Thin Solid Films 626 (2017) 76–84

Contents lists available at ScienceDirect

Thin Solid Films

j ourna l homepage: www.e lsev ie r .com/ locate / ts f

Physicochemistry of point defects in fluorine doped zinc tin oxidethin films

B. Salameh ⁎,1, A.M. Alsmadi 2, F. El AkkadDepartment of Physics, Kuwait University, 13060 Safat, Kuwait

⁎ Corresponding author.E-mail address: b.salameh@ku.edu.kw (B. Salameh).

1 On leave from the Department of Applied Physics, TaJordan.

2 On leave from Department of Physics, The Hashemite

http://dx.doi.org/10.1016/j.tsf.2017.02.0210040-6090/© 2017 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 5 November 2016Received in revised form 2 February 2017Accepted 7 February 2017Available online 09 February 2017

Zinc tin oxide (ZTO) and F-doped zinc tin oxide (FZTO) films with Zn concentration up to 35 at.% were preparedby chemical spray pyrolysis technique. The X-ray diffraction results showed an expansion in the lattice of tinoxide by either doping with fluorine or adding Zn due to the incorporation of fluorine into oxygen vacancies orthe replacement of the host Sn atoms by Zn, respectively. The X-ray photoelectron spectroscopy results of theFZTOfilms yield oxygen vacancy concentration [VO] in the range 1021–1022 cm−3 and substitutionalfluorine con-centration [FO] in the range (1.71–9.66) × 1020 cm−3. For relatively low Zn concentration the electron concentra-tion measured using Hall effect is close to [FO] but lower than [VO] by two orders of magnitude. The resultssuggested neutral oxygen vacancies. The overall results showed that tin is in tetravalent oxidation state in thewhole range of studied Zn concentrations. All films under investigation show high transparency in the visiblerange (T ≥ 82%). In addition, the optical transmittance shows a tail in the near IR region due to free carrier absorp-tion. The optical energy gap of the FZTO films falls in the range 3.86 eV–4.45 eV and exhibits a UV shift with theincrease in free carrier concentration due to the Burstein-Moss effect.

© 2017 Elsevier B.V. All rights reserved.

Keywords:Thin filmsSpray pyrolysisZinc tin oxideFluorine-doped zinc tin oxidePoint defectsBurstein-Moss effect

1. Introduction

Due to their combinedhigh optical transparency in the visible regionand their good electrical conductivity, transparent conducting oxides(TCO) have numerous technological applications such as transparentelectrodes in photovoltaic cells, liquid crystal displays, light emitting di-odes, heat reflecting mirrors and gas sensors [1–3]. In the last decades,different metal oxide semiconductors like SnO2, ZnO, In2O3 and TiO2have been extensively used as TCO thin films [4,5]. Doping these oxideswith specific elements like F, Al, B and Cl could increase the electricalconductivity whilemaintaining the high optical transparency in the vis-ible range [6,7]. Therefore they could be more attractive for many opto-electronic applications. Among the different transparent conductingoxides, zinc tin oxide (ZTO) and fluorine doped zinc tin oxide (FZTO)films are promising candidates for many applications. They have lowelectrical resistivity, high optical transmittance, good thermal stability,high mechanical strength and low processing cost [8,9].

Several techniques have been used for preparing ZTOfilms includingpulsed laser deposition [10], sputtering [11], atomic laser deposition

fila Technical University, Tafila,

University, Zarqa, Jordan.

[12] and spray pyrolysis [13,14] among others. The latter technique isknown to be simple, reproducible, cheap, and adaptable to large-scaleproduction. Yet only few reports were devoted to study spray depositedZTO films and even fewer were dedicated to their electrical and opticalproperties [13,14]. Moreover, the question of physicochemistry of pointdefects in ZTO and FZTO films has not been investigated previously. In arecent work, El Akkad et al. [15,16] have obtained experimentalevidence that oxygen vacancies are neutral in SnO2 films at room tem-perature. Their results are in good agreement with recent theoreticalpredictions [17], but in contrary to what had long been believed to bethe case [18]. Additionally, detailed analysis of the optical propertiesrevealed the presence of optical transitions involving un-identified de-fects in F:SnO2 thin films [16]. This calls for a close investigation of therole of point defects and their influence on the physical properties of hy-brid systems involving SnO2 such as ZTO. RadheshyamRai [19] reportedthat doping SnO2 with transition metal oxides influences dramaticallythe defect chemistry behavior of SnO2. In addition, they found that thesubstitution of tin ions by zinc ions create more oxygen vacancies.Concerning FZTO films, only three reports were found in the literaturethat is devoted to study this TCO. Pandey et al. [9] investigated the effectof annealing temperature on the structural, electrical and optical perfor-mance of amorphous FZTO thin films prepared by radio-frequencymag-netron sputtering technique. Jun-Hyuck et al. [20] described in detailsthe preparation procedure of FZTO from aqueous solution and provideda brief description of their properties. Park et al. [21] studied the

http://crossmark.crossref.org/dialog/?doi=10.1016/j.tsf.2017.02.021&domain=pdfhttp://dx.doi.org/10.1016/j.tsf.2017.02.021mailto:b.salameh@ku.edu.kwJournal logohttp://dx.doi.org/10.1016/j.tsf.2017.02.021http://www.sciencedirect.com/science/journal/00406090www.elsevier.com/locate/tsf

Table 1The crystallite size (D) calculated from the XRDmeasurements and the atomic percentageof C, Sn, Zn, F, O for the investigated ZTO and FZTO films obtained from the XPS measure-ments at an etching time of 60 s.

Sample D (nm) C Sn Zn F O

TO 20 36.1 18.5 0.0 0.00 45.4ZTO1 15 73.1 4.6 1.9 0.00 20.5ZTO2 – 35.0 19.7 8.9 0.00 36.4ZTO3 – 75.1 6.5 3.1 0.00 15.4FTO 20 39.0 18.1 0.0 0.70 42.2FZTO1 26 46.8 16.0 0.9 0.37 35.9FZTO2 28 21.2 25.0 3.1 0.31 50.4FZTO3 30 66.5 9.7 1.5 0.53 21.8FZTO4 – 71.7 4.2 2.3 0.29 21.6

Table 2Concentration of Zn ([Zn]), substitutionalfluorine ([F]) and electrons (n). Carriersmobility(μ) and, optical energy gap (Eg), of the investigated ZTO and FZTO films.

