DOI: 10.1002/chem.201402998

Full Paper

& Chemical Vapor Deposition

Synthesis and Structural Characterization of b-KetoiminateStabilized Gallium Hydrides for Chemical Vapor Deposition Applications Peter Marchand,[a] David Pugh,[b] Ivan P. Parkin,[a] and Claire J. Carmalt*[a]

Abstract: Bis-b-ketoimine ligands of the form [(CH2)n{N(H)C(Me) CHC(Me)=O}2] (LnH2, n = 2, 3 and 4) were employed in the formation of a range of gallium complexes [Ga(Ln)X] (X = Cl, Me, H), which were characterised by NMR spectroscopy, mass spectrometry and single-crystal X-ray diffraction analysis. The b-ketoimine ligands have also been used for the stabilisation of rare gallium hydride species [Ga(Ln)H] (n = 2 (7); n = 3 (8)), which have been structurally characterised for the first time, confirming the formation of five-coordinate, monomeric species. The stability of these hydrides has been probed through thermal analysis, revealing stability at temperatures in excess of 200 8C. The efficacy

Introduction Gallium oxide has found use in a diverse range of industrially and technologically important applications. There are five known crystallographic phases of Ga2O3, with the most thermodynamically stable being monoclinic b-Ga2O3 on account of its low density.[1] b-Ga2O3 is a wide band-gap material (4.2– 4.9 eV),[2] which is electrically insulating at room temperature, however displays n-type semiconductivity at elevated temperatures.[3] At temperatures above 500 8C, Ga2O3 acts as a sensor for reducing gases such as CO and H2,[4] whereas at temperatures above 900 8C it can be used as a catalytically inactive oxygen sensor.[3] More recently, Ga2O3 nanowires have been found to show a reversible response to O2 and CO at much lower working temperatures in the range of 100–500 8C.[5] Lanthanum gallate (LaGaO3) doped with various metals was observed to have superior oxygen-ion-conductivities in comparison to other materials such as yttria-stabilised zirconia (YSZ), making these ma-

[a] Dr. P. Marchand, Prof. I. P. Parkin, Prof. C. J. Carmalt Materials Chemistry Centre, Department of Chemistry University College London 20 Gordon Street, London, WC1H 0AJ (UK) Fax: (+ 44) 020-7679-7463 E-mail: [email protected] [b] Dr. D. Pugh Department of Chemistry, University of Southampton Highfield, Southampton, SO17 1BJ (UK) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201402998. Chem. Eur. J. 2014, 20, 10503 – 10513

of all the gallium b-ketoiminate complexes as molecular precursors for the deposition of gallium oxide thin films by chemical vapour deposition (CVD) has been investigated through thermogravimetric analysis and deposition studies, with the best results being found for a bimetallic gallium methyl complex [L3{GaMe2}2] (5) and the hydride [Ga(L3)H] (8). The resulting films (F5 and F8, respectively) were amorphous as-deposited and thus were characterised primarily by XPS, EDXA and SEM techniques, which showed the formation of stoichiometric (F5) and oxygen-deficient (F8) Ga2O3 thin films.

terials highly desirable for use as solid electrolytes in solid oxide fuel cells (SOFCs).[6] In addition, gallium oxide thin films have been used as white-light emitting phosphors for use in light-emitting devices (LEDs),[7] as well as a potential deep-UV transparent conducting oxide.[8] Further application includes use within catalytic zeolite systems to enhance the conversion of lower alkanes into aromatic molecules.[9] Given the range of applications of thin films of gallium oxide, it is no surprise that a number of techniques have been explored for the deposition of the material. Common fabrication techniques such as sputtering,[10] sol-gel processes,[11] spray pyrolysis,[12] screen printing,[13] atomic layer deposition,[14] as well as chemical vapour deposition (CVD) processes including low-pressure (LP),[15] atmospheric-pressure (AP)[16] and aerosol-assisted (AA)[17] CVD, have all been reported for the production of thin films of Ga2O3. Amongst the many available techniques for the deposition of Group 13 metal oxides, CVD is favourable due to the production of high quality, uniform films that are largely free from contamination. AACVD relies on the solubility of the chosen precursor rather than depending on its volatility, thus broadening the range of potential precursors that can be employed. Furthermore, the solution-based process offers an additional means with which to control film morphology and hence its properties, and is therefore of great interest for the deposition of materials.[18] Interest in the use of single-source precursors for the CVD of main group materials has been driven by their potential for the repeatable deposition of thin films with the desired stoichiometries and good homogeneity.[19] This has prompted a sig-

