General Synthesis of Homoleptic Indium Alkoxide Complexes and the Chemical Vapor Deposition of Indium Oxide Films

A general synthetic route to homoleptic indium alkoxide complexes was developed, and one of the
new compounds was used as a precursor to transparent, conductive indium oxide films. The amide complex
In[N-t-Bu(SiMe3)]3 reacted with t-BuOH, EtMe2COH, Et2MeCOH and i-PrMe2COH to yield the dimers [In(μ-OR)(OR)2]2 (R = t-Bu, CMe2Et, CMeEt2, and CMe2i-Pr) in high yield. Similar reactions of In[N-t-Bu(SiMe3)]3 with the less bulky alcohols i-PrOH and Et2HCOH yielded, respectively, insoluble [In(O-i-Pr)3]n
and the tetramer In[(μ-OCHEt2)2In(OCHEt2)2]3, which has a six-coordinate central indium atom surrounded
by three four-coordinate indium atoms. The compounds [In(O-i-Pr)3]n and In[(μ-OCHEt2)2In(OCHEt2)2]3 were
also prepared by reacting [In(μ-O-t-Bu)(O-t-Bu)2]2 with an excess of the respective alcohols. Attempts to
prepare the previously reported oxo cluster In5(μ5-O)(μ3-O-i-Pr)4(μ2-O-i-Pr)4(O-i-Pr)5 by thermally decomposing
[In(O-i-Pr)3]n failed. The reaction between In[N-t-Bu(SiMe3)]3 and 2,6-diisopropylphenol afforded the bis tert-butylamine adduct In(O-2,6-i-Pr2C6H3)3(H2N-t-Bu)2. The evidence suggests that the tert-butylamine ligands
in In(O-2,6-i-Pr2C6H3)3(H2N-t-Bu)2 resulted from a secondary reaction between HN-t-Bu(SiMe3) and 2,6-diisopropylphenol. The powerful donor p-(dimethylamino)pyridine (p-Me2Npy) reacted with [In(μ-O-t-Bu)(O-t-Bu)2]2 to yield 5-coordinate In(O-t-Bu)3(p-Me2Npy)2 and with the more sterically encumbered complex
[In(μ-OCMeEt2)(OCMeEt2)2]2 to yield four-coordinate In(OCMeEt2)3(p-Me2Npy). In addition, [In(μ-O-t-Bu)(O-t-Bu)2]2 reacted with 2,2,6,6-tetramethyl-3,5-heptanedione (t-Bu2-β-diketone) to afford (t-BuO)2In(μ-O-t-Bu)2In(t-Bu2-β-diketonate)2, which has four- and six-coordinate indium centers and virtual C2 symmetry. X-ray
crystallographic studies were carried out for [In(μ-O-t-Bu)(O-t-Bu)2]2, In[(μ-OCHEt2)2In(OCHEt2)2]3, In(O-2,6-i-Pr2C6H3)3(H2N-t-Bu)2·1/2C7H9, In(O-t-Bu)3(p-Me2Npy)2·1/2Et2O, In(OCMeEt2)3(p-Me2Npy), and (t-BuO)2In(μ-O-t-Bu)2In(t-Bu2-β-diketonate)2. The t-amoxide complex [In(OCMe2Et)3]2 and oxygen were used
as precursors to deposit transparent, highly conductive indium oxide films on silicon, glass, and quartz substrates
at substrate temperatures of 300−500 °C in a low-pressure chemical vapor deposition process. A backscattering
spectrum indicated the film deposited at 500 °C was stoichiometric In2O3 (O/In = 1.46 ± 0.07). The films
were transparent in the visible region (>75%) and had resistivities as low as 9.1 × 10-4 Ω cm. X-ray diffraction
studies indicated the films deposited on glass were cubic and highly (100) oriented.