Crystal producing apparatus, crystal producing method, substrate producing method, gallium nitride crystal, and gallium nitride substrate

A crystal producing apparatus includes a crystal forming unit and a crystal growing unit. The crystal forming unit forms a first gallium nitride (GaN) crystal by supplying nitride gas into melt mixture containing metal sodium (Na) and metal gallium (Ga). The first GaN crystal is sliced and polished to form GaN wafers. The crystal growing unit grows a second GaN crystal on a substrate formed by using a GaN wafer, by the hydride vapor phase epitaxy method, thus producing a bulked GaN crystal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to and incorporates by reference the entire contents of Japanese priority documents, 2006-270363 filed in Japan on Oct. 2, 2006 and 2007-198607 filed in Japan on Jul. 31, 2007.

BACKGROUND

1. Technical Field

This disclosure relates to a technology for producing a gallium nitride (GaN) crystal and a GaN substrate.

2. Description of the Related Art

Indium gallium aluminum (InGaAln)-based devices (group-III nitride semiconductor) are popularly used as ultraviolet light sources, purple light sources, blue light sources, or green light sources. Such group-III nitride semiconductor devices are typically formed on a substrate made of sapphire or silicon carbide (SiC) by metalorganic chemical vapor deposition (MOCVD) method or molecular beam epitaxy (MBE) method.

However, there is considerable difference between the coefficients of thermal expansion and the lattice constants of the substrate, which is made of sapphire or SiC, and the group-III nitride semiconductor. Such differences in the physical properties of the substrate and the group-III nitride semiconductor result in causing crystal defects (imperfections) in the group-III nitride semiconductor. If the group-III nitride semiconductor has crystal defects, performance of the device, such as a light-emitting device, made from the group-III nitride semiconductor degrades, e.g., device lifetime is shortened, large driving power is necessary, and the like.

Some of the conventional light emitting devices are made from a conducting substrate so that it was possible to obtain an electrode from such a conducting substrate. However, because sapphire is insulating, it is difficult to obtain an electrode from such an insulating substrate, requiring obtaining an electrode from a group-III nitride semiconductor. To obtain an electrode from a group-III nitride semiconductor, a device needs to be large, resulting in increasing necessary costs. Furthermore, if the device size increases, the substrate may warp because of the differences in the physical properties of the sapphire substrate and the group-III nitride semiconductor.

In a group-III nitride semiconductor device formed on a sapphire substrate, separation of chips by use of cleavages is difficult, so that it is difficult to obtain a desired resonator facet for a laser diode (LD). For counteracting above problems, in one approach, the resonator facet is formed by the dry etching method, or the method of separating a sapphire substrate in a manner similar to cleavage after polishing the sapphire substrate to make it as thick as 100 micrometers (μm) or less. In this approach, however, it is difficult to form a resonator facet and to conduct chip separation in a single process, unlike a process for forming a conventional LD. As a result, manufacturing costs increase due to the necessity of extra processes.

Another approach for reducing crystal defects is to selectively grow a group-III nitride semiconductor on a sapphire substrate in a longitudinal direction. Although the occurrence of crystal defects can be reduced in this approach, it is still difficult to solve problems related to insulation properties or cleavage of a sapphire substrate.

In still another approach, the substrate is made of gallium nitride (GaN). In other words, the substrate is made of the same material as the crystal grown on the substrate. For example, Japanese Patent No. 3788037 discloses a technology for producing a GaN substrate. The GaN substrate is produced by growing a GaN crystal on a gallium arsenic (GaAs) substrate by hydride vapor phase epitaxy (HVPE) method, and then, slicing grown GaN crystal.

However, the GaN crystal is formed using coalescence and bending of dislocation to reduce the occurrence of dislocations. Therefore, it is difficult to obtain desired flatness on a surface of a GaN crystal, resulting in making it difficult to produce a bulked GaN crystal in desired quality with less defect density.

Furthermore, when a GaN crystal is polished and sliced, mechanical damage and etching easily occurs in a crystal grain boundary or a region of dislocation. Thus, it is difficult to produce a wafer in desired quality with preferable flatness of its surface.

BRIEF SUMMARY

According to an aspect of the present invention, there is provided an apparatus that produces a group-III nitride crystal. The apparatus includes a crystal growing unit that grows by vapor phase epitaxy method a second nitride crystal on a first group-III nitride crystal group-III that is formed by flux method.

According to another aspect of the present invention, there is provided a method of producing a group-III nitride crystal. The method includes growing a second group-III nitride crystal by vapor phase epitaxy method on a first group-III nitride crystal that is formed by flux method.

According to still another aspect of the present invention, there is provided a gallium nitride crystal in bulks. The gallium nitride crystal includes a first gallium nitride crystal formed by flux method; and a second gallium nitride crystal grown on the first gallium nitride crystal.

The above and other aspects, features, advantages and technical and industrial significance of this disclosure will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings. Same reference numerals are assigned to the same or substantially the same units, members, portions, and the like, and explanations thereof are omitted.

FIG. 1is a schematic diagram of a crystal producing apparatus1000that produces a bulked gallium nitride (GaN) crystal according to an embodiment of the present invention. The crystal producing apparatus1000includes a crystal forming unit100and a crystal growing unit500. The crystal forming unit100forms a GaN crystal by supplying nitride gas into melt mixture containing metal sodium (Na) and metal gallium (Ga). In other words, the crystal forming unit100forms a GaN crystal by the flux method. A GaN crystal formed by the crystal forming unit100will be called “a first GaN crystal” for convenience of explanation.

The crystal growing unit500grows a GaN crystal on the first GaN crystal by vapor phase epitaxy method. The GaN crystal grown on the first GaN crystal will be called “a second GaN crystal” for convenience of explanation.

As shown inFIG. 2, the crystal forming unit100includes a crucible10; a reaction container20; a bellows30; a supporting unit40; heating units50,60; temperature sensors51,61,71,81; heater/coolers70,80; gas supply pipes90,200; valves110,150; a pressure regulator120; gas canisters130,220; an exhaust pipe140; a vacuum pump160; a pressure sensor170; a pipe180; a thermocouple190; a flowmeter210, a vibration applying unit230; a shifting mechanism240; a vibration detecting unit250; a temperature control unit260; and a floating cover300.

Nitride gas is filled in each of the gas canisters130and220.

The crucible10is a container of which peripheral portion is in a circular shape and made of boron nitride (BN) or austenite-based stainless copper (SUS316L). The crucible10stores therein melt mixture270containing metal Na and metal Ga.

The reaction container.20encloses the crucible10. In other words, the crucible10is placed inside the reaction container20. The reaction container20includes a body portion21, a cover portion22, and a melt reservoir23. Each of the body portion21, the cover portion22, and the melt reservoir23is made of SUS316L. The body portion21and the cover portion22are sealed together by metal O-ring. The melt reservoir23is arranged on a bottom portion of the body portion21, and stores therein alkali metal melt280.

The bellows30is connected to the cover portion22of the reaction container20in a gravity direction DR1. The bellows30holds the supporting unit40and blocks between an inside and an outside of the reaction container20. Furthermore, the bellows30expands and contracts in accordance with a movement of the supporting unit40in the gravity direction DR1.

The heating unit50surrounds a peripheral surface20A of the reaction container20. The heating unit50includes a heater (not shown) and a current source (not shown). In the heating unit, the current source supplies current to the heater in response to a control signal CTL1received from the temperature control unit260. When current flows in the heater, the crucible10and the reaction container20are heated.