Sample [Zn] [F] n μ Eg

(at.%) (at.%) (cm−3) (cm−3) (cm2/Vs) (eV)

TO 0 0.00 0.00 7.21 × 1019 6.3 3.97ZTO1 28.8 0.00 0.00 3.13 × 1017 28 3.77ZTO2 31.1 0.00 0.00 1.12 × 1017 32 3.63ZTO3 32.3 0.00 0.00 9.84 × 1016 35 3.53FTO 0.0 1.93 5.35 × 1020 5.62 × 1020 14 4.42FZTO1 5.5 1.15 3.20 × 1020 3.69 × 1020 7.0 4.40FZTO2 11.1 0.62 1.71 × 1020 1.60 × 1020 8.0 4.45FZTO3 13.4 2.74 7.59 × 1020 4.25 × 1019 23 4.13FZTO4 35.6 3.49 9.66 × 1020 9.90 × 1018 6.0 3.86

77B. Salameh et al. / Thin Solid Films 626 (2017) 76–84

electrochemical characteristics of the FZTO films prepared usingMOCVD. Yet, no investigationswere reported on the physical propertiesof spray deposited FZTO films.

In this paper, a comprehensive study of the structural, electrical, op-tical and chemical properties of ZTO and FZTO films prepared by thespray pyrolysis method is introduced. The overall results are correlatedto the preparation conditions in an attempt to throw more light on therole of point defects and to provide information that may help in opti-mizing the properties of this transparent conducting oxide for differentdevice applications.

2. Experimental procedures

ZTO and FZTOfilmswere prepared by chemical spray pyrolysis tech-nique. The starting solution was amixture of SnCl4: propanol: H2Owithmolar ratio 1:9:2. Solutions containing up to 50% Zn were prepared byadding the desired amount of Zn in the form of ZnCl2. Doping with Fwas accomplished by adding the required amount of F in the form ofNH4F to the spray solution. The thin films were prepared by sprayingthe solution on well cleaned borosilicate glass substrates which weremaintained at temperatures in the range of 430–500 °C using IR heaterand temperature controller. Sprayingwas performed using N2 as carriergaswith a pressure of 2.5 psi in pulses of duration ~1 s and time interval~40 s in order to allow the substrate to be reheated to the preparationtemperature. The distance between the spray bottle and the substratewas maintained at approximately 25 cm.

The structural properties of ZTO and FZTO films were carried outusing X-ray diffractometer type Siemens D5000 with Cu Kα radiation(λ = 1.5406 Å) with Bragg-Brentano geometry. The compositionalanalysis of the films were carried out using X-ray photoelectron spec-troscopy (XPS) model Thermo ESCALAB 250Xi spectrometer usingmonochromator with Al Kα radiation (1486.6 eV) with X-ray spot size380 μm. The spectral acquisition and processing were carried outusing Avantage V 4.74 data system. The parameters used in the XPSanalyses are: Analysis chamber pressure 10−9 Torr, step size 0.1 eV,dwell time 100 ms, and pass energy of 20 eV. All binding energy (BE)values were determined using the C 1s peak at 284.6 eV which origi-nates from adventitious carbon as the binding energy calibration refer-ence. Etching was performed using an argon ion gun with voltage of2 kV, current of 2 μA, and raster size of 2 mm2.

Room temperature electrical resistivity and Hall measurementswere carried out using the Van Der Pau method in MMR technologiestype system. For this, four Al contacts each of area 2 mm2 and thickness50 nm were deposited on the sample surface by thermal evaporation.Leads to the external circuit were made by soldering gold wires to theAl contacts using indium. Currents in the range 0.5–5.0 mA and a mag-netic field of 0.3 Tesla were used.

Transmission spectra were recorded in the wavelength range from200 to 2500 nm using a double beam spectrophotometer type Cary5000UV–Vis-NIR and a Shimadzu Solid Spec-3700UV- Vis-NIR Spectro-photometer where a borosilicate glass substrate was used as areference.

3. Results and analysis

The fluorine and zinc concentrations in the starting solution werevaried in a wide range. The concentration of the fluorine and zinc inthe thin films were determined by analyzing the XPS spectra as willbe discussed in Section 3.2. The investigated thin films were dividedinto four categories TO, ZTO, FTO and FZTO, and they were labeled asgiven in Tables 1 and 2.

3.1. Structural characterization

The crystal structure of ZTO and FZTO filmswith variable concentra-tions of Zn and F was investigated using XRD measurements. Fig. 1(a)

shows the XRD spectra for ZTO films with Zn concentration in therange 0–31 at.%. The spectra show that below 31 at.% the films possessa single phase polycrystalline behavior with tetragonal rutile structure.The crystallites in the TO films exhibit mixed preferential orientationalong the (110) and (200) planes. Upon increasing the zinc concentra-tion, the intensity of the (110) peak decreases, while that of the (200)peak increases. This indicates a change in the preferential orientationof the grains. It is also observed that the crystalline quality decreasesby increasing Zn concentration. These results agree with the observa-tions reported on ZTO films prepared by spray pyrolysis [13,22].

Fig. 1(b) shows theXRD spectra for the FZTOfilms. The spectra showthat the films with low Zn concentration (0–13 at.%) possess a singlephase polycrystalline feature with tetragonal rutile structure. The zincfree film (FTO) exhibits mixed preferential orientation of crystallitesalong the (110) and (200) planes similar to the case of undoped TOfilms (Fig. 1(a)). Upon increasing the zinc concentration, a change inthe preferential orientation occurs which is associated with the emer-gence of the (211) and (301) peaks. This is to be comparedwith the ori-entation along the (200) plane in absence of fluorine (i.e. in ZTO films).Therefore, it seems that the presence of fluorine in ZTO films has a rolein determining the orientation of the crystallites. This may be due to thelocal distortion of the lattice associated with the incorporation of F intooxygen sites.

