WO2010084681A1

WO2010084681A1 - Group iiib nitride crystal manufacturing method

Info

Publication number: WO2010084681A1
Application number: PCT/JP2009/071232
Authority: WO
Inventors: 崇行平尾; 真岩井; 克宏今井
Original assignee: 日本碍子株式会社
Priority date: 2009-01-23
Filing date: 2009-12-21
Publication date: 2010-07-29
Also published as: JP5651480B2; JPWO2010084681A1

Abstract

A seed crystal substrate is prepared by forming a gallium nitride thin film on a surface of a sapphire substrate, and is placed inside a growing container (12). In addition, metallic gallium and metallic sodium are weighed out such that their molar ratio is 25‑32:68‑75 and are placed inside the growing container (12). The growing container (12) is placed inside a reaction container (20), an inlet pipe (22) is connected to the reaction container (20), and the reaction container (20) is filled with nitrogen gas from a nitrogen gas tank (42) via a pressure regulator (40). Then the system is regulated until a specific nitrogen gas pressure is achieved inside the reaction container (20), and target temperatures are set such that the temperature of a lower heater (46) is higher than that of an upper heater (44), and a gallium nitride crystal is grown. As a result, a flow in the molten mixture inside the growing container (12) is created in the direction along the surface of the seed crystal substrate due to thermal convection.

Description

Method for producing group 3B nitride crystals

The present invention relates to a method for producing a group 3B nitride crystal such as gallium nitride.

In recent years, it has been actively researched to produce semiconductor devices such as blue LEDs, white LEDs, blue-violet semiconductor lasers using 3B group nitrides such as gallium nitride and to apply the semiconductor devices to various electronic devices. Conventional gallium nitride-based semiconductor devices are mainly manufactured by a vapor phase method. Specifically, a gallium nitride thin film is heteroepitaxially grown on a sapphire substrate or silicon carbide substrate by metal organic vapor phase epitaxy (MOVPE) or the like. In this case, since the thermal expansion coefficient and the lattice constant of the substrate and the gallium nitride thin film are greatly different, high-density dislocations (a kind of lattice defects in the crystal) are generated in the gallium nitride. For this reason, it has been difficult to obtain high-quality gallium nitride having a low dislocation density by the vapor phase method. On the other hand, in addition to the gas phase method, a liquid phase method has also been developed. The flux method is one of the liquid phase methods. In the case of gallium nitride, the temperature required for crystal growth of gallium nitride is reduced to about 800 ° C. and the pressure is reduced to several MPa to several hundred MPa by using metallic sodium as the flux. be able to. Specifically, nitrogen gas is dissolved in a mixed melt of metallic sodium and metallic gallium, and gallium nitride becomes supersaturated and grows as crystals. In such a liquid phase method, dislocations are less likely to occur than in a gas phase method, so that high-quality gallium nitride having a low dislocation density can be obtained.

Research and development related to the flux method is also actively conducted. For example, Patent Document 1 discloses a method for producing a group 3B nitride crystal for the purpose of improving the crystal growth rate and the crystallinity / uniformity of a semiconductor crystal. Specifically, a method of crystal growth of gallium nitride on a seed crystal substrate by disposing a seed crystal substrate diagonally or straightly in a mixed melt of metal sodium and metal gallium is disclosed. According to this method, since the mixed melt flows along the crystal growth surface by thermal convection, the mixed melt is sufficiently and uniformly supplied to each part of the crystal growth surface.

JP 2008-290929 (for example, paragraph 0009)

However, according to the manufacturing method of Patent Document 1, although a gallium nitride crystal having a large grain size (area surrounded by grain boundaries) can be obtained, an area having a low dislocation density, such as an etch pit density (EPD), is obtained. An area with an order of 10 ⁴ / cm ² or less may not exist. When a gallium nitride crystal having a high dislocation density is used, for example, in a power control device to which a high voltage is applied, there are many holes penetrating in the thickness direction, and leakage current may flow through the holes. Therefore, there is a problem that a high voltage cannot be applied. On the other hand, even if a gallium nitride crystal having a low dislocation density is present, there is a possibility that a leakage current may flow through the grain boundary if the grain size is small, so that a high voltage cannot be applied.