The heating unit60is arranged near a bottom surface20B of the reaction container20. The heating unit60includes a heater (not shown) and a current source (not shown). In the heating unit60, the current source supplies current to the heater in response to a control signal CTL2received from the temperature control unit260. When current flows in the heater, the crucible10and the reaction container20are heated. The heating units50and60heat the crucible10and the reaction container20to a crystal growth temperature.

The temperature sensor51is arranged near the heating unit50. The temperature sensor51detects a temperature T1of the heater in the heating unit50, and outputs a signal indicative of the detected temperature T1to the temperature control unit260.

The temperature sensor61is arranged near the heating unit60. The temperature sensor61detects a temperature T2of the heater in the heating unit50, and outputs a signal indicative of the detected temperature T2to the temperature control unit260.

The heater/cooler70surrounds the melt reservoir23. The heater/cooler70includes a heating member (not shown) and a cooling member (not shown). The heating member includes a heater (not shown) and a current source (not shown). The current source supplies current to the heater whereby the heater heats the melt reservoir23. The cooling member includes an air blower (not shown) that blows cool air to the melt reservoir23thereby cooling the melt reservoir23. The heater/cooler70can heat the melt reservoir23to an evaporation suppression temperature, or can cool the melt reservoir23to a condensation temperature in response to a control signal CTL3received from the temperature control unit260. The evaporation suppression temperature is a temperature at which vapor pressure PNa—Gaof metal Na evaporated from the melt mixture270substantially corresponds to vapor pressure PNaof metal Na evaporated from the alkali metal melt280. The condensation temperature is a temperature at which metal Na vapor is condensed to be metal melt.

The heater/cooler80surrounds a condensing area90A of the gas supply pipe90. The heater/cooler80includes a heating member (not shown) and a cooling member (not shown). The heating member includes a heater (not shown) and a current source (not shown). The current source supplies current to the heater whereby the heater heats the condensing area90A. The cooling member includes an air blower (not shown) that blows cool air thereby cooling the condensing area90A. The heater/cooler80can cool the condensing area90A to the condensation temperature, or can heat the condensing area90A to an evaporation acceleration temperature in response to a control signal CTL4received from the temperature control unit260. The evaporation acceleration temperature is a temperature at which metal Na is transported to a different area by chemical vapor transport.

The temperature sensor71is arranged near the heater/cooler70. The temperature sensor71detects a temperature (T3) of either the heating member or the cooling member of the heater/cooler70, and outputs the detected temperature T3to the temperature control unit260.

The temperature sensor81is arranged near the heater/cooler80. The temperature sensor81detects a temperature (T4) of either the heating member or the cooling member of the heater/cooler80, and outputs the detected temperature T4to the temperature control unit260.

The gas supply pipe90includes gas supply pipes91and92. The gas supply pipe90supplies nitride gas from the gas canister130into the reaction container20via via the pressure regulator120and alkali metal melt290.

The gas supply pipe91includes the condensing area90A. One end of the gas supply pipe91communicates with the bellows30, and the other end communicates with the gas supply pipe92via the valve110.

One end of the gas supply pipe92communicates with the gas supply pipe91via the valve110, and the other end communicates with the gas canister130.

The valve110controls flow of nitride gas in the gas supply pipe90. In other words, the valve110allows or does not allow flow of the nitride gas from the gas supply pipe92to the gas supply pipe91. Thus, the valve110is configured to separate the gas supply pipe90into the gas supply pipes91and92, and to connect the gas supply pipes91and92with each other.

The pressure regulator120is arranged in the gas supply pipe92near the gas canister130. The pressure regulator120regulates at a predetermined pressure the pressure of nitride gas flowing in the gas supply pipe92from the gas canister130.

One end of the exhaust pipe140communicates with the reaction container20, and the other end communicates with the vacuum pump160. Thus, the exhaust pipe140conveys gas from the reaction container to the vacuum pump160.

The valve150is arranged in the exhaust pipe140. The valve150controls flow of gas from the reaction container20to the vacuum pump160.

The vacuum pump160sucks gas from the reaction container20via the exhaust pipe140and the valve150to form a vacuum in the reaction container20.

The pressure sensor170is attached to the reaction container20, and detects pressure inside the reaction container20.

One end of the gas supply pipe200communicates with the pipe180, and the other end communicates with the gas canister220. The flowmeter210is arranged in the gas supply pipe200. Thus, the gas supply pipe200supplies nitride gas from the gas canister220to the pipe180via the flowmeter210.

The flowmeter210adjusts the flow rate of the nitride gas flowing in the gas supply pipe200in response to a control signal CTL5received from the temperature control unit260.

The vibration applying unit230includes piezoelectric element or the like, and applies vibration having a predetermined frequency to the supporting unit40.

The vibration detecting unit250includes an acceleration pick-up or the like. The vibration detecting unit250detects vibration of the supporting unit40and outputs to the shifting mechanism240a vibration detection signal BDS indicative of vibration information of the supporting unit40.

FIGS. 3A and 3Bare enlarged views of parts of the supporting unit40, the pipe180, and the thermocouple190.

The supporting unit40includes a cylinder member41. One end of the cylinder member41is inserted into a space24inside the reaction container20via an opening portion (not shown) arranged on the cover portion22. A seed crystal5is attached on a bottom surface41B of the cylinder member41. The supporting unit40supports a GaN crystal6(seeFIG. 3B) grown on the one end of the cylinder member41.

The pipe180conveys the nitride gas from the gas supply pipe200, i.e., from the gas canister220, through the inside of the supporting unit40to the reaction container20to cool the seed crystal5and the GaN crystal6. A bottom surface180A of the pipe180is arranged in such a manner that the bottom surface180A faces the bottom surface41B of the cylinder member41. A plurality of holes181are formed on the bottom surface180A of the pipe180. Nitride gas supplied into the pipe180is blown to the bottom surface41B of the cylinder member41through the holes181. Nitride gas output to an inside of the cylinder member41is then output to an outside of the crystal forming unit100through an opening portion (not shown) of the cylinder member41.

The thermocouple190is arranged inside the cylinder member41in such a manner that one end190A of the thermocouple190is in contact with the bottom surface41B of the cylinder, member41. The thermocouple190detects a temperature T5of each of the seed crystal5and the GaN crystal6, and outputs the detected temperature T5to the temperature control unit260.

With such an arrangement, thermal conductivity between the GaN crystal6and the cylinder member41increases. As a result, the temperature T5of the GaN crystal6can be accurately detected with the thermocouple190, and the GaN crystal6can be appropriately cooled by nitride gas blown to the bottom surface41B.

Returning to the explanation ofFIG. 2, the shifting mechanism240is attached to the supporting unit40on an upper side of the bellows30. The shifting mechanism240shifts up or down the supporting unit40so that the seed crystal5comes in contact with a gas-liquid interface2between the space24and the melt mixture270in response to the vibration detection signal BDS output from the vibration, detecting unit250.

The shifting mechanism240includes, as shown inFIG. 4, a corrugated member241, a gear242, a shaft member243, a motor244, and a control unit245. The corrugated member241has a substantially triangular cross-sectional shape, and is fixed onto a peripheral surface41A of the cylinder member41. The gear242is fixed onto, one end of the shaft member243, and is engaged with the corrugated member241. The other end of the shaft member243is coupled with a-axis (not shown) of the motor244.

The motor244rotates the gear242in a direction represented by arrows246or247based on a control by the control unit245. The control unit245controls the motor244to rotate the gear242based on the vibration detection signal BDS output from the vibration detecting unit250.

