Reports: AC5
47014-AC5 Thin Film Growth of Amorphous Metal-Phosphorus Alloys
The continued goals of the research are to investigate fundamental aspects of the chemical vapor deposition (CVD) of thin films of amorphous metallic alloys. We have now made significant progress in the synthesis and characterization of new molecular precursors as well as preliminary CVD screening of promising candidates for the CVD growth of thin films of metal phosphorus alloys. The screening studies have been carried out with a simple hot wall CVD reactor which was built during the grant period.
1. Organometallic Precursor Synthesis. As described in last year's report we have investigated the synthesis, characterization and testing of new metal complexes which bear simple phosphine ligands (as the source of P) in combination with bis-trifluoromethyl pyrazolate (Pz) or 3,5-dimethyl pyrazolate (Me2Pz) ligands. These pyrazolate ligands provide a reasonable level of volatility and can form stable leaving groups during the growth/decomposition process. In addition to the compounds described in last years report we have now prepared and characterized the following platinum group metal compounds: For Ru: Ru(η1-Me2Pz)2(η1-HMe2Pz)2(PMe3)2 (1) and Ru2(μ-Pz)4(H2O)Cl (2); for Rh: Rh2(μ-Pz)2(P(OMe)3)4 (3), Rh2(μ-Pz)2(C2H4)4 (4), Rh2(μ-Pz)(μ-Cl)(PMe3)4 (5), Rh(μ-Pz)4Li2(THF)2 (6), Rh(η1-Pz)(PMe3)3(η2-O2) (7) and Rh(η1-Pz)2(η1-HPz)2(Cl)2 (8). Interestingly, complex 6 represents the rare formation of a mononuclear Rh(II) complex. Using the related bis-tert-butyl pyrazolate ligand (t-Bu2Pz) dinuclear Rh2(μ-t-Bu2Pz)2 (9) is formed and with 3,5-dimethyl pyrazolate (Me2Pz) the binary Rh(III) anion [Rh(η1-Me2Pz)6]3- (10) can be isolated in high yield. In addition, despite the fact that Rh(I) and Ir(I) often adopt stable, 4-coordinate, planar 16 electron structures we have isolated and structurally characterized three examples of mononuclear 18 electron cations [Rh(PMe3)5]+ or [Ir(PMe3)5]+ as either their Pz or [B(C6F5)4]- salts.
We have also investigated the PMe3/Pz chemistry of Co which is proving to be quite fascinating and quite complex. Several potential CVD precursors have been prepared and characterized. For the known complex, Co(PMe3)4, (H.-F. Klein, Angew. Chem. Int. Ed. 1971, 10, 343), detailed thin film growth studies have been completed. New Pz/PMe3 complexes prepared and characterized include: Co(PMe3)3(η1-Pz) (11), Co(PMe3)3(η1-Pz)Cl (12) and Co(OPMe3)2(η1-Pz)2 (13). Interestingly the reaction of Co(PMe3)4 with PzH in hydrocarbon solutions gives good yields of a mixture of two complexes which we have, so far, been unable to separate, CoH(PMe3)4.Co(PMe3)(η1-Pz)3 (14). The two Co compounds in this mixture co-crystallize together from hexane and also co-sublime under vacuum. All of the compounds reported here have been characterized spectroscopically as well as by single crystal X-ray diffraction studies.
2. Film Growth and Characterization Studies. We have initiated a systematic evaluation of the most promising compounds as CVD precursors for the growth of amorphous metal-phosphorus alloys. In collaboration with the research group of Professor John Ekerdt (UT Chemical Engineering) detailed thin film growth studies have been completed on Co(PMe3)4. Unlike our previous studies on cis-H2Ru(PMe3)4 which produces thin films of amorphous ruthenium-phosphorus alloys (RuP) under a variety of conditions, Co(PMe3)4 exhibits a more complex CVD profile. Thus, thin nanocrystalline (hcp) Co-based films were grown by CVD on SiO2. The films incorporate both P and C when the substrate temperature ranges from 225 °C to 325 °C, and feature crystallites sized < 15 nm. The P within the deposited film is mostly elemental with some being phosphidic, and the C exists mostly in the carbidic form along with some being graphitic. The Co is correspondingly predominantly metallic. Upon annealing to 400 °C for 3 h, some extent of both Co2P crystallite precipitation and minimal allotropic transformation to (fcc) Co is observed, and the amount of carbidic C is reduced relative to graphitic C, but the nanocrystalline microstructure of the film is preserved, still primarily composed of (hcp) Co-based crystallites. Preliminary CVD screening studies have also been carried out on Co(PMe3)3(η1-Pz) (11) and CoH(PMe3)4. Co(PMe3)(η1-Pz)3 (14). In the case of Rh(PMe3)3(η1-Pz) as the precursor an interesting solid state transformation takes place. Thin films of metastable amorphous RhP films can be grown with this compound at a deposition temperature of 300 °C. After two weeks at room temperature the films undergo a transformation to crystalline and XRD studies reveal that a mixture of rhodium phosphide (fcc) Rh2P and Rh metal (fcc) are present is roughly equal amounts. The size of the crystallite (fcc) Rh2P is approximately 8 Ǻ.
