Method for producing group iii-nitride semiconductor

Abstract

The present invention pertains to a method for producing a group III nitride semiconductor in which a seed crystal (1) is brought into contact with a mixed melt (101), which was obtained by mixing a group III metal and a flux, while supplying a nitrogen-containing gas to the mixed melt (101), the seed crystal (1) is drawn up while maintaining the contact state, and a crystal (2) comprising a group III nitride semiconductor is grown in a columnar shape on the seed crystal (1), the method comprising: a first step for controlling the draw-up speed of the seed crystal (1) so that the diameter of the crystal (2) gradually decreases; and a second step for, after the abovementioned first step, performing control such that the diameter of the crystal (2) becomes larger than the diameter of the crystal (2) in the first step by setting the draw-up speed of the seed crystal to be slower than that in the first step or stopping the draw-up of the seed crystal.

Description

Method for manufacturing III-nitride semiconductor

[0001] The present invention relates to a method for manufacturing a III-nitride semiconductor.

[0002] As a method for manufacturing GaN, the Na flux method is known. In the Na flux method, nitrogen is dissolved in a mixed melt of Ga and Na to grow GaN in a liquid phase. In the Na flux method, generally, a seed substrate is placed in the mixed melt and GaN is grown on the seed substrate.

[0003] Patent Document 1 describes a method of growing a crystal in a columnar shape by pulling up a seed crystal while bringing the seed crystal into contact with a mixed melt of Ga and Na.

[0004] Japanese Unexamined Patent Application Publication No. 2004-292286

[0005] However, in the method of Patent Document 1, the dislocations of the seed crystal also propagated to the grown crystal, and the quality of the crystal was not good.

[0006] The present invention has been made in view of such a background, and aims to provide a method for manufacturing a III-nitride semiconductor capable of improving crystal quality.

[0007] One aspect of the present invention is a method for manufacturing a III-nitride semiconductor, comprising: supplying a gas containing nitrogen to a mixed melt of a group III metal and a flux, bringing a seed crystal into contact with the mixed melt, pulling up the seed crystal while maintaining the contacted state, and growing a crystal made of a III-nitride semiconductor on the seed crystal in a columnar shape, the method including: a first step of controlling a pulling-up speed of the seed crystal so that a diameter of the crystal gradually decreases; and a second step of, after the first step, controlling the diameter of the crystal to be larger than the diameter of the crystal in the first step by making the pulling-up speed of the seed crystal slower than that in the first step or by stopping the pulling-up of the seed crystal.

[0008] In the above aspect, the pulling-up speed of the seed crystal is controlled so that the diameter of the crystal gradually decreases. Therefore, the dislocation density of the crystal can be reduced.

[0009] As described above, according to the above embodiment, it is possible to provide a method for manufacturing a group III nitride semiconductor that can improve crystal quality.

[0010] This is a flowchart of the manufacturing method for a group III nitride semiconductor in the first embodiment. This is a schematic diagram showing the seed crystal and the shape of the crystal at each stage in the first embodiment. This is a schematic diagram showing the seed crystal and the shape of the crystal at each stage in the first embodiment. This is a schematic diagram showing the seed crystal and the shape of the crystal at each stage in the first embodiment. This is a schematic diagram showing the seed crystal and the shape of the crystal at each stage in the first embodiment. This is a schematic diagram showing the seed crystal and the shape of the crystal at each stage in the first embodiment. This is a schematic diagram showing the seed crystal and the shape of the crystal at each stage in the first embodiment. This is a block diagram showing the configuration of the manufacturing apparatus for a group III nitride semiconductor in the second embodiment. This is a schematic diagram showing the configuration of the circuit. This is a schematic diagram showing the configuration of the circuit. This is a flowchart for controlling the contact state between the crystal and the mixed melt. This is a flowchart for controlling the diameter of the crystal. This is a photograph showing the overall view of the grown crystal. This is an SEM image showing the top of the grown crystal. This is an SEM image showing the middle of the grown crystal. This is an SEM image showing the bottom of the grown crystal.

[0011] A method for manufacturing a group III nitride semiconductor is a method for manufacturing a group III nitride semiconductor, comprising: supplying a nitrogen-containing gas to a mixed molten mixture of a group III metal and a flux, bringing a seed crystal into contact with the mixed molten mixture, and pulling up the seed crystal while maintaining the contact, thereby growing a columnar crystal made of a group III nitride semiconductor on the seed crystal, the method comprising: a first step of controlling the pulling speed of the seed crystal so that the diameter of the crystal gradually decreases; and a second step after the first step of controlling the crystal diameter to be larger than the diameter of the crystal in the first step by slowing the pulling speed of the seed crystal down to a rate slower than the decrease in diameter, or by stopping the pulling of the seed crystal.