When the gear242rotates in a direction represented by the arrow246, the supporting unit40shifts upward in the gravity direction DR1. On the other hand, when the gear rotates in a direction represented by the arrow247, the supporting unit40shifts downward in the gravity direction DR1. In other words, the supporting unit40shifts upward or downward in the gravity direction DR1because of the rotation of the gear242in a direction represented by the arrows246or247. A length of the corrugated member241in the gravity direction DR1corresponds to a distance of movement of the GaN crystal6in an upward direction or a downward direction shifted by the supporting unit40.

FIG. 5is an exemplary timing chart of the vibration detection signal BDS. The vibration detection signal BDS contains signal component SS1when the supporting unit40is not in contact with the melt mixture270, contains signal component SS2when the supporting unit40is in contact with the melt mixture270, and contains signal component SS3when portion of the supporting unit40is dipped into the melt mixture270.

When the supporting unit40is not in contact with the melt mixture270, the supporting unit40largely vibrates due to vibration applied by the vibration applying unit230, so that the vibration detection signal BDS contains the signal component SS1with relatively large amplitude. On the other hand, when the supporting unit40is in contact with the melt mixture270, the supporting unit40does not vibrate so much because of viscosity of the melt mixture270even when vibration is applied by the vibration applying unit230. Therefore, in such a situation, the vibration detection signal BDS contains the signal component SS2with relatively small amplitude. When portion of the supporting unit40or the GaN crystal6is dipped into the melt mixture270, the supporting unit40or the GaN crystal6hardly vibrates because of viscosity of the melt mixture270even when vibration is applied by the vibration applying unit230. Therefore, in such a situation, the vibration detection signal BDS contains the signal component SS3with amplitude smaller than that of the signal component SS2.

Referring back toFIG. 4, the control unit245detects signal components of the vibration detection signal BDS upon receiving the vibration detection signal BDS from the vibration detecting unit250. Upon detecting the signal component SS1, the control unit245controls the motor244to shift the supporting unit40downward in the gravity direction DR1until the signal component SS1changes to the signal component SS2or SS3.

Specifically, upon detecting the signal component SS1, the control unit245controls the motor244to rotate the gear242in a direction represented by the arrow247. The motor244rotates the gear242via the shaft member243in the direction represented by the arrow247based on a control by the control unit245. As a result of the above control, the supporting unit40shifts downward in the gravity direction DR1.

When the signal component SS1changes to the signal component SS2or SS3, the control unit245controls the motor244to stop rotating the gear242. The motor244stops rotating the gear242under the control of the control unit245. As a result of the above control, the supporting unit40stops its shifting, so that the one end of the supporting unit40is in contact with the gas-liquid interface2, or the one end is dipped into the melt mixture270. On the other hand, upon receiving the vibration detection signal BDS containing the signal component SS2or SS3, the control unit245controls the motor244to stop shifting the supporting unit40.

As described above, the shifting mechanism240shifts the supporting unit40in the gravity direction DR1based on the vibration detected by the vibration detecting unit250so that the one end of the supporting unit40is in contact with the surface of the melt mixture270, or the one end of the supporting unit40is dipped into the melt mixture270.

The floating cover300is made of boron nitride (BN) and the like, and arranged on a surface of the melt mixture270around the supporting unit40. The floating cover300prevents evaporation of metal Na from the melt mixture270.

FIG. 6is a plan view of the floating cover300. The floating cover300is in a torus shape. That is, the floating cover300has a hole301at the center. The floating cover300has an inner diameter r and an outer diameter R. The inner diameter r is such that a value α is added to a diameter of the supporting unit40. The value α is such that corresponds to a clearance formed between the supporting unit40and the floating cover300for melting nitride gas with the melt mixture270. The outer diameter R substantially corresponds to an inner diameter of the crucible10.

The one end of the supporting unit40is maintained in contact with the surface of the melt mixture270or dipped in the melt mixture270via the hole301.

The temperature control unit260generates the control signal CTL1for heating the crucible10and the reaction container20to the crystal-growth temperature based on the temperature T1, and generates the control signal CTL2for heating the crucible10and the reaction container20to the crystal-growth temperature based on the temperature T2. The temperature control unit260generates the control signal CTL3for controlling a temperature of the melt reservoir23to an evaporation suppression temperature Tevc or a condensation temperature Tcoh based on the temperature T3. The temperature control unit260generates the control signal CTL4for controlling a temperature of the condensing area90A to the condensation temperature Tcoh or an evaporation acceleration temperature Tev based on the temperature T4. The temperature control unit260generates the control signal CTL5for flowing nitride gas with a flow rate appropriate to change a temperature T5of the seed crystal5or the GaN crystal6to be lower than a temperature of the melt mixture270around the seed crystal5based on the temperature T5.

The temperature control unit260outputs the control signal CTL1to the heating unit50, the control signal CTL2to the heating unit60the control signal CTL3to the heater/cooler70, and the control signal CTL4to the heater/cooler80. The temperature control unit260outputs the generated control signal CTL5to the flowmeter210.

FIG. 7is a timing chart of the temperature of each of the crucible10, the reaction container20, the melt reservoir23, and the condensing area90A.FIGS. 8A and 8Bare schematic diagrams for explaining state variation in each of the crucible10, the reaction container20, the melt reservoir23, and the condensing area90A between timings t1and t3shown inFIG. 7.FIG. 9is a schematic diagram for explaining state variation in each of the crucible10and the reaction container20at timing t3shown inFIG. 7.FIG. 10is a graph for explaining a relation between the temperature of the GaN crystal6and the flow rate of nitride gas according to the embodiment.

In the graph shown inFIG. 7, a curve k1represents the combined temperature of the crucible10and the reaction container20, curves k2and k3represent the temperatures of the GaN crystal6, a curve k4represents the temperature of the melt reservoir23, and a curve k5represents the temperature of the condensing area90A.

The heating units50and60heat the crucible10and the reaction container20until timing t5so that, as represented by the curve k1, the temperature of the crucible10and the reaction container20rises , and is maintained at 800° C. The heater/cooler70heats the melt reservoir23until timing t5so that, as represented by the curve k4, the temperature of the melt reservoir23rises and is maintained at the evaporation suppression temperature Tevc. The heater/cooler80heats the condensing area90A until timing t5so that, as represented by the curve k5, the temperature of the condensing area90A rises and is maintained at the condensation temperature Tcoh.

At timing t0, i.e., when the heating units50and60start heating the crucible10and the reaction container20, as shown inFIG. 8A, metal Na7and metal Ga8are present in the crucible10. On the other hand, at timing t0, i.e., when the heater/cooler70starts heating the melt reservoir23and the heater/cooler80starts heating the condensing area90A, as shown inFIG. 8A, the metal Na7is present in the melt reservoir23, while the metal Na7is not present in the condensing area90A.

When the temperature of the crucible10and the reaction container20reaches 98° C. at timing t1, the metal Na7in the crucible10melts and mixes with the metal Ga8that has already melted at about 30° C. Subsequent1y, an intermetallic compound of Ga and Na is generated, so that the melt mixture270is generated by the intermetallic compound in the crucible10at 560° C. or higher. The temperature of the crucible10and the reaction container20as represented by the curve k1inFIG. 7reaches 800° C. at timing t3.

