JP4451265B2 - Group III element nitride crystal substrate and group III element nitride semiconductor device manufacturing method - Google Patents

Description

  The present invention relates to a group III element nitride crystal substrate and a method for manufacturing a group III element nitride semiconductor device, and a group III element nitride crystal substrate and a group III element nitride semiconductor device obtained thereby.

  Group III element nitride compound semiconductors such as gallium nitride (GaN) (hereinafter sometimes referred to as Group III element nitride semiconductors or GaN-based semiconductors) are attracting attention as materials for semiconductor elements that emit green, blue, and ultraviolet light. ing. Blue laser diodes (LD) are applied to high-density optical discs and displays, and blue light-emitting diodes (LEDs) are applied to displays and lighting. Further, ultraviolet LD is expected to be applied to biotechnology and the like, and ultraviolet LED is expected to be an ultraviolet light source for fluorescent lamps.

A group III element nitride semiconductor substrate (for example, a GaN substrate) for an LD or LED is usually formed on a sapphire substrate by heteroepitaxially growing a group III element nitride crystal using a vapor phase epitaxial growth method. Yes. The dislocation density of the crystal obtained by this method is usually 10 ⁸ cm ⁻² to 10 ⁹ cm ⁻² , and reduction of the dislocation density is an important issue. In order to solve this problem, efforts have been made to reduce the dislocation density. For example, an ELOG (Epitaxial lateral overgrowth) method has been developed. According to this method, the dislocation density can be lowered to about 10 ⁵ cm ⁻² to 10 ⁶ cm ^−2, but the manufacturing process is complicated.

On the other hand, a method of performing crystal growth in a liquid phase instead of vapor phase epitaxial growth has been studied. However, since the equilibrium vapor pressure of nitrogen at the melting point of a group III element nitride single crystal such as GaN or AlN is 10000 atm (10000 × 1.013 × 10 ⁵ Pa) or more, conventionally, a GaN single crystal has been grown in the liquid phase. In order to achieve this, a condition of 8000 atm (8000 × 1.013 × 10 ⁵ Pa) at 1200 ° C. has been required. On the other hand, it has recently been clarified that GaN single crystals can be synthesized at a relatively low temperature and low pressure of 750 ° C. and 50 atm (50 × 1.013 × 10 ⁵ Pa) by using Na flux.

Recently, a mixture of Ga and Na was melted at 800 ° C. and 50 atm (50 × 1.013 × 10 ⁵ Pa) in a nitrogen gas atmosphere containing ammonia, and this melt was used for a growth time of 96 hours. A single crystal having a maximum crystal size of about 1.2 mm is obtained (see, for example, Patent Document 1).

Also reported is a method of growing a single crystal by liquid phase epitaxy (LPE) after forming a GaN crystal layer on the sapphire substrate by metal organic chemical vapor deposition (MOCVD). Has been.
JP 2002-293696 A

  Also in the liquid crystal growth method of a nitride crystal using at least one of an alkali metal and an alkaline earth metal as a flux, mixing of impurities becomes a problem. This is because the carrier density and the like may change when impurities are introduced. Moreover, alkali metals and alkaline earth metals, which are fluxes, are elements that are particularly apt to be mixed in semiconductor processes.

  Accordingly, an object of the present invention is to provide a substrate obtained by growing a group III element nitride crystal in at least one of an alkali metal and an alkaline earth metal, and a semiconductor device manufactured using the substrate. In order to provide a technique for preventing diffusion of impurities.

In order to achieve the above object, the method for producing a group III element nitride crystal substrate of the present invention is a production method having the following substrate forming step (a) and a heat treatment step (b).
(A) In a molten liquid (flux) containing at least one of an alkali metal and an alkaline earth metal, nitrogen of the nitrogen-containing gas is allowed to react with a group III element to crystallize to form a group III element nitride crystal substrate Substrate forming step.
(B) A heat treatment step in which impurities existing in the substrate obtained in the step (a) are converted into an inorganic compound by heat treatment.

The method for producing a group III element nitride semiconductor device of the present invention is a method for producing a group III element nitride semiconductor device in which a group III element nitride crystal layer is laminated on a group III element nitride crystal substrate. And (d) a substrate forming step, (e) a crystal layer forming step, and (f) a heat treatment step.
(D) In a molten liquid (flux) containing at least one of an alkali metal and an alkaline earth metal, nitrogen of the nitrogen-containing gas is allowed to react and crystallize to form a group III element nitride crystal substrate. Substrate forming step.
(E) A crystal layer forming step of forming the group III element nitride crystal layer on the substrate obtained in the step (d).
(F) A heat treatment step in which impurities present in the substrate obtained in the step (e) are converted into an inorganic compound by heat treatment.

