Patent Application
20100307418
The present invention relates to a vapor phase epitaxy apparatus (MOCVD apparatus) for a group III nitride semiconductor, and more specifically, to a vapor phase epitaxy apparatus for a group III nitride semiconductor including a susceptor for holding a substrate, a heater for heating the substrate, a raw material gas-introducing portion, a reactor, and a reacted gas-discharging portion.
A metal organic chemical vapor deposition method (MOCVD method) has been employed for the crystal growth of a nitride semiconductor as frequently as a molecular beam epitaxy method (MBE method). In particular, the MOCVD method has been widely employed in apparatuses for the mass production of compound semiconductors in the industrial community because the method provides a higher crystal growth rate than the MBE method does and obviates the need for a high-vacuum apparatus or the like unlike the MBE method. In recent years, in association with widespread use of blue or ultraviolet LEDs and of blue or ultraviolet laser diodes, numerous researches have been conducted on increases in apertures and number of substrates each serving as an object of the MOCVD method in order that the mass productivity of gallium nitride, gallium indium nitride, and gallium aluminum nitride may be improved.
Such vapor phase epitaxy apparatuses are, for example, vapor phase epitaxy apparatuses each having a susceptor for holding a substrate, an opposite face of the susceptor, a heater for heating the substrate, a reactor formed of a gap between the susceptor and the opposite face of the susceptor, a raw material gas-introducing portion for providing the reactor with a raw material gas, and a reacted gas-discharging portion as described in Patent Documents 1 to 6. In addition, the following two kinds have been mainly proposed for the form of the vapor phase epitaxy apparatus. That is, a form in which a crystal growth surface is directed upward (face-up type) and a form in which a crystal growth surface is directed downward (face-down type) have been proposed. In the vapor phase epitaxy apparatus of each form, a substrate is installed horizontally and a raw material gas is introduced from a lateral direction of the substrate.
[Patent Document 1] JP 11-354456 A
[Patent Document 2] JP 2002-246323 A
[Patent Document 3] JP 2004-63555 A
[Patent Document 4] JP 2006-70325 A
[Patent Document 5] JP 2007-96280 A
[Patent Document 6] JP 2007-243060 A
An organometallic compound gas as a raw material for a group III metal and ammonia as a nitrogen source have been generally used as raw material gases for a group III nitride semiconductor. Those raw material gases are introduced from bombs for raw materials and the like into a reactor through tubes independent of each other with their flow rates each adjusted by a massflow controller. For example, Patent Document 4 discloses that, with regard to a face-down type vapor phase epitaxy apparatus, an organometallic compound and ammonia as raw materials are mixed immediately in front of a substrate in a reactor before being used in a reaction.
When the organometallic compound and ammonia are mixed immediately in front of the substrate as described above, however, these raw material gases are not sufficiently mixed even on the surface of the substrate, and hence it becomes difficult to perform crystal growth over the entirety of the substrate uniformly. In view of the foregoing, the following vapor phase epitaxy apparatus has been proposed in, for example, Patent Document 3. In the vapor phase epitaxy apparatus described in the document, a gas channel is designed so that ammonia and an organometallic compound may be mixed in advance before being supplied to a reactor and the mixed gas may be supplied to a substrate. However, even the invention has not solved the following problem. That is, the growth reaction rate of a crystal is slow when crystal growth is performed.
Vapor phase epitaxy apparatuses are mainly used in crystal growth for LED's, ultraviolet laser diodes, or electronic devices. In addition, as described above, the apertures of substrates serving as objects of the crystal growth have been increasing in recent years in order that the productivity of the crystal growth may be improved. However, an increase in size of each of the substrates involves the following problem. That is, the growth reaction rate of a group III nitride semiconductor on the substrate slows down and the uniformity of a crystalline film thickness distribution in the surface of the substrate deteriorates.