When the temperature of the melt reservoir23reaches 98° C. at timing t2, the metal Na7present in the melt reservoir23melts, and the alkali metal melt280is generated in the melt reservoir23. Subsequent1y, a temperature of the melt reservoir23as represented by the curve k4inFIG. 7reaches the evaporation suppression temperature Tevc. The temperature of the condensing area90A as represented by the curve k5inFIG. 7reaches the condensation temperature Tcoh at timing t3.

In the process of heating the crucible10and the reaction container20to 800° C. and heating the melt reservoir23to the evaporation suppression temperature Tevc, vapor pressure PNaof metal Na evaporated from the alkali metal melt280and vapor pressure PNa—Gaof metal Na evaporated from the melt mixture270gradually increase. The metal Na evaporated from the melt mixture270and/or the alkali metal melt280gets condensed in the condensing area90A, which is at the condensation temperature Tcoh that is lower than temperatures of any one of the crucible10, the reaction container20, and the melt reservoir23. As a result, the alkali metal melt290gets condensed in the condensing area90A.

At timing t3, the vapor pressure PNa—Gaof alkali metal evaporated from the melt mixture270substantially corresponds to the vapor pressure PNaof alkali metal evaporated from the alkali metal melt280(seeFIG. 8B). As a result, it is possible to suppress variation of mixture ratio between metal Na and metal Ga in the melt mixture270due to evaporation of metal Na from the melt mixture270and alkali metal melt280.

At this point, when a temperature of the condensing area90A is higher than a melting point of metal Na, and at which Na does not practically evaporates, it is possible to consider that diffusion of metal Na to a side of the valve110does not affect to mixture ratio of metal Na and metal Ga in the melt mixture270. As a result, it is possible to suppress more variation of mixture ratio between metal Na and metal Ga in the melt mixture270. The temperature at which Na does not practically evaporates is, i.e., in a range between 200° C. and 300° C. The vapor pressure of Na at 200° C. is about 1.8×10−2Pa, while the vapor pressure of Na at 300° C. is about 1.8 Pa. Although diffusion of metal Na may occur due to evaporation even at a temperature higher than 300° C., it is possible to suppress variation in the mixture ratio of metal Na and metal Ga in the melt mixture270. Therefore, the condensation temperature Tcoh is preferably set in a temperature range between 200° C. and 300° C.

Because the floating cover300is arranged on the surface of the melt mixture270, evaporation of metal Na from the melt mixture270can be suppressed when a timing, at which Na evaporates from the melt mixture270in the crucible10, has passed. Therefore, it is possible to suppress more variation in the mixture ratio of metal Na and metal Ga in the melt mixture270.

Nitride gas4of which pressure is regulated by the pressure regulator120is supplied into the space24via the gas supply pipe90(seeFIG. 9).

At timing t3, at which the temperature of the crucible10and the reaction container20reaches 800° C., the shifting mechanism240shifts the supporting unit40up or down based on the vibration detection signal BDS output from the vibration detecting unit250until the one end of the supporting unit40comes contact with the melt mixture270as explained above.

When temperature of the crucible10and the reaction container20is about 800° C., which is a high-temperature state, the nitride gas4in the space24is absorbed in the melt mixture270through medium of metal Na. At this point, density of nitride or density of group-III nitride in the melt mixture270is densest around the gas-liquid interface2between the space24and the melt mixture270. Therefore, the GaN crystal6grows from the seed crystal5.

When nitride gas is not supplied into the pipe180, the temperature T5is kept at 800° C., which is the same as that of the melt mixture270. However, the seed crystal5or the GaN crystal6is cooled by supplying nitride gas into the pipe180, and the temperature T5is set lower than that of the melt mixture270to increase degree of supersaturation of nitride or group-III nitride in the melt mixture270around the seed crystal5or the GaN crystal6.

Specifically, the temperature T5is maintained at a temperature Ts1, which is lower than 800° C., as represented by the curve k2inFIG. 7, after timing t3. The temperature Ts1is, e.g., 790° C. A process of setting the temperature T5to the temperature Ts1is explained below.

When the temperatures T1and T2, which are respectively detected by the temperature sensors51and61, reach 800+α° C., i.e., a temperature of a heater included in each of the heating units50and60when the temperature of the crucible10and the reaction container20is set at 800° C., the temperature control unit260generates the control signal CTL5for flowing nitride gas with flow rate for setting the temperature T5to the temperature Ts1, and outputs the control signal CTL5to the flowmeter210.

The flowmeter210causes nitride gas to flow from the gas canister220to the pipe180via the gas supply pipe200at such a flow rate that the temperature T5is maintained at the temperature Ts1. The temperature T5decreases from 800° C. substantially in proportion to the flow rate of nitride gas. When the flow rate of nitride gas reaches flow rate fr1standard cubic centimeters per minute (sccm), the temperature T5reaches the temperature Ts1(seeFIG. 10).

The flowmeter210causes nitride gas to flow into the pipe180at flow rate fr1. Nitride gas supplied to the pipe180is blown to the bottom surface41B of the cylinder member41from the holes181of the pipe180.

Accordingly, the seed crystal5or the GaN crystal6is cooled via the bottom surface41B. As the seed crystal5or the GaN crystal6cools, the temperature T5drops to the temperature Ts1at timing t4. Subsequent1y, the temperature T5is maintained at the temperature Ts1until timing t5.

Each of the temperatures T1and T2of the heater in the heating units50and60has a predetermined difference α° C. from a temperature of the melt mixture270. When the temperature T5of the GaN crystal6drops below 800° C., the temperature control unit260controls the heating units50and60using the control signals CTL1and CTL2, so that the temperatures T1and T2reach 800+α° C.

The temperature T5of the seed crystal5or the GaN crystal6is preferably controlled to decrease, as represented by the curve k3shown inFIG. 7, after timing t3. In other words, the temperature T5decreases from 800° C. to a temperature Ts2, which is lower than the temperature Ts1, in a period from timing t3to timing t5. At this point, the flowmeter210increases flow rate of nitride gas to be flown, as represented by the curve k6shown inFIG. 10, from zero to flow rate fr2, which is larger than the flow rate fr1, based on the control signal CTL5output from the temperature control unit260. When the flow rate of nitride gas reaches the flow rate fr2, the temperature T5is set to the temperature Ts2, e.g., 750° C., which is lower than the temperature Ts1.

As described above, by gradually increasing a difference between a temperature (i.e., 800° C.) of the melt mixture270and the temperature T5of the GaN crystal6, a degree of supersaturation of nitride or group-III nitride in the melt mixture270around the GaN crystal6gradually increases, resulting in continuing crystal growth of the GaN crystal.

FIG. 11is a schematic diagram for explaining a relation between pressure of nitride gas (hereinafter, “nitride gas pressure”) and temperature of the melt mixture270for growing GaN crystal according to the embodiment. InFIG. 11, the temperature is represented on the horizontal axis and pressure is represented on the vertical axis. On the graph, a region REG1represents an area in which the GaN crystal6melts. A region REG2represents an area in which a number of self-nucleation growth is conducted on an inner bottom surface or an inner side surface of the crucible10in contact with the melt mixture270, and the GaN crystal6in a columnar shape grown in a direction of c-axis (<0001>) is formed. A region REG3represents an area in which the GaN crystal6is grown from the seed crystal5. A region REG4represents an area in which a number of self-nucleation growth is conducted and the GaN crystal6in a plate shape grown from the c-plane is formed, similar to that of the region REG2.

The GaN crystal6is grown at the temperature-pressure conditions in the region REG2or REG4, while the GaN crystal6is grown from the seed crystal5at the temperature-pressure conditions in the region REG3.