  In part of the crystal obtained by growing a group III element nitride crystal in a flux containing at least one of an alkali metal and an alkaline earth metal, for example, threading dislocation (for example, screw dislocation or edge dislocation) When lattice defects exist, impurities such as alkali metals and alkaline earth metals may be mixed there. Thus, in liquid phase growth, alkali metals and alkaline earth metals, especially Na and Li may be mixed in the threading dislocations and lattice defect regions in the crystal, and this is diffused. Was first discovered. This contamination can be confirmed by, for example, SIMS (Secondary ion mass spectroscopy). The present invention solves this problem of liquid phase growth. That is, according to the present invention, since the substrate is subjected to heat treatment as described above, even if impurities such as alkali metal or alkaline earth metal are mixed in the liquid phase grown substrate, these are converted into inorganic compounds, The diffusion can be prevented. Therefore, the substrate and semiconductor device obtained by the present invention are of high quality. Furthermore, if the substrate obtained by the method of the present invention is used, it is possible to prevent impurities in the substrate from being scattered in the manufacturing apparatus or the like during the device formation process, so that stable device fabrication can be realized and obtained. Semiconductor devices are also very high quality.

  In the substrate forming step (a), examples of the alkali metal used include Na, Li, K, Rb, Cs, and Fr, and examples of the alkaline earth metal include Ca, Mg, Sr, Be, and Ba. It is done. These may be used alone or in combination of two or more. Among these, it is preferable to use Na, Li, Ca, and Mg, and it is particularly preferable to use Na and Li in combination or to use them alone. For example, when the substrate is a GaN substrate, it is preferably formed in a melt containing Na and Li. When the substrate is an AlN substrate, one selected from Li, Na and Sn and Ca is included. It is preferable to form in a melt. Moreover, the said melt may contain other metals, such as Sn, for example in addition to an alkali metal and alkaline-earth metal as a flux component.

Examples of the group III element include Ga, Al, and In, preferably Ga and Al, and the substrate is Al _u Ga _v In _1-uv N (where 0 ≦ u ≦ 1, 0 ≦ v ≦ 1). , U + v ≦ 1), preferably a group III element nitride crystal substrate. Further, it is preferable that the group III element is Ga, the substrate is a GaN substrate, the group III element is Al, and the substrate is an AlN substrate.

  In the substrate forming step (a), a group III element nitride crystal may be grown on a support substrate prepared in advance. In this case, it is preferable that a seed layer is formed on the support substrate, and the seed layer is formed of a crystal having the same composition as the group III element nitride crystal substrate. The support substrate may be any one of a GaAs substrate having a (111) surface, a Si substrate having a (111) surface, a sapphire substrate having a (0001) surface, and a SiC substrate having a (0001) surface. Such a substrate is preferable. In addition, when the group III element nitride crystal is grown on the support substrate or the seed layer as described above, the heat treatment in the heat treatment step (b) is performed after the support substrate or the seed layer is removed (after the self-standing substrate is manufactured). It is preferable to apply. Examples of the removing method include a laser lift-off method, a grinding process, and a polishing process.

  As described above, impurities may exist in the substrate obtained by the substrate forming step (a). Examples of the impurities include alkali metals, alkaline earth metals and Sn, which are flux components, crystals, And the like derived from constituent materials such as crucibles, reaction vessels, and other members used in the production of the above. Examples of the alkali metal include Na, Li, K, Rb, Cs and Fr, and examples of the alkaline earth metal include Ca, Mg, Sr, Be and Ba. Among these, in particular, when Na, Li, Ca, Mg, etc. are mixed in the substrate, for example, the carrier density of the device structure (for example, a group III element nitride crystal layer) formed on the substrate is increased. May have a major impact. As described above, the impurities are often mixed into threading dislocation sites, lattice defect sites, and the like, and the dislocation sites and lattice defect sites are, for example, in the plane of cathodoluminescence generated by irradiating the substrate with an electron beam. It is possible to evaluate by observing the distribution. In this case, the site is observed as a dark spot because of its low luminous intensity.

