Michael P. Beer

The adduct formed between dimethylcadmium and 1,4-dioxane, 1,4-dioxanedimethylcadmium(II): a crystallographic and spectroscopic study

Abstract The reaction of dimethylcadmium with 1,4-dioxane gives a 1:1 adduct, 1 . Recently it has been shown that the growth of CdS by metal organic chemical vapour deposition (MOCVD) is improved if the commonly used dimethylcadmium is replaced by 1 . The nature of 1 in the solid state was unknown and so a sample of 1 was sublimed in vacuo to give colourless crystals, on which an X-ray diffraction study was carried out. The cadmium atom is four coordinate, being bound to two methl groups (CdC 2.09(2) A) and two oxygen atoms (CdO(1) 2.88(2) A, CdO(4*) 2.75(2) A) from different 1,4-dioxane molecules, giving rise to an unusual one-dimensional polymeric structure. Evidence for an association between dimethylcadmium and 1,4-dioxane in benzene solution was obtained from a spectroscopic study. 1 is not transported in the gas-phase in sizeable quantities, thus casting doubt upon the role conventionally assigned to it in reducing side reactions in MOCVD.

Growth of II–VI compounds by metal-organic chemical vapour deposition. Structural characterisation of dimethylcadmium-N,N,N′,N′-tetramethylethane-1,2-diamine by gas-phase electron diffraction and its use in the growth of epitaxial layers of CdS and CdSe upon GaAs

Aspects of the use of Lewis acid–base adducts of dimethylcadmium in the growth of CdY (Y = S or Se) by metal-organic chemical vapour deposition (MOCVD) have been investigated. The adduct dimethylcadmium-N,N,N′,N′-tetramethylethane-1,2-diamine, Me2Cd·Me2NCH2CH2NMe2(1), was made and its structure determined by gas-phase electron diffraction. In 1 the cadmium atom is four-co-ordinate, being bound to two methyl groups [rα(Cd–C)= 2.11(2)A] and two nitrogen atoms [rα(Cd—N)= 2.47(5)A] with ∠C—Cd—C and ∠N—Cd—N being 132(11) and 84(3)°, respectively. The electron diffraction study provides the first unequivocal evidence for the existence of an adduct of dimethylcadmium being transported in the gas phase. Further evidence for the existence of 1 in the gas phase is provided by mass spectrometry and gas-phase infrared spectroscopy. Compound 1 was subsequently employed in the MOCVD process to grow visibly good-quality layers of CdY (Y = S or Se) on gallium arsenide substrates, but without overcoming the problem of pre-reaction experienced in the growth of CdY (Y = S or Se) by MOCVD.

Extended X-ray absorption fine structure studies of the amorphous tungsten sulfides and selenides, WS5, WSe5 and WS3

Extended X-ray absorption fine structure (EXAFS) and S K-edge absorption studies of the amorphous tungsten sulfide, WS5, and EXAFS studies of the amorphous tungsten selenide, WSe5, suggest these compounds can be formulated as WV(S22–) and WV(Se22–)2.5. The observed metal–metal distances of ca. 2.75 A are consistent with the formation of a metal–metal bond between these d1 metal centres; the metal–metal bond would explain the observed diamagnetism of these compounds. The Se—Se bond length of 2.34 A, determined from the Se K-edge EXAFS studies, is typical of a diselenide group. Comparison of the S K-edge absorption spectrum for WS5 with spectra from related compounds suggests that the sulfur is in the –1 oxidation state. This is in agreement with the observed absorption in the infrared at 518 cm–1 which we assign to an S–S stretch in a disulfide group. The S K-edge absorption spectrum of WS3 suggests that this compound contains both S–I and S–II. W LIII-edge EXAFS studies of WS3 show much greater disorder in the W—S bonding than in WS5, which can be attributed to the presence of sulfur in two oxidation states. Models based on two possible formulations, WV(S2–)2(S22–)0.5 and WIV(S2–)(S22–), give almost equally good fits to the experimental data. The observed diamagnetism of WS3 can be explained by the formation of W—W bonds of ca. 2.75 A, which are found in the EXAFS studies. Possible structural models for WS5 and WSe5 are presented.

