Structural and Dynamic Properties of Gallium Alkoxides.

A comparison of chlorido-gallium functionalized alkoxides as precursors for aerosol-assisted chemical vapor deposition (AACVD) was carried out. Variable-temperature (VT)-NMR studies were used to probe the fluxional behavior of these alkoxides in solution, and hence their utility as precursors. The synthesis involved the initial isolation of the dimer [GaCl(NMe2)2]2 via a salt metathesis route from GaCl3 and 2 equiv of LiNMe2. This dimer was then reacted with 4 equiv of HOCH2CH2CH2NEt2, resulting in the formation of Ga[μ-(OCH2CH2CH2NEt2)2GaCl2]3 (1). Mass spectrometry and VT-NMR confirmed the oligomeric structure of 1. Tuning of the ligand properties, namely, the chain length and substituents on N, resulted in formation of the monomers [GaCl(OR)2] (R = CH2CH2NEt2, (2); CH2CH2CH2NMe2, (3)). VT-NMR studies, supported by density functional theory calculations, confirmed that the ligands in both 2 and 3 possess a hemilabile coordination to the gallium center, owing to either a shorter carbon backbone (2) or less steric hindrance (3). Both 2 and 3 were selected for use as precursors for AACVD: deposition at 450 °C gave thin films of amorphous Ga2O3, which were subsequently annealed at 1000 °C to afford crystalline Ga2O3 material. The films were fully characterized by X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, UV-visible spectroscopy, and energy dispersive X-ray analysis.

Introduction

In recent years, the research surrounding metal oxide thin films, particularly those containing group 13 elements, has expanded greatly due to these materials having properties crucial to the function and performance of a wide variety of electronic, optical, and chemically active devices, including photovoltaic cells,[1] gas sensors,[2] transparent conducting oxides (TCOs),[3] as well as screen displays.[4] Gallium oxide (Ga2O3) is a material that exists in various forms, with two prevailing due to their increased thermodynamic stability, α-Ga2O3 and β-Ga2O3 being metastable and stable, respectively. The β-Ga2O3 form has a wide band gap of ~4.7 eV[5] and performs well as an n-type semiconductor in the temperature range of 450–550 °C.[6] Recent work has shown the use of gallium oxide thin films as solar-blind photodetectors,[7] in which high selectivity for ultraviolet (UV) light in the presence of visible light affords one of the highest responsivity reported (1.8 × 105 A W–1) when compared to other photodetectors such as AlGaN, GaN, and MgZnO. Other recent work highlighting the utility of Ga2O3 includes improving the sensitivity of the gas sensing ability of ZnO toward NO2 when doped with Ga2O3.[8] Many routes toward gallium oxide thin films have been outlined in the literature using a variety of techniques, such as aerosol-assisted chemical vapor deposition (AACVD),[9−12] spin coating,[13] and pulsed layer deposition,[14] to name but a few.[15] Homoleptic oxygen containing gallium complexes have been investigated as precursors to Ga2O3, including β-diketonates,[11] [Ga(acac)3][16,17] and [Ga(dbm)3][18] (acac = acetylacetonate, and dbm = dibenzoylmethanoate), where tris-β-diketonate gallium(III) complexes have been isolated. Others include β-ketoiminates,[13,19−21] of the general formula [4-[2-(R)imino]-2-pentanone] gallium(III), (R = CH2CH2CH2OCH2CH3, CH(CH3)2, CH2CH2OCH3, CH2CH2CH2OCH3, and CH2CH2OCH2CH3),[13,21] and further contributions (Figure ) have recently detailed the synthesis of oxygen-rich gallium complexes with dimethylmalonate and cyclopropane-1,1-dimethylester ligands for use in catalysis[22] and di(tert-butyl)gallium species with β-diketonato ligands, which were further functionalized with molecular oxygen to form metal-stabilized peroxides.[23]

Figure 1

Examples of oxygen-rich gallium compounds previously reported. Gallium β-diketonates: (A) [Ga(acac)3][16,17] and (B) [Ga(dbm)3].[18] Gallium β-ketoiminate: (C) [Ga(4-[2-(isopropyl)imino]-2-pentanone)3].[13] Gallium dimethylester: (D) [Ga(cyclopropane-1,1-dimethylester)3]3+.[22] Gallium alkoxides: (E) [Ga(OBu)3]2[24] and (F) [GaCl2(OCH2CH2NMe2)]2.[25]