In addition, another problem arises. That is, the number of channels for the selection of gas flow rate conditions for crystal growth is small. In recent years, group III nitride semiconductors have shown remarkable development, and their crystal structures have become more and more complicated because additionally good performance has been requested. For example, a blue LED formed of the simplest structure is formed of n-type GaN, InGaN, GaN, AlGaN, and p-type GaN. In addition, a superlattice structure has also been frequently used in recent years for the purpose of additionally increasing the output of an LED. Raw material gas conditions for obtaining crystals each having good film quality vary in those various layers, and the flow rate of a raw material gas is optimized in each layer. As described above, however, one introducing tube is provided for each of ammonia and an organometallic compound in a vapor phase epitaxy apparatus that has been conventionally well known, and hence the optimization of a gas flow rate is largely restricted. In other words, an optimum condition has been determined by changing the absolute value of the flow rate of each of ammonia and the organometallic compound. However, it is hard to say that each layer grows under an optimum condition by such method in which the number of selection channels is small.
Therefore, a problem to be solved by the present invention is to provide a vapor phase epitaxy apparatus which: can realize a high growth reaction rate of a group III nitride semiconductor on a substrate and a good crystalline film thickness distribution in the surface of the substrate (film thickness uniformity); and has a large number of channels for the selection of raw material gas flow rate conditions.
The inventors of the present invention have made various studies with a view to obtaining a vapor phase epitaxy apparatus capable of growing a group III nitride semiconductor with good reaction efficiency in view of such circumstances. As a result, the inventors have found such a fact as described below. When a vapor phase epitaxy reactor is constituted so as to include a first mixed gas ejection orifice capable of ejecting a mixed gas obtained by mixing three kinds, i.e., ammonia, an organometallic compound, and a carrier gas at an arbitrary ratio, and a second mixed gas ejection orifice capable of ejecting two or three kinds selected from ammonia, the organometallic compound, and the carrier gas at an arbitrary ratio, optimum conditions for respective layers such as GaN, InGaN, and AlGaN can be easily controlled, and as a result, a high crystal growth rate and a good crystalline film thickness distribution in a surface can be obtained. Thus, the inventors have reached a vapor phase epitaxy apparatus of a group III nitride semiconductor of the present invention.
That is, the present invention is a vapor phase epitaxy apparatus of a group III nitride semiconductor, the apparatus having: a susceptor for holding a substrate; an opposite face of the susceptor; a heater for heating the substrate; a reactor formed of a gap between the susceptor and the opposite face of the susceptor; a raw material gas-introducing portion for supplying a raw material gas to the reactor; and a reacted gas-discharging portion, in which the raw material gas-introducing portion includes a first mixed gas ejection orifice capable of ejecting a mixed gas obtained by mixing three kinds, i.e., ammonia, an organometallic compound, and a carrier gas at an arbitrary ratio, and a second mixed gas ejection orifice capable of ejecting two or three kinds selected from ammonia, the organometallic compound, and the carrier gas at an arbitrary ratio.
The vapor phase epitaxy apparatus of the present invention is constituted so as to include the first mixed gas ejection orifice capable of ejecting the mixed gas obtained by mixing three kinds, i.e., ammonia, the organometallic compound, and the carrier gas at an arbitrary ratio, and the second mixed gas ejection orifice capable of supplying two or three kinds selected from ammonia, the organometallic compound, and the carrier gas at an arbitrary ratio to the reactor. As a result, the mixed gas in which the flow rate and concentration of each gas are optimally controlled can be supplied from each of the first mixed gas ejection orifice and the second mixed gas ejection orifice (which may hereinafter be abbreviated as “mixed gas ejection orifices”) to the surface of the substrate in the reactor, and optimum conditions can be easily controlled upon crystal growth of the respective layers such as GaN, InGaN, and AlGaN. Accordingly, the uniformity of the film thickness distribution, and reaction rate, of the group III nitride semiconductor can be improved.
The present invention is applied to a vapor phase epitaxy apparatus for a group III nitride semiconductor having a susceptor for holding a substrate, an opposite face of the susceptor, a heater for heating the substrate, a reactor formed of a gap between the susceptor and the opposite face of the susceptor, a raw material gas-introducing portion for providing the reactor with a raw material gas, and a reacted gas-discharging portion. The vapor phase epitaxy apparatus of the present invention is a vapor phase epitaxy apparatus for performing the crystal growth of a nitride semiconductor mainly formed of a compound of one kind or two or more kinds of metals selected from gallium, indium, and aluminum, and nitrogen. In the present invention, an effect can be sufficiently exerted particularly in the case of such vapor phase epitaxy that a plurality of substrates of such sizes as to have diameters of 3 inches or more are held.
