Molecular Design of Precursors for the Chemical Vapor Deposition of

Chapter 27

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on April 3, 2018 | https://pubs.acs.org Publication Date: December 1, 2005 | doi: 10.1021/bk-2005-0917.ch027

Molecular Design of Precursors for the Chemical Vapor Deposition of Group 13 Chalcogenides Claire J . Carmalt, Emily S. Peters, Simon J. King, John D. Mileham, and Derek A. Tocher Department of Chemistry, University College London, 20 Gordon Street, London WC1H 0AJ, United Kingdom

A range of aluminum and gallium alkoxide and thiolate complexes have been synthesized and characterized. These complexes are of interest as precursors to group 13 oxides and sulfides. Thermal gravimetric analysis was used to study the decomposition of some of the compounds. The formation of thin films of alumina and gallium oxide on glass substrates was achieved via low pressure chemical vapor deposition. The films were analyzed by SEM/EDAX, powder XRD, XPS and band gap measurements.

Introduction Traditional synthetic routes to many materials involve high temperatures and/or the use of toxic and pyrophoric reagents. The formation of a range of materials, including metal nitrides, sulfides and phosphides via single-source chemical vapor deposition (CVD) has been of interest (7-3). Single-source precursors offer several advantages over traditional routes. These include lower deposition temperatures, reduced contamination from carbon and removal of the use of toxic/pyrophoric reagents. In general, a successful single-source precursor should contain the elements of the desired material (e.g. Ga and Ο for Ga C>3), possess easily removable ligands and exhibit sufficient volatility. 2

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© 2006 American Chemical Society Lattman and Kemp; Modern Aspects of Main Group Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

377 Alumina thin films offer, in principle, great potential in a number of applications including as wear resistant coatings, insulating layers, optical filters and corrosion protective coatings (4). The conventional C V D method for preparing alumina thin films involves the hydrolysis of A1C1 with a mixture of H and C 0 at 700-1000 C (5). The formation of alumina films from other precursors, such as the pyrophoric AlMe have also been described (6). However, highly volatile and non-pyrophoric precursors are preferred for the deposition of alumina films in order to make the process safer. The versatility of metal alkoxides as precursors to metal oxides via CVD processes is widely recognized; they are easy to prepare and purify and are intrinsically noncorrosive so can be stored almost indefinitely in a dry atmosphere (7). Metal alkoxides are ideal for use in low pressure (LP)CVD as a result of their high volatility. Despite the interest in alumina there are few reports of the use of organoaluminum alkoxides for the deposition of A I 2 O 3 thin films (8). Gallium oxide (Ga 0 ) is considered to be one of the most ideal materials for application as thin film gas sensors at high temperature (9). Gallium oxide is an electrical insulator at room temperature and semiconducting above 400 °C; it is chemically and thermally stable. As well as detecting oxygen gas at temperatures above 900 °C, a gallium oxide thin film operates as a surfacecontrol-type sensor to reducing gases below 900 °C (70). Therefore, it may be possible to switch the function of the sensor with temperature (77). CVD is the most practical method for preparing thin films for large-scale applications. CVD has a number of advantages over other deposition techniques as it offers the potential for good film uniformity and composition control, large area growth and excellent step coverage. Despite the interest in gallium oxide for thin film gas sensor applications there have been very few reports using CVD to deposit Ga 0 films. The precursors [Ga(hfac)3] (hfac = hexafluoroacetoacetonate), [GaiOCHiCF^MHNMe^] and [Ga(O Bu) ] have been used to deposit amorphous G a 0 thin films at ~ 450 °C (12, 13, 14). Group 13 sulfides (e.g. A1 S ) are also of interest and are reported to have potential applications as alternatives to II/VI materials for photovoltaic and optoelectronic devices (75). The synthesis and characterization of a range of potential precursors to aluminum and gallium oxides and sulfides are described herein. The decomposition of some of the compounds has been studied using thermal gravimetric analysis and the formation of thin films of alumina and gallium oxide is described. 3

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Synthesis of Molecular Precursors A requisite for metal alkoxides to be precursors in CVD processes are that they should be sufficiently volatile and sublimation should occur at as low a temperature as possible. The presence of hydride ligands in place of alkoxide groups, e.g. [AlH(OR) ] versus [Al(OR) ] , can increase the volatility of the precursor. Another method for increasing the volatility of metal alkoxides is to 2

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Lattman and Kemp; Modern Aspects of Main Group Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 2005.