Though these β-diketonato-gallium complexes were not used for the deposition of gallium oxide, the merit of these gallium-oxygen systems is evidenced by their ability to stabilize highly reactive metal–carbon bonds which can be further functionalized to increase the stoichiometry of oxygen of the compound.[23] β-Diketoiminate complexes form a conjugated six-membered ring intramolecularly through the O and N donor atoms and the metal center, increasing the thermal stability of the complex and lowering the melting point.[21] The R group bound to the imide N atom of the β-diketoiminates can be further functionalized depending on the requirements of the precursor and the material to be deposited.[26]

Gallium alkoxides have also shown potential as precursors to Ga2O3 thin films.[25] When comparing alkoxides to β-diketoiminates, increased functionalization can also be obtained from alkoxide precursors, where donor-functionalized alcohols featuring an amine group allow the N atom to tether to the metal center, and the R groups associated with the amine, as well as the length of the alcohol’s carbon backbone, can be varied to improve solubility and stability.[27] A benefit associated with the use of functionalized alkoxides of gallium (specifically bis(alkoxides), which can be difficult to synthesize and are uncommon compared to their diorganoalkoxometallane analogues)[10] is that they are monomeric in nature, as the pendant donor atom’s electronic contribution saturates the metal’s coordination sphere, reducing the tendency for oligomerization to occur, making them excellent precursors for CVD.[25,27,28]

Previously, we have investigated the stepwise route to alkyl/chloro-gallium donor-functionalized alkoxides from the reactions of gallium chloride with lithium dimethylamide[27] and lithium hexamethyldisilazide,[25] including an in-depth crystal structure analysis. Other works report of functionalization through ethers,[29] and more recently, even thioethers to deposit oxysulfide thin films.[30,31]Gallium/copper mixed-metal thiolate compounds,[32] used for potential gallium copper sulfide materials, were shown to have simple room-temperature NMR spectra; however, low-temperature NMR studies revealed the complex dissociative mechanisms that these precursors undergo in solution. This information can be used to understand thermolytic pathways during deposition to the target material. Variable-temperature (VT)-NMR studies, to the best of our knowledge, have not been used to understand the fluxional behavior of gallium bis(alkoxides) in solution (though recently gallium complexes from methylidenemalonates have been probed with VT-NMR to understand the differences in stereochemistry and hence the reactivity of catalytically active gallium species)[33] or, furthermore, how these mechanisms can be used as an aid in the understanding of precursor thermolysis. In this work, we extend the study through the synthesis of chloro-gallium bis(alkoxides): Ga[μ-(OCH2CH2CH2NEt2)2GaCl2]3 (1), and monomers of the form [GaCl(OR)2] (R = CH2CH2NEt2 (2), CH2CH2CH2NMe2 (3)). A study in solution via VT-NMR, supported by density functional theory (DFT) calculations, has facilitated the comparison of these compounds’ effectiveness as precursors toward Ga2O3 thin films via AACVD.

Experimental Section

General Procedures

All syntheses were performed under a dry, oxygen-free argon atmosphere using standard Schlenk techniques or in an MBraun Unilab glovebox. All solvents used were stored in alumina columns and dried with anhydrous engineering equipment, such that the water concentration was 5–10 ppm. The gallium amide [GaCl(NMe2)2]2 was prepared according to standard literature procedures.[27] All other reagents were procured commercially from Aldrich, and the alcohols were degassed by three freeze–pump–thaw cycles and stored over 4 Å molecular sieves. 1H and 13C NMR spectra were obtained on Bruker NMR spectrometers Avance III 400, Avance 500, and Avance Neo 700. Spectra were recorded using CD2Cl2, toluene-d8, or C6D6 which were dried and degassed over molecular sieves prior to use. 1H and 13C{1H} chemical shifts relative to TMS were calibrated using the residual solvent peak. High- and low-temperature NMR calibrations were carried out using standard samples of 80% 1,2-ethanediol in DMSO-d6 and 4% CH3OH in CD3OD, respectively. Data acquisition and processing were performed using standard Bruker TopSpin software. The mass spectra were obtained using a Micromass 70-SE spectrometer using chemical ionization (CI) with methane reagent gas, or using an LCT Premier QTOF spectrometer using electrospray ionization (ESI). Single crystals of 2 and 3 were analyzed on a Bruker SMART APEX diffractometer using graphite monochromated Mo Kα radiation (0.71073 Å) at 150.15 K. Using Olex2,[34] we solved the structure with the olex2.solve[35] structure solution program using Charge Flipping and refined with the ShelXL[36] refinement package using Least Squares minimization. Crystallographic information is available from the CSD, CCDC numbers: CCDC1917612 (2) and CCDC1917613 (3).

