JP2011207640A - Method for producing group iii nitride semiconductor crystal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for producing a GaN crystal by an Na flux method, in which the dislocation density is reduced without causing an inclusion in the GaN crystal.SOLUTION: A self-supporting substrate comprising GaN is used as a seed crystal 18, and a GaN crystal 100 is grown on the seed crystal 18 at a growth rate of not more than 7 μm/h. The GaN crystal 100 is grown in a step-flow mode, and a dislocation propagating from the seed crystal 18 is bent in the GaN crystal 100 to reduce the dislocation density. Then a GaN crystal 102 is grown on the GaN crystal 100 at a growth rate faster than 7 μm/h and slower than 25 μm/h.

Description

  The present invention relates to a method for producing a group III nitride semiconductor crystal on a seed crystal by a flux method, and relates to a production method capable of obtaining a group III nitride semiconductor crystal with few dislocations.

  A so-called Na flux method is known as a method for producing a group III nitride semiconductor crystal such as GaN. In this method, a mixed melt of Na (sodium) and Ga (gallium) is reacted with nitrogen at about 800 ° C. and several tens of atmospheres to grow GaN crystals.

  In this Na flux method, a seed substrate is used, for example, a template substrate in which GaN is grown on a sapphire substrate by HVPE method, MOCVD method, or the like, or a GaN free-standing substrate is used.

  In the Na flux method, there is a method described in Patent Document 1 as a method for controlling the growth rate of the group III nitride semiconductor crystal. Patent Document 1 describes a method for producing a homogeneous GaN crystal by controlling the growth rate by the flow rate of nitrogen and keeping the growth rate constant. However, there is no description that the GaN crystal is grown by intentionally changing the growth rate.

JP2009-263169

  The present inventor examined the growth rate of the GaN crystal in the Na flux method, and at the growth rate of 25 to 30 μm / h, many inclusions may occur, and this inclusion prevents dislocation propagation. Thus, it has been found that the dislocation density of the GaN crystal decreases. Here, the inclusion means that the mixed melt used in the Na flux method is taken in during the growth of the GaN crystal, and the mixed melt remains in the GaN crystal. Further, at the growth rate of 8 to 15 μm / h, the inclusion was decreased, but the dislocation density was not decreased. That is, it was impossible to achieve both suppression of inclusion generation and reduction of dislocation density.

  Accordingly, an object of the present invention is to provide a method for producing a group III nitride semiconductor crystal by a flux method, in which no inclusion is generated in the group III nitride semiconductor crystal and the dislocation density can be reduced. is there.

  According to a first aspect of the present invention, there is provided a group III nitride semiconductor crystal in which a mixed melt containing at least a group III metal and an alkali metal is reacted with a gas containing at least nitrogen to grow a group III nitride semiconductor on a seed crystal. In the manufacturing method, the first step of starting the growth of the group III nitride semiconductor crystal at a growth rate of 7 μm / h or less, and after the first step, the growth rate is increased stepwise or continuously to be higher than 7 μm / h. And a second step of growing the group III nitride semiconductor crystal at a high growth rate.

Here, the group III nitride semiconductor is a semiconductor represented by the general formula Al _x Ga _y In _z N (x + y + z = 1, 0 ≦ x, y, z ≦ 1), and a part of Al, Ga, and In Is replaced with other group 13 elements (group 3B elements) B or Tl, and part of N is replaced with other group 15 elements (group 5B elements) P, As, Sb, Bi. Including replacements. More generally, GaN, InGaN, AlGaN, or AlGaInN containing at least Ga is shown.

  The group III metal is at least one of Ga, Al, and In. In particular, it is desirable to manufacture a GaN crystal using only Ga.

  As the alkali metal, Na (sodium) is usually used, but K (potassium) may be used, or a mixture of Na and K may be used. Furthermore, Li (lithium) or an alkaline earth metal may be mixed. In addition, dopants are added to the mixed melt for purposes such as controlling the physical properties of III-nitride semiconductors for crystal growth, properties such as magnetism, promoting crystal growth, suppressing miscellaneous crystals, and controlling the growth direction. May be. For example, when C (carbon) is added, effects of suppressing miscellaneous crystals and promoting crystal growth can be obtained. Further, Ge (germanium) or the like can be used as the n-type dopant, and Zn (zinc) or the like can be used as the p-type dopant.

  The gas containing nitrogen is a gas of a compound containing nitrogen as a constituent element, such as nitrogen molecules or ammonia, and may be a mixed gas thereof, or may further contain an inert gas such as a rare gas. .