The main object of the present invention is to provide a group 3B nitride crystal having a large grain size and a low dislocation density.

The present inventors have found that the flow direction of the mixed melt in the growth vessel and the concentration of metal gallium in the mixed melt affect the grain size and the dislocation density, and have completed the present invention. .

That is, in the method for producing a Group 3B nitride crystal of the present invention, a seed crystal substrate is immersed in a mixed melt having a concentration of Group 3B metal of 22 to 32 mol% in a growth vessel, and the surface of the seed crystal substrate is aligned. The group 3B nitride is crystallized on the seed crystal substrate by supplying nitrogen gas to the growth vessel while generating a flow in the direction of the mixed melt.

According to the method for producing a group 3B nitride crystal of the present invention, when growing a group 3B nitride crystal, a nitrogen gas is supplied to the growth vessel while generating a flow in a direction along the surface of the seed crystal substrate in the mixed melt. Grain size tends to increase. Specifically, the grain size of the group 3B nitride crystal can be set to include a circle of φ1 mm. The reason for this is not clear, but crystals generally grow easily along the normal direction of the surface of the seed crystal substrate, so if the mixed melt is stationary, crystallization occurs at multiple locations on the surface of the seed crystal substrate. It is considered that the boundaries between many crystals eventually become grain boundaries and the grain size does not increase. On the other hand, if the mixed melt flows in a direction along the surface of the seed crystal substrate, growth along the normal direction of the surface is suppressed and growth in a direction parallel to the surface is promoted. Therefore, it is considered that the crystal tends to be large and the grain size is large.

Further, when a mixed melt in which such a flow is generated is used, the dislocation density usually tends to be high, but by setting the concentration of the group 3B metal in the mixed melt to 22 to 32 mol%, The dislocation density can be kept low. Specifically, the order of the etch pit density (EPD) in the circle of φ1 mm described above can be suppressed to 10 ⁴ / cm ² or less. The reason for this is not clear, but according to the examples described later, the dislocation density increases when the concentration of the group 3B metal is lower than 22 mol% or higher than 32 mol%. It can be kept low. In consideration of such an effect of suppressing the dislocation density, 25 to 30 mol% is preferable, and 25 to 28 mol% is more preferable.

In the method for producing a group 3B nitride crystal of the present invention, a mechanism in which the dislocation density decreases and the grain size increases will be described below with reference to FIGS. The following mechanism is inference based on the results of Examples and Comparative Examples described later. The mixed melt will be described by taking as an example a case where a sodium flux obtained by melting Ga which is a group 3B metal is used.

First, comparing the case where the Ga concentration is less than 22 mol% and the case where the Ga concentration is 22 to 32 mol%, since the former is less soluble in N _{2 due} to less Ga in the flux, the GaN concentration at saturation (See FIG. 15 (a)), and as a result, the generation amount of nuclei as a starting point of crystal growth on the seed crystal substrate increases (see FIG. 15 (b)), whereas in the latter, the flux Since N ₂ is difficult to dissolve due to the large amount of Ga in the interior, the GaN concentration at the time of saturation is low (see FIG. 16A), and as a result, the generation amount of nuclei is reduced (see FIG. 16B). Conceivable. Here, since the dislocations existing in the seed crystal substrate are considered to penetrate the nucleus in the vertical direction, the amount of dislocation increases when the amount of nucleus generation is large, and the amount of dislocation decreases when the amount of nucleus generation is small. For these reasons, it is considered that the dislocation density is high when the Ga concentration is less than 22 mol%, and the dislocation density is low when the Ga concentration is 22 to 32 mol%.