AACVD

Depositions were carried out under dinitrogen (99.99% from BOC). The synthesized precursors were placed into an inert AACVD glass bubbler and dissolved in an appropriate dry solvent. Two glass substrates were placed into a horizontal bed cold-wall reactor chamber, one resting on top of the graphite heating block, the second resting 0.5 cm above, ensuring a laminar flow of gas. The glass substrates were cleaned prior to deposition with isopropyl alcohol, water, and acetone and allowed to dry in air before the deposition took place. The temperature of the graphite block was controlled with a thermocouple composed of platinum-rhodium rods. An ultrasonic humidifier containing a piezoelectric device (functioning at 20 kHz) was used to generate an aerosol mist of the precursor. The mist was carried into the reaction chamber via a flow of dry dinitrogen gas, which was optimized at a flow rate of 1 L min–1.

Film Analysis Methods

XRD for thin films was carried out with a Bruker D8 Discover X-ray diffractometer using monochromatic Cu Kα1 and Cu Kα2 radiation of wavelengths 1.54056 and 1.54439 Å, respectively, emitted in an intensity ratio of 2:1, voltage = 40 kV; current = 40 mA. SEM/EDXA was performed using a Philips XL30 FEG with an electron beam accelerating energy of 30 kV. XPS profiling was performed using a Thermo Scientific K-Alpha XPS system using monochromatic Al Kα radiation at 1486.6 eV X-ray source. CasaXPS software was used to analyze binding energy of the data, referenced to a C 1s peak at 284.8 eV. UV/vis/NIR transmission spectra were recorded using a PerkinElmer Lambda 950 spectrometer in the range of 300–1400 nm with a background of air, and Tauc plots from this data were used to calculate the band gaps. Raman spectra were acquired using a Reinshaw Raman 1000 system using a helium neon laser of wavelength 632.8 nm and objective lens at 50× magnification. The Raman system was calibrated against the emission lines of neon.

Synthesis of Ga[μ-(OCH2CH2CH2NEt2)2GaCl2]3 (1)

HOCH2CH2CH2NEt2 (27.8 mg, 0.21 mmol) was added to a stirred solution of [GaCl(NMe2)2]2 (20.0 mg, 51.7 μmol) in hexane (20 cm3) at −78 °C. The reaction mixture was left to stir for 24 h. The solvent was then removed under reduced pressure, yielding a colorless oil of (1). Extended synthesis in the Supporting Information (SI). 1H NMR δ/ppm (toluene-d8) 25 °C: 3.86 (t, 4H (−OCH2), J = 5.82 Hz), 2.72 (m, 8H (−N(CH2CH3)2), 4H (−CH2N), 1.72 (quin, 4H (−OCH2CH2), J = 6.18 Hz), 1.04 (t, 12H, (−N(CH2CH3)), J = 7.14 Hz). 60 °C: 4.07 (s, 4H (−OCH2)), 2.65–2.49 (m 8H (−N(CH2CH3)2), 4H (−CH2N)), 1.63 (s, 4H (−OCH2CH2)), 1.02–0.91 (m, 12H, (−N(CH2CH3))). 13C{1H} NMR δ/ppm (toluene-d8) 25 °C: 64.8 (−OCH2), 52.8 (−CH2CH2N), 46.4 (−N(CH2CH3)2), 38.5 (−OCH2CH2), 10.1 (−N(CH2CH3)2). 2D NMR spectrum is included in the SI. ESI (ES−): m/z = 1273 (Ga4Cl6O6N6C42H96). (ES+): 414 (Ga2Cl2O2N2C10H24), 398 (Ga2Cl2O2NC10H22), 133 (ONC7H19).

