J. Mater. Sci. Technol., 2012, 28(3), 268–274.

Synthesis, Crystal Structural and Electrical Conductivity Properties

of Fe-Doped Zinc Oxide Powders at High Temperatures

Hakan Colak1,2)† and Orhan Turkoglu1)

1) Department of Chemistry, Science of Faculty, University of Erciyes, Kayseri, 38039, Turkey

2) Deparment of Chemistry, Science of Faculty, University of Cankırı Karatekin, Ballıca Campus, Cankırı, 18200, Turkey

[Manuscript received December 31, 2010, in revised form March 5, 2011]

The synthesis, crystal structure and electrical conductivity properties of Fe-doped ZnO powders (in the rangeof 0.25–15 mol%) were reported in this paper. I-phase samples, which were indexed as single phase witha hexagonal (wurtzite) structure in the Fe-doped ZnO binary system, were determined by X-ray diffraction(XRD). The solubility limit of Fe in the ZnO lattice is 3 mol% at 950 C. The above mixed phase was observed.And the impurity phase was determined as the cubic-ZnFe2O4 phase when compared with standard XRD datausing the PDF program. This study focused on single I-phase ZnO samples which were synthesized at 950 Cbecause the limit of the solubility range is the widest at this temperature. The lattice parameters a and c ofthe I-phase decreased with Fe-doping concentration. The morphology of the I-phase samples was analyzedwith a scanning electron microscope. The grain size of the I-phase samples increased with heat treatment anddoping concentration. The electrical conductivity of the pure ZnO and single I-phase samples was investigatedusing the four-probe dc method at 100–950 C in air atmosphere. The electrical conductivity values of pureZnO, 0.25 and 3 mol% Fe-doped ZnO samples at 100 C were 2×10−6, 1.7×10−3 and 6.3×10−4 S·cm−1,and at 950 C they were 3.4, 8.5 and 4 S·cm−1, respectively.

KEY WORDS: II-VI semiconductors; Zinc oxide and doped zinc oxide; Four point probe method

1. Introduction

Zinc oxide (ZnO) powders are very important ma-terials due to the many interesting properties in-herent in this material, such as dielectric, piezo-electric, pyroelectric, semiconducting, acousto-optic,optical, electro-optical, nonlinear optical, photo-electrochemical and electrical properties[1]. ZnO isone of the most versatile and technologically inter-esting semiconducting materials because of its typicalproperties such as resistivity control over the range of10−3–10−5 Ω·cm, transparency in the visible range,high electrochemical stability, direct band gap, ab-sence of toxicity and abundance in nature[2]. ZnO nor-mally occurs in the hexagonal wurtzite crystal struc-ture with a=0.32488 nm and c=0.52066 nm in thestandard data (JCPDS, 36-1451). Electron doping in

† Corresponding author. Assist. Prof., Ph.D.; Tel.: +90506 9868893; Fax: +90 376 2181031; E-mail address:hakan colak@hotmail.com (H. Colak).

nominally undoped ZnO has been attributed to Zn in-terstitials and oxygen vacancies[3]. High purity ZnOcrystals exhibit strong n-type conductivity with theelectrons moving in the conduction band as chargecarriers. This characteristic has traditionally been at-tributed to native defects such as oxygen vacanciesand zinc interstitials[4,5].

ZnO has both good electronic and optical prop-erties because of a stoichiometric deviation due tothe existence of intrinsic defects such as O vacan-cies and Zn interstitials. However, these propertiesof pure ZnO are unstable due to the adsorption ofatmospheric oxygen, which decreases the conductiv-ity, and they cannot meet the increasing needs forpresent day applications. To stabilize them againstsuch changes and enhance the properties of the ZnO,doping is necessary and this purpose was achieved byadding some dopants[6,7]. Moreover, doping leads toan increase in the conductivity of ZnO. ZnO dopingwas achieved by replacing Zn2+ atoms with the atomsof the dopant elements. The efficiency of the dopant

H. Colak et al.: J. Mater. Sci. Technol., 2012, 28(3), 268–274. 269

element depends on its electronegativity and the dif-ference between its ionic radius and the ionic radiusof zinc[8]. Extensive studies have been carried outto modify the properties of ZnO for different applica-tions. Doping with transitional metal elements leadsto many interesting properties of ZnO[9]. In addi-tion, thermal annealing is a widely used method toimprove crystal quality and to study structural defectsin materials. For semiconductors, thermal annealingis also used to activate dopants. During the annealingprocess, dislocations and other structural defects willmove in the material and adsorption/decompositionwill occur on the surface, thus the structure and thestoichiometric ratio of the material will change, whichinfluences the electrical and optical properties of ZnO.The structural, electrical and optical properties arevital for semiconductor devices. It is necessary to in-vestigate how these properties are affected by thermalannealing[10].

