WO2014078263A1

WO2014078263A1 - Methods of producing trimethylgallium

Info

Publication number: WO2014078263A1
Application number: PCT/US2013/069556
Authority: WO
Inventors: Liam P. SPENCER; James C. Stevens; Deodatta Vinayak Shenai-Khatkhate
Original assignee: Dow Global Technologies Llc; Rohm And Haas Electronic Materials Llc
Priority date: 2012-11-14
Filing date: 2013-11-12
Publication date: 2014-05-22

Abstract

Methods of preparing compounds of the formula: Me₃M, wherein M is gallium, indium or thallium; the methods comprising reacting at least three equivalents of a halomethane compound with a compound of the formula (R)₃M; wherein each R is independently C₂-C₈ alkyl.

Description

Methods of Producing Trimethylgallium

Background of the Invention

Trimethylgallium (TMG) is an expensive metal-organic precursor used in the electronics and solar cell industries. It is commonly used in metal-organic chemical vapor deposition (MOCVD) processes, where a gas phase epitaxial surface reaction occurs at high temperatures, and in other deposition processes. In the synthesis of gallium-based semiconductor materials such as GaN and GaAs, a dilute organogallium precursor, typically TMG is reacted in a controlled fashion with an N- or As-atom source at the surface of a substrate. The product of the reaction decomposes at this surface to provide a thin film with the desired properties.

As is known in the art, ultra-high purity TMG is almost always required because the optoelectronic properties of the final products made from it are extremely sensitive to trace levels of dopants and impurities. So, not only must the reagents used be of ultra-high purity, but certain specific impurities must be rigorously excluded in order to achieve the desired device properties. For example, when making TMG for use in the preparation of semiconductors, it is extremely important to minimize levels of Si and O, as the presence of these impurities (and others) negatively impacts the crystal growth stage and thus, the functionality of the final product.

Fukin, K. K.; Frolov, I. A. Tr. Khim. Khim. Tekhnol. 1973, 4, 40, teach a method for making TMG that employs an exchange reaction between GaCl₃ and trimethylaluminum (TMA) (Scheme 1). However, this method suffers from at least two serious defects. First, only one methyl group is transferred from the TMA to the gallium. Consequently, at least three equivalents of TMA, which is an expensive reagent, are required to make one equivalent of TMG. Second, trace impurities in the TMA require a distillation step prior to use which can cause up to 25% loss of the product. Although this route is expensive, it does provide a high purity product after distillation. And since this process occurs in the absence of any ethers or other coordinating solvents, the product is free of oxygen impurities and thus, may be used in MOCVD. Scheme 1. Direct alkylation of GaC13 with TMA.

GaCI₃ + 3 Me₃AI ► GaMe₃ + 3 Me₂AICI

toluene

Another method used to prepare TMG is taught in EP 1247813 and employs a ligand-modified exchange reaction that employs TMA as a key reagent. In this approach, a tri-w-propylamine (TPA) adduct of TMA is first generated, followed by distillation to remove the silicon impurities. A significant benefit of this reaction is the more efficient transfer of methyl groups from aluminum to gallium, i.e., the use of the amine adduct of TMA allows for two of the three methyl groups of TMA to be transferred to gallium. A further advantage is that the reaction produces an insoluble MeAlCl₂(NPr₃) byproduct that helps to drive the reaction to completion. Though alkyl group transfer still remains incomplete (only 2 of the 3 available -CH₃ groups are transferred), this method requires the use of 50% less TMA per mole TMG produced, when compared to methods that transfer only one methyl group per molecule of TMA. And using less TMA lowers the overall raw material costs associated with the preparation of the TMG.

Scheme 2. Production of TMG from a ligand-modified methyl exchange reaction.

MeaAI + N Pr₃ Me₃AI(N Pr₃)

2 GaCI₃ + 3 M e3AI(NPr₃) 2 GaMe₃ + 3 MeAICI₂(NPr₃) toluene

Still another method used to make TMG is described in U.S. Patent No. 4,604,473. This method utilizes ether adducts of Grignard reagents (Scheme 3). Provided that the ether has a high boiling point, this approach yields adduct-free TMG in roughly 68% yields. While this method does not involve TMA, the purity of the TMG remains an issue, due to trace contamination of the TMG product with the high boiling dialkyls ether solvent.

