Signature of strong localization and crossover conduction processes in doped ZnO thin films: synergetic effect of doping fraction and dense electronic excitations

Ga_xZn_1-xO thin films with varying Ga fraction within the solubility limit were irradiated with high-energy heavy ions to induce electronic excitations. The films show good transmittance in the visible region and a reduction of about 20% in transmittance was observed for irradiated films at higher ion fluences. The Urbach energy was estimated and showed an augmenting response upon increase in doping fraction and ion irradiation, this divulges an enhancement of localized states in the bandgap or disorder in the films. The evolution of such localized states plays a vital role in charge transport and thus the temperature dependent electrical conductivity of irradiated thin films was studied to elucidate the dominant conduction mechanisms. The detailed analysis unfolds that in the high-temperature regime (180 K <T< 300 K), the charge conduction was dominated by thermally activated band conduction followed by the nearest neighbor hopping (NNH) mechanism. Whereas in the lower temperature regime (25 K <T< 170 K), the conduction mechanism was governed by Mott-VRH (variable range hopping) followed by Efros-Shklovskii (ES)-VRH. A sudden and steep rise in resistivity below 30 K was observed for GZO films with higher doping fraction at higher ion fluence and proclaims the presence of strong localization of carriers.