Although there are a few reports on the doping ele-ments in the ZnO varistor, the effects of some dopants,such as Fe, on the electrical conductivity of the ZnOvaristor still remain unclear. This is because dopingeffects have been studied in quite different systems un-der different experimental conditions[11]. In this work,metallic iron-doped ZnO powders were synthesized athigh temperatures by using the solid state reactionmethod. Then, their structural properties were stud-ied and the dc electrical conductivity of the ZnO solidsolution samples was measured at high temperatures.The solid state reaction was chosen because of its re-producibility, easy control and sufficent products formeasurement[12].

2. Experimental

Synthesized iron (Fe) doped ZnO powders at hightemperatures were measured by using the standardsolid state reaction method. At first, solid mixturesof Fe-ZnO (0.25≤ x ≤15 mol%) were prepared by mix-ing and homogenizing with stoichiometric amounts ofpure ZnO and metallic Fe in an agate mortar. Asstarting materials, commercially pure ZnO powders(99.9% Fluka) and metallic iron powders (99.5% AlfaAesar) were used. The powder mixtures were firstcalcined at 600 and 650 C for 24 h. After grindingand homogenization, the pre-annealed mixtures wereheat treated at 700, 750, 800, 850, 900 and 950 C,respectively. All these heat treatments were madein air atmosphere for 48 h, in alumina crucibles andwithout any compaction procedure. At the end ofeach heat treatment procedure, the annealed powderswere slowly cooled in the furnace by switching it off(uncontrolled). Before and after heat treatment, eachof the samples was ground in an agate mortar to ho-mogenize powder size. The samples were then ana-lyzed with X-ray powder diffraction (XRD). The XRDdata of the samples were recorded on a computer-interfaced Bruker AXS D8 advanced diffractometeroperating in Bragg–Brentano geometry (CuKα radi-

ation, graphite monochromator, 40 kV and 40 mA)over a 10 deg.≤2θ≤90 deg. angular range. The diver-gence and receiving slits of 1 and 0.1mm, respectively,were located on the diffractometer. The diffractionpatterns were scanned in 0.002 deg. (2θ) steps and thediffracted beams were counted with a NaI(Tl) scintil-lation detector. The Diffrac Plus and Win-Index pro-grams were used to obtain information about the crys-tal structures of the samples. Then, for a comparisonwith standard data, the PDF program (Maint PowderDiffraction Database Manager Software) was used. Inall cases, the XRD patterns of the samples, which wereobtained as single phase, could be indexed on the ba-sis of a hexagonal cell in the Fe–ZnO system. Thesingle phase was named as I-phase. The other sam-ples which were not indexed with a hexagonal crys-tal structure were named as II-phase. Samples whichwere out of the solubility range were heterogeneoussolid mixtures (I+II). The morphologies of both an-nealed (950 C) and non-annealed undoped ZnO sam-ples and I-phase samples (950 C) were observed byscanning electron microscopy (SEM, LEO 440) oper-ated at 20 kV. Before SEM measurement, the I-phasesamples in the Fe–ZnO system were pressed into pel-lets of 0.1 cm in thickness (t) and 1.3 cm in diameter(d) under ∼5 ton pressure (∼369 MPa) and were cal-cined at 950 C for 24 h in air. After heat treatment,the XRD measurements showed that no phase changein the hexagonal samples was observed. The averagegrain sizes of the I-phase samples were calculated bythe image-pro plus 5.0 program from the SEM micro-graphs.