Further purification of the ether-contaminated TMG produced can be achieved by the formation of a TMG-amine or TMG-phosphine adduct, isolation of the adduct, and thermal dissociation of the Lewis acid-base adduct to release TMG. Although this approach successfully yields TMG with sufficient purity, the cost and time associated with the preparation and use of the Grignard reagent and the additional purification efforts needed to purify the resulting TMG render this method uncompetitive, when compared to other TMG manufacturing approaches.

Scheme 3. Preparation of TMG from ether adducts of Grignard reagents.

Mel + Mg ► MeMgl

DIPE

GaCI₃ + 3 MeMgl ► Me₃Ga

DIPE

Another method of making TMG that does not utilize TMA, but does utilize Grignard reagents, is taught in European Patent No. 1,705,719. This method involves the alkylation of alkyl gallium sesquihalide derivatives of the general formula R_mGa₂X6-_m (R = methyl or ethyl and m is an integer from 1 to 5) can be achieved by employing alkyl lithium compounds, alkyl magnesium halides (Scheme 4), trialkylaluminum reagents, and dialkyl zinc compounds. Such reactions yield trialkylgallium products in high yield but has thus far been performed in ethereal solvents. As a result, the utility of this process remains questionable for the reasons described above, especially because of an oxygen-containing solvent.

Scheme 4. Grignard route to TMG.

R_mGa₂X₆-m + 6-m RMgl ► 2 R₃Ga

Ethereal

solvent

Synthetic approaches that do not involve TMA and Grignard reagents have also been reported. Alkylating agents such as MeLi (Kovar, R.A.; Derr, H.; Brandau, D.; Calloway, J.O. Inorg. Chem. 1975, 14, 2809) ZnMe₂ (Kraus, C.A.; Toonder, F.E. Proc. Natl. Acad. Sci. USA 1933, 19, 292), and dialkyl mercury reagents (Coates, G.E., J. Chem. Soc. 1951, page 2003) have been examined, as has the use of Ga/Mg alloys (U.S. Patent No. 5,248,800); however we did not deem any of these methods to be commercially viable.

These prior-art methods used ethereal solvents to facilitate the formation of trialkylgallium products. While using these solvents typically results in the isolation of products in high yields, the use of an oxygen containing ether solvent is undesirable due to the formation of Lewis acid-base adducts with trialkylgallium or trialkylindium compounds. Although separation techniques such as high temperature distillation have been attempted to release the base-free trialkylgallium species, it has been found that this approach does not result in products that possess a sufficient purity for use the MOCVD process. The presence of oxygen-containing impurities, which come from the use of oxygenated solvents during the synthesis, impairs the use of these metalorganic compounds by modifying the desired electronic properties of the resulting semiconductors during the crystal growth stage. In the above examples, elemental gallium or a gallium(III) halide was used as the source of gallium in the alkylgallium products.

In spite of the above efforts, additional methods of making TMG are needed. Especially those that do not utilize oxygen containing solvents or Grignard reagents.

Summary of the Invention

Disclosed herein are methods of making TMG, trimethylindium (TMI) or trimethylthallium, comprising an alkyl redistribution reaction between a trialkylgallium, trialkylindium or trialkylthallium compound and a methyl halide, wherein the three alkyl portions 1) comprise at least two carbons and 2) are the same or different.

In one aspect, disclosed herein are alkyl redistribution methods of preparing compounds of the formula:

Me₃M, wherein M is gallium, indium, or thallium; the methods comprising reacting at least three equivalents of a halomethane compound with a compound of the formula (R)₃M; wherein each R is independently C₂-C8 alkyl.

In a second aspect, compounds made according to the above methods are used in vapor deposition processes such as, but not limited to, Metal- Organic Chemical Vapor Deposition (MOCVD), Metal-Organic Vapor Phase Epitaxy (MOVPE), Metal-Organic Molecular Beam Epitaxy (MOMBE), and Atomic Layer Deposition (ALD) processes. Brief Description of the Figures

Figure 1 is the 1H NMR Spectrum of the reaction product from TEG and 4 equiv. of Mel in CD₃C₆D₅.

