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Structure and electrical properties of zinc oxide base iron doped ceramics
expand article infoAlexey V. Pashkevich‡, Aleksandr K. Fedotov‡, Yuliya V. Kasyuk‡, Liudmila A. Bliznyuk§, Julia A. Fedotova‡, Nikita A. Basov§, Alexander S. Fedotov‡, Ivan A. Svito‡, Evgheni N. Poddenezhny|
‡ Belarusian State University, Minsk, Belarus
§ National Practical Center of National Academy of Science of for Material Science, Minsk, Belarus
| P.О. Sukhoi Gomel State Technical University, Gomel, Belarus

Corresponding author: Aleksandr K. Fedotov ( fedotov@bsu.by )

© 2018 Alexey V. Pashkevich, Aleksandr K. Fedotov, Yuliya V. Kasyuk, Liudmila A. Bliznyuk, Julia A. Fedotova, Nikita A. Basov, Alexander S. Fedotov, Ivan A. Svito, Evgheni N. Poddenezhny.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation: Pashkevich AV, Fedotov AK, Kasyuk YV, Bliznyuk LA, Fedotova JA, Basov NA, Fedotov AS, Svito IA, Poddenezhny EN (2018) Structure and electrical properties of zinc oxide base iron doped ceramics. Modern Electronic Materials 4(3): 87-95. https://doi.org/10.3897/j.moem.4.3.39540
Open Access

Abstract

The search for new economically advantageous technologies of new zinc oxide based composite ceramic materials and the study of their structure and properties attract special attention today. These ceramics have a number of advantages as compared with materials prepared by more expensive technologies, due to the possibility to fabricate items having different shapes and sizes and particularly to vary their morphology, structure and phase composition. This allows controlling their functional properties by varying the powder particle size in charge, the temperatures, durations and atmospheres of synthesis and heat treatment, and the types of doping impurities in the ceramics. The structure and electrical properties of (FexOy)10(ZnO)90 ceramics (0 ≤ x ≤ 3; 1 ≤ y ≤ 4) synthesized in air using single- and two-stage synthesis methods have been studied. FeO, α-Fe2O3 and Fe3O4 powders or (α-Fe2O3 + FeO) mixture have been used for ZnO doping. X-ray diffraction, gamma-ray resonance spectroscopy and Raman spectroscopy data suggest that at average iron concentrations of 1–3 at.% the ceramic specimens contain at least three phases: the Zn1-δFeδO solid solution with a wurtzite structure, the ZnFe2O4 ferrite phase with a spinel structure and FexOy residual iron oxides which were used as doping impurities. Scanning electron microscopy and energy dispersion X-ray analysis have shown that the wurtzite phase grain size in the ceramic specimens decreases from several decades of microns for single-stage synthesis to submicron sizes for two- stage synthesis. We show that iron addition to ZnO induces a compression of the wurtzite phase crystal lattice, the compression of lattice magnitude being proportional to the oxygen content in the FexOy iron oxide doping agent. The temperature dependences of the electrical resistivity suggest that deep donor centers with an activation energy of about 0.37 eV are formed in the Zn1-δFeδO wurtzite phase. The temperature dependences of the electrical resistivity of electrons for undoped ZnO in the 6–300 K range and for doped (FeO)10(ZnO)90 ceramic synthesized in one stage exhibit a variable activation energy below 50 K which indicates a heavily disordered structure.

Keywords

zinc oxide, ceramics, iron doping, phase structure, electrical properties

1. Introduction

The search for new economically advantageous technologies and the study of the structure and properties of new zinc oxide base composite ceramic materials currently draw special attention [1]. These ceramics have a number of advantages over materials the technologies of which are more expensive, due to the possibility to fabricate items having different shapes and sizes and particularly to vary their morphology, structure and phase composition. This allows controlling their functional properties by varying the powder particle size of the charge, the temperatures, durations and atmospheres of synthesis as well as following heat treatment and the types of doping impurities in the ceramics [2–4].

Zinc oxide can be used as the base material for the fabrication of conventional devices (e.g. variable resistors [2]) or fundamentally new device types. For example, ZnO doping with magnetic impurities allows producing promising materials having novel electronic, optical and magnetic properties [5–12] including magnetic field controlled ones [13, 14].

