The present invention relates to a method for producing a trialkyl gallium. With the advancement of mobile phones and optical communication technologies, demand is rapidly growing for compound semiconductors for use in high speed electronic devices such as high electron mobility transistors (HEMTs), heterojunction bipolar transistors (HBTs), semiconductor lasers, DVDs, optical devices such as white and blue super high-intensity LEDs, and other applications. In general, alkyi derivatives of group 12 and group 13 metals, and in particular the methyl or ethyl derivatives are often used as metalorganic precursors for compound semiconductors. A great demand exists for, in particular, trialkyl gallium for the production of compound semiconductors by MOCVD with group 15 elements, such as nitrogen, arsenic, and the like.

A typical example of a prior-art method for producing a trialkyl gallium is reacting an alkyi halide with a gallium-magnesium mixture or gallium-magnesium alloy. This method is advantageous in allowing commercially readily available metallic gallium and metallic magnesium of high purity to be used as received, and in not requiring the use of a reagent for which care must be taken. Many prior-art methods use a gallium-magnesium alloy because the use of a gallium-magnesium alloy as a starting material results in the production of a trialkyl gallium in higher yields than the use of a gallium-magnesium mixture.

Methods using a gallium-magnesium alloy require a process of preparing an alloy by heating. However, it is difficult to prepare a uniform gallium-magnesium alloy. In most cases, alkyi iodides are used to react with the alloy. Alkyi iodides are the most reactive among the alkyi halides. However, alkyi iodides are more expensive than alkyi bromides and alkyi chlorides. In addition, since the boiling points of trimethyl gallium and methyl iodide are close, it is difficult to isolate trimethyl gallium from methyl iodide. Furthermore, alkyi iodides and alkyi bromides are toxic and/or ozone-depleting.

It is therefore desired to provide a method for the production of trialkyl gallium that can be performed in high yields and with acceptable rate using alkyi chlorides instead of alkyi bromides or iodides.

Another method for producing trialkyl gallium is based on the reaction of gallium trihalide with a Grignard reagent or with trialkyl aluminum. Processes starting from gallium trihalide are, however, economically less attractive because gallium trihalide is rather expensive. Furthermore, gallium trihalide is very corrosive and hygroscopic, requiring specific equipment and handling without air contact.

Processes for the preparation of trialkyl gallium by reacting metallic gallium, not mixed or alloyed with magnesium, with an alkyi chloride have been described. However, their performance was extremely poor; see Storawieski and Klabunde, Appl. Organometallic Chem. 3 (1989) 3 pp. 219-224.

It has now been found that trialkyl gallium can be prepared from metallic gallium and alkyi chloride provided that a suitable catalyst is used. Suitable catalysts are found to be Lewis acids.

The present invention therefore relates to a process for the preparation of trialkyl gallium comprising the steps of (i) reacting metallic gallium with alkyi chloride in the absence of metallic magnesium and in the presence of a Lewis acid catalyst and (ii) reacting the product of step (i) with a metal alkyi.

This process thus allows for the preparation of trialkyl gallium in high yield starting from metallic gallium and an alkyi chloride. Furthermore, the process of the present invention does not require the use of ether solvents that would require pyrolysis to obtain trialkyl gallium.

The first step in the process according to the present invention involves the reaction of metallic gallium with alkyl chloride in the presence of a Lewis acid catalyst. No Mg or other alkaline earth or alkali metal is required in this step.

The product resulting from this step is alkylgallium sesquichloride; a mixture of alkylgallium dichloride and dialkylgallium chloride. Any metallic gallium can be used. For instance, a commercially available gallium having a purity of 99.9% (3N) or better may be used. Commercially available gallium has a purity of up to 7N.

The electrical properties and optical properties of a compound semiconductor produced by MOCVD are greatly affected by the purity of the starting organometallic compound. Therefore, it is desirable that a trialkyl gallium of high purity is produced. Since the purity of the resulting trialkyl gallium is affected by the purity of the starting gallium, it is preferable to use gallium of high purity.