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Full Paper nificant amount of research into the development of novel Results and Discussion precursor systems, in particular those containing pre-existing M O bonds, for the deposition of metal oxides. To date, the Synthesis field of single-source precursors to gallium oxide thin films is largely dominated by mono- and donor-functionalised alkoxThe synthesis of chloro-gallium b-ketoiminate complexes ides and b-diketonates.[20] More recently, b-ketoimines have re1 and 2 has been previously reported, and was achieved through deprotonation of the pro-ligands LnH2 (n = 2 and 3; ceived some interest in this regard, mainly due to the potential for additional functionalisation of the imino residue allowing Scheme 1) with NaH followed by salt metathesis with for the formation of monomeric species with enhanced thermal stabilities and solubilities. Furthermore, gallium hydrides are an attractive class of compound for application as molecular precursors, with advantages including i) low molecular masses, ii) clean decomposition pathways due to the weakness of the Ga H bond, iii) ready M H bond cleavage with respect to M C and M Cl bonds and iv) low levels of carbon contamination within deposited films as a result of the absence of direct metal carbon bonds within the precursor.[21] Although their thermal instability has limited the use of gallium hydrides as molecular precursors, progress in the last 25 years has led to the isolation of some systems that exhibit considerable robustness. In particular, gallane adducts stabilised by N-heterocyclic car- Scheme 1. The synthesis of gallium b-ketoiminate derivatives 1–6. benes have been shown to have thermal stabilities as high as GaCl3.[24–25] The tetramethylene derivative of the pro-ligand, 214 8C.[22] Examples of mixed gallium hydride species containing M O bonds also exist in the form of mono- and donorL4H2, was synthesised by condensation of acetylacetone with functionalised alkoxides, with the latter being successfully uti1,4-diaminobutane, which was subsequently converted into lised in the deposition of Ga2O3 thin films.[23] Recent studies the corresponding gallium complex 3 through the same salt have demonstrated the use of bis-b-ketoimines as suitable metathesis pathway (Scheme 1). Colourless, crystalline blocks ligand systems for the formation and isolation of monomeric that were suitable for analysis by single-crystal X-ray diffraction gallium species, with the notable formation of the b-ketoimiwere formed by slow solvent diffusion of hexane into a concennate-stabilised gallium hydride complex [Ga(L2)H] (7).[24] Altrated CH2Cl2 solution of the crude product. The molecular though the application of the hydride as a molecular precursor structure of 3 is shown in Figure 1 and selected bond lengths to Ga2O3 by AACVD was reported, structural characterisation and angles are given in Table 1. The monomeric complex 3 crystallised as a five-coordinate species in the triclinic Pwas not obtained and no systematic study of the potential of 1 space group, with the gallium cation in a slightly distorted the bis-b-ketoiminate systems as precursors was carried out. trigonal bipyramidal geometry (t = 0.86).[26] Chloride, as the Herein, we report the synthesis and structural characterisation of a range of gallium bis-b-ketoiminates with chloride, methyl largest ligand, predictably occupies one of the equatorial sites, and hydride co-ligands. Thermal analysis and deposition studwith the other two sites being occupied by one oxygen and ies of the gallium b-ketoiminate precursors has been used to one nitrogen of the chelating b-ketoiminate ligand. The axial investigate their efficacy towards the formation of Ga2O3 by positions are occupied by the remaining donor atoms of the b-ketoiminate ligand, with the O(1)-Ga-N(2) angle bending AACVD. away from linearity to 175.34(6)8. Bond lengths within the complex are consistent with those found within the propylene analogue 2.[24] As expected, bond lengths within the ketoimine Chem. Eur. J. 2014, 20, 10503 – 10513

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Full Paper Whereas salt metathesis provides a clean route to the formation of a range of gallium b-ketoiminate complexes, an exploration of a range of alternative co-ligands was of interest with a view to their application as molecular precursors to Ga2O3 through CVD. Compounds 4–6 were synthesised by the methane elimination reaction of GaMe3 with pro-ligands LnH2 (n = 2, 3 and 4), according to Scheme 1. In all cases, the evolution of gas was observed at ca. 60 8C. Needle-like crystals of 4 were isolated by slow cooling of a hot hexane solution of the crude compound. Crystallographic analysis revealed the formation of a five-coordinate, monomeric complex shown in Figure 2. In contrast to 3, the gallium centre in 4 lies in a distorted squarebased pyramidal geometry (t = 0.19). The basal plane is formed of the N2O2 unit of the ketoiminate ligand, with the substituent methyl group occupying the axial position. This is in contrast to the geometries seen for the chloride complexes 1–3, which all tended towards trigonal bipyramidal geometries. Figure 1. ORTEP diagram of compound 3. Thermal ellipsoids at 50 % probability, hydrogen atoms omitted for clarity.

Table 1. Selected bond lengths [] for 3, 4, 7 and 8. Bond

[Ga(L4)Cl] (3)

Ga(L2)Me (4)

[Ga(L2)H] (7)

[Ga(L3)H] (8)

Ga O

1.881(1) 1.951(1) 1.960(2) 2.031(2) 2.2632(5) 1.295(2) 1.309(2) 1.309(2) 1.325(2)

1.942(4) 1.947(4) 2.030(5) 2.034(5) 1.971(5) 1.295(6) 1.302(6) 1.303(7) 1.312(7)

1.937(2) 1.957(2) 2.024(2) 2.043(2) 1.56(3) 1.288(3) 1.304(3) 1.299(3) 1.312(3)

1.915(2) 2.023(2) 1.994(2) 2.082(2) 1.75(3) 1.290(3) 1.303(4) 1.314(3) 1.313(4)

Ga N Ga X C O C N

moieties are intermediate between those of single and double bonds, suggesting a delocalisation of electron density.[27] Ga O bond lengths show good agreement with similar gallium b-diketonate complexes,[28] and are longer than Ga O bond lengths within gallium donor-functionalised alkoxide complexes.[29] This is a result of the bidentate, monoanionic nature of the bonding, making each individual Ga O bond weaker. The 1H and 13C{1H} NMR spectra of 3 were in agreement with the observed solid-state structure. Of note was the absence of the broad NH resonance in the 1H NMR spectrum, characteristic of protonated b-ketoimines, which acted as a good indication that deprotonation in situ was complete and that no proligand remained within the reaction mixture. The methylene groups of the tetramethylene bridging moiety were observed as two broad singlets at 3.52 and 1.78 ppm. This is likely to be a result of Berry pseudo-rotation of the complex in the solution phase, leading to movement in the tetramethylene bridging group and broad, “averaged” resonances being observed within the spectrum. Mass spectrometry showed the presence of the parent ion at m/z 354, in addition to various ions resulting from the fragmentation of the complex, in particular the loss of Cl (m/z 319) and CH3 (m/z 339) groups. Chem. Eur. J. 2014, 20, 10503 – 10513

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Figure 2. ORTEP diagram of compound 4. Thermal ellipsoids at 50 % probability, hydrogen atoms omitted for clarity.