[0012] The above method for manufacturing a group III nitride semiconductor may further include a third step after the first step, in which the pulling speed of the seed crystal is controlled so that the diameter of the crystal enlarged in the first step is maintained.

[0013] In the above-described method for manufacturing a group III nitride semiconductor, in the first step, the resistance value of a circuit consisting of a power supply, a first terminal connected to the power supply and in contact with a seed crystal, and a second terminal connected to the power supply and in contact with the mixed molten metal is measured. If the resistance value is greater than or equal to a predetermined value, the seed crystal is pulled down until the resistance value falls below the predetermined value, and then the seed crystal is pulled up again. If the resistance value is below the predetermined value, the pulling speed of the seed crystal may be maintained.

[0014] In the above-described method for manufacturing a group III nitride semiconductor, in the first step, the resistance value of a circuit consisting of a power supply, a first terminal connected to the power supply and in contact with a seed crystal, and a second terminal connected to the power supply and in contact with the mixed molten metal is measured. If the resistance value is greater than or equal to a predetermined value, the seed crystal is pulled down until the resistance value falls below the predetermined value, and then the seed crystal is pulled up again. If the resistance value is below the predetermined value, the pulling speed of the seed crystal may be maintained.

[0015] In the above-described method for manufacturing a group III nitride semiconductor, in the first step, the resistance value of a circuit comprising a power supply, a first terminal connected to the power supply and in contact with a seed crystal, and a second terminal connected to the power supply and in contact with the mixed molten metal is measured, the diameter of the crystal is estimated from the resistance value, and the estimated value of the crystal diameter is compared with the target value of the crystal diameter. If the estimated value is greater than the target value, the seed crystal pulling speed is increased; if the estimated value is less than the target value, the seed crystal pulling speed is decreased; and if the estimated value and the target value are equal, the seed crystal pulling speed is maintained.

[0016] In the above-described method for manufacturing a group III nitride semiconductor, in the third step, the resistance value of a circuit consisting of a power supply, a first terminal connected to the power supply and in contact with the seed crystal, and a second terminal connected to the power supply and in contact with the mixed molten metal is measured, the diameter of the crystal is estimated from the resistance value, the estimated value of the crystal diameter is compared with the target value of the crystal diameter, and if the estimated value is greater than the target value, the seed crystal pulling speed is increased; if the estimated value is less than the target value, the seed crystal pulling speed is decreased; and if the estimated value and the target value are equal, the seed crystal pulling speed is maintained.

[0017] In the above-described method for manufacturing a group III nitride semiconductor, the seed crystal is held so that its c-axis direction is vertical, and the seed crystal may be pulled up in the c-axis direction.

[0018] (First Embodiment) 1. Overview of the Flux Method In the method for producing a group III nitride semiconductor in the first embodiment, a group III nitride semiconductor is grown by the flux method. The flux method is a method in which a gas containing nitrogen is supplied to a mixed melt containing an alkali metal which will be used as a flux and a group III metal which will be used as a raw material, and the mixture is dissolved, and a group III nitride semiconductor is epitaxially grown in the liquid phase.

[0019] The raw material is a Group III metal, which is at least one of gallium (Ga), aluminum (Al), or indium (In). The composition of the Group III nitride semiconductor grown can be controlled by the ratio of these metals, and GaN, AlN, InN, AlGaN, InGaN, AlGaInN, etc., can be grown. The present invention is particularly suitable for growing GaN.

[0020] The alkali metal flux is usually sodium (Na), but potassium (K) may also be used, or a mixture of Na and K. Furthermore, lithium (Li) or alkaline earth metals may also be added.

[0021] Carbon (C) may be added to the mixed melt. Adding C can accelerate the crystal growth rate. In addition, dopants other than C may be added to the mixed melt for purposes such as controlling the physical properties of the group III nitride semiconductor being grown, such as conductivity and magnetism, promoting crystal growth, suppressing miscellaneous crystals, and controlling the growth direction. For example, germanium (Ge) can be used as an n-type dopant, and magnesium (Mg), zinc (Zn), calcium (Ca), etc. can be used as p-type dopants.

[0022] Nitrogen-containing gases are gases of nitrogen molecules or compounds that contain nitrogen as a constituent element, such as ammonia. Mixed gases containing multiple types of nitrogen-containing compounds are also acceptable. Furthermore, nitrogen-containing gases may be mixed with inert gases such as noble gases.