378 use the 'donor functionalization' concept (16). Using this concept a ligand type can be developed which unifies the advantages of both steric demands and σdonor stabilization. The reaction of alane with alcohols and thiols and the formation of aluminum and gallium 'donor functionalized' alkoxides are described below.


Reactivity of Alane with Alcohols and Thiols The reaction between [AlH (OEt )] (generated in situ from L1AIH4 and (/ 7)) and 1 equivalent of /-PrOH in diethyl ether at room temperature resulted in the isolation of colorless crystalline 1 (18), as shown in Scheme 1. 3

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Scheme 1 An X-ray structure determination showed that the pentanuclear complex [Αΐ5(μ4-0)(μ-0-/-ΡΓ) Η6] (1) had formed (Figure 1). The formation of compound 1 could be the result of hydrolysis, however oxoalkoxides have been isolated previously, even where exceptional precautions against adventitious hydrolysis were taken. For example, triisopropoxides of a number of metals are actually pentanuclear oxoaggregates of formula [Μ5(μ5-0)(μ3-0-/-ΡΓ) (μ-0-/-ΡΓ)4(0-/7

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r)s] (e.g. M = In, Se, Yb) (79, 20). The central, four-coordinate, oxygen atom in 1 adopts a distorted square pyramidal geometry, with Al(4) occupying the apical position, but with the basal site opposite to Al(3) vacant. The Al(2)-0-Al(2A) angle is 154.09(15)° whilst the remaining A l - O - A l angles are in the range 97.88(8) to 99.02(8)°. The Al-O(isopropoxide) distances involving Al(2), Al(3) and Al(4) are typical (1.843(2)-1.858(2) A), whereas those to A l ( l ) are significantly shorter by 0.04 A ; conversely, the A l - 0 bonds to the central oxygen atom are 0.04 A longer with that to the "apical" Al(4) being the longest. H

Figure 7. Molecular structure of the C symmetric complex 1. (Reproduced by permission of The Royal Society of Chemistry (RSC) on behalf of the Centre National de la Recherche Scientifique (CNRS). Copyright 2002.) s

The reaction of [AlH (NMe Et)] and one equivalent of ROH (R = C HF 2,3,5,6) in diethyl ether resulted in the formation of the dihydrido complex [H Al(OR)(NMe Et)] (2) (Scheme 1). The *H N M R spectrum of 2 shows the expected 1:1 ratio of amine:alkoxide peaks. The presence of the hydride group was confirmed by the FT-IR spectra of 2, which showed characteristic absorption and deformation bands at 1897 and 772 cm" , respectively (27). Based on the presence of the intense band at 1897 cm" in the IR spectrum, the structure of 2 is most probably monomeric (Scheme 1) (22). The monohydrido complexes [HAl(C HF -2,3,5,6) (NMe Et)] (3) and [HAl(OC H Me -2,6) (NMe Et)] (4) were isolated from the reaction of [AlH (NMe Et)j and two equivalents of ROH (R = C6HF -2,3,5,6 or C H Me23