  The growth rate can be controlled by temperature, pressure, the molar ratio of the group III metal to the entire mixed melt, and the like. The total mixed melt is the total number of moles of the group III metal and the alkali metal combined, and the total number of moles including the dopant other than the group III metal and the alkali metal contained in the mixed melt. Good.

  The growth rate of the group III nitride semiconductor crystal in the first step is desirably 1 μm / h or more. If the growth rate is slower than this, it takes a long time to grow a group III nitride semiconductor crystal, which is not desirable because of a problem in productivity. A more desirable growth rate in the first step is 3 to 7 μm / h.

  The thickness of the group III nitride semiconductor crystal grown in the first step is preferably 0.4 mm or more. If the thickness is 0.4 mm or more, the dislocation density can be sufficiently reduced. However, the thickness of the group III nitride semiconductor crystal grown in the first step is desirably 2 mm or less. If it is thicker than this, the effect of reducing the dislocation density in the first step is saturated, and there is no significance in increasing the thickness. More desirably, the thickness is 0.5 to 1.5 mm.

In order to facilitate step flow growth of GaN in the first step, the dislocation density of the seed crystal is preferably 1 × 10 ⁸ / cm ² or less, and particularly preferably a group III nitride semiconductor free-standing substrate, A template substrate may be used. The template substrate is a substrate in which a group III nitride semiconductor layer is formed on a dissimilar substrate such as a sapphire substrate by an HVPE method, an MOCVD method, or the like.

  The growth rate of the group III nitride semiconductor crystal in the second step is desirably 8 to 20 μm / h. Inclusion does not occur, and a dislocation density equivalent to that of the GaN crystal grown in the first step can be obtained. A more desirable growth rate in the second step is 8 to 15 μm / h.

  A second invention is a method for producing a group III nitride semiconductor crystal according to the first invention, wherein the second step has a growth rate faster than 7 μm / h and slower than 25 μm / h. .

  The third invention further comprises a third step of growing a group III nitride semiconductor crystal at a growth rate of 25 μm / h or more after the second step in the second invention. It is a manufacturing method of a semiconductor crystal.

  In a fourth invention according to the first to third inventions, in the first step, the molar ratio of the group III metal to the alkali metal in the mixed melt is 13% or less, so that the group III nitride semiconductor crystal In the second step, the group III metal is supplied to the mixed melt, and the molar ratio of the group III metal to the alkali metal in the mixed melt is set to 18 to 30%. A method for producing a group III nitride semiconductor crystal, characterized in that the growth rate of the group nitride semiconductor crystal is higher than 7 μm / h.

A fifth invention is characterized in that, in the first to fourth inventions, the surface on which the seed crystal group III nitride semiconductor is grown has a dislocation density of 1 × 10 ⁸ / cm ² or less. This is a method for producing a group III nitride semiconductor crystal.

  A sixth invention is a method for producing a group III nitride semiconductor crystal according to the first to fifth inventions, wherein the seed crystal is a group III nitride semiconductor free-standing substrate.

  According to a seventh invention, in the first to sixth inventions, the first step is a step of growing a group III nitride semiconductor crystal on the seed crystal by a thickness of 0.4 mm or more. This is a method for producing a group III nitride semiconductor crystal.

  As in the first invention, by starting the growth of the group III nitride semiconductor crystal at a growth rate of 7 μm / h or less, the growth of the group III nitride semiconductor crystal becomes step flow growth, and the dislocation of the seed crystal becomes III. Dislocations do not propagate in the direction perpendicular to the main surface of the group III nitride semiconductor crystal and can be reduced during the growth of the group nitride semiconductor crystal, reducing the dislocation density of the group III nitride semiconductor crystal. Can do. Then, after the dislocation density is reduced, the growth rate is made higher than 7 μm / h, whereby a Group III nitride semiconductor crystal having a high quality, a large thickness, and a small inclusion can be obtained.

  Further, as in the second invention, a high-quality group III nitride semiconductor crystal with less inclusion can be obtained by increasing the growth rate in the second step to be higher than 7 μm / h and lower than 25 μm / h. .

  According to the third invention, a thicker and better quality group III nitride semiconductor crystal can be obtained.

  Further, as in the fourth invention, the growth rate of the group III nitride semiconductor crystal can be controlled by the molar ratio of the group III metal to the alkali metal in the mixed melt, and the molar ratio should be 13% or less. Since the viscosity of the mixed melt is reduced and the growth rate is 7 μm / h or less, step flow growth can be caused.

