4. Conclusion
In summary, optical and structural properties of ultrathin Al2O3/ZnO
nanolaminates, deposited on Si and glass substrates have been studied.
Regression analysis of ellipsometric spectra have shown that absorption
peak decrease and blue shifted with the decrease of bilayers thickness in
the stack. Such behavior reveals the formation of disordered structures
with decreasing the thickness of bilayers. The degree of disordering has
been analyzed by calculating the Urbach tail at the band edge of the absorption
spectra of Al2O3/ZnO nanolaminates. Analysis of Urbach energy
have shown that structures with bilayer spacing of 2 nm and 1.2 nm
have noticeably higher Urbach energy values that structures with
thicker bilayers thicknesses. The evaluated Urbach energy value of
about 0.5 eV and higher for ZnO layer was the indication of the amorphous
structure formation. These results were supported by XRD data,
no peaks pointing to crystalline structure have been found, as well
as the absence of PL exitonic peaks, which was detected in thicker
samples [10,11]. Finally, we conclude that on the basis of the analysis
of optical methods such as transmission, spectroscopic ellipsometry
and photoluminescence of ultrathin nanolaminates, the amorphous
nature of them has been proven. These results allowing not only to
determine optical properties of Al2O3/ZnO nanolaminates but also
to get some inside on the formation processes of ultrathin films of
Al2O3/ZnO.