[0023] 2. Seed crystal In the first embodiment, a seed crystal 1 is attached to the tip of a vertically movable lifting shaft 200. Then, the seed crystal 1 is brought into contact with a mixed molten liquid 101 held in a crucible 100 to grow a group III nitride semiconductor on the seed crystal 1.

[0024] Seed crystal 1 is made of a group III nitride semiconductor of any composition, such as GaN, AlGaN, or AlN. Seed crystal 1 is made of a group III nitride semiconductor with the same composition as the group III nitride semiconductor that is usually grown by the flux method. Seed crystal 1 may be formed by any method, such as the MOCVD method, HVPE method, MBE method, or flux method.

[0025] The seed crystal 1 may have any shape. However, it is preferable that the seed crystal 1 has a shape that has a face that facilitates crystal growth, such as a +c face or a (10-11) face. It is also preferable to hold the seed crystal 1 so that the face that facilitates crystal growth is vertically downward. For example, the seed crystal 1 may be a rectangular parallelepiped with one of its six faces being a +c face. In this case, the seed crystal 1 is held so that the +c face is vertically downward.

[0026] The seed crystal 1 is attached to the tip of the lifting shaft 200 with its c-axis direction aligned with the axis direction of the lifting shaft 200. Therefore, the seed crystal 1 can move up and down in the c-axis direction.

[0027] 3. Method for Manufacturing a Group III Nitride Semiconductor Next, the method for manufacturing a Group III nitride semiconductor in the first embodiment will be described with reference to the figures. Figure 1 is a flowchart of the method for manufacturing a Group III nitride semiconductor in the first embodiment. As shown in Figure 1, the method for manufacturing a Group III nitride semiconductor in the first embodiment includes a first diameter expansion step S1, a diameter reduction step S2, a second diameter expansion step S3, and a diameter maintenance step S4.

[0028] 3-1. First Diameter Enlargement Process First, the first diameter enlargement process S1 is performed. In the first diameter enlargement process S1, the furnace atmosphere is replaced with an inert gas, the furnace is heated, and then the furnace is evacuated to sufficiently reduce outgassing components such as oxygen inside the furnace.

[0029] Next, predetermined amounts of alkali metals and Group III metals are weighed in a glove box where the atmosphere, including oxygen and dew point, is controlled. Then, the weighed predetermined amounts of alkali metals and Group III metals are placed into crucible 100. Additives such as carbon may be added as needed.

[0030] Next, the crucible 100 containing the raw materials is placed in the reaction vessel, and a seed crystal 1 is attached to the tip of the lifting shaft 200 inside the reaction vessel. Then, the reaction vessel is evacuated, and a gas containing nitrogen is supplied to the reaction vessel. When the pressure inside the reaction vessel reaches the crystal growth pressure, the furnace is heated to the crystal growth temperature. The crystal growth temperature is, for example, 700°C to 1000°C, and the crystal growth pressure is, for example, 2 MPa to 10 MPa. During the heating process, the solid alkali metals and solid group III metals in the crucible 100 melt into liquids, forming a mixed melt 101. At this stage, the seed crystal 1 is withdrawn so that it does not come into contact with the mixed melt 101.

[0031] Next, when the reaction vessel reaches the crystal growth temperature and pressure, and the nitrogen dissolved in the mixed molten metal 101 becomes supersaturated, the lifting shaft 200 is moved downward, thereby pulling down the seed crystal 1 in the c-axis direction. Then, as shown in Figure 2, the mixed molten metal 101 in the crucible 100 is brought into contact with the seed crystal 1. Once the mixed molten metal 101 and the seed crystal 1 are in contact, the downward pulling of the seed crystal 1 is stopped.

[0032] By maintaining this state, a crystal 2 of a group III nitride semiconductor is grown from the seed crystal 1. As shown in Figure 3, the crystal 2 grows into a frustum-shaped hexagon. The six sides of the frustum-shaped hexagon have {10-11} planes, and the top surface (the surface in contact with the mixed molten metal 101) has a +c plane. The first diameter expansion step S1 is performed until the diameter of the crystal 2 reaches a predetermined value. Here, the diameter of the crystal 2 is the diameter of the circumscribed circle of the cross section perpendicular to the c-axis direction of the crystal 2. The first diameter expansion step S1 is performed, for example, until the diameter of the crystal 2 reaches 1 to 300 mm.

[0033] 3-2. Diameter Reduction Process Next, the diameter reduction process S2 is performed. Once the diameter of crystal 2 has increased to a predetermined value, the process moves from the first diameter expansion process S1 to the diameter reduction process S2.

