NOVELTY - In the production of layers of rare earth metal oxide (I) on silicon-containing surfaces (A) by application of a precursor compound (II) for (I) to (A) followed by thermal decomposition of (II), the precursor (II) consists of (a) a metal nitrate or (b) a complex of a metal nitrate with an organic ligand or water. (N.B. Although claim 1 relates to rare earth metal oxides, other claims and the disclosure suggest that the process is applicable to any metal oxide).    USE - The silicon-containing surface (A) to be coated is specifically of silicon dioxide, hydrogen-terminated silicon and/or hydroxyl-terminated silicon or of silicon (all claimed); the silicon dioxide-based substrates include glasses. The coating layers of rare earth metal oxide (I) are useful as protective layers for (A) (due to their mechanical properties) and as dielectric intermediate layers (having high dielectric constants) in electrical semiconductor elements, especially in a metal oxide semiconductor field effect transistor (MOSFET's) or dynamic random access meory (DRAM). The process specifically includes converting the obtained metal oxide layer into a field effect transistor (claimed). The coatings of (I) may also be useful in optical elements, such as light-emitting diodes (LED's) and solar cells.    ADVANTAGE - Rare earth metal oxide layers are obtained by a simple procedure, which avoids the disadvantages of conventional chemical vapor deposition (CVD) procedures (i.e. low volatility of organometallic precursors and adverse effects on electrical properties and stability due to introduction of carbon atoms). The nitrate precursors (II) are chemically stable (i.e. not subject to decomposition at normal storage temperatures) and readily available.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for:    (1) a silicon-containing surface (A) with a metal oxide layer formed by the process;    (2) apparatus for producing metal oxide (not restricted to rare earth metal oxide) layers by the process, comprising a device (specifically an immersion bath or pulsed spray device) for applying a solution of precursor to be thermally decomposed to form the metal oxide layer, where the solution to be applied contains a metal nitrate (specifically a rare earth or transition metal nitrate); and    (3) the use of a metal nitrate compound of formula Mn(NO3)n.xL (II') in a process as above.    L = water or organic ligand, specifically a 1-6C alkyl compound, 1-6C alcohol, tetrahydrofuran, acetonitrile or dimethoxyethane;    M = one or more of La, Sc, Y, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Tm, Yb, Ln, Hf, Lu, Ti, Zr, V, Nb, Ta, Cr, Mo, W, Mn, Re, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, Hg, Ga, In, Tl, Ge, Sn and Pb;    x and n are not defined.