JP3472976B2 - Method and apparatus for forming group III nitride semiconductor - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a group III nitride semiconductor using a metal organic chemical vapor deposition method (MOCVD method), and more particularly to a method for producing mixed crystal AlGaN.

[0002]

2. Description of the Related Art In recent years, GaN-based semiconductors have been vigorously researched and developed as materials for short-wavelength semiconductor devices, especially for short-wavelength semiconductor lasers. As a method for improving the light emission efficiency of this semiconductor laser, LED, etc., a double hetero structure (hereinafter DH structure) or a separation confinement hetero structure (hereinafter SC
H structure) or the like is used. These structures are basically manufactured by laminating materials having different bandgap values.

In group III nitride semiconductors, for example, AlN
When a so-called mixed crystal system such as Al _x Ga _1-x N, which is a solid solution of GaN, is used, the bandgap value according to the value of x is 3.4 eV of GaN.
To AlN up to 6.2 eV can be prepared.
Of the group III nitride semiconductor epitaxial film production methods, the best film quality is obtained by metalorganic chemical vapor deposition (MOCVD method), which produces the mixed crystal epitaxial film such as AlGaN. Well known as a method.

In the film formation of a III-V group compound in the MOCVD method, a group III organic compound (MO) and a group V hydride are introduced as a source gas into a reaction tube and brought into contact with a heated substrate. By reacting the thermal decomposition product of the raw material gas with the raw material gas, a group III-V compound epitaxial film is grown on the substrate. The MOCVD apparatus is roughly divided into a vertical furnace that guides the gas flow in the vertical direction and a horizontal furnace that guides the gas flow in the horizontal direction according to the reaction tube structure.

As shown in FIG. 1, an MOCVD apparatus using a vertical furnace introduces a source gas into a reaction tube 2 from an introduction tube 1 and brings it into contact with a substrate 4 placed on a susceptor 3 from above to perform epitaxial growth. Is to do. Susceptor 3
Can be rotated to improve the homogeneity of the epitaxial film on the substrate. Further, the reacted residual gas and the unreacted source gas are discharged from the lower part of the reaction tube 2. This vertical furnace has the advantage that the structure is relatively simple and the device is small. On the other hand, because a fluid retention point (stagnation point) 5 is formed immediately below the introduction pipe 1 and the gas flow is in the opposite direction to thermal convection, a small experimental furnace to a large-scale apparatus for mass production can be used. It is difficult to expand the design.

As shown in FIG. 2, in the MOCVD apparatus using the horizontal furnace, the surface of the substrate 4 and the flow direction of the raw material gas are parallel to each other, and the direction of the gas flow does not largely change unlike the vertical furnace. Even in the case of this horizontal furnace, the homogeneity of the epitaxial film can be improved by rotating the substrate by the susceptor 3. The horizontal furnace has the disadvantage that the installation area of the device becomes large, but since the shape of the gas flow is simple, it is relatively easy to expand the design from an experimental machine to a practical machine, and to manufacture compound semiconductors. It is widely used as a mass production epitaxial furnace.

[0007] Trimethylgallium (hereinafter TMG) containing group III elements, trimethylaluminum (hereinafter TMA), and trimethylindium (hereinafter TM) are commonly used as a raw material of a group III nitride semiconductor used in the MOCVD method.
Examples include organic metals such as I) and ammonia containing a Group V element (hereinafter NH ₃ ). A mixed crystal epitaxial film can be prepared by reacting two or more kinds of group III element raw materials with a group V element raw material at a predetermined ratio and depositing them on a substrate.

The optical characteristics of a light emitting element such as a semiconductor laser using a mixed crystal system such as Al _x Ga _1-x N are determined by the mixed crystal ratio x of the material and the homogeneity inside the material.

