Structural, optical, and conducting properties of crystalline ZnO:Co thin films grown by reactive electron beam deposition Osman Gürbüz, Sadık Güner n , Ömer Büyükbakkal, Serkan Çalışkan Department of Physics, Fatih University, Büyükçekmece 34500, Istanbul, Turkey article info Keywords: Wurtzite structure Schrerrer equation Williamson-Hall method Reactive Electron Beam Deposition technique XRD UV–vis spectrophotometer abstract We deposited an undoped ZnO and 6 different Co doped ZnO (ZnO:Co) thin films on fused silica (SiO 2 ) substrates with 100 nm thickness at substrate temperature of 125 1C using a Reactive Electron Beam Deposition technique. Energy-Dispersive X-ray Spectroscopy (EDS) was used to analyze the elemental composition rates of films. Elemental Co concentration varies from 4.62 to 28.77 at. %. The surface morphologies and grain sizes of thin films were investigated by Scanning Electron Microscope (SEM). The crystal and phase structures of the ZnO:Co thin films were characterized using X-ray diffraction (XRD). The films have single crystal and polycrystalline structures due to Co concentrations. Theoretical crystallite size and strain calculations were performed by applying the Scherrer and Williamson–Hall (W–H) methods. The grain sizes are 2–4 times greater than the crystalline sizes for ZnO:Co films. Optical properties of the films were studied by absorbance measurements using a UV–vis spectrophotometer. The analysis of the optical absorption spectra indicated that the energy band gap of the bulk ZnO film increased from 3.22 eV to maximum 4.17 eV upon Co deposition. Co 2 þ ion replaces Zn 2 þ ion in the structure without causing any remarkable defect for its hexagonal Wurtzite structure. Electrical conducting properties were investigated by using a Four Point Probe (FPP) technique. The conductivity depends on crystalline quality and Co concentration. & 2014 Elsevier B.V. All rights reserved. 1. Introduction The nanocrystalline structured zinc oxide (ZnO) thin layers of films have a significant role in spintronic, electronic, and photo- voltaic device fabrication [1,2]. The crystal structure of ZnO exists in three forms, hexagonal Wurtzite with a ¼ 3.25 Å and c ¼ 5.12 Å, cubic zinc blende, and rock salt, while the thermodynamically stable phase is Wurtzite [1]. In either cubic zinc blende or hexagonal lattice, each anion is surrounded by 4 cations at the corners of a tetrahedron. ZnO is a semiconducting material and native doping is n-type due to oxygen vacancies. It has many useful properties such as good transparency, high electron mobility, direct band gap, strong room-temperature luminescence, etc. The room temperature Hall mobility in ZnO single crystals is in the order of 200 cm 2 V 1 s 1 [3]. The wide band gap value (E g ¼ 3.35 eV at 300 K) makes it a good candidate for the production of green, blue–ultraviolet, and white light-emitting diodes. ZnO has a large exciton binding energy (60 meV) that ensures an efficient excitonic emission at room temperature and room temperature ultraviolet (UV) luminescence has been declared in thin films [4,5]. The optical and transport properties of a semiconducting material have their dependence on both internal factors including excitonic effects due to the Coulomb interaction [6–8] and external ones. External ones are related to dopants, implantation, impu- rities or point defects, which usually create electronic states in the band gap, and therefore influence both optical absorption and emission processes. Al, Ag, N, In, etc. are the elements to be doped or implanted to change optical and electronic transport properties [9,10]. The incorporation of Al or Ag into ZnO can lead to an increase in the carrier concentration [11]. When these elements are doped, the ZnO protects its Wurtzite crystal lattice. The optical properties are already used in emerging applications for transpar- ent electrodes in liquid crystal displays and in energy-saving or heat-protecting windows, and electronic applications of ZnO as thin-film transistors [12]. ZnO is doped or implanted for many other purposes with 3d transition metal ions (TM) especially Co and others like Fe, Mn, Cr, Ni to get diluted magnetic semiconductors (DMS). Numerous theoretical and experimental analyses revealed that TM ions doped ZnO can have ferromagnetic property [13–15] and exhibit photocatalytic activity [16,17]. Some studies claimed that magne- tization arises from Co clusters or secondary phase formation Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials http://dx.doi.org/10.1016/j.jmmm.2014.01.079 0304-8853 & 2014 Elsevier B.V. All rights reserved. n Corresponding author. Tel.: þ90 212 8663300x2048; fax: þ90 212 8663302. E-mail address: [email protected](S. Güner). Please cite this article as: O. Gürbüz, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j. jmmm.2014.01.079i Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
