presence or absence of the aluminum nitride film was confirmed, and the case where the aluminum nitride film was significantly etched was judged as nitrogen polarity.

  First, in order to remove metal ions on the surface of the aluminum nitride crystal substrate, it was immersed in 20% high-purity hydrochloric acid at room temperature for 5 minutes. Next, the aluminum nitride crystal substrate was immersed for 10 minutes in a 45 mass% potassium hydroxide aqueous solution heated to 50 ° C. on a hot plate and etched. At this time, an interference color was observed on the aluminum nitride crystal substrate before etching, but the interference color disappeared quickly when immersed in an aqueous potassium hydroxide solution. In the case of Al polarity, the interference color is not lost because it has strong chemical durability against an aqueous potassium hydroxide solution. As a result, the present aluminum nitride crystal substrate was confirmed to be nitrogen polar. After the etching, in order to remove potassium hydroxide remaining on the substrate surface, the substrate was neutralized by dipping in 20% high-purity hydrochloric acid at room temperature for 10 minutes. Next, the aluminum nitride crystal substrate was washed in ultrapure water and dried by dry nitrogen blowing.

  As a result of cross-sectional observation of the etched substrate using a field emission scanning electron microscope, it was determined that almost no pre-grown aluminum nitride on the sapphire surface remained, and the aluminum nitride crystal substrate had nitrogen polarity.

  Hereinafter, the results of epitaxial growth of aluminum nitride crystals by the HVPE method will be described in the case where there is an etching using an aqueous ammonia solution on the aluminum nitride crystal substrate, that is, the case where the surface oxide layer is not removed.

Comparative Example 1
This is a comparative example in which an aluminum nitride crystal is epitaxially grown on an aluminum nitride crystal substrate without etching.

  A reaction tube having a cross section shown in FIG. 4 was used. The aluminum trichloride gas was supplied in accordance with Japanese Patent Application Laid-Open No. 2003-303774 by generating aluminum trichloride gas by reacting metal aluminum with hydrogen chloride gas. Therefore, a hot wall type resistance heating device is used as the heating device, and includes two zones of a temperature region in which the aluminum trichloride gas is generated and a temperature region in which the generated aluminum trichloride gas and the nitrogen source gas are reacted. A heating device capable of temperature control was used.

  The aforementioned aluminum nitride crystal substrate cleaved to 1 × 1 cm was placed on an alumina susceptor. From the start of temperature increase, a carrier gas for forming the epitaxial growth layer was circulated in the reaction tube. That is, hydrogen gas was supplied from the halide gas supply nozzle, and the linear velocity of the gas at the tip at this time was set to 290 cm / s. Further, a total of 750 SCCM of hydrogen gas was supplied as a carrier gas for aluminum trichloride gas and ammonia gas as a nitrogen source. Nitrogen gas was supplied to the barrier nozzle so as to be 0.6 times the linear velocity of aluminum trichloride gas. The inside of the reaction tube is in a state where a gas having a total flow rate of 2250 SCCM is supplied, and the reaction tube temperature is raised to 1100 ° C. in this state.

Subsequently, in order to maintain the linear velocity, aluminum trichloride gas was supplied from a halide gas supply nozzle by mixing ammonia gas as a nitrogen source gas with a carrier gas. The supply pressure of aluminum trichloride was 5 × 10 ⁻⁴ atm, the supply pressure of ammonia gas was 1 × 10 ⁻³ atm, and growth of aluminum nitride crystals was started. The aluminum nitride crystal was epitaxially grown by holding for 60 minutes in this state.

  After growing for 60 minutes, the supply of aluminum trichloride was stopped and the temperature of the heating device was started to drop. In order to prevent re-decomposition of the aluminum nitride grown on the substrate, ammonia gas was passed through the reaction tube until the temperature of the heating device decreased to 550 ° C. Thereafter, it was confirmed that the heating apparatus had dropped to near room temperature, and the substrate was taken out from the reaction tube.

Next, the substrate weight was weighed, and the average film thickness of the aluminum nitride crystal epitaxially grown on the substrate was calculated from the weight change before and after growth, the substrate area, and the density of aluminum nitride. The density of aluminum nitride was 3.27 g / cm ³ . The average film thickness of aluminum nitride grown on the substrate was 1.9 μm.

  When the rocking curve of the X-ray diffractometer was measured, the half-value width of the Tilt component was 42.6 min and the half-value width of the Twist component was 37.2 min.

Example 1
In this embodiment, the aluminum nitride crystal substrate is etched with a weakly basic solution to remove the surface oxide layer, and then an aluminum nitride crystal is epitaxially grown thereon.

