Lithium β-diketonates are conventional Li CVD precursors, traditionally used due to their air stability and reasonable volatility. However, they are solids, presenting difficulties in evaporation rate stability from solid phase. As an example, Li(thd) (combined with K(thd) ) was applied for the deposition of lithium-potassium niobate films on K,Li tantalate substrates by LPCVD [[i]].
[i] K. Chikuma, A. Onoe and A. Yoshida, Jpn. J. Appl. Phys., Part 1, 1998, 37, 5582-5587
According to [4], Li(thd) decomposes under vacuum, and was suggested to be less useful precursor than f.e. [Li(OtBu)6]. Di Carolis succeeded to improve a thermal stability of Li(thd) precursor by complexation it with crown ether (18-crown-6).
Li(thd) (or Li(DPM)) was applied for the deposition by low pressure MOCVD of Li2O films (and polycristalline lithium niobate LiNbO3 layers when combined with Nb(thd)2Cl3
as Nb source), using N2 or Ar carrier gas mixed with 50 % of O2 at 5 Torr. Li2O films were grown on alumina substrates under Ar+O2 atmosphere, whereas under N2+O2 atmosphere Li2CO3 were growing, according to XRD and
ESCA analyses. On Si or SiO2 substrates, both Li2O and Li2CO3 layers reacted with substrates forming lithium silicates. The optimum condition for preparing LiNbO3 films were found when feeding both Li(thd) and Nb(thd)2Cl3 precursors, from a film composition map mapped as a function of the reaction temperature vs Li mol % in feed gas.[[i], [ii]]
[i] S.-Ch. Jung, N. Imaishi, Kor. J. Chem. Eng., Vol.16, No. 2, 229-233, Electrochem. Soc. Proc 1996, Vol 96-5, p253, The growth of LiNbO3 thin film by LPMOCVD using β-diketonate complexes DOI: 10.1007/BF02706841
[ii] Sang-Chol Jung, Nobuyuki Imaishi,, The Reports of the Institute of Advanced Material Study: Kyushu University, 1997, pp. 7-12 (SEE N97-29549 01-23). “Low Pressure MOCVD of Li2O or Li2CO3 thin films from Li(DPM)”
Lithium 2,2,6,6-tetramethyl-3,5-heptanedionate Li(thd) has been applied for ALD growth of lithium-containing layers.
Thus, alternate pulsing of Li(thd) and O3 in a temperature range of 185–300 °C produced lithium carbonate (Li2CO3) films. The film composition was confirmed by TOF-ERDA. Polycrystalline lithium carbonate films were grown at 225 °C according to XRD. A Surface roughness of 19 nm was found by AFM for 120 nm Li2CO3 film grown at 225 °C.
Lithium lanthanate thin films were grown by ALD from Li(thd), La(thd)3 and ozone. Film composition was varied by controlling the number of Li(thd) (Li2CO3) and La(thd)3 (La2O3) sub-cycles (Fig.) [30]
Lithium 2,2,6,6-tetramethylheptane-3,5-dionate Li(thd) was applied for the CVD deposition of LiNbO3 films in several precursor combinations, however, a number of difficulties exist with each method of manufacturing LiNbO3
1) two-source solid precursor MOCVD process using Li(thd) and Nb(thd)4 precursors. This method suffers from the premature oxidation of the Li(thd), producing a Li2CO3 soot deposited into the thin film and degrading optical quality/ reducing the growth rate of the film. This method also requires a 7-to-3 ratio of Li(thd) / Nb(thd)4 in order to obtain 1:1 stoichiometry (LiNbO3).
2) two-source MOCVD process using (Li(thd)) and Nb(OMe)5 - produces a rough LiNbO3 layer surface (Curtis et al., U.S. Pat. No. 3,911,176)
3) single-source precursor generated in situ by the reaction of Li(thd) and Nb(OEt)5 ( toluene solution) - results in a certain amount of defects in the thin film surface due to the gas phase decomposition of the precursor
Both thermal stability of the precursor during sublimation, and in the very hot vicinity of the substrate, are of
concern for MOCVD process. The substrate in a MOCVD reactor for producing LiNbO3 is ca.700° C, whereas Li(thd) is thermally stable to ~400° C. The Li2CO3 soot is produced by oxidation of a certain amount of the Li(thd) in the hot vicinity of the substrate,
and the soot is then deposited onto the substrate and is absorbed into the thin film. A prior approach to solving this problem was to use commercially available Li(thd) prepared by the literature method [[i]]. The quantity of soot is minimized by reducing
the overall growth temperature (to reduce temperature in the vicinity of the substrate), however, still an unacceptable quantity of soot absorbed into the LiNbO3 film, and low deposition temperatures result in films with worse crystalline properties.
