Lead tetraalkyls including tetraethyllead PbEt4, tetraphenyllead PbPh4 and others, were successfully used as CVD precursors.
Synthesis:
1. Synthesis from lead dichloride and alkylsmagnesium chloride:
6 PbCl2 + 12 RMgCl → 6 “PbR2” + 12 MgCl2
6 “PbR2” → 3 PbR4 + 3 Pb
2. Synthesis from sodium-lead alloy and alkyl halides:
4 NaPb + 4 RCl → PbR4 + 3 Pb + 4 NaCl
3. Electrolysis of alkyls on lead anode : Pb + 4 R- → PbR4
4. Synthesis from lead acetate, ethyl iodide and triethylaluminum:
6 Pb(OAc)2 + 6 EtI + 6 AlEt3 → 6 PbEt4 + 4 Al(OAc)3 + Al2I6 (CdEt2: catalyst)
5. Tetramethyllead has been synthesized by decomposition of trimethyllead hydride
Me3PbH → 3 PbMe4 + Pb + 2 H2
Lead tetraalkyls are poisonous, thermally labile - decomposing via PbR3 + R formation (homolytical decomposition mechanism). [[i]]
[i] Prof. Dr. Heinrich Lang, The Chemistry of Metal CVD, International Research Training Group, “Materials and Concepts for Advanced Interconnects”
Tetramethyllead PbMe4 due to its is potentially applicable as precursor for MOCVD of Pb-containing layers.
Tetraethyl lead PbEt4 is volatile liquid , having relatively high vapor pressure (0.1 torr at 17.8°C). The dependence of vapor
pressure on temperature is shown on Figure [[i]]
[i] Dr. Anton Carl Greenwald, « MOCVD OF LEAD-GERMANATE FOR NON-VOLATILE RAMs », A Final Report , submitted by SPIRE CORPORATION, Contract Number: N00014-90-C-0127, SBIR, Phase I, 1990, http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA230014
Tetraethyl lead PbEt4 (99.9999% pure from Eagle Pitcher Laboratories) was applied for the growth of the PbO thin films by MOCVD (by using nitrous oxide N2O (40-400 ppm (at.%) in Ar as oxidant). The deposition rate of PbO at substrate temperature 300 -500C was ~50-100 nm/h.The deposition rate fell off sharply at both higher and lower temperatures. At the lower temperatures, the compound was not reacting with the oxidant or decomposing without the oxidant. At the higher temperatures, lead oxide has a small but sufficient vapor pressure to be evaporated off the surface of the sample.
The deposition rate of metallic lead Pb was
measured separately with pure H2 as a carrier gas. At 600°C, the deposition rate was 100nm/ hour, comparable to that observed with Ar at lower temperatures. At 700°C, the deposition rate fell to 40nm/hour. There was no oxygen added to these test runs,
but pure lead should easily melt at these temperatures. This means that there was sufficient residual oxygen during these tests so an alloy was formed that would not change state at elevated temperatures. The drop in deposition rate at 700°C was related
to either too little oxygen available to stabilize the matrix, or to partial evaporation of the lead oxide formed.[[i]]
[i] Dr. Anton Carl Greenwald, « MOCVD OF LEAD-GERMANATE FOR NON-VOLATILE RAMs », A Final Report , submitted by SPIRE CORPORATION, Contract Number: N00014-90-C-0127, SBIR, Phase I, 1990, http://oai.dtic.mil/oai/oai?verb=getRecord&metadataPrefix=html&identifier=ADA230014
Tetraethyl lead PbEt4, combined with Ti(OiPr)4, was applied as precursor for the deposition of ferroelectric PbTiO3 thin films by thermal MOCVD under the reduced pressure of 6 Torr. The deposition behavior of PbO layers was examined independently. Homogeneous nucleation of the reaction species in the vapor phase was suppressed under the reduced pressure. The films prepared at 500°-650°C consisted of PbTiO3 of the single perovskite phase; on MgO(100) highly c-axis oriented and epitaxial films were grown. The deposition rate of the film was varied from 5- 100 nm/min by adjusting the source temperature and the flow rate of the carrier gas. [[i],[ii]]
Tetraethyl lead PbEt4, combined with titanium isopropoxide Ti(OiPr)4, was applied for deposition of device-quality ferroelectric thin PbTiO3 films by MOCVD. The conditions suitable for the deposition of perovskite PbTiO3 on fused silica and Pt-coated alumina substrates were identified. The stoichiometry of the PbTiO3 films was variable over a wide range with two types of microstructures (faceted or nodular), as determined by electron microprobe analysis. Under optimal deposition conditions the MOCVD PbTiO3 films exhibited nominally phase-pure perovskite XRD patterns (with preferred crystallographic orientation which was deposition conditions-dependent). However, the lattice parameters determined by XRD (a = 3.91 and c = 4.09 A) indicated a slightly smaller degree of tetragonality in MOCVD PbTiO3 compared to the bulk material. [[iii]]