It is also noticed that by increasing Zn concentration a decrease inthe crystalline quality occurs. It appears from Fig. 1(a) and (b) thatZTO and FZTO films have amorphous structure for Zn concentrationabove about 30%.

The XRD peaks were shifted toward lower Bragg angles after addingF or Zn to tin oxide indicating an expansion of the lattice. An example ofthis shift is shown in Fig. 2(a) for the (200) peak. Similar shift in the XRDpeaks after F-doping has been reported previously for TO [15]. The ex-pansion of the lattice following F doping cannot be attributed to the re-placement of the host oxygen atoms by fluorine since fluorine has ionicradius (1.33 Å) which is smaller than that of oxygen ion (1.4 Å). Previ-ous investigations have shown that non-intentionally doped TO thinfilms grown by chemical spray pyrolysis technique contains high

Fig. 1. XRD spectra for (a) ZTO films with different Zn concentration and (b) F doped ZTOfilms with different Zn concentration.

Fig. 2. The 200 peak (a) in TO, FTO and ZTO films and (b) in FZTO films with similar F anddifferent Zn concentrations. (For interpretation of the references to color in this figure, thereader is referred to the web version of this article.)

Fig. 3. Full-range XPS survey spectrum for F doped ZTO thin films, the main peaks areindicated.

78 B. Salameh et al. / Thin Solid Films 626 (2017) 76–84

concentration of oxygen vacancies (V0) [1]. Similar results will be pre-sented in Section 4 for our films. Therefore, the observed lattice expan-sion is likely due to the incorporation of fluorine into oxygen vacanciesleading to the formation of substitutional fluorine defects (F0). On theother hand, the lattice expansion observed after the addition of Zn toTO (Fig. 2(a)) can be attributed to the substitution of Sn+4 by Zn+2.These ions have ionic radii of 0.71 Å and 0.74 Å, respectively so thatthe lattice is expected to expand slightly by the addition of Zn in TOfilms.

A significant shift in the XRDpeaks toward lower Bragg angles is alsoobserved in FZTO films with the increase in the fluorine concentration.Fig. 2(b) shows the (200) peak for samples FZTO2 and FZTO3 wherebythe fluorine concentration varies by 342% while the Zn concentrationvaries by only 21%. The peak shift toward lower Bragg angles impliesan expansion of the lattice which can be attributed to the samemecha-nism proposed for the case of FTO films (insertion of fluorine into oxy-gen vacancies).

The average crystallite size (D) was calculated using the Scherrerformula [23]

D ¼ 0:9λβ cosθ

ð1Þ

where λ is the wavelength of radiation (1.5406 Å), β is the full width athalfmaximumof theXRDpeak and θ is the peakposition. The calculatedvalues of the crystallite size range between 15 nm and 30 nm and arelisted in Table 1.

3.2. X-ray photoelectron spectroscopy results

Fig. 3 shows the full-range XPS survey spectrum of a representativeFZTO film. The only detected peaks correspond to O, Sn, C, Zn and F. TheZTO films show the same spectrum without the fluorine peak. The

79B. Salameh et al. / Thin Solid Films 626 (2017) 76–84

existence of adventitious carbon which is observed in all investigatedfilms is due to contamination from the environment during the deposi-tion of the films. The observation of contaminated carbon was reportedby many other researchers [15,24]. After etching, the carbon signal de-creases significantly and the other signals remain clear. The elementalXPS core level spectra were analyzed for all samples under investiga-tion. Fig. 4 shows the high resolution C 1s XPS signal which can bedeconvoluted into two Gaussian peaks: CA peak at 284.55 eV and CBpeak at 287.20 eV. These peaks are assigned to hydrocarbon groups(C\\C and C\\H bonds) and to C\\O bonds, respectively. Similar peakswere reported by other research groups at the same binding energies[15,25,26].

Typical high resolution XPS core level spectra of F 1s, Sn 3d, Zn 2pand O 1s of FZTO thin film are shown in Fig. 5. The F 1s signal(Fig. 5(a)) can be deconvoluted into two Gaussian peaks: the principalpeak (FA) located at 684.78 eV and the auxiliary peak (FB) located at688.14 eV. All investigated films show this double-peak feature. Theprincipal peak (FA) was observed by many research groups in FTO[15,16,25,27] and in FZTO [9] films prepared by different techniquesand it was assigned to Sn\\F bond. The intensity of this peak has beenshown to be proportional to the amount of NH4F in the spray solution.Consequently, it was used to calculate the concentration of substitution-al fluorine in the film [15,16]. On the other hand, the chemical stateresulting in the appearance of FB peak is unknown but naturally itmust be attributed to another fluoride phase. Similar fluorine FB peakwas observed previously in FTO films prepared by spray pyrolysis [15].

As shown in Fig. 5(b), the Sn spectrum consists of two distinctivespin-orbit peaks 3d5/2–3d3/5 located at 486.69 eV and 495.13 eV, respec-tively. These values agree well with the values reported in the literaturefor Sn\\O bond inwhich Sn is in a tetravalent-oxidation state as in SnO2[9,15,28]. Furthermore, no shift in the position of the Sn peaks was ob-served by changing the Zn concentration, which indicates a tetravalentoxidation state of Sn for the entire range of investigated Zn concentra-tions. A confirmation of this conclusion will be presented inSection 4.1. Fig. 5(c) shows the two spin orbit peaks of Zn (2p3/2 - 2p1/2) that are located at 1020.93 eV and 1044.01 eV, respectively. The ener-gy difference between Zn 2p3/2 and Zn 2p1/2 due to spin-orbit splitting isabout 23.08 eV which is very close to the standard value for ZnO [29–31]. This shows that the oxidation state of the Zn is mainly Zn2+.