  The procedure for the weak basic solution etching of the aluminum nitride crystal substrate was as follows. First, the above-mentioned aluminum nitride crystal substrate surface was degreased and washed in acetone for 3 minutes. A commercially available 25% aqueous ammonia solution was used for the etching. The pH at room temperature was 12.8. Next, etching was performed by immersing the aluminum nitride crystal substrate in a 25% aqueous ammonia solution heated to 50 ° C. on a hot plate for 10 minutes. After completion of the etching, the substrate was washed with ultrapure water and immediately dried with dry nitrogen blowing. This board | substrate was conveyed to the HVPE apparatus, and was installed on the susceptor. The subsequent growth procedure of the aluminum nitride crystal on the substrate is the same as in Comparative Example 1.

  The average film thickness of aluminum nitride epitaxially grown on the substrate was 1.3 μm. When the X-ray rocking curve was measured, the half width of the Tilt component was 27.6 min, the half width of the Twist component was 30.6 min, and the crystallinity was improved as compared with the case where the aluminum nitride crystal substrate was not etched.

Example 2
Aluminum nitride crystals were epitaxially grown in the same manner as in Example 1 except that the aluminum nitride crystal substrate was etched with an aqueous ammonia solution for 20 minutes. As a result, the average film thickness of the aluminum nitride crystal was 1.4 μm. When the rocking curve was measured, the half width of the Tilt component was 22.2 min, the half width of the Twist component was 28.8 min, and the crystallinity was even better than when the etching time was 10 min.

  Separately, an aluminum nitride crystal substrate subjected to the same aqueous ammonia etching was prepared, and a cross-sectional observation was performed with a field emission scanning electron microscope. As a result, the thickness of the aluminum nitride pre-grown layer was 0.66 μm. Since the original film thickness was 0.76 μm, it was etched by 0.1 μm after 20 minutes of etching. This thickness is such that the surface oxide layer of the aluminum nitride crystal substrate shown in FIG. 1 is sufficiently removed. It can be understood that the reason why the quality of the aluminum nitride crystal epitaxially grown on the aluminum nitride crystal substrate is improved is that the surface oxide layer of the substrate is removed.

Comparative Example 2
An aluminum nitride crystal was epitaxially grown in the same manner as in Example 1 except that the aluminum aqueous solution etching of the aluminum nitride crystal substrate was performed for 30 minutes. As a result, the average film thickness of the aluminum nitride crystal was 1.4 μm. When the rocking curve was measured, the half width of the Tilt component was 61.2 min, and the half width of the Twist component was 41.4 min. The crystal quality was lowered rather than the untreated case. The cause of this is that etching proceeds relatively easily with a weakly basic aqueous solution in an aluminum nitride crystal substrate having nitrogen polar aluminum nitride crystals, but in this case, etching proceeds more than the surface oxide layer. It is expected that the surface was rough.

  The above facts are summarized in Table 1 and Figure 5.

The figure which shows the measurement result of the element profile in the depth direction of the aluminum nitride template crystal substrate (X-ray photoelectron spectroscopy) Schematic diagram of template substrate and aluminum nitride single crystal substrate Schematic diagram of the crystal structure (wurtzite crystal) and polarity of aluminum nitride Schematic diagram of gas confluence and reaction zone of halide vapor phase epitaxial growth apparatus used in the present invention Relationship between aqueous ammonia etching time and crystal quality

Explanation of symbols

21 Aluminum nitride crystal film (pre-grown crystal layer)
22 Surface oxide layer (region)
23 Base material substrate 24 Aluminum-based nitride crystal layer (epitaxial growth crystal layer)
25 Aluminum nitride crystal substrate 41 Reaction tube 42 Heating device 43 Susceptor 44 Substrate 45 Halide gas supply nozzle 46 Barrier nozzle

Claims (4)

An aluminum nitride characterized by removing an oxide layer existing on the surface of an aluminum nitride crystal substrate having nitrogen polarity by etching with a weakly basic aqueous solution and then epitaxially growing an aluminum nitride crystal on the substrate Crystal production method. 2. The method for producing an aluminum-based nitride crystal according to claim 1, wherein the aluminum nitride crystal substrate is a template substrate in which an aluminum nitride crystal is coated on an aluminum nitride single crystal substrate or a base material substrate. The method for producing an aluminum-based nitride crystal according to claim 1 or 2, wherein the weakly basic aqueous solution is an aqueous ammonia solution. An aluminum-based nitride crystal multilayer substrate obtained by the production method according to claim 1.

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