[0034] In the diameter reduction step S2, the lifting shaft 200 is moved upward, thereby pulling up the seed crystal 1 in the c-axis direction. The pulling speed of the seed crystal 1 is set to a range that maintains contact between the mixed molten metal 101 and the crystal 2. It is preferable that only the +c plane and the {10-11} plane at the tip of the crystal 2 are in contact with the mixed molten metal 101.

[0035] When the seed crystal 1 is pulled up in this manner, the side surface of crystal 2 is in contact with the mixed molten metal 101 for a certain period of time, and crystal growth occurs from the side surface of crystal 2. However, after that, the side surface of crystal 2 is exposed from the mixed molten metal 101 due to the pulling up of the seed crystal 1. At this point, crystal growth does not occur in the areas of seed crystal 1 or crystal 2 that are not in contact with the mixed molten metal 101. Therefore, by growing crystal 2 while pulling up seed crystal 1, the lateral growth of crystal 2 is restricted. As a result, crystal 2 can be grown into a hexagonal prism shape.

[0036] Furthermore, in the diameter reduction step S2, the pulling speed of the seed crystal 1 is increased so that the diameter of crystal 2 gradually decreases. When the seed crystal 1 is pulled at high speed, the time that the side surface of crystal 2 is in contact with the mixed molten metal 101 is shortened, so the lateral growth of crystal 2 is further restricted. As a result, as shown in Figure 4, the diameter of crystal 2 gradually decreases. Hereinafter, the portion of crystal 2 whose diameter is decreasing will be referred to as crystal 2A. The diameter reduction step S2 is performed until the diameter of crystal 2 decreases to a predetermined value. For example, the diameter reduction step S2 is performed until the diameter of crystal 2A is between 0.1 and 30 mm.

[0037] Dislocations from seed crystal 1 propagate to crystal 2A and extend along the c-axis. Here, because the diameter of crystal 2A is gradually reduced, the dislocations are bent outwards from crystal 2A. As a result, dislocations in crystal 2A can escape to the sides of crystal 2A, thereby reducing the dislocation density of crystal 2A.

[0038] The rate at which seed crystal 1 is pulled up may be increased continuously or in stages.

[0039] 3-3. Second diameter expansion process Next, the second diameter expansion process S3 is performed. Once the diameter of crystal 2 has decreased to a predetermined value, the process moves from the diameter reduction process S2 to the second diameter expansion process S3.

[0040] In the second diameter enlargement step S3, the movement of the lifting shaft 200 is stopped, thereby stopping the downward pulling of the seed crystal 1 in the c-axis direction. Then, as shown in FIG. 5, the crystal 2 grows into a hexagonal frustum shape in the same manner as in the first diameter enlargement step. Since the pulling up of the seed crystal 1 is stopped, the side surface of the crystal 2 is always in contact with the mixed melt 101 during the second diameter enlargement step. Therefore, the diameter of the crystal 2 increases. The second diameter enlargement step S3 is performed until the diameter of the crystal 2 becomes a predetermined value. The second diameter enlargement step S3 is performed, for example, until the diameter of the crystal 2 becomes 1 to 300 mm.

[0041] 3-4. Diameter maintenance step Next, a diameter maintenance step S4 is performed. When the diameter of the crystal 2 has become a predetermined value, the process shifts from the second diameter enlargement step S3 to the diameter maintenance step S4.

[0042] In the diameter maintenance step S4, the lifting shaft 200 is moved upward, thereby pulling up the seed crystal 1 in the c-axis direction. The pulling-up speed of the seed crystal 1 in the diameter maintenance step S4 is set within a range where the contact between the mixed melt 101 and the crystal 2 is maintained. Further, the pulling-up speed of the seed crystal 1 in the diameter maintenance step S4 is made slower than the pulling-up of the seed crystal 1 in the diameter reduction step S2 so that the diameter of the crystal 2 in the second diameter enlargement step S3 is maintained. Thereby, a columnar crystal 2 (hereinafter referred to as crystal 2B) with a uniform diameter can be grown. The pulling-up of the seed crystal 1 in the diameter maintenance step S4 may be performed continuously or stepwise.

[0043] Here, the crystal 2A grown in the diameter reduction step S2 has a reduced dislocation density as described above. Therefore, there are few dislocations inherited from the crystal 2A to the crystal 2B. Therefore, the dislocation density of the crystal 2B is also reduced.

[0044] When the crystal 2 has grown to a predetermined length, the seed crystal 1 is further pulled up so that the crystal 2 is not in contact with the mixed melt 101. Thereby, crystal growth is terminated.

