Tuning field-emission characteristics of ZnO nanorods through defect engineering via O+ ion irradiation

Vertically aligned ZnO nanorods grown by a wet chemical method were implanted with O + ions with three different ion fluences: ( Φ ) = 5 × 10 14, 5 × 10 15, and 5 × 10 16 ions/cm 2. It is observed that the concentration of Oxygen vacancies (O V) introduced by implantation first increases from 25.94 % to 54.76 % with increasing Φ and decreases beyond Φ = 5 × 10 15 ions/cm 2. We attribute this to the knocking out of oxygen atoms from the host matrix, which gets saturated due to the presence of an ample amount of O + ions inside the host matrix beyond Φ = 5 × 10 15 ions/cm 2 and further confirmed by extended x-ray absorption fine structure measurements. Therefore, the abundant O V becomes more delocalized followed by overlapping with the maxima of valence bands resulting in the narrowing of the bandgap of ∼ 0.4 eV. The appearance of an additional Raman peak at ∼ 575 cm − 1 in Raman spectra further confirmed the presence of impurity states. It is evident that at a fixed J = 100  μA/cm 2, the turn-on field increases from 3.61 V/ μm to 6.61 V/ μm with increasing Φ, and as a result of this, the field-enhancement factor ( β) decreases. We attribute this increase in turn-on field as a consequence of charge trapping in deep-level states created by O V.