Transcript
Structural, optical, and conducting properties of crystalline ZnO:Cothin films grown by reactive electron beam deposition
Osman Gürbüz, Sadık Güner n, Ömer Büyükbakkal, Serkan ÇalışkanDepartment of Physics, Fatih University, Büyükçekmece 34500, Istanbul, Turkey
a r t i c l e i n f o
Keywords:Wurtzite structureSchrerrer equationWilliamson-Hall methodReactive Electron Beam DepositiontechniqueXRDUV–vis spectrophotometer
a b s t r a c t
We deposited an undoped ZnO and 6 different Co doped ZnO (ZnO:Co) thin films on fused silica (SiO2)substrates with �100 nm thickness at substrate temperature of 125 1C using a Reactive Electron BeamDeposition technique. Energy-Dispersive X-ray Spectroscopy (EDS) was used to analyze the elementalcomposition rates of films. Elemental Co concentration varies from 4.62 to 28.77 at. %. The surfacemorphologies and grain sizes of thin films were investigated by Scanning Electron Microscope (SEM).The crystal and phase structures of the ZnO:Co thin films were characterized using X-ray diffraction(XRD). The films have single crystal and polycrystalline structures due to Co concentrations. Theoreticalcrystallite size and strain calculations were performed by applying the Scherrer and Williamson–Hall(W–H) methods. The grain sizes are 2–4 times greater than the crystalline sizes for ZnO:Co films. Opticalproperties of the films were studied by absorbance measurements using a UV–vis spectrophotometer.The analysis of the optical absorption spectra indicated that the energy band gap of the bulk ZnO filmincreased from 3.22 eV to maximum 4.17 eV upon Co deposition. Co2þ ion replaces Zn2þ ion in thestructure without causing any remarkable defect for its hexagonal Wurtzite structure. Electricalconducting properties were investigated by using a Four Point Probe (FPP) technique. The conductivitydepends on crystalline quality and Co concentration.
& 2014 Elsevier B.V. All rights reserved.
1. Introduction
The nanocrystalline structured zinc oxide (ZnO) thin layers offilms have a significant role in spintronic, electronic, and photo-voltaic device fabrication [1,2]. The crystal structure of ZnO existsin three forms, hexagonal Wurtzite with a¼3.25 Å and c¼5.12 Å,cubic zinc blende, and rock salt, while the thermodynamicallystable phase is Wurtzite [1]. In either cubic zinc blende orhexagonal lattice, each anion is surrounded by 4 cations at thecorners of a tetrahedron.
ZnO is a semiconducting material and native doping is n-typedue to oxygen vacancies. It has many useful properties such as goodtransparency, high electron mobility, direct band gap, strongroom-temperature luminescence, etc. The room temperature Hallmobility in ZnO single crystals is in the order of 200 cm2 V�1 s�1[3].The wide band gap value (Eg¼3.35 eV at 300 K) makes it a goodcandidate for the production of green, blue–ultraviolet, and whitelight-emitting diodes. ZnO has a large exciton binding energy(60 meV) that ensures an efficient excitonic emission at room
temperature and room temperature ultraviolet (UV) luminescencehas been declared in thin films [4,5].
The optical and transport properties of a semiconductingmaterial have their dependence on both internal factors includingexcitonic effects due to the Coulomb interaction [6–8] and externalones. External ones are related to dopants, implantation, impu-rities or point defects, which usually create electronic states in theband gap, and therefore influence both optical absorption andemission processes. Al, Ag, N, In, etc. are the elements to be dopedor implanted to change optical and electronic transport properties[9,10]. The incorporation of Al or Ag into ZnO can lead to anincrease in the carrier concentration [11]. When these elementsare doped, the ZnO protects its Wurtzite crystal lattice. The opticalproperties are already used in emerging applications for transpar-ent electrodes in liquid crystal displays and in energy-saving orheat-protecting windows, and electronic applications of ZnO asthin-film transistors [12].