[0045] As described above, the method for manufacturing a group III nitride semiconductor in the first embodiment includes a diameter reduction step S2 in which the crystal 2 is grown so that the diameter gradually decreases. Therefore, a columnar crystal 2 with a low dislocation density can be grown.

[0046] (First Modified Form of the First Embodiment) In the second diameter enlargement step S3 in the first embodiment, although the pulling up of the seed crystal 1 is stopped, the seed crystal 1 may be pulled up at a pulling up speed slower than that in the diameter reduction step S2. In this case, as shown in FIG. 7, the diameter of the crystal 2 gradually increases. If a portion of the crystal 2 where the diameter gradually increases is defined as the crystal 2C, the crystal 2C is located between the crystal 2A and the crystal 2B. When enlarging the diameter of the crystal 2 while pulling up the seed crystal 1, the pulling up speed of the seed crystal 1 may be made equal to the pulling up speed of the seed crystal 1 in the diameter maintaining step S4. Also, similarly in the first diameter enlargement step S1, the seed crystal 1 may be pulled up at a pulling up speed slower than that in the diameter reduction step S2.

[0047] (Second Modified Form of the First Embodiment) When using a seed crystal 1 with a large diameter, the first diameter enlargement step S1 may be omitted.

[0048] (Second Embodiment) FIG. 8 is a block diagram showing the configuration of a manufacturing apparatus for a group III nitride semiconductor in the second embodiment. The manufacturing apparatus for a group III nitride semiconductor in the second embodiment is obtained by adding a control unit 201, a resistance measurement unit 202, a contact detection unit 203, a diameter estimation unit 204, terminals 205, 206, and a power source V to the lifting axis 200 of the manufacturing apparatus for a group III nitride semiconductor in the first embodiment. The manufacturing apparatus for a group III nitride semiconductor in the second embodiment is used in the manufacturing method for a group III nitride semiconductor in the first embodiment. Also, the manufacturing apparatus for a group III nitride semiconductor in the second embodiment can control the presence or absence of contact between the crystal 2 and the mixed melt 101, and the diameter of the crystal 2.

[0049] The control unit 201 controls the lifting and lowering of the lifting axis 200 based on the presence or absence of contact between the seed crystal 1 or the crystal 2 and the mixed melt 101, and the estimated value of the diameter of the crystal 2.

[0050] The resistance measurement unit 202 measures the resistance value R of a circuit composed of the power source V and the terminals 205, 206 connected to the power source V. The terminal 205 is in contact with the seed crystal 1. The terminal 206 is in contact with the mixed melt 101.

[0051] Figures 9 and 10 schematically show the circuit configuration. As shown in Figure 9, when the crystal 2 is not in contact with the mixed molten liquid 101, nitrogen is present between the crystal 2 and the mixed molten liquid 101. Therefore, no current flows between terminals 205 and 206. Consequently, the resistance value R of the circuit becomes very high.

[0052] On the other hand, as shown in Figure 10, when the crystal 2 is in contact with the mixed molten metal 101, the mixed molten metal 101 exists between the crystal 2 and the terminal 206. Since the mixed molten metal 101 is made of metal, it is conductive. Therefore, current flows between terminals 205 and 206. As a result, the resistance value R of the circuit is significantly lower than when the crystal 2 is not in contact with the mixed molten metal 101. Furthermore, the resistance value R changes depending on the distance between the crystal 2 and the terminal 206. Therefore, if the diameter of the crystal 2 changes, the resistance value R also changes.

[0053] To reduce the circuit's resistance R, the seed crystal 1 is preferably made of a group III nitride semiconductor doped with n-type impurities. This increases the conductivity of the seed crystal 1, thereby reducing the circuit's resistance R. The n-type impurity is preferably oxygen.

[0054] The contact detection unit 203 detects whether or not there is contact between the seed crystal 1 or crystal 2 and the mixed melt 101 based on the resistance value R of the circuit measured by the resistance measurement unit 202. Details of the detection operation will be described later.

[0055] The diameter estimation unit 204 estimates the diameter of the crystal 2 from the resistance value R of the circuit measured by the resistance measurement unit 202. For example, the relationship between the resistance value R and the diameter of the crystal 2 can be determined by simulation or measurement to create a calibration curve, and the diameter of the crystal 2 can be estimated from the resistance value R using that calibration curve.

[0056] Next, the method for manufacturing a group III nitride semiconductor in the second embodiment will be described. In the method for manufacturing a group III nitride semiconductor in the second embodiment, the contact between the crystal 2 and the mixed melt 101 is maintained during the diameter reduction step S2 and the diameter maintenance step S4 in the first embodiment. In addition, the diameter maintenance step S4 in the second embodiment is controlled so that the diameter of the crystal 2 reaches a target value. Otherwise, it is the same as the first embodiment.