Second, comparing the case where the Ga concentration is less than 22 mol% and the case where the Ga concentration is 22 to 32 mol%, in the former, the amount of nuclei generated is large, so the interval between adjacent nuclei is narrow (FIG. 15 ( b)), the latter is considered to have a wide interval (see FIG. 16B). Since the nucleus is considered to be a truncated pyramid, crystal growth includes growth in a direction perpendicular to the C plane (C axis growth) and growth in a direction perpendicular to the side surface (lateral growth). In the former, the lateral growth width is narrow, so the C-axis direction growth has priority over the lateral growth, and in the latter, the lateral growth is wide, so the lateral growth is promoted. When the lateral growth is promoted, dislocations generated from adjacent nuclei collide with each other and become a grain-sized end (that is, a grain boundary), and many of the dislocations gather and converge there. Therefore, when the Ga concentration is less than 22 mol%, the dislocation density increases and the grain size decreases (see FIG. 15C), and when 22 to 32 mol%, the dislocation density decreases and the grain size decreases. (See FIG. 16C).

Note that when the Ga concentration exceeds 32 mol%, the dislocation density increases, but the mechanism is considered as follows. That is, when the Ga concentration exceeds 32 mol%, it is considered that the amount of nuclei generated is too small and lateral growth becomes dominant, and there is almost no growth in the C-axis direction, and a sword mountain type crystal grows. At this time, since the GaN concentration at the time of saturation is too low, the grains are separated too much from each other, so that dislocations generated from adjacent nuclei are difficult to associate with each other. As a result, the width of the grain boundary is widened, and dislocations that should have converged at the grain boundary remain without being converged, and the dislocation density is increased.

Here, examples of the group 3B nitride include boron nitride (BN), aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN), and thallium nitride (TlN). preferable. As the seed crystal substrate, for example, a sapphire substrate, a silicon carbide substrate, a silicon substrate or the like on which a thin film of the same type as the Group 3B nitride is formed may be used, or the same type as the Group 3B nitride may be used. A substrate may be used. The flux may be appropriately selected from various metals according to the type of the group 3B metal. For example, when the group 3B metal is gallium, an alkali metal is preferable, metal sodium or metal potassium is more preferable, and metal sodium is preferable. Further preferred.

In the method for producing a group 3B nitride crystal of the present invention, when generating a flow in a direction along the surface of the seed crystal substrate in the mixed melt, the seed crystal substrate is inclined with respect to the horizontal direction in the growth vessel. Preferably, the lower part is higher than the upper part with respect to the temperature of the mixed melt in the growth container. By doing so, the mixed melt flows along the surface of the seed crystal substrate by thermal convection, so there is no need to use an external power source such as a motor. Therefore, the configuration of the manufacturing apparatus is simplified. Also, even when the seed crystal substrate is placed horizontally, the mixed melt may flow laterally along the surface of the seed crystal substrate due to thermal convection, but the seed crystal substrate should have an angle with respect to the horizontal direction. When supported, the mixed melt easily flows along the surface of the seed crystal substrate by thermal convection, so that it is easy to ensure an appropriate flow rate. At this time, the seed crystal substrate may be supported at preferably 10 to 90 °, more preferably 45 to 90 °. In this way, the flow rate of the mixed melt can be increased.

Thus, when using the thermal convection of the mixed solution, it is preferable to set the temperature of the mixed melt so that the lower portion is 1 to 8 ° C. higher than the upper portion. If the temperature is lower than 1 ° C., thermal convection does not occur so much and it is difficult to obtain the effect of increasing the grain size. If it exceeds 8 ° C., the flux is transported along the inner wall to the upper part of the growth container having a low temperature, and it becomes difficult to secure a sufficient amount of flux necessary for the growth. In addition, since the supersaturation degree at the gas-liquid interface is too higher than the region where the seed crystal substrate is disposed, miscellaneous crystals are likely to be generated at the gas-liquid interface, and gallium nitride is deposited on the seed crystal substrate. Since it will be inhibited, it is not preferable. Furthermore, since the gas-liquid interface is lower in temperature than the growth region, the dissolution rate of nitrogen is slow, and the growth rate is slow. In addition, a partition plate that prevents gas flow between the upper and lower portions of the growth vessel, an upper heater that heats the upper portion of the growth vessel, and a lower heater that heats the lower portion of the growth vessel is provided around the growth vessel. The set temperature of the lower heater may be higher than that of the upper heater. In this way, compared to the case without a partition plate, a temperature difference is likely to occur between the upper part and the lower part of the mixed melt, and the degree of thermal convection can be easily controlled by the temperature difference of the heater.