[0009]

MOCVD of nitride semiconductors
A problem in forming a film by the method is a parasitic reaction that occurs between an alkyl compound (TMG, TMA in this case) as a group III raw material and NH ₃ as a group V raw material. The parasitic reaction is a phenomenon in which the raw materials mixed in the reaction furnace start and advance the reaction before they reach the substrate. In AlGaAs system, parasitic reaction hardly occurs between raw materials. In the case of this AlGaAs system, the Al mixed crystal ratio (AlAs ratio: Al _x Ga in the solid phase of the formed epitaxial film)
_1-x As x) is the ratio of TMA in the gas phase {TMA / (TMA + TMG)}
The mixed crystal ratio in the formed solid phase can be controlled by the gas component ratio. On the other hand, in the case of AlGaN system, a significant parasitic reaction occurs and the yield is significantly reduced.
There are problems that reproducibility cannot be obtained in the mixed crystal ratio ( _x when Al _x Ga _1-x N is used), or x value varies inside the film, resulting in poor homogeneity. The reason why the parasitic reaction occurs violently in the AlGaN system is that NH ₃ is a strong Lewis base while the alkyl compound of the group III raw material is a relatively strong Lewis acid. When these molecules meet each other even in the gas phase at about room temperature, they bond with each other and form (CH ₃ ) ₃ -M: NH ₃ (provided that M: Al or
It forms a white solid compound at room temperature called an adduct of the form Ga). The melting temperature is about 56.7 for (CH ₃ ) ₃ -Al: NH _3.
℃, (CH ₃ ) ₃ -Ga: NH ₃ about 30 ℃.

As shown in FIG. 2, it is assumed that an AlGaN-based film is formed by a conventional MOCVD apparatus for AlGaAs-based film formation. As in the case of the AlGaAs system, TMG or TMA and NH ₃ which are the raw material gases are introduced into the pipe 7 upstream of the reactor 6, and when these raw material gases are mixed in the pipe 7, an adduct is deposited inside the pipe 7. Adhere to the wall of the pipe and inflate the pipe 7. Therefore, these source gases are heated to a certain temperature in the reaction furnace 6,
For example, they are mixed at a temperature above the melting temperature of the adduct described above.

By the way, an alloy containing Ga formed from TMG is used.
Even if the duct is finally decomposed, it cannot become GaN.
The adduct containing Al formed from MA is decomposed and finally
Can be AlN. That is, the ada formed in the gas phase
(CH₃)₃Al: NH₃Is heated as it travels through the reactor
When the temperature reaches about 70 ℃, CH_FourDischarge
[(CH₃)₂AlNH₂] Amide species are formed. Furthermore, [(CH
₃)₂AlNH₂]₃It becomes a cyclic trimer compound. This ring
When the compound is heated, CH_FourAre sequentially discharged
To finally form AlN. On the other hand, in the case of Ga,
Corresponding (CH₃)₃Ga: NH ₃Or [(CH₃)₂GaNH₂]₃Such as
The material has a much weaker binding force than that of Al and can be heated.
However, it is difficult to finally reach GaN.

If TMA is mixed with NH ₃ at the reactor inlet, solid AlN is produced in the gas phase. If this reaction is remarkable, a large amount of TMA is introduced into the reaction tube. However, the AlN crystal film does not grow on the substrate and is deposited on the wall surface of the reaction furnace and the pipes and filters arranged downstream. Here, since the AlN formation reaction is a gas phase reaction, the spatial density (product of both densities) of the reactants (here, TMA molecule and NH ₃ molecule) is used as the reaction factor. Therefore, the pressure inside the reaction tube was reduced.
The reaction can be suppressed by using so-called low pressure CVD.

Generally, the organic metal raw material is a methyl group (CH
_3- ) contains a large amount of carbon. If carbon is taken into the formed film as an impurity, various characteristics of the device such as light emission characteristics and electric characteristics are deteriorated. Normally, carbon is
Since a carbon source such as a methyl group is discharged as CH ₄ to the outside by atomic hydrogen supplied from a group V hydride which is a group V raw material, it is not incorporated into the formed film. Therefore, in low pressure (for example, 76 Torr: 1/10 atmospheric pressure) CVD, carbon uptake increases unless a large amount of Group V hydride is introduced. In addition, since Al ₃ O ₃ -based film uses NH ₃ which has low thermal decomposability as a group V material, a large amount of N
H ₃ needs to be introduced. On the other hand, if the NH ₃ partial pressure is increased, the above parasitic reaction is promoted.