Despite washing with hexane, 1H and 13C{1H} NMR spectroscopic analysis of the crystals showed the presence of a small amount of the free ligand. However, the resonances corresponding to the product were in agreement with the findings of the structural characterisation and could be easily distinguished from those corresponding to the free ligand. The formation of a monomethyl complex was confirmed by the presence of a three-proton singlet appearing at 0.46 ppm, corresponding to Ga CH3.[30] Mass spectrometry showed the protonated parent ion at m/z 307, as well as the loss of CH3 indicated by a fragment at m/z 291. Elemental analysis of the sample gave values close to those expected, although a slight discrepancy was found, likely due to the presence of residual free ligand within the sample. In an attempt to form the propylene and tetramethylene analogues of the mono-metallic methyl gallium complex, the re-

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Full Paper actions between trimethyl gallium and the free ligands L3H2 and L4H2 were carried out by applying the same method used for the synthesis of 4. 1H NMR spectra of both reaction products showed the presence of two sets of resonances, the first corresponding to the free ligand (including the presence of the characteristic NH proton) and the second presumably corresponding to the methane elimination product. In both cases, the ratio between the two sets of resonances was 1:1. Furthermore, the peak corresponding to the organometallic protons (Ga Me) showed an integration of 12 H, as opposed to the expected 3 H for the formation of the desired product, indicating that mono-substitution had taken place, with each equivalent of the free ligand reacting with two equivalents of GaMe3, resulting in the formation of the bimetallic species 5 and 6, as shown in Scheme 1. To confirm this, the reactions were repeated with a ligandto-metal stoichiometry of 1:2, resulting in the complete formation of the bimetallic species. The absence of any resonances corresponding to the presence of the pro-ligands (in particular that of the NH protons) within the 1H NMR spectrum confirmed that the reaction had gone to completion. Peaks at 0.38 ppm correspond to the organometallic methyl protons, which show a slight upfield shift with respect to the unreacted starting material GaMe3 ( 0.18 ppm).[31] Mass spectrometry of 5 confirmed the formation of the bimetallic product by the detection of a fragment at m/z 419, corresponding to the molecular ion with loss of a CH3 group. A fragment at m/z 321 corresponds to the loss of GaMe3 (perhaps forming [Ga(L3)Me] in the fragmentation process) and loss of a further CH3 resulted in a fragment detected at m/z 305. The mass spectrum of 6 showed an identical fragmentation pattern, again confirming the formation of the bimetallic species. Attempts to form crystals of 5 and 6 by numerous methods were unsuccessful, in contrast to the relatively facile recrystallisation of 4, owing to the oily nature of the bimetallic species compared with the solid mono-metallic complex. The synthesis of gallium hydride derivative 7 has been reported and discussed in a previous publication,[24] however, neither its structural characterisation nor the synthesis of its propylene analogue were reported. Moreover, apart from application in the AACVD of Ga2O3 thin films, no investigation into its thermal stability was carried out. Although initial attempts at crystallisation afforded only microcrystalline material, pale-yellow crystals of 7 were grown by vapour diffusion of hexane into a CH2Cl2 solution of the crude product. The propylene analogue 8 was also synthesised by the same method as 7, by direct reaction of L3H2 with a freshly prepared ethereal solution of [GaH3(NMe3)] (Scheme 1). In this case, the crude product was dissolved in a minimal volume of toluene and cooled to 20 8C, yielding colourless platelet crystals after four days. The synthesis of the tetramethylene analogue [Ga(L4)H] was investigated by the same synthetic pathway outlined for both 7 and 8, however numerous attempts led only to the formation of a small amount of grey precipitate within the reaction medium. Isolation of the solution, removal of the solvent and NMR spectroscopic analysis of the resulting off-white solid product showed only the presence of L4H2, similar to the obChem. Eur. J. 2014, 20, 10503 – 10513

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servations made in the attempted synthesis of [In(L2)H], which has been previously reported.[24] Given the successful synthesis and isolation of 7 and 8 without decomposition of the gallane starting material, it is likely that some reaction occurred with the tetramethylene pro-ligand, however, any product formed was insufficiently stable to allow its isolation, leading to the formation of gallium metal as a grey precipitate. Initial characterisation of 8 was carried out by 1H NMR spectroscopy, with the formation of the hydride being indicated by the absence of any NH proton of the pro-ligand and by the presence of a very broad singlet at 5.83 ppm. This peak broadening is due to the two quadrupolar gallium nuclei, 69Ga and 71 Ga, with I = 3/2 and is characteristic of hydrogen atoms forming terminal bonds to gallium.[23c, 32] IR spectroscopic analysis showed a broad band at 1867 cm 1, characteristic of a Ga H stretch.[23a] Figure 3 shows the molecular structures of hydride complexes 7 and 8, which both crystallised into the monoclinic P21/c space group. Analysis of the geometries around the galli-

Figure 3. ORTEP diagram of a) 7 and b) 8. Thermal ellipsoids at 50 % probability, hydrogen atoms (except H(1)) are omitted for clarity.

um centres by calculation of their respective t values revealed that 7 exhibits a distorted square-based pyramidal geometry (t = 0.35), whereas the cation of 8 lies in a distorted trigonal bipyramidal geometry (t = 0.61). In both cases, the degree of distortion is much greater than observed in chloride and methyl complexes, which typically showed geometries closer to the idealised extremes. Applying the same geometrical analysis to the corresponding ethylene-bridged chloride complex 1 reported by Vohs et al.[25] gave a t value of 0.03, showing a near-perfect square-based pyramidal geometry. All three gallium complexes of the form [Ga(L2)X] (X = Cl (1), Me (4) and H (7)) were, therefore, found to exhibit square-based pyramidal geometry (albeit to different degrees of distortion), whereas those of the form [Ga(L3)X] (X = Cl (2), Me (8)) were both found to have trigonal bipyramidal geometries. This observation is attributed to the lack of flexibility offered by L2 compared with L3 and is further demonstrated by N Ga N bite angles found within the complexes (Table 2). The constraint of the ethylene-bridged ligand L2 prohibits the bite angle approaching 908, resulting in a square-based pyramidal geometry. In contrast, the additional flexibility offered by L3 allows bite angles closer to 908, thus

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Full Paper Table 2. Comparison of N-Ga-N bite angles and complex geometries (represented by t values) for five-coordinate gallium b-ketoiminate complexes. Complex

t

Bite angle [8]

0.03 81.65(5) [Ga(L2)Cl] (1) [Ga(L2)Me] (4) 0.19 80.4(2) 0.35 79.58(8) [Ga(L2)H] (7) square-based pyramidal

Complex [Ga(L3)Cl] (2) [Ga(L4)Cl] (3) [Ga(L3)H] (8) trigonal

t

Bite angle [8]