  The heat treatment in the heat treatment step (b) is preferably carried out under conditions for converting the impurities into inorganic compounds. Examples of the conditions for converting the impurity into an inorganic compound include a solution containing an element capable of reacting with the impurity and in an atmosphere. Examples of the element include chlorine, sulfur, oxygen, bromine and the like. Specific examples of the conditions include an acidic solution, an oxygen atmosphere, an air atmosphere, a water vapor atmosphere, water, a sulfide-containing gas, and a chlorine gas atmosphere, and these may be performed alone. Two or more types may be used in combination. Examples of the acidic solution include hydrochloric acid solution, bromic acid solution, sulfuric acid solution, and nitric acid solution. Examples of the sulfide-containing gas include hydrogen sulfide gas in an atmosphere in which sulfur is sublimated. Among these heat treatment methods, a hydrochloric acid solution, an oxygen atmosphere, and a water vapor atmosphere are preferable. For example, when heat treatment is performed under such conditions, diffusion of impurities can be prevented by converting the impurities into inorganic compounds according to the environment to make the compounds more stable.

  The temperature of the heat treatment is preferably, for example, normal temperature to boiling point (for example, 100 ° C. in the case of water) in a liquid, and preferably 100 ° C. to 1000 ° C. More preferably, the temperature is 900 ° C., and the pressure of the heat treatment is preferably performed under atmospheric pressure, for example. In addition, for example, the heat treatment time is preferably 30 minutes or more in a liquid, more preferably 1 hour or more, and preferably 30 minutes or more in a gas phase, more preferably 1 It's over time. The upper limit of the heat treatment time is not particularly limited, but is preferably 24 hours, for example.

When the heat treatment in the heat treatment step (b) is carried out, for example, in a hydrochloric acid solution, a hydrogen chloride aqueous solution having a concentration of 10% to 40% is heated to room temperature to 109 ° C. (boiling point) and carried out for 30 minutes to 6 hours. It is preferable to do. For example, when implemented in an oxygen atmosphere, the oxygen partial pressure is 1 atm (1 × 1.013 × 10 ⁵ Pa) to ⁵ atm (5 × 1.013 × 10 ⁵ Pa) in an environment of 100 ° C. to 1000 ° C. It is preferable to carry out for 30 minutes to 6 hours. For example, when carried out in a water vapor atmosphere, 30 atmospheres under an environment pressure of 1 atm (1 × 1.013 × 10 ⁵ Pa) to ⁵ atm (5 × 1.013 × 10 ⁵ Pa), 100 ° C. to 1000 ° C. It is preferable to carry out for 6 minutes.

When, for example, Na exists as an impurity in the substrate and the heat treatment in the heat treatment step (b) is performed in a hydrochloric acid solution or a chlorine gas atmosphere, NaCl is formed and the heat treatment step is performed in a bromic acid solution. When formed in a sulfuric acid solution, Na ₂ SO ₄ is formed; when performed in a nitric acid solution, NaNO ₃ is formed; when operated in an oxygen atmosphere, Na ₂ O is formed. When carried out in an air atmosphere, Na ₂ O or NaOH is formed, when carried out in a water vapor atmosphere, Na ₂ O or NaOH is formed, and when carried out in water, NaOH is formed and sulfide-containing gas is formed. When carried out in, Na ₂ S is formed.

When, for example, Li is present as an impurity in the substrate and the heat treatment in the heat treatment step (b) is performed in a hydrochloric acid solution or a chlorine gas atmosphere, LiCl is formed and performed in a bromic acid solution. When LiBr is formed and carried out in a sulfuric acid solution, Li ₂ SO ₄ is formed, when carried out in a nitric acid solution, LiNO ₃ is formed, and when carried out in an oxygen atmosphere, Li ₂ O is formed. When carried out in an air atmosphere, Li ₂ O or Li ₃ N is formed, when carried out in a water vapor atmosphere, LiOH is formed, and when carried out in water, LiOH is formed in a sulfide-containing gas. When implemented, Li ₂ S is formed.

  When the substrate obtained in the substrate forming step (a) is, for example, a GaN substrate, the heat treatment in the heat treatment step (b) is preferably performed in an environment of hydrochloric acid solution, oxygen atmosphere, or water vapor atmosphere. . When the substrate obtained in the substrate forming step (a) is, for example, an AlN substrate, the heat treatment in the heat treatment step (b) is performed in an environment of hydrochloric acid solution, oxygen atmosphere, or water vapor atmosphere. Is preferred.

  The heat treatment may be performed on the entire substrate surface or only on the substrate surface of the semiconductor device formation surface. By applying to the entire surface of the substrate, the diffusion of impurities can be further prevented, and even when applied only to the substrate surface of the semiconductor device formation surface, the diffusion of impurities can be sufficiently prevented.

