Optimization of Ge substrates for ZnO deposition and their application for CO2 detection

The present work focuses on the deposition of ZnO thin films on crystalline and porous Germanium substrates for hazardous gas detection. The porous Ge substrates were obtained by means of surface exfoliation by using H+ implantation and subsequent annealing. Morphological analysis revealed the formation of pores in the range between 1 and 5 mu m. The deposition of ZnO thin films has been made on crystalline and porous Ge substrates utilizing the dip coating method. Structural and morphological characterizations confirmed the formation of highly porous polycrystalline ZnO film exclusively on porous Ge substrates. All the obtained samples were tested for their sensitivities at 0.1, 0.5, 1 and 2% of CO2 concentrations. The ZnO films deposited on porous Ge exhibited an increment of sensing response by a factor of 2, which is correlated to the increment of the superficial area induced by the substrate. Maximum sensing response similar to 50% was achieved for 2% of CO2 when measured at 300 degrees C. ZnO over porous Ge films showed complete resistance recovery, whereas ZnO over crystalline Ge showed a drift in the recovery resistance. ZnO deposited porous Ge showed the fastest response and recovery time similar to 25 and 10s, respectively.