0.82 89.64(5) 0.86 93.63(6) 0.61 87.77(8) bipyramidal

complexes 2 and 8 display trigonal bipyramidal geometries. Increasing the carbon chain length of the bridging group further, as in chloride complex 3, results in a further increase in the bite angle (93.63(6)8) and a trigonal bipyramidal geometry is again observed. As would be expected for main-group complexes, it has been found that the preferred geometry within these systems is trigonal bipyramidal, however, when geometric constraints are enforced by the ligand system, square-based pyramidal geometries are observed. Selected bond lengths for complexes 7 and 8 are given in Table 1. The Ga H bond length of 1.56(3)  observed in 7 was in keeping with literature values for gallium hydride species.[33] However, within 8, the Ga H bond was found to be significantly longer (1.75(3) ). Although this bond is also longer than those found in the majority of structurally characterised gallium hydride complexes, it is comparable to that of the amido-bridged dimer [H2GaNEt2]2 (1.72 )[34] and is likely to be a reflection of the uncertainty in accurately locating hydrogen compared with heavier atoms. The remaining bond lengths within the structures of 7 and 8 are consistent with those in the b-ketoiminate complexes previously discussed. In particular, C C, C O and C N bond lengths within the six-membered ring formed by the gallium centre and b-ketoiminate moieties show a delocalisation of electron density, as has been discussed for chloride complex 3. The hydride complexes showed good stability at room temperature over a period of a few weeks under an inert atmosphere. However, the complexes were susceptible to hydrolysis upon exposure to air and moisture. Thermogravimetric analysis was carried out to establish the thermal properties and decomposition characteristics, however, it is pertinent to note that the onset of decomposition of 8 was not observed until approximately 235 8C, representing considerable thermal stability for such species and thus presenting hydrides as feasible precursors for the deposition of gallium oxide by CVD.

Figure 4. Thermogravimetric analysis of gallium b-ketoiminate complexes. a) Complexes 1, 4 and 7 (L2); b) 2, 5 and 8 (L3).

tion chamber in the gas phase. All compounds showed a sizeable temperature window between melting and decomposition, suggesting their suitability for CVD (Table 3). Bimetallic species 5 and 6 showed melting points that were significantly lower than those for the monomeric species (65 and 73 8C, respectively), whereas their decomposition onset temperatures were comparable (215–230 8C), representing

Thermal analysis To gain an understanding of the general efficacy of b-ketoiminates towards the deposition of Ga2O3, thermogravimetric analysis and differential scanning calorimetry studies were carried out on complexes 1–8 between 25 and 600 8C (Figure 4). One important characteristic of a molecular precursor for thermal CVD processes is a sufficient temperature window between the melting point and the onset of decomposition to allow for the successful transport of the precursor to the reacChem. Eur. J. 2014, 20, 10503 – 10513

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Table 3. Melting points and decomposition onset temperatures [8C] of compounds 1–8 determined by DSC and TGA, respectively. Compound

Melting point [8C]

Decomp. onset temp. [8C]

1 2 3 4 5 6 7 8

200 161 191 120 65 73 126 127

260 260 245 210 215 230 215 235

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Full Paper a very large temperature window of approximately 150 8C. As discussed, the decomposition onset temperatures found for the hydride complexes 7 (215 8C) and 8 (235 8C) represent significant stability for such species and indicate that such compounds are feasible candidates for CVD application, particularly due to the low mass and clean decomposition offered by the hydride co-ligand. Figure 4 shows representative TGA mass loss profiles for the complexes based on ethylene (Figure 4 a) and propylene-bridged (Figure 4 b) ligand derivatives. Chloride complexes 1–3 showed higher decomposition temperatures (up to 100 8C higher for complete decomposition) and pathways involving multiple steps. Furthermore, residual masses were higher than would be necessary for full decomposition to Ga2O3, suggesting that unwanted ligand substituents were retained. The TGA profile for hydride complex 7 shows an initial mass loss at approximately 100 8C, which can be attributed to the loss of residual solvent (toluene) within the sample. No further mass loss was subsequently observed until the onset of decomposition at 215 8C, with decomposition occurring by a relatively clean pathway thereafter. The residual mass (43 %) was again higher than necessary for complete decomposition to Ga2O3 (32 %). The propylene derivative 8, however, showed a very clean decomposition profile and the total mass loss observed was 71 %, compared to a theoretical mass loss of 69 % for decomposition to the oxide. In our earlier attempts to form similar hydride species based on b-diketonate ligand moieties, the only gallium-containing products isolated were tris-diketonates of the form [Ga(bdk)3] (bdk = b-diketonate).[35] However, of note was the isolation of a doubly-reduced b-diol from the reaction mixture owing to the hydride reduction of the b-diketonate ligand. A grey precipitate was also observed within the reaction mixture, attributed to gallium metal. These observations provide evidence that such systems are prone to the formation of metallic gallium, with the hydride acting as a reducing agent, and it is therefore likely that decomposition of 7 and 8 yields metallic gallium in addition to Ga2O3, given the reducing potential of the hydride species. Indeed, this consideration could account for the small discrepancy between the experimental and calculated mass losses for 8, although this greater than anticipated mass loss could also be attributed to sublimation of the precursor. The best decomposition profiles were observed for the methyl derivatives 4–6, with clean, single-step decompositions occurring between 200–400 8C and major decomposition steps reaching completion at ca. 350 8C. Total mass loss was greater than necessary for decomposition to Ga2O3, suggesting either sublimation of the precursor prior to decomposition, or decomposition to gallium metal. Whereas the TGA profiles present the methyl complexes as suitable precursors, the bimetallic species 5 and 6 are likely to deposit oxygen-deficient films owing to the gallium-to-oxygen ratio of 1:1 within the precursor molecule (cf. Ga2O3 = 1:1.5). Aerosol-assisted chemical vapour deposition We had previously attempted to utilise 7 in an LPCVD process, resulting in decomposition of the precursor before sufficient Chem. Eur. J. 2014, 20, 10503 – 10513