1 is an explanatory diagram showing an overall configuration of a crystal plate manufacturing apparatus 10. FIG. It is explanatory drawing (sectional drawing) of the growth container 12. FIG. 2 is a photograph of a fluorescence microscope image of the gallium nitride crystal of Example 1. FIG. 2 is an appearance photograph of a gallium nitride crystal etched in Example 1. FIG. It is a photograph which shows the expansion visual field image of an area with many etch pits and an area with few. It is an external appearance photograph of the gallium nitride crystal etched in Example 1, Comprising: The area with few etch pits is shown by a circle with φ1 mm. It is a photograph of the enlarged visual field image of an area with many etch pits of Example 2, a small area, and an area where bunching is seen. It is a photograph of the enlarged visual field image of an area with many etch pits of Example 3, a small area, and an area where bunching is seen. It is a photograph of the enlarged visual field image of an area with many etch pits of Example 4, an area with few, and a bunching. It is a photograph of the enlarged visual field image of the area where a bunching of comparative example 1 is seen, and an area with many etch pits. It is a photograph of the enlarged visual field image of the area where a bunching of comparative example 2 is seen, and an area with many etch pits. 5 is a graph in which EPD of each area of Examples 1 to 4 and Comparative Examples 1 and 2 is plotted on the vertical axis and the value of x is plotted on the horizontal axis. It is a photograph of the fluorescence microscope image of the gallium nitride crystal at the time of growing on soaking conditions without providing a temperature gradient in Example 1. It is explanatory drawing which shows the whole structure of the crystal plate manufacturing apparatus. It is explanatory drawing of the crystal growth mechanism in case Ga concentration is less than 22 mol%. FIG. 6 is an explanatory diagram of a crystal growth mechanism when a Ga concentration is 22 to 32 mol%.

A suitable apparatus for carrying out the method for producing a group 3B nitride crystal of the present invention will be described below with reference to FIGS. FIG. 1 is an explanatory view showing the overall configuration of the crystal plate manufacturing apparatus 10, and FIG. 2 is an explanatory view (cross-sectional view) of the growth vessel 12.

As shown in FIG. 1, the crystal plate manufacturing apparatus 10 includes a growth vessel 12, a reaction vessel 20 that houses the growth vessel 12, an electric furnace 24 in which the reaction vessel 20 is arranged, a nitrogen cylinder 42, and stainless steel. And a pressure controller 40 provided in the middle of the pipe connecting the reaction vessel 20.

The growth container 12 is a bottomed cylindrical alumina crucible. As shown in FIG. 2, a seed crystal substrate 18 in which a thin film 16 of the same type as the group 3B nitride is formed on the surface of the sapphire substrate 14 is disposed in the growth container 12. The seed crystal substrate 18 is arranged so that the surface has an angle with respect to the horizontal direction (that is, obliquely). Further, the growth container 12 accommodates a group 3B metal and flux. What is necessary is just to select suitably according to the kind of 3B group metal from various metals, for example, when a 3B group metal is a gallium, as a flux, an alkali metal is preferable as a flux, and metal sodium and metal potassium are more preferable. More preferably, sodium metal is used. The group 3B metal or flux becomes a mixed melt by heating.

The reaction vessel 20 is made of stainless steel and has an inlet pipe 22 into which nitrogen gas can be introduced. The lower end of the inlet pipe 22 is located in the reaction vessel 20 and in the upper space of the growth vessel 12. The upper end of the inlet pipe 22 is connected to the pressure controller 40.