The electrical conductivity (σdc) measurements ofthe I-phase samples (950 C) were made using thestandard four-probe dc method. The circular sampleswith a hexagonal structure, which were mentionedabove as the pelleted samples, were used for the elec-trical conductivity measurements. To reduce contactresistance, fine platinum wires were directly attachedto the surface of the samples. The ohmic character ofthe wire contacts was checked prior to each measure-ment. The contacts were positioned symmetricallywith respect to the center of the circular pellet and thecontact separations (s) were 0.2 cm. The conductivitymeasurements were made between 100 and 950 C forI-phase samples synthesized at 950 C. The increasein furnace temperature was initial in 100 C (in therange 100–400 C) steps, then in 50 C (450–950 C)steps in air. During the measurements, the sampletemperature was determined by a thermocouple lo-cated 5 mm away from the sample. This thermocou-ple had a cold (0 C) junction. All experimental datawere recorded by a Keithley 2400 sourcemeter and aKeithley 2700 electrometer which were controlled bya computer.

3. Results and Discussion

3.1 Structural properties

Because data on the solubility of Fe ions in ZnO

270 H. Colak et al.: J. Mater. Sci. Technol., 2012, 28(3), 268–274.

Table 1 Observed phases in the binary system of Fe-doped ZnO (in the range of 0.25–15 mol%)

Temp./Cx (Fe addition)/mol%

0.25 0.50 0.75 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

700 I I I I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II

750 I I I I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II

800 I I I I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II

850 I I I I I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II

900 I I I I I I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II

950 I I I I I I I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II I+II

Notes: I—Fe-doped ZnO (hexagonal structure), II—cubic-ZnFe2O4, I+II—heterogeneous solid mixtures

Fig. 1 XRD patterns of Fe-doped ZnO: (a) undopedZnO (950 C), (b) 0.25 mol% Fe-doped ZnO, (c)3 mol% Fe-doped ZnO, (d) 6 mol% Fe-doped ZnO,(e) 15 mol% Fe-doped ZnO (950 C) (∗: the peaksof II-phase, ZnFe2O4)

are not known, annealing experiments at varying tem-peratures were performed[13]. Fig. 1 shows the XRDpatterns of pure ZnO (950 C, 48 h), 0.25, 3, 6 and15 mol% Fe-doped ZnO samples (950 C, 48 h). Inthis figure, all the XRD patterns show typical peakpatterns, which can be indexed as the ZnO wurtzitestructure. The peak appears at 2θ=34.44 deg., whichcorresponds to the (002) directions of the ZnO hexag-onal wurtzite structure[14]. For 6 and 15 mol% Fe-doped ZnO samples, XRD patterns show extra peaks,which indicate ZnFe2O4, (PDF No. 01–1108)[15–18].

The XRD patterns of other single I-phase samples,which were synthesized at other temperatures, werequite similar to the patterns of the I-phase samplesgiven in Fig. 1. At 950 C, the XRD measurementsrevealed that doping with more than 3 mol% causedthe coexistence of the I and II phase. No indication ofiron metal impurities was observed in the samples. Inthe Fe–ZnO binary system, the observed single phasesand heterogeneous solid mixtures depending on thereaction temperature and amounts of Fe doping arepresented in Table 1. The widest range solubilityin the ZnO lattice of Fe ions is at 950 C, and atthis temperature the solubility limit is 3 mol%. Atlower temperatures the solubility range is lower than

950 C. The low solubility of Fe may be satisfyinglyexplained by the heterovalence of the Fe ions in theZnO host lattice[13]. The mobility of Fe ions whichdiffuse in ZnO tetrahedral sites increases with tem-perature. Heat treatment temperature was found tohave an effect on the solubility of Fe ions in ZnO[19].In addition, both the ionic radii and the valence stateare important factors in determining the solubility ofthe dopants. A smaller deviation of these factors fromthose of Zn2+ would be favorable for the dopants tohave a higher solubility. This study focused on singleI-phase ZnO samples synthesized at 950 C due tothe limit of the solubility range, which is the widestat this temperature. The other I-phase samples syn-thesized at temperatures lower than 950 C, and themultiphase samples (I+II) were excluded from thisstudy.

