Al-doped ZnO powdered materials: Al solubility limit and IR absorption properties
Graphical abstract
Comparison of optical properties of pure ZnO and AZO for different temperatures and atmospheres.
Introduction
Al-doped ZnO compounds are the most conventional transparent conductive oxides (TCOs), which are useful as transparent electrodes (opto-electronic devices) [1], [2], [3], [4] or as thermal insulator films in smart windows (low emissive windows) [4]. Substitution of aluminium for zinc remains nevertheless quite difficult because of the difference in oxidation state, ionic radius and coordination preference. In oxides, Al3+ and Zn2+ are mostly found in octahedral and tetrahedral sites respectively. Actually, the solubility limit of aluminium in zincite matrix is so low that substitution is still questionable. In recent years, many researchers have attempted to provide a more accurate estimation of the Al3+ solubility limit in ZnO [5], [6], [7], [8], [9]. In literature, this solubility limit has been studied on powder as well as on thin films. On the one hand, for powder, the Al3+ solubility limit has been already characterized by various techniques such as X-ray diffraction [5], 27Al NMR [6], electron diffraction [7] or secondary ion mass spectrometry [8]. Whatever the method used, the thermodynamical solubility limit varies from 0.3 to 0.5 mol.% according to authors. However, this value can be thought either over- or sub-estimated because of quantification limits of the different techniques. On the other hand, Al-doped ZnO compounds are often characterized in their final shape that is TCO thin films, elaborated from magnetron sputtering or other physical vapour deposition processes or more scarcely by dip/spin coating… Investigation of the composition of these thin films using X-ray diffraction remains difficult because of various artefacts, which hinder the observation of possible other minor phases: (i) small amount of matter due to film thickness, (ii) strong intensity of diffraction peaks caused by epitaxial film growth, (iii) presence of the substrate peaks when the latter is a crystallized material, etc. XRD studies with small incidence angle and with long acquisition time are indeed needed. Nevertheless, Al3+ solubility limit on thin films is better confirmed, in an indirect way, from electrical measurements (film conductivity). It can be reminded that for TCOs, conductivity is directly linked to carrier concentration i.e. here, aluminium doping rate since a supplementary free electron is created upon the non-aliovalent substitution of Al3+ for Zn2+ [9]. From conductivity measurements, several reports [9], [10] seem to show that it is possible to dissolve up to 1 mol.% of Al in zinc oxide. The proposed values are higher than for powder, what can be linked to wrong estimations or, on the contrary, to experimental conditions used in the deposition process (high energy, plasma, low partial pressure of oxygen…), which enable a better homogenisation of elements and also the introduction of much aluminium in ZnO.
In the first part of this work, the Al solubility limit has been investigated on powder synthesized via a soft chemical route and post-annealed at various temperatures under air. An assessment of Al solubility limit, in these conditions, could be proposed from X-ray diffraction and 27Al NMR analyses.
In a second step, the characteristics of TCOs consisting of visible transmission/infrared absorption [11], [12], [13], [14] are linked to one characteristic parameter: the plasma frequency ωp, comparable to a “cut-off frequency” with regard to the electronic transport properties and therefore refractive index of the material [14]. In the conduction band, free electrons could be compared to a charges gas, which can react in two different ways with the electromagnetic wave: (i) if the incident wave is very energetic, i.e. with a frequency higher than ωp, electrons “do not have enough time to respond to the perturbation” and thus the wave goes through the material without any interaction, the material is therefore transparent, (ii) on the contrary, when the incident wave is less energetic i.e. lower than plasma frequency, the radiation is absorbed. The plasma frequency is related to the carrier concentration but also to their mobility: an adequate TCO should be a compromise between carrier concentration and mobility. Both phenomena (transparency or absorption) involve a peculiar variation of optical indexes with plasma frequency, which is characterized by an abrupt decrease of the index n in combination with an abrupt increase of the absorption coefficient k [14]. By using diffuse reflection collected with an integrating sphere for which absorption and diffuse reflection are complementary, their sum being equal to unity, a qualitative assessment of TCOs' properties is possible on powder since over the plasma frequency a decrease of the diffuse reflection upon an increase of the index k should be observed. In the second part of this paper, diffuse reflectance of Al-doped ZnO prepared in various conditions will be discussed in order to illustrate the real impact of inserted Al3+ rate on the material's optical properties.
Section snippets
Experimental part
A polyesterification reaction based on the Pechini process [15] was used for the preparation of Al-doped ZnO powder. This synthesis process is based on the chelation of metal ions by an α-hydroxyl acid such as citric acid in order to form stable homogeneous solutions. When mixed with a polyalcohol such as ethylene glycol, these solutions enable the polyesterification and the formation of a viscous brown resin by heating on a hotplate and removing excess solvent. The synthesis was performed with
Investigation of the Al limit of solubility in ZnO matrix
First, various Al-doped ZnO compositions, thermally treated at 850 °C under air, were analyzed by X-ray diffraction (Cu Kα1 exclusively). The corresponding patterns are reported in Fig. 1. These analyses show the presence of an additional phase from 1 mol.% of aluminium: ZnAl2O4 exhibiting the spinel-type structure (JCPDS no. 05-0669). Further thermal treatments, either at 1200 °C under air or at 850 °C under argon, have been performed in order to assess the impact of temperature and atmosphere on
Conclusion
Introduction of Al3+ dopant inside ZnO lattice is difficult from steric, electronic and geometric, i.e. natural coordination preference, and leads consequently to very low solubility limit (under 0.3 mol.%) according to XRD investigations, whereas NMR analyses leads rather to a limit well below 0.1 mol.%. Finally even if each characterization technique is close to its limits, this study shows the very low Al solubility in the zincite matrix. In spite of this, an indirect method, by optical
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