Detailed microstructure analysis of as-deposited and etched porous ZnO films

Abstract

ZnO nanostructured materials in thin film forms are of particular interest for photovoltaic or photocatalysis processes but they suffer from a lack of simple methods for optimizing their microstructure. We have demonstrated that microporous ZnO thin films with optimized inter grain accessibility can be produce by radio frequency magnetron sputtering process and chemical etching with 2.75 mM HCl solution for different duration. The as-deposited ZnO thin films were first characterized in terms of structure, grain size, inter grain space, open cavity depth and total thickness of the film by XRD, AFM, SEM, profilometry and optical measurements. A specific attention was dedicated to the determination of the surface enhancement factor (SEF) by using basic geometrical considerations and images treatments. In addition, the porous fraction and its distribution in the thickness have been estimated thanks to the optical simulation of the experimental UV-Visible-IR spectrums using the Bruggeman dielectric model and cross section SEM images analysis respectively. This study showed that the microstructure of the as-deposited films consists of a dense layer covered by a porous upper layer developing a SEF of 12-13 m 2 m −2. This two layers architecture is not modified by the etching process. The etching process only affects the upper porous layer in which the overall porosity and the inter-grain space increase with the etching duration. Column diameter and total film thickness decrease at the same time when the films are soaked in the HCl bath. The microporous structure obtained after the etching process could generate a great interest for the interfaces electronic exchanges for solar cells, photocatalysis and gas sensors applications.

Integration of P-CuO Thin Sputtered Layers onto Microsensor Platforms for Gas Sensing

“…In the case of copper oxides that can have multiple valences of copper, like in tenorite CuO (Cu II ), paramélaconite Cu 4 O 3 (mixed Cu I /Cu II ) or cuprite Cu 2 O (Cu I ), high deposition pressure could lead to a reduction [ 60 ] of the CuO target and the deposition of a phase with lower valences states. Moreover, as the layer had to be integrated by a wet process, a low deposition pressure of 0.5 Pa was preferred to obtain dense [ 61 ] oxide layer. These deposition conditions are then adequate to avoid the filling of the intergranular porosity of the sensitive layer with dye or any residue obtained during photolithographic process but should lead to layers with not optimized sensitivities.…”

Highly Sensitive Sputtered ZnO:Ga Thin Films Integrated by a Simple Stencil Mask Process on Microsensor Platforms for Sub-ppm Acetaldehyde Detection

“…The thicknesses of deposited thin films have been set to 50 nm on microsensors and 100 nm on glass substrates for structural characterizations. A pressure of 2 Pa was set to promote the intergranular porosity [ 55 ].…”

Oxygen Reduction Investigation on Sputtered ZnO Layers with Nano‐granular Structure

“…200 nm, prepared by thermal oxidation) [25] compared to native films (several nanometers thin) under droplets of zinc sulfate. [30][31][32][33][34][35][36][37][38] To the best of our knowledge, no synthesis of dense nanogranular ZnO layers using "one-step" sputtering (with no annealing post-treatment) has been reported before. In contrast, Thomas et al evidenced a protective effect of ZnO patinas grown under artificial seawater droplets.…”

“…[26] Finally, sputtering was chosen for its ability to producing thin oxide films as "model systems" in academic research with a large variety of nanostructures depending on the selected deposition parameters, such as target composition, nature of the reactive gases and power (among others). [30][31][32][33][34][35][36][37][38] To the best of our knowledge, no synthesis of dense nanogranular ZnO layers using "one-step" sputtering (with no annealing post-treatment) has been reported before. To obtain sputtered ZnO films with no preferred orientation, researchers modified the pressure of oxygen on the deposition procedure.…”