The O 1s signal (Fig. 5(d)) can be deconvoluted into two Gaussianpeaks: (OA) located at 530.3 eV and (OB) located at 531.03 eV. Thebinding energy of the peak OA lies within the range of previously re-ported values for the Zn\\O and Sn\\O bonds in ZnO [32,33] and

Fig. 4. High resolution XPS spectra and deconvoluted components for C 1s peak. (Forinterpretation of the references to color in this figure, the reader is referred to the webversion of this article.)

SnO2 [34,35], respectively. The OB peak can be assigned to the O\\Cbond [15,36].

Fig. 6 shows the depth profiles for Zn and F obtained using argon ionetching on ZTO, FTO and FZTO thin films. In ZTO films, Zn shows an ini-tial decrease in its concentration with depth until an etching time ofabout 30 s. Beyond this time, a very little change occurs. Upon addingfluorine to form FZTO, the profile of Zn becomes almost flat as shownin Fig. 6(a). This implies that doping ZTO with fluorine stabilizes theZn depth profile. In Fig. 6(b) it is shown that the F concentration inFTO films fluctuate until an etching time of 60 s then it increase slightlybeyond this time. Upon adding Zn to form FZTO, the profile of F becomesalmostflat beyond10 s. This also implies that adding Zn to FTO stabilizesthe fluorine depth profile. Since the coexistence of Zn and F in FZTOfilms stabilizes the profile of both elements, this indicates that there isa mutual interaction between these two elements in the FZTO lattice.Complex defects involving both elements can possibly be created. Thesimplest example, is the center (Zn2+,F−) which acts as a singleacceptor.

The atomic percentage of C, Sn, Zn, F and O for the investigated thinfilms obtained from the analysis of the XPS spectra is given in Table 1.The concentrations of zinc and fluorine in the samples are calculatedusing the following equations: [Zn] = Zn/cation (at.%); wherecation = Sn + Zn. For fluorine, [F] = F/2Sn (at.%) (see Section 4.2).

3.3. Electrical properties

The Van der Pauwmethod was used for the room temperature Halleffect and the electrical conductivity measurements of the investigatedZTO and FZTO films. The obtained parameters (carrier concentration(n) and electron mobility (μ)) are summarized in Table 2. All filmsshowed n-type conductivity. The results show an increase in the elec-tron concentration by doping ZTO films by fluorine. For example, com-paring sample ZTO3 with sample FZTO4 which have almost the sameZn content (~ 34 ± 0.17 at.%) shows that by adding about 3.5 at.% fluo-rine the electron concentration increased by two orders of magnitude.An increase in the electron concentration is also observed upon dopingTO by fluorine (Table 2). This increase in the electron concentration inboth cases is attributed to the formation of substitutional fluorine de-fects (F0) as mentioned in Section 3.1.

On the other hand, we observed a decrease in the carrier con-centration by two orders of magnitude by adding about 28 at.% Znto TO (samples TO and ZTO1: Table 2). Similar behavior is also ob-served for FZTO samples. The carrier concentration decreasessharply by the addition of 13 at.% Zn or more to FTO. The substitu-tion of the tetravalent Sn+4 by the divalent Zn+2 ions creates ZnSnacceptor defects which cause a reduction in electron concentrationby compensation.

The carrier mobility is plotted as a function of carrier concentrationfor the ZTO and FZTO films in Fig. 7. The figure shows a gradual decreasein the mobility with increasing the carrier concentration in agreementwith theoretical predictions [27,37,38]. The dominant scattering mech-anism controlling the room temperature mobility in TO and ZTO filmshas been reported to be due to ionized impurities for electron concen-tration in the range 1020 cm−3 or higher [27,37]. For lower carrier con-centration other scattering mechanisms, mainly grain-boundaryscattering, have a pronounced contribution [39]. Themobility is expect-ed to decrease with increasing the carrier concentration due to thesecombined scattering mechanisms [38].

3.4. Optical properties

The optical transmission spectra in the wavelength range200–2500 nm of the investigated films is shown in Fig. 8. All filmsshow high transparency in the visible range with an average opticaltransmittance higher than 82%. A sharp drop in the transmittance atthe fundamental absorption edge is observed at around 300 nm for all

Fig. 5.High resolutionXPS spectra and deconvoluted components for FZTOfilm showing (a) F 1s, (b) Sn 3d, (c) Zn2p and (d)O 1s peaks. (For interpretation of the references to color in thisfigure, the reader is referred to the web version of this article.)

80 B. Salameh et al. / Thin Solid Films 626 (2017) 76–84

investigated films. At higher wavelengths, interference peaks are ob-served in the wavelength range 650≤λ≤1200 nm followed by a tailforλ≥1400nmand n ≥ 4.25 × 1019 cm−3. This decrease in the transmis-sion is a characteristic phenomenon of free carrier absorption.

Fig. 6. Depth profile of zinc (a) and fluorine (b) in SnO2, ZTO and FZTO films.

The absorption coefficient (α) of the investigated filmswas calculat-ed using the relation [40]:

α ¼ 1�d ln 1�T� � ð2Þwhere d is the film thickness.

The classical formula of the absorption coefficient (αf) due to freecarriers absorption is given by [41]

α f ¼q2λ2

τm�8π2nrc3nð Þ ð3Þ

where τ is the relaxation time,m* is the effectivemass of the charge car-rier, nr is the refractive index and c is the speed of light.

Fig. 9 shows the absorption coefficient (α) as a function of the carrierconcentration (n) for four selected wavelengths in the free carrier's ab-sorption region. It is noticed that the relation is linear for all values of λas predicted by Eq. (3). This confirms that free carrier absorption isdominant in this region.

Fig. 7. Electron mobility versus electron concentration for the investigated ZTO and FZTOfilms.

Fig. 8. Optical transmittance spectra of (a) ZTO and (b) FZTO films. (For interpretation ofthe references to color in this figure, the reader is referred to the web version of thisarticle.)

Fig. 10.Dependenceof (αhν)2 on thephoton energy hν of (a) ZTO and (b) FZTOfilms. (Forinterpretation of the references to color in this figure, the reader is referred to the webversion of this article.)