ZnO is doped or implanted for many other purposes with 3dtransition metal ions (TM) especially Co and others like Fe, Mn, Cr,Ni to get diluted magnetic semiconductors (DMS). Numeroustheoretical and experimental analyses revealed that TM ionsdoped ZnO can have ferromagnetic property [13–15] and exhibitphotocatalytic activity [16,17]. Some studies claimed that magne-tization arises from Co clusters or secondary phase formation
Contents lists available at ScienceDirect
journal homepage: www.elsevier.com/locate/jmmm
Journal of Magnetism and Magnetic Materials
http://dx.doi.org/10.1016/j.jmmm.2014.01.0790304-8853 & 2014 Elsevier B.V. All rights reserved.
n Corresponding author. Tel.: þ90 212 8663300x2048; fax: þ90 212 8663302.E-mail address: [email protected] (S. Güner).
Please cite this article as: O. Gürbüz, et al., Journal of Magnetism and Magnetic Materials (2014), http://dx.doi.org/10.1016/j.jmmm.2014.01.079i
Journal of Magnetism and Magnetic Materials ∎ (∎∎∎∎) ∎∎∎–∎∎∎
[18–20], while others concluded that an intrinsic carrier-mediatedmechanism was responsible for the ferromagnetism [21–23]. Theformation of O vacancies or defects like Zn interstitials is anotherreason for ferromagnetism in ZnO nanostructures [24–26].A trapped electron in the O vacancy and a magnetic impurityaround it constitute a bound magnetic polaron (BMP) with a largenet magnetic moment. This case suggests that structural analysesshould be done on ZnO and Co doped ZnO films in detail tounderstand the source of magnetism entirely.
Many researchers have fabricated DMS films in the single orpolycrystalline forms by different chemical vapor (CVD) andphysical vapor deposition (PVD) techniques [27–35]. In this study,we mainly focus on producing single crystalline ZnO and ZnO:Comonolayer films. The structure, optical absorption, and electricalconductivity of fabricated films were investigated in detail. Wewill present the magnetic investigations as a separate study andutilize the results of the present work.
2. Experimental details
Reactive electron beam deposition (REBD) technique wasapplied to grow a ZnO and 6 distinct ZnO:Co films on fused silica(SiO2) substrates that have 1 cm2 area and 1 mm thickness. Thestarting materials were commercial ZnO tablets (purity 99.9%, KurtJ. Lesker) and metallic cobalt (Co) pellet (99.99%, Kurt J. Lesker).The base chamber and deposition system were made by MantisDeposition Ltd. The substrates were cleaned with isopropanol andethanol mixture in an ultra-sonic cleaner, and then rinsed indistilled water and alcohol again. The chamber was evacuated,using a roughing and a turbo molecular pump, to a pressurebelow 3�10�7 mbar. The pressure during deposition was about2�10�6 mbar. The substrates were heated at 125 1C by Jouleeffect. A thermo-couple (K-type Chromel-Alumel) placed in
contact with the substrate was used to monitor the substratetemperature.
We measured the deposition rate and thickness of the filmswith a quartz crystal monitor (QCM). The thickness of each filmwas justified by a NanoMap 500LS profilometer. The film deposi-tion rate ranged from 0.1 to 1 Å/s and the oxygen flow rate was0.6 sccm. The Co concentration x varies (Zn1�xO:Cox) between4.62 at. % and 28.77 at. % in the structure. The substrates wererotated at a frequency of 10 rev/min to increase the uniformityof films.
The elemental composition rate of films was determined by anInca Energy Dispersive X-ray Spectroscopy (EDS) system. Thecrystal structure of films was analyzed by conventional θ–2θ XRDstudies. The studies were carried out in a Rigaku SmartLabdiffractometer that uses Cu Kα radiation (λ¼1.5406 Å). All mea-surements were performed by using the same input parameters,for example, 2θ values were taken between 201 and 801. Thesurface morphology of the films was observed by a Jeol 7001scanning electron microscope (SEM).
Optical absorbance measurements were carried out in thewavelength range from 200 to 1000 nm by using a ThermoScientific, Evolution 300 UV–vis spectrophotometer.
The DC conductivity measurements are performed to specifythe electric conductivity of materials by a Four Point Probe Method(FPP) [36].
3. Results and discussion
3.1. Structural analysis and surface morphology
The chemical composition rates of the films were determinedby the EDS analytical technique. The spectrum recorded from ZnO:Co that includes 16.70 at. % of Co ion is seen in Fig. 1. In the
Fig. 1. EDS spectrum recorded from ZnO:Co-4. It includes 16.70 at. % Co.
