METHOD FOR PURIFICATION OF TELLURIUM AND SELENIUM ALKYLS

Abstract: A method for providing purified tellurium and selenium alkyls and dialkyls which comprises forming an adduct of the alkyl with at least one compound of a group 11 or 12 metal and thermally dissociating the said adduct to yield a purified form of the adduct where the purified product is used in semiconducts such as cadmium mercury telluride for infra-red detectors.

Description: Method for Purification of Tellurium and Selenium Alkyls This invention relates to methods for the purification of tellurium and selenium alkyls, particularly the dialkyls of these metals. Tellurium and selenium alkyls are important compounds used in the semiconductor industry, for example in the preparation of Cadmium Mercury Telluride (CMT) used in opto-electronic applications. In many such uses the alkyls are decomposed, usually thermally to form the element, which may be itself deposited on a substrate, or combined with other elements in compounds. It is a very important requirement of such alkyls that they be extremely pure, as the presence of even minute quantities of impurities, eg at a ppm level can have a detrimental effect on the properties of the semiconductor. Present methods of purification of tellurium and selenium alkyls generally involve fractional distillation and are unable to easily reach the abovementioned levels of purity. Another known method of purifying metal alkyls involves the formation of an adduct that has properties which allow it to be decomposed in such a way that the pure alkyl is formed as a decomposition product. The use of adducts in this way to purify tellurium and selenium alkyls is as yet unknown, although some work has been done to investigate various adducts of these alkyls. See for example G E Coates J Chem Soc (1951) pp 2003 - 2013. It is known that some of these adducts dissociate with relative ease, but none have been previously known to dissociate in such a way as to yield the pure alkyls. The present invention seeks to overcome the problems of prior art methods of purification of tellurium and selenium alkyls by novel routes using adducts, some of which are themselves novel. According to the present invention there is provided a method of purification of a tellurium or selenium alkyl characterised by forming an adduct of the alkyl with at least one compound of a Group 11 or 12 metal (Cu, Ag, Au, Zn, Cd, Hg) and thermally dissociating the said adduct to yield the alkyl. Preferably the at least one . compound of a group 11 or 12 metal is an inorganic compound. The method is suitable for the purification of all tellurium and selenium alkyls, but is particularly suitable for the metal dialkyls. The alkyl group may be straight or branched chain, and is preferably a lower alkyl eg containing up to -4- carbon atoms. These lower alkyls are most useful for semiconductor production. Preferred Group 11 or 12 metals are cadmium, mercury and copper. The compound is preferably a halide or nitrate, most preferably halides, these being bromides, chlorides and iodides. The most preferred cσmpounts are therefore: Cul, Cdl„, CdBr , HgCl„, Cu 2 I 2 , Hglg. Preferably the method is carried out in the following way: (i) A solution of the alkyl in a suitable solvent is prepared, ani this is added to a solution of the metal compounl in a suitable solvent, pre f erably an organic solvent. 1 ' referred solvents in both cases are polar aliphatic orpanic solvents containing 1 to 10 carbon atoms, preferably 1 to 5. It is desirable to use a solvent for the alkyl that is also capable o" d issolving the metal compound. Suitable sovents for the alkyl inclu-e ethers, such as THF but especially diethyl ether; alcohols, especially ethanol: an! nitrileε especially acetonitrile These solvents are also suitable .for the metal compound. Typically the alkyl is dissolve! in an ether an! is added at an appropriate rate to a solution of the metal compound in an alcohol. Preferably the a mount of alkyl and metal compoun'. u^e are in approximately stoichioinetric ratios . (ii) When reaction appears to be complete, excess solvents may be separated by filtration from any precipitated product, or alternately or additionally the bulk of the solvent may be