A novel explanation for the increased conductivity in annealed Al-doped ZnO: an insight into migration of aluminum and displacement of zinc

Abstract

A combined experimental and first-principles study is performed to study the origin of conductivity in ZnO:Al nanoparticles synthesized under controlled conditions via a reflux route using benzylamine as a solvent. The experimental characterization of the samples by Raman, nuclear magnetic resonance (NMR) and conductivity measurements indicates that upon annealing in nitrogen, the Al atoms at interstitial positions migrate to the substitutional positions, creating at the same time Zn interstitials. We provide evidence for the fact that the formed complex of Al and Zn corresponds to the origin of the Knight shifted peak (KS) we observe in Al NMR. As far as we know, the role of this complex has not been discussed in the literature to date. However, our first-principles calculations show that such a complex is indeed energetically favoured over the isolated Al interstitial positions. In our calculations we also address the charge state of the Al interstitials. Further, Zn interstitials can migrate from Al and possibly also form Zn clusters, leading to the observed increased conductivity.

Co‐doping effects of (Al, Ti, Mg) on the microstructure and electrical behavior of ZnO‐based ceramics

“…Instead, there is a signal at the unusual chemical shift of about 185 ppm. The deviation from the normal range has been attributed to the Knight shift caused by the conduction electrons . Momot et al hypothesize that it stems from Al in Al Zn ‐Zn i complexes where Al substitutes Zn, and Zn migrates to an interstitial position .…”

“…The deviation from the normal range has been attributed to the Knight shift caused by the conduction electrons. 52 Momot et al hypothesize that it stems from Al in Al Zn -Zn i complexes where Al substitutes Zn, and Zn migrates to an interstitial position. 52 The intensity of this signal has been found in our previous study and correlates with the conductivity evolution as a function of the sintering atmosphere.…”

“…52 Momot et al hypothesize that it stems from Al in Al Zn -Zn i complexes where Al substitutes Zn, and Zn migrates to an interstitial position. 52 The intensity of this signal has been found in our previous study and correlates with the conductivity evolution as a function of the sintering atmosphere. 19 When the co-doping elements of Al ions are varied, only a smaller influence on the conductivity was obtained compared to the atmosphere effect, the central transitions of static 67 Zn spectra are shown in Figure 11B.…”

Enhanced Sol–Gel Route to Obtain a Highly Transparent and Conductive Aluminum-Doped Zinc Oxide Thin Film

“…In an inclusive investigation through performing 27 Al NMR spectrometry on AZO powders [93], Damm et al reported that while before the reductive annealing, the relative occupancy of Al 3+ in octahedral positions is more or less equal to the tetrahedral ones, after the reduction, annealing a dominant substitutional tetrahedral occupancy of Al 3+ is observed. It was also confirmed by Momot et al that upon the reductive annealing, a rearrangement of the Al 3+ coordination in the ZnO lattice happens by the migration of the Al 3+ ions at interstitial positions to the substitutional positions [94]. This “dopant activation” contributes to boosting the conductivity up through an increment of the active charge carriers.…”

Possibilities of Increasing the Usability of Sputtered AZO Films as a Transparent Electrode

“…Avandhut et al excluded these defects as a source of doping in ZnO using solid‐state NMR measurements and DFT calculations . It has been reported that Zn interstitial clusters or Zn‐Al complexes can also be responsible for n‐type conductivity in zinc oxide.…”