[0057] In the diameter reduction step S2 and the diameter maintenance step S4, the seed crystal 1 is pulled up. At this time, if the speed at which the seed crystal 1 is pulled up is too fast, or if the liquid level fluctuates due to evaporation of the mixed melt 101 or consumption of Ga, the crystal 2 may separate from the mixed melt 101. If the crystal 2 is not in contact with the mixed melt 101, the growth of the crystal 2 will stop. Therefore, it is not possible to continuously grow the crystal 2. However, since the crystal 2 and the mixed melt 101 are held inside the reaction vessel, it is difficult to directly observe the contact state between the crystal 2 and the mixed melt 101.

[0058] Therefore, in the second embodiment, during the diameter reduction step S2 and the diameter maintenance step S4, it is determined at any given time whether the crystal 2 is in contact with the mixed melt 101 by the method described below, and the lifting shaft 200 is controlled based on this determination. This controls the process so that the state in which the crystal 2 is in contact with the mixed melt 101 is maintained.

[0059] The details of this control will be explained based on the flowchart in Figure 11. First, as shown in step S11, the contact detection unit 203 determines whether the resistance value R measured by the resistance measurement unit 202 is greater than or equal to a predetermined value. If the resistance value R is greater than or equal to the predetermined value, the process proceeds to step S12; if the resistance value R is less than the predetermined value, the process proceeds to step S14.

[0060] Step S11 is a determination of whether or not the crystal 2 is in contact with the mixed molten metal 101. If the crystal 2 is not in contact with the mixed molten metal 101, no current flows through the circuit, so the resistance value R is very high. On the other hand, if the crystal 2 is in contact with the mixed molten metal 101, current flows through the circuit because the mixed molten metal 101 is a metal. Therefore, the resistance value R is significantly lower than when the crystal 2 is not in contact with the mixed molten metal 101.

[0061] Therefore, by appropriately determining a predetermined value in step S11, it is possible to determine whether or not the crystal 2 is in contact with the mixed molten liquid 101. In this case, if the resistance value R is greater than or equal to the predetermined value, it is determined that the crystal 2 is not in contact with the mixed molten liquid 101, and if the resistance value R is less than the predetermined value, it is determined that the crystal 2 is in contact with the mixed molten liquid 101. In this manner, the contact detection unit 203 detects whether or not the crystal 2 is in contact with the mixed molten liquid 101.

[0062] If the resistance value R is greater than or equal to a predetermined value, the control unit 201 controls the lifting shaft 200 to pull down the seed crystal 1 until the resistance value R becomes less than the predetermined value, as shown in step 12. In other words, the seed crystal 1 is pulled down until the crystal 2 comes into contact with the mixed melt 101.

[0063] Next, as shown in step S13, the control unit 201 controls the lifting shaft 200 to pull up the seed crystal 1 at a predetermined speed. In other words, once the crystal 2 comes into contact with the mixed molten metal 101, the seed crystal 1 is pulled up again. Here, it is preferable that the pulling speed be slower than the pulling speed of the seed crystal 1 immediately before step 12. This prevents the crystal 2 from being in contact with the mixed molten metal 101 again.

[0064] If the resistance value R is less than a predetermined value, the control unit 201 controls the lifting shaft 200 to maintain the pulling speed of the seed crystal 1, as shown in step S14. In other words, the control unit 201 controls the lifting shaft 200 to maintain the state in which the crystal 2 is in contact with the mixed melt 101.

[0065] As described above, when the pull-up of the seed crystal 1 causes the crystal 2 to be in a state where it is not in contact with the mixed melt 101, it is possible to control the crystal 2 to come into contact with the mixed melt 101. Therefore, the seed crystal 1 can be pulled up while the crystal 2 is in contact with the mixed melt 101, and as a result, the crystal 2 can be grown continuously.

[0066] Next, the operation of controlling the diameter of crystal 2 will be explained based on the flowchart in Figure 12. The diameter control of crystal 2 is performed in the second diameter expansion step S3 and the diameter maintenance step S4.

[0067] First, as shown in step S21, the diameter estimation unit 204 estimates the diameter of the crystal 2 from the resistance value R. The resistance value R changes depending on the distance between the crystal 2 and the terminal 206. Also, the distance between the crystal 2 and the terminal 206 changes depending on the diameter of the crystal 2. Therefore, it is possible to estimate the diameter of the crystal 2 from the resistance value R.