Synthesis of [GaCl(OCH2CH2NEt2)2] (2)

HOCH2CH2NEt2 (4.69 g, 40.2 mmol) was added to a stirred solution of [GaCl(NMe2)2]2 (3.79 g, 9.80 mmol) in diethyl ether (40 cm3) at −78 °C. The reaction mixture was left to stir for 24 h. The solution was then concentrated under reduced pressure to a minimum (ca. 10 cm3) and cooled to −18 °C. Colorless crystals of [GaCl(OCH2CH2NEt2)2] (2) were obtained (5.95 g, 90%). Anal. Calcd. for C12H28ClN2O2Ga: C, 42.68; H, 8.36; N, 8.30. Found: C, 40.42; H, 8.05; N, 7.21%. Mp: 81 °C. 1H NMR δ/ppm (CD2Cl2) 3.72 (m, 4H (−OCH2)), 3.08 (m 4H (−N(CH2CH3)2)), 2.96 (m, 4H (−N(CH2CH3)2)), 2.65 (m, 8H (−N(CH2CH3)2 and −OCH2CH2)), 1.10 (t, 6H, (−N(CH2CH3)2) J = 7.2 Hz), 0.96 (t, 6H, (−N(CH2CH3)2), J = 7.2 Hz). 13C{1H} NMR δ/ppm (CD2Cl2) −20 °C: 57.9 (−OCH2), 53.3 (−OCH2CH2), 44.7 (−N(CH2CH3)2), 40.4 (−N(CH2CH3)2), 9.3 (−N(CH2CH3)2), 6.1 (−N(CH2CH3)2). LRMS (CI+, CH4): m/z = 337 ([M]+), 301 ([M – Cl]+).

Synthesis of [GaCl(OCH2CH2CH2NMe2)2] (3)

HOCH2CH2CH2NMe2 (2.18 g, 21.2 mmol) was added to a stirred solution of [GaCl(NMe2)2]2 (2.00 g, 5.17 mmol) in diethyl ether (40 cm3) at −78 °C. The reaction mixture was left to stir for 24 h. The solution was then concentrated under reduced pressure to a minimum (ca. 10 cm3) and cooled to −18 °C. Colorless crystals of [GaCl(OCH2CH2CH2NMe2)2] (3) were obtained (1.13 g, 71%). Anal. Calcd. for C10H24ClGaN2O2: C, 38.79; H, 7.82; N, 9.05. Found: C, 38.19; H, 7.90; N, 8.54%. Mp: 81 °C. 1H NMR δ/ppm (CD2Cl2) −20 °C: 4.09 (m, 2H (−OCH2)), 3.96 (m, 2H (−OCH2)), 3.01 (m, 2H (−OCH2CH2CH2), 2.41 (m, 6H (−N(CH3)) and 2H (−OCH2CH2CH2)), 2.30 (s, 6H (−N(CH3)), 1.92 (m, 2H (−OCH2CH2CH2), 1.39 (m, 2H (−OCH2CH2CH2). 13C{1H} NMR δ/ppm (CD2Cl2) −20 °C: 66.3 (−OCH2), 60.3 (−OCH2CH2CH2), 47.2 (−N(CH3), 44.2 (−N(CH3), 28.5 (CH2CH2CH2). LRMS (CI+, CH4): m/z = 309 ([M]+).

Results and Discussion

Compound Synthesis

The synthesis of 1, 2, and 3 outlined in Scheme was carried out based on a modified literature procedure.[27] The reaction of [GaCl(NMe2)2]2 with 4 mol equiv of the corresponding donor-functionalized alcohol in dry Et2O at −78 °C was allowed to warm to room temperature and stirred for 24 h. This solution was then reduced in vacuo and stored in a freezer at −18 °C, affording a pale viscous oil in the case of 1 and colorless single crystals of 2 and 3.