In addition, as in the fifth invention, the present invention is effective when a seed crystal having a dislocation density of 1 × 10 ⁸ / cm ² or less is used. This is because when the dislocation density is higher than 1 × 10 ⁸ / cm ² , step flow growth hardly occurs and the dislocation density cannot be sufficiently reduced by the present invention.

  Further, as in the sixth invention, a group III nitride semiconductor free-standing substrate can be used as a seed crystal. Such a self-supporting substrate usually has a lower dislocation density than the group III nitride semiconductor of the template substrate, and step flow growth is likely to occur. Therefore, the effect of reducing the dislocation density according to the present invention is high.

  Further, if the group III nitride semiconductor crystal is grown to 0.4 mm or more in the first step as in the seventh invention, the effect of preventing the propagation of dislocations by step flow growth becomes sufficiently high, and the group III nitride The dislocation density of the semiconductor crystal can be sufficiently reduced.

The figure which showed the structure of the manufacturing apparatus 1 used for manufacture of the GaN crystal of Example 1. FIG. The figure which showed the manufacturing process of the GaN crystal of Example 1. FIG. The figure which showed the manufacturing process of the GaN crystal of Example 1. FIG.

  Hereinafter, specific examples of the present invention will be described with reference to the drawings. However, the present invention is not limited to the examples.

  FIG. 1 is a diagram showing a configuration of a manufacturing apparatus 1 used in the GaN crystal manufacturing method of Example 1. The production apparatus 1 includes a pressure vessel 10, a reaction vessel 11, a crucible 12, a heating device 13, supply pipes 14 and 16, exhaust pipes 15 and 17, a Ga supply pipe 19, a Ga holding container 20, It is constituted by.

  The pressure vessel 10 is made of cylindrical stainless steel and has pressure resistance. A supply pipe 16 and an exhaust pipe 17 are connected to the pressure vessel 10. A reaction vessel 11 and a heating device 13 are disposed inside the pressure vessel 10. The reaction vessel 11 has heat resistance. A crucible 12 is disposed in the reaction vessel 11. The crucible 12 is, for example, W (tungsten), Mo (molybdenum), BN (boron nitride), alumina, YAG (yttrium aluminum garnet), or the like. The crucible 12 holds a mixed melt 21 containing Ga and Na, and a seed crystal 18 is held in the mixed melt 21. A supply pipe 14 and an exhaust pipe 15 are connected to the reaction container 11, and ventilation in the reaction container 11, supply of nitrogen, and reaction container 11 are performed by valves (not shown) provided in the supply pipe 14 and the exhaust pipe 15. Control the pressure inside. Further, nitrogen is also supplied from the supply pipe 16 to the pressure vessel 10, and the pressure in the pressure vessel 10 is adjusted by adjusting the supply amount of nitrogen and the exhaust amount by valves (not shown) of the supply pipe 16 and the exhaust pipe 17. The pressure in the reaction vessel 11 is controlled to be substantially the same. Further, the temperature in the reaction vessel 11 is controlled by the heating device 13. Further, a Ga supply pipe 19 is connected to the reaction container 11, and liquid Ga can be supplied from the Ga holding container 20 into the crucible 12 through the Ga supply pipe 19.

  In addition, if the thing with pressure resistance is used as the reaction vessel 11, the pressure vessel 10 is not necessarily required. Further, a device capable of stirring the mixed melt 21 held in the crucible 12 by rotating or swinging the crucible 12 is provided, and the mixed melt 21 is stirred by growing the mixed melt 21 during the growth of the GaN crystal. It is preferable that the concentration distribution of Na, Ga, and nitrogen in the inside be uniform. A GaN crystal can be grown uniformly. Alternatively, the Ga supply pipe 19 may be connected to the crucible 12 to supply liquid Ga directly into the crucible 12. Further, a lid may be provided on the crucible 12 in order to prevent evaporation of Na during GaN crystal growth.

  Next, a method for manufacturing a GaN crystal using the manufacturing apparatus 1 will be described.

First, a free-standing substrate made of GaN is prepared as a seed crystal 18. The surface of the free-standing substrate on which GaN is grown has a dislocation density of 1 × 10 ⁸ / cm ² or less.