Figure 2 is 1H spectrum of the reaction mixture from TEG and 10 equiv. of Mel in

CD C₆D5, at time equals 0.

Figure 3 A is 1H NMR spectrum of the reaction mixture from TEG and 10 equiv. of Mel in CD₃C₆D₅, after 48 hours.

Figure 3B is 13 C{ 1 H} NMR spectrum of the reaction mixture from TEG and 10 equiv. of Mel in CD₃C₆D₅, after 48 hours.

Figure 4 is Time resolved 1H NMR spectrum of the reaction between TMG and 10 equiv. of Etl.

Detailed Description

In one embodiment, the the halomethane compound comprises methyl iodide, methyl bromide, methyl chloride, or combinations thereof. A preferred halomethane is chloromethane.

When conducting the alkyl redistribution reaction, at least three equivalents of halomethane should be used. While lesser amounts may be used, the reaction product will be a mixture of mono, bis, and tri-methylated compounds. Thus it is preferred that at least four equivalents of halomethane are used. More preferably more than 5 equivalents are used. In one preferred embodiment, at least 8 equivalents are used. In another preferred embodiment, at least 10 equivalents are used. While 20 or more equivalents of halomethane may be used, it is believed that using such a large excess is not cost effective.

In one embodiment, the methods disclosed herein are conducted at a temperature of at least 30 °C up to and including the boiling point of the halomethane compound. If the reaction is conducted in a sealed container, then temperatures higher than the boiling point (at atmospheric pressure) may be used.

Preferably, the reaction between the halomethane and the compound of formula (R)₃M is conducted under an inert atmosphere, i.e., an atmosphere that does not react with the starting materials or the reagents. Examples include nitrogen, a noble gas, or a mixture thereof.

In one embodiment, the reactions occur in a pressurized vessel at a pressure greater than 1 atmosphere. In another aspect, the pressure is less than 5 atmospheres.

The reactions disclosed herein do not require the use of a solvent. It is preferred that a solvent not be used. In the most preferred embodiments, a solvent is not used.

In one embodiment, the R groups are the same and are selected from the group consisting of C₂-C₁₀ alkyl groups. More preferably, the R groups are C₂-C8 alkyl groups. Still more preferably, the R groups are C₂-C₆ alkyl groups. Even more preferably, the R groups are ethyl groups, in such a case the compound is Et₃M.

In another embodiment, at least two of the R groups are different, and all of the R groups are selected from the group consisting of C₂-C₁o alkyl groups. More preferably, at least two of the R groups are different and are independently C₂-Cg alkyl groups. Still more preferably, at least two of the R groups are different and are independently C₂-C₆ alkyl groups. Even more preferably, only two of the R groups are ethyl groups. Or in another embodiment, only one R group is an ethyl group.

The methods disclosed herein may be conducted in either a batch or continuous process.

In one embodiment, M is gallium.

In another embodiment, M is indium.

In another embodiment, M is thallium.

In one embodiment, the triethylmetal compound (Et M) is formed by reacting a metal hydride (MH₃) and ethylene. Such reactions are typically performed in the presence of an organic solvent. Furthermore, the reactions typically involve a heating step.

In one embodiment, the trimethyl compounds made according to the methods described herein have a purity of 99.9999%.

In another embodiment, they also have less than 0.1 ppm of metallic and oxygenated impurities. Preferably, the trimethyl compounds have a purity of 99.9999% and less than 0.1 ppm of metallic and oxygenated impurities.

The (R)₃M compounds may be purified using methods known in the art, such as distillation.

As indicated by 1 H and 13 C{ 1 H} NMR spectroscopy and described in Example 1, the reaction between TEG and 4 equivalents of Mel at 70°C proceeded to yield a mixture of ethyl- and methylgallium-containing species and ethyl iodide (the 1H NMR spectrum may be seen in Figure 1). When the amount of Mel was increased to 10 equivalents, TMG was formed quantitatively (Example 2 and Figure 2).