The electric furnace 24 includes a hollow cylindrical body 26 in which the reaction vessel 20 is disposed, and an upper lid 28 and a lower lid 30 that block the upper opening and the lower opening of the cylindrical body 26, respectively. The electric furnace 24 is a three-zone heater type, and is divided into three zones, an upper zone 34, an intermediate zone 35, and a lower zone 36, by two ring-shaped

partition plates

32 and 33 provided on the inner wall of the cylindrical body 26. It has been. An upper heater 44 is embedded in an inner wall surrounding the upper zone 34, an intermediate heater 45 is embedded in an inner wall surrounding the middle zone 35, and a lower heater 46 is embedded in an inner wall surrounding the lower zone 36. Each

heater

44, 45, 46 is controlled so as to have a target temperature set individually in advance by a heater control device (not shown). The reaction vessel 20 is accommodated so that the upper end is located in the upper zone 34 and the lower end is located in the lower zone 36.

The pressure controller 40 performs control so that the pressure of the nitrogen gas supplied to the reaction vessel 20 becomes a preset target pressure.

A usage example of the crystal plate manufacturing apparatus 10 of the present embodiment configured as described above will be described. This crystal plate manufacturing apparatus 10 is used for manufacturing a group 3B nitride by a flux method. Hereinafter, a case where a gallium nitride crystal plate is manufactured will be described as an example.

First, as the seed crystal substrate 18, a sapphire substrate 14 having a gallium nitride thin film 16 formed on the surface thereof is prepared and placed in the growth vessel 12. At this time, the seed crystal substrate 18 is supported at an angle with respect to the horizontal direction. Further, metallic gallium is prepared as the group 3B metal and metallic sodium is prepared as the flux, and these are weighed to a desired molar ratio and accommodated in the growth vessel 12. The growth vessel 12 is placed in the reaction vessel 20, the inlet pipe 22 is connected to the reaction vessel 20, and nitrogen gas is charged into the reaction vessel 20 from the nitrogen cylinder 42 via the pressure controller 40. The reaction vessel 20 is accommodated from the upper zone 34 in the cylindrical body 26 of the electric furnace 24 through the middle zone 35 to the lower zone 36, and the lower lid 30 and the upper lid 28 are closed. The pressure controller 40 controls the inside of the reaction vessel 20 to have a predetermined nitrogen gas pressure, and a heater control device (not shown) causes the upper heater 44, the middle heater 45, and the lower heater 46 to have predetermined target temperatures, respectively. To grow a gallium nitride crystal. The nitrogen gas pressure is preferably set to 1 to 7 MPa, more preferably 2 to 6 MPa. The average temperature of the three heaters is preferably set to 700 to 1000 ° C, more preferably set to 800 to 900 ° C. The growth time of the gallium nitride crystal may be appropriately set according to the heating temperature and the pressure of the pressurized nitrogen gas, and may be set in the range of several hours to several hundred hours, for example.

In this embodiment, in order to generate heat convection in the mixed melt in the growth vessel 12, each target temperature is set so that the temperature of the lower heater 46 is higher than that of the upper heater 44 and the middle heater 45. Due to the heat convection generated in this way, the mixed melt flows along the surface of the thin film 16 of the seed crystal substrate 18 as shown by the one-dot chain line arrow in FIG. Specifically, it is preferable to set the temperatures of the upper, middle and lower heaters 44 to 46 so that the temperature of the mixed melt is 1 to 8 ° C. higher in the lower part than in the upper part.

According to this embodiment described in detail above, when growing a group 3B nitride crystal, nitrogen gas is supplied to the growth vessel 12 while generating a flow in the direction along the surface of the seed crystal substrate 18 in the mixed melt. The grain size tends to increase because of the supply. Specifically, the grain size of the group 3B nitride crystal can be set to include a circle of φ1 mm. Further, when a mixed melt in which such a flow is generated is used, the dislocation density tends to be high, but the concentration of the group 3B metal in the mixed melt is preferably 22 to 32 mol% (preferably 25 to 30 mol). %, More preferably 25 to 28 mol%), the dislocation density can be kept low. Specifically, the order of the etch pit density (EPD) in the circle of φ1 mm described above can be suppressed to 10 ⁴ / cm ² or less.