Tetraethyl lead PbEt4, combined with Ti(OiPr)4 and O2 as Ti and O sources, was applice as lead precursor for the growth of perovskite PbTiO3 thin films on Si, Si/TiO2 and Si/Pt substrates by carrier-gas-free MOCVD, which resulted in high grow rates (∼6.0 μm/h) at low growth temperature (450°C) growth. However, the C–V characteristics of low-temperature-grown perovskite PbTiO3 layers did not always show normal ferroelectric behavior. The difference in growth behaviour with or without carrier gas was compared.[[iv] ]
Tetraethyl lead (Pb(C2H5)4), combined with Ti(OiPr)4 and O2, was used for the preparation of lead titanate (PbTiO3) films by a low-pressure CVD. The influence of O2 to the film growth rate, crystalline property, and composition was
studied. At 823 K (550°C) the layers exhibiting only TiO2 XRD diffraction pattern was formed without adding O2, whereas after adding 1 vol.% of O2 the stoichiometric PbTiO3 films were formed. However, increasing surplus O2 concentration not only decreased
the Pb content, but also caused concentration profile of Pb in the layers, as found by RBS. Raising the substrate temperature to
873 K (600°C), the deviation of Pb composition was reduced. According to the thermodynamic calculations of the stability domain of the Pb–O system, the formation
of high valence lead oxides having high volatility during film deposition are responsible for the Pb-deficiency phenomenon in the CVD process.[[v]]
[i]M. Okada, S. Takai, M. Amemiya, K. Tominaga, Jap. Journal Appl. Phys., vol.28, part 1, Number 6, “Preparation of c-Axis-Oriented PbTiO3 Thin Films by MOCVD under Reduced Pressure”, http://iopscience.iop.org/article/10.1143/JJAP.28.1030/meta
[ii] M. Okada et al.. Jpn. J. Appl. Phys., 2, 718 (1990).
[iii]S. L. Swartz , D. A. Seifert , G. T. Noel & T. R. Shrout , Ferroelectrics , Vol 93, 1989 , Iss. 1 p. 37-43, “Characterization of MOCVD PbTiO3 thin films”
https://doi.org/10.1080/00150198908017318
http://www.tandfonline.com/doi/abs/10.1080/00150198908017318
[iv] Chulsoo Byuna, Jin Wook Jang, Young-June Cho, Kun Jai Lee, Byong-Whi Lee, Thin Solid Films, Volume 324, Issues 1–2, 1 July 1998, Pages 94-100,
“Low temperature synthesis of PbTiO3 thin films by MOCVD without carrier gas”
https://www.sciencedirect.com/science/article/pii/S0040609097012194
[v] Lu Sheng Hong, Chung Chieh Wei , Materials Letters, Vol. 46, Iss. 2–3, November 2000, Pages 149-153, « Effect of oxygen pressure upon composition variation during chemical vapor deposition growth of lead titanate films from tetraethyl lead and titanium tetraisopropoxide », https://doi.org/10.1016/S0167-577X(00)00159-2
Tetraethyl lead (Pb(C2H5)4), in combination with Ti(OiPr)4 as Ti source and NO2 as oxidant, was applied as Pb precursor for the preparation of PbTiO3 thin films on (0001)
sapphire by photo-MOCVD method. The substrate temperature, the flow rate of NO2 and photo-irradiation were the key factors influencing the structural nature and growth rate of PbTiO3 layers. When the photo-irradiation was present, perovskite PbTiO3 films were
prepared at substrate temperatures >530°C. From the influence of the irradiated light wavelength on the films structure it was concluded that the photochemical processes including NO2 photolysis affected the low temperature growth of PbTiO3. Higher
growth rates were obtained for the photo-deposited layers, as compared to the films obtained using the thermal MOCVD method (at substrate temperatures < 550°C). The electrical properties of the obtained PbTiO3 films were measured. [[i]]
[i] T. Katayama, M. Fujimoto, M. Shimizu, T. Shiosaki, J. Cryst. Growth, 1991, Vol. 115, Iss. 1–4, p.289-293, « Photo-MOCVD of PbTiO3 thin films », https://doi.org/10.1016/0022-0248(91)90755-T
Tetraethyllead Pb(C2H5)4 combined with Zr(DPM)4 and Ti(OiPr)4 was applied for the preparation of strongly [001] oriented lead zirconate-titanate
[Pb(ZrxTi1−x)O3] thin films having tetragonal structure (0<x<0.52) on (100)Pt/(100) MgO substrates by MOCVD. Dense and noncolumnar growth with good surface morphology was demonstrated by SEM. The relative dielectric constant of the layers was 200–350 at RT, and was less dependent on composition x than for PZT bulk ceramics. Typical D‐E hysteresis loops occurring with PZT bulk ceramics were observed, with remnant
polarization 30–40 μC/cm2, and coercive field decreasing from 65 to 40 kV/cm with increase in Zr content. The pyroelectric
coefficients without poling treatment were ~3×10−8 C/cm2 K, almost same as poled PZT bulk ceramics. [[i]]
[i] Y. Sakashita, T. Ono, H. Segawa, K. Tominaga, M. Okada, J. Appl. Phys, (1991), 69, 8352, « Preparation and electrical properties of MOCVD‐deposited PZT thin films »,
https://doi.org/10.1063/1.347397 , http://aip.scitation.org/doi/abs/10.1063/1.347397