Crystal Formation and Electric Transportation of (Co,Al) Co-doped ZnO with Various Defect Concentrations*

Electric transportation of Al- and Co-doped ZnO, Co_0.05Al_0.02Zn_0.93O (CAZO) film as a function of the hydrogen content H₂% in the sputtered gas during growth process is studied. As H₂% is increased, the concentration of defect for hydrogen to take out part of oxygen content in the sputtering process is also increased. Therefore, the optical transmittance decreases with increasing H₂%. Using TEM investigation, we discovered that the films are in nano-rod, columnar, and polycrystalline formats for H₂%=0, 1% and 50% respectively, indicating that the generated defects lower the crystal ordering due to possible huge defect strain. XRD measurements show perfect ZnO structure for the H₂%=0 sample, while manifesting mixing phase with additional CoO nano-crystals for H₂%=50%. The resistance of H₂%=0 sample is infinite. The H₂%=1% sample exhibits the lowest resistance and the other H₂% > 1% samples show higher resistances. All resistance to temperature curves can be fitted by VRH model, indicating that defect concentration is the key factor to introduce defect states for hopping conduction. However, when the defect concentration is high, excess scattering increases the resistance.