Optical, electrical and acetone sensing properties of a 3D mesh of Ge quantum wires and nanopores in Al2O3 matrix doped with Nb and Ta

Three-dimensional meshes of semiconductor quantum wires (QWs) and nanopores (NPs) show exciting properties and are very suitable for various applications such as sensors, catalysts, photovoltaic cells etc. Here we investigate the structural, optical, electrical, and acetone-sensing properties of a thin film containing a 3D mesh of Ge QWs and NPs in an amorphous alumina matrix doped with Nb and Ta. The materials based on Ge QWs are produced by self-assembled growth using magnetron sputtering thin film deposition. The materials based on NPs are produced by specific thermal annealing of the Ge QW- based materials. The annealing causes the evaporation of Ge from the alumina matrix, leaving a 3D mesh of nanopores with the same geometrical properties as had the Ge QWs present before the annealing. We compare the properties of these two types of materials and explore the effects of Nb- and Ta-doping. We show that doping drastically changes the materials' optical, electrical, and photocurrent generation properties based on Ge QWs. Their conductivity significantly increases with doping. However, doping diminishes the photo-generation properties because it causes a substantial increase in the dark current. The materials based on NPs have entirely different optical properties, with a very low extinction coefficient for an extensive range of energies and a significantly lower refraction index. The conductivity of the annealed materials is several orders of magnitude smaller than in the Ge-based materials. However, the NPs-based materials, i.e. the Nb- and Ta–doped nanoporous alumina, show a high sensitivity of resistance to exposure to acetone vapour. Especially the Nb-doped film shows an excellent predisposition for acetone sensing. It exhibits hydrophilic properties, and its resistance drops firmly immediately after exposure to acetone vapour. Therefore, it can be potentially used as active material in gas sensors.