Zinc oxide base composite ceramics can be used for the fabrication of thermoelectric transducers [15], yet this will require increasing their electrical conductivity and reducing their heat conductivity which can be achieved e.g. by doping.

Doping of ZnO base composite ceramic materials with transition elements is of special research interest [5–12]. The properties of these materials depend not only on the concentration of incorporated impurities but also on whether they are distributed in the matrix lattice (wurtzite) homogeneously or form submicron size magnetic clusters with a specific phase and magnetic structure. Hybrid nanocomposite structures of this type can be used for the fabrication of various devices e.g. adsorption gas sensors [16–20].

There are numerous literary experimental data on the effect of doping with transition or other elements on the structure and properties of single crystals, polycrystalline films, nanostructured powders, nanowires or tetrapods [5–12, 21–23]. However, ZnO base compositions synthesized using ceramic technologies have not yet been studied completely (except ceramics for variable resistors and optical applications [2, 7]). The effect of magnetic impurities (including iron) in ZnO base ceramic compositions and variation of synthesis conditions on the chemical composition, structure and magnetic properties of the forming phases has not yet been studied in the required detail. Furthermore there is insufficient information on the effect of the type of the forming intrinsic defects and defect complexes on the electron transport and other properties of the ceramics. Iron often incorporates in zinc oxide during the growth and is one of the most abundant d-group transition metal impurities in this material [21, 24–26]. Calculations [27] showed that in the perfect ZnO the iron ions having the Fe2+ charge state should give relatively shallow donors. However the presence of random acceptors in real specimens (e.g. nitrogen for ZnO synthesized in air) transforms Fe2+ to Fe3+ leading to the formation of deep donor levels [27].

The aim of this work is to study the effect of synthesis method and type of doping agents (FexOy iron oxides) on the phase composition and electrical properties of ZnO base ceramics.

2. Materials and experimental

The ZnO base specimens were synthesized using the conventional open air powder sintering technique for ceramics [2, 7]. The source components for charge preparation were ultrahigh-purity ZnO, FeO, α-Fe2O3 and Fe3O4 powders and (α-Fe2O3 + FeO) mixtures (Table 1). The weights of the charge components in the studied specimens were calculated using the (FexOy)10(ZnO)90 compound formula in which the weight fraction of the iron oxide powders was 10 wt.%.

Table 1.Download as CSV 

Specimen synthesis methods.

Spec. No. Specimen* Synthesis method Preliminary/Final Sintering
Temperature, °C Time, h
1 ZnO Sintering 1100 2
2 (ZnO)90(FeO)10-1 Single-Stage Synthesis 1200 2
3 (ZnO)90(Fe2O3)10-1
4 (ZnO)90(Fe3O4)10-1
5 (ZnO)90(FeO)10-2 Two-Stage Synthesis 900/1200 2/2
6 (ZnO)90(Fe2O3)10-2
7 (ZnO)90(Fe3O4)10-2
8 (ZnO)90(Fe2O3 + FeO)10-2 Two-Stage Synthesis 4/48

* “1” and “2” in the specimen notations refer to single- and two-stage synthesis, respectively.

The powders were mixed in a stainless steel attritor with hard-alloy balls in the presence of a humidifier (alcohol). The initial oxide particle sizes (as-grinded for 12 h) were 10–50 μm. For charge preparation the source oxides were dried at 500–900 °C approaching constant weight, following which 3 wt.% PVA glue binder was added to the charge. Then the mixture was uniaxially compacted at 200 MPa to 10–18 mm diam. 2–5 mm thickness tablets. For single-stage synthesis the as-compacted tablets were sintered in air at 1200 °C for 2 h. For two-stage synthesis the tablets were preliminarily sintered at 900 °C for 2 h, then ground again to powder, mixed with the binder, compacted and annealed in open air at 1200 °C for 2 h. As can be seen from Table 1 the synthesis method for Ceramic No. 8 differed from the others. After synthesis or annealing the specimens were cooled in the furnace at a 200–300 K/h rate.

