Anomalous amorphous SiO2 films

Anomalous SiO₂ films have been prepared by sputtering Si in a mixture of Ar-10% O₂ at 77 K. The same sputtering conditions at room temperature yield normal SiO₂ which means that the anomaly is produced by the low temperature deposition. The anomaly reveals itself in several physical properties. The density of the anomalous SiO₂ is 1.72 as compared with 2.20 for bulk and the dielectric constant is about 50% larger than bulk and with a much stronger temperature dependence. The infrared (ir) spectrum of the anomalous SiO₂ is only slightly different from bulk SiO₂ but esr experiments reveal about 3 × 10¹⁸ spins cm which do not exist in bulk SiO₂. These anomalous films are extremely stable: upon heating only a small amount of oxygen (1 part in 10⁵) evolves at 440°C but the density and IR spectrum remain unchanged up to 1300°C. Annealing at 1500°C completely removes the ESR signal and returns the ir spectrum and the density to that of cristobalite. An electron diffraction and transmission electron microscopy study reveals that the anomalous SiO₂ films consist of essentially bulk like SiO₂ clusters about 250 Å in diameter separated by a low density network. The low density network undoubtedly contains unbound O atoms and the SiSi bonds which give rise to the esr signal. The structural model can account for all the anomalous properties.