Effect of doping- and field-induced charge carrier density on the electron transport innanocrystalline ZnO

Abstract

The charge transport properties of thin films of sol–gel processed undoped and Al-dopedzinc oxide nanoparticles with variable doping level between 0.8 and 10 at.% wereinvestigated. The x-ray diffraction studies revealed a decrease of the average crystallitesizes in highly doped samples. We provide estimates of the conductivity and the resultingcharge carrier densities with respect to the doping level. The increase of chargecarrier density due to extrinsic doping was compared to the accumulation of chargecarriers in field effect transistor structures. This allowed us to assess the scatteringeffects due to extrinsic doping on the electron mobility. The latter decreases from4.6 × 10−3 to4.5 × 10−4 cm2 V−1 s−1 with increasing doping density. In contrast, the accumulation leads to an increasing mobility up to1.5 × 10−2 cm2 V−1 s−1. The potential barrier heights related to grain boundaries between the crystallites werederived from temperature dependent mobility measurements. The extrinsic doping initiallyleads to a grain boundary barrier height lowering, followed by an increase due todoping-induced structural defects. We conclude that the conductivity of sol–gel processednanocrystalline ZnO:Al is governed by an interplay of the enhanced charge carrier densityand the doping-induced charge carrier scattering effects, achieving a maximum at 0.8 at.%in our case.