Referring back toFIG. 7, when crystal growth of the GaN crystal6is completed, i.e., at timing t5, the heating units50and60stop heating the crucible10and the reaction container20. Accordingly, the temperature of the crucible10and the reaction container20drops below 800° C. as represented by the curve k1, and the temperature T5of the GaN crystal6drops below the temperature Ts1or Ts2.

On the other hand, the heater/cooler70cools the melt reservoir23as represented by the curve k4. The temperature of the melt reservoir23is maintained at the condensation temperature Tcoh during a period from timing t6to timing t7. The heater/cooler80heats the condensing area90A as represented by the curve k5. The temperature of the condensing area90A is maintained at the evaporation acceleration temperature Tev during a period from timing t6to timing t7. The evaporation acceleration temperature Tev is a temperature at which the alkali metal melt290is transported from the condensing area90A to different areas by chemical vapor transport.

During a period from timing t6to timing t7, a temperature of the melt reservoir23is maintained at the condensation temperature Tcoh, while a temperature of the condensing area90A is maintained at the evaporation acceleration temperature Tev. As a result, the alkali metal melt290evaporates and flows from the condensing area90A to the melt reservoir23by chemical vapor transport.

At timing t7, the alkali metal melt290is not present in the condensing area90A. Because the valve110is in closed state, vapor containing metal Na evaporated from the alkali metal melt290does not diffuse toward the pressure regulator120while the alkali metal melt290flows from the condensing area90A to the melt reservoir23.

At timing t7, when the alkali metal melt290has completely flown to the melt reservoir23, the heater/cooler70cools the melt reservoir23and the heater/cooler80cools the condensing area90A. Accordingly, the crucible10, the reaction container20, the melt reservoir23, and the condensing area90A are cooled to the room temperature after timing t7.

The crystal growing unit500includes, as shown inFIG. 12, a reactor501, a heater502, gas introduction pipes503and504, a Ga storage unit505, a susceptor507, a shaft508, and a gas exhaust pipe510.

The reactor501is substantially cylindrical. The heater502is also cylindrical, and it is arranged around the reactor501. The heater502heats the reactor501to a temperature between 800° C. and 1050° C.

The gas introduction pipes503and504are arranged on a top portion of the reactor501. One end of the gas introduction pipe503is arranged above the Ga storage unit505. One end of the gas introduction pipe504is arranged in a position between a bottom surface of the Ga storage unit505and a, top surface of the susceptor507.

The gas introduction pipe503introduces hydrogen chloride (HCl) and hydrogen (H2) (hereinafter, “HCl +H2”) into the reactor501, and blows HCl+H2to the Ga storage unit505. The gas introduction pipe504introduces ammonia (NH3) and hydrogen (H2) (hereinafter, “NH3+H2”) into the reactor501, and emits NH3+H2to an outside of the gas introduction pipe504in a downward side of the Ga storage unit505.

The Ga storage unit505is arranged inside the reactor501, and it stores therein Ga melt506. The susceptor507is attached to one end of the shaft508. The susceptor507holds a GaN crystal509in wafer shape, and rotates or shifts up or down the GaN crystal509in accordance with the rotation and shifting of the shaft508.

The shaft508is attached to a bottom of the reactor501in rotatable and up-down movable manner. The shaft508supports the susceptor507and rotates and shifts up or down the susceptor507.

The gas exhaust pipe510is arranged at the bottom of the reactor501, and it emits exhaust gas to an outside of the reactor501.

When inside of the reactor501is heated with the heater502to a temperature between 800° C. and 1050° C., metal Ga in the Ga storage unit505melts because the melting point of Ga is about 29.8° C., so that the Ga melt506is generated. When HCl+H2introduced from the gas introduction pipe503is blown to the Ga melt506, gallium chloride (GaCl) is generated.

Mixed gas containing GaCl and H2carrier gas is transported downwardly in a space of the reactor501. The mixed gas containing GaCl and H2carrier gas is mixed with another mixed gas containing NH3and H2output from the gas introduction pipe504in a position below a bottom surface of the Ga storage unit505, so that GaN is generated.

Because the GaN crystal509held on the susceptor507is heated to a temperature between 800° C. and 1050° C., GaN generated by chemical vapor reaction gets attached to the GaN crystal509, so that a GaN crystal in the same quality as that of the GaN crystal509is grown.

The shaft508rotates and shifts downward during crystal growth, so that the GaN crystal509rotates and shifts downward accordingly. As a result, it is possible to maintain a constant distance between the gas introduction pipe504and the surface of the GaN crystal, even when thick GaN crystal is grown on the GaN crystal509by the HVPE method. Thus, it is possible to produce a bulked GaN crystal in constant quality.

FIG. 13is a flowchart for explaining a method of producing a GaN crystal with the crystal producing apparatus1000.

At step S1, the crystal forming unit100forms a first GaN crystal, i.e., the GaN crystal6(seeFIG. 16A) by supplying nitride gas to mixed gas containing metal Na and metal Ga in a manner described above. At this point, a crystal in a columnar shape, i.e., longer in the c-axis direction than in the a-axis direction, is formed. The speed of growth of the GaN crystal6(a speed of crystal growth by the flux method) is, e.g., about 1 μm/h.

At step S2, the first GaN crystal is sliced (seeFIG. 16B) and polished to obtain a plurality of GaN crystals661to667.

At step S3, the crystal growing unit500grows a second GaN crystal670by the HVPE method on a slice of the first GaN crystal661obtained at step S2, producing a bulked GaN crystal700(seeFIG. 16C). The speed of growth of the second GaN crystal (a speed of crystal growth by the HVPE method) is, e.g., about 100 μm/h.

At step S4, the bulked GaN crystal is sliced into a plurality of GaN wafers, and the slices (wafers) of the GaN are polished. Each of the wafers serves as a GaN substrate.

As described above, according to the embodiment, the first GaN crystal is formed by the flux method, the first GaN is sliced and polished, and the second GaN crystal is grown on the sliced and polished first GaN crystal by the HVPE method to produce a bulked GaN crystal. Because the first GaN crystal has low dislocation density, which is equal to or smaller than 105cm−2, in a desired quality, the second GaN crystal grown on the first GaN crystal by the HVPE method becomes such that has low dislocation density, which is equal to or smaller than 105cm−2, in a desired quality.

FIG. 14is a flowchart for explaining detailed procedures at step S1shown inFIG. 13. The gas supply pipe90is separated into the gas supply pipes91and92. The crucible10, the reaction container20, and the gas supply pipe91are introduced into a glove box (not shown), in which argon (Ar) gas is filled. Metal Na is input into the melt reservoir23of the reaction container20in Ar gas atmosphere (step S11). The Ar gas is such that amount of moisture is equal to or less than 1 part per million (ppm), and amount of oxygen is equal to or less than 1 ppm (same condition is applied in the following explanations).

Metal Na and metal Ga are put into the crucible10in Ar gas atmosphere (step S12). Mixture ratio between metal Na and metal Ga is set 5:5.

The floating cover300is put into the crucible10in Ar gas atmosphere (step S13), and the crucible10in which metal Na and metal Ga are present is placed inside the reaction container20.

The crucible10, the reaction container20, and the gas supply pipe91are taken out of the glove box (not shown), and replaced at respective predetermined positions of the crystal forming unit100with Ar gas filled in each of the crucible10, the reaction container20, and the gas supply pipe91.