removed by evaporation. (iii) The solid adduct product is washed with an appropriate solvent, eg the ether used in the reaction, and excess volatiles removed in vacuo. (iv) The adduct is thermally dissociated in vacuo to yield the pure alkyl which may be collected in a cooled receiver. Suitable dissociation temperatures will depend upon the adduct and are easily determined experimentally. Generally temperatures in the range 80° - 150°C are suitable. It is preferred that the preparation and subsequent manipul¬ ation of the adduct is performed under an inert atmosphere such as nitrogen. Appropriate conditions for the preparation of any particular tellurium or selenium alkyl can be easily determined experimentally generally following the procedure outlines above. Alkyls purified by the process of the invention are in many instances in a state of purity suitable for their use directly in the preparation of semiconductors, for example the epitaxial deposit of CMT. The invention will now be described by way of example only. Table 1 below lists various adducts which were prepared using the method of the two preparative examples and some of their characteristics. Table 2 below lists the microanalytical data for the various adducts. Certain of the adducts describe'! below are novel, ie: ((C 2 H 5 ) 2 Te) 2 GdI 2 G 2 H ) 2 Te(G 3r 2 ) 2 ((G 2 H ) 2 Te) 2 C 3r 2 (iso-C K 7 ) 2 Te) 2 GdI 2 ;÷i . =0-2,K ?e) :d3r 9 Table 1- Adducts for the purification of dialkyltellurium compounds Adduct employed Thermal behaviour (°C) % Yield of dialkyltellurium 37 67 65 47 50 51 66 ( iI r,Tfi),. . C1J-„ ' 5 ~~ n is variable uαd tl ir compound maybe polymeric Table 2: Preparation of Adducts of R ? Te with Gro i' H <>■ 12 Heta ] compoun.li. Di lkyltellurium Metal Co d Solvent Employed* R 2 Te: MX n Analysis (%) (R 2 Te) MX ratio in found (theoretical) the adduct Di-isopropyltellurium Cdl 2 Ethanol l(CH 3 ) 2 CH} 2 Te CdBr : Ethanol ( i ~ \'. ciO To C-il, ϋthnnol * In each case, an ethereal solution of R 2 Te was added to the metal halide in the solvent shown. Preparative Example 1 . Purification of iethyltelluri inn using oαciπilum io i c-le A solution of dietl-yltellurlum (25.6 g, 0.1373 gmol) In diethylether (50 cm 3 ) Has added dropwise to a solution of cadmium Iodide (24.1 g, 0.065 gmol) in ethanol (350 cm 3 ) to obtain a white precipitate which dissolved on warming to 50 3 C. The reaction mixture was cooled and stirred at room temperature for 30 minutes. The volume Has reduced to ca 100 cm 3 In vacuo and the product was obtained as white crystals on cooling to -30°C overnight. The crystals were isolated from the reaction mixture by filtration under a nitrogen atmosphere and by successive washings with petroleum ether cooled to -°C. The crystals were pumped to remove any volatile impurities for lh at room temperature. Hicroanalytical data suggested that the product had a limiting empirical formula (Et 2 Te) 2 CdI 2 . The pure adduct, (Et 2 Te) 2 Cdr 2 (34.95 g, 0.0473 gmol) Has then heated to 100°C. Pure dlethyltellurium distilled and was collected in vacuo in a cold trap (11.00 g, 0.0592 gmol, 62.52X yield). The adduct was found to dissociate between 85-90°C. Prepara ti ve Example 2 Fi-tr-i-Ci--3i3.t--.iori of diethyltelluriuπi usinfl Copper* ( I ) iodide A solution of diethyltellurium (6.6 g, 0.0355 gmol) in diethylether (25 cm 3 ) was added dropwise to a solution of copper(I) iodide (3.5 g, 0.0183 gmol) in acetonitrile (50 cm 3 ). A pale pink mlcrocrystalline precipitate formed during the addition. The excess diethyltellurium and acetonitrile were removed by filtration under a nitrogen atmosphere and by successive Washings with petroleum ether cooled to 4^C. The adduct obtained was pumped for lh at room temperature to remove any volatile impurities, Microanalytlcal studies revealed a limiting empirical formula (Et 2 Te)CuI. The adduct, (Et 2 Te)CuI was then heated to 150 0 in vacuo , Pure diethyltellurium distilled and was collected in a pold trap. No sublimation of the adduct was found to occur up to 150°C. The dissociation of the adduct was found to occur between 110-130 β C. Preparative Example 3 Purification of di-isopropyltellurium using cadmium iodide A solution of di-isopropyl tellurium (26.34g, 0.1253 g