The synthesis and spectroscopic characterization of bis(γ-methoxypropyl) cadmium

Abstract The preparation of the hitherto-unknown compound (CH 3 O(CH 2 ) 3 ) 2 Cd is reported. It is a colourless oil, which has been shown by mass spectrometry to vaporize without decomposition. A thorough vibrational and multinuclear magnetic resonance study suggests strongly that there is no internal coordination of the O atoms to the Cd atom, whereas such coordination is observed in the zinc-containing analogue.

Growth of II–VI compounds by metal–organic chemical vapour deposition. Propylene sulfide: a novel sulfur-containing precursor for MOCVD growth of ZnS

As part of our continuing investigations into the problem of pre-reaction in MOCVD systems, layers of ZnS have been grown onto GaAs substrates using Me2Zn and the novel sulfur-containing precursor propylene sulfide. Layers have been produced in the temperature range 300–550 °C and have been characterized by XRD. The results indicate that the layers with the highest degree of crystallinity are grown towards the higher end of this temperature range. Most significantly, no pre-reaction is observed between these precursors.

Studies of adducts of main group organometallic compounds by Fourier transform Raman spectroscopy

Abstract Fourier transform (FT)-Raman spectroscopy has been used to study species that play a part in Metal Organic Vapour Phase Epitaxy (MOVPE). These compounds include N-donor adducts of (CH 3 ) 2 Cd and (CH 3 ) 3 Ga and the dimeric species [(CH 3 ) 2 GaN(CH 3 ) 2 ] 2 which is a likely intermediate in the thermal production of the wide band gap semi-conductor GaN from (CH 3 ) 3 Ga.N(CH 3 ) 2 H. A search using FT-Raman spectroscopy for possible adducts of (CH 3 ) 2 Cd and (C 2 H 5 ) 2 Cd with Te-donor molecules is also described. It is found that FT-Raman spectroscopy with near-infrared excitation is a particularly suitable method for studying adducts containing nitrogen heterocycles which show strong fluorescence under visible irradiation. Thus the Raman spectra of the adducts (CH 3 ) 2 Cd.2,2′-bipyridyl and (CH 3 ) 2 Cd.1,10-phenanthroline, which are bright yellow, have been measured.

An FTIR and FT-Raman study of some novel organo-cadmium compounds

Abstract Infrared and Raman spectra are reported for the compounds (CH3OCH2CH2CH2)2Cd (A), CH3CdOCH2CH2SCH3 (B) and Cd(OCH2CH2SCH3)2 (C). These spectra demonstrate that, as expected, A contains a linear CCdC moiety and C a linear OCdO moiety. It is shown that there is no intramolecular bonding between the O and Cd atoms in A or between the S and Cd atoms in B or C. Spectra of A have been recorded for all of gas (IR), liquid (Raman) and solid (IR) phases and show that there is no significant change in structure of this molecule on changing phase. A preliminary assessment suggests that none of these compounds are likely to be particularly useful precursors for the formation of cadmium-containing semiconductor materials by MOCVD (Metal Organic Chemical Vapour Deposition).

The molecular structure of diethyl cadmium determined by gas-phase electron diffraction

Abstract The molecular structure of diethyl cadmium (C2H5)2Cd, has been studied by gas-phase electron diffraction. The molecule was found to consist of a central linear CCdC fragment terminated at each end by a methyl group From consideration of the data it seems that there is a substantial degree of rotation about the CdC bonds Results from a model in which free rotation about the CdC bonds was assumed yielded the following values for the molecular parameters: rg(CdC) = 2.133(6) A, rg(CC) = 1.537(7) A, ∠CdCC = 115.81(1.1)°.