Further, since the mixed melt flows along the surface of the seed crystal substrate 18 by thermal convection, it is not necessary to use an external power source such as a motor, and the configuration of the manufacturing apparatus is simplified.

Furthermore, since the seed crystal substrate 18 is supported so as to have an angle with respect to the horizontal direction, the mixed melt easily flows along the surface of the seed crystal substrate 18 by thermal convection, so that it is easy to ensure an appropriate flow rate. At this time, the seed crystal substrate 18 may be supported at preferably 10 to 90 °, more preferably 45 to 90 °. In this way, the flow rate of the mixed melt can be increased.

Furthermore, since the

partition plates

32 and 33 are provided inside the electric furnace 24, the upper part of the mixed melt in the growth vessel 12 accommodated in the reaction vessel 20 is compared with the case where these partition plates are not provided. It is easy to control the temperature difference between the upper and lower heaters, and the degree of thermal convection can be easily controlled by the temperature difference between the upper, middle and lower heaters 44 to 46.

In the above-described embodiment, thermal convection is used to generate a flow in the direction along the surface of the seed crystal substrate 18 in the mixed melt. However, a rotary table with a shaft that is rotated by an external motor is used as the electric furnace 24. The reaction vessel 20 that is provided inside and accommodates the growth vessel 12 is placed on this turntable and rotated to cause the mixed melt in the growth vessel 12 to flow in the direction along the surface of the seed crystal substrate 18. Also good. A specific example is shown in FIG. Since the crystal plate manufacturing apparatus 110 of FIG. 14 is the same as the crystal plate manufacturing apparatus 10 except that the reaction vessel 20 is rotatable, only the differences from the crystal plate manufacturing apparatus 10 will be described below. The reaction vessel 20 is placed on a disc-shaped turntable 50 having a rotary shaft 52 attached to the lower surface. The rotating shaft 52 has an internal magnet 54 and rotates as the external magnet 56 arranged in a ring shape outside the cylindrical casing 58 is rotated by an external motor (not shown). The inlet pipe 22 inserted into the reaction vessel 20 is cut in the upper zone 34. For this reason, when the rotation shaft 52 rotates, the reaction vessel 20 placed on the turntable 50 also rotates without any trouble. Further, nitrogen gas filled in the electric furnace 24 from the nitrogen cylinder 42 via the pressure controller 40 is introduced into the reaction vessel 22 from the inlet pipe 22. By using this crystal plate manufacturing apparatus 110, a flow in the direction along the surface of the seed crystal substrate 18 can be generated in the mixed melt in the growth vessel 12. In addition, it is preferable to determine the posture of the seed crystal substrate in the growth vessel 12 so that the spiral flow generated in the mixed melt is parallel to the surface of the seed crystal substrate 18.

Example 1
A gallium nitride crystal plate was produced using the crystal plate manufacturing apparatus 10 shown in FIG. The procedure will be described in detail below. First, in a glove box in an argon atmosphere, a 10 × 15 mm seed crystal substrate 18 is stood against the side wall so that the angle with respect to the horizontal direction is 60 ° in the growth vessel 12, and metal gallium and metal sodium are mixed at a molar ratio. Weighed so that Ga: Na = x: (100−x), x = 28, and placed in the growth vessel 12. The growth vessel 12 was placed in the reaction vessel 20, and the reaction vessel 20 was placed in the cylindrical body 26 of the electric furnace 24 while performing a nitrogen purge, and the upper lid 28 and the lower lid 30 were closed and sealed. Thereafter, a gallium nitride crystal was grown under predetermined growth conditions. In this example, the growth conditions were a nitrogen pressure of 4.5 MPa and an average temperature of 875 ° C., and the growth was performed for 100 hours. The set temperature of the upper heater 44 and the middle heater 45 is 865 ° C., the set temperature of the lower heater 46 is 885 ° C., and the temperature gradient (ΔT) from the upper end of the upper heater 44 to the lower end of the lower heater 46 is set to 20 ° C. did. At this time, the temperature difference between the gas-liquid interface in the mixed melt in the growth vessel 12 and the bottom portion of the growth vessel was about 5 ° C. By providing such a temperature gradient, thermal convection of the mixed melt in the growth vessel 12 was generated. As a result, the mixed melt convects from the bottom to the top along the surface of the thin film 16 of the seed crystal substrate 18 as shown by the one-dot chain arrow in FIG. After the reaction is completed, the reaction vessel 20 is naturally cooled to room temperature, the reaction vessel 20 is opened, the growth vessel 12 is taken out, ethanol is introduced into the growth vessel 12, metal sodium is dissolved in ethanol, and then the grown gallium nitride crystal plate is removed. It was collected.