  This seed crystal 18 is placed on the bottom surface of the crucible 12, Na and Ga are placed in the crucible 12, the crucible 12 is placed in the reaction vessel 11 and sealed, and the reaction vessel 11 is placed in the pressure vessel 10. Placed and sealed. Na or Ga may be placed in the crucible 12 in a solid state, or liquid Na and Ga may be placed in the crucible 12, or liquid Na and Ga may be mixed and then placed in the crucible 12. Also good. The molar ratio of Ga to Na was 7%. Next, it heated with the heating apparatus 13, the mixed melt 21 of Ga and Na was produced in the crucible 12, and the temperature of the mixed melt 21 was 860 degreeC. In addition, nitrogen was supplied into the reaction vessel 11 through the supply pipe 14 and the exhaust pipe 15 to set the pressure in the reaction vessel 11 to 4.2 MPa. In addition, nitrogen was also supplied into the pressure vessel 10 through the supply pipe 16 and the exhaust pipe 17 so that the pressure in the pressure vessel 10 became approximately the same as the pressure in the reaction vessel 11. The seed crystal 18 is held in the mixed melt 21. This temperature and pressure were maintained for 100 hours, and a GaN crystal 100 having a thickness of 0.6 mm was grown on the surface of the seed crystal 18. The growth rate of GaN is 6 μm / h.

Here, the molar ratio of Ga to the entire mixed melt 21 is 7%, and the amount of Ga in the mixed melt 21 is small. Therefore, the viscosity of the mixed melt 21 is small, and the conditions are such that it is easy to grow laterally. Therefore, step flow growth is likely to occur. Step flow growth is a crystal growth mode in which there is little crystal growth from the terrace on the crystal surface, and the crystal grows laterally from the step (direction horizontal to the crystal surface). In addition, since the amount of Ga in the mixed melt 21 is small, the growth rate of GaN is as slow as 6 μm / h, which is also a condition for the GaN crystal 100 to be easily step flow grown. Furthermore, the fact that the dislocation density on the side surface on which the GaN crystal 100 of the seed crystal 18 is grown is 1 × 10 ⁸ / cm ² or less is also a factor that facilitates the generation of step flow growth. As a result, the GaN crystal 100 grown on the seed crystal 18 has a dominant step flow growth. By this step flow growth, most of the dislocations 101 propagating from the seed crystal 18 are bent in the lateral direction (the direction parallel to the main surface of the GaN crystal 100) in the GaN crystal 100 and perpendicular to the main surface of the GaN crystal 100. Propagation of dislocations in the direction is inhibited (see FIG. 2). As a result, the surface of the GaN crystal 100 grown on the seed crystal 18 has a lower dislocation density than the surface of the seed crystal 18.

  In the process of growing the GaN crystal 100, the growth rate of the GaN crystal 100 is set to 6 μm / h by setting the molar ratio of Ga to the entire mixed melt 21 to 7%. In order to facilitate the generation and reduce the dislocation density, the growth rate of the GaN crystal 100 may be set to 7 μm / h or less. At this time, if the molar ratio of Ga to the entire mixed melt 21 is set to 13% or less. Good. A more preferable growth rate of the GaN crystal 100 is 1 to 7 μm / h. In order for the growth rate of the GaN crystal 100 to fall within this range, the molar ratio of Ga to the entire mixed melt 21 may be 4 to 13%. In the above process, the GaN crystal 100 is grown to a thickness of 0.6 mm. However, in order to sufficiently reduce the dislocation density of the GaN crystal 100, it may be grown to a thickness of 0.4 mm or more. More desirably, the thickness is 0.5 to 1.5 mm.

  Next, the liquid Ga in the Ga holding container 20 was supplied into the crucible 12 through the Ga supply pipe 19, and the molar ratio of Ga to the entire mixed melt 21 in the crucible 12 was set to 18 to 30%. Then, the temperature of the mixed melt was 860 ° C., the pressure was 3.7 MPa, and the GaN crystal 102 was further grown on the GaN crystal 100 for 100 hours. The growth rate of the GaN crystal 102 is 15 μm / h, and the combined thickness of the GaN crystal 100 and the GaN crystal 102 is about 2 mm. When supplying the liquid Ga into the crucible 12, the amount of Ga in the crucible 12 is continuously increased so that the growth rate is continuously increased from 6 μm / h to 15 μm / h. Good.

  Here, since dislocations in the GaN crystal 100 are sufficiently reduced in the previous step, dislocations do not increase even if the growth rate of the GaN crystal 102 is made higher than the growth rate of 7 μm / h. Moreover, since the growth rate of the GaN crystal 102 is slower than 25 μm / h, the occurrence of inclusion can be suppressed. Therefore, the high-quality GaN crystal 102 can be grown at a faster rate than the GaN crystal 100. Since the growth rate of the GaN crystal 100 is slower than that of the GaN crystal 102, the occurrence of inclusion in the GaN crystal 100 is naturally suppressed.