Scheme 1

Synthesis of Donor-Functionalized Gallium Compounds 1–3

The reaction of 3 equiv of gallium chloride bis(dimethylamide) with 6 equiv of HOCH2CH2CH2NEt2 formed the oligomer 1. Several attempts to isolate a monomer were unsuccessful (detailed in SI). In the 1H spectrum of 1 recorded at room temperature in CD2Cl2 (SI), the methylene protons do not show nonequivalence, resonating at the same frequency, which suggests that the OCH2CH2CH2NEt2 fragment has not formed a ring-like structure and remained as an open chain with μ-2 bridging oxygens and the rest of the alcohol as a nondonating pendant. Cooling led to a large number of broad lines in the CD2Cl2 spectrum. Such spectral behavior is different from that observed for 2 and 3 (see below) and is indicative of complex multisite exchange. Both positive and negative ESI conditions were used to help identify possible fragments from oligomer 1. Under ESI conditions, possible oxonium/ammonium species could be detected in the positive mode, or oxygen/nitrogen anions in the negative mode. Mass spectrometry of 1 showed a specific isotopic pattern of 11 peaks, with the highest molecular ion fragment containing four gallium and six chlorine atoms occurring at 1279 m/z. The expected molecular ion fragment of the proposed structure of Ga[μ-(OCH2CH2CH2NEt2)2GaCl2]3 can be found at 1273 m/z; however, under ESI conditions, it is possible that protonated alcohol sites caused by the ESI solvent could have given rise to the mass of 1279 m/z. Other fragments containing specific isotopic patterns for two gallium and chlorine atoms in the positive mode at 414 and 398 m/z suggested heteroatom uncoupling between the nitrogen and ethyl groups or alkyl backbone, respectively, which is not uncommon under mass spectrometry conditions and similar to other group 13 alkoxide species previously reported in the literature.[37−40]

In order to probe this interesting reactivity, the reaction of 3 equiv of gallium chloride bis(dimethylamide) with 12 equiv of the smaller backbone donor-functionalized alcohol HOCH2CH2NEt2 gave crystalline material in excellent yield (90%) which was suitable for single crystal X-ray analysis. Compound 2 crystallized in the monoclinic P21/c space group, and as shown in Figure , it is comparable to similar gallium chlorido bis(alkoxides) previously reported.[27] This alteration of the chain length from HOCH2CH2CH2NEt2 (used in the synthesis of 1) to HOCH2CH2NEt2 successfully facilitated the isolation of a monomer with the gallium atom adopting a distorted trigonal bipyramidal geometry, which is similar compared to the previously reported analogous gallium chloride bis(alkoxide) with the formula [GaCl(OCH2CH2NMe2)2], which crystallized in the orthorhombic Pna21 space group.[41] This compound was also monomeric, and similarly adopted a trigonal bipyramidal geometry, with −NMe2 groups lying in the axial positions, and oxygen atoms of each ligand and the chlorine atom residing in the axial positions.[41]

Figure 2

Molecular structure of compound 2. Thermal ellipsoids at 50% probability, hydrogen atoms omitted for clarity.

In compound 2, the oxygen atoms of each alkoxide ligand as well as the chloride occupy the equatorial positions while the nitrogen atoms of the donor-functionalized group reside in the axial positions, typical for compounds of this type. The donor-functionalized alkoxide ligands form a five-membered ring with the gallium center. The N(1)–Ga(1)–N(2) bond angle is 170.36(5)°, deviating from 180°. The sum of the bond angles in the equatorial plane of 2 is 359.95° and hence planar. The two equatorial Ga–O bond lengths (Table ) are less alike than when compared to the analogous compound with methyl groups at the N position, [GaCl(OCH2CH2NMe2)2] (1.8235(13) and 1.8302(14) Å).[41]

Table 1

Selected Bond Lengths (Å) and Angles (deg) for Compounds 2 and 3

	2	3
lengths (Å)
M–O	1.8339(13)	1.8235(13)
	1.8403(12)	1.8302(14)
M–N	2.1802(14)	2.2109(17)
	2.2073(15)	2.1725(16)
M–Cl	2.2324(5)	2.2296(6)
angles (deg)
O–M–O	127.64(6)	129.34(6)
N–M–N	170.36(5)	171.32(6)
O–M–N	85.08(5)	88.50(6)
	90.66(6)	86.83(6)
	92.34(5)	88.16(6)
	83.41(5)	89.19(6)
Cl–M–O	115.10(4)	114.89(5)
	117.21(5)	115.74(5)
Cl–M–N	95.16(4)	93.76(5)
	94.48(4)	94.92(5)

The Ga–N distances are 2.1802(14) and 2.2073(15) Å and are significantly shorter than the sum of the van der Waals radii; hence, these Ga–N interactions can be considered dative. The degree of trigonality, τ, is a measure of distortion for a five-coordinate complex between the two ideals of either a square-based pyramidal (τ = 0) or a trigonal bipyramidal (τ = 1) geometry.[42] For compound 2, τ = 0.71, suggesting that the metal center is deviated severely away from a trigonal planar geometry.