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volatilisation took place to allow transfer of the precursor to the reaction chamber.[24] To overcome this problem, aerosol-assisted delivery of the precursors was used. Generation of an aerosol from a precursor solution facilitates the transport of the precursor to the reaction chamber by use of a carrier gas, circumventing the high temperatures necessary for thermal CVD and preventing decomposition of the precursor prior to it entering the reaction chamber. Despite the suggestion from TGA that decomposition of the chloride precursors occurs between 300 and 500 8C, which is within the desired temperature window for the deposition of Ga2O3 on glass, no deposition was observed for any of the chloride complexes under any experimental conditions employed. The substrate temperature was varied between 400 and 600 8C, flow rates between 0.2 and 1 L min 1 and quantities of precursor between 0.2 and 0.6 g. It was initially thought that the lack of film growth was due to insufficient solubility of the precursor in toluene, owing to the presence of a residue of the precursor within the AACVD flask after the nebulisation and transport of the solvent to the CVD reactor. However, use of a number of alternative solvents (CH2Cl2, tetrahydrofuran (THF) and MeOH), in which the solubility of the precursors was sufficient for no residue to be left, did not lead to any observable deposition. Depositions from precursors 5 and 8 were successful in the formation of thin, transparent films, observable by the appearance of interference fringes on the silica-coated glass substrates. Depositions were optimised on the basis of substrate coverage, with optimum conditions being given in Table 4. All films were determined to be adherent by the Scotch tape test, however they were easily scratched by a brass or stainless steel stylus.

Table 4. Optimised AACVD conditions for the deposition of Ga2O3 from single-source precursors 5 and 8 and XPS determined elemental ratios.[a] Film No.

Precursor

Mass [g]

Solvent volume [mL]

Deposition time [h]

Substrate temp. [8C]

F5 F8

5 8

0.2 0.6

30 40

2 2.5

550 500

[a] All depositions were carried out using precursor solutions in anhydrous toluene and N2 as a carrier gas at a flow rate of 0.5 L min 1.

Glancing angle powder X-ray diffraction was carried out on films F5 and F8 as an initial tool for characterisation. However, in both cases, the as-deposited films were found to be amorphous, as expected for the growth of Ga2O3 at substrate temperatures below approximately 700 8C.[29, 36] Energy dispersive X-ray analysis (EDXA) confirmed the presence of gallium within the films, however the gallium-to-oxygen ratios could not be accurately measured due to breakthrough to the underlying glass substrate. From SEM imaging (Figure 5), both films were found to be made up of spherical particles, indicating that film growth had occurred through a Volmer–Weber-type island growth mechanism.

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Figure 5. SEM images of Ga2O3 films of a) F5 ( 30 000 magnification) and b) F8 ( 40 000 magnification).

XPS analysis further confirmed the presence of both gallium and oxygen within the films by the presence of Ga 3d and O 1s peaks, respectively (Figure 6, Table 5). For reference, a Ga2O3 standard was also analysed by XPS (Figure 6 a). Peaks at 19.87, 19.54 and 19.41 eV for F5, F8 and standard, respectively, were consistent with literature values for Ga 3d5/2 within Ga2O3.[37] In all samples, including the standard, the presence of another gallium species was indicated by the appearance of a doublet at 18.21 and 18.66 eV, likely to be metallic gallium by comparison to the Ga 3d5/2 binding energy of 18.4 eV reported for elemental gallium.[38] The observation of this impurity was also reported by Ghosh et al. in the XPS analysis of a Ga2O3 standard.[39] The oxygen 1s peak of Ga2O3 is reported to appear at a binding energy of approximately 531 eV,[40] therefore peaks observed at 530.44, 530.35 and 530.52 eV were attributed to O 1s of the native oxide. A small additional O 1s peak at approximately 532 eV within the XPS spectra for the films and standard were attributed to the presence of trace amounts of other oxygen-containing impurities within the samples.[40b] Gallium-to-oxygen ratios (Table 5) were calculated from the XPS data, showing the standard sample and film F5 to be near stoichiometric Ga2O3 (Ga/O ratio 0.67 and 0.62, respectively), with slight discrepancies being attributed to trace impurities at the surface level. Film F8, however, was found to be oxygen-deficient (Ga/O ratio 0.95). Oxygen-deficient film F8 showed low to moderate transparency in the visible region, as determined from the UV/Vis/IR transmission spectrum shown in Figure 7. The sample showed approximately 20–60 % transmission in the visible region, which decreased towards the UV wavelengths. The spectrum of the near-stoichiometric film F5, however, showed much improved transparency (60–80 % in the visible). Whereas our previous report of Ga2O3 thin films deposited from 7 showed comparatively higher transparency (> 80 % in the visible),[24] these films had been post-treated by annealing in air at 1000 8C. It is therefore likely that the reduced transmission in F5 and F8 arises from carbon contamination. Indeed, XPS analysis revealed a carbon content at the surface of the as-deposited films, though accurate quantification was unreliable due to the presence of surface contaminants.

Conclusion A range of gallium species utilising bis-b-ketoimine ligand systems have been synthesised and the crystal structures of a number of such species have been reported. The geometries Chem. Eur. J. 2014, 20, 10503 – 10513

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Figure 6. XPS peaks for Ga 3d. a) Ga2O3 standard; b) film F5; c) film F8.

displayed by the complexes range between the idealised extremes of trigonal bipyramidal and square-based pyramidal geometries and are heavily influenced by the nature of the ligand system, in particular by the length of the bridging carbon chain between the two b-ketoimine moieties. In particular, the stabilising ability of these ligands towards rare gallium hydride species has been demonstrated through the synthesis of monomeric hydride species 7 and 8, the crystal structures of which are also reported. Moreover, the complexes showed considerable thermal stability, with the onset of decomposition occurring above 200 8C. The general instability of such gallium

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Full Paper [GaH3(NMe3)],[42] were synthesised by literature procedures, as were ligands [(CH2)n{N(H)C(Me) CHC(Me)=O}2] (LnH2, n = 2, 3 and 4) with adaptations.[43] Details of the ligand syntheses can be found in the Supporting Information.

Table 5. Ga 3d5/2 and O 1s XPS peaks and gallium-to-oxygen ratios for samples F5, F8 and a Ga2O3 standard. Literature values included for reference. Sample

Binding energy [eV] O 1s Ga 3d5/2

Ga/O

F5 F8 Ga2O3 (standard) Ga2O3 (lit.)