Attempts to convert TMG to triethylgalllium by treating TMG with 10 equivalents of ethyl iodide (48 hours at 70°C) failed to generate any triethylgallium. See Figure 3.

While using methyl iodide as the halomethane is convenient (because it is a liquid) when making TMG from triethylgallium, its use causes problems when attempting to purify the TMG. The overall reaction for the conversion of triethylgallium to TMG using 10 equiv. of methyl iodide is shown below in Scheme 5:

Scheme 5:

70°C

Et₃Ga + 10 Mel Me₃Ga + 3 Etl + 7 Mel

Upon using distillation to purify the product, the TMG (boiling point of 56 °C) is commonly contaminated with methyl iodide (boiling point of 42-43 °C) and ethyl iodide (boiling point of (71-73 °C). Thus other methods of purifying the TMG may be used, as shown in Example 3, which utilizes a chelant, followed by removal of the volatiles, thermal decomposition of the chelated product and distillation of the now non-chelated TMG.

The inventors surprisingly found that the problems associated with the use of methyl iodide can be obviated if chloromethane is used as the halomethane (as in Scheme 6, below). Without wishing to be bound by a particular theory, one plausible explanation is that the chloromethane reagent, which is a gas at room temperature (boiling point of -24 °C) and the EtCl that is produced (boiling point of 12 °C) during the reaction, both have boiling points that are significantly lower than that of the TMG (boiling point of 56 °C), and hence are more easily removed. Scheme 6:

70°C

Et₃Ga + 10 MeCI Me₃Ga + 3 EtCI + 7 MeCI

As a result, it is more cost effective to distill the TMG prepared according to Scheme 6, than it is TMG prepared according to Scheme 5.

Example 1 : TMG Synthesis from TEG and 4 equivalents of Mel

To a 40 mL Scintillation vial fitted with a pressure-release disk was loaded with triethylgallium (1.0 g, 6.4 mmol) and methyl iodide (1.6 mL, 25.6 mmol). The reaction was heated to 70°C and stirred overnight, whereupon an aliquot was diluted with 1 mL C₆D6 and examined by 1H NMR spectroscopy (400 MHZ, 16 scans). See Figure 1 , which shows a mixture of different compounds present in the reaction mixture, due to an incomplete reaction.

Example 2: TMG Synthesis from TEG and 10 equivalents of Mel

To a 40 mL Scintillation vial fitted with a pressure-release disk was loaded with triethylgallium (1.0 g, 6.4 mmol) and methyl iodide (4.0 mL, 64 mmol). The reaction was heated to 70°C and stirred overnight at a pressure of 1.1 atm, whereupon an aliquot was

1 13 1

diluted with 1 mL C₆D6 and examined by H and C{ H} NMR spectroscopy. See Figures 2 (which shows the reaction mixture at time = 0) and 3 A and 3B, which do not show any starting material (which shows complete consumption of the starting material after 48 hours).

1H NMR (C₆D₆): 2.69 (q, J = 6 Hz, -CH₂I), 1.42 (t, J = 6 Hz, -CH₃CH₂), -0.09 (s, -GaCH₃). ¹³C{ 1H} NMR (C₆D₆): 20.5 (-C¾), 0.9 (-GaC), - 1.5 (- H₂I), -24.2 (- H₃I).

Example 3: Isolation of TMG Produced from Alkyl Exchange Reaction

To the crude reaction mixture obtained above was added 4,4'-methylenebis(N,N- dimethylaniline) (MBDA) (811 mg, 3.2 mmol). After removal of the solvents in vacuo, a colorless solid was obtained that was transferred to a 10 mL Bantamware glass flask and attached to a micro-distillation apparatus. Under inert atmosphere and vacuum, the flask was slowly heated to 45°C under 10 mTorr whereupon a distillate was collected into a cooled receiving flask (-30°C). When the distillation was complete, the receiving flask was evacuated briefly at -30°C and then slowly warmed to room temperature. The liquid

1 13 1

collected was analyzed by H and C{ H} NMR spectroscopy (85% yield).