The structure and phase composition of the ceramics were studied at room temperature using X-ray diffraction (XRD) on a DRON-3 M automated instrument in CuKα radiation and using scanning electron microscopy (SEM) in secondary electron mode on Oxford Instruments LEO 1455VP and Tescan Vega 3LMU instruments. The 20 scanning step for XRD was max. 0.03, the exposure time being min. 5 sec. The X-ray diffraction patterns were processed using the FullProf software based on the Ritweld profile analysis method [28].

The element concentrations in the composites were measured using selected area electron probe X-ray analyzers for energy dispersive X-ray analysis (EDXA) installed on the scanning electron microscope.

The porosity of the specimens was studied under an Olimpus GX41 microscope (Japan) with the AutoScan 005 software.

Mössbauer spectra were taken in the 20–300 K range on an MS4 Mössbauer spectrometer (SEE Co., USA) for powdered specimens (57Fe isotope, transmission mode, 57Co/Rh source (20 mCi)). The spectra were approximated using the Rancourt method based MOSMOD software [29].

Raman spectra were taken on a Nanofinder High-End LOTIS TII confocal spectrometer (Belarus, Japan). The signal was excited with a solid state laser (532 nm, 20 mW). The laser radiation was focused on the specimen surface with a 50× objective (digital aperture 0.8+). The incident laser radiation power was reduced to 2 mW for the avoidance of heat damage. The backscattered light was dispersed by a 600 mm-1 diffraction grating for achieving a spectral resolution of not worse than 3 cm-1. Spectral calibration was based on gas discharge lamp lines for achieving an accuracy of not worse than 3 cm-1. The signal acquisition time was 30 s. The photodetector was a cooled silicon CCD matrix.

The temperature dependences of the electrical resistivity ρ(T) of the ceramics were measured for rectangular specimens (2–3 mm in width and thickness and 7–10 mm in length) cut out from the synthesized ceramic tablets, with silver current and potential contacts deposited on the specimen surfaces. The ρ(T) curves were measured on a High Field Measurement System instrument (Cryogenic Ltd, London) in the 6–300 K range. The test specimens were mounted in a special probe with a system of thermometers and heaters. The probe with soldered-up specimens was placed in a close-cycle refrigerator cryostat with a superconducting solenoid and connected to the automatic measurement system.

The current passing through the specimen was controlled with a Keithley 6430 Sub-Femtoamp Remote SourceMeter which provided for specimen electrical resistivity measurements in the 100 μOhm – 20 GOhm range with an accuracy of not worse than 0.1%. The specimen temperature was controlled with LakeShore thermal diodes calibrated accurate to a 0.0005 K and having a reproducibility of 0.001 K thus allowing temperature stabilization and measurement with an accuracy of not worse than 0.005 K using a LakeShore 331 measurement controller. The relative error of the electrical resistivity measurements was within 5%, being mainly determined by the specimen dimension measurement error and the sizes of the electric potential contacts.

3. Results and discussion

3.1. Structure and phase composition of composite ceramics

Table 2 shows types and weight fractions of doping oxides and main structural parameters of the test ceramics: porosity, EDXA-determined specimen-average iron concentration and average δ-iron concentration in the solid solution of the main (wurtzite) Zn1-δFeδO phase, average Fe concentration in sample and concentration (delta) in Wurtzite phase and lattice parameters of the ferrite phase and residual doping impurities forming during the synthesis.

Table 2.Download as CSV 

Main structural parameters of the specimens.

No. Specimen Porosity Average Fe concentration in ZnO, at.% Fe(δ) concentration in Zn1-δFeδO, at.% Lattice parameters, nm
Wurtzite phase Ferrite phase
1 ZnO 14.6 0 0 a = 0.32498
c = 0.52020 –
2 (ZnO)90(FeO)10-1 17.0 1.2 0.77 a = 0.32595
c = 0.52073 –
3 (ZnO)90(Fe2O3)10-1 15.5 2.3 0.82 a = 0.32381
c = 0.51822 –
4 (ZnO)90(Fe3O4)10-1 18.7 2.87 0.69 a = 0.32372
c = 0.51805 –
5 (ZnO)90(FeO)10-2 18.3 1.0 0.81 a = 0.32418 0.83892
c = 0.51900
6 (ZnO)90(Fe2O3)10-2 17.1 2.81 0.66 a = 0.32367 0.84074
c = 0.51809
7 (ZnO)90(Fe3O4)10-2 16.5 2.98 0.79 a = 0.32377
c = 0.51835 0.84322
8 (ZnO)90(Fe2O3 + FeO)10-2 17.0 5.04 0.87 a = 0.32346
c = 0.51758 0.84077

