Optical Bandgap and Electric Transportation of Co- and Al- co-doped ZnO with Low Defect Concentration*

The defects in Co- and Al- co-doped ZnO, Co_0.05Al_0.02Zn_0.93 (CAZO), films were generated during sputtering growth by mixing low percentage of hydrogen, 0.1~1% denoted as CAZO-H2%, along with argon gas at 450^oC. For comparison, an extremely low defect film, except the native oxygen vacancies, was grown under a mixture of oxygen with argon gas, which is denoted as CAZO-O2. The optical band gaps and electric resistivities were measured as a function of the H₂%. We found the CAZO-O2 is insulating while the resistance drops to a single digit (less than 10 Ω) with the addition of a very small amount of H₂, 0.1%, in the growth atmosphere and reaches the minimum for CAZO-1%. Combining previous data with higher defect concentrations, the lowest resistance was observed for the CAZO-1% sample. The optical bandgap, E_op, however, shows the opposite trend that increases smoothly with the defect concentration from E_op=3.4eV for CAZO-O2 sample, similar to the bandgap of pure ZnO, to 3.61eV for CAZO-1.1%. The role of defects on the bandgap and resistance will be presented.