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4 October 2005 Influence of the amorphous character of Al2O3 layers on their use in the deep UV spectral range
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Amorphous layers of metal oxides deposited by Plasma Ion Assisted Deposition (PIAD) are widely used in the field of optical coatings due to their salient properties which enable the deposition of complex multilayer stacks. However, their use in the Deep UV spectral range is restricted as the range of transparency is limited by the absorption due to the first electronic band transition. The only oxide suitable for applications at 193 nm seems to be Al2O3 for which a band gap energy of 8.7 eV (143 nm) is reported for the crystalline state. Yet for thin layers of Al2O3 no work reports the making of absorption free layers at 193 nm. In this study we investigate how the amorphous structure of PIAD-deposited Al2O3 thin films influences the electronic structure and as a consequence of that the absorption behaviour for wavelengths close to the absorption edge. The electronic structure is worked out by a theoretical approach where in a first step the geometric structure is simulated using a Monte Carlo approach. Using this geometric structure the electronic structure is calculated by the tight-binding method in a second step. With these data absorption spectra are calculated and compared to measurements on PIAD Al2O3 layers. The experimental data for the start of the absorption lie on the longer wavelength side of the limit set by the amorphous structure - a fact, that encourages further work on the optimization of the deposition parameters.
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Bernhard von Blanckenhagen, Diana Tonova, and Thorsten Koslowski "Influence of the amorphous character of Al2O3 layers on their use in the deep UV spectral range", Proc. SPIE 5963, Advances in Optical Thin Films II, 59630K (4 October 2005);

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