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spectrum, the most intensive peak belongs to Si. The other peaksbelong to Zn, Co, and O elements and their intensities are relatedwith the composition rates. The elemental analysis results of allfilms are given in Table 1 in terms of at. %. The maximum Coconcentration is 28.77 at. % that belongs to ZnO:Co-6.
The XRD technique is a simple and powerful tool to determinethe crystal structure and to estimate the crystallite size and latticestrain [37]. The XRD patterns of all ZnO and ZnO:Co films areshown in the Fig. 2. The θ–2θ patterns of the ZnO film and four ofZnO:Co films involve a single peak around 34.41 that is diffractedfrom (002) planes. The crystalline orientation of these films isparallel to the substrate normal that is along c-axis orientation[38]. There is no remarkable shift at 2θ value. This case showsthat there are no CoO clusters which induce a significant move-ment to the lower angles of (002) peak due to larger radius ofCo2þ ion ranging from 0.65 Å to 0.745 Å [31]. As reported in theliterature, the Co2þ ions substitute for Zn2þ ions in the tetragonalsite without forming a secondary phase [39].
The XRD pattern recorded from ZnO:Co-5 includes two morepeaks. These additional peaks were detected approximately at2θ¼31.361 and 36.921 which can be defined respectively to (100)and (101) planes of ZnO phase [40,41]. The optimum conditions fordeposition rate of source materials, chamber vacuum, and sub-strate temperature are needed to have single crystal structuredfilms. Further, there is a solubility limit of Co in the ZnO lattice inthe present deposition conditions. Hence, the XRD analyses exhibitthe single crystalline nature of 5 films and polycrystalline nature ofZnO:Co-5 film in hexagonal lattice structure. One can claim thatthe ZnO:Co-6 film has almost amorphous structure. A roughestimation indicates that around 20 at. % of Co concentration, apreferred crystallographic orientation is not observed any more.At the recorded XRD spectra from all seven films, there is noobserved peak originated due to Co clusters or CoO structures.
The intensity of XRD peaks depend on many factors such aslong range crystal quality and thickness of films. Here, theintensity of all recorded peak decreases with increasing Co con-centration which means that the crystallinity decreases since allfilms have almost same thickness.
In order to calculate the average crystallite size, D from the XRDpatterns, we used Scherrer's equation [9]
D¼ Kλβ cos θ
ð1Þ
where D is the particle size in nanometers, λ is the wavelength ofthe radiation (1.540 Å for CuKα), K is the shape factor equal to0.9 for hexagonal lattice, β is the peak width at half maximum(FWHM) intensity in radians and θ is the peak position.
The distance between adjacent crystal planes, d, was calculatedfrom the well known Bragg's equation. The hexagonal latticeparameters, such as c, can be calculated by using a Miller indices(hkl) based equation given in the following expression;
dhkl ¼43h2þhkþk2
a2þ l2
c2
!�1=2
ð2Þ
here, dhkl is the interplanar spacing, the indices h¼k¼0 andl¼2 for (002) peak; therefore
dhkl ¼cl
ð3Þ
c¼ dhkll¼ 2dhkl ð4Þ
The calculated lattice parameters are given in Table 2. Thelattice constants of bulk ZnO are a¼3.250 Ǻ and c¼5.206 Ǻ, whichgives a c/a ratio �1.60 as given in Ref. [1]. Our films have c valuesbetween 5.16 and 5.20 Ǻ. This means the strain (ε), which is thedegree of deformation from ideal lattice, is very small. The straininduced in the films due to crystal imperfection and distortion canbe calculated using the Williamson–Hall (W–H) equation
ε¼ β
4 tan θð5Þ
It is seen that the strain varies as tan θ while crystallite size variesas 1= cos θ [42].
Assuming that the particle sizes and strain contributions to linebroadening are independent of each other, and the resultant peakwidth is simply obtained by the sum of Eqs. (1) and (5):
β¼ KλD Cos θ
þ4ε tan θ ð6Þ
Table 1Substrate temperature, thickness, and Co concentration rates of fabricatedZnO:Co films.
By substituting tan θ¼ Sin θ= Cos θ in the above equation,we obtain
β Cos θ¼ KλD
þ4 ε Sin θ ð7Þ
The above equations are known as modified forms of the W-Hequation, namely, uniform deformation model (UDM). A plot isdrawn with 4sin θ along the x-axis and β cos θ along the y-axis forZnO film as shown in Fig. 3. From the linear fit for the data,the crystallite size for each film was estimated from the y-intercept, and the strain ε, from the slope of linear fit. That isY¼�0.0005xþ0.0073 as seen in Fig. 3. Hence from the correlationwith Eq. (7), the crystallite size is calculated as D¼Kλ/0.0073¼19 nm for undoped ZnO.