The gas supply pipe92is connected to the valve110. The valve150is then opened while the valve110is closed, so that Ar gas is output from each of the crucible10, the reaction container20, and the gas supply pipe91by the vacuum pump160. Internal pressure of each of the crucible10, the reaction container20, and the gas supply pipe91is reduced to a predetermined pressure, which is equal to or less than 0.133 pascal (Pa), by using the vacuum pump160. Subsequent1y, the valve150is closed, and the valve110is opened to fill nitride gas from the gas canister130into the crucible10and the reaction container20via the gas supply pipe90. At this point, nitride gas is supplied to the crucible10and the reaction container20so that internal pressure of each of the crucible10and the reaction container20is regulated to around 0.1 mega pascal (MPa) by the pressure regulator120.

When internal pressure of the reaction container20detected by the pressure sensor170reaches around 0.1 MPa, the valve110is closed and the valve150is opened, so that the nitride gas filled in each of the crucible10, the reaction container20, and the gas supply pipe91is emitted by using the vacuum pump160. At this point, internal pressure of each of the crucible10, the reaction container20, and the gas supply pipe91is reduced to a predetermined pressure, which is equal to or less than 0.133 Pa, by using the vacuum pump160.

A process of vacuuming of the crucible10, the reaction container20, and the gas supply pipe91, and a process of filling nitride gas into the crucible10, the reaction container20, and the gas supply pipe91are repeated for a several times.

Subsequently, internal pressure of each of the crucible10, the reaction container20, and the gas supply pipe91is reduced to a predetermined pressure by using the vacuum pump160. Then, the valve150is closed and the valve110is opened to fill nitride gas into the crucible10, the reaction container20, and the gas supply pipe91so that internal pressure of each of the crucible10, the reaction container20, and the gas supply pipe91reaches 1.01 MPa (step S14).

The heating units50and60heat the crucible10and the reaction. container20to 800° C. (a crystal growth temperature) (step S15), and the heater/cooler70controls a temperature of the alkali metal melt280at which vapor pressure PNaof metal Na evaporated from the alkali metal melt280substantially corresponds to vapor pressure PNa—Gaof metal Na evaporated from the melt mixture270(step S16).

The heater/cooler80controls a temperature of an area, i.e., the condensing area90A, which is around the space24of the crucible10and the reaction container20in the gas supply pipe90that supplies nitride gas, to the condensation temperature Tcoh (step S17).

At this point, because a melting point of metal Na stored in the melt reservoir23is about 98° C., the metal Na melts during a process of heating the melt reservoir23to the evaporation suppression temperature Tevc, forming the alkali metal melt280. As a result, a gas-liquid interface1(seeFIG. 2) is generated. The gas-liquid interface1is positioned at an interface between the space24in the reaction container20and the alkali metal melt280.

During a process of heating the crucible10and the reaction container20to 800° C., metal Na and metal Ga present in the crucible10is condensed to be liquid, generating the melt mixture270containing metal Na and metal Ga in the crucible10. The floating cover300floats on the generated melt mixture270, forming a clearance between the melt mixture270and the supporting unit40.

When a temperature of the melt reservoir23is reaching the evaporation suppression temperature Tevc, and a temperature of the crucible10is reaching 800° C., the vapor pressure PNaof metal Na evaporated from the alkali metal melt280and the vapor pressure PNa—Gaof metal Na evaporated from the melt mixture270gradually increases, increasing metal Na vapor present in the space24of the reaction container20. Portion of metal Na vapor present in the space24is diffused to the condensing area90A of which temperature is lower than that of the crucible10and the reaction container20, and condensed as the alkali metal melt290in the condensing area90A. At this point, the vapor pressure PNasubstantially corresponds to the vapor pressure PNa—Ga.

The shifting mechanism240dips the seed crystal5into the melt mixture270in a manner described above (step S18). When the temperature of the crucible10and the reaction container20is about 800° C., which is a high-temperature state, nitride gas present in the space24is absorbed into the melt mixture270through medium of metal Na. Accordingly, crystal growth from the seed crystal5to the GaN crystal6is started. The temperature of the crucible10and the reaction container20, and nitride gas pressure in the reaction container20are such that are present in the region REG3described in connection withFIG. 11.

Subsequent1y, the temperature of the crucible10and the reaction container20is maintained at 800° C., a temperature of the alkali metal melt280in the melt reservoir23is maintained at the evaporation suppression temperature Tevc, and a temperature of the condensing area90A is maintained at the condensation temperature Tcoh, for a predetermined time period, i.e., a few hours (step S19).

When crystal growth of the GaN crystal6is started, the temperature T5of the GaN crystal6is. set to be the temperatures T1or T2, which is lower than that of the melt mixture270(800° C.), in a manner described above (step S20).

When growth of the GaN crystal6proceeds, nitride gas in the space24is consumed, resulting in reducing amount of the nitride gas in the space24. Accordingly, internal pressure P1in the space24gets lower than internal pressure P2in the gas supply pipe90(i.e., P1<P2), generating differential pressure between the space24and the gas supply pipe90. Due to the differential pressure, nitride gas in the gas supply pipe90is supplied into the space24through medium of the alkali metal melt290, i.e., metal Na melt. In other words, nitride gas is supplied into the space between the crucible10and the reaction container20(step S21). At this point, even when the alkali metal melt290is present in such a manner that the alkali metal melt290blocks entire cross sectional surface in a-axis direction of the gas supply pipe90, it is possible to introduce nitride gas into the space24by sweeping aside the alkali metal melt290by using the differential pressure of the nitride gas because the alkali metal melt is liquid.

Subsequent1y, the GaN crystal6is shifted in a manner described above so that the GaN crystal6comes in contact with the melt mixture270(step S22). As a result, the GaN crystal6in a large size is formed.

After a predetermined time has elapsed, the temperature of the crucible10and the reaction container20is lowered (step S23), terminating formation of the first GaN crystal using flux method, and process control proceeds to step S2.

After step S23, a temperature of the melt reservoir23is maintained at the condensation temperature Tcoh, and a temperature of the condensing area90A is maintained at the evaporation acceleration temperature Tev. As a result, the alkali metal melt290is transported from the condensing area90A to the melt reservoir23by chemical vapor transport. Subsequent1y, a GaN crystal is formed by the flux method in accordance with a flowchart shown inFIG. 14.

As described above, a material for forming a GaN crystal by the flux method is supplied in such a state that the alkali metal melt290condensed in the condensing area90A is transported to the melt reservoir23.

Although it is explained that the first GaN crystal is formed at the temperature-pressure condition in the region REG3, it is possible to form the first GaN crystal at the temperature-pressure condition of the region REG2or REG4. In this case, the seed crystal5is not fixed at one end of the supporting unit40, and the one end is alternatively in contact with a surface of the melt mixture270or dipped into the melt mixture270. Then, a plurality of crystal cores are formed on the one end of the supporting unit40, the crystal cores are formed into one crystal by geometrical selection, resulting in growing the GaN crystal6.

When using the temperature-pressure condition in the region REG2or REG3, it is possible to grow the GaN crystal6on a bottom surface or a side surface of the crucible10that is in contact with the melt mixture270as well as on the one end of the supporting unit40. Furthermore, when using the temperature-pressure condition in the region REG4, a crystal core is not attached to a bottom surface of the floating cover300and an internal surface of the crucible10, so that it is possible to shift up and down the floating cover300in accordance with increase or decrease of amount of the melt mixture270.

FIG. 15is a flowchart for explaining detailed procedures at step S3shown inFIG. 13. After step S2, the sliced and polished first GaN crystal is maintained on the susceptor507(step S31), and metal Ga is input into the Ga storage unit505.