As shown in FIG. 3, as a method of reducing the influence of parasitic reaction, the gas flow path in the reaction tube is divided into two by a partition plate so that the source gases are mixed near the substrate on which the film is formed. In addition, (1) the operating pressure is set to normal pressure to maintain the basic film quality, while the flow velocity of the gas flowing in the reaction furnace 6 is increased so that the gas reaches from the mixing start position to the substrate position. In order to shorten the time or (2) reduce the influence of the parasitic reaction, the reactor is operated under reduced pressure, and the deterioration of the film quality due to carbon incorporation is compensated by introducing a large amount of NH ₃ . However, in (1), the gas consumption increases because the gas flow velocity is increased. Further, although the cross-sectional area of the gas flow path may be reduced, it becomes difficult to increase the size of the device for mass production. In (2), N
Even if the H ₃ flow rate is increased, the NH ₃ partial pressure cannot be raised above the pressure in the reaction furnace, so it is impossible to sufficiently reduce the carbon incorporation into the film under reduced pressure.

The present invention has been made in view of the above-mentioned points, and an object thereof is to reduce the influence of a parasitic reaction and increase the flow rate of the raw material gas, and to obtain a homogeneous mixed crystal having a predetermined mixed crystal ratio. A film forming apparatus and a method for forming a group III nitride mixed crystal.

[0016]

A method of forming a group III nitride semiconductor film according to the present invention by a vapor phase epitaxy method is a method in which ammonia gas and organic gallium gas are premixed and guided toward the substrate. An organic aluminum gas is further mixed with the mixed gas in the vicinity of the substrate and is supplied onto the substrate.

A Group III nitride semiconductor film forming apparatus for forming a Group III nitride semiconductor film on a substrate by a vapor phase growth method according to the present invention includes a reaction chamber capable of accommodating the substrate and a gas in the reaction chamber. A first gas supply pipe whose discharge end is open in the vicinity of the upper part of the substrate; and a gas discharge end in the reaction chamber is open in the vicinity of the upper part of the substrate and below the first gas supply pipe. The second
A gas supply pipe, a first introduction pipe for supplying an organoaluminum gas from outside the reaction chamber to a gas introduction end of the first gas supply pipe, and a reaction chamber outside the reaction chamber for a gas introduction end of the second gas supply pipe. And an introduction means for supplying an organic gallium gas and an ammonia gas from the above.

[0018]

[Function] TMG is mixed with NH ₃ to form an adduct, but since the solid adduct does not adhere to the inner wall or the like and is infarcted in the duct because it is at an ambient temperature above the solidification temperature of at least 30 ° C. Absent. Further, since TMG reaches the substrate without passing through a region where AlN solid-phase nuclei are present at a high concentration, Ga can be efficiently deposited on the substrate.

In the present invention, TEA (triethylaluminum) can be used as the organic Al and TEG (triethylgallium) can be used as the organic Ga.

[0020]

According to the present invention, a homogeneous Group III nitride mixed crystal having a predetermined mixed crystal ratio is formed without reducing the influence of parasitic reaction on the film formation and increasing the flow rate of the source gas. Can be membrane.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION As shown in FIG. 4, a quartz glass flow channel 8 having a rectangular cross section through which a raw material gas flows has two flow passages inside, that is, an upper first gas supply pipe 9
And a second gas supply pipe 10 below. TMA and TMG, which are Group III raw materials, are carrier gases (hereinafter carrier gas)
A first gas introduction pipe 11 and / or a second gas supply pipe, which is diluted and transported by the first gas supply pipe 9 and is connected to the first gas supply pipe 9.
The gas is introduced into the flow channel 8 through the second gas introduction pipe 12 connected to 10. NH ₃ which is a group III raw material is introduced into the flow channel 8 from a third gas introduction pipe 13 connected to the second gas supply pipe 10. The sapphire substrate 4 is installed on the susceptor 3 made of graphite.
Is subjected to high frequency heating by an RF coil (not shown) provided outside the quartz reaction tube outer cylinder 14 having a cylindrical cross section. The raw materials TMG, TMA, and NH ₃ are heated on the substrate and AlG
A crystalline film of aN is deposited on the substrate. Also, the reaction tube outer cylinder 14
In order to prevent dirt from adhering to the inner wall and the outer wall of the flow channel, hydrogen gas is flown from the fourth gas introduction pipe 15 between the reaction tube outer cylinder 14 and the flow channel 8 as a counter flow.
The flow rates of all gases such as carrier gas and NH ₃ are strictly controlled by an MFC (mass flow controller). An exhaust pump and a pressure control valve (both not shown) are connected to the downstream side of the reaction furnace to keep the pressure inside the reaction furnace and the flow channel constant regardless of the gas flow rate into the reaction furnace. .