  In the step of growing the GaN crystal 102, the growth rate of the GaN crystal 102 is set to 15 μm / h by setting the molar ratio of Ga to the entire mixed melt 21 to 18 to 30%. Should be faster than 7 μm / h and slower than 25 μm / h. A more desirable growth rate of the GaN crystal 102 is 8 to 15 μm / h.

  Thereafter, the GaN crystal may be grown on the GaN crystal 102 at a growth rate of 25 μm / h or more.

  Next, heating and pressurization are stopped to return to normal temperature and normal pressure, the crystal growth of the GaN crystal 102 is terminated, the seed crystal 18 and the GaN crystals 100 and 102 grown on the seed crystal 18 are taken out, and the adhered Na Is removed with ethanol or the like.

  According to the GaN crystal manufacturing method of Example 1 described above, a GaN crystal having no inclusion and having a dislocation density lower than that of the seed crystal 18 can be manufactured.

  In Example 1, the growth rate of GaN is controlled mainly by changing the molar ratio of Ga to Na in the mixed melt. However, by controlling the temperature and pressure without controlling the molar ratio of Ga, the GaN The growth rate may be controlled, or the growth rate of GaN may be controlled by controlling any two or more of temperature, pressure, and molar ratio of Ga to Na in the mixed melt. Further, the growth rate may be controlled by adding impurities. In Example 1, Ga is supplied during growth to control the molar ratio of Ga. However, GaN crystals are grown at a low Ga molar ratio, and Ga is added after the growth is completed. The GaN crystal may be grown again with a high Ga molar ratio.

In Example 1, a GaN free-standing substrate is used as a seed crystal, but a template substrate may be used. The template substrate is a substrate in which a GaN layer is formed on a dissimilar substrate such as a sapphire substrate by HVPE method, MOCVD method or the like. Also in the case of using a template substrate, it is desirable that the dislocation density on the surface of the GaN layer of the template substrate is 1 × 10 ⁸ / cm ² or less to facilitate step flow growth.

  Moreover, although Example 1 was a manufacturing method of a GaN crystal, this invention is applicable also to manufacture of group III nitride semiconductors other than GaN, for example, AlGaN, InGaN, AlGaInN.

  The group III nitride semiconductor according to the present invention can be used as a growth substrate in the production of electronic devices made of group III nitride semiconductors (for example, light emitting devices such as LEDs and LDs, and field effect transistors such as HEMTs). .

10: Pressure vessel 11: Reaction vessel 12: Crucible 13: Heating device 14, 16: Supply tube 15, 17: Exhaust tube 18: Seed crystal 19: Ga supply tube 20: Ga holding vessel 100, 102: GaN crystal 101: Dislocation

Claims (7)

In a method for producing a group III nitride semiconductor crystal, a mixed melt containing at least a group III metal and an alkali metal is reacted with a gas containing at least nitrogen to grow a group III nitride semiconductor on a seed crystal.
A first step of starting growth of a group III nitride semiconductor crystal at a growth rate of 7 μm / h or less;
After the first step, a second step of growing the group III nitride semiconductor crystal at a fast growth rate higher than 7 μm / h by increasing the growth rate stepwise or continuously;
A method for producing a Group III nitride semiconductor crystal, comprising:   2. The method for producing a group III nitride semiconductor crystal according to claim 1, wherein the second step has a growth rate faster than 7 μm / h and lower than 25 μm / h. 3.   The method for producing a group III nitride semiconductor crystal according to claim 2, further comprising a third step of growing a group III nitride semiconductor crystal at a growth rate of 25 µm / h or more after the second step. . In the first step, the growth rate of the group III nitride semiconductor crystal is set to 7 μm / h or less by setting the molar ratio of the group III metal to the entire mixed melt to 13% or less.
In the second step, the Group III metal is supplied to the mixed melt, and the molar ratio of the Group III metal to the alkali metal in the mixed melt is set to 18 to 30%. Make the growth rate faster than 7 μm / h,
The method for producing a group III nitride semiconductor crystal according to any one of claims 1 to 3, wherein: 5. The surface according to claim 1, wherein a surface on which the group III nitride semiconductor of the seed crystal is grown has a dislocation density of 1 × 10 ⁸ / cm ² or less. A method for producing a group III nitride semiconductor crystal.   6. The method for producing a group III nitride semiconductor crystal according to any one of claims 1 to 5, wherein the seed crystal is a group III nitride semiconductor free-standing substrate.   7. The III according to claim 1, wherein the first step is a step of growing a group III nitride semiconductor crystal with a thickness of 0.4 mm or more on the seed crystal. A method for producing a group nitride semiconductor crystal.

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