Both the OCH2 and NCH2 methylene protons of the OCH2CH2N fragment of the donor-functionalized ligand of 2 resonate at two different frequencies in the low-temperature 1H NMR spectrum recorded at 228 K (SI). In addition, the methylene protons of the N(CH2CH3)2 groups resonate at two different frequencies in the same spectrum (SI). The nonequivalence of all the methylene protons in 2 at 228 K is attributed to the formation of a five-membered ring as a result of the coordination from the O and N heteroatoms of the bidentate ligand OCH2CH2NEt2 to the Ga atom. As this cycle is nonplanar (SI), the pseudo-axial and pseudo-equatorial orientations of methylene protons in the slow exchange regime on the NMR time scale at 228 K are expected to lead to their nonequivalence in the 1H NMR spectrum.

Since compound 2 was successfully isolated as a monomer due to the decrease in the chain length of the carbon backbone (c.f. compound 1), the next variable to investigate was the substituent on the donor N atom. The reaction of 3 equiv of gallium chloride bis(dimethylamide) with 12 equiv of HOCH2CH2CH2NMe2 gave the monomer 3 in good yield (71%), with X-ray quality single crystals being isolated (Figure , Table ).

Figure 3

Molecular structure of compound 3. Thermal ellipsoids at 50% probability, hydrogen atoms omitted for clarity.

Table 2

Crystallographic Data for Compounds 2 and 3

	2	3
chemical formula	[GaClC12H28O2N2]	[GaClC10H24O2N2]
Fw (g mol–1)	337.53	309.48
crystal system	monoclinic	monoclinic
space group	P21/c	P21/c
a (Å)	15.924(2)	10.0997(17)
b (Å)	7.0297(9)	9.9824(17)
c (Å)	15.4616(19)	15.543(2)
α (deg)	90	90
β (deg)	114.538(2)	113.718(9)
γ (deg)	90	90
V (Å3)	1574.5(3)	1434.7(4)
Z	4	4
ρcalcd (g cm–3)	1.424	1.433
μ (mm–1)	1.915	2.094
reflns collected	12615	11541
unique reflns	3676	3362
Rint	0.0354	0.0317
R1 and wR2 [I > 2σ(I)]	0.0304, 0.0776	0.0325, 0.0853
R1 and wR2 [all data]	0.0343, 0.0800	0.0406, 0.0901

The oxygen atoms of each alkoxide ligand and the chloride group occupy the equatorial positions while the nitrogen atoms of the donor-functionalized group reside in the axial positions, despite the lengthening of the carbon backbone. The donor-functionalized alkoxide ligands form a six-membered ring with the gallium center. The N(1)–Ga(1)–N(2) bond angle is 171.32(6)°, again slightly deviating from 180°. The sum of the bond angles in the equatorial plane of 3 are planar (359.97°).

When compared directly to compound 2, the Ga–O1 and Ga–O2 bond lengths were significantly different, with the Ga–O1 and Ga–O2 bond lengths in 3 being shorter (Table ). This is likely due to the longer backbone relieving strain in the (−GaOCH2CH2CH2NMe2−) ring system. The Ga–N distances are 2.2109(17) and 2.1725(16) Å, which are again significantly shorter than the sum of the van der Waals radii; therefore, these Ga–N interactions can be considered dative. For compound 3, τ = 0.70, suggesting again that the metal center is severely distorted away from an ideal trigonal planar geometry.

NMR and DFT Study

Featureless broad lines, implying a fluxional coordination sphere, were observed in the 1H NMR spectrum at 296 K of compound 3. On cooling, the spectrum sharpened significantly at 207 K (Figure ) to show a spectrum analogous to that observed for 2 at 228 K (see SI).

Figure 4

VT-NMR of compound 3 in toluene-d8 over a temperature range from 207 to 384 K. Spectra have been staggered for clarity.

