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

TL;DR

This study investigates how doping and electric field effects influence electron transport in nanocrystalline ZnO films, revealing a complex interplay between increased charge carriers and scattering that affects conductivity.

Contribution

It provides a detailed analysis of doping-induced charge carrier changes and their impact on electron mobility and grain boundary barriers in sol-gel processed ZnO:Al.

Findings

Charge carrier density increases with doping level.

Electron mobility decreases as doping increases.

Maximum conductivity occurs at 0.8 at% doping.

Abstract

Charge transport properties of thin films of sol--gel processed undoped and Al-doped zinc oxide nanoparticles with variable doping level between 0.8 at% and 10 at% were investigated. The X-ray diffraction studies revealed a decrease of the average crystallite sizes in highly doped samples. We provide estimates of the conductivity and the resulting charge carrier densities with respect to the doping level. The increase of charge carrier density due to extrinsic doping were compared to the accumulation of charge carriers in field effect transistor structures. This allowed to assess the scattering effects due to extrinsic doping on the electron mobility. The latter decreases from 4.6*10^-3 cm^2/Vs to 4.5*10^-4 cm^2/Vs with increasing doping density. In contrast, the accumulation leads to an increasing mobility up to 1.5*10^-2 cm^2/Vs. The potential barrier heights related to grain boundaries between the crystallites were derived from temperature dependent mobility measurements. The extrinsic doping initially leads to a grain boundary barrier height lowering, followed by an increase due to doping-induced structural defects. We conclude that the conductivity of sol--gel processed nanocrystalline ZnO:Al is governed by an interplay of the enhanced charge carrier density and the doping-induced charge carrier scattering effects, achieving a maximum at 0.8 at% in our case.