  Furthermore, it is preferable to have a cleaning step (c) for performing a cleaning process on the substrate after the heat treatment step (b). By performing the cleaning treatment, for example, unreacted impurities (for example, alkali metal such as sodium metal) present on the substrate surface portion and impurities formed as an inorganic compound by the heat treatment can be removed. Examples of the washing treatment include washing with water, heat treatment in pure water, washing with an acid solution (including heat treatment), washing with an organic solvent, and the like. Examples of the organic solvent include acetone, Solfine (registered trademark), isopropyl alcohol (IPA), and the like. Examples of the acid solution include hydrochloric acid solution and hydrofluoric acid. Among these, the heat treatment in a hydrochloric acid solution is preferable. As other cleaning treatments, for example, at room temperature, acetone cleaning, solfine cleaning and IPA cleaning are each performed for 5 minutes, followed by rinsing with water, followed by fluoric acid cleaning for 10 minutes, rinsing with water for 10 minutes, and drying. Cleaning process. For example, when the thickness of the substrate is 500 μm, the substrate surface portion may be near the thickness from the substrate surface to 10 μm.

  The group III element nitride crystal substrate of the present invention is a group III element nitride crystal substrate manufactured by the substrate manufacturing method of the present invention. Since the substrate of the present invention has been subjected to the heat treatment step (b), the impurity density near the substrate surface is smaller than the impurity density inside the substrate. For example, the impurity density in the vicinity of the substrate surface is preferably one digit or more lower than the impurity density inside the substrate, and more preferably two or more digits lower. In addition, the said impurity density can be calculated | required by SIMS (secondary ion mass spectrometry), for example. For example, when the thickness of the substrate is 500 μm, the substrate surface portion may be in the vicinity of the thickness from the substrate surface to 10 μm, or may be in the vicinity of the thickness of 50 μm. Further, the inside of the substrate is a portion other than the substrate surface portion.

It is preferable to manufacture a semiconductor device by forming a group III element nitride crystal layer on the group III element nitride crystal substrate of the present invention. In the group III element nitride crystal substrate of the present invention, impurities mixed in the substrate are converted into an inorganic compound by the heat treatment in the heat treatment step (b), so that they diffuse into the crystal layer formed on the substrate. Can be prevented. Therefore, the obtained semiconductor device has excellent characteristics and extremely high quality without affecting the carrier density of the crystal layer (device structure). The crystal layer is preferably a group III element nitride crystal represented by Al _u Ga _v In _1-uv N (where 0 ≦ u ≦ 1, 0 ≦ v ≦ 1, u + v ≦ 1), for example, It is preferable to form by vapor phase growth method such as MOCVD method. Further, after the Group III element nitride crystal layer is formed on the substrate, the heat treatment may be further performed.

  Next, in the semiconductor device manufacturing method of the present invention, the substrate forming step (d) can be performed in the same manner as the substrate forming step (a), and the heat treatment of the heat treatment step (f) It can be performed in the same manner as the heat treatment in the heat treatment step (b).

  Further, in the crystal layer forming step (e), a semiconductor device is manufactured, and the obtained semiconductor device is divided into a plurality of chips, and in the heat treatment step (f), the chip-shaped semiconductor device is heat treated. It is preferable to apply. Further, the heat treatment is preferably performed on the entire semiconductor device formed into chips, and in particular, it is preferably performed on the surface exposed by forming into chips. Thereby, it is possible to further prevent impurities from diffusing from a portion (for example, threading dislocations in the substrate) that contains a particularly large amount of the impurity exposed by the chip formation.

  Furthermore, it is preferable to have a cleaning step (g) for performing a cleaning process on the semiconductor device after the heat treatment step (f). By performing the cleaning treatment, for example, unreacted impurities (for example, alkali metal such as sodium metal) present on the surface portion of the substrate and the impurity inorganic compound formed by the heat treatment can be removed. The cleaning process conditions are as described above.

  Next, the semiconductor device of the present invention is a group III element nitride semiconductor device manufactured by the manufacturing method of the present invention. The semiconductor device is preferably a laser diode, a light emitting diode or a field effect transistor, for example.

  Next, an example of the manufacturing method of the board | substrate of this invention is demonstrated using FIG.

First, as shown in FIG. 1 (a), in a nitrogen-containing gas (preferably a pressurized atmosphere of 100 atm (100 × 1.013 × 10 ⁵ Pa) or less), at least one of an alkali metal and an alkaline earth metal A group III element nitride crystal is grown in a melt containing a group III element and nitrogen to form a group III element nitride crystal substrate 13. As the nitrogen-containing gas, for example, a nitrogen-containing gas containing at least one of nitrogen gas and ammonia gas can be used. The melt is prepared, for example, by putting a material into a crucible and heating. After preparing the melt, the group III element nitride crystal is grown by bringing the melt into a supersaturated state. Preparation of the melt and crystal growth are performed at a temperature of about 700 ° C. to 1100 ° C. and a pressure of about 1 atm (1 × 1.013 × 10 ⁵ Pa) to 50 atm (50 × 1.013 × 10 ⁵ Pa), for example. It can be carried out. The surface of the obtained group III element nitride crystal 13 may be planarized by, for example, machining (polishing) and mechanochemical polishing, or the polished substrate may be subjected to heat treatment.