Thus, it is desirable to have a solid source MOCVD process for producing a high quality thin film LiNbO3 with no defects, no soot particles, and a required ratio of precursor quantities which approximates the stoichiometrically
expected ratio.
[i] Hammond, G. S, Nonhebel, D. C., and Wu, C-W. S., Inorg. Chem., 2, 73 (1963).
Lithium tetramethylheptanedionate Li(thd), in combination with Nd(thd)4, was applied for the growth of textured c-axis oriented LiNbO3 films on silicon substrates by the solid-source
MOCVD, for their potential use for the waveguiding applications. Optical confinement in the LiNbO3 films was achieved by using thermally grown SiO2 layers were used as cladding layers. Either increasing the growth temperature and/or decreasing the growth rate
allowed to vary the texture direction from the 006 to the 012 direction. Under optimal growth conditions, pure 006 texturing was achieved without the aid of an electric field, nor using a SiN(x) buffer layer. The
variation in film growth rate strongly influenced the crystallinity and surface rms roughness of c-axis oriented films, f.e. by increasing the growth rate allowed to decrease the (006) rocking curve FWHM values to less
than 2 deg. Increase of growth rate also decreased the surface roughness (rms values as low as 1.5 nm were achieved). However, too high growth rate lead to increased roughness due to gas phase nucleation, what was increasing
the optical losses; the best films had optical losses of only 4.5 dB/cm at a wavelength of 632.8 nm.[[i]]
[i] S.Y. Lee, R.S. Feigelson, J. Mater. Research, 1999, Vol. 14, no. 6, pp. 2662-2667 , "c-axis lithium niobate thin film growth on silicon using solid-source metalorganic chemical vapor deposition"
Lithium tetramethylheptanedionate Li(thd), combined with Nb(OMe)5, was applied as MOCVD growth of LiNbO3, however, it
produced a rough layer surface [[i]]
[i] Curtis et al., U.S. Pat. No. 3,911,176
Lithium dipivaloylmethanate Li(thd) [or Li(DPM)], combined with niobium pentaethoxide [Nb(OEt)5], was applied as precursor for the growth of LiNbO3 thin films on (001) and (012) sapphire substrates by MOCVD, and their crystalline structures were investigated by XRD and pole figure analyses. Strong (001) LiNbO3 orientation involving twin structures was obtained on (001) sapphire substrates, whereas highly (100)-oriented films were grown on the (012) oriented sapphire, with FWHM of the X-ray rocking curves of being 0.16° and 0.21°, respectively. Highly (100)-oriented LiNbO3 films had unique morphologies with long and narrow grains, which aligned in plane on the (012) sapphire substrates, and the c-axis of the LiNbO3 being parallel to the short axis of the grains, as was determined by field emission scanning electron microscopy (FE-SEM). [[i]]
The growth of epitaxial lithium niobate films on sapphire by thermal CVD from the metalorganic compounds Li(C11H19O2) (Li(thd) and Nb(OC2H5)5 was reported to be within the range of operating conditions for the growth of pure polycrystalline LiNbO3 on silicon substrates. The composition of the epitaxially grown LiNbO3 film was similar to the LiNbO3 solid solution in the phase diagram of the Li–Nb composite oxide obtained from a molten solution.[ii]
Li(dpm) (or Li(thd) (vaporized at 197°C), combined with Nb(OC2H5)5 (vaporized at 105°C) , was used as precursor for the growth of lithium niobate (LiNbO3) thin films on Al2O3(001) substrates by MOCVD (O2 was used as oxidant). The c-axis oriented and epitaxially grown stoichiometric LiNbO3 thin films were grown on Al2O3(001) substrates after optimization of the film deposition conditions. The stoichiometric LiNbO3 thin films had refractive index 2.24 at wavelength 632.8 nm (close to that of bulk crystal). [[iii]]
Lithium dipivaloylmethanate
Li(thd) combined with niobium(V) ethoxide Nb(OEt)5 (more precisely, an organometallic compound formed by their reaction was used as a single-source precursor), was applied for the deposition of lithium niobate thin films on (0001) sapphire by MOCVD. The epitaxial
nature of the films was established by XRD and RBS.[[iv]]