The purity of gallium is preferably 99.999% (5N) or better, and particularly preferably 99.9999% (6N) or better. High-purity gallium having a purity of 5N or better is commercially available as described above. It is difficult to buy Ga with a purity below 4N.

Suitable alkyl chlorides are Ci-io alkyl chlorides, and preferably Ci _-4 alkyl chlorides. Specific examples of suitable alkyl chlorides are methyl chloride, ethyl chloride, n- propyl chloride, isopropyl chloride, n-butyl chloride, isobutyl chloride, sec-butyl chloride, tert-butyl chloride, and combinations thereof. Methyl chloride and ethyl chloride are the most preferred alkyl chlorides. Preferred Lewis acid catalysts are aluminium, gallium, and tin-containing Lewis acids, more preferably aluminium, gallium, and tin bromides, iodides, sulfates, triflates, alkyl sulfonates (e.g. methyl sulfonates) and aryl sulfonates (e.g. p-toluene sulfonates and phenyl sulfonates). Specific examples of suitable Lewis acids are GaBr ₃ , Gal ₃ , AIBr ₃ , SnBr ₄ , AI ₂ (S0 ₄ ) ₃ , AI(MeS0 ₃ ) ₃ , AI(PhS0 ₃ ) ₃ , AI(pTolS0 ₃ ) ₃ , Ga ₂ (S0 ₄ ) ₃ , Ga(MeS0 ₃ ) ₃ , Ga(PhS0 ₃ ) ₃ , and Ga(pTolS0 ₃ ) ₃ .

More preferred Lewis acid catalysts are GaBr ₃ , Gal ₃ , AIBr ₃ , and SnBr ₄ . AIBr ₃ is the most preferred catalyst.

The Lewis acid catalyst can be added to the reaction mixture as such, or can be formed in situ by adding, e.g., or Br ₂ to the reaction mixture. The latter is especially advantageous for the in situ formation of Gal ₃ , and GaBr ₃ .

The reaction is preferably performed at a temperature in the range 30-200°C, preferably 35-150°C, most preferably 40-120°C.

At these temperatures, gallium is in liquid form (melting point: 29.8°C). This means that solvents are not required, which is a further advantage of the process of the present invention.

However, if the presence of solvents is for some reason desired, then it is preferred to use hydrocarbon solvents. Examples of suitable solvents are saturated aliphatic hydrocarbons like pentane, hexane, heptane, octane, nonane, decane, undecane, and dodecane; saturated alicyclic hydrocarbons like cyclohexane and cycloheptane; and aromatic hydrocarbons like toluene, xylene, trimethylbenzene, ethylbenzene, ethyltoluene, and indene. Preferred hydrocarbons are those that are easily separable from the resulting trialkyl gallium, more preferably by having a boiling point that differs significantly from that of the trialkyl gallium.

Alkylchloride is preferably used in an amount of 1 -10000, more preferably 100- 5000, and most preferably 130-500 mole% relative to metallic gallium. The catalyst is preferably used in an amount of 0.00001 -1000, more preferably 0.1 - 100, and most preferably 2-30 mole% relative to metallic gallium.

The process is carried out by introducing gallium, alkyl chloride, catalyst, and optionally solvent into a reaction vessel under inert gas atmosphere. These compounds can be added in any form and in any order. Preferably, gallium is added first, followed by alkyl chloride. If the alkyl chloride is gaseous under the addition conditions, it may be added by purging the reactor with the alkyl chloride and finally feeding the alkyl chloride into the reactor under atmospheric or higher pressures. It can also be added as a liquid under pressure or at temperature conditions at which the alkyl chloride is liquid. Alternatively, the alkyl chloride can be added as a solution in a solvent that is compatible with the reaction mixture, preferably a hydrocarbon solvent, or it can be bubbled through molten Ga under vacuum in the presence of the Lewis acid catalyst.