The calculated parameters for the films that have one preferredorientation are given in Table 2. The negative strain values are verysmall which is why they indicate the small magnitude of shrinkagefor the hexagonal crystal lattice.
The surface morphology of the films was analyzed by highresolution SEM. Fig. 4 shows SEM images of the ZnO, ZnO:Co-3,ZnO:Co-4, and ZnO:Co-5 films. ZnO film has long range uniformitywithout any remarkable defect in the structure. The observedgranular sizes and calculated crystallite sizes are approximatelyequal. The granular size and number of defects increase withhigher Co concentration in the doped films. The observed sizefrom the SEM image of ZnO:Co-5 film is smaller than 100 nm formost granules. The polycrystalline nature of this film is seen verywell in Fig. 4.
3.2. Optical properties
Optical properties of fabricated films were determined byUV–vis measurements in 200–1000 nm wavelength region. Fig. 5shows absorbance spectra recorded from all films. The absorbancecharacteristics in the 200–550 nm region are as expected; i.e.,
when Co content increases, the absorbance increases accordingly.The spectra involve a peak at 362 nm, which is a characteristicexcitonic peak in a range between 340 and 370 nm for ZnO:Cosamples [43]. With increasing Co ratio, we observe that excitoniccharacter increases in spectra. It is known that the dopant ionscreate defects in the crystal lattice that act as trapping sites of theexcitons, promoting recombination of electron–hole pairs, result-ing in reduced photocatalytic activity [44,45]. The absorption edgeof the peak showed a blue shift with increasing dopant concen-tration. While the ZnO film has absorption edge around 347 nm,the ZnO:Co-6 film has it around 332 nm.
The observed blueshift assigns the changes at the energy bandgap (Eg) of the films. The Eg of the films can be calculated from theTauc relation [46]. The Tauc relation for a direct band gap materialis given by the expression
αhv¼ Aðhv�EgÞn ð8Þ
where α is the absorption coefficient, and A is the absorbance.Since ZnO is a direct band gap material the power n is equal to 0.5.The expression for A is
A¼ αd¼ � ln T ð9Þ
here, d is the film thickness and T is the transmittance. An (αE)2 vsE graph is plotted. In the graph, a straight line is fitted for thestraight region. The extrapolation of this straight line to the E axisgives the value of the band gap. The plotted graphs for the ZnO:Co-4 and ZnO:Co-5 films and extrapolated Eg values are shown inFig. 6.
The ZnO film has Eg¼3.22 eV; this value fits well with thespecified values for pure ZnO given as in the references [4,5].The evaluated Eg values for ZnO:Co-1–6 films are 3.96, 3.74, 4.08,3.78, 3.96, and 4.06 eV in order. We do not see a direct correlationbetween the increasing Co concentration and band gap values.However, the reason for getting higher Eg values is explained byTalaat et al. The energy band gap increases due to active transi-tions involving 3d levels in Co2þ ions and strong sp–d exchangeinteractions between the itinerant ‘sp’ carriers (band electrons)and the localized ‘d’ electrons of the dopant [47].
The films that have Co concentration greater than 10.43 at. %have a characteristic absorption region between 600 and 800 nm.In the literature, this characteristic region is given in the550–700 nm wavelength interval [48]. There are three observedabsorption peaks in this characteristic region that originate frominter atomic d–d transitions in Co2þ ions. The 3d levels in theCo2þ ions split due to tetrahedral crystal field formed by O ionspresent in the ZnO structure. These transitions correspond to thetransitions from 4A2 (E) ground state toward 2A1(G), 4T1 (P), and 2E(G) excited states. This situation also proves the replacement of Cocations instead of Zn2þ ions as in the 2þ state [48–50]. Thetheoretical analyses on the physical origin of observed shift (forcharacteristic absorption region) in our studies are still beingconducted.
Table 2Geometric parameters of ZnO and ZnO:Co films.
