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(SiMe 3 )] 3 reacted with t -BuOH, EtMe 2 COH, Et 2 MeCOH and i -PrMe 2 COH to yield the dimers [In( μ -OR)(OR) 2 ] 2 (R = t -Bu, CMe 2 Et, CMeEt 2, and CMe 2 i -Pr) in high yield. Similar reactions of In[N- t -Bu(SiMe 3 )] 3 with the less bulky alcohols i- PrOH and Et 2 HCOH yielded, respectively, insoluble [In(O- i -Pr) 3 ] n and the tetramer In[( μ -OCHEt 2 ) 2 In(OCHEt 2 ) 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 -OCHEt 2 ) 2 In(OCHEt 2 ) 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 In 5 ( μ 5 -O)( μ 3 -O- i -Pr) 4 ( μ 2 -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(SiMe 3 )] 3 and 2,6-diisopropylphenol afforded the bis tert -butylamine adduct In(O-2,6- i -Pr 2 C 6 H 3 ) 3 (H 2 N- t -Bu) 2 . The evidence suggests that the tert -butylamine ligands in In(O-2,6- i -Pr 2 C 6 H 3 ) 3 (H 2 N- t -Bu) 2 resulted from a secondary reaction between HN- t -Bu(SiMe 3 ) and 2,6-diisopropylphenol. The powerful donor p -(dimethylamino)pyridine ( p -Me 2 Npy) reacted with [In( μ -O- t -Bu)(O- t -Bu) 2 ] 2 to yield 5-coordinate In(O- t -Bu) 3 ( p -Me 2 Npy) 2 and with the more sterically encumbered complex [In( μ -OCMeEt 2 )(OCMeEt 2 ) 2 ] 2 to yield four-coordinate In(OCMeEt 2 ) 3 ( p -Me 2 Npy). In addition, [In( μ -O- t -Bu)(O- t -Bu) 2 ] 2 reacted with 2,2,6,6-tetramethyl-3,5-heptanedione ( t -Bu 2 - β -diketone) to afford ( t -BuO) 2 In( μ -O- t -Bu) 2 In( t -Bu 2 - β -diketonate) 2, which has four- and six-coordinate indium centers and virtual C 2 symmetry. X-ray crystallographic studies were carried out for [In( μ -O- t -Bu)(O- t -Bu) 2 ] 2, In[( μ -OCHEt 2 ) 2 In(OCHEt 2 ) 2 ] 3, In(O-2,6- i -Pr 2 C 6 H 3 ) 3 (H 2 N- t -Bu) 2 ·1/2C 7 H 9, In(O- t -Bu) 3 ( p -Me 2 Npy) 2 ·1/2Et 2 O, In(OCMeEt 2 ) 3 ( p -Me 2 Npy), and ( t -BuO) 2 In( μ -O- t -Bu) 2 In( t -Bu 2 - β -diketonate) 2 . The t -amoxide complex [In(OCMe 2 Et) 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 In 2 O 3 (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.