19.87 19.54 19.41 19.6[36]

0.62 0.95 0.67 –

530.44 530.35 530.52 530.70[41]

All manipulations were carried out under an inert atmosphere using standard Schlenk techniques. For hydride syntheses, glassware was flame-dried prior to use. Nitrogen (99.99 %) was obtained from BOC and passed through activated molecular sieves to remove moisture. All solvents were dried over activated alumina by the Grubbs method using anhydrous engineering equipment, such that the water concentration was 5–10 ppm.[44]

Complex [Ga(L4)Cl] (3)

Figure 7. UV/Vis/IR transmission spectrum for films F5 and F8.

hydrides has previously limited their feasible use as singlesource precursors in thin-film deposition, thus the isolation of thermally stable hydrides is of great significance for their application in this area. The general efficacy of b-ketoiminate precursors has been investigated through the deposition of Ga2O3 thin films from methyl (5) and hydride (8) precursor derivatives. In both cases, the deposition of gallium- and oxygen-containing thin films was confirmed through EDX and XPS analysis. The formation of oxygen-deficient films from the hydride precursor 8 is a likely result of hydride acting as a reducing agent, resulting in the co-deposition of gallium metal and gallium oxide within the resultant films. In the absence of hydride, films deposited from methyl derivative 5 showed near-stoichiometric Ga2O3, despite the gallium-to-oxygen ratio of 1:1 within the precursor. The work demonstrates the potential of b-ketoiminate based precursor systems for the deposition of metal oxide materials by CVD and further investigation into alternative ligand derivatives is ongoing.

Experimental Section

NaH (1.0 g, 42.0 mmol) was suspended in THF (50 mL) and added to a stirring solution of L4H2 (5.0 g, 19.8 mmol) in THF (50 mL) and the mixture was stirred overnight. Solvent was removed from the resulting suspension in vacuo to afford a pale-yellow solid product, L4Na2. Subsequently, L4Na2 (0.59 g, 2.0 mmol) was suspended in THF (20 mL) and added slowly to a stirring solution of GaCl3 (0.35 g, 2.0 mmol) in THF (20 mL) at 78 8C. The mixture was stirred for approximately 30 min before being warmed slowly to RT and subsequently heated to reflux for 16 h. The resulting yellow supernatant was separated from the white solid precipitate (NaCl) by filtration and the solvent was removed in vacuo to afford a solid yellow crude product. The product was then dissolved in minimal CH2Cl2 (ca. 3 mL), filtered into a narrow Schlenk flask and layered with hexane (ca. 10 mL). The product was left overnight, during which time colourless crystalline blocks formed. Yield: 0.71 g (67 %); 1H NMR (600 MHz, CDCl3, 25 8C, TMS): d = 5.10 (s, 2 H; CH), 3.52 (br s, 4 H; CH2CH2CH2CH2), 2.06 (s, 6 H; CH3CO), 2.03 (s, 6 H; CH3CN), 1.78 ppm (br s, 4 H; CH2CH2CH2CH2); 13C{1H} NMR (600 MHz, CDCl3, 25 8C, TMS): d = 182.9 (CO), 174.9 (CN), 99.5 (CH), 48.7 (CH2CH2CH2CH2), 28.6 (CH2CH2CH2CH2), 26.6 (CH3CN), 22.5 ppm (CH3CO); MS: m/z: 354 [M], 339 [M CH3], 319 [M Cl], 311 [M CH3CO]; elemental analysis calcd (%) for C14H22N2O2Ga: C 47.30, H 6.24, N 7.88; found: C 46.72, H 6.20, N 6.17.

Complex [Ga(L2)Me] (4) L2H2 (7.8 g, 34.8 mmol) was dissolved in anhydrous toluene (30 mL) and added slowly to a stirring solution of GaMe3 (4.0 g, 34.8 mmol) in toluene (30 mL) at 78 8C. The mixture was stirred at 78 8C for approximately 30 min before being warmed slowly to RT. The reaction mixture was then heated to 80 8C in an oil bath and stirred at this temperature for 16 h. The resulting dark-yellow/orange solution was filtered and toluene was removed under reduced pressure to give a dark-yellow waxy product. The product was dissolved in a minimum volume of hexane with gentle heating and allowed to cool to RT, during which time needle-like, yellow crystals formed. Yield: 8.16 g (77 %); 1H NMR (600 MHz, CDCl3, 25 8C, TMS): d = 4.91 (s, 2 H; CH), 3.37–3.46 (m, 4 H; CH2CH2), 1.99 (s, 6 H; CH3CO), 1.94 (s, 6 H; CH3CN), 0.45 ppm (s, 3 H; GaCH3); 13C{1H} NMR (600 MHz, CDCl3, 25 8C, TMS): d = 182.1 (CH3CO), 171.0 (CH3CN), 97.5 (CH), 45.9 (CH2CH2), 26.9 (CH3CN), 21.6 (CH3CO), 7.1 ppm (GaCH3); MS: m/z: 307 [M+H] + , 291 [M CH3] + ; elemental analysis calcd (%) for C13H21GaN2O2 : C 50.85, H 6.89, N 9.12; found: C 49.23, H 7.29, N 9.24.

General procedures GaCl3 (10 mesh beads, 99.99 %), LiH, NaH and amines were purchased from Sigma–Aldrich and b-diketones from Alfa Aesar: all were used without further purification. GaMe3 was supplied by SAFC Hitech Ltd. Gallium complexes 1, 2 and 7,[24] along with Chem. Eur. J. 2014, 20, 10503 – 10513

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Complex [L3(GaMe2)2] (5) L3H2 (0.6 g, 2.5 mmol) was dissolved in anhydrous toluene (20 mL) and added slowly to a stirring solution of GaMe3 (0.6 g, 5.0 mmol) in toluene (20 mL) at 78 8C. The mixture was stirred at 78 8C for