For the wurtzite ZnO structure, the lattice con-stants determined at room temperature by various ex-perimental measurements and theoretical calculationswere in good agreement. The lattice constants of thesamples mostly ranged from 0.32483 to 0.32501 nm forthe a parameter and from 0.52023 to 0.52050 nm forthe c parameter. The deviation from that of the idealwurtzite crystal is probably due to lattice stabilityand ionicity[20]. Table 2 shows the dependence of the

Table 2 Unit cell parameters of I-phase ZnO sam-ples at 950 C

x (Fe addition)/mol%Unit cell parameters/nm

a c

0.25 0.32501 0.52050

0.50 0.32499 0.52047

0.75 0.32493 0.52033

1 0.32488 0.52026

2 0.32488 0.52025

3 0.32483 0.52033

lattice parameters a and c, respectively, on the addi-tion ratio of metallic Fe. The lattice parameters ofthe Fe-doped ZnO system are smaller than those ofpure ZnO, both a and c parameters decrease with thesubstitution of Fe ions with Zn ions. Fe ions are sub-stituted with Zn2+ ions which are in the tetrahedralsites of the wurtzite structure of ZnO. It is consideredthat valence state of Fe in ZnO is both +2 and +3,namely Fe in the ZnO matrix exists in a mixed va-lence state. It has been reported that local magneticprobes like electron paramagnetic resonance and

H. Colak et al.: J. Mater. Sci. Technol., 2012, 28(3), 268–274. 271

Fig. 2 SEM images of I-phase samples for pure ZnO (non-annealed) (a), pure ZnO (annealed) (b), 0.25 (c),0.50 (d), 0.75 (e), 1 (f), 2 (g) and 3 mol% (h) Fe-doped ZnO samples at 950 C

Mossbauer spectroscopy indicate the presence of Fein ZnO in both valence states, Fe2+ and Fe3+. Andthe presence of Fe3+ in ZnO is due to possible holedoping in the system by cation (Zn) vacancies[21]. Ithas also been reported in another study[22] that usu-ally, if Fe is present in the substitutional site in adefect-free ZnO crystal, the valence state of Fe willbe +2. In this report, also, the X-ray photoelectronspectroscopy (XPS) result confirmed the presence ofuncoupled Fe3+ within the sample. In the Fe–ZnObinary system, Fe3+ ions are present due to the ex-istence of cation vacancy. Cation vacancy near Fecan promote Fe2+ into Fe3+ and also mediate theFe2+–Fe2+ exchange interaction. Since the transi-

tion metal (TM) doping percentage is slightly on thehigher side toward the cationic percolation thresh-old, Fe2+–Fe3+ exchange, although being less in num-ber in comparison to the Fe2+–Fe2+ interaction, mayalso be possible[22]. However, the ionic radius (four-coordinated) of the Fe2+ ion is 0.074 nm and thatof the Fe3+ is 0.064 nm while the ionic radius of theZn2+ is 0.074 nm[23]. Therefore, the decrease in thelattice parameters a and c can be explained by theionic radii difference[24].

Fig. 2 shows the SEM micrographs of both an-nealed (950 C) and non-annealed undoped ZnO andI-phase ZnO samples (950 C). In the SEM micro-graphs of undoped ZnO samples (Fig. 2(a) and (b)),

272 H. Colak et al.: J. Mater. Sci. Technol., 2012, 28(3), 268–274.

there is a homogeneous grain distribution. From theseimages the average grain sizes were calculated andwere found to be 0.63 and 1.03 µm, respectively[19].The grain sizes of the annealed sample are obviouslylarger than those of the non-annealed samples, whichcould be caused by strain relaxation. The anneal-ing process clearly produces a recovery of the crystalstructure and an increase in the grain size[1,25]. Ascan be seen in the SEM micrographs, the grains ofthe I-phase samples are homogeneously distributed.

The grain size of all synthesized Fe-doped ZnOsamples are given in Table 3. From the SEM images,

Table 3 Grain size values of I-phase ZnO samplesat 950 C

x (Fe addition)/mol% Grain size/µm

ZnO∗ 0.67

ZnO 1.03

0.25 1.32

0.50 1.46

0.75 1.73

1 1.93

2 2.51

3 2.60

Note: ∗—non-annealed

the grain size of the Fe-doped ZnO samples slightlyincreases with the addition of Fe because the substi-tution of Fe2+ and Fe3+ for Zn2+ increases the activ-ity of ZnO by means of distortion of the ZnO lattice,which is beneficial to grain growth in Fe-doped ZnOsamples[26]. The grains are regular in size and shape.Some pores can be noticed in the photographs. Whenincreasing the dopant concentration to 3 mol%, theporosity increased[24]. In addition, the surface of thesamples is smooth.