81B. Salameh et al. / Thin Solid Films 626 (2017) 76–84

The energy band gap (Eg) was obtained using the following relation[42,43]:

αhνð Þm ¼ A hν−Eg� � ð4Þ

where m= 2 for direct optical transitions, A is a constant and hν is thephoton energy. Fig. 10 shows the variation of (αhν)2 versus hν for theinvestigated films. The linear relationship in the fundamental absorp-tion edge region indicates that the gap is direct for all investigatedfilms. The values of the energy gap obtained by extrapolation of thestraight lines to the hν axis are listed in Table 2. The values of Eg forthe ZTO and FZTO films are in the range 3.53–4.45 eV.

The dependence of the band gap on the Zn concentration in the filmis depicted in Fig. 11. There is a clear trend in the variation of the energygap with Zn concentration. The energy gap decreases with the increase

Fig. 9. The absorption coefficient (α) as a function of electron concentration (n) fordifferent wavelengths in the free carrier absorption region for FZTO films. (Forinterpretation of the references to color in this figure, the reader is referred to the webversion of this article.)

of the Zn content. This may be attributed to the reduced value of theband gap of ZnO (Eg = 3.2 eV) relative to that of SnO2 (Eg = 3.8 eV)or may be due to the formation of Urbach tails at high impurity concen-tration. A similar trend has also been observed for the ZTO films pre-pared by spray pyrolysis [13,22,44].

The optical energy gap of the FZTO films increased from 3.86 to4.45 eV, as the charge carrier concentration increased from 9.90×1018

to 1.60×1020 cm−3. This broadening of the optical energy gap can be at-tributed to Burstein-Moss (B-M) shift as a result of the penetration ofthe Fermi level into the conduction band [45]. The measured energy

Fig. 11. The optical energy gap dependence on the Zn concentration of the ZTO and FZTOfilms.

82 B. Salameh et al. / Thin Solid Films 626 (2017) 76–84

gap Eg in this case can be written as:

Eg ¼ E0g þ ΔEBM ð5Þ

where Eg0 is the intrinsic energy gap andΔEBM is the energy differencebetween the Fermi level and the conduction band edge. The magnitudeof ΔEBM is given by [46]

ΔEBM ¼ h2

8m�3nπ

� �2=3 ð6Þ

where m* is the carriers effective mass. The variation of the measuredenergy gap with n2/3 for the investigated FZTO films is shown inFig. 12. The relation is linear for relatively low electron concentrationsin good agreement with Eq. (6). This confirms that the B-M effect is re-sponsible for the observed UV shift in the band gap. By further increasein the carrier concentration, the energy gap deviates to lower values rel-ative to the B-M line (sample FZTO1) possibly due to the formation ofdensity of states tails at the bands edges as a result of perturbations inthe local potential associated with the random distribution of impuri-ties. These band tails lead to a large number of band-to-tail and tail-to-tail transitions causing narrowing of the optical energy gap [47].

4. Discussion

In this section, we evaluate the concentration of oxygen vacanciesand of substitutional fluorine (FO) and compare those with the electronconcentration measured using Hall effect in order to understand therole of point defects responsible for the optoelectronic properties inFZTO films.

4.1. Oxygen vacancies

The ternary ZTO system is assumed to have the chemical formulaZnSnOx for the entire range of Zn concentration. In order to calculate xone should take into account the presence of adventitious carbon inthe samples and the reaction of this carbon with oxygen [36]. The for-mation of C\\O bonds is clear from the observation of the XPS peaksCB and OB as shown in Figs. 4 and 5d, respectively. The presence ofsuchbondshas also been reported previously [15,25]. Therefore, oxygenis shared between Sn, Zn and C, and the total oxygen concentration Otcan be written as Ot = OSn + OZn + OC, where OSn, OZn and OC are,the oxygen concentrations involved in bonds with Sn, Zn and C, respec-tively. Based on the XPS results (Section 3.2), carbon in\\C\\O and zincin Zn\\O act as divalent ions while tin acts as a tetravalent ion as in

Fig. 12. Variation of the optical energy gap with n2/3 for the investigated FZTO thin film.The linear dependence at low carrier concentration agrees with Burstein-Moss effectand the reason for the disagreement at higher values of n is possibly due to theformation of density of states tails at the bands edges.

SnO2. Therefore, if Osn = x {Sn} then OZn = ½ × {Zn} and OC = ½ × [ε{C}], where {Sn}, {Zn} and {C} are the atomic percentage of Sn, Zn andC, respectively and ε is the fraction of C involved in oxygen bonds(0≤ε≤1). This leads to the equation:

x ¼ 2 Otf g2 Snf g þ Znf g þ ε Cf g ð7Þ

Based on the above equation and using the XPS data for the FZTOsamples (Table 1), xwas computed and the deviation from stoichiome-try δ = 2 − x was calculated as a function of the Zn content. Fig. 13shows a plot of δ vs [Zn] with ε as a variable. We used values of ε be-tween 0.2 and 0.4 since these values were deduced from the ratio be-tween the intensity of the CB peak (due to C\\O bond) and the totalintegrated intensity of the carbon XPS peak (see Fig. 4). It is clear fromFig. 13 that δ falls in the range between 0 and 1 which confirms thatSn is in the tetravalent oxidation state (Sn4+) as in SnO2 − x in thewhole range of the studied Zn concentrations. This conclusion supportsthe interpretation of the XPS results.

The oxygen vacancy concentration [VO] is calculated using [VO] ≈ δP, with P = 2.77 × 1022 cm−3 mol−1 [16]. Fig. 14 shows [VO] as a func-tion of Zn concentration in the FZTO films for ε in the range 0.2–0.4. Forcomparison, the figure depicts also the concentration of free carriers de-termined using Hall measurements. It is clear that the vacancy concen-tration falls in the range (1021–1022 cm−3) that is about two orders ofmagnitude higher than the free electron concentration (1018–1020 cm−3). We believe that this discrepancy is unlikely to be due tocompensation of electrically active VO donors since this would implythe presence of acceptors with concentration in the range 1021–1022 cm−3 in FTO. Instead, this result can be taken as an evidence forthe electrical neutrality of the oxygen vacancies in agreement with pre-vious reports [14,16]. The source of electrons in TO and ZTO is likely tobe another native defect such as interstitial tin or a more complex in-trinsic centers.