1H NMR (C₆D₆): 6.97 (d, J = 6 Hz, 4H), 6.81 (d, J = 6 Hz, 4H), 3.71 (s, 2H), 2.33 (s, 12H), -0.17 (s, 18H).

¹³C{ 1H} NMR (C₆D₆): 148.5, 136.3, 129.4, 118.8, 44.6, 40.3, -4.8.

Example 4: Probing the reaction of TMG and Etl

To a 40 mL Scintillation vial fitted with a pressure-release disk was loaded with trimethylgallium (60 mg, 0.52 mmol), mesitylene (10 mg) and ethyl iodide (820 mg, 5.2 mmol). The reaction was heated to 70°C and stirred overnight whereupon an aliquot was diluted with 1 mL C₆D6 and examined by 1H spectroscopy. See Figure 4. No reaction was observed in the time frame examined.

Example 5: Trimethylindium Synthesis from triethylindium and Mel

To a 40 mL Scintillation vial fitted with a pressure-release disk was loaded with triethylindium (1.0 g, 4.95 mmol) and methyl iodide (7.03 g, 49.5 mmol). The reaction was heated to 70°C and stirred overnight, whereupon an aliquot was diluted with 1 mL C₆D6 and

1 13 1

examined by H and C{ H} NMR spectroscopy. The product was purified by evacuating the flask to remove volatile components and then sublimation of the residue to obtain trimethylindium as a colorless solid (526 mg, 66%).

1H NMR (C₆D₆): -0.15 (s, -InCH₃).

¹³C{ 1H} NMR (C₆D₆): 0.1 (-InC). Example 6: Synthesis of GaH₃ (Prophetic)

A Hastelloy accelerating rate calorimetric sphere is charged with Ga metal (1.00 g, 14.3 mmol), hydrogen (2.87 g, 1.43 mol) and subsequently sealed. The sphere is stirred with a Teflon stir bar and heated to 200 °C for 48 hours to produce the product.

Example 7: Synthesis of GaEt from GaH (Prophetic)

GaH₃ is transferred to a Hastelloy accelerating rate calorimetric sphere and then charged with ethylene. The sealed device is sealed and stirred with a stir bar whilst heating to 200 °C for 48 hours to produce GaEt₃.

Example 8: One pot synthesis of GaEt₃ from Ga, H₂, and ethylene (Prophetic)

A Hastelloy accelerating rate calorimetric sphere is charged with Ga metal, hydrogen and ethylene and then sealed. The sphere is stirred with a stir bar and heated to 200 °C for 48 hours to produce the product.

Claims

What is claimed is:

1. Methods of preparing compounds of the formula:

Me₃M, wherein M is gallium, indium or thallium; the methods comprising reacting at least three equivalents of a halomethane compound with a compound of the formula (R)₃M; wherein each R is independently C₂-C8 alkyl.

2. Methods according to claim 1, wherein the halomethane comprises methyl iodide, methyl bromide, methyl chloride, or combinations thereof.

3. Methods according to claims 1 or 2, wherein at least 8 equivalents of halomethane are used.

4. Methods according to claims 1-3, wherein the reaction is conducted at atmospheric pressure and the temperature of the reaction is at least 30 °C up to and including the boiling point of the halomethane.

5. Methods according to claims 1-4, wherein at least 10 equivalents of halomethane are used.

6. Methods according to claims 1-5, wherein the halomethane is chloromethane.

7. Methods according to claims 1-6, wherein the reaction between the halomethane and the Et₃M is conducted under an inert atmosphere.

8. Methods according to claims 1-7, wherein the reaction occurs in a pressurized vessel at a pressure greater than 1 atmosphere.

9. Methods according to claims 1-8, wherein (R)₃M is Et₃M.

10. Methods according to claim 9 , wherein the Et₃M is formed by reacting MH₃ and ethylene.

11. Methods according to claims 1-10, wherein M is gallium.

12. Methods according to claims 1-10, wherein M is indium.

13. Methods according to claims 1-10, wherein M is thallium.