3.2 Electrical properties

The total electrical conductivity is calculated us-ing the following equation:

σT =I

VG−1 (1)

where G is the geometric resistivity correctionfactor[27]. Both the values and the variation of theelectrical conductivity of the pelleted powder ZnOsamples are in connection with their structure and itschanges. On the other hand, thermal treatments ofthe respective samples modify their structural charac-teristics and consequently, their electrical properties.On this basis, the study of the temperature depen-dence of the electrical properties of ZnO samples, mayoffer useful information about the possible changes ofthe structural characteristics of ZnO samples[28].

The temperature dependence of electrical conduc-tivity (σdc) of the I-phase ZnO pelleted powder sam-ples at 950 C was studied in the temperature range of100–950 C. logσT–103/T graphs were drawn in orderto show the conductivity change with respect to tem-perature. The graphs were compared with each other

and the conductivity characteristics of the sampleswere evaluated. The electrical conductivity plots vstemperature of undoped ZnO, 0.50, 0.75 and 2 mol%Fe-doped ZnO (950 C) samples are given in Fig. 3.The graphic curves of the other single I-phase sampleswere quite similar to the curves of the single I-phasesamples given in this figure. As seen in Fig. 3, the elec-trical conductivity of I-phase samples increases withincreasing temperature and shows the semiconduct-ing behavior of pure and Fe-doped ZnO samples. Itis well known that the electrical conductivity of ZnOsamples is controlled by intrinsic defects (oxygen va-cancy and interstitial zinc atoms) generated at hightemperature[11].

Also, for ZnO samples, increasing electrical con-ductivity with temperature can be explained by thefollowing:

ZnO ↔ Znxi + 1/2O2(g) (2)

Znxi ↔ Zn•i + e− (3)

Zn•i ↔ Zn••i + e− (4)

After ionization, carrier concentration is increasedby two extra electrons[29]. The electrical conductionof the doped ZnO follows a mechanism in which theelectron or hole hops from one localized site to thenext. Whenever it is transferred to another site, thesurrounding molecules respond to this perturbationwith structural changes and the electron or hole istemporarily trapped in the potential well leading toatomic polarization. The electron resides this site un-til it is thermally activated to migrate to another site.Another aspect of this charge hopping mechanism isthat the electron or hole tends to associate with localdefects[30].

It is observed that conductivity slightly decreaseswith increasing Fe concentration (Fig. 4). It is wellknown that the electrical conductivity of ZnO sam-ples at room temperature is due to intrinsic defectscreated by oxygen vacancies. These defects introducedonor states in the forbidden band slightly below theconduction band and hence result in the conductingbehavior of ZnO. The electrical conductivity is con-trolled by the intrinsic defects generated during syn-thesis and by the presence of dopants. It is known thatduring the sintering process at high temperature, oxy-gen vacancies will be produced which are responsiblefor electrical conductivity. From the observed result ofdecrease in conductivity of ZnO on Fe doping, it seemsthat the Fe-doping affects the defect chemistry of theZnO. Therefore, It is believed that Fe in ZnO acts asa deep donor and decreases the concentration of in-trinsic donors. This reduction in intrinsic donor con-centration increases with increasing Fe content, whichin turn decreases the electrical conductivity[31].