  Then, as shown in FIG. 1B, the group III element nitride crystal substrate 13 is preferably subjected to heat treatment in, for example, a hydrochloric acid solution heated to 80 ° C. for 0.5 hour (in FIG. An example is shown by 15). This is because there may be some threading dislocations 14 in the group III element nitride crystal substrate 13, and impurities may be mixed in the portion and these may diffuse, but by this heat treatment, for example, It is possible to prevent impurities such as metallic sodium from diffusing from the group III element nitride crystal substrate 13. Alternatively, it may be reacted with acids other than hydrochloric acid to form a stable salt (for example, NaBr if the impurity is Na), which is further removed by washing with water or heat treatment in pure water. May be.

Further, as another heat treatment condition, for example, the heat treatment is performed in oxygen gas to change the impurities into oxides to make the compound more stable, and further, it is removed by washing with water or heat treatment in pure water. May be. For example, if the impurity is metallic sodium, it becomes sodium oxide (Na ₂ O), and since this compound has a high melting point and is a more stable material than metallic sodium, diffusion is prevented. Similarly, heat treatment can be performed in an atmosphere in which sulfur is sublimated to change the impurities into sulfides. For example, if the impurity is metallic sodium, it can be changed to sodium sulfide (Na ₂ S). Also in this case, it may be removed by reaction by washing with water or heat treatment in pure water.

  By performing such a heat treatment, it is possible to reduce the diffusion of impurities from the substrate, so that the semiconductor device fabricated on the obtained substrate can reduce the diffusion of impurities and prevent deterioration of characteristics such as leakage current. Can do.

  Next, an example of the semiconductor device manufacturing method of the present invention will be described with reference to FIG.

First, as shown in FIG. 2A, a composition formula Al _u Ga _v In _1-uv N (where 0 ≦ u ≦ 1, 0 ≦ v ≦ 1, u + v ≦ 1) is formed on a support substrate 11 such as a sapphire substrate. ) Is formed. The seed layer 12 is a seed crystal, and is preferably a crystal represented by, for example, GaN or Al _u Ga _1-u N (where 0 ≦ u ≦ 1). The seed layer 12 can be formed by, for example, the MOCVD method, the MBE method, the HVPE method, or the like. Note that another semiconductor layer may be included between the support substrate 11 and the seed layer 12.

Next, as shown in FIG. 2B, at least one of an alkali metal and an alkaline earth metal in a nitrogen-containing gas (preferably a pressurized atmosphere of 100 atm (100 × 1.013 × 10 ⁵ Pa) or less). Then, the seed layer 12 is brought into contact with a melt containing group III element and nitrogen, and LPE-GaN 13a is grown on the seed layer 12. In this case, the group III element nitride crystal substrate 13 including the support substrate 11, the seed layer 12, and the LPE-GaN 13a is used. Note that the support substrate 11 may be removed after the formation of the LPE-GaN 13a, and the strain of the LPE-GaN 13a can be relaxed by removing the support substrate 11. Examples of the removal method include a laser lift-off method, a grinding process, and a polishing process.

  Next, as shown in FIG. 2C, the LPE-GaN 13a is heat-treated in the same manner as described above (the heat-treated portion is indicated by 15).

  Then, as shown in FIG. 2D, a device structure such as a semiconductor laser, a light-emitting diode, or a high-frequency device is formed on the LPE-GaN 13a by, for example, forming the GaN layer 16 or the AlGaN layer 17 by MOCVD. Thus, a highly reliable semiconductor device can be realized.

Hereinafter, the present invention will be described in more detail with reference to examples. In the following examples, the fabrication of a group III element nitride semiconductor device using a GaN crystal will be described. For example, the composition formula Al _x Ga _y In _{1 -xy} N such as Al _x Ga _1-x N or AlN is used. A group III element nitride crystal represented by (where 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, x + y ≦ 1) can also be formed by a similar method.

First, as shown in FIG. 3A, the temperature of the support substrate 21 made of sapphire is heated to about 1020 ° C. to 1100 ° C., and then trimethylgallium (TMG) and NH ₃ are supplied onto the substrate. Thus, a semiconductor seed layer 22 made of GaN was formed.

