Optoelectronic Properties of F-co-doped PTO Thin Films Deposited by Spray Pyrolysis

F-co-doped Palladium Tin Oxide (PTO) thin films were pyrolytically deposited on glass substrate at 450C using an alcoholic precursor solution consisting of Tin (IV) Chloride (SnCl4.5H2O), Palladium Chloride (PdCl2) and Ammonium Fluoride (NH4F). A resistivity of 0.3-6.9×10Ωcm was obtained in F-co-doped PTO films prepared with a Pd content of 3.68at% and F content of 0 – 23.96at% under optimized conditions. The optical properties were studied in the UV/VIS/NIR region. The optical bandgap of the films laid in the range 3.945 – 4.014 eV. Using dispersion analysis with Drude and Kim terms, optical constants were determined from spectro-photometric measurements for films on glass.


Introduction
Transparent conducting oxide (TCO) films produced using binary co mpounds such as SnO 2 , In 2 O 3 and ZnO doped with an impurity are in practical use [1]. Doped SnO 2 is used in a wide range of applicat ions which include the field of sensors, opacities, transparent electrodes in solar panels and other electrochro mic devices, thin film magnetic recording med ia and as material fo r Li-ion batteries [2,3]. However, these developments require improvements in electrical, optical and chemical properties of SnO 2 thin films used. New transparent conducting oxide films using ternary compounds such as Zn 2 SnO 4 , ZnSnO 3 , MgIn 2 O 4 , Zn 2 In 2 O 5 and In 4 Sn 3 O 12 have been reported [1]. In addition to the use of ternary compounds, mult ico mponent oxides such as ZnO-In 2 O 3 , In 2 O 3 -SnO 2 , Zn 2 In 2 O 5 -In 4 Sn 3 O 12 and MgIn 2 O 4 -Zn 2 In 2 O 5 were proposed [1]. The use of ternary and multico mponent oxides whose physical and chemical properties can be controlled by changing their chemical co mpositions, may be used to improve the properties of SnO 2 and its applications.
Sn O 2 film is h igh ly t ransparent in the v isib le reg ion, chemically inert, mechan ically hard and can resist h igh temp eratu res [4, 5 , 6 ] as it is o n ly attacked by h ot concentrated alkalis [3]. It belongs to a class of materials that co mb in es h ig h electrical co n d u ctiv ity with o ptical transp aren cy and th erefo re cons titutes an imp o rtant component for the optoelectronic applications [2]. The efficiency in these applications is usually improved by suitably doping the tin oxide for examp le, doping with Sb and F increases the conductivity of t in o xide [3,7,8,9]. Tin oxide is a crystalline solid with a tetragonal crystal lattice. It is a wide gap, non-stoichiometric semiconductor and behaves as a degenerate n-type semiconductor with a low resistivity ( Ωc m) [9]. It can exist in two structures belonging to an indirect band gap of about 2.6 eV [10] and direct band gap that ranges fro m 3.6 eV to 4.6 eV at room temperature [9,11]. Studies have shown that F doped SnO 2 films have higher electrical conductivity, optical transmission and infrared reflect ion than the tin oxide films prepared with other dopants [7] wh ile Pd doped SnO 2 films have high sensitivity to reducing gases when used as metal gas sensors [12].
Some of the most popular methods of depositing Tin o xide thin films include the follo wing: Plasma enhanced Chemical Vapour Deposition [2], Electron beam Evaporation [6], Reactive sputtering [13], Sol-gel [14], Electronspun [15], Inkjet printing [16] and Sp ray pyrolysis [4,5,7,12]. Of these methods the spray pyrolysis method represents the less expensive method since it can produce large area, high-quality and low cost thin films [7]. It is also suitable for substrates with complex geo metry and can be used for a variety of o xide materials [17]. A previous study [30] has shown that the co-doping of SnO 2 based ceramics with Sb 5+ and Zn 2+, noticeably enhances the properties of SnO 2 by combin ing the advantages of Sb 5+ which increases the electrical conductivity and Zn 2+ which greatly improves the ceramic density.
In the present study, we report on the deposition and the characterizat ion of single and co-doped SnO 2 thin films prepared by spray pyrolysis technique for optoelectronic applications. The aim of th is work is to g ive an insight on how fixed deposition conditions influence the optical and electrical properties of the produced films with a thickness of 132.5±10 n m. The optical characteristics (transmittance and reflectance) have been evaluated in the UV-Vis-NIR spectral range. In addition, calculat ions have been carried out in order to determine the carrier densities as well as the optical mobilit ies by using the Drude model.