The catalyst or - in case of in situ catalyst formation - its precursor can be introduced before or after addition of alkyl chloride. If added after the addition of alkyl chloride, it may be dosed during the reaction in several portions. Alternatively, it is added in one portion at the start.

The second step of the process of the present invention involves the reaction of the alkylgallium sesquichloride formed in the first step with a metal alkyl to form trialkylgallium. As a by-product, dialkyl metal chloride, alkyl metal chloride, or metal halide chloride will be formed.

Examples of suitable metal alkyls are aluminium alkyls, magnesium alkyls, and lithium alkyls. More preferably, the metal alkyl is an aluminium alkyl, even more preferably trialkyl aluminium. The alkyi groups of the metal alkyi may comprise 1 to 10 carbon atoms, more preferably 1 to 4 carbon atoms. The alkyi groups of the metal alkyi are preferably the same as the alkyi groups of the alkyi chloride used in the previous step. Methyl is the most preferred alkyi; trimethyl aluminium and triethyl aluminium are the most preferred metal alkyls.

The metal alkyi is preferably used in an amount of 10-10000, more preferably 30- 500, and most preferably 70-400 mole% relative to the gallium metal used in the first step. Also during this step, solvents are not required. However, if the presence of solvents is for some reason desired, then it is preferred to use hydrocarbon solvents. Examples of suitable solvents are saturated aliphatic hydrocarbons like pentane, hexane, heptane, octane, nonane, decane, undecane, and dodecane; saturated alicyclic hydrocarbons like cyclohexane and cycloheptane; and aromatic hydrocarbons like toluene, xylene, trimethylbenzene, ethylbenzene, ethyltoluene, and indene. Preferred hydrocarbons are those that are easily separable from the resulting trialkyl gallium, more preferably by having a boiling point that differs significantly from that of the trialkyl gallium. The reaction temperature is suitably selected to efficiently carry out the reaction in consideration of the type of alkylgallium sesquichloride, metal alkyi, and optional solvent used, and other factors. The reaction is usually carried out at about 0° to about 200°C, preferably about 20° to about 150°C, and more preferably about 30° to about 120°C.

After the reaction, the temperature of the reactor may be increased in order to distill the produced trialkyl gallium. Both process steps can be performed in the same reactor. Alternatively, the intermediate alkylgallium sesquichloride can be distilled, re-crystallized or otherwise purified before its use in step (ii), if so desired. The trialkyi gallium obtained by the process of the present invention can be suitably used for the preparation of semiconductor devices, e.g. gallium nitride-based semiconductors.

The alkylgallium sesquichloride resulting from the first step of the process finds use as precursor for semiconductor fabrication and for the synthesis of other Ga compounds. The invention therefore also relates to a process for the preparation of alkylgallium sesquichloride.

EXAMPLES Example 1

Ga (12.00 g, 172.1 mmol) was loaded into a 50 ml 3-necked reaction flask. The flask was purged with MeCI (4 cycles of evacuation and filling with MeCI) and placed into an oil bath. A MeCI overpressure of 0.3 bar was applied. The oil bath was heated to 100°C and l ₂ (4.37 g, 17.2 mmol) was added to the Ga within 40 min under stirring.

The reaction mixture was stirred for 13 hours at 100°C. The obtained reaction mixture had appearance of a transparent amber-coloured liquid with a small amount of unreacted Ga. The resulting alkylgallium sesquichloride crystallized from the reaction mixture in ca. 40 hours at room temperature.

The flask, which contained crystallized alkylgallium sesquichloride, was transferred into a glovebox. Trimethyl aluminium (TMAL, 3 ml) was added to melt the mixture and, after removal of unreacted Ga with a pipette (0.85 g; meaning 93% conversion to alkylgallium sesquichloride in the first reaction step), was transferred into a 100 ml 2-necked flask placed into an oil bath. The remaining TMAL (total of 37.2 g, 516.4 mmol) was added into the flask within 12 min.