Hereinafter, the vapor phase epitaxy apparatus of the present invention is described in detail with reference to
It should be noted that
As illustrated in each of
Here, the first mixed gas ejection orifice and the second mixed gas ejection orifice described above are the ejection orifices of channels for mixed gases of two types independent of each other, and are of constitutions different from such constitutions that mixed gases of the same type are ejected from two ejection orifices.
For example, the raw material gas-introducing portion 6 illustrated in each of
It should be noted that, in the raw material gas-introducing portion of each of
In the raw material gas-introducing portion 6, a portion where the raw material gases are mixed is typically set so as to be in front of the tip of each mixed gas ejection orifice 8 at a distance of 5 cm or more and 100 cm or less. In particular, a site where ammonia and the organometallic compound are mixed is constituted so as to be preferably in front of the tip of each mixed gas ejection orifice 8 at a distance of 5 cm or more and 100 cm or less, or more preferably in front of the tip of the mixed gas ejection orifice 8 at a distance of 10 cm or more and 50 cm or less. When the distance is shorter than 5 cm, the respective raw material gases may not be sufficiently mixed up to the tip of each mixed gas ejection orifice 8. In addition, when the distance is longer than 100 cm, adducts produced from the raw material gases may react with each other to an extent more than necessary. In addition, a diffusing plate or the like can also be used in the portion where the raw material gases are mixed for mixing the raw material gases effectively. It should be noted that, even when the portion where the gases are mixed is to be installed outside the vapor phase epitaxy apparatus in such case as described above, the portion where the gases are mixed can be regarded as part of the vapor phase epitaxy apparatus of the present invention.
In addition, the number of the mixed gas ejection orifices 8 in the raw material gas-introducing portion 6 is not limited to two, and any number of the ejection orifices may be used as long as the number is two or more. When an excessively large number of the ejection orifices are provided, however, an investigation on the optimization of the flow rate of a raw material gas requires a long time period. In addition, the structure of the raw material gas-introducing portion 6 becomes complicated. Even in the case where the number of the ejection orifices is four or more, influences on the growth rate of crystal growth and film thickness uniformity in the surface of the substrate remain nearly unchanged as compared with those in the case where the number of the ejection orifices is three. By reason of the foregoing, the number of the mixed gas ejection orifices 8 is preferably two or three. In the case where the number of the ejection orifices is three or more, a tube for a gas containing ammonia, a tube for a gas containing the organometallic compound, and a tube for the carrier gas are installed in the gas channels through respective massflow controllers as in the case where the number of the ejection orifices is two.
Further, as illustrated in each of
The gas ejection orifices (the mixed gas ejection orifices 8 or the mixed gas ejection orifices 8 and the carrier gas ejection orifice 17) can be sequentially provided in a vertical direction. As illustrated in each of
The raw material gas-introducing portion 6 in the present invention is preferably provided with means (equipment) for cooling each of the mixed gas ejection orifices 8 and the carrier gas ejection orifice 17. In the vapor phase epitaxy of a group III nitride semiconductor, the inside of the reactor is typically heated to about 700° C. to about 1200° C. for crystal growth. Accordingly, the temperature of the raw material gas-introducing portion 6 also increases to about 600° C. to about 1100° C. unless cooling is performed. As a result, the raw material gases decompose in the raw material gas-introducing portion 6. In order that the decomposition may be suppressed, as illustrated in each of
However, a method of cooling each mixed gas ejection orifice 8 is not limited to such means as described above. That is, a method involving providing the cooling means for the uppermost portion of the raw material gas-introducing portion 6 or a method involving partially bonding the respective sites of the raw material gas-introducing portion 6 with a member having good thermal conductivity and providing the cooling means for one site of the raw material gas-introducing portion 6 to perform the cooling so that all members of the raw material gas-introducing portion 6 may be indirectly cooled is also permitted instead of the method involving providing the cooling means for the lowermost portion of the raw material gas-introducing portion 6 as illustrated in each of FIGS. 3 to 6.
