TITANIUM ALKOXIDES - β-DIKETONATES
One of disadvantages of titanium tetrakis(alkoxides) for MOCVD applications is their high moisture sensitivity.Titanium alkoxo-β-diketonato complexes are less reactive with air moisture, some of them such as Ti(OiPr)2(acac)2, Ti(OMe)2(thd)2, Ti(OiPr)2(thd)2, Ti(OtBu)2(thd)2, have been applied as MOCVD precursors of Ti-containing layers.
Noncorrosive Ti alkoxides-β-diketonates have higher thermal decomposition temperature and thus larger ALD window where a self-regulated growth rate is achieved.
Titanium bis(isopropoxide) bis(2,2,6,6-tetramethylheptane-3,5-dionate) Ti(OiPr)2(thd)2
Fig. Chemical formula of Ti(OiPr)2(thd)2
Titanium bis(isopropoxide) bis(2,2,6,6-tetramethylheptane-3,5-dionate) Ti(OiPr)2(thd)2 (M = 532.58) is white crystalline powder, density d = 1.0 g/cm3, moisture sensitive (decomposes in the atmosphere containing H2O and O2, soluble in the organic solvents (f.e in ethylcyclohexane (ECH) possible solution concentrations are 0.01-0.45M), melting point 170-184.6°C (other data mp.160°C).
1H NMR spectrum Ti(OiPr)2(thd)2
Fig. 1H NMR spectrum of Ti(OiPr)2(thd)2
The 1H NMR spectrum Ti(OiPr)2(thd)2 contains peaks at 1.4ppm (CH3 of tBu group) and 3.9 ppm (CH3 of iPr group) and 7.45 ppm (CH3 of iPr group). (Fig. )
Volatility and thermal properties of Ti(OiPr)2(thd)2
Fig. TGA/DSC curves of Ti(OiPr)2(thd)2
Ti(OiPr)2(thd)2 sublimes at 150°C/ 0.01Torr, vapor pressure 1 Torr/ 160°C
Vapor pressure equation: logP(Pa) = 14.855 – 5435/T(K)
The TGA/DSC curves (atm. press.) of the Ti(OiPr)2(thd)2 precursor are presented in Fig. 5.4.2. The most of the precursor evaporates at the temperatures ~190-280°C, with an endothermic peak in the DTA curve at ~255°C. A residue of ~7% is left after the experiment, indicating that some of the precursor decomposes without volatilisation.
Ti(OiPr)2(thd)2 for PbTiO3 by MOCVD
Ti(OiPr)2(thd)2 was applied as Ti precursor for the growth of PbTiO3 layers in the PbTiO3-Pb(Mg0.33Nb0.66)O3 (PT-PMN) srructures on Si and Pt/Ti/SiO2/Si substrates by MOCVD using ultrasonic nebulization of precursor solution. PT-PMN films contained less pyrochlore phase (which even in small quantities decreases dielectric constant and worsens ferroelectric and piezoelectric properties) at higher Ti content; nearly single phase perovskite films were obtained at the composition of 80PMN-20PT. The PT-PMN films with perovskite phase showed a typical butterfly type C-V curve verifying the ferroelectricity, relative dielectric constant was about 60.
Ti(OiPr)2(thd)2 for Nb-doped Pb(Zr,Ti)O3 by MOCVD
Ti(OiPr)2(thd)2 was reported to be applied as a titanium source precursor for MOCVD of good quality Nb-doped Pb(Zr,Ti)O3 (PNZT) thin films. Ti(OiPr)2(thd)2 precursor was chemically compatible and stable with other metal precursors (like f.eNb(OiPr)4(thd)). Single-phase perovskite Nb-doped PZT films having good ferroelectric properties were obtained using Ti(OiPr)2(thd)2 as Ti precursor [i]
[i] I.-S. Chen, J. F. Roeder, T. E. Glassman, T. H. Baum, Chem. Mater., 1999, 11 (2), pp 209–212, DOI: 10.1021/cm980503s, « Liquid Delivery MOCVD of Niobium-Doped Pb(Zr,Ti)O3 Using a Novel Niobium Precursor », https://pubs.acs.org/doi/abs/10.1021/cm980503s
Titanium bis(mpd) bis(methd) Ti(mpd)2(methd)2
Ti(mpd)(methd)2 in combination with Zr(methd)4 and Pb(methd)2, mixed in a cocktail source dissolved in the ECH (ethylcyclohexane) solvent, was applied as Ti precursor for the growth of ferroelectric Pb(Zr,Ti)O3 thin films by liquid delivery MOCVD on 6-inch Pt/Ti/SiO2/Si wafers at 550°C. Ti(mpd)(methd)2 precursor had decomposition temperature around 300°C, close to the Zr and Pb precursors. The cocktail sources (including Ti(mpd)(methd)2) in the ECH solvent were perfectly vaporized above 280°C, and were stable for at least 6 months. The deposited Pb(Zr,Ti)O3 films were crystallized into perovskite PZT phase with appropriate composition, however undesirable PbPtx alloy admixture formed at the interface between bottom Pt electrodes and PZT layer at 550°C (with reactor pressure 3 Torr, vaporizer temperature 280°C/ vaporizer pressure 115 Torr). The polarization properties of the deposited PZT thin films were not sufficient due to the formation of PbPtx layer.[[i], [ii]]
MOCVD PbPtx alloy phase existed in Pb(Zr, Ti)O3 films deposited at 500°C using Ti(mpd)(methd)2 as Ti precursor disappeared by post-annealing at 600°C and the annealed Pb(Zr, Ti)O3 film showed reasonable hysteresis properties ( 2Pr =56 μ C/cm2 and 2Ec = 181 kV/cm). [iii]
[i] Y. Otani , N. Abe, Y. Ueda , M. Miyake , S. Okamura & T. Shiosaki , Integr. Ferroelectrics, An International Journal , Vol. 46, 2002 - Issue 1, p.115-124, https://doi.org/10.1080/10584580215398
[ii] Y. Otani , N. Abe, Y. Ueda , M. Miyake , S. Okamura & T. Shiosaki , Integrated Ferroelectrics, An International Journal, Vol. 51, 2003 - Issue 1, p.63-72, https://doi.org/10.1080/10584580390229824 , « Fabrication of Pb(Zr,Ti)O 3 Thin Films by Liquid Delivery MOCVD Using a Cocktail Source with Pb(METHD) 2 , Zr(METHD) 4 and Ti(MPD)(METHD) 2 « , www.tandfonline.com/doi/abs/10.1080/10584580390229824
[iii] Y. Otani, S. Okamura, T. Shiosaki, J. Electroceram., 2004, Vol. 13, Iss. 1–3, pp 15–22, “Recent Developments on MOCVD of Ferroelectric Thin Films”, https://link.springer.com/article/10.1007/s10832-004-5069-z
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