The nonequivalent pairs of signals observed clearly in the low-temperature 1H NMR spectra of 3 can be seen to coalesce as the temperature was raised (Figure ). Coalescence is defined as the temperature at which a single peak with a flat top is observed, which can be seen in Figure . The temperature dependence of the NMR spectrum of the NMe2 protons showing two singlets at 2.25 and 2.35 ppm at 207 K provides convenient means to follow the chemical exchange process as a function of temperature (Figure ) and estimate the free energy of activation. Using the chemical shift difference value of 40 Hz at 213 K and the standard expression (eq )(where k is the rate constant, kB is Boltzmann’s constant, T is the coalescence temperature, h is Planck’s constant, and ΔG⧧ is the Gibbs free energy of activation) for an equally populated two-site exchange model,[43] the free energy of activation (ΔG⧧) at the coalescence temperature of 256 K (Figure ) is estimated to be 53 ± 1 kJ mol–1.

Figure 5

VT-NMR of 3 in toluene-d8 over a temperature range from 250 to 259 K used to determine the coalescence temperature.

The fluxionality due to the hemilabile coordination of the donor-functionalized alkoxide ligands can cause the nitrogen and gallium atoms to dissociate–associate in a dynamic fashion, producing the broad spectrum observed at 296 K (Figure ). This fluxionality displayed by compounds 2 and 3 is not surprising; five-coordinate trigonal bipyramidal systems are capable of polytopal rearrangements in the coordination sphere, classically portrayed by PF5, with more modern examples including titanium, cobalt, iron, and antimony complexes.[44−47] Following these types of rearrangements, bidentate ligands with weakly coordinated donor groups lying in the axial positions could plausibly undergo dissociation, when bound to a labile metal center—in this case, gallium(III). This disconnection from either one or both of the hemilabile alkoxide ligands allows an inversion of the configuration at the metal center, followed by reattachment of the two nitrogen-donor groups. On the basis of the X-ray structure of 3 and the 1:1 ratio of the two dynamically exchanging species observed at low temperatures from the NMR spectra in toluene-d8 (Figures and 5), a possible solution-state exchange pathway is shown in Figure using the DFT M06/def2-TZVP optimized geometries (Gaussian 09).[48]

Figure 6

Dynamic behavior of the alkoxide ligand arms in 3, illustrated using DFT optimized molecular geometries. Conformation 3a is viewed along the Cl-Ga axis (top left), and from the side (middle left). Conformation 3b is viewed along the Cl-Ga axis (top right) and from the side (middle right). Conformation 3c is viewed from the side (bottom). Color key: gallium, pink; chlorine, green; oxygen, red; nitrogen, blue; carbon, gray.

The symmetry of 3b corresponds to that determined by single crystal X-ray diffraction in the solid state. On the assumption that the Ga···N close contacts can be considered as bonds in 3, two six-membered rings in their chair conformations can be visualized with a common Ga atom (Figure ). Then, a possible mechanism, by which the rearrangement shown in Figure can occur, involves interconversions of the six-membered rings (depicted as an exchange between 3a to 3c in Figure ), followed by rotations about the O–Ga bonds (depicted as an exchange between 3c to 3b in Figure ). The calculated free energy of 3c relative to 3a (and 3b) is +13.3 kJ mol–1 at the M06/def2-TZVP level of theory, suggesting that the presence of a very small proportion of 3c in solution cannot be ruled out, hence supporting the possibility of the above considered mechanism in Figure . Other possible mechanisms such as direct interconversion of the two six-membered rings, involving the ligands sweeping over or around the metal center, are also possible, although they may be energetically less likely.

This deviation from the structural assignment resolved by X-ray single crystal analysis has also been reported by Nguyen et al.[49] in which thiolate ligands are rapidly exchanged around a gallium center between bridging and terminal positions in solution and is confirmed with low-temperature NMR experiments. To summarize, the 2-carbon backbone in compound 2 shows some fluxionality, and when moving to a 3-carbon backbone in 3, the fluxionality is increased. When comparing to 1 which also has a 3-carbon backbone, as well as an −NEt2 group rather than the −NMe2 group in 3, the fluxionality is increased such that oligomerization occurs instead, and a monomer cannot be isolated. Given the monomeric nature of 2 and 3, these two compounds were investigated further as precursors for the CVD of Ga2O3.