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380 2,6) in diethyl ether (Scheme 1). The *H NMR spectra of 3 exhibits broad peaks suggesting that a fluxional process is occurring in solution at room temperature. The 1:2 ratio of amine.alkoxide peaks is observed as expected and the FT-IR shows characteristic A l - H bands (1896 and 776 cm' ). Analytical and spectroscopic data confirmed the formation of compound 4 (FT-IR 1843 and 760 cm" ). The observed sharp band in the IR spectra of 3 and 4 are consistent with monomeric structures (Scheme 1) (22). Crystals of (5) were obtained on recrystallization of 4, and from the 1:3 reaction of [AlH (NMe Et)] with 2,6-Me2C H30H. Analytical and spectroscopic data for 5 were consistent with the formation of the triphenoxide [Al(OC H Me2-2,6)3(NMe2Et)]. The X-ray structure of 5 (Figure 2) shows the coordination of the central aluminum to be aflattenedtetrahedral, the N-A1-0 angles ranging between 102.49(6) and 107.56(6)°, whereas those for O - A l - 0 are between 111.73(6) and 116.36(6)°. The A l - 0 distances are unexceptional, ranging between 1.7007(12) and 1.7172(12) A [the A l - N distance is 1.9736(14) Â]. The structure of 5 is similar to related compounds, such as the ketone adduct [Al(OC6H2-/-Bu -2,6-Me-4)(0=C(C H9)-/-Bu-4)](2y). 1

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Figure 2. The Molecular Structure of 5. The reaction of [AlH (OEt )] and one equivalent of 2,6-Me2C H SH in diethyl ether at room temperature afforded colorless crystals of [Al(SC6H Me22,6) (OEt )] (6) (23). Analytical and spectroscopic data for 6 were consistent with the formation of [Al(SC6H Me -2,6)3(OEt2)] rather than that of the 3

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381 expected product [H Al(SC6H3Me2-2,6)] . The X-ray structure of 6 is shown in Figure 3. The structure, ignoring the two ethyl groups, has pseudo C3 symmetry about the O - A l direction creating a propeller-like geometry. The three 2,6dimethylphenyl ring systems are oriented approximately orthogonally (77, 82 and 96°) to the S plane. The "all down" geometry observed here is in contrast to the "one up, two down" conformation seen in compound 5. The geometry at aluminum is best described as flattened tetrahedral, the metal center being displaced towards the "basal" S face, lying only 0.43 Â out of this plane in the direction of the oxygen atom. The angles subtended at aluminum are in the range 97.34(7)-120.81(5) , and the sum of the three S-Al-S angles is 349°. The Al-S distances are unexceptional, ranging between 2.227(1) and 2.238(1) Â [the A l - 0 distance is 1.907(2) A], The propeller-like geometry results in one of the methyl groups of each 2,6-dimethylphenyl ring system being directed into the face of its nearest neighbor aryl ring, the shortest Η···π distance being 2.78 A. 2

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Figure 3. Molecular Structure of 6. (Reproduced with permission from referen 23. Copyright 2003 Elsevier.)

In order to obtain compound 6, the reaction mixture formed between LiAlH and AICI3 to produce [AlH (OEt )] was allowed to stir for 30 min before the addition of 2,6-Me C H3SH. However, if 2,6-Μθ2θ6Η 8Η is added immediately to the slurry of L1AIH4 and AICI3 in diethyl ether, colorless crystals of a new compound [Li(OEt )3][Al(SC H3Me -2,6)4] (7) are obtained (27). The formation of 7 is the result of incomplete reaction of aluminum(III) chloride with L1AIH4 before the addition of the thiol. Spectroscopic data shows only the 4

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382 formation of 7 rather than a mixture of products. A single crystal X-ray diffraction analysis showed the anion in 7 to be the S -symmetric homoleptic complex shown in Figure 4. The geometry at aluminum is distorted tetrahedral with unique angles of 104.85(3) and 119.16(6)°, reflecting a slight flattening of the tetrahedron about the S axis [which bisects the S-Al-S(OA) angle], and an Al-S distance of 2.253(1) A. The [(Et 0) Li] cation is disordered. 4

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Figure 4. The structure of the homoleptic, Srsymmetric anion 7. (Reprod with permissionfromreference 23. Copyright 2003 Elsevier.)