The heater502heats the reactor501to a temperature between 800° C. and 1050° C. (step S32), and material gas (Nh3+H2, HCl+H2) is supplied into the reactor501(step S33)

The second GaN crystal is grown on the first GaN crystal (step S34), and a bulked GaN crystal is formed, terminating a process at step S3and proceeding process control to step S4.

It is possible to produce a number of GaN substrates by forming a plurality of GaN wafers from the first GaN crystal, and by introducing the GaN wafers in a plurality of the crystal growing units500, respectively, to form a bulked GaN crystal.

Furthermore, it is possible to repeatedly use the first GaN crystal by exclusively slicing a portion corresponding to the second GaN crystal when a bulked GaN crystal is sliced.

Moreover, it is possible to use a GaN wafer sliced and polished from the second GaN crystal that has grown by the HVPE method as an HVPE substrate. In other words, when a GaN crystal having low dislocation density and less tilt grain boundary is used as a substrate, a GaN crystal grown on the substrate also has low dislocation density and less tilt grain boundary, so that it is possible to repeatedly use the second GaN crystal as a substrate.

Furthermore, it is possible to produce a bulked GaN crystal by growing the second GaN crystal on the first GaN crystal without slicing the first GaN crystal.

In this case, a process performed at step S2is omitted, and the first GaN crystal formed by the flux method is maintained on the susceptor507until process control proceeds to step S31.

A process of forming the first GaN crystal at the temperature-pressure condition in the region REG3, and producing a GaN substrate having an m-plane as a principal surface after slicing the first GaN crystal is explained with reference toFIGS. 17A to 17E.

A hexagonal columnar GaN crystal800(first GaN crystal) elongated in a c-axis direction is formed from a seed crystal at the temperature-pressure condition in the region REG3by the flux method (FIG. 17A).

The GaN crystal800is sliced along the m-plane, and polished to form a GaN wafer801having the m-plane as a principal surface (FIG. 17B).

A bulked GaN crystal870is produced by growing a GaN crystal810(second GaN crystal) on the m-plane of the GaN wafer801by the HVPE method (FIG. 17C).

The bulked GaN crystal870is sliced along the m-plane (FIG. 17D), and polished to produce GaN substrates811to820, each having the m-plane as a principal surface (FIG. 17E).

A process of forming the first GaN crystal at the temperature-pressure condition in the region REG3, and producing a GaN substrate having an a-plane as a principal surface after slicing the first GaN crystal is explained with reference toFIGS. 18A to 18E.

A hexagonal columnar GaN crystal800(first GaN crystal) elongated in a c-axis direction is formed from a seed crystal at the temperature-pressure condition in the region REG3by the flux method (FIG. 18A).

The GaN crystal800is sliced along the a-plane, and polished to form a GaN wafer802having the a-plane as a principal surface (FIG. 18B).

A bulked GaN crystal880is produced by growing a GaN crystal830(second GaN crystal) on the a-plane of the GaN wafer801by the HVPE method (FIG. 18C).

The bulked GaN crystal880is sliced along the a-plane (FIG. 18D), and polished to produce GaN substrates831to840, each having the a-plane as a principal surface (FIG. 18E).

A process of forming the first GaN crystal at the temperature-pressure condition in the region REG3, and producing a GaN substrate having a c-plane as a principal surface after slicing the first GaN crystal is explained with reference toFIGS. 19A to 19E.

A hexagonal columnar GaN crystal800(first GaN crystal) elongated in a c-axis direction is formed from a seed crystal at the temperature-pressure condition in the region REG3by the flux method (FIG. 19A).

The GaN crystal800is sliced along the c-plane, and polished to form a GaN wafer803having the a-plane as a principal surface (FIG. 19B).

A bulked GaN crystal890is produced by growing a GaN crystal850(second GaN crystal) on the c-plane of the GaN wafer801by the HVPE method (FIG. 19C).

The bulked GaN crystal890is sliced along the c-plane (FIG. 19D), and polished to produce GaN substrates851to857, each having the c-plane as a principal surface (FIG. 19E).

A process of forming the first GaN crystal at the temperature-pressure condition in the region REG4, and producing a GaN substrate having a c-plane as a principal surface without slicing the first GaN crystal is explained with reference toFIGS. 20A to 20D.

A large plate-shaped GaN. crystal804(first GaN crystal) is formed by floating a crystal on a surface of the melt mixture270at the temperature-pressure condition in the region REG4by the flux method (FIG. 20A).

The c-plane of the GaN crystal804is polished.

A bulked GaN crystal895is produced by growing a GaN crystal860(second GaN crystal) on the c-plane of the GaN crystal804by the HVPE method (FIG. 20B).

The bulked GaN crystal895is sliced along the c-plane (FIG. 20C), and polished to produce GaN substrates861to867, each having the c-plane as a principal surface (FIG. 20D).

A process of forming the first GaN crystal at the temperature-pressure condition in the region REG3, and producing a GaN substrate having a c-plane as a principal surface without slicing the first GaN crystal is explained with reference toFIGS. 21A to 21D.

A hexagonal columnar GaN crystal900(first GaN crystal) elongated in a c-axis direction is formed from a seed crystal at the temperature-pressure condition in the region REG3by the flux method (FIG. 21A).

A bulked GaN crystal910containing the GaN crystal is produced by growing a GaN crystal (second GaN crystal) on the GaN crystal900by the HVPE method (FIG. 21B).

The bulked GaN crystal910is sliced along the c-plane (FIG. 21C), and polished to produce GaN substrates911to922, each having the c-plane as a principal surface (FIG. 21D).

A process of forming the first GaN crystal at the temperature-pressure condition in the region REG3, and producing a GaN substrate having an m-plane as a principal surface without slicing the first GaN crystal is explained with reference toFIGS. 22A to 22D.

The hexagonal columnar GaN crystal900(first GaN crystal) elongated in a c-axis direction is formed from a seed crystal at the temperature-pressure condition in the region REG3by the flux method (FIG. 22A).

The bulked GaN crystal910containing the GaN crystal900is produced by growing a GaN crystal (second GaN crystal) on the GaN crystal900by the HVPE method (FIG. 22B).

The bulked GaN crystal910is sliced along the m-plane (FIG. 22C), and polished to produce GaN substrates930to933, each having the m-plane as a principal surface (FIG. 22D).

A process of forming the first GaN crystal at the temperature-pressure condition in the region REG3, and producing a GaN substrate having an a-plane as a principal surface without slicing the first GaN crystal is explained with reference toFIGS. 23A to 23D.

The hexagonal columnar GaN crystal900(first GaN crystal) elongated in a c-axis direction is formed from a seed crystal at the temperature-pressure condition in the region REG3by the flux method (FIG. 23A).

The bulked GaN crystal910containing the GaN crystal900is produced by growing a GaN crystal (second GaN crystal) on the GaN crystal900by the HVPE method (FIG. 23B).

The bulked GaN crystal910is sliced along the a-plane (FIG. 23C), and polished to produce GaN substrates930to933, each having the a-plane as a principal surface (FIG. 23D).

A process of forming the first GaN crystal at the temperature-pressure condition in the region REG4, and producing a GaN substrate having a c-plane as a principal surface without slicing the first GaN crystal is explained with reference toFIGS. 24A to 24D.

A platelet GaN crystal901(first GaN crystal) is formed by floating a crystal on a surface of the melt mixture270at the temperature-pressure condition in the region REG4by the flux method (FIG. 24A).

