A novel apparatus for inorganic thin film deposition by rapid expansion from supercritical solution (RESS) is described. Metal oxide thin films were deposited from supercritical water solutions via ...free jet expansion from a laser-drilled orifice in vacuum. Adherent, highly reflective SiO$\sb2$, GeO$\sb2$, and $\rm\lbrack Si\sb{x}Ge\sb{1-x}O\sb2$) films were deposited by direct impingement of the RESS free jet onto a heated Si(100) substrate. Supercritical water solutions were prepared by hydrothermal dissolution of silica gel and by dissolution of hex-GeO$\sb2$ in water at 25$\sp\circ$C. Also, tetraethylorthosilicate (TEOS) in an ethanol/water solution was used as a precursor for deposition of SiO$\sb2$ and $\rm\lbrack Si\sb{x}Ge\sb{1-x},O\sb2$) films, thereby allowing the monosilicic acid concentration in the pre-expansion fluid to be controlled precisely. Mixed oxide films derived from solutions of dissolved GeO$\sb2$ and TEOS reflect the stoichiometry of the parent solutions, as evidenced by AES. FTIR spectra of the $\rm\lbrack Si\sb{x}Ge\sb{1-x}O\sb2$) films contain new metal-oxygen stretching bands that are assigned to vibrational model of intertetrahedral Si-O-Ge linkages. A strong Si-O-Ge antisymmetric stretching (v$\sb{\rm as}$) band near 1000 cm$\sp{-1}$ evidences a high degree of atomic-scale mixing of the component oxides. Film morphologies are dependent on the pre-expansion solute concentration and the solution flow rate. Films comprised of uniform spheroidal particles 50-200 nm in size resulted from low solution flow rates and sufficiently high solute concentrations. Low solute concentrations resulted in smooth thin films without distinct particles, and high solution flow rates resulted in porous films. The free jet expansion of dilute silica-supercritical water solutions was modeled as an isentropic expansion of real-fluid water. Homogeneous nucleation rates of SiO$\sb2$ from supercritical water were calculated and were found to be significant even for very low supersaturations. High silica concentrations result in solute nucleation in the single phase (supercritical) region of the expansion, and the films produced are comprised of spheroidal particles $\ge$50 nm in diameter. Furthermore, in this regime the particle size is inversely related to the initial solute concentration. Low solute concentrations result in nucleation after the water binodal is crossed, and complex film morphologies result due to complications arising from a two-phase solvent expansion.
