Selective hydrogen sensing by cobalt doped ZnO thin films: A study on carrier reversal conductivity

The n to p type carrier reversal conductivity is observed in cobalt doped zinc oxide (Co_ZnO) thin films with cobalt content from 0.001 to 0.5 wt% in hydrogen (H2) gas. Maximum response (220.6 %) and p-type conductivity is achieved in H2 gas at lower operating temperature (200oC) using 0.01 wt% Co_ZnO thin film where n-type conductivity is observed at higher operating temperatures which provide selective H2 sensing. The n and p type conductivity are observed at 250oC but the response (%) is less. Below 250oC it shows fully p-type conductivity in presence of H2 gas. By decreasing operating temperature, O2" molecular oxygen ions might be dominant to chemi-adsorb and creates OH" on sensor surface by reacting with H2 gas which increase the sensor resistance and shows p-type conductivity. The effect of interstitial defects and oxygen vacancies are also studied for n to p carrier reversal using photo luminescence spectra of undoped and Co_ZnO thin films.