A bulked GaN crystal950containing the GaN crystal901is produced by growing a GaN crystal (second GaN crystal) on the GaN crystal901by the HVPE method (FIG. 24B).

The bulked GaN crystal950is sliced along the c-plane (FIG. 24C), and polished to produce GaN substrates951to958, each having the c-plane as a principal surface (FIG. 24D).

In such a condition that the first GaN crystal is in plate shape, it is possible that a principal crystal surface of the first GaN crystal, and a principal crystal surface of a GaN substrate sliced from a bulked GaN crystal can be different from each other. In other words, it is possible to produce a GaN substrate with a desired principal crystal surface.

As described above, according to the crystal producing apparatus1000, a crystal producing method of the present invention is realized in above explained processes of producing a bulked GaN crystal. Furthermore, a substrate producing method of the present invention is realized in above explained processes of producing a GaN substrate.

The crystal producing apparatus1000includes the crystal forming unit100that forms the first GaN crystal by supplying nitride gas into melt mixture containing alkali metal Na and group-III metal Ga, and the crystal growing unit500that grows the second GaN crystal on the first GaN crystal by the HVPE method. Because the first GaN crystal is homogeneous and has low dislocation density, which is equal to or smaller than 105cm−2, in a preferable quality, and the second GaN crystal is grown on such first GaN crystal, the second GaN crystal that is homogeneous and has low dislocation density can be grown. Therefore, it is possible to produce a bulked GaN crystal that is homogeneous and has low dislocation density with less cost.

A speed of crystal growth of the second GaN crystal can be set faster than that of the first GaN crystal. As a result, it is possible to produce a bulked GaN crystal that is homogeneous and has low dislocation density with a short manufacturing time.

The GaN substrate is formed by slicing a bulked GaN crystal that is homogeneous and has low dislocation density. Therefore, it is possible to produce a GaN substrate in preferable quality and having a large area with a desired crystal principal surface, such as c-plane. (polar face), m-plane (non-polar face), a-plane (non-polar face), or other non-polar faces. Thus, it is possible to realize a nitride gallium substrate having a desired principal surface in preferable quality with less manufacturing costs.

Although it is explained that the crystal growth temperature in the flux method is 800° C. in the above embodiment, the crystal growth temperature in the flux method can be such that is within the regions REG2and REG3. Furthermore, nitride gas pressure can be such that is within the regions REG2and REG3.

The crystal forming unit100can be such that does not include the vibration applying unit230, the shifting mechanism240, and the vibration detecting unit250. In such situation, the GaN crystal6is not shifted upward or downward, and alternatively, the supporting unit40is set in such a manner that the bottom surface41B of the supporting unit40comes in contact with the melt mixture270containing melted metal Na and melted metal Ga in the crucible10. Therefore, the GaN crystal6is grown from the seed crystal5. As a result, it is possible to form the GaN crystal6in a large size.

Furthermore, the crystal forming unit100can be such that does not include the pipe180, the thermocouple190, the gas supply pipe200, the flowmeter210, and the gas canister220. In such situation, the temperature T5of the GaN crystal6is not controlled to be lower than that of the melt mixture270. However, the GaN crystal6is in contact with the melt mixture270by the supporting unit40, so that the GaN crystal6is grown from the seed crystal5in such a state that variation in mixture ratio between metal Na and metal Ga is controlled. As a result, it is possible to form the GaN crystal6in a large size.

The crystal forming unit can be such that does not include the pipe180, the thermocouple190, the gas supply pipe200, the flowmeter210, the gas canister220, the vibration applying unit230, the shifting mechanism240, and the vibration detecting unit250. In such situation, the GaN crystal6is not shifted upward or downward, and the temperature T5of the GaN crystal6is not controlled to be lower than that of the melt mixture270. However, the GaN crystal6is supported by the supporting unit40in such a manner that the GaN crystal6comes in contact with the melt mixture270containing melted metal Na and melted metal Ga in the crucible10. Therefore, the GaN crystal6is grown from the seed crystal5in such a state that variation in mixture ratio between metal Na and metal Ga is controlled. As a result, it is possible to form larger GaN crystal.

According to the embodiment, it is explained that metal Na and metal Ga are input in the crucible10in Ar gas atmosphere, and metal Na is input into the melt reservoir23and the condensing area90A in Ar gas atmosphere. However, it is possible to input metal Na and metal Ga into the crucible, and input metal Na into the melt reservoir23and the condensing area90A in other gas atmosphere, such as helium (He), neon (Ne), or krypton (Kr), instead of Ar atmosphere. It is normally preferable to input metal Na and metal Ga into the crucible10, and input metal Na into the melt reservoir23and the condensing area90A, in inactive gas atmosphere or in nitride gas atmosphere. In such situation, inactive gas atmosphere or nitride gas atmosphere is such that amount of moisture is equal to or less than 1 ppm, and amount of oxygen is equal to or less than 1 ppm.

Furthermore, although it is explained that metal Ga and metal Na are mixed with each other, it is possible to mix metal Ga with other alkali metal, such as lithium (Li) or potassium (K), or alkaline earth metal, such as magnesium (Mg), calcium (Ca), or strontium (Sr).

Moreover, it is possible to use other compounds containing nitride, such as sodium azide or ammonia, instead of nitride gas.

Furthermore, although Ga is explained as group-III metal, it is possible to use boron (B), aluminum (Al), or indium (In) as group-III metal.

In other words, the crystal forming unit works sufficient1y as long as the crystal forming unit forms group-III nitride crystal using melt mixture in which alkali metal or alkaline earth metal is mixed with group-III metal (including boron).

It is possible to use nitride gas or other mixed gas instead of H2as carrier gas in the HVPE method. The crystal growth temperature can be others as long as a GaN crystal can be grown.

Furthermore, although it is explained that the HVPE method is employed as a chemical vapor method, other chemical vapor methods are applicable. For example, it is possible to apply other chemical vapor methods, such as gallium hydride vapor phase epitaxy (GaH-VPE) in which gallium hydride (GaH) and ammonia are used as material, a method of growing GaN by carbothermal reduction and nitridation of gallium oxide (Ga2O3) and causing Ga2O3to react with ammonia, or sublimation method.

A GaN substrate produced by the crystal producing apparatus1000is used for forming semiconductor devices such as light emitted diodes, semiconductor lasers, photodiodes, or transistors.

According to an aspect of the present invention, in a crystal producing apparatus and a crystal producing method, a first group-III nitride crystal is formed by the flux method, and a second group-III nitride crystal is grown on the first group-III nitride crystal by chemical vapor method. Because the first group-III nitride crystal is homogeneous and in preferable quality, and the second group-III nitride crystal is grown on such first group-III nitride crystal, it is possible to grow a homogeneous second group-III nitride crystal in desired quality. Therefore, it is possible to produce a homogeneous bulked group-III nitride crystal in desired quality with less cost.

According to another aspect of the present invention, a bulked group-III nitride crystal produced in the crystal producing method is sliced. As a result, it is possible to produce a group-III nitride substrate having a desired principal surface with less cost.

According to still another aspect of the present invention, a second nitride gallium crystal is grown on a homogeneous first nitride gallium crystal in desired quality, and is formed by the flux method. Therefore, it is possible to produce a homogeneous bulked nitride gallium crystal in desired quality with less manufacturing costs.

According to still another aspect of the present invention, a bulked nitride gallium crystal is sliced to form a nitride gallium substrate. As a result, it is possible to produce a nitride gallium substrate having a desired principal surface in desired quality with less cost.