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Full Paper approximately 30 min before being warmed slowly to RT. The reaction mixture was then heated to approximately 80 8C and stirred at this temperature for 16 h. The yellow solution was cooled to RT, filtered and the solvent was removed from the filtrate in vacuo. Recrystallisation of the resulting yellow solid powder was unsuccessful from a range of solvent and temperature combinations. Yield: 0.63 g (58 %); 1H NMR (600 MHz, CDCl3, 25 8C, TMS): d = 4.85 (s, 2 H; CH), 3.23–3.27 (m, 4 H; CH2CH2CH2), 1.96 (s, 6 H; CH3CO), 1.91 (s, 6 H; CH3CN), 1.70–1.80 (m, 2 H; CH2CH2CH2), 0.38 ppm (s, 12 H; Ga(CH3)2); 13C{1H} NMR (600 MHz, CDCl3, 25 8C, TMS): d = 180.4 (CO), 171.5 (CN), 98.2 (CH), 46.4 (CH2CH2CH2), 30.5 (CH2CH2CH2), 26.4 (CH3CN), 20.9 (CH3CO), 7.3 ppm (Ga(CH3)2); MS: m/z 421 [M CH3], 321 [M GaMe3], 305 [M GaMe3 CH3]; elemental analysis calcd (%) for C17H32N2O2Ga2 : C 46.84, H 7.40, N 6.43; found: C 45.30, H 8.01, N 6.28.

Complex [L4(GaMe2)2] (6) L4H2 (0.6 g, 2.5 mmol) was dissolved in anhydrous toluene (20 mL) and added slowly to a stirring solution of GaMe3 (0.6 g, 5.0 mmol) in toluene (20 mL) at 78 8C. The mixture was stirred at 78 8C for approximately 30 min before being warmed slowly to RT. The reaction mixture was then heated to approximately 80 8C and stirred at this temperature for 16 h. The yellow solution was cooled to RT, filtered and the solvent was removed from the filtrate in vacuo. Recrystallisation of the resulting yellow solid powder was unsuccessful from a range of solvent and temperature combinations. Yield: 0.89 g (79 %); 1H NMR (600 MHz, CDCl3, 25 8C, TMS): d = 4.85 (s, 2 H; CH), 3.28 (br s, 4 H; CH2CH2CH2), 1.96 (s, 6 H; CH3CO), 1.92 (s, 6 H; CH3CN), 1.53 (br s, 2 H; CH2CH2CH2), 0.38 ppm (s, 12 H; Ga(CH3)2); 13 1 C{ H} NMR (600 MHz, CDCl3, 25 8C, TMS): d = 180.0 (CO), 171.3 (CN), 98.3 (CH), 48.6 (CH2CH2CH2), 27.9 (CH2CH2CH2), 26.4 (CH3CN), 21.2 (CH3CO), 7.4 ppm (Ga(CH3)2); MS: m/z: 435 [M CH3], 335 [M GaMe3], 319 [M GaMe3 CH3]; elemental analysis calcd (%) for C18H34N2O2Ga2 : C 48.05, H 7.62, N 6.23; found: C 47.15, H 7.81, N 6.15.

Complex [Ga(L3)H] (8) A freshly prepared solution of [GaH3(NMe3)] (5.0 mmol) in Et2O (50 mL) was cooled to 78 8C and added to a suspension of L3H2 (1.2 g, 5.0 mmol) in Et2O (20 mL) at 78 8C. The suspension was stirred for 15 min, then warmed to RT and stirred for 16 h. During this time, a pale-yellow solution with a white suspension formed. The reaction was filtered, volatiles were removed in vacuo and the crude product was dissolved in minimal toluene. The toluene solution was cooled to 20 8C, from which colourless crystals were formed overnight. Yield: 0.91 g (59 %; based on GaCl3); 1H NMR (600 MHz, C6D6, 25 8C, TMS): d = 5.83 (v br s, 1 H; GaH), 4.85 (s, 2 H; CH), 2.80–2.89 and 2.79–2.85 (m; CH2CH2CH2, each 2 H), 2.15–2.19 (m, 1 H; CH2CH2CH2), 1.98 (s, 6 H; COCH3), 1.36 (s, 6 H; CNCH3), 1.12– 1.16 ppm (m, 1 H; CH2CH2CH2); 13C{1H} NMR (600 MHz, C6D6, 25 8C, TMS): d = 182.95 (CO), 171.1 (CN), 97.9 (CH), 49.0 (CH2CH2CH2), 26.8 (CNCH3), 26.5 (CH2CH2CH2), 21.2 ppm (COCH3); MS: m/z: 305 [M H + ]; IR (KBr): n˜ = 1867 cm 1 (br, Ga-H); elemental analysis calcd (%) for C13H21N2O2Ga: C 50.85, H 6.89, N 9.12; found: C 49.56, H 7.31, N 8.48.

Aerosol-assisted chemical vapour deposition Films were grown on SiO2-coated float-glass substrates supplied by Pilkington NSG, of dimensions approximately 90  45  4 mm. Prior to use, the substrates were cleaned by using water and detergent, propan-2-ol and acetone and were dried in air. For each deposiChem. Eur. J. 2014, 20, 10503 – 10513

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tion, two substrates were placed within the reactor with the lower surface of the ‘top-plate’ sitting at a distance of approximately 8 mm above the upper surface of the ‘bottom-plate’. The reactor was heated to the relevant temperature by a graphite block containing a Whatman cartridge heater and the temperature of the block was monitored by using a Pt-Rh thermocouple. All deposition temperatures quoted refer to this temperature. The temperature of the top-plate was generally found to be approximately 50– 70 8C lower than that of the bottom-plate. After reaching the desired temperature, the temperature within the reactor was allowed to equilibrate for 30 min prior to commencing the deposition. The precursors were dissolved in anhydrous solvent using standard Schlenk techniques, from which an aerosol was generated at RT by use of an ultrasonic humidifier. The aerosol was carried into the reactor by passing nitrogen through a brass baffle to obtain laminar flow. The total time for each deposition was dependent upon the solvents, volumes and flow rates being used. Depositions were carried out at substrate temperatures ranging between 400–600 8C.