mol.) 3 in diethyl ether (150 cm ) was added dropwise to a solution of 3 cadmium iodide (22.50g, 0.0614 g mol.) in ethanol (200 cm ). The reaction mixture was cooled to -30°C overnight and the product obtained as white needle crystals. The crystals were isolated from the reaction mixture by filtration under a nitrogen atmosphere and by successive washings with petroleum ether, pre-cooled to 4°C. The crystals were pumped for 1 hour at room temperature to remove any volatile impurities. Overall yield 25.14g (51.47%). Microanalysis of the crystals suggested that the product had a limiting empirical formula (iPrTe) ? (CdI-) . The pure adduct (iPrTe)„(CdI 2 ) , (25.14g, 0.0316 g mol.) was heated to about 175°C. Pure di-isopropyl tellurium distilled and was collected in vacuo in a cold trap (7.17g, 0.0335 g mol., 53% yield). The adduct was found to dissociate between 125°C- 135°C. Preparative Example 4 Purification of diethyltellurium using Cadmium Bromide A solution of diethyltellurium (23.32g, 0.1256 g mol) in 3 diethylether (150 cm ) was added dropwise to a solution of cadmium bromide (CdBr 2> 4H 2 0, 88.62g, 0.2574 g mol) in ethanol (200 cm 3 ). A yellowish white microcrystalline precipitate formed during the addition, which was found to redissolve on warming the reaction mixture to 50°C. The solution was cooled and stirred at room temperature for 30 minutes. The colume was then reduced to ca 3 200 cm in vacuo and the product was obtained as white crystals on cooling to -30°C overnight. The crystals were isolated from the reaction mixture by filtration under a nitrogen atmosphere and by successive washings with petroleum ether pre-cooled to 4°C. The crystals were pumped for 1 hour at room temperature to remove any volatile impurities. Overall yield was 93.6% (85.92g). Microanalysis of the crystals suggested that the product had a limiting empirical formula (Et p TeMCdBr J) . The pure adduct, (Et Te) (CdBr) , (85.92g, 0.092 g mol) was heated to 140°C. Pure diethyltellurium distilled and was collected in vacuo in a cold trap (14.66g, 0.0789 g mol, 85.85% yield) . The adduct was found to dissociate between 80-90°C. Preparative Example 5 Purification of diethyltellurium using Mercury (II) Iodide A solution of diethyltellurium (4.78g 0.025 g mol) was added . dropwise to a red solution of mercury (II) iodide (6.16g, 3 0.013 g mol) in ethanol ( 250 cm ) to obtain a pale yellow crystalline product. The product was isolated from the reaction mixture by filtration under a nitrogen atmosphere and by successive washings with petroleum ether pre-cooled to 4°C. The crystals were pumped to remove any volatile impurities for 1 hour at room temperature. Microanalytical data suggested that the product had a limiting emperical formula (Et Te) Hgl . The pure adduct (EtTe) Hgl (5-90g, 7.146 m mol) was then heated to 150°C. Pure diethyl tellurium distilled and was collected in vacuo in a cold trap maintained at -196°C (1.22g, 6.573 m mol, 46.99% yield). The adduct was found to dissociate between 120- 140°C. Preparative Example 6 Purification of diethyltellurium using Mercury II Chloride A solution of diethyltellurium (5.71g, 0.0307 g mol) was added dropwise to a solution of mercury (II) Chloride (7.39g, 0.0272 gmol) 3 in diethyl ether (250 cm ) to obtain a pale yellow crystalline product. The product was isolated from the reaction mixture by filtration under a nitrogen atmosphere and by siccessive washings with petroleum ether precooled to 4°C. The crystals were pumped to remove any votalite impurities for 1 hour at room temperature. Microanalytical data suggested that the product had a limiting empirical formula Et„Te HgCl„. The pure adduct Et T HgCl„ (11.65g, 0.0254 g mol) was then heated to 120°C. Pure diethyltellurium distilled and was collected in vacuo in a cold trap maintained at the liquid nitrogen temperature. The adduct was found to melt at ca 100°C and dissociate between 100-120°C. Frerarative Example 7 Purification of di-isopropyltellurium using cadmium iodide " . A solution of di-isopropyltellurium (6l S g. 0.2^3 g mol) in diethyl ether (50 cm ) was aided dropwise to a solution o^ ca d mium iodide (130.53 g, 0. 