Figure 1 shows typical examples of SEM cleavage surface images for single- and two-stage synthesized ceramic specimens. It can be seen from Fig. 1 that the grain size of the single-stage synthesized ceramics ranges from several to decades of microns (Fig. 1 a, c, e, g). After two-stage synthesis the predominant grain size decreased to the submicron range (Fig. 1b, d, f, h) although as can be seen from Fig. 1b, d some large grains persisted after the second synthesis stage. The XRD study showed that the wurtzite structure remained in the Zn1-δFeδO solid solutions regardless of the FexOy doping impurity type. According to the EDXA data presented in Table 2 the iron content in the wurtzite phase solid solution of most ceramics studied was in the 0.66 < δ < 0.81 at.% range. The only exclusion was ceramic Specimen No. 8 synthesized at a higher temperature and during a longer time.

Figure 1.

Typical SEM images of the grain structure at the cleavage surfaces of (FexOy)10(ZnO)90 ceramics synthesized using (a, c, e, g) single-stage synthesis method and (b, d, f, h) two-stage synthesis method with the doping impurities being (a and b) FeO, (c and d) α-Fe2O3, (e and f) Fe3O4 and (g and h) Fe2O3. Inset: Large magnification SEM images of separate specimen areas.

According to the XRD data the (FexOy)10(ZnO)90 doped ceramics contained, along with the Zn1-δFeδO wurtzite structure solid solution lines, several additional peaks marked with arrows in Fig. 2. The lattice parameters of the phases calculated by approximation from the XRD spectra are also shown in Table 2. The observed decrease in the a and c lattice parameters of the wurtzite phase after ZnO doping seems to be caused by partial substitution of Zn ions for iron ones in this phase. It should also be noted that the а/с lattice parameter ratio remains almost the same for all the doping impurities studied, i.e. the cell is not distorted.

Figure 2.

Typical XRD spectra for undoped ZnO and (FexOy)10(ZnO)90 composite ceramics synthesized in two stages using different FexOy doping impurities: (a) ZnO, (b) (ZnO)90(FeO)10-2, (c) (ZnO)90(Fe2O3)10-2 and (d) (ZnO)90(Fe3O4)10-2. Figure 2a shows Miller’s indices for the zinc oxide wurtzite lattice. Arrows in Fig. 2b–d show the additional peaks observed in the spectra corresponding to the ZnFe2O4 ferrite phase or FexOy residual oxides.

Based on X-ray pattern fitting we can attribute the additional peaks (marked with arrows in Fig. 2) to the ZnFe2O4 ferrite phase and residual FexOy iron oxides.

The presence of additional phases in the (FexOy)10(ZnO)90 composite ceramic structure (along with Zn1-δFeδO solid solutions having a wurtzite structure) was also detected by EDXA, Mössbauer and Raman spectroscopies. EDXA showed that Zn1-δFeδO solid solution grains with δ < 1 at.% (dark regions in Fig. 3) coexist with relatively large regions (5–30 μm or even larger) containing 2–3 wt.% iron (bright regions in Fig. 3). According to Table 1, the presence of these iron-rich regions with a ZnFe2O4 ferrite structure and/or FexOy residual oxides increases the average iron content in the (FexOy)10(ZnO)90 ceramic specimens to 1–3 at.%.

Figure 3.

Example of EDXA iron distribution in (ZnO)90(FeO)10-2 composite ceramic (Table 1, Specimen 5).