Decomposition Studies Thermal gravimetric analysis (TGA) of 3 (10 °C/min from 20 to 500 °C; under N ) showed a total weight loss of 78%, which is less than the 88% weight loss expected if decomposition to A1 0 has occurred. The TGA of 4 also showed that incomplete decomposition has occurred as the total weight loss (77%) is less than that expected (85%). Three weight losses were observed in the TGA of 3 and 4, the first of which (19% and 23% respectively) corresponds to the loss of NMe Et (calculated as 17% and 21% respectively). The decomposition properties of [Al(SC H Me -2,6)3(OEt )] (6) were also studied by TGA (Figure 5). The decomposition of 6 is clean and shows a total weight loss of 73%. This is in good agreement with the calculated value for decomposition of 6 to A1 S (71%). Two distinct weight losses are observed, the first (19%) corresponds to the loss of solvent from 6 (20%). The second (54%) corresponds 2

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Donor-Functionalized Aluminum and Gallium Alkoxides The reaction of Et M (M = A l or Ga) with one equivalent of HOCH CH NMe resulted in the formation of white crystalline solids (compounds 8 and 9), according to Scheme 2. Analytical and spectroscopic data were consistent with the formation of the monoalkoxides [Et Al(OCH CH NMe )] (8) and [Et Ga(OCH CH NMe )] (9). On the basis of mass spectroscopic data these compounds are thought to be dimeric via alkoxy bridges in the solid-state. By TGA, compound 9 gave a mass loss of 71% with a clean decomposition. This sample showed the best deposition characteristics. Therefore LPCVD using compounds 8 and 9 were studied. 3

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Thin-Film Growth LPC VD of 8 and 9 were investigated, the details of which are described in the Experimental section. Both complexes formed gray/white colored films on glass under LPCVD conditions at 500 °C. SEM analysis of the films produced from 8 showed that these films consist of agglomerates of approximately 0.5 μπι. The EDAX analysis showed the presence of aluminum and oxygen in the films. Significant breakthrough of the coating to the underlying glass was observed and so accurate quantitative analysis by EDAX was difficult. X-ray photoelectron spectroscopy (XPS) of the film revealed the presence of aluminum and oxygen. The A l 2p binding energy shift of 74.69 eV and the Ο 2p shift of 532.79 eV are in good agreement with previous literature measurements (Al 2p, 74.7 eV; Ο 2p 531.6 eV) (24). The films showed negligible carbon content by both EDAX and XPS (0-1%). Overall films grown from 8 have an approximate composition of A I 2 O 3 . The films were amorphous according to powder XRD. SEM analysis of the films produced from 9 showed the formation of spherical particles of approximately 0.5-2 μπι in size. EDAX analysis showed the presence of gallium and oxygen in the films. However, the films were amorphous according to powder XRD, typical for G%0 films grown at this temperature (75). Therefore, the films were annealed at 900 °C for 24 hr, resulting in crystalline p-Ga 0 films as shown by powder XRD (Figure 6). X ray photoelectron spectroscopy (XPS) of the film revealed the presence of gallium, oxygen and carbon. However, the carbon concentration was significantly reduced after the first etch, indicating that the carbon contamination was graphitic. The Ga 2p binding energy shift was 1119.2 eV and the Ο 2p shift was at 532.8 eV for films grownfromcompound 9. The XPS peak profiles are consistent with a single environment for gallium and oxygen. The binding energy shifts are in agreement with previous literature values (14). Overall films grown from 9 have an approximate composition of Ga 0 . Conducting a Tauc plot of the UV/Visible data indicated that G%0 prepared by 3

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385 LPCVD had an indirect bandgap of 4.6 eV, comparable to other literature values for Ga 0 of 4.2-4.9 eV (14). 2

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Figure 6. X-ray powder diffraction pattern of the annealed Ga Oi film grown from compound 9. 2

Experimental

General Procedures All manipulations were performed under a dry, oxygen-free dinitrogen atmosphere using Schlenk techniques or in a MBraun Unilab glove box. A l l solvents were distilled from appropriate drying agents prior to use (sodium toluene and hexanes; sodium/benzophenone for diethyl ether). All other reagents were procured from Aldrich and used without further purification. Microanalytical data were obtained at University College London (UCL). The synthesis of compounds 1,5 and 6 were described in references 18 and 23.

