Organic-inorganic hybrids have recently attracted substantial attention due to the potential of combining distinct properties of organic and inorganic components within a single molecular composite. The natural quantum well structures formed by the organic-inorganic perovskites, for example, exhibit a number of interesting and potentially useful properties. Several members of this family exhibit a semiconductor-metal transition as a function of increasing perovskite sheet thickness (or well width) (see D. B. Mitzi, C. A. Feild, W. T. A. Harrison, and A. M. Guloy, Nature 369, 467 (1994); and D. B. Mitzi, S. Wang, C. A. Feild, C. A. Chess, and A. M. Guloy, Science 267, 1473 (1995)). Strong, wavelength tunable photoluminescence (see T. Ishihara, J. Takahashi, and T. Goto, Solid State Commun. 69, 933 (1989), and G. C. Papavassiliou and I. B. Koutselas, Synthetic Metals 71, 1713 (1995)) and optical third harmonic generation (see J. Calabrese, N. L. Jones, R. L. Harlow, N. Herron, D. L. Thorn, and Y. Wang, J. Am. Chem. Soc. 113, 2328 (1991)) have also been noted at room temperature, as a result of the radiative decay of excitons (with exceptionally large binding energies and oscillator strength) in the inorganic sheets. Recently, a heterostructure electroluminescent (EL) device has been demonstrated (see M. Era, S. Morimoto, T. Tsutsui, and S. Saito, Appl. Phys. Lett. 65, 676 (1994)) which employs (C.sub.6 H.sub.5 C.sub.2 H.sub.4 NH.sub.3).sub.2 PbI.sub.4 as the active light-emitting component, with intense electroluminescence of more than 10,000 cd/m.sup.2 observed at liquid nitrogen temperature. Besides the perovskite family, other organic-inorganic hybrids exhibit a range of interesting properties including superconductivity, non-linear optical and catalytic properties.
Of particular importance for many experimental studies and potential applications is the need for simple and inexpensive thin film deposition techniques. One of the key difficulties in depositing thin films of organic-inorganic hybrids is the distinctly different character of the organic and inorganic components with regard to potential film forming processes. Organic materials tend to be soluble in solvents which are not, in general, the same as those appropriate for the inorganic component, making it often impractical to find a suitable solvent to enable solution deposition techniques (e.g. spin-coating). Additionally, organic compounds tend to decompose at relatively low temperature, whereas inorganic materials often do not effectively evaporate until much higher temperatures.
Recently, a new dip-processing technique has been demonstrated (see K. Liang, D. B. Mitzi, and M. T. Prikas, Chem. Mater. 10, 403 (1998)) for the deposition of organic-inorganic perovskites, in which a metal halide film (deposited by thermal evaporation) reacts with organic cations contained in a dipping solution. While this technique is potentially useful for certain compounds and applications, the resulting film surfaces are often locally rough and a suitable solvent is still required for the dipping process. An entirely vacuum compatible two-source thermal evaporation technique has also been demonstrated (see Era M., T. Hattori, T. Taira, and T. Tsutsui, Chem. Mater. 9, 8 (1997)) in which the organic and metal halide salts are individually thermally evaporated. However, control of the organic salt evaporation rate, and therefore the balancing of the two deposition rates, is difficult since the organic ammonium halide salt generally does not evaporate as a molecular species, but rather dissociates into the free organic amine and hydrogen halide gas.
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