As shown in Table 1, the following three types of comparative experiments were conducted with respect to the type of gas flowing through the gas introduction pipe.

[0023]

[Table 1] Gas introduction pipe conditions Conditions Conditions 1st H ₂ TMA, TMG, H ₂ TMA, H ₂ 2nd TMA, TMG, H ₂ H ₂ TMG, H ₂ 3rd NH ₃ , H ₂ NH ₃ , H ₂ NH ₃ , H ₂ 4th H ₂ H ₂ H ₂ Here, TMA introduction amount is 7 μmol / min (molar flow rate per minute), TMG introduction amount is 37 μmol / min, NH ₃ flow amount is 5 SLM.
(Standard Liter per Minute: volumetric flow rate in standard state), first and second gas supply pipes 9 as carrier gas,
The flow rate of hydrogen gas introduced to 10 is 7 SLM and 10 SLM,
The hydrogen gas flowing as a counter flow was fixed at 8 SLM. The pressure inside the reaction tube outer cylinder 14 was maintained at 300 Torr. The vapor phase ratio of Al under these conditions {TMA / (TMA + TM
G)} is 0.16, and if no parasitic reaction occurs, this ratio
An AlGaN mixed crystal should be deposited. The substrate was not rotated, the substrate temperature was fixed at 1050 ° C., and the film formation time was fixed at 1 hour. The length of the first and second gas supply pipes 9 and 10 is the second
The length is sufficient to ensure that the gases introduced from the second and third gas introduction pipes 12 and 13 are mixed in the gas supply pipe 10. That is, the NH ₃ gas and the TMG gas are introduced into the second gas supply pipe 10 from the second and third gas introduction pipes 12 and 13, and are mixed while flowing in the second gas supply pipe 10, and the open ends of the second gas supply pipe 10 are mixed. The length is such that the concentration distribution of the mixed gas is substantially uniform. This length was set to 20 cm in consideration of the gas flow rate and the like. Further, a stirrer for gas mixing may be installed in the second gas supply pipe 10. Further, the second and third gas introduction pipes 12, 13 and the second
Connection part with the gas supply pipe 10, that is, the second gas supply pipe 10
The temperature at and near the gas introduction end of is always above room temperature, 30 ° C
Maintained above.

Under the conditions, the mixed gas of TMA and TMG is introduced from the second gas introduction pipe 12 into the second gas supply pipe 10, and the third gas
NH ₃ introduced through the gas introduction pipe 13 was mixed at the entrance of the second gas supply pipe 10. When the changes in the Al mixed crystal ratio and the film thickness were measured along the gas flow direction on the center line of the film forming apparatus (here, the Al mixed crystal ratio x was determined by the Vegard's law from the (0002) reflection peak position of the X-ray. The film thickness was measured by SEM (scanning electron microscope). The measurement method is the same.), The growth rate of the film thickness was extremely slow, the parasitic reaction was severe, and almost no Al was formed in the formed film. It was not captured.

Under the conditions, the mixed gas of TMA and TMG is introduced into the first gas supply pipe 9 from the first gas introduction pipe 11, and is supplied from the third gas introduction pipe through the second gas supply pipe 10 immediately before the substrate. It was mixed with NH ₃ being. When the changes in the Al mixed crystal ratio and the film thickness along the gas flow direction on the center line of the film forming apparatus were measured, the film thickness was thin on the upstream side, and the Al mixed crystal ratio was larger than the vapor phase ratio of Al. On the downstream side, on the contrary, the film thickness is large and the Al mixed crystal ratio is Al.
Was smaller than the gas phase ratio of. Since more than 80% of the supplied Group III raw materials are Ga (TMG), even if TMA is lost due to a parasitic reaction, film formation from TMG should be ensured.

