Comparison of the Atomic Layer Deposition of Tantalum Oxide Thin Films Using Ta(NtBu)(NEt2)3, Ta(NtBu)(NEt2)2Cp, and H2O.

The growth characteristics of Ta2O5 thin films by atomic layer deposition (ALD) were examined using Ta(NtBu)(NEt2)3 (TBTDET) and Ta(NtBu)(NEt2)2Cp (TBDETCp) as Ta-precursors, where tBu, Et, and Cp represent tert-butyl, ethyl, and cyclopentadienyl groups, respectively, along with water vapor as oxygen source. The grown Ta2O5 films were amorphous with very smooth surface morphology for both the Ta-precursors. The saturated ALD growth rates of Ta2O5 films were 0.77 Å cycle-1 at 250 °C and 0.67 Å cycle-1 at 300 °C using TBTDET and TBDETCp precursors, respectively. The thermal decomposition of the amido ligand (NEt2) limited the ALD process temperature below 275 °C for TBTDET precursor. However, the ALD temperature window could be extended up to 325 °C due to a strong Ta-Cp bond for the TBDETCp precursor. Because of the improved thermal stability of TBDETCp precursor, excellent nonuniformity of ∼2% in 200 mm wafer could be achieved with a step coverage of ∼90% in a deep hole structure (aspect ratio 5:1) which is promising for 3-dimensional architecture to form high density memories. Nonetheless, a rather high concentration (∼7 at. %) of carbon impurities was incorporated into the Ta2O5 film using TBDETCp, which was possibly due to readsorption of dissociated ligands as small organic molecules in the growth of Ta2O5 film by ALD. Despite the presence of high carbon concentration which might be an origin of large leakage current under electric fields, the Ta2O5 film using TBDETCp showed a promising resistive switching performance with an endurance cycle as high as ∼17 500 for resistance switching random access memory application. The optical refractive index of the deposited Ta2O5 films was 2.1-2.2 at 632.8 nm using both the Ta-precursors, and indirect optical band gap was estimated to be ∼4.1 eV for both the cases.

Nanoenergetic materials: boron nanoparticles from the pyrolysis of decaborane and their functionalisation.

The surfaces of boron nanoparticles 10-150 nm in diameter, prepared by gas phase pyrolysis of decaborane vapour at 1 atm and 700-900 degrees C, can be halogenated by treatment with Br2 or XeF2; the surface halogenation somewhat increases the onset temperature for the oxidation of the particles under O2.

Mono(cycloheptatrienyl) tantalum chemistry: synthesis and characterization of new tantalum halide, hydride, and alkyl species.

The new tantalum(II) complex (eta (6)-C 7H 8)TaCl 2(PMe 3) 2 ( 1) was synthesized by the reduction of TaCl 5 with n-butyllithium in the presence of PMe 3 and cycloheptatriene. Compound 1 adopts a four-legged piano stool structure in which the tantalum center is bound to a eta (6)-cycloheptatriene ring in addition to two chlorides and two phosphine ligands in a transoid arrangement. Treatment of 1 with methyllithium results in a loss of the equivalents of HCl and formation of the eta (7)-cycloheptatrienyl complex (eta (7)-C 7H 7)TaCl(PMe 3) 2 ( 2), whereas treatment of 1 with sodium or sodium borohydride affords small amounts of the eta (5)-cycloheptadienyl complex (eta (5)-C 7H 9)TaCl 2(PMe 3) 2 ( 3). Compound 2 adopts a three-legged piano stool structure; the eta (7)-C 7H 7 ring is fully aromatic and planar. The molecular structure of 3 is similar to that of 1, except for the eta (5) binding mode of the seven-membered ring. Treatment of the previously described sandwich compound (C 5Me 5)Ta(C 7H 7) with allyl bromide affords the tantalum(V) product (C 5Me 5)Ta(C 7H 7)Br ( 4), which reacts with LiAlH 4 to give the tantalum(V) hydride (C 5Me 5)Ta(C 7H 7)H ( 5). Compound 4 also reacts with alkylating agents to generate the methyl, allyl, and cyclopropyl complexes (C 5Me 5)Ta(C 7H 7)Me ( 6), (C 5Me 5)Ta(C 7H 7)(eta (1)-CH 2CHCH 2) ( 7), and (C 5Me 5)Ta(C 7H 7)(c-C 3H 5) ( 8). Compounds 4- 8 all adopt bent sandwich structures in which the dihedral angle between the two carbocyclic rings is 34.9 degrees for the bromo compound 4, 26.6 degrees for the hydride 5, 33.1 degrees for the methyl compound 6, 34.2 degrees for the allyl compound 7, and 37.5 degrees for the cyclopropyl compound 8. (1)H and (13)C NMR data are reported for the diamagnetic compounds.

Synthesis and characterization of the cycloheptatrienyl tantalum "mixed-sandwich" compounds (c5r5)ta(c7h7).

The molecule (cycloheptatrienyl)(cyclopentadienyl)tantalum, (C(5)H(5))Ta(C(7)H(7)) (1), and its methylcyclopentadienyl and pentamethylcyclopentadienyl analogues (C(5)H(4)Me)Ta(C(7)H(7)) (2), and (C(5)Me(5))Ta(C(7)H(7)) (3) have been synthesized by magnesium reduction of the corresponding (C(5)R(5))TaCl(4) species in the presence of cycloheptatriene. The crystal structures of 2 and 3 show that the two rings are planar and essentially parallel to each other. Interestingly, the Ta-C distances to the C(7)H(7) ring are significantly shorter (by about 0.1 A) than those to the cyclopentadienyl ring; the difference reflects stronger bonding to the C(7)H(7) ring. A comparison with the structures of other (C(5)R(5))M(C(7)H(7)) shows that the M-C distances to the seven-membered ring are especially sensitive to the d-orbital energies of the metal center and its ability to engage in delta bonding with the ring. For 1-3, the EPR spectra at room temperature consist of octets due to the tantalum nuclear spin. Both A(iso) and g(iso) increase as the number of methyl groups on the cyclopentadienyl ring increases. EPR spectra of 1-3 as frozen glasses correspond to axial symmetry, and the hyperfine couplings and g factors are deduced from simulations.

Rhenium oxohalides: synthesis and crystal structures of ReO3Cl(THF)2, ReOCl4(THF), Re2O3Cl6(THF)2, and Re2O3Cl6(H2O)2.

Convenient methods to prepare solvated rhenium oxochlorides are described; these compounds should serve as useful starting materials for rhenium chemistry. Treatment of perrhenic acid, HReO(4), with chlorotrimethylsilane or with thionyl chloride, followed by addition of tetrahydrofuran, forms the new oxochloride complexes ReO(3)Cl(THF)(2) and ReOCl(4)(THF), respectively. Small amounts of two dinuclear oxochlorides, which evidently resulted from adventitious hydrolysis, were also isolated: Re(2)O(3)Cl(6)L(2), where L = THF or H(2)O. All four compounds were characterized by X-ray crystallography. The rhenium(vii) complex ReO(3)Cl(THF)(2) adopts a distorted octahedral geometry in which the three oxo ligands are in a facial arrangement; the rhenium(vi) complex ReOCl(4)(THF) adopts a trans octahedral structure. The two dinuclear rhenium(vi) compounds both have a single, nearly linear, bridging oxo group; on each Re center, the three terminal chlorides adopt a mer arrangement, and the terminal oxo and the coordinated Lewis base are mutually trans. The water ligand in the aqua complex is hydrogen bonded to nearby THF molecules. IR data are given.