Sample name Diffraction peak FWHM (rad) c (Å) Scherrer's method D (nm) ε W–H UDM method D (nm)
We conducted in-plane bulk resistivity measurements byapplying the FPP method. The bulk resistivity of each film wascalculated by substituting the detected data in the equation 10
ρ¼ π
ln 2
� � Vl
� �t ð10Þ
where ρ is the resistivity of the film (in Ω-cm), I is the currentapplied between the two outer points (in mA), V is the voltagedrop across the inner points (in mV), and t is the thickness of thefilm. Then, the electrical conductivities were determined bycalculating the reciprocal resistivity. The results are given inTable 3.
It is seen that the electrical conductivity increases with increas-ing Co concentration except for the last two films, ZnO:Co-5 andZnO:Co-6. The conductivity exhibits decrement although thesefilms have higher Co concentrations. However, they have shortrange polycrystalline nature. So, we can say that conductivitydepends not only on Co concentration, but also depends on
Fig. 4. Some selected SEM micrographs belong to (a) undoped ZnO, (b) ZnO:Co-3 (10.43 at. % Co), (c) ZnO:Co-4 (16.70 at. % Co), and (d) ZnO:Co-5 (23.14 at. % Co) films.
Fig. 5. Absorbance spectra of ZnO and all ZnO:Co films deposited at 125 1Csubstrate temperature.
0
50
100
150
200
250
1 2 3 4 7
1 2 3 4
5 6
5 6 70
50100150200250300350400450
(αhν
)2 (a.u
.)
ZnCo16,70O
3,78 eV
hν (eV)
hν (eV)
(αh ν
)2 (a.u
.)
ZnCo23,14O
3,96 eV
Fig. 6. Tauc plots for (a) ZnO:Co-4 (16.70 at% Co) and (b) ZnO:Co-5 (23.14 at% Co)thin films. The extrapolation of straight line to E axis gives the value of Eg.
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crystalline structure quality. In the XRD spectra, good crystalstructure is specified by narrow FWHM values of the diffractionpeaks. When we look at Tables 2 and 3, the films that have smallFWHM values also have good conducting properties. In the litera-ture, the ZnO films produced by different methods in singlecrystalline or polycrystalline forms have electric resistivity valuesin a very wide interval between 10�4 and 1012 Ω cm [51,52]. Theserecorded results depend on both crystallinity and the number ofcarrier concentration.
4. Conclusion
The structural, optical, and conducting properties of a ZnO and6 distinct ZnO:Co thin films grown by a Reactive Electron BeamDeposition techniquewere investigated. EDSmeasurements show thatdoping magnitude of elements there is no any remarkable except forCo. SEM investigations revealed that the Co doped films have greatergranular size. This analysis was supported by XRD investigations too.Most films have a preferred crystalline orientation along c directionwhile the ZnO:Co-5 and ZnO:Co-6 have polycrystalline nature. Around20 at. % of Co concentration, a preferred crystallographic orientation isnot observed any more. The average crystallite size varies between19.0 nm as the minimum for ZnO film with respect to the W–H UDMmethod and 28.7 nm for ZnO:Co-2 film with respect to the Scherrermethod. These sizes are in the range of obtained results by othergroups [38,43]. The crystallite size calculation by the Scherrer methodgives approximately 10% greater values for all ZnO:Co films withrespect to the W–H UDM method.
The optical spectra involve an excitonic peak around 362 nmthat is reported as a characteristic peak for ZnO:Co films between340 and 370 nm [43]. With increasing Co concentration, theintensity of exciton peak increased and a blueshift was observedat the peak position. The energy band gap of films was calculatedfrom the Tauc relation. The undoped ZnO has Eg¼3.24 eV; thisvalue fits well with the specified values given for bulk ZnO in thereferences [4,5]. The Eg values for all ZnO:Co films vary in therange of 3.74–4.08 eV. There is no one to one relation betweenincreasing Co concentration and band gap values as reported inthe literature [47,48]. The absorbance spectra involve anothercharacteristic absorption region between 600 and 800 nm thatoriginate from interatomic d–d transitions in Co2þ ions. Thisregion is given between 550 and 700 nm in the literature [48].
The film ZnO:Co-4 (Co, 16.70 at. %) has the highest conductivityand good crystallinity among the produced films. When weanalyzed the data given in Tables 2 and 3, we decided that theelectrical conductivity primarily depends on the crystalline qualitywhile the carrier concentration has secondary effect.
Acknowledgments
This work was supported by TUBİTAK and the ScientificResearch Fund of Fatih University under the Project numbers110T855 and P50011202_B respectively.
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Table 3The measured bulk resistivity and calculated electrical conductivities.