A photograph of a fluorescence microscope image of the gallium nitride crystal of Example 1 is shown in FIG. The photograph of the fluorescence microscope image shows the fluorescence emitted when irradiated with ultraviolet rays having a wavelength of 330 to 385 nm. Although FIG. 3 is displayed in gray scale for the sake of convenience, the grain boundaries can be confirmed from the impurity band emission that actually shines pale, and the approximate grain size can be estimated. From FIG. 3, it was confirmed that a large gallium nitride crystal containing a circle having a grain size of at least φ1 mm was obtained.

Further, the surface (Ga surface) of the gallium nitride crystal of Example 1 is dialed and etched by immersion in an acidic solution (mixed solution of sulfuric acid: phosphoric acid = 1: 3 (volume ratio)) at 250 ° C. for about 2 hours. Processed. After etching, differential interference images were observed using an optical microscope, and etch pits due to dislocations were observed. An appearance photograph of the etched gallium nitride crystal is shown in FIG. The appearance photograph was created by observing a differential interference image of the etched gallium nitride crystal using an optical microscope and combining several tens of images. The irregular shape is due to the fact that the crystal was cracked at the time of cooling after growth, and that etching was also performed from the side surface (surface perpendicular to the Ga surface) of the crystal. Further, the black groove is a trace of the crack enlarged by etching. The light blue (gray for monochrome display in FIG. 4) portion is a portion with few dislocations or no dislocations where pits were not vacant even after etching.

Further, etch pit density (EPD) was calculated with an enlarged field of view of 100 μm square. The observed enlarged field image is shown in FIG. The EPD was evaluated as follows. The differential interference image observation described above was performed, and pits (etch pits) due to dislocations were visually determined. Specifically, the EPD was calculated for each area by dividing into (1) an area where there were many etch pits, (2) an area where there were few etch pits, and (3) an area where bunching was seen. Bunching refers to a phenomenon in which a step density fluctuates due to a difference in the atomic step growth rate of each crystal surface, resulting in a macroscopically observable step. The EPD was obtained by calculating the number of etch pits in each area of 100 μm square. The etch pit is formed as a hexagonal pyramid pit because the center of the dislocation is etched deeper. Etch pits exist with a size of several μm to several tens of μm, but this is considered to be due to the difference in size depending on the type of dislocation (from the largest to the screw dislocation, mixed dislocation, and edge dislocation). ). For these reasons, the EPD of each area is a value obtained by dividing the total number of various etch pits by the area. For the sake of convenience, the EPD was set to <10 ¹ / cm ² for areas where no etch pits were confirmed, such as areas with few etch pits in Example 1. Moreover, in Example 1, the area where the bunching of said (3) was seen was not confirmed.

Based on the EPD results, an area with less φ1mm and less etch pits is shown in FIG. In FIG. 6, in the appearance photograph of FIG. 4, an area having φ1 mm and few etch pits (an area where the EPD order is 10 ⁴ / cm ² or less) is indicated by a circle. Thus, it can be seen that the gallium nitride crystal plate obtained in Example 1 is a size that encloses a circle having a grain size of φ1 mm, and the order of EPD in the circle is 10 ⁴ / cm ² or less. .

