Under‐stoichiometric thin silica layers SiOx with different stoichiometric degree 1 ≤ x ≤ 2, were prepared by thermal evaporation of silicon monoxide in vacuum and in ambient oxygen atmosphere of various pressure onto crystalline silicon substrates. The chemical composition has been determined by Fourier transform infrared spectroscopy (FTIR). A special formula is derived to correlate the stoichiometric degree x with the wavenumber of the main TO stretching mode (Si–O–Si) in silica, finally to determine the actual composition values x of the layers. Cathodoluminescence (CL) of these layers shows the development of typical amorphous SiO2 luminescence bands at the composition threshold x > 1.5 and then onwards to x = 2. These luminescence bands were observed at 4.3, 2.7, 2.15, and 1.9 eV. The green–yellow luminescence (2.15 eV) is strongly increasing with the annealing temperature up to 1300 °C and is assigned to phase separation of SiOx into Si and SiO2 and formation of hexamer silicon rings in the understoichiometric silica network. Finally we observe Si nanoclusters by means of transmission elec‐ tron microscopy (TEM) micrographs. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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High resolution transmission electron microscopy, scanning transmission electron microscopy and cathodoluminescence have been used to investigate Si and Ge cluster formation in amorphous silicon dioxide layers. Commonly, cathodoluminescence emission spectra of pure SiO 2 are identified with particular defect centers within the atomic network of silica including the nonbridging oxygen hole center associated with the red luminescence at 650 nm (1.9 eV) and the oxygen deficient centers with the blue (460 nm; 2.7 eV) and ultraviolet band (295 nm; 4.2 eV). In Ge + ion implanted SiO 2 an additional violet emission band appears at 410 nm (3.1 eV). The strong increase of this violet luminescence after thermal annealing is associated with formation of low-dimension Ge aggregates like dimers, trimers and higher formations, further growing to Ge nanoclusters. On the other hand, pure silica layers were modified by heavy electron beam irradiation (5 keV; 2.7 A/cm 2 ) leading to electronic as well as thermal dissociation of oxygen and appearance of under-stoichiometric SiOx . This SiOx will undergo a phase separation and we observe Si cluster formation with a most probable cluster diameter of 4 nm. Such largely extended Si clusters will diminish the SiO 2 related luminescence and Si crystal related luminescence in the near IR appears.