AACVD Experiments

The monomeric compounds 2 and 3 were successfully used to deposit Ga2O3 at an optimized temperature of 450 °C. A dinitrogen carrier gas was used at 1 L min–1 to deliver the aerosol of either 2 or 3 using dry toluene as the solvent. Table shows the deposition conditions used and the resulting properties of the films from each of the precursor.

Table 3

AACVD Conditions and Ga2O3 Thin Film Properties Derived from UV/vis Data Deposited from Compounds 2 and 3

precursor	temperature (°C)	transmittance (%)	band gap (eV)
2	450	75	3.8
3	450	74	3.7

Transmittance and band gap calculations of the annealed thin films are displayed in Table Optical measurements were conducted over a range from 250 to 2000 nm on quartz samples. Measurements for glass samples are not reported as the transmittance for glass overlaps with that of Ga2O3. Transmittance measurements revealed the annealed Ga2O3 thin films transmitted 74–75% of visible light (380–780 nm) compared to 76% of plain quartz.

Films grown on silica coated float glass from 2 and 3 were amorphous; energy dispersive X-ray analysis (EDXA) confirmed negligible carbon contamination (<0.1 atm %), also confirmed by Raman spectroscopy (SI). The composition of the annealed films was also determined by EDXA measurements, the results of which indicated the presence of oxygen, gallium, and carbon. The presence of gallium and oxygen in a 1:1.5 ratio confirmed the Ga2O3 stoichiometry. The presence of residual chlorine or nitrogen was not observed. To obtain crystalline Ga2O3 material, films deposited from 2 on quartz substrates were annealed at 1000 °C for 12 h in air. Glancing angle X-ray powder diffraction confirmed that the annealing process gave crystalline films, consistent with reference standards for monoclinic β-Ga2O3 (Figure ).

Figure 7

XRD pattern of the monoclinic β-Ga2O3 film deposited on quartz by AACVD of compound 2 after annealing at 1000 °C for 12 h.

SEM analysis was conducted on both the as-deposited and annealed thin films. SEM images of Ga2O3 thin films deposited from precursors 2 and 3 are shown in Figure a,b. As-deposited Ga2O3 samples were seen to have a very smooth morphology. Features were barely visible with particle sizes ranging from 10 to 30 nm. After annealing, the crystallite size increased to 100 nm, which is expected as annealing causes the film to sinter (see SI for a histogram of the distribution of the grain size). Side-on SEM measurements were used to determine the film thickness (∼10 μm) and displayed a uniform thickness along the entire edge of the glass substrate (SI). Additionally, the X-ray photoelectron spectroscopy (XPS) for a Ga2O3 thin film deposited from compound 3 (Figure c) confirmed the presence of gallium in the thin films.

Figure 8

SEM images of gallium oxide thin films (a) 2 and (b) 3 deposited by AACVD in toluene at 450 °C. Images (x) i show the as-deposited amorphous films on silica coated float glass. Images (x) ii show the annealed films on quartz. (c) XPS of Ga 2p for a Ga2O3 thin film deposited from compound 3.

Conclusions

Gallium chlorido bis(alkoxide) precursors (1–3) were synthesized with monomer formation in the case of 2 and 3, and oligomer formation in 1. In all cases, the synthetic route to these compounds involved the reaction of a gallium chlorido dimethylamide dimer with 4 mol equiv of the corresponding functionalized alcohol. The anticipated fluxionality of compounds 2 and 3 in solution was probed and confirmed by VT-NMR measurements. The fluxionality trend observed in the synthesis of these precursors suggested that increasing the ligand backbone length and the size of the substituents on N leads to increased dynamic behavior. The result of this implied a long-chain backbone with bulky N substituents, as is the case for 1, provides so much flexibility that no monomeric unit was formed. Contrastingly, a smaller backbone (2) afforded some fluxionality and could be isolated as a monomer, whereas smaller N substituents (3) afforded a highly fluxional monomeric compound. In order to understand how 3 exhibits highly fluxional behavior in solution, DFT calculations were also carried out to consider a possible mechanism behind the observed fluxionality in 3. Compounds 2 and 3 were used to successfully deposit Ga2O3 via AACVD at an optimized temperature of 450 °C to give the expected amorphous material which could be annealed at 1000 °C to give crystalline gallium oxide.