Physical Measurements NMR spectra were recorded on Brtiker AMX400 spectrometers at UCL. The NMR spectra are referenced to CD2CI2, which was degassed and dried prior


386 ,3

to use; *H and C chemical shifts are reported relative to SiMe (0.00 ppm). Mass spectra (CI) were run on a micromass ZABSE instrument, and IR spectra on a Nicolet 205 instrument. TGA of the compounds were obtained from the Thermal Methods Laboratory at Birkbeck College. EDXA/SEM results were obtained on a Hitachi S570 instrument using the K E V E X system. Glancing angle X-ray diffraction analysis of the films was obtained using a Siemens D5000 diffractometer in reflection mode (1.5° incidence) using C u K (λ « 15406 Â) radiation. X-ray photoelectron spectra were recorded using a VG ESCALAB 220i X L instrument using focused (300 mm spot) monochromatic Al Κα radiation at a pass energy of 20 eV. Scans were acquired with steps of 50 meV. A flood gun was used to control charging and the binding energies were referenced to an adventitious C Is peak at 284.8 eV. Depth profile measurements were obtained by using argon beam sputtering. 4


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Synthesis of2 A solution of HOC6HF -2,3,5,6 (0.63 g, 3.77 mmol) in diethyl ether (10 mL) was added dropwise to [AlH (NMe Et)] (0.5 mL, 3.77 mmol) in diethyl ether (10 mL) at room temperature. The evolution of hydrogen was observed and the mixture was allowed to stir for 1 hour. The volume of the solution was reduced under vacuum to ca. 5 mL. Cooling to -20 °C afforded a white powder of 2 (0.58 g, 79%), mp 42 °C. H NMR (CD C1 ): δ 1.26 (t, J = 14 Hz, 3H, NCH C# ), 2.60 (s, 6H, ^(CH^\ 2.97 (q, J= 25 Hz, 2H, NCi/ CH ), 6.53 (spt, «7=21 Hz, 1H, C e F ^ , ΑΉ not detected. C{ li} NMR (CThCh): δ 8.8 (s, NCH CH ), 43.6 (s, N(CH )2), 94.5 (s, NCH CH ), 139.3 (s, p-OC F H), 141.7 (s, m-OC F H), 145.6 (s, o-OC F H), 148.0 (s, ipso-OC ¥M). IR (nujol, cm" ): 3079 w, 1897 s, 1650 s, 1515 s, 1261 s, 1169 s, 1108sbr, 1038 w, 1020w, 944 s, 925 w, 820 w, 805 m, 772 m, 738 m, 722 m, 687 w, 570 w, 480 w, 428w. 4

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Synthesis of 3 Compound 3 was prepared in a similar manner to 2 but using HOC HF 2,3,5,6 (1.25 g, 7.54 mmol) and [AlH (NMe Et)] (0.5 mL, 3.77 mmol). Cooling to -20 °C afforded a white powder of 3 (1.01 g, 75%), mp 46-48 °C. H NMR (CD C1 ): δ 1.34 (t, J = 14 Hz, 3H, NCH C# ), 2.72 (s, 6H, N(C// )2), 3.07 (d br, J = 11 Hz, 2H, NC// CH ), 6.53 (s br, 2H, CtftH), AlH not detected. C{ H} NMR (CD C1 ): δ 8.8 (s, NCH CH ), 43.0 (s, N(CH )2), 94.7 (s, NCH CH ), 95.4 (s, NCH CH ), 139.4 (s, p-OC F H), 141.7 (s, m-OCeF H), 145.6 (s, o-OC F H), 148.0 (s, i>so-Oe F H). IR (nujol, cm ): 3075 w, 1896 s, 1651 s, 1505 s br, 1280 w, 1261 m, 1168 s, 1109 s br, 1037 m, 993 w, 940 vs, 821 w, 806 w, 773 w, 727 w, 702 w, 686 m, 644 m, 568 m, 538 m, 531 m, 517 w, 481 s, 458 w. 6

