Influence of oxygen vacancies on surface charge potential and transportation properties of Al-doped ZnO nanostructures produced via atomic layer deposition

Abstract

High quality Al-doped ZnO (AZO) films are advantageous for many applications. Their properties can be tuned by controlling the doping concentration and intrinsic defect density. In this work, high-quality AZO films have been synthesized using the atomic layer deposition (ALD) technique. Throughout the doping cycle and post-annealing treatments under various atmospheres, the oxygen-related vacancies were controlled. The effect of oxygen vacancies on the charge transportation and surface potential were studied. The O 1s X-ray photoelectron spectrometry (XPS) spectra of the AZO film were deconvoluted into three components related to the O2− species, corresponding to the oxygen in the ZnO lattice (OL); oxygen vacancies or defects (OV); and chemisorbed or dissociated (OC) oxygen species. In the case of the as-deposited films and films annealed under various atmospheres, a co-relationship between the OV and mobility (μ) can be determined. In the case of the N2-annealed film, the OV fraction is at its maximum value, while the other components are at their minimum values. Among the as-deposited films, the maximum OV fraction is obtained when a Al:Zn ratio (RAl/Zn) of 7% is used. When RAl/Zn=7%, the μ value of the as-deposited AZO film is enhanced from 12.1cm2V−1s−1 (RAl/Zn=3%), to 18.5cm2V−1s−1. It shows its potential application as photoanode. The Hall Effect and the XPS analysis of the film reviled a co-relationship between OV and μ with the Al-doping concentration or post annealing atmosphere. Kelvin probe atomic force microscopy (KPFM) was used to evaluate the surface charge potentials of the films. The N2-annealed AZO film with RAl/Zn=3% (AZO-3-N2) exhibited the maximum negative potential (−115.79mV); however, the film with RAl/Zn=7% (AZO-7-N2) exhibited the maximum positive potential (797.23mV). Hence, the properties of these films may directly pertain to the bit readout signal and reliability of charge storage and memory applications. © 2017 Elsevier B.V.