Next, after forming a resist pattern on the surface of the semiconductor seed layer 22 by photolithography, the semiconductor seed layer 22 is patterned by dry etching using Cl ₂ gas as shown in FIG. 3B. . The dry etching can be performed using an inductively coupled reactive dry etching (RIE) apparatus.

Next, after removing the resist pattern, in a nitrogen atmosphere (preferably under a pressurized atmosphere of 100 atm (100 × 1.013 × 10 ⁵ Pa) or less), a molten solution containing Ga, Na, and nitrogen is added to the semiconductor. By bringing the surface of the seed layer 22 into contact and maintaining the melt in a supersaturated state, LPE-GaN 23a was selectively grown on the semiconductor seed layer 22 as shown in FIG. In this case, the group III element nitride crystal substrate 23 including the support substrate 21, the semiconductor seed layer 22, and the LPE-GaN 23a is used. Note that a mask may be formed on the side surface of the semiconductor seed layer 22 and the surface of the support substrate 21 when the group III element nitride crystal is grown.

FIG. 4 shows an example of an oscillating LPE apparatus used for producing a group III element nitride crystal using a melt. The oscillating LPE apparatus 300 includes a stainless steel growth furnace 301 and can withstand an atmospheric pressure of 50 atm (50 × 1.013 × 10 ⁵ Pa). The growth furnace 301 is provided with a heater 302 and a thermocouple 303 for heating. The crucible fixing base 304 is disposed in the growth furnace 301, and a mechanism that rotates about the rotation shaft 305 is attached thereto. A crucible 306 made of boron nitride (BN) or alumina (Al ₂ O ₃ ) is fixed in the crucible fixing base 304. In the crucible 306, a melt 307 and a seed crystal 308 are disposed. As the crucible fixing base 304 rotates, the molten liquid in the crucible 306 moves to the left and right, and thereby the molten liquid can be stirred. The atmospheric pressure is adjusted by the flow rate adjuster 309. Nitrogen gas, which is a raw material gas, or a mixed gas of ammonia gas (NH ₃ gas) and nitrogen gas is supplied from a raw material gas tank (not shown), and after impurities are removed by a gas purification unit, the inside of the growth furnace 301 Sent to.

  Hereinafter, an example of crystal growth using this apparatus will be described.

(1) First, Ga and Na which is a flux are weighed by a predetermined amount and set in a crucible. Ga having a purity of 99.9999% (six nines) or more is preferably used, and purified Na is preferably used. Na can be purified by heating and melting Na in a substituted glove box (which may be N ₂ , Ar, Ne, Xe, etc.) to remove oxides and the like appearing on the surface layer. Na may be purified by a zone refining method. In the zone refining method, impurities are precipitated by repeating melting and solidification of Na in a tube, and the purity of Na can be increased by removing it.

  (2) Next, in order to melt the raw material in the crucible, the temperature in the electric furnace is raised to 800 ° C. At this stage, as shown, the seed crystal substrate is not present in the melt. In order to mix Ga and Na, the crucible is swung so that the melt does not adhere to the seed crystal substrate. In order to prevent GaN oxidation, for example, nitrogen gas is preferably used as the atmospheric gas.

  (3) Next, the seed crystal substrate is placed in the melt by rotating the crucible around the rotation axis, and crystal growth is started.

(4) During crystal growth, the crucible is swung at a speed of one cycle per minute in order to stir the melt. However, during the growth, the seed crystal substrate is present in the melt, the crucible is kept at 800 ° C., the pressure is kept at 40 atm (40 × 1.013 × 10 ⁵ Pa), and, for example, LPE growth is performed for 10 hours. .

  (5) After the growth is completed, the crucible is rotated as shown in the figure, the substrate is taken out of the melt, and the melt temperature is lowered.

  According to this method, not only Na but also Li, K, Ca, Sr, Ba flux, or mixed flux of alkali metal and alkaline earth metal can be used to obtain the same effect. For example, in a mixed flux of Na and Ca, it is possible to grow crystals at a lower pressure by mixing about 10% of Ca.

  After the crystal growth, as shown in FIG. 3D, the support substrate 21 and the seed layer 22 were removed to obtain LPE-GaN 23a. By removing the support substrate and the like in this manner, the strain in the LPE-GaN 23a could be relaxed. In the obtained LPE-GaN 23a, as described above, threading dislocations 24 were observed from the semiconductor seed layer 22 toward the substrate surface, and dislocations were also observed in a portion where the laterally grown GaN crystals collide with each other. When impurity analysis was performed by SIMS, there were many impurities such as sodium in the threading dislocation 24 region.