The Raman spectra of the specimens also suggest the presence of not only the wurtzite phase (Fig. 4) but also additional iron containing phases (Fig. 5). Comparison of the Raman peak positions in Fig. 5 with literary data [23] suggests the presence of two cubic phases one of which is identified as ZnFe2O4 zinc ferrite and the others can be interpreted as residual source FexOy doping impurities.

Figure 4.

Examples of Raman spectra for the wurtzite phase in (a) indoped ZnO and (b) (ZnO)90(FeO)10-2, (c) (ZnO)90(Fe2O3)10-2 and (d) и (ZnO)90(Fe3O4)10-2 doped composite ceramics.

Figure 5.

Examples of Raman spectra for phase inclusions in (a) (ZnO)90(FeO)10-2, (b) (ZnO)90(Fe2O3)10-2 and (c) (ZnO)90(Fe3O4)10-2 composite ceramics.

The Mössbauer data confirm the XRD, EDXA and Raman data indicating the presence of additional phases with high iron content in the experimental ceramic specimens. To analyze the phase transformations occurring during the synthesis of the composite ceramics studied we measured the Mössbauer spectra both for the initial powder mixtures before compacting and for the as-synthesized ceramic specimens with different doping impurities for single- and two-stage synthesis methods (Fig. 6).

Figure 6.

Mössbauer spectra of (a) (ZnO)90(α-Fe2O3)10 and (c) (ZnO)90(Fe3O4)10 initial powders and (b) (ZnO)90(Fe2O3)10-2 and (d) (ZnO)90(Fe3O4)10-2 respective composite ceramics synthesized in two stages.

As can be seen from Fig. 6 a and b the Mössbauer spectra of the source powders mixtures ((a) (ZnO)90(α-Fe2O3)10 and (b) (ZnO)90(Fe3O4)10) contain magnetically split sextets [31]. The (ZnO)90(FeO)10 powder mixture also proved to exhibit a magnetically split sextet. The Mössbauer spectra of all the as-synthesized ceramic specimens changed significantly and mainly contained a non-magnetic doublet (Fig. 6b, d) pertaining to ZnFe2O4 zinc ferrite [30] which was also detected in these ceramics by EDXA (Fig. 3).

3.2. Electrical properties of composite ceramics

Below we present the electrical properties of the (FexOy)10(ZnO)90 ceramics depending on the synthesis method (single- or two stage) and FexOy doping impurity type. The room temperature electrical resistivity ρ300 data for the specimens are summarized in Table 3. Figure 7 shows the temperature dependences of the electrical resistivity ρ(T) for undoped (Curve 1) and doped (Curves 2–8) ceramic specimens.

Table 3.Download as CSV 

Main electrical parameters of the specimens.

No. Specimen Average Fe concentration in (FexOy)10(ZnO)90, at.% Fe(δ) concentration in Zn1-δFeδO, at.% ρ at 300 K, Ohm · m ΔЕσ at 300 K, eV
1 ZnO 0 0 3.67 · 101 0.05–0.07
2 (ZnO)90(FeO)10-1 1.2 0.77 7.51 · 100 0.36
3 (ZnO)90(Fe2O3)10-1 2.3 0.82 1.32 · 100 0.27
4 (ZnO)90(Fe3O4)10-1 2.87 0.69 2.86 · 105 0.34
5 (ZnO)90(FeO)10-2 1.0 0.81 7.89 · 102 0.36
6 (ZnO)90(Fe2O3)10-2 2.81 0.66 3.18 · 103 0.37
7 (ZnO)90(Fe3O4)10-2 2.98 0.79 6.12 · 102 0.37
8 (ZnO)90(FeO + Fe2O3)10-2 5.04 0.87 4.37 · 101 0.24
Figure 7.

Temperature dependences of electrical resistivity for (1) undoped and (2–8) doped (ZnO)90(FexOy)10 ceramic specimens. Curve numbering corresponds to specimen numbering in Table 3.

As can be seen from Fig. 7, zinc oxide doping with iron leads to an increase in the electrical resistivity near room temperature ρ300 for most of the specimens. The only exclusion is the (ZnO)90(FeO)10-1 specimen synthesized in one stage whose electrical resistivity at T > 200 K proved to be lower than that of the undoped ZnO specimens which, as follows from Fig. 7, is most likely due to its heavily inhomogeneous structure.