It should be noted that the form of the susceptor 2 in the present invention is, for example, a disk shape having spaces for holding a plurality of substrates in its peripheral portion as illustrated in
Upon performance of crystal growth on the substrate with the vapor phase epitaxy apparatus of the present invention, the organometallic compound (such as trimethyl gallium, triethyl gallium, trimethyl indium, triethyl indium, trimethyl aluminum, or triethyl aluminum, or a mixed gas of them) and ammonia serving as the raw material gases, and the carrier gas (hydrogen or an inert gas such as nitrogen, or a mixed gas of them) are supplied by the respective external tubes to the raw material gas-introducing portion 6 of such vapor phase epitaxy apparatus of the present invention as described above. Further, the gases are each supplied from the raw material gas-introducing portion 6 to the reactor 5 under substantially optimum flow rate and concentration conditions.
Next, the present invention is described specifically by way of examples. However, the present invention is not limited by these examples.
Such a vapor phase epitaxy apparatus as illustrated in
It should be noted that the raw material gas-introducing portion was of such a constitution as illustrated in
(Vapor Phase Epitaxy Experiment)
Gallium nitride (GaN) was grown on the surfaces of the substrates with such vapor phase epitaxy apparatus. After the circulation of cooling water through the flow channel for flowing a coolant of the opposite face (flow rate: 18 L/min) had been initiated, each substrate was cleaned by increasing the temperature of the substrate to 1050° C. while flowing hydrogen. Subsequently, the temperature of each sapphire substrate was decreased to 510° C., and then a buffer layer formed of GaN was grown so as to have a thickness of about 20 nm on the substrate by using trimethyl gallium (TMG) and ammonia as raw material gases, and hydrogen as a carrier gas.
After the growth of the buffer layer, the supply of only TMG was stopped and the temperature was increased to 1050° C. After that, ammonia (flow rate: 30 L/min) and hydrogen (flow rate: 5 L/min) were supplied from the ejection orifice in an upper layer, TMG (flow rate: 40 cc/min), ammonia (flow rate: 10 L/min), and hydrogen (flow rate: 30 L/min) were supplied from the ejection orifice in a middle layer, and nitrogen (flow rate: 30 L/min) was supplied from the ejection orifice in a lower layer so that undoped GaN might be grown for 1 hour. It should be noted that all growth including that of the buffer layer was performed while each substrate was caused to rotate at a rate of 10 rpm.
After the nitride semiconductor had been grown as described above, the temperature was decreased, and then the substrates were taken out of the reaction vessel. After that, GaN thicknesses were measured. As a result, the GaN thickness at the center of each substrate was 3.95 μm. The foregoing shows that a GaN growth rate at the center of the substrate was 3.95 μm/h. In addition,
Gallium nitride (GaN) was grown on the surfaces of the substrates with the same vapor phase epitaxy apparatus as in Example 1. After the circulation of cooling water through the flow channel for flowing a coolant of the opposite face (flow rate: 18 L/min) had been initiated, each substrate was cleaned by increasing the temperature of the substrate to 1050° C. while flowing hydrogen. Subsequently, the temperature of each sapphire substrate was decreased to 510° C., and then a buffer layer formed of GaN was grown so as to have a thickness of about 20 nm on the substrate by using trimethyl gallium (TMG) and ammonia as raw material gases, and hydrogen as a carrier gas.
After the growth of the buffer layer, the supply of only TMG was stopped and the temperature was increased to 1050° C. After that, ammonia (flow rate: 35 L/min) and hydrogen (flow rate: 5 L/min) were supplied from the ejection orifice in an upper layer, TMG (flow rate: 40 cc/min), ammonia (flow rate: 5 L/min), and hydrogen (flow rate: 30 L/min) were supplied from the ejection orifice in a middle layer, and nitrogen (flow rate: 30 L/min) was supplied from the ejection orifice in a lower layer so that undoped GaN might be grown for 1 hour. It should be noted that all growth including that of the buffer layer was performed while each substrate was caused to rotate at a rate of 10 rpm.