[0068] Next, as shown in step S22, the control unit 201 compares the estimated diameter value estimated by the diameter estimation unit 204 with the target diameter value. If the estimated diameter value is greater than the target value, the process proceeds to step S23. If the estimated diameter value is less than the target value, the process proceeds to step S24. If the estimated diameter value is equal to the target value, the process proceeds to step S25.

[0069] If the estimated diameter is larger than the target value, the control unit 201 increases the pulling speed of the seed crystal 1 by a predetermined value, as shown in step S23. This reduces the estimated diameter and brings it closer to the target value.

[0070] If the estimated diameter is smaller than the target value, the control unit 201 slows down the pulling speed of the seed crystal 1 by a predetermined value, as shown in step S24. This increases the estimated diameter and brings it closer to the target value.

[0071] If the estimated diameter is equal to the target value, the control unit 201 maintains the pulling speed of the seed crystal 1, as shown in step S25. This increases the estimated diameter and brings it closer to the target value. Note that "equal to the target value" does not mean exactly equal; a difference of a certain degree of error is acceptable. For example, if the estimated value is in the range of 90-110% of the target value, the estimated value and the target value may be considered equal.

[0072] By repeatedly performing the crystal diameter control operation shown in the flowchart of Figure 12, a columnar crystal 2 with a constant diameter at the target value can be grown.

[0073] As described above, according to the second embodiment, the contact state between the crystal 2 and the mixed melt 101 can be maintained, making it easy to grow the crystal 2 into a columnar shape. Furthermore, it becomes easy to bring the diameter of the crystal 2 closer to the target value or to maintain the target value.

[0074] (First Modified Form of the Second Embodiment) In the second embodiment, control is performed to maintain the contact state between the crystal 2 and the mixed melt 101 as shown in the flowchart of Figure 11 during the diameter reduction step S2 and the diameter maintenance step S4. However, this control may be performed in only one of the diameter reduction step S2 or the diameter maintenance step S4. In the diameter reduction step S2, the pulling speed of the seed crystal 1 is fast because the diameter of the crystal 2 is reduced, and therefore the possibility of the crystal 2 separating from the mixed melt 101 is also increased. Therefore, it is preferable to perform this control at least in the diameter reduction step S2. Furthermore, the control shown in the flowchart of Figure 11 may also be performed in the first diameter expansion step S1 and the second diameter expansion step S3.

[0075] (Second Modification of the Second Embodiment) In the second embodiment, the diameter of the crystal 2 is controlled in the second diameter expansion step S3 and the diameter maintenance step S4 as shown in the flowchart of Figure 12. However, this control may be performed in only one of the second diameter expansion step S3 or the diameter maintenance step S4. If this control is performed only in the second diameter expansion step S3, the final pulling speed in the second diameter expansion step S3 should be maintained in the diameter maintenance step S4 as well. Furthermore, the diameter control shown in the flowchart of Figure 12 may also be performed in the first diameter expansion step S1 and the second diameter expansion step S3.

[0076] Experiment 1 Next, the experimental results relating to the second embodiment will be described. First, a seed crystal 1 in the shape of a rectangular parallelepiped measuring 1 mm × 1 mm × 2 mm was prepared. Then, crystal 2 was grown on seed crystal 1 using the Na flux method. The growth temperature was 900°C and the growth pressure was 4.0 MPa.

[0077] The growth of crystal 2 was carried out in the following three stages. First, crystal 2 was grown on seed crystal 1 with seed crystal 1 stopped. Next, crystal 2 was grown while seed crystal 1 was pulled up at a rate of 300 μm per hour. Next, crystal 2 was grown with seed crystal 1 stopped again. During the growth of crystal 2, the tip of crystal 2 was kept in contact with the mixed molten liquid 101. Whether or not crystal 2 was in contact with the mixed molten liquid 101 was determined by measuring the resistance value R by forming a circuit between seed crystal 1 and the mixed molten liquid 101, as shown in the second embodiment.

[0078] Figure 13 is a photograph of the grown crystal 2. Figures 14-16 are SEM images of the upper, middle, and lower parts of the grown crystal 2, respectively. The upper part of crystal 2 corresponds to the area where crystal 2 was grown after seed crystal 1 was initially stopped, the middle part of crystal 2 corresponds to the area where crystal 2 was grown while seed crystal 1 was being pulled up, and the lower part of crystal 2 corresponds to the area where crystal 2 was grown after seed crystal 1 was stopped again.