By and large, as can be seen from Fig. 7 the (FexOy)10(ZnO)90 specimens can be divided in two groups by ρ(T) dependence pattern.

One group (Curves 3–7) have linear (Ln ρ) dependences on inverse temperature (1/T) in Arrhenius coordinates in the 150–300 K range. These specimens include all the two-stage synthesized ceramics and those of the single-stage synthesized ones which were doped with Fe2O3 or Fe3O4 oxides or (FeO + Fe2O3) mixture. The linear sections of the Lg ρ – (1/Т) functions for the specimens have close conductivity activation energies ΔЕσ ≈ 0.37 eV as determined from the linear section slopes of the Ln ρ(1/Т) curves in Fig. 7.

The second group of curves in Fig. 7 (Specimens 1, 2 and 8) refer to undoped zinc oxide ceramics (Specimen 1) and (FeO)10(ZnO)90 specimens synthesized using the single-stage (Specimen 2) and two-stage (Specimen 8) methods which had lower resistivities at T > 150 K as noted above. As can be seen from Fig. 7, Specimens 1, 2 and 8 have cooling-reduced conductivity activation energy which is in the 0.005–0.05 eV range at 6–50 K.

We will now discuss the experimental data on the structure and electrical properties of the ceramics. Table 3 suggests that in the 150–300 K range the temperature dependences of the electrical resistivity of iron doped ceramics 3–8 obey the following law:

(1)

with the conductivity activation energy ΔЕσ ~0.37 eV. This Еσ value is far greater than the literary data, i.e., 0.25 ± 0.02 for polycrystalline ZnO films with 0.2 wt.% iron content, which were attributed [26] to the Fe3+ ionization energy.

Note that multiple researchers reported ionization or conductivity activation energies of ~0.15–0.4 eV for undoped zinc oxide which were attributed to the presence of the impurity band and/or tails of localized states [32] and to intrinsic donor defects [33–35] (e.g. zinc vacancies forming during ceramic sintering in air). On the other hand the experimentally revealed deep level can be associated with the formation of the Fe–O complexes during ceramic synthesis and heat treatment. This should increase the concentration of oxygen vacancies which are formed due to leaving by oxygen of zinc oxide lattice sites towards iron ions. However, taking into account the doping level of the experimental specimens (above 1020 m-3 for the wurtzite phase), the revealed deep donor centers could as well be attributed to the presence of some quantity of double-charged iron ions [36]. The deep donor was reported [36] to originate from the interaction of solute Fe2+ iron ions with the intrinsic defects (e.g. due to the incorporation of these ions into the zinc sublattice) in the wurtzite crystal lattice which however is in conflict with our Mössbauer data.

As noted above the ρ(T) dependences in Arrhenius coordinates for the undoped specimens and a doped one (Fig. 7 Curves 1 and 2) suggest that the activation energy decreases with decreasing temperature. This is commonly attributed either to electron hopping through localized states with variable hopping range (the so-called VHR mode) or to the presence of tails of localized states or large-scale potential relief caused by an inhomogeneous distribution of impurities and defects. The electron hopping conductivity is described by the well-known Mott relationship [32, 37].

(2)

where α = 0.25 for Mott’s mechanism in bulk semiconductors and α = 0.5 for Shklovskii-Efros’ hopping. The ρ01 and T0 parameters in Eq. (2) are the characteristic parameters of the respective VHR hopping conductivity mode.

The relevance of the ρ(T) curves to some variant of Eq. (2) usually becomes clear from their representation in the so-called Mott coordinates Ln ρ – (1/T)α. As can be seen from Fig. 8 b the Mott representation of the ρ(T) curves for temperatures below 15 K causes linearization of the ρ(T) curves over a wider range of low temperatures in accordance with the Shklovskii-Efros law [37]. This indicates possible formation of the Coulomb gap in the vicinity of the Fermi level for VHR conductivity through localized states. A similar behavior was observed for polycrystalline films [39]. It should be noted that the linearization of the ρ(T) curves in Mott’s coordinates was absent for doped Specimen No. 2.