  The obtained LPE-GaN 23a was processed into a circular wafer by slicing and grinding, and both surfaces of the wafer were processed into mirror surfaces by diamond polishing or mechanochemical polishing.

Next, as shown in FIG. 3 (e), by subjecting the LPE-GaN 23a to heat treatment at 500 ° C. for 0.5 hours in an electric furnace filled with water vapor, the substrate surface portion (for example, around 10 μm), For example, impurities such as alkali metals were converted into inorganic compounds. Thereafter, the substrate was subjected to a cleaning treatment and dried to obtain a group III element nitride crystal substrate in which the concentration of impurities such as sodium on the substrate surface was reduced. As an atmosphere in the electric furnace, chlorine gas (HCl), oxygen gas, sulfide gas (H ₂ S), or the like can be used in addition to water vapor. Metal heat can be changed to sodium oxide by heat treatment in oxygen gas. Sodium oxide (Na ₂ O) can be removed by washing with water or heat treatment in pure water to cause a reaction. Similarly, heat treatment can be performed in an atmosphere in which sulfur is sublimated to change to sodium sulfide (Na ₂ S). As shown in FIG. 3F, for example, the GaN layer 26 and the AlGaN layer are formed by MOCVD. By forming 27, the semiconductor device of the present invention can be manufactured. By applying such heat treatment to a GaN substrate produced by the alkali metal flux method, diffusion of alkali metal to the light emitting part of a semiconductor laser or a light emitting diode or each electrode part of an FET transistor can be prevented. The characteristics can be improved.

A method for producing a field effect transistor by forming a group III element nitride crystal layer on a group III element nitride crystal substrate obtained as described above will be described with reference to FIG. The group III element nitride crystal substrate 43 exhibited an electrical resistance close to that of an insulator with an electric resistance of, for example, 10 ¹⁰ Ω or more. The LPE-GaN 43a was heat-treated, and then a GaN layer 46 and an AlGaN layer 47 were formed by MOCVD. Further, a source electrode 48, a Schottky gate electrode 49, and a drain electrode 50 were formed thereon. By applying a voltage to the gate electrode 49, the concentration of the two-dimensional electron gas formed at the interface between the GaN layer 46 and the AlGaN layer 47 is controlled, and the operation as a transistor is performed.

  Since the field effect transistor obtained by the method of the present invention is heat-treated on the base substrate 43, the field effect transistor has high insulation, can reduce leakage current during transistor operation, and has excellent high frequency characteristics. A transistor can be realized.

  As described above, the GaN substrate obtained by LPE growth of the GaN crystal using the substrate on which the semiconductor seed layer is formed has been described. However, the GaN substrate obtained by slicing the crystal obtained by growing the bulk bulk using the GaN crystal as a seed is used May be. Since the crystals grown in this way are not selectively grown, there is a possibility that impurities such as alkali metals or alkaline earth metals may be included in randomly generated dislocations, etc. Thus, diffusion of impurities can be prevented.

  As shown in FIG. 6 (a), a semiconductor laser wafer 81 in which an electrode was formed by epitaxial growth on a GaN substrate produced by liquid phase growth using sodium as a flux to process the laser structure was produced. This was processed into a bar as shown in FIG. In the figure, the device structure 83 may be formed on the LPE-GaN substrate 82, and the electrode 84 may be further formed. Next, as shown in FIG.6 (c), the bar which coated the end surface of the light emission part was chip-ized. Such a bar or chip-like semiconductor device was heat-treated (500 ° C.) in an electric furnace filled with water vapor, and then heat-treated in water or pure water. Thereby, the diffusion of impurities contained in the substrate could be prevented.

  According to the present invention, even if impurities such as alkali metals and alkaline earth metals are mixed in the group III element nitride substrate formed by liquid phase growth, these are converted into inorganic compounds by a heat treatment step. Therefore, stable device fabrication that can prevent the diffusion can be realized. Therefore, the manufacturing method of the present invention can be applied to various semiconductor devices such as laser diodes, light emitting diodes, and field effect transistors.

1A to 1B are cross-sectional views showing an example of a method for manufacturing a substrate according to the present invention. 2A to 2D are process diagrams showing an example of a method for manufacturing a semiconductor device of the present invention. 3A to 3E are process diagrams showing another example of the semiconductor device manufacturing method of the present invention. FIG. 4 is a configuration diagram of an example of a manufacturing apparatus used in the semiconductor device manufacturing method of the present invention. FIG. 5 is a cross-sectional view showing the configuration of another example of the semiconductor device of the present invention. 6A to 6C are process diagrams showing still another example of the semiconductor device manufacturing method of the present invention.