Sample Preparation
The substrates used were ordinary float glass slides which were 1.2 mm thick and measuring 2.5 by 7.6 cm 2 . The cleaning procedure involved rubbing the glass slides gently on both sides using a cotton swab soaked in foam made fro m a mixture of deionized water, liquid detergent and sodium hydroxide in the rat io of 3:2:1. They were then drag wiped using a lens cleaning t issue held at an angle of 45 degrees before being wiped with Isopropyl alcohol and acetone respectively. Lastly, the substrates were ultrasonically cleaned in distilled water for 30 minutes before drying with spray of pressurized air. Spray pyrolysis technique was used to deposit the films at a substrate temperature of 450±10℃ using compressed air at 1 bar as the atomization gas. The experimental set up of the in-house-made spray pyrolysis system is as shown in Figure 1. It consisted of a fu me chamber, hot plate, spray nozzle of diameter ~1mm, input gas valve, gas compressor, gas flow meter, conduit tube, thermocouple and pressure gauge. SnO 2 films were produced from a precursor solution consisting of Tin (iv) chloride (99%) p repared by dissolving comp letely 6g of stannic chloride in 100ml of ethanol (99.9%) [3,5,6]. The PTO films were prepared by dissolving PdCl 2 (59-60%Pd) in ethanol (99.9%) then added to the spraying solution at a constant volume. For F:PTO films, varying volumes of a solution of 0.5g NH 4 F (99%) in d istilled water was added to the starting solution [16,18].
The spraying was done under the following conditions: • Substrate temperature: 450±10℃ • Carrier-gas: Co mpressed air at 1 bar • Solution flow rate: 6ml/ min • No zzle-to-substrate distance: 33cm horizontally 32+3cm vert ically After spraying, the films were left to cool with the hot plate before removal for transmittance and reflectance measurements.

Sample Characterizati on
Spectral normal t ransmittance T(λ) and near normal reflectance R(λ) were measured in the wavelength range 300 < λ < 2500n m, on a co mputerized double beam solid-spec 3700DUV Shimadzu Spectrophotometer equipped with 198851 Bariu m Su lphate (BaSO 4 ) integrating sphere. Bariu m Sulphate plate was used as a reference for the calculation of optical properties such as band gap, absorption coefficient and refractive index. The thickness of the as-deposited samples of F:PTO were estimated by fitt ing the experimental spectral data to theoretical spectral data based on Drude and Kim analysis using the SCOUT software [19] in the wavelength range 300n m -2500n m. Electrical resistivity of the films was calculated fro m t wo adjustable parameters of Drude: plasma frequency Ω p and damping constant γ.

Anal ysis of Optical Properties
In order to compare the transparency of PTO thin films with various F-doping levels, the optical spectra in the UV-VIS-NIR reg ion and near infrared reg ion of the samples were measured. The optical t ransparency of PTO films and F:PTO films for both experimental and computed spectral are shown in Figure 2.
The films exh ibited high transparency in the visible region with an average transmittance of 83%. Maximu m transmittance of 85% for 19.11at% F:PTO at wavelength of 540n m was recorded. The two weird peaks at about 900n m and 1600n m occur due to change of detectors since the SolidSpec-3700DUV spectrophotometer used in this study, is equipped with 3 detectors: a photomultip lier tube detector for the ultraviolet and visible reg ions, and InGaAs and PbS detectors for near in frared region. The photomult iplier tube detector can be switched to the InGaAs detector in the wavelength range from 700n m to 1000n m (the default switching wavelength is 870n m). The In GaAs detector can be switched to a PbS detector in the range from 1600n m to 1800n m (the default switching wavelength is 1650n m). The transparency at wavelengths of 355 -495n m had a maximu m of 81.7% for 7.30at% F:PTO which decreased with increasing F doping concentration to 64.2% for 23.96 at % F:PTO. Th is is due to the increase in fundamental absorption as photon striking increases with increase in carrier concentration [20]. The maximu m transmittance observed in the 19.11at% F:PTO films may be attributed to a decrease in diffuse and multip le reflections caused by the increase in g rain size and a reduction in light-scattering effect [21]. The film with the lowest resistivity had the lowest transmission, especially in the near infrared. A sharp fall in transmission at about 310 n m is due to the absorption of the glass substrate [14].
The optical constants are reported in Figure 3 showing spectral refractive index n(λ) and extinct ion coefficient k(λ) for PTO film and F:PTO films. Dispersion analysis using a model for the dielectric susceptibility of the film consisting of Drude [22] and Kim terms [23] was used to model the measured reflectance and transmittance. The charge carriers set free by the donors or acceptors can be accelerated by very litt le energies and hence do respond to applied electric fields with frequencies in the infrared region [19]. Drude is a free electron contribution which describes the intraband contributions to the optical properties. This The one oscillator contribution developed by Kim contains four adjustable parameters: TO Ω resonance frequency, p Ω oscillator strength, τ Ω damp ing constant and Gauss-Lorentz-switch constant σ . σ may vary between zero and infinity. For σ = 0, a Gaussian line shape is achieved. A large value of σ (larger than 5) leads to a Lorentzian line shape. The Kim oscillator models the weak broad interband absorption in the measured wavelength range. The interband dielectric susceptibility described by Kim is given by [20]; Model parameters were determined fro m the best fit between computed and experimental data using Scout software [19]. The best fit gives us directly the optical constants of the film under study. Figure 4 shows the experimental spectra and the fitted simulated spectra of 7.30at% F:PTO. The thicknesses of the five samples obtained from fitting the experimental spectral data to theoretical spectral data based on dispersion analysis using the SCOUT software in the wavelength range 0.3 -2.5 μm was found to be 132.5 ± 10 n m. This was verified by the thicknesses obtained from SEM cross sections. The thicknesses obtained fro m the two different methods agree within a discrepancy of not more than 5%.
In Figure 3, the refractive index of PTO was found to be around 1.98 at 500 n m. It was also observed that the refract ive index of all the films decreases with wavelength and then attains almost a constant value towards higher wavelengths [6].    Figure 5 shows spectral absorption coefficient (α) for PTO and F:PTO with four fluorine doping levels. Fo r all the films, the absorption edge lies in the UV region and varies with the charge concentration. The samples show a slight decrease in the absorption coefficient for α < 10 4 cm -1 for λ > 1000 n m for all samples apart fro m 13.61at% F:PTO film wh ich tends to increase as shown in Figure 5(i). The dependence on α on hν region is shown in Figure 5(ii). It is clear that the value of α increases with increasing photon energy in the range 0.5 e V -1.3 eV but decreases for 13.61at% F:PTO film. This may be due to variation in fluorine concentration in the films.
The optical bandgap, E g , was determined using the standard formu la [22,23]:  Figure 5. Absorption coefficient (α) against wavelength (i) and Absorption coefficient (α) against Energy of PT O film and F:PTO films prepared at 450±10 0 C Figure 6 shows plots of (αhν)2 versus photo energy, hν, in the h igh absorption region. Ext rapolation of the curve to hν = 0 gave the direct band gap of PTO and F:PTO films in the range 3.945 eV -4.014 eV. The bandgaps for 7.30at% Where α=2 k/λ is the absorption coefficient, hν, the photon energy, and n=1/ 2 accounts for the fact that the d irectly allo wed transitions across the bandgap are expected to dominate F:PTO and 13.61at% F:PTO films are found to be wider than that of PTO, 19.11at% F:PTO and 23.96at% F:PTO. The increase in the energy gap is correlated with the Moss-Burstein effect and many body effects since the absorption edge of the films shifts to shorter wavelength [1]. It shows that Eg , for the transparent thin film, increases with the carrier concentration. There is a good agreement with literature where the values are varying between 3.9 and 4.5 eV depending on the dopants and preparation method [26][27][28][29]. This indicates an optimu m level of Fluorine doping on the PTO film, causes a widening effect on the band gap and is attributed to be due to the gradual increase in carrier concentration and mobility [20,21].   [17,24]. This is due to the decrease in the number of charge carriers for h igher F doping concentrations.