Many researchers have studied the effects of 3dtransition metal impurities on the electrical conduc-tivity of the ZnO varistor. They investigate that 3dtransition metals could enhance the excess oxygenconcentration in the grain boundary region and a

H. Colak et al.: J. Mater. Sci. Technol., 2012, 28(3), 268–274. 273

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

-5.6

-4.8

-4.0

-3.2

-2.4

-1.6

-0.8

0.0

0.8log

/Scm

-1)

1/T 103 / K-1

(a)

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6-3.0

-2.4

-1.8

-1.2

-0.6

0.0

0.6

log

(/S

cm-1)

1/T 103 / K-1

(b)

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6-4.0

-3.2

-2.4

-1.6

-0.8

0.0

0.8

log

(/S

cm-1)

1/T 103 / K-1

(c)

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6-4.0

-3.2

-2.4

-1.6

-0.8

0.0

0.8

1.6

log

(/S

cm-1)

1/T 103 / K-1

(d)

Fig. 3 Electrical conductivity plots for undoped ZnO (a), 0.25 (b), 0.75 (c) and 2 mol% (d) Fe-doped ZnO (950 C)

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5-6

-5

-4

-3

-2

-1

0

1

2

3

Conductivity at 100 oC Conductivity at 500 oC Conductivity at 950 oC

log

(/S

cm-1)

Fe / mol%

Fig. 4 Electrical conductivity variation for Fe-dopedZnO I-phase ZnO samples (950 C)

potential barrier is formed preferentially. Therefore,the electrical conductivity of Fe-doped ZnO samplesis apparently lower than that of undoped ZnO sam-ples, and the grain boundary is more resistive thanthe grain[32–34]. Also, it was observed from the SEMmicrographs that Fe-doping produces a large numberof pores which may be the main reason for the signif-icant decrease in the conductivity of the consideredsamples[35].

In general, for a semiconducting material, dc con-ductivity increases exponentially with temperature in-dicating that conductivity is a thermally activatedprocess. For doped semiconductors with low concen-tration of donors, the hopping transport of carriers

is expected between the nearest donors at low tem-peratures. The total electrical conductivity can beexpressed as follows;

σdc = σ0 exp(−Ea/kT ) (5)

where σdc is the electrical conductivity at any temper-ature, Ea is the activation energy, which correspondsto the energy difference between the donor level andthe conduction level, σ0 is the pre-exponential factor,k is the Boltzmann constant, and T is the absolutetemperature[8,11,30,36].

The activation energy can be calculated from theconductivity equation shown below,

lnσT = −Ea

k

1T

+ lnσ0 (6)

The slope of the linear part of the Arrhenius curveof the lnσT–1/T graph is equal to –Ea/k. For thetemperature range the I-phase ZnO samples haveArrhenius-type electrical conductivity. From thesecurves, drawn for all I-phase ZnO samples, the ac-tivation energies Ea were calculated (Fig. 5).

The increase in the activation energy of Fe-dopedZnO can be explained as follows: when ZnO is sub-stituted with Fe ions which are in the 3+ ion valencestate, they replace the Zn2+ ions and occupy zinc in-terstitial sites. The substituted Fe atoms cannot beeasily ionized like zinc atoms because of their higherionization potential (2 eV). Hence, the donor concen-tration is lowered by the addition of Fe which resultsin the decrease of electrical conductivity. Thus, a

274 H. Colak et al.: J. Mater. Sci. Technol., 2012, 28(3), 268–274.

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.50.09

0.18

0.27

0.36

0.45

0.54

0.63E

a / eV

Fe / mol%

Fig. 5 Effect of doping concentration on activation en-ergy for I-phase ZnO samples (950 C)

higher value of activation energy (Ea) is found for Fe-doped ZnO samples and it increases with increasingiron concentration (x)[31].

4. Conclusion

Fe-doped ZnO powders were synthesized by us-ing the solid state reaction method with metallic Fepowder and commercially pure ZnO powder. TheXRD analysis results indicated that all powder sam-ples in the Fe–ZnO binary system (0.25≤x≤3 mol%,at 950 C) had a wurtzite structure, namely the solu-bility limit of Fe in the ZnO lattice at this temperaturewas 3 mol%. The lattice constants of the I-phase ZnOsamples decreased with Fe doping concentration. Thegrain size of the I-phase ZnO samples increased withboth heat treatment and the amount of Fe doping.For pure and Fe-doped ZnO samples (950 C), elec-trical conductivity increased with heat treatment anddecreased with increasing doping concentration. Inaddition, the activation energy of I-phase ZnO sam-ples increased with increasing dopant concentration.

Acknowledgement

This work was financially supported by the ResearchFoundation of Erciyes University (Kayseri, Turkey).

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