Explanation of symbols

11 21 41 Support substrate 12 22 42 Seed layer 13a 23a LPE-GaN
13 23 43 Group III element nitride crystal substrate 14 24 44 Threading dislocation 15 25 45 Heat treatment region 16 26 46 GaN layer 17 27 47 AlGaN layer 48 Source electrode 49 Schottky gate electrode 50 Drain electrode 81 Wafer 82 LPE-GaN substrate 83 Device Structure 84 Electrode 85 Dividing surface 86 Light emitting surface 300 Oscillating LPE device 301 Growth furnace 302 Heater 303 Thermocouple 304 Crucible fixing base 305 Rotating shaft 306 Crucible 307 Melting liquid 308 Seed crystal 309 Flow rate regulator 310 Source gas

Claims (16)

A method for producing a group III element nitride crystal substrate, comprising the following (a) substrate forming step and (b) heat treatment step.
(A) In a melt (flux) containing at least one of an alkali metal and an alkaline earth metal, nitrogen of the nitrogen-containing gas is reacted with a group III element to crystallize to form a group III element nitride crystal substrate Substrate forming step.
(B) A heat treatment step in which impurities present in the substrate obtained by the substrate formation step of (a) are converted into an inorganic compound by heat treatment.   The substrate manufacturing method according to claim 1, wherein the heat treatment in the heat treatment step (b) is performed in a solution or atmosphere containing at least one element selected from the group consisting of chlorine, sulfur, oxygen, and bromine.   The heat treatment in the heat treatment step (b) is performed in at least one environment selected from the group consisting of an acidic solution, an oxygen atmosphere, a water vapor atmosphere, water, a sulfide-containing gas, and a chlorine gas atmosphere. The method for manufacturing a substrate according to claim 1.   The method for producing a substrate according to claim 3, wherein the acidic solution is at least one selected from the group consisting of a hydrochloric acid solution, a bromic acid solution, a sulfuric acid solution, and a nitric acid solution.   The method for manufacturing a substrate according to claim 3, wherein the sulfide-containing gas is at least one of an atmosphere in which sulfur is sublimated and a hydrogen sulfide gas.   The substrate according to any one of claims 1 to 5, wherein at least one of a threading dislocation site and a lattice defect site is present in the substrate obtained by the substrate forming step (a), and the impurity is present in the site. Production method.   The substrate manufacturing method according to claim 1, wherein the impurity is at least one of an alkali metal and an alkaline earth metal.   The substrate manufacturing method according to claim 1, wherein the impurity is at least one selected from the group consisting of Na, Li, Ca, and Mg. The method for manufacturing a substrate according to claim 1, further comprising a cleaning step (c) below after the heat treatment step (b).
(C) A cleaning step of cleaning the substrate that has been subjected to the heat treatment in the heat treatment step of (b).   The method for producing a substrate according to claim 1, wherein the alkali metal in the melt is at least one selected from the group consisting of Na, Li, K, Rb, Cs, and Fr.   The method for manufacturing a substrate according to claim 1, wherein the melt includes at least one selected from the group consisting of Na, Li, Ca, and Mg. The Group III element is at least one selected from the group consisting of Ga, Al and In, and the substrate is Al _u Ga _v In _1-uv N (where 0 ≦ u ≦ 1, 0 ≦ v ≦ 1, u + v The method for producing a substrate according to claim 1, wherein the substrate is a group III element nitride substrate represented by ≦ 1).   The method for manufacturing a substrate according to claim 1, wherein the group III element is Ga, and the substrate is a GaN substrate.   The method for producing a substrate according to claim 1, wherein the melt contains at least one of Na and Li, and the substrate is a GaN substrate.   The method for manufacturing a substrate according to claim 1, wherein the melt includes one selected from the group consisting of Li, Na, and Sn and Ca, and the substrate is an AlN substrate. A method of manufacturing a group III element nitride semiconductor device in which a group III element nitride crystal layer is laminated on a group III element nitride crystal substrate, the substrate forming step of (d) below, crystal layer formation of (e) The manufacturing method which has a heat treatment process of a process and (f).
(D) In a melt (flux) containing at least one of an alkali metal and an alkaline earth metal, nitrogen of the nitrogen-containing gas is reacted with a group III element to crystallize to form a group III element nitride crystal substrate Substrate forming step.
(E) A crystal layer forming step of forming a group III element nitride crystal layer on the substrate obtained in the substrate forming step of (d).
(F) A heat treatment step in which impurities existing in the substrate obtained by the substrate formation step of (d) are converted into an inorganic compound by heat treatment.

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