Anal ysis of Electrical Properties
Electrical properties were analy zed by use of Drude model basing on the results fro m the spectrophotometer. The polarization P induced by an externally applied electric field E, of the probing light wave in a ho mogeneous material is given by the electric susceptibility χ; P=εoχE (5) The dielectric function ε connects the dielectric displacement and the electric field vector and is closely related to the susceptibility χ; D=εoεE (6) where ε= 1+χ (7) The frequency dependence of the susceptibility is very characteristic for a material since it incorporates vibrations of the electronic system and the atomic cores as well as contributions fro m free charge carriers. The doping of semiconductors leads to free charge carriers wh ich can be investigated by IR spectroscopy. The response of the free carriers to oscillat ing electric fields can be described to a good approximat ion by the Drude model. The parameters of that model relate the concentration of the charge carriers and their mobility to properties of the dielectric function. After a model parameter fit of the simulated spectrum to measured data the carrier concentration and the mobility or resistivity can be computed [19]. The Drude model relates the macroscopic susceptibility to the microscopic quantities carrier concentration n and mobility μ; (8) and (9) where e is the elementary charge(1.6×10 -19 C), εo is the permittiv ity of the free space (8.854×10 -12 As/Vm) and m the effective mass of the charge carriers (m=0.3m o ) [25] and is the mass of an electron. Resistivity can also be computed using the formula [16]; (10) Figure 8 shows resistivity and carrier concentration as a function of the F content for PTO and F:PTO films with F content of 0 -23.96at%. The distribution of resistivity is mainly related to that of carrier concentration. In particu lar, the resistivity markedly increased as the F content was increased fro m 0at% and reached a maximu m at a F content of 7.30at%. It then decreased rapidly as the F content was further increased, reaching a minimu m at a F content of 13.61at%.

Figure 8. (i) Resistivity versus F Concentration (ii) Carrier concentration versus F Concentration
The carrier concentration in itially decreased as the F content was increased to 7.30 at%. It is well known that a free electron acting as a carrier in SnO 2 films is caused by two kinds of donors: a native defect such as an oxygen vacancy and a Fion on a substitutional site of an O 2ion [17,18]. Therefore, the drastic decrease of carrier concentration obtained in F:PTO films prepared with F content of 7.30 at % is possibly related to the following two origins: oxidation enhancement of films and carrier compensation due to doped Pd acting as