65 g mol) in ethanol (ca 250 cm ) to afford a clear orange solution. The reaction mixture was cooled to -30 C overnight and the product was obtained as white needle shape-, crystals. These crystals were isolate! from the reaction mixture by filtration under a nitrogen atmeosphere, and by successive washings with petroleum ether precooled to 'l G. The crystals were pumped for 1 hour at room temperature to remove any volatile impurities. Hicroanalysis of the product suggeste! that the product had a- limiting composition (i-C_H_)„Te.CdI_. The pure adduct (192.06 g, 0.331 mol) was heated to ca. 1?0°G Pure !i-isoprop l ellurium distillei and was collected in vacuo in a cold trap maintained at -196°C. Yiel " . was 31-26g, 50 Si . based on the starting (i-C^H 7 )_Te. Preparative Example 3 Puri f ication of di-isopropyltellurium using ca' iu^ broπiile A solution of di-isopropyltellurium (^ 62 g, 0.0216 £mol) in ethanol (50 cm ) was added dropwise to a solution of cadmium bromi 'e (CdBr^H^O, 9.9I g) in ethanol (50 cm ) A yellow-white micro- crystalline precipitate formed during the addition The pro ' uct wan isolate ' from the reaction mixture by filtration under a nitrogen atmosphere and by successive washings with petroleum ether pre- cooled to 4 G. The cryεtls were pumped for 1 hour at room temper- m ature to remove any volatile impurities. Hicroanalysis of the crystals suggestei that the pro d uct br ^ a limiting composition CdBr ((i-C_H_)_Te)„ . The pure adduct (2.5 g) was heated to ca.l25°G Pure -'i-iso- propyltεllurium ^istille-ϊ and was collected in vacuo in a col ~ trap mmaaiinnttaaiinneedd aatt -1°6 G. Yiel ! was l.oo g. 65. 6 based on the weight of the adduct SUBSTITUTE SHEET

Claims: Claims 1. A method of purification of a tellurium or selenium alkyl characterised by forming an adduct of the alkyl with at least one compound of a group 11 or 12 metal (Cu, Ag, Au, Zn, Cd, Hg) and thermally dissociating the said adduct to yield the alkyl. 2. A method of purification according to claim 1 characterised in that the tellurium or selenium alkyl.is a dialkyl. 3. A method of purification according to claim 1 or 2 characterised in that each alkyl group in the tellurium or selenium alkyl contains 1 to ^ carbon atoms. ^. A method of purification according to any one of claims 1, 2 or 3 characterised in that the alkyl is a tellurium alkyl. 5- A method according to claim 1 characterised in that the compounc' of a group 11 or 12 metal is a halide or nitrate thereof. 6. A method of purification according to claim 5 characterised in that the metal compound is a chloride, bromide or iodide. 7. A method of purification according to claim 6 characterised in that the at least one compound of a group 11 or 12 metal is selected from Cul, Cdl 2 , CdBr 2 , HgCl 2 Cu^ and Hgl 2 . 8. A method of purification according to claim 1 characterised in that the adduct is forme'! by reaction between the alkyl and the metal compound in an organic solvent. 9. A method according to claim 3 characterised in that the solvent is selecte! from ethers, alcohols an', nitriles and mixtures thereo^. 10. A methoi of purification according to claim 1 characterise-' in that the a d duct is dissociated at 8θ C C - 150°G. 11. A method of purification according to claim 1 characterised in that ' r1 iethyl tellurium is purified by forming an adduct with a compound selected from Cdl ? , CdBr ? , Hg01 ? , Cul ? , and Hgl ? ani then thermally dissociating the adduct and collecting the alkyl. 12. A method of purification according to claim 1 characterised in that diisop opyl tellurium is purified by forming an adduct with a compoun 3 selected from Gil„ and CdBr and then thermally dissociating the aiduct and collecting the alkyl. SUBSTITUTE SHEET m- 13- An adduct of dialkyl tellurium having a formula, selected from following: ((C 2 H 5 ) 2 Te) 2 C I 2 (G 2 H 5 ) 2 e(G Br 2 ) 2 ((G 2 H 5 ) 2 Te) 2 C Br 2 ((i-C 3 H 7 ) 2 Te) 2 CdI 2 ((i-C 6 H 7 ) 2 Te) 2 Cd-Br E (i-C ; -,H 7 ) 2 TeCdI 2 HEET

Inventors: COLE-HAMILTON DAVID JOHN, MULLIN JOHN BRIAN, SHENAI-KHATKHATE DEODATTA VINA, WEBB PAUL, COLE-HAMILTON, DAVID, JOHN, MULLIN, JOHN, BRIAN, SHENAI-KHATKHATE, DEODATTA, VINAYAK, WEBB, PAUL
Applicants: SECR DEFENCE BRIT, THE SECRETARY OF STATE FOR DEFENCE IN HER BRITANNI
Application year: 1988
Main classification: C07C165
IPC codes: C07C391, C07C395