[i] K. Shiratsuyu, A. Sakurai, K. Tanaka, Yu. Sakabe, Jpn. J. Appl. Phys. 38 (1999) pp. 5437-5441, « Preparation and Characterization of Epitaxial LiNbO3 Thin Films by Metal-Organic Chemical Vapor Deposition »
[ii] Y. Akiyama, K. Shitanaka, H. Murakami, Y.-S. Shin, M. Yoshida, N. Imaishi, Thin Solid Films, 2007, Vol. 515, Iss. 12, p. 4975-4979, The Third International Symposium on Dry Process (DPS 2005) Epitaxial growth of lithium niobate film using metalorganic chemical vapor deposition
[iii] R. Morohashi, N. Wakiya, T. Kiguchi, T. Yoshioka, J. Tanaka, K. Shinozaki, Key Engineering Materials 2009, (Vol. 388), p.179-182, « Preparation of Epitaxial LiNbO3 Thin Film by MOCVD and Its Properties »
[iv] Alex A. Wernberg, Henry J. Gysling, Albert J. Filo, Thomas N. Blanton, Appl. Phys. Lett. 62, 946 (1993); http://dx.doi.org/10.1063/1.108528, http://scitation.aip.org/content/aip/journal/apl/62/9/10.1063/1.108528 « Epitaxial growth of lithium niobate thin films from a single‐source organometallic precursor using metalorganic chemical vapor deposition »
Li(DPM) (in combination with Nb(DPM)2Cl3 or Nb(OC2H5)5) was applied as lithium precursor for the
growth of polycrystalline LiNbO3 films by thermal CVD at 5 Torr at temperature range of 873–1023 K (600-750°C). It was determined that the range of operation conditions for growing pure LiNbO3 was wider using Nb(OC2H5)5, compared to that using Nb(DPM)2Cl3
as the Nb source. The reactivity of Nb(OEt)5 was found to be considerably higher than that of Nb(DPM)2Cl3; the distribution of LiNbO3 growth rate in the reactor and step coverage was governed by the Nb2O5 growth process.[[i]]
[i] Y. S. Shin, M. Yoshida, Y. Akiyama, N. Imaishi and S. C. Jung, Jpn. J. Appl. Phys. 42 (2003) pp. 5227-5232 Preparation of Lithium Niobate Thin Film by Thermal Chemical Vapor Deposition
Li-dipivaloylmethanate Li(DPM) (or Li(thd) , in combination with [K(DPM)] as K source and pentaethoxyniobium [Nb(OEt)5] as Nb source, was for the deposition of epitaxial films of potassium
lithium niobate (KLN) (K,Li)NbO3 on (K,Li)TaO3 single-crystal substrates by low-pressure MOCVD. XRD was used to study the crystallinity of the films; reciprocal space mapping demonstrated that films of good epitaxial quality was obtained. The surface of the
deposited layers was smooth enough to be used as a waveguide; several guided modes were observed for both TE and TM waves by the prism coupler method. Film refractive indices at various wavelengths were determined; the phase matching property of the waveguide
was discussed. The propagation loss of 0.5 dB/cm at a wavelength of 633 nm was determined using estimation by the cut-back method.[[i]]
[i] Kiyofumi Chikuma, Atsushi Onoe and Ayako Yoshida, Jpn. J. Appl. Phys. 37 (1998) pp. 5582-5587, “Waveguiding Epitaxial Potassium Lithium Niobate Single-Crystal Films Deposited by Metalorganic Chemical Vapor Deposition”
Air stable Li(thd) complex was applied as precursor for the MOCVD growth of Li-containing materials like LiCoO2 (and compared with sec-butyllithium LisecBu as precursor). The desired materials could be easily obtained using Li(thd), whereas with LisecBu insufficient
lithium was transported to the reactor zone [[i]]
[i] P Fragnaud, R Nagarajan, DM Schleich, D Vujic, J. Power Sources, 1995, Vol.54, Iss. 2, p.362–366, « Thin-film cathodes for secondary lithium batteries »
Lithium tetramethylheptanedionate Li(thd) was successfully applied for the growth of thin films of the active cathode materials LiMn2O4 and LiCoO2 by low pressure CVD or spray pyrolysis technique (the desired material were easily obtained using air-stable Li(thd) complex, whereas studies using tert-butyllithium LitBu were abandoned). Temperatures over 600 °C were necessary in order to prepare the spinel phase LiMn2O4 , whereas
LiCoO2 phase was readily prepared at 450-650 °C temperatures. LiCoO2 and LiMn2O4 were successfully prepared by the spray pyrolysis technique at temperatures as low as 400 °C. [[i]]
[i] P Fragnaud, R Nagarajan, DM Schleich, D Vujic, J. Power Sources, 1995,Vol. 54, Iss. 2, p. 362–366, « Thin-film cathodes for secondary lithium batteries »