Therefore, from the gas supply conditions in the conditions
Deposition experiment excluding only TMA supply (in this case, a binary G
aN will be deposited), and the film was uniformly deposited to a thickness similar to the maximum thickness of the film obtained in the condition experiment. In other words, under the conditions, it is considered that the deposition from TMG is reduced on the upstream side of the film-formed substrate due to the influence of the parasitic reaction caused by TMA. The reason for this is that both TMG and TMA, which are Group III raw materials, diffuse from the upper channel. The TMA diffusing downward from the first gas supply pipe 9 causes a parasitic reaction with NH ₃ in the relatively low temperature region 16 and forms a polymer of TMA and NH _{3 in} the gas phase. As the temperature increases downstream, CH4 is discharged from the polymer, eventually forming very fine AlN solid phase nuclei in the gas stream. On the other hand, TMG and NH ₃ can also form a polymer, but the binding force is
Since it is smaller than that in the case of Al, it decomposes when approaching the high temperature region, and in the vapor phase it is in a non-equilibrium state without forming the solid-state nuclei of GaN. However, not only AlN but also non-equilibrium T
Polymers of MG and NH ₃ can also precipitate GaN. Therefore, when both TMG and TMA are supplied from the side far from the substrate, they cannot reach the substrate without passing through the low temperature region 16, and not only AlN but also GaN.
Also precipitates in the vapor phase, and the semiconductor film is not formed on the substrate. The substrate is heated to a high temperature around 1000 ° C., and a large temperature gradient is generated in the gas flow on the substrate. The particles in the gas with a large temperature gradient are subjected to the force (Soret effect) to discharge them from the high temperature region, so the particles generated by the parasitic reaction are the inner walls of the water-cooled reactor tube and It adheres to the wall surface of the exhaust pipe and hardly deposits on the substrate. further,
On the upstream side, since the supply amount of the group III element raw material is reduced, GaN with weak bonding force becomes more dominant than evaporation, and only AlN with strong bonding force is deposited, resulting in a high Al mixed crystal ratio. it is conceivable that.

The conditions were made in view of the results of the conditions, and only TMA is transferred from the first gas supply pipe to the first gas supply pipe 9, and TMG is transferred from the second gas supply pipe to the second gas supply pipe 1.
Supplied to 0. Because TMG is mixed with NH ₃ in the reactor inlet portion, (CH _{3) 3} -M: may form the shape of the adduct of NH _3,
Since the temperature of the portion is kept at room temperature or higher, which is the melting point of the adduct, that is, at 30 ° C. or higher, the adduct does not deposit on the inner wall of the flow channel. The TMA flowing through the first gas supply pipe 9 comes into contact with NH ₃ introduced from the second gas supply pipe 10 in the low temperature region 16,
A parasitic reaction that forms an adduct is generated, and this is further decomposed to generate fine solid-phase nuclei.However, TMG can reach the substrate without passing through the low-temperature region 16 in which the solid-state nuclei exist. Does not deposit GaN. When the changes in the Al mixed crystal ratio and the film thickness along the gas flow direction on the center line of the film forming apparatus were measured, an almost constant mixed crystal ratio and film thickness were obtained over a range of about 2 inches. The mixed crystal ratio is the above-mentioned Al vapor phase ratio of 0.16,
It was uniform over the entire membrane. The film was evenly formed to a thickness similar to the maximum film thickness of the film obtained in the condition experiment.

As another embodiment of the present invention, as shown in FIG. 5, the second gas introducing pipe 12 and the third gas introducing pipe 13 are connected to one end of the common introducing pipe 17 outside the reaction tube outer cylinder 14. It The other end of the common introduction pipe 17 is connected to the second gas supply pipe 12.
Here, TMG is supplied to the second gas introduction pipe 12 and N is supplied to the third gas introduction pipe 13.
Introduce H ₃ . Second gas introduction pipe 12 and third gas supply pipe 13
A heating chamber 18 having a heater (not shown) is provided so as to surround the connection portion of the heating chamber 18, and the temperature in the heating chamber 18 is equal to or higher than the room temperature of the mixed gas in the common introducing pipe, that is, 30
Set to heat above ℃. Since the heated gas is guided to the second supply pipe without lowering to room temperature or lower, that is, 30 ° C. or lower, adhesion of the generated adduct to the pipe wall is prevented.