4.2. Substitutional fluorine

The electron concentration can be comparedwith the concentrationof the substitutional fluorine [FO] which is calculated from the XPS re-sults using the Eq.equation [FO] = FOSnP where F/OSn is the concentrationof fluorine in oxygen sites in units of atomic percent and P =2.77 × 1022 cm−3 mol−1. However since oxygen is a very chemically ac-tive element, its concentration measured by XPS involves bonds otherthan the Sn\\O bond. If these bonds are excluded, erroneous results inthe calculation of [FO] will be obtained. Consequently, the ratio F/OSn isreplaced by F/(2Sn) since {OSn} ≅ 2{Sn} in all samples (Section 4.2).

Fig. 13. The calculated deviation from stoichiometry (δ) as a function of the Znconcentration for the investigated ZTO and FZTO films. (For interpretation of thereferences to color in this figure, the reader is referred to the web version of this article.)

Fig. 14. (Color online) the concentration of oxygen vacancies [VO] and of free carriers as afunction of Zn concentration for the FZTO films. (For interpretation of the references tocolor in this figure, the reader is referred to the web version of this article.)

83B. Salameh et al. / Thin Solid Films 626 (2017) 76–84

The obtained values of [FO] are listed in Table 2. We found that the con-centration of substitutional fluorine is very close to that of free electronsfor relatively low concentration of Zn (below about 13 at.% Zn). Thisconfirms that the substitutional fluorine is the main source of electronsin FZTO samples and that the oxygen vacancies are electrically neutralas predicted theoretically by Singh et al. [17] and as suggested by the re-sults in Section 4.1. For Zn concentrations ≥ 13 at.%, themeasured valuesof the electron concentration deviate and fall below the FO concentra-tion (Table 2) which is suggestive of electrical compensation

It wasmentioned in Section 3.3 that for ZTO containing about ~34±0.17 at.% (samples ZTO3 and FZTO4), the addition of about ~3.5 at.% Fleads to a sharp increase in the electron concentration from~1017 cm−3 to ~1019 cm−3. However, 3.5 at.% F implies9.7 × 1020 cm−3 of substitutional fluorine donor concentration whichis about 10 times higher than the electron concentration (Table 2).This indicates that partial electrical compensation occurs also by F dop-ingwhich suggests that F acts as amphoteric impurity in ZTO. The abilityof F to form complex defects including acceptors besides its known roleas a donor in FTO has been reported by a number of investigators [16,27]. Particularly, the possibility of formation of complex acceptor de-fects involving F and Zn in FZTO has been proposed in Section 3.2.

5. Conclusions

The structural, chemical, optical and electrical properties of ZTO andFZTOfilmsprepared by spraypyrolysis technique are studied. XRDmea-surements showed that the films are polycrystalline with tetragonal ru-tile structure and grain size in the range (15–30) nm. The analysis of theXPS results taking into account the formation of oxygen bonds with Sn,Zn and adventitious carbon revealed the presence of oxygen vacancyconcentration about 100 times higher than the free electron concentra-tion in ZTO and FZTOfilms. This strongly suggests that oxygen vacanciesare electrically neutral in this type of films. The analysis also showedthat for Zn concentration up to 35 at.%, tin is in tetravalent oxidationstate and the chemical formula for ZTO films is ZnSnOx with x = 2 −δ where δ is a measure of the deviation from stoichiometry. On theother hand, the increase in the free electron concentration in FZTOfilms reduces the optical transmittance in the near IR range due tofree carrier absorption and produces a UV shift in the energy band gapdue to the Burstein-Moss effect. Optical energy gap values in the range3.53 eV–4.45 eV have been determined for ZTO and FZTO films.

Acknowledgments

This work was supported and funded by research administration ofKuwait University (Project No. SP04/14). We thank Mr. ManeeshMathai for his help in the electrical and optical measurements. We also

acknowledge the support of the general facility of the Faculty of Science(Projects GS 02/08, GS03/01 and GS01/10) and Semiconductor researchfacility, research administration (project GE01/08).

References

[1] P.D. Paulson, B.E. McCandless, R.W. Birkmire, Optical properties of Cd1 − xZnxTefilms in a device structure using variable angle spectroscopic ellipsometry, J. Appl.Phys. 95 (2004) 3010–3019.

[2] C.H. Yang, S.C. Lee, S.C. Chen, T.Ch. Lin, The effect of annealing treatment on micro-structure and properties of indium tin oxides films, Mater. Sci. Eng. B 129 (2006)154–160.

[3] B. Murali, M. Madhuri, S.B. Krupanidhi, Solution processed Cu2CoSnS4 thin films forphotovoltaic applications, Cryst. Growth Des. 14 (2014) 3685–3691.

[4] A. Slonopas, M. Melia, K. Xie, T. Globus, J.M. Fitz-Gerald, P. Norris, Factors limitingdoping efficiency of iridium in pulsed laser deposited TiO2 transparent conductingoxide, J. Mater. Sci. 51 (2016) 8995–9004.

[5] S.B. Qadri, H. Kim, H.R. Khan, A. Piqué, J.S. Horwitz, D. Chrisey, W.J. Kim, E.F. Skelton,Transparent conducting films of In2O3–ZrO2, SnO2–ZrO2 and ZnO–ZrO2, Thin SolidFilms 377 (2000) 750–754.

[6] J.C. Lee, E. Park, N.G. Subramaniam, J.E. Lee, J.W. Lee, J.C. Lee, T.W. Kang, Non-metallicelement (chlorine) doped zinc oxide grown by pulsed laser deposition for applica-tion in transparent electrode, Curr. Appl. Phys. 12 (2012) S80–S84.

[7] C.Y. Hsu, Y.C. Lin, L.M. Kao, Y.C. Lin, Effect of deposition parameters and annealingtemperature on the structure and properties of Al-doped ZnO thin films, Mater.Chem. Phys. 124 (2010) 330–335.