Analysis techniques Thermogravimetric analysis was carried out with a Netzsch system from 20 to 600 8C. Samples of approximately 10–20 mg were analysed under an inert atmosphere of helium in sealed aluminium pans. Scanning electron microscopy (SEM) was performed to determine surface morphology and film thickness with a JEOL JSM6301F field emission SEM at an accelerating voltage of 5 keV. Images were captured with SEMAfore software. Samples were cut to approximately 5  5 mm coupons and coated with a fine layer of gold to avoid charging. X-ray photoemission spectroscopy (XPS) was performed with a Thermo Scientific K-alpha photoelectron spectrometer using monochromatic AlKa radiation. Survey scans were collected in the range 0–1100 eV (binding energy) at a pass energy of 160 eV. Higher resolution scans were recorded for the principal peaks of Ga (3d), O (1s) and C (1s) at a pass energy of 50 eV. Peak positions were calibrated to carbon and plotted by using the CasaXPS software. Energy dispersive (EDX) analysis was carried out with a JEOL JSM-6301F field emission instrument with an acceleration voltage of 20 kV. UV/Vis transmittance and reflectance spectra were obtained with a PerkinElmer Lambda 950 spectrometer using an air background and were recorded between 250 and 2000 nm.

Crystallography Crystals were obtained as described above. Details of the crystallographic data collection and refinement are given in Table 6. Diffractometers: for compounds 3 and 7, Rigaku R-Axis Spider including curved Fujifilm image plate and a graphite monochromated sealed tube Mo generator (l1 = 0.71073 ) was used; for compound 4, a Bruker SMART APEX CCD diffractometer using graphite monochromated MoKa radiation (l1 = 0.71073 ) was used; for compound 8, a Rigaku AFC12 goniometer equipped with an enhanced sensitivity (HG) Saturn724 + detector mounted at the window of an FR-E + SuperBright molybdenum rotating anode generator (l1 = 0.71073 ) with VHF Varimax optics (100 mm focus) was used. For compounds 3, 7 and 8: cell determination, data collection, data reduction, cell refinement and absorption correction were carried out by using CrystalClear-SM Expert 3.1 b18.[45] For compound 4, cell determination, data reduction and integration were carried out with SAINT + and absorption corrections were applied by using SADABS.[46] Structure solution and refinement were carried out by using WinGX and software packages.[47] H atoms attached to C atoms were placed in geometrically assigned positions, with

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Full Paper Table 6. Crystallographic data for the compounds reported in this paper. Compound

3

4

7

8

formula M [g mol 1] T [K] crystal system space group (No.) a [] b [] c [] a [8] b [8] g [8] U [3] Z m(MoKa) [mm 1] F(000) total reflns unique reflns Rint GooF on F2 R1b [Io > 2s(Io)] R1 (all data) wR2b [Io > 2s(Io)] wR2 (all data)

C14H22ClGaN2O2 355.51 120(2) triclinic P-1 (2)

C13H21GaN2O2 307.04 150(2) monoclinic P21/c (14)

C12H19GaN2O2 293.01 120(2) monoclinic P21/c (14)

C13H21GaN2O2 307.04 100(2) monoclinic P21/c (14)

8.646(1) 9.218(1) 11.043(2) 79.726(3) 77.402(3) 65.573(3) 778.2(2) 2 1.942 368 9706 3531 0.034 1.084 0.026 0.028 0.063 0.065

8.148(2) 13.014(3) 14.885(3) 90 110.30(1) 90 1480.4(6) 4 1.855 640 12 073 3432 0.094 0.892 0.060 0.117 0.138 0.160

7.9220(2) 12.0209(3) 13.5746(5) 90 93.045(2) 90 1290.88(7) 4 2.124 608 13 496 2956 0.041 1.121 0.037 0.039 0.095 0.096

7.4450(5) 15.1760(9) 12.4920(8) 90 97.112(1) 90 1400.6(2) 4 1.961 640 9263 2895 0.032 1.116 0.041 0.044 0.113 0.116

C H distances of 0.95 (CH), 0.98 (CH3) or 0.99  (CH2) and refined by using a riding model, with Uiso(H) = 1.2 Ueq(C) (CH, CH2) or 1.5 Ueq(C) (CH3). The Ga H protons in 7 and 8 were located in the Fourier difference map and allowed to refine freely with Ueq(H) = 1.2 Ueq(Ga). enCIFer was used to prepare CIFs for publication.[48] CCDC-994235 (3), -994236 (4), -994237 (7) and -994238 (8) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Acknowledgements We would like to thank the EPSRC (grant nos. EP/F035675 and EP/H00064X) and UCL (Impact Studentship Scheme) for financial support. We would also like to thank SAFC Hitech Ltd. for supplying GaMe3 and InMe3, Pilkington NSG for supplying the float glass, and Drs. Peter Horton, Mateusz Pitak and Graham Tizzard at the EPSRC National Crystallography Service for data set collection of compounds 3, 7 and 8. Keywords: chelates · chemical vapour deposition · gallium oxides · hydrides · thin films [1] a) L. M. Foster, H. C. Stumpf, J. Am. Chem. Soc. 1951, 73, 1590 – 1595; b) M. Marezio, J. P. Remeika, J. Chem. Phys. 1967, 46, 1862 – 1865. [2] a) T. Harwig, G. J. Wubs, G. J. Dirksen, Solid State Commun. 1976, 18, 1223 – 1225; b) Z. Hajnal, J. Miro, G. Kiss, F. Reti, P. Deak, R. C. Herndon, J. M. Kuperberg, J. Appl. Phys. 1999, 86, 3792 – 3796. [3] M. Fleischer, H. Meixner, Sens. Actuators B 1991, 4, 437 – 441. [4] a) M. Fleischer, H. Meixner, Sens. Actuators B 1992, 6, 257 – 261; b) M. Fleischer, H. Meixner, Sens. Actuators B 1995, 26, 81 – 84. [5] Z. F. Liu, T. Yamazaki, Y. Shen, T. Kikuta, N. Nakatani, Y. X. Li, Sens. Actuators B 2008, 129, 666 – 670. [6] A. Cneyt Tas¸, P. J. Majewski, F. Aldinger, J. Am. Ceram. Soc. 2000, 83, 2954 – 2960. Chem. Eur. J. 2014, 20, 10503 – 10513

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Received: April 8, 2014 Published online on July 17, 2014

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