After the nitride semiconductor had been grown as described above, the temperature was decreased, and then the substrates were taken out of the reaction vessel. After that, GaN thicknesses were measured. As a result, the GaN thickness at the center of each substrate was 3.85 μm. The foregoing shows that a GaN growth rate at the center of the substrate was 3.85 μm/h. In addition,
A vapor phase epitaxy apparatus was produced in the same manner as in Example 1 except that the constitution of the raw material gas-introducing portion was changed to such a constitution as illustrated in
After a nitride semiconductor had been grown, the temperature was reduced and each substrate was taken out of a reaction vessel. Then, the thickness of the GaN film was measured. As a result, the thickness of the GaN film at the center of each substrate, a GaN growth rate, the thickness distribution of the GaN film in the surface of a 3-inch substrate, and a fluctuation in film thickness in the surface were substantially identical to those of Example 1. As described above, a crystal having a high crystal growth rate and a good crystalline film thickness distribution in a surface was obtained even in the 3-inch substrate.
A vapor phase epitaxy apparatus was produced in the same manner as in Example 1 except that the constitution of the raw material gas-introducing portion was changed to such a constitution as illustrated in
After a nitride semiconductor had been grown, the temperature was reduced and each substrate was taken out of a reaction vessel. Then, the thickness of the GaN film was measured. As a result, the thickness of the GaN film at the center of each substrate, a GaN growth rate, the thickness distribution of the GaN film in the surface of a 3-inch substrate, and a fluctuation in film thickness in the surface were substantially identical to those of Example 2. As described above, a crystal having a high crystal growth rate and a good crystalline film thickness distribution in a surface was obtained even in the 3-inch substrate.
A vapor phase epitaxy apparatus was produced in the same manner as in Example 1 except that the ejection orifice in the upper layer was changed to an ejection orifice capable of ejecting ammonia and a carrier gas at an arbitrary ratio, the ejection orifice in the middle layer was changed to an ejection orifice capable of ejecting an organometallic compound and a carrier gas at an arbitrary ratio, and the ejection orifice in the lower layer was changed to an ejection orifice capable of ejecting a carrier gas in the production of the vapor phase epitaxy apparatus of Example 1. A horizontal distance between the tip of each gas ejection orifice and a substrate, and the position at which the respective gases were mixed were identical to those of Example 1.
(Vapor Phase Epitaxy Experiment)
Gallium nitride (GaN) was grown on the surfaces of the substrates with such vapor phase epitaxy apparatus. After the circulation of cooling water through the flow channel for flowing a coolant of the opposite face (flow rate: 18 L/min) had been initiated, each substrate was cleaned by increasing the temperature of the substrate to 1050° C. while flowing hydrogen. Subsequently, the temperature of each sapphire substrate was decreased to 510° C., and then a buffer layer formed of GaN was grown so as to have a thickness of about 20 nm on the substrate by using trimethyl gallium (TMG) and ammonia as raw material gases, and hydrogen as a carrier gas.
After the growth of the buffer layer, the supply of only TMG was stopped and the temperature was increased to 1050° C. After that, ammonia (flow rate: 40 L/min) and hydrogen (flow rate: 5 L/min) were supplied from the ejection orifice in an upper layer, TMG (flow rate: 40 cc/min) and hydrogen (flow rate: 30 L/min) were supplied from the ejection orifice in a middle layer, and nitrogen (flow rate: 30 L/min) was supplied from the ejection orifice in a lower layer so that undoped GaN might be grown for 1 hour. It should be noted that all growth including that of the buffer layer was performed while each substrate was caused to rotate at a rate of 10 rpm.
After the nitride semiconductor had been grown as described above, the temperature was decreased, and then the substrates were taken out of the reaction vessel. After that, GaN thicknesses were measured. As a result, the GaN thickness at the center of each substrate was 3.70 μm. The foregoing shows that a GaN growth rate at the center of the substrate was 3.70 μm/h. The value was smaller than the GaN growth rate of each of Example 1 and Example 2. In addition,
As described above, the vapor phase epitaxy apparatus of the present invention can improve the uniformity of the film thickness distribution, and reaction rate, of a group III nitride semiconductor.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2009-138586 | Jun 2009 | JP | national |