[8] J.H. Ko, I.H. Kim, D. Kim, K.S. Lee, T.S. Lee, B. Cheong, W.M. Kim, Transparent andconducting Zn-Sn-O thin films prepared by combinatorial approach, Appl. Surf.Sci. 253 (2007) 7398–7403.

[9] P. Pandey, S.H. Cho, D.K. Hwang, W.K. Choi, Structural and electrical properties offluorine-doped zinc tin oxide thin films prepared by radio-frequency magnetronsputtering, Curr. Appl. Phys. 14 (2014) 850–855.

[10] P. Görrn, F. Ghaffari, Th. Riedl, W. Kowalsky, Zinc tin oxide based driver for highlytransparent active matrix OLED displays, Solid State Electron. 53 (2009) 329–331.

[11] M.G. McDowell, R.J. Sanderson, I.G. Hill, Combinatorial study of zinc tin oxide thin-film transistors, Appl. Phys. Lett. 92 (2008) 013502.

[12] J. Lindahl, C. Hägglund, J.T. Wätjen, M. Edoff, T. Törndahl, The effect of substrate tem-perature on atomic layer deposited zinc tin oxide, Thin Solid Films 586 (2015)82–87.

[13] S. Vijayalakshmi, S. Venkataraj, M. Subramanian, R. Jayavel, Physical properties ofzinc doped tin oxide films prepared by spray pyrolysis technique, J. Phys. D. Appl.Phys. 41 (2008) 35505.

[14] A.I. Martinez, B.A. Garcia, D.R. Acosta, Properties of transparent zinc tin oxideconducting films prepared by chemical spray pyrolysis, 27th international confer-ence on the physics of semiconductors, AIP Conf. Proc. 772 (2005) 187–189.

[15] F. El Akkad, S. Joseph, Physicochemical characterization of point defects in fluorinedoped tin oxide films, J. Appl. Phys. 112 (2012) 23501.

[16] F. El Akkad, T.A. Paulose, Optical transitions and point defects in F:SnO2 films: Effectof annealing, Appl. Surf. Sci. 295 (2014) 8–17.

[17] A.K. Singh, A. Janotti, M. Scheffler, C.G. Van deWalle, Sources of electrical conductiv-ity in SnO2, Phys. Rev. Lett. 101 (2008) 055502.

[18] L. Chinnappa, K. Ravichandran, K. Saravanakumar, G. Muruganantham, B. Sakthivel,The combined effects of molar concentration of the precursor solution and fluorinedoping on the structural and electrical properties of tin oxide films, J. Mater. Sci.Mater. Electron. 22 (2011) 1827–1834.

[19] R. Rai, Study of structural and electrical properties of pure and Zn-Cu doped SnO2,Adv. Mater. Lett. 1 (2010) 55–58.

[20] J. Jun-Hyuck, Y.H. Hwang, B.S. Bae, Bias-temperature-illumination stability of aque-ous solution processed fluorine doped zinc tin oxide (ZTO:F) transistor,Electrochem. Solid-State Lett. 15 (2012) H123–H125.

[21] J.H. Park, D. Byun, J.K. Lee, Employment of fluorine doped zinc tin oxide (ZnSnOx:F)coating layer on stainless steel 316 for a bipolar plate for PEMFC, Mater. Chem. Phys.128 (2011) 39–43.

[22] V. Bilgin, S. Kose, F. Atay, I. Akyuz, The effect of Zn concentration on some physicalproperties of tin oxide films obtained by ultrasonic spray pyrolysis, Mater. Lett. 58(2004) 3686–3693.

[23] B.D. Cullity, Elements of X-ray Diffraction, second ed. Adison-Wesley, MA, 1978.[24] B. Thomas, B. Skariah, Spray deposited Mg-doped SnO2 thin film LPG sensor: XPS

and EDX analysis in relation to deposition temperature and doping, J. AlloysCompd. 625 (2015) 231–240.

[25] S. Wu, S. Yuan, L. Shi, Y. Zhao, J. Fang, Preparation, characterization and electricalproperties of fluorine-doped tin dioxide nanocrystals, J. Colloid Interface Sci. 346(2010) 12–16.

[26] J.H. Park, D.J. Byun, J.K. Lee, Electrical and optical properties of fluorine-doped tinoxide (SnOx:F) thin films deposited on PET by using ECR–MOCVD, J. Electroceram.23 (2009) 506–511.

[27] A.I. Martínez, L. Huerta, J.M. O-Rueda de Leon, D. Acósta, O. Malik, Physicochemicalcharacteristics of fluorine doped tin oxide films, J. Phys. D. Appl. Phys. 39 (2006)5091–5096.

[28] H.J. Ahn, H.C. Choi, K.W. Park, S.B. Kim, Y.E. Sung, Investigation of the structural andelectrochemical properties of size-controlled SnO2 nanoparticles, J. Phys. Chem. B108 (2004) 9815–9820.

[29] W. Li, L. Fang, H. Ruan, G. Qin, P. Zhang, H. Zhang, L. Ye, Ch. Kong, Oxygen vacanciesinduced ferromagnetism in Ag–N codoped ZnO thin films, Mater. Lett. 143 (2015)128–130.

http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0005http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0005http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0005http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0005http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0005http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0010http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0010http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0010http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0015http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0015http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0015http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0015http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0020http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0020http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0020http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0020http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0025http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0025http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0025http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0025http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0025http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0025http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0025http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0025http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0025http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0030http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0030http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0030http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0035http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0035http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0035http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0040http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0040http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0040http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0045http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0045http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0045http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0050http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0050http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0055http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0055http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0060http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0060http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0060http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0065http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0065http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0065http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0070http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0070http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0070http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0075http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0075http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0080http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0080http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0080http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0085http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0085http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0085http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0090http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0090http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0090http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0090http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0095http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0095http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0095http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0100http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0100http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0100http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0105http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0105http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0105http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0105http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0110http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0110http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0110http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0115http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0120http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0120http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0120http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0120http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0125http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0125http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0125http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0130http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0130http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0130http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0130http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0135http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0135http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0135http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0140http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0140http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0140http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0140http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0145http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0145http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0145

84 B. Salameh et al. / Thin Solid Films 626 (2017) 76–84

[30] V. Devi, M. Kumar, D.K. Shukla, R.J. Choudhary, D.M. Phase, P. Kumar, B.C. Joshi,Structural, optical and electronic structure studies of Al doped ZnO thin films,Superlattice. Microst. 83 (2015) 431–438.