[Brief description of drawings]

FIG. 1 is a sectional view conceptually showing a semiconductor film forming apparatus.

FIG. 2 is a sectional view conceptually showing a semiconductor film forming apparatus.

FIG. 3 is a sectional view conceptually showing a semiconductor film forming apparatus.

FIG. 4 is a sectional view of a semiconductor film forming apparatus of the present invention.

FIG. 5 is a cross-sectional view of a semiconductor film forming apparatus of the present invention.

[Explanation of symbols for main parts]

1 Introductory pipe 2 reaction tubes 3 susceptor 4 substrates 5 retention point 6 Reactor 7 piping 8 flow channels 9 First gas supply pipe 10 Second gas supply pipe 11 First gas introduction pipe 12 Second gas introduction pipe 13 Third gas introduction pipe 14 Reaction tube outer cylinder 15 Fourth gas introduction pipe 16 Low temperature region 17 Common introduction tube 18 heating chamber

─────────────────────────────────────────────────── --Continued from the front page (56) References JP-A-6-216030 (JP, A) JP-A-2-211620 (JP, A) JP-A-2-291114 (JP, A) JP-A-9- 266173 (JP, A) (58) Fields surveyed (Int.Cl. ⁷ , DB name) H01L 21/205 C23C 16/34 C23C 16/455 H01L 33/00

Claims (9)

(57) [Claims] 1. A method for forming a group III nitride semiconductor film by a vapor phase epitaxy method, which comprises mixing an ammonia gas and an organic gallium gas in advance and guiding the mixed gas toward a substrate. A method of forming a Group III nitride semiconductor, further comprising: mixing an organoaluminum gas in the vicinity of the substrate and supplying the mixture onto the substrate. 2. The organic aluminum gas is TMA (trimethylaluminum) or TEA (triethylaluminum), and the organic gallium gas is TMG.
The film forming method for a group III nitride semiconductor according to claim 1, wherein the film is (trimethylgallium) or TEG (triethylgallium). 3. The method for forming a Group III nitride semiconductor according to claim 2, wherein the temperature of the mixed gas of the ammonia gas and the organic gallium gas is maintained at 30 ° C. or higher. 4. A Group III nitride semiconductor film forming apparatus for forming a Group III nitride semiconductor film on a substrate by vapor phase epitaxy, comprising: a reaction chamber capable of accommodating the substrate; and a gas exhaust in the reaction chamber. A first gas supply pipe whose end opens near the upper part of the substrate; and a gas discharge end in the reaction chamber near the upper part of the substrate and below the first gas supply pipe. A second gas supply pipe, a first introduction pipe for supplying an organoaluminum gas from outside the reaction chamber to a gas introduction end of the first gas supply pipe, and a reaction chamber for a gas introduction end of the second gas supply pipe A group III nitride semiconductor film forming apparatus comprising: an introducing unit that supplies an organic gallium gas and an ammonia gas from the outside. 5. The III introducing pipe according to claim 4, wherein the introducing means comprises second and third introducing pipes for supplying the organic gallium gas and the ammonia gas to the gas introducing end of the second gas supplying pipe. Group nitride semiconductor film forming apparatus. 6. The introducing means is communicatively coupled at one end outside the reaction chamber to second and third introducing pipes for introducing an organic gallium gas and an ammonia gas, respectively, and at the other end of the second gas supply pipe. A common introducing pipe communicatively coupled to the gas introducing end, and a heating means for surrounding and heating the common introducing pipe including a communicative coupling portion of the second and third introducing pipes and the common introducing pipe. The film-forming apparatus for group III nitride semiconductors according to claim 4. 7. The Group III nitride semiconductor film forming apparatus according to claim 4, wherein the substrate and the first and second gas supply pipes extend in a substantially horizontal direction. 8. The first gas supply pipe is above the second gas supply pipe, and the first and second gas supply pipes are juxtaposed to each other.
Group III nitride semiconductor film deposition system. 9. The II according to claim 4, wherein the second gas supply pipe has a length sufficient to mix the ammonia gas and the organic gallium gas.
Group I nitride semiconductor film deposition system.