(Examples 2 to 4)
In Examples 2 to 4, a gallium nitride crystal plate was obtained in the same manner as in Example 1 except that metal gallium and metal sodium were weighed so that the values of x in Example 1 were x = 22, 25, and 32, respectively. Manufactured. Also for these, differential interference image observation was performed in the same manner as in Example 1, the etch pits were visually determined, and the EPD of each of the above areas (1) to (3) was obtained. The results of Examples 2 to 4 are shown in FIGS. 7 to 9, respectively.

(Comparative Examples 1 and 2)
In Comparative Examples 1 and 2, a gallium nitride crystal plate was manufactured in the same manner as in Example 1 except that metal gallium and metal sodium were weighed so that the value of x in Example 1 was x = 18 and 36, respectively. did. Also for these, differential interference image observation was performed in the same manner as in Example 1, the etch pits were visually determined, and the EPD of each of the above areas (1) to (3) was obtained. The results of Comparative Examples 1 and 2 are shown in FIGS.

(Evaluation)
FIG. 12 is a graph in which the EPD of each area of Examples 1 to 4 and Comparative Examples 1 and 2 is plotted on the vertical axis and the value of x is plotted on the horizontal axis. From FIG. 12, in the case of Examples 1 to 4 (that is, x = 22 to 32), there was an area where the order of EPD in a circle of φ1 mm was 10 ⁴ / cm ² or less, whereas Comparative Example 1, In the case of 2 (that is, x = 18, 36), such an area did not exist. Further, in Examples 1 to 4 and Comparative Examples 1 and 2 (that is, x = 18 to 36), the grain size is a size including a circle of φ1 mm, and the grain size tends to increase as x increases. On the other hand, when the growth was carried out under a soaking condition without providing a temperature gradient (ΔT) in Example 1, the grain size was 0.2 to 0. 0 as shown in the fluorescence microscope image of FIG. It was large enough to contain a 3mm circle. As is apparent from FIG. 13, when grown under soaking conditions, the emission of impurity bands due to grain boundaries was large and the grain size was small.

This application is based on Japanese Patent Application No. 2009-012962 filed on January 23, 2009, and the contents of which are incorporated herein by reference in their entirety.

The present invention can be used for semiconductor devices such as blue LEDs, white LEDs, and blue-violet semiconductor lasers in addition to high-frequency devices represented by power amplifiers.

Claims

In the growth container, the seed crystal substrate is immersed in a mixed melt having a concentration of the group 3B metal of 22 to 32 mol%, and a flow in the direction along the surface of the seed crystal substrate is generated in the mixed melt. 3B nitride is crystallized on the seed crystal substrate by supplying nitrogen gas to
A method for producing a group 3B nitride crystal.
The concentration of the 3B metal is 25-30 mol%;
The method for producing a group 3B nitride crystal according to claim 1.
In generating a flow in the direction along the surface of the seed crystal substrate in the mixed melt, the seed crystal substrate is supported in the growth container at an angle with respect to the horizontal direction, and the growth melt is mixed. For the temperature of the liquid, make the lower part higher than the upper part.
A process for producing a group 3B nitride crystal according to claim 1 or 2.
The temperature of the mixed melt in the growth vessel is set so that the lower part is 1-8 ° C. higher than the upper part.
A process for producing a group 3B nitride crystal according to claim 3.
Around the growth vessel, there are a partition plate that prevents gas flow between the upper and lower portions of the growth vessel, an upper heater that heats the upper portion of the growth vessel, and a lower heater that heats the lower portion of the growth vessel Provided, the set temperature of the lower heater is made higher than the upper heater,
A process for producing a group 3B nitride crystal according to claim 3 or 4.
The group 3B metal is metal gallium, and the group 3B nitride is gallium nitride.
The method for producing a group 3B nitride crystal according to any one of claims 1 to 5.