[31] M. Shatnawi, A.M. Alsmadi, I. Bsoul, B. Salameh, G. Alnawashi, F. Al-Dweree, F. ElAkkad, Magnetic and optical properties of Co-doped ZnO nanocrystalline particles,J. Alloys Compd. 655 (2016) 244–252.

[32] T.P. Rao, M.C.S. Kumar, Resistivity stability of Ga doped ZnO thin films with heattreatment in air and oxygen atmospheres, J. Cryst. Process Technol. 2 (2) (2012)72–79.

[33] W.T. Yen, Y.C. Lin, P.C. Yao, J.H. Ke, Y.L. Chen, Effect of post-annealing on the opto-electronic properties of ZnO:Ga films prepared by pulsed direct current magnetronsputtering, Thin Solid Films 518 (2010) 3882–3885.

[34] D.H. Kim, J.H. Kwon, M. Kim, S.H. Hong, Structural characteristics of epitaxial SnO2films deposited on a- and m-cut sapphire by ALD, J. Cryst. Growth 322 (2011)33–37.

[35] W.S. Choi, The fabrication of tin oxide films by atomic layer deposition using tetrakis(ethylmethylamino) tin precursor, Trans. Electr. Electron. Mater. 10 (2009)200–202.

[36] S. Delpeux, F. Beguin, R. Benoit, R. Erre, N. Manolova, I. Rashkov, Fullerene core star-like polymers. Preparation from fullerenes and monoazidopolyethers, Eur. Polym. J.34 (1998) 905–915.

[37] C. Agashe, O. Kluth, J. Hüpkes, U. Zastrow, B. Rech, M.Wuttig, Efforts to improve car-rier mobility in radio frequency sputtered aluminum doped zinc oxide films, J. Appl.Phys. 95 (2004) 1911–1917.

[38] J.G. Lu, Z.Z. Ye, Y.J. Zeng, L.P. Zhu, L. Wang, J. Yuan, B.H. Zhao, Q.L. Liang, Structural,optical, and electrical properties of (Zn,Al)O films over a wide range of composi-tions, J. Appl. Phys. 100 (2006) 73714–73900.

[39] M. Chen, Z.L. Pei, X. Wang, Y.H. Yu, X.H. Liu, C. Sun, L.S. Wen, Intrinsic limit of elec-trical properties of transparent conductive oxide films, J. Phys. D. Appl. Phys. 33(2000) 2538–2548.

[40] J. Tauc, R. Grigorovici, A. Vancu, Optical properties and electronic structure of amor-phous germanium, Phys. Status Solidi B 15 (1966) 627–637.

[41] J.I. Pankove, Optical Processes in Semiconductors, Dover Publication, Mineola, NY,USA, 1975.

[42] X.F. Chen, G. He, M. Liu, J.W. Zhang, B. Deng, P.H. Wang, M. Zhang, J.G. Lv, Z.Q. Sun,Modulation of optical and electrical properties of sputtering-derived amorphousInGaZnO thin films by oxygen partial pressure, J. Alloys Comp. 615 (2014) 636–642.

[43] G. He, J. Gao, H. Chen, J. Cui, Z. Sun, X. Chen, Modulating the interface quality andelectrical properties of HfTiO/InGaAs gate stack by atomic-layer-deposition-derived Al2O3 passivation layer, ACS Appl. Mater. Interfaces 6 (2014) 22013–22025.

[44] K. Ravichandran, K. Thirumurugan, N.J. Begum, S. Snega, Investigation of p-typeSnO2:Zn films deposited using a simplified spray pyrolysis technique, Superlattice.Microst. 60 (2013) 327–335.

[45] E. Burstein, Anomalous optical absorption limit in InSb, Phys. Rev. 93 (1954)632–633.

[46] I. Hamberg, C.G. Granqvist, K.F. Berggren, B.E. Sernelius, L. Engström, Band-gap wid-ening in heavily Sn-doped In2O3, Phys. Rev. B 30 (1984) 3240–3249.

[47] J.W. Zhang, G. He, L. Zhou, H.S. Chen, X.S. Chen, X.F. Chen, B. Deng, J.G. Lv, Z.Q. Sun,Microstructure optimization and optical and interfacial properties modulation ofsputtering-derived HfO2 thin films by TiO2 incorporation, J. Alloys Comp. 611(2014) 253–259.

http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0150http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0150http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0150http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0155http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0155http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0155http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0160http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0160http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0160http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0165http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0165http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0165http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0170http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0170http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0170http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0175http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0175http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0175http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0180http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0180http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0180http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0185http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0185http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0185http://refhub.elsevier.com/S0040-6090(17)30110-4/rf2050http://refhub.elsevier.com/S0040-6090(17)30110-4/rf2050http://refhub.elsevier.com/S0040-6090(17)30110-4/rf2050http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0190http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0190http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0190http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0195http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0195http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0200http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0200http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0205http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0205http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0205http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0210http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0210http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0210http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0210http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0210http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0215http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0215http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0215http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0215http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0220http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0220http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0225http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0225http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0225http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0225http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0230http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0230http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0230http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0230http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0230http://refhub.elsevier.com/S0040-6090(17)30110-4/rf0230

Physicochemistry of point defects in fluorine doped zinc tin oxide thin films1. Introduction2. Experimental procedures3. Results and analysis3.1. Structural characterization3.2. X-ray photoelectron spectroscopy results3.3. Electrical properties3.4. Optical properties

4. Discussion4.1. Oxygen vacancies4.2. Substitutional fluorine

5. ConclusionsAcknowledgmentsReferences