SEM Investigation of ZnO and CdO–ZnO Layers Grown by Sol-Gel Technology and a Multifractal Analysis of their Surface Depending on Synthesis Conditions

Introduction. Super-thin films of zinc oxide regarded as transparent electrodes can be integrated in effective semiconductor heterostructures for use in modern infrared photo electronics and solar power installations. The most important parameter of zinc oxide thin layers is their surface nanorelief, which can be effectively studied using SEM spectroscopy. SEM images allow for a quantitative description of the surface depending on the synthesis conditions using the method of multifractal analysis. Such an approach reveals quantitative relationships between the fractal parameters of the surface topography of the layers in these systems and the temperature regimes used for their final annealing in conventional sol-gel technology.

Aim. To reveal quantitative relationships between the fractal parameters of the surface topography of layers in the Zn–O & Zn–Cd–O systems and the temperature conditions of their final annealing. The MFA method was used for a quantitative description of the surface state depending on the synthesis conditions.

Materials and methods. Super-thin films in the ZnO and ZnO–CdO systems were synthesized using a modified sol-gel technology. The temperature-concentration ranges of the parameters of the modified technological process, which allows high-quality layers of the material to be reproducibly obtained on a glass substrate, were determined. The surface morphology was investigated by SEM spectroscopy depending on the temperature of the final annealing of the layers. SEM images of the surface served as a basis for multifractal analysis (MFA) of the surface area and volume of nanoforms, which are formed on the surface of the obtained layers thus determining their surface relief.

Results. Renyi’s numbers and the parameters of fractal ordering in MFA were chosen as fractal parameters for describing the nano-geometry of the layer surface. MFA was applied to the description of both the surface areas and volumes of nanoforms. Quantitative correlations between Renyi’s numbers, as well as the parameters of fractal ordering for the areas and volumes of surface nanoforms, and the temperature of the final annealing were found.

Conclusion. The numerical values of Renyi’s numbers for the surface and volume characteristics of the surface of layers were used to assess the effect of the fractality of the surface on the molar surface energy of the film. Consideration of the fractal geometry of nanoforms with their characteristic sizes smaller than 5·103μm shows the possibility of both an increase in the surface energy of the resulting film and its decrease when changing the characteristic sizes of nanoforms. The latter effect is due to the formation of a highly porous surface at the nano level

surface structure, crystal morphology, second electron microscopy, oxides, zinc compounds

1. Yang L., Zhoua J., Larbot A., Persin M. Preparation and characterization of nano-zinc oxide. J. of Materials Processing Technology. 2007, vol. 189, iss. 1-3, pp. 379-383. doi:10.1016/j.jmatprotec.2007.02.007

2. Handbook of Sol-Gel Science and Technology: Processing, Characterization and Applications. Ed. by S. Sakka (Prof. Emeritus of Kyoto University). Kluwer Academic Publishers, Boston, Dordrecht, London, 2005, vol. 1–3.

3. Ponomareva A. A., Moshnikov V. A., Maslova O. A., Yuzyuk Yu. I., Suchaneck G. Effect of thermal annealing on the surface of sol-gel prepared oxide film studied by atomic force microscopy and Raman spectroscopy. Glass Phys. Chem. 2014, vol. 40, iss. 1, pp. 99-105. doi: 10.1134/S1087659614010192

4. Raoufi D. Fractal analyses of ITO thin films: A study based on power spectral density. Physica B. Condensed Matter. 2010, vol. 405, iss. 1, pp. 451–455. doi: 10.1016/j.physb.2009.09.005

5. Moskvin P. P., Skyba G. V., Dobriakov V. L., Kolodii M. A., Rashkovetskyi L. V., Kolomys O. F., Rarata S. V. Sol-gel synthesis, surface morphology and spectral properties of ZnO ultrathin films on a silicon single crystal. Voprosy khimii і khimicheskoi tekhnologii. 2018, vol. 4, pp. 36-42. (In Ukr.) Available at: http://vhht.dp.ua/wp-content/uploads/pdf/2018/4/Moskvin.pdf (accessed 12.11.2020)

6. Moskvin P., Kryzhanivskyу V., Rashkovetskyi L., Lytvyn P., Vuichik M. Multifractal analysis of areas of spatial forms on surface of ZnxCd(1−x)Te–Si (111) heterocompositions. J. Cryst. Growth. 2014, vol. 404, pp. 204-209. doi: 10.1016/j.jcrysgro.2014.07.012

7. Moskvin P., Kryzhanivskyy V., Rashkovetskyi L., Lytvyn P. Multifractals spectrums for volumes of spatial forms on surface of ZnxCd(1–x)Te–Si (111) heterostructures and estimation of the fractal surface energy. J. Cryst. Growth. 2016, vol. 450, pp. 28-33. doi: 10.1016/j.jcrysgro.2016.05.035

8. Moskvin P., Kryzhanivskyy V., Rashkovetskyi L., Morozov A., Lytvyn P. Invariance of multifractal spectrums of spatial forms on the surface of ZnxCd(1-x)Te–Si heterocompositions synthesized by electron beam epitaxy and hot wall epitaxy. J. Cryst. Growth. 2017, vol. 475, pp.144-149. doi: 10.1016/j.jcrysgro.2017.06.010

9. Moskvin P. P., Kryzhanivskyy V. B., Rashkovetskyi L. V., Lytvyn P., Vuichik N. V. Multifractal parameterization of spatial forms on the surface of ZnxCd(1–x)Te–Si heterocompositions& its relationship with the conditions of the synthesis of layers. Russian J. of Physical Chemistry. 2014, vol. 88, iss. 8, pp. 1375–1381. doi: 10.1134/S0036024414080196

10. Moskvin P. P., Kryzhanivskyy V. B., Rashkovetskyi L. V., Vuichik N. V. Multifractal parametrization for the volume of space forms on surfaces of ZnxCd(1–x)Te–Si(111) heterocompositions and estimating the energy of a surface with fractal structure. Russian J. of Physical Chemistry A. 2016, vol. 90, iss. 5, pp. 780-787. doi: 10.1134/S003602441605023X

11. Moskvin P., Balytska N., Melnychuk P., Rudnitskyi V., Kyrylovich V. Special features in the application of fractal analysis for examining the surface microrelief formed at face milling. Eastern-European J. of Enterprise Technologies. 2017, vol. 86, iss. 2/1, pp. 9-15. doi: 10.15587/1729-4061.2017.96403

12. Feder J. Fractals, Physics of Solids and Liquids, Plenum Press. New York, 1988, 183 p.

13. Vstovsky G. V. Transform information: a symmetry breaking measure. Found. Phys. 1997, vol. 27, iss. 10, pp. 1413-1444. doi: 10.1007/BF02551520

14. Kolmakov А. G., Vstovsky G. V., Bunin I. G. Introduction to multifractal parameterization of material structures. M., Research Center "Regular and Chaotic Dynamics", 2001, 116 p. (In Russ.)

15. Vstovsky G. V. Elements of information physics. M., MGIU, 2002, 260 p.