Figure 8.

Temperature dependences of electrical resistivity for (1) undoped and (2) doped inhomogeneous ceramics in Mott’s coordinates for (a) α = 0.25 and (b) α = 0.5 in Eq. (2).

As can be seen from Curve 1 in Fig. 8 a heating of the undoped ceramic specimen to 20 K < T < 40 K initiates a ρ(T) curve transition from the Shklovskii-Efros law to the Mott law with α = 0.25 in Eq. (2). Above 50 K, undoped ZnO and the heavily inhomogeneous doped single-stage synthesized specimens (Fig. 9 a, Curve 2) exhibit a ρ(T) curve transition to dependences of the following type

Figure 9.

Diagram of the density of states for n conductivity doped ZnO as a function of electron energy for high intrinsic defect concentrations forming the impurity band and the tails of localized states near conduction band bottom.

(3)

where the conductivity activation energy ΔЕ ≈ (Ep – EF) is not equal to the impurity ionization energy and is temperature dependent. Here EF is the Fermi level and Ep is the percolation level. These dependences are typically attributed to specimen inhomogeneity [39, 40] and often observed in polycrystalline undoped zinc oxide films [34, 35].

As noted above the ρ(T) curve pattern described by Eq. (3) is often associated with the formation of tails of localized states near the conduction band bottom and the development of large-scale potential profile. Therefore in accordance with earlier data [12, 41] the ρ(T) curves observed below 150 K in doped ZnO of n type of conductance can be described using the band energy diagram shown in Fig. 9. This diagram illustrates that the behavior of the ρ(T) curves for heating of an inhomogeneous specimen in the experimental temperature range can be interpreted as a sequential transition from Shklovskii-Efros’ conductivity and then Mott’s conductivity described by Eq. (2) which occur by hopping through localized “defect band” states to percolation conductivity described by Eq. (3) that is caused by the formation of large-scale potential roughness by intrinsic defects and impurities.

It should be noted that a combination of hopping conductivity attributed to Mott’s law described by Eq. (2) at below 40 K with conventional band conductivity described by Eq. (1) at above 50 K was observed earlier in intrinsic ZnO single crystals [35] containing shallow intrinsic defects, e.g. interstitial zinc and/or oxygen vacancies which produce shallow donor levels with an ionization energy of about 0.05 eV.

4. Conclusion

The structure, type of doping Fe oxide and temperature were shown to affect the electrical properties of (FexOy)10(ZnO)90 ceramics (0 ≤ x ≤ 3; 1 ≤ y ≤ 4) prepared using single- and two-stage synthesis methods in air. The doping agents were FeO, Fe2O3 and Fe3O4 iron oxides or (Fe2O3 + FeO) mixture. Carried out X-ray microfluorescent analysis, X-ray diffraction and Raman spectroscopy showed that at average iron concentrations of 1–3 at.% the ceramic specimens contain at least three phases: the Zn1-δFeδO solid solution with the iron content δ ≤ 0,81 at.%, the ZnFe2O4 ferrite phase with a spinel structure and FexOy residual iron oxides. Scanning electron microscopy showed that the wurtzite phase grain size in the ceramic specimens decreases from several decades of microns for single-stage synthesis to submicron sizes for two-stage synthesis. Two-stage synthesis also improved the homogeneity of the grain size and composition distribution. Iron introduction in the ZnO crystal lattice produces deep donor centers with an activation energy of about 0.37 eV presumably due to the interaction of the iron ions with the intrinsic zinc oxide defects. The electrical resistivity vs temperature dependences ρ(T) for undoped zinc oxide in the 6–300 K range exhibit a variable activation energy indicating a heavily disordered structure and possible formation of large-scale potential relief. In the latter case an increase in the temperature of the ceramic specimens initiates a transition from Shklovskii-Efros’ and Mott’s hopping conductivity with a variable activation energy (6–30 K) to percolation conductivity (50–100 K) and band conductivity (above 200 K).

Acknowledgements

The Authors express their gratitude to the State Research Program “Physical Materials Science, New Materials and Technologies” (Belarus) for financial support.

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