[0079] As shown in Figures 13-16, it was found that the upper and lower parts of crystal 2 grown with seed crystal 1 stopped had a frustum-shaped hexagon with {10-11} planes on the sides and a c-plane on the bottom. Furthermore, the middle part of crystal 2 grown while pulling up seed crystal 1 had a columnar shape. Therefore, it was found that crystal 2 can be grown into a columnar shape by growing it while pulling up seed crystal 1. In addition, it was found that the diameter of crystal 2 differed in its upper, middle, and lower parts. Therefore, it was found that the diameter of crystal 2 can be controlled by the pulling speed of seed crystal 1.

[0080] Furthermore, since the diameter of crystal 2 decreases from the top to the middle, it is thought that dislocations extending in the c-axis direction escape from the sides of crystal 2. The lower part of crystal 2 is the part grown after the dislocations have escaped, so it is thought that the dislocation density in the lower part of crystal 2 is lower than that in the upper part of crystal 2.

[0081] (Other variations) In the first and second embodiments, the seed crystal 1 is oriented vertically in the c-axis direction, and the seed crystal 1 is pulled up in the c-axis direction, but the pulling direction is not limited to the c-axis direction. For example, it may be in the [10-11] direction, the m-axis direction, or the a-axis direction.

[0082] The lifting axis 200 may be rotated, thereby rotating the seed crystal 1 while growing the crystal 2. This allows for more uniform crystal growth.

Claims

1. A method for producing a group III nitride semiconductor, comprising: supplying a nitrogen-containing gas to a mixed molten metal of a group III metal and a flux, bringing a seed crystal into contact with the mixed molten metal, and pulling up the seed crystal while maintaining the contact, thereby growing a columnar crystal made of a group III nitride semiconductor on the seed crystal, the method comprising: a first step of controlling the pulling speed of the seed crystal so that the diameter of the crystal gradually decreases; and a second step after the first step of controlling the pulling speed of the seed crystal to be slower than in the first step, or stopping the pulling of the seed crystal, so that the diameter of the crystal becomes larger than the diameter of the crystal in the first step.

2. The method for producing a group III nitride semiconductor according to claim 1, further comprising a third step of controlling the pulling speed of the seed crystal after the first step so as to maintain the diameter of the crystal enlarged by the second step.

3. The method for manufacturing a group III nitride semiconductor according to claim 1 or 2, wherein in the first step, the resistance value of a circuit comprising a power supply, a first terminal connected to the power supply and in contact with the seed crystal, and a second terminal connected to the power supply and in contact with the mixed molten liquid is measured, and if the resistance value is greater than or equal to a predetermined value, the seed crystal is pulled down until the resistance value becomes less than the predetermined value, and then the seed crystal is pulled up again, and if the resistance value is less than the predetermined value, the pulling speed of the seed crystal is maintained.

4. The method for manufacturing a group III nitride semiconductor according to claim 2, wherein in the third step, the resistance value of a circuit comprising a power supply, a first terminal connected to the power supply and in contact with the seed crystal, and a second terminal connected to the power supply and in contact with the mixed molten liquid is measured, and if the resistance value is greater than or equal to a predetermined value, the seed crystal is pulled down until the resistance value becomes less than the predetermined value, and then the seed crystal is pulled up again, and if the resistance value is less than the predetermined value, the pulling speed of the seed crystal is maintained.

5. A method for manufacturing a group III nitride semiconductor according to claim 1 or 2, wherein in the first step, the resistance value of a circuit comprising a power supply, a first terminal connected to the power supply and in contact with the seed crystal, and a second terminal connected to the power supply and in contact with the mixed molten metal is measured, the diameter of the crystal is estimated from the resistance value, the estimated diameter of the crystal is compared with a target diameter of the crystal, the pulling speed of the seed crystal is increased if the estimated value is greater than the target value, the pulling speed of the seed crystal is decreased if the estimated value is less than the target value, and the pulling speed of the seed crystal is maintained if the estimated value and the target value are equal.

6. The method for manufacturing a group III nitride semiconductor according to claim 2, wherein in the third step, the resistance value of a circuit comprising a power supply, a first terminal connected to the power supply and in contact with the seed crystal, and a second terminal connected to the power supply and in contact with the mixed melt is measured, the diameter of the crystal is estimated from the resistance value, the estimated diameter of the crystal is compared with a target diameter of the crystal, the pulling speed of the seed crystal is increased if the estimated value is greater than the target value, the pulling speed of the seed crystal is decreased if the estimated value is less than the target value, and the pulling speed of the seed crystal is maintained if the estimated value and the target value are equal.

7. The method for producing a group III nitride semiconductor according to claim 1 or claim 2, wherein the seed crystal is held such that its c-axis direction is vertical, and the pulling of the seed crystal is in the c-axis direction.

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