The obtained reaction mixture was distilled to obtain 18.25 g of a transparent product. ¹ H NMR indicated that this product contained about 4 mol% Mel and 96 mol% trimethyl gallium. The yield of raw trimethyl gallium was 92% based on the amount of Ga used in the first step, and 99% based on Ga reacted in the first step.

Example 2

A 50 ml 3-necked flask containing Ga (8.00 g, 1 14.7 mmol) was purged with MeCI (ca. 0.3 bar) and heated to 50°C. Bromine (ca. 0.8 ml, 2.52 g, 15.8 mmol) was added with a syringe into the flask within 22 min.

After addition of all the Br ₂ , the temperature of the oil bath was increased to 100°C (the temperature inside the reaction mixture was 95°C). After 20 h of stirring at 100°C, 0.73 g of Ga stayed unreacted (91 % conversion). The unreacted Ga was removed from the reaction mixture with a pipette.

The flask, which contained crystallized alkylgallium sesquichloride, was transferred into a glovebox and placed into an oil bath. A condenser with a receiving flask was connected to the flask and TMAL (24.82 g, 344.1 mmol) was gradually added to the reaction mixture. In the beginning of the addition the clear transparent mixture turned into a grey suspension.

The obtained reaction mixture was distilled to obtain 10.36 g (90.2 mmol, 79% yield based on Ga used in the reaction with MeCI, 86.5% yield based on reacted Ga) of the clear transparent product. According to peak area in ¹ H NMR spectrum the product had 99% purity; MeBr (0.9 mol%) being the major impurity.

Example 3

A 100 ml 2-necked flask containing Ga (12.00 g, 172.1 mmol) and AIBr ₃ (12.00 g, 45.0 mmol) was purged with MeCI (ca. 0.3 bar). After filling the flask with MeCI the AIBr ₃ turned liquid. Every hour the reaction flask was shortly flushed with MeCI to remove side gas products. On the first day the reaction mixture was stirred at 100°C for 5 hours and ca. 0.5 g of Ga remained unreacted. After keeping the reaction mixture at room temperature overnight, the reaction with MeCI at 100°C was continued on the next day for two more hours. A very small amount of Ga (<0.05 g) could still be seen in the transparent liquid product. Total reaction time at 100°C was 7 hours.

The flask, which contained crystallized alkylgallium sesquichloride, was transferred into a glovebox and placed into an oil bath. A condenser with a receiving flask was connected to the flask and TMAL (37.2 g, 516.4 mmol) was gradually added to the reaction mixture.

After addition of ca. 10 ml TMAL the reaction temperature increased from 26 to ca. 80°C and the clear transparent solution turned into a grey suspension. After addition of all the TMAL, the temperature of the oil bath was increased to 120°C and a distillate (b.p. 62-73°C) was collected to obtain 17.02 g (148.2 mmol, 86% yield based on Ga used in the first step) of a clear transparent product. The distillation started when the temperature of the oil bath was 104°C. According to ¹ H NMR the product had 97% purity; 1.5% of the total peak area was contributed by MeBr, which corresponded to 4.5 mol% MeBr contamination. After keeping the bottoms at room temperature overnight without stirring a small amount of grey solids precipitated and the bottoms were clear.

Comparative Example

A 50 ml stainless steel reactor (no stirring) was loaded with Ga (0.77 g, 1 1 mmol), closed, pressurized with MeCI to 3.9 atm. At room temperature (0.4 g, 7.9 mmol MeCI) and heated to 40°C for 3 days. After cooling the reactor to room temperature, the pressure was 3.8 atm. The reactor only contained Ga metal; no product was formed.

This experiment shows that for reacting Ga metal with alkyl chloride, a Lewis acid catalyst is essential. Without such catalyst (as in the present Comparative Example), no reaction takes place.