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387 Synthesis of4/5 Compound 4 was prepared in a similar manner to 2 but using 2,6M e ^ H j O H (1.84 g, 15.1 mmol) and [AlH (NMe Et)] (1.0 mL, 7.54 mmol). Cooling to -20 °C afforded colourless crystals of 4 (1.24 g, 48% yield), mp 5860 °C. Data for 4: Anal. Calc. for GoHjoNCfeAl: C, 69.94; H, 8.81; N , 4.08. Found: C, 68.72; H, 8.24; N , 4.15. Ή NMR (CDzCfc): δ 1.39 (t, J = 10 Hz, 3H, NCH C# ), 2.28 (s, 12H, OCeHKCftk), 2.75 (s, 6H, N(Cft)j), 3.15 (q, 7 = 20 Hz, 2H, NCtf CH ), 6.75 (t, J = 17 Hz, 2H, /j-OCeflKCH^), 7.01 (d, J = 8 Hz, 4H, m - O C s W H j ^ ) . C{'H} N M R (CD C1 ): δ 8.4 (s, NCH CH ), 18.1 (s, OC H (CH3) ), 18.3 (s, N(CH )2), 43.3 (s, NCH CH ), 118.4 (s, p-OCeH^CH^), 127.0 (s, p-OC^CUÙ), 128.6 (s, o-OCeHKCHj», 155.9 (s, ipsoOCeHjiCH^). IR (nujol, cm ): 1843 s, 1594 s, 1429 w, 1281 s, 1237 s, 1182 m, 1093 s, 1036 m, 997 m, 919 s, 897 s, 760 s, 749 s, 679 s br, 532 m, 402 w. Compound 5 was prepared either from the recrystallization of 4 in diethy ether or via the route described above for 4 but using a 1:3 ratio of [AlHjiNMe^t)]: 2,6-Me C H OH. Data for 5: Ή N M R (CDiCfe): δ 1.28 (t, J = 10 Hz, 3H, NCH Cft), 2.12 (s, 18H, OC H (C// )2), 2.75 (s, 6H, ^(CHjh), 3.24 (q, J = 20 Hz, 2H, NC// CH ), 6.61 (t, J = 17 Hz, 3H,p- O C V / ^ C H ^ ) , 6.87 (d, J= 8 Hz, 6H, m- OC H (C# )2). C{'H} N M R (CEfeCfe): δ 7.5 (s, NCH CH ), 17.9 (s, OCeH^CH^ and N(CH )2), 44.1 (s, NCH CH ), 118.6 (s, /j-OCeH^CH^), 127.0 (s, p-OCeHKCH^), 128.8 (s, o - O C e H ^ C H ^ ) , 155.2 (s, ipsoOC6H (CH )2). IR (nujol, cm' ): 1589 m, 1562 s, 1475 s, 1430 m, 1279 s, 1234 s, 1175 m, 1092 s, 1029 m, 979 s, 933 s, 851 s, 786 s, 654 s, 609 s, 550 m. 3

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Synthesis of 8 HOCH CH NMe (2.34 g, 26.3 mmol) was added dropwise to a solution of [Et Al] (3.00 g, 26.3 mmol) in toluene (30 mL) at -78 °C with stirring. The reaction mixture was allowed to warm slowly to room temperature and stirred for a further 2 h to yield a colourless solution. Removal of the solvent in vacuo afforded a white solid. The solid was redissolved in hexane (~5 mL) and cooled to -20 °C. Compound 8 was obtained as white crystals after several days at this temperature (4.56 g, yield 100%). Anal. Calc. for C H NOA1: C, 55.47; H, 11.64; N , 8.06. Found: C, 54.31; H, 11.88; N , 8.90. Ή NMR (CDzCfe): δ -0.30 (quartet, J = 8.1 Hz, 4H, A1C# CH ), 1.00 (t, quartet, J = 8.1 Hz, 6H, AlCH Cft), 2.30 (s, 6H, NC// ), 2.54 (t, J = 5.4 Hz, 2H, OCH C# N), 3.70 (t, J = 5.4 Hz, 2H, OC# CH N, 2H). IR (nujol, cm" ): 2852 vs, 2794 m, 1464 vs, 1416 m, 1378 s, 1355 m, 1272 m, 1185 w, 11096 vs, 1073 w, 1031 m, 988 m, 904 m, 791 m, 674 m, 635 m, 575 m, 517 m, 443 m, 408 m. Mass Spec (CI): (m/z) 346 ([M]), 317 ([M]-Et), 173 (Et2Al(OCH CH NMe ), 144 (EtAl(OCH CH NMe )), 72 (CH CH NMe ). 2

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388 Synthesis of 9 Compound 9 was prepared in a similar manner to 8 but using HOCH CH NMe (0.64 mL, 6.37 mmol), [Et Ga] (0.95 mL, 6.37 mmol) and hexane rather than toluene. Compound 9 was obtained as a pale white microcrystalline solid after several days at -20 °C (1.14 g, yield 83%). Anal. Calc. for C H NOGa: C, 44.49; H, 9.33; N , 6.49. Found: C, 44.38; H, 9.40; N , 6.59. Ή NMR (CD C1 ): δ 0.28 (quartet, J = 8.1 Hz, 4H, GaC// CH ,), 1.08 (t, J = 8.1 Hz, 6H, GaCH C# ), 2.24 (s, 6H, NC# ), 2.40 (t, J = 5.4 Hz, 2H, OCH C// N), 3.67 (t, J = 5.4 Hz, 2H, OC# CH N). C{ H} NMR (CD C1 ): δ 3.8 (GaCH CH ), 10.6 (GaCH CH ), 45.2 (NCH ), 58.9 (OCH CH N), 61.8 (OCH ). IR (nujol, cm" ): 2856 vs, 2789 m, 1460 vs, 1420 m, 1379 s, 1356 m, 1273 m, 1186 w, 1101 vs, 1072 w, 1036 m, 999 m, 954 m, 932 w, 893 m, 785 m, 629 m, 554 m, 503 m, 435 m. Mass Spec (CI): (m/z) 433/432 ([M]), 403 ([M]-Et), 344 ([M]-(OCH CH NMe )), 216 (Et Ga(OCH CH NMe ), 186 (EtGa(OCH CH NMe )), 127 (Et Ga), 72 (CH CH NMe ). 2

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Low pressure CVD experiments 0.5 g of compound was loaded into the sealed end of a glass tube (400 mm length χ 16 mm diameter) in the glovebox. Glass substrates (100 mm χ 7 mm χ 1 mm) were placed along the inside of the tube. The tube was then put in a furnace such that 30 cm was inside the furnace and the end containing the sample protruded by 4 cm. The tube was heated to a temperature of 600°C under vacuum (0.1 Torr). The tube was slowly drawn into the furnace until the sample started to melt. It was held at that point until all of the precursor had decomposed and then the furnace was allowed to cool to room temperature. A l l the compounds produced white/gray films deposited on the glass substrates. The films were analyzed by EDAX/SEM, band gap measurements, XRD and XPS.

X-rqy structure determination Crystal data for 5 has been deposited in the Cambridge Structural Database (CCDC 239164).

Acknowledgements

We are grateful to Epichem Ltd for [AlH (NMe Et)J, EPSRC for studentships (ESP, JDM) and the Nuffield foundation for an undergraduate research bursary (SJK). We would like to dedicate this article to Professor A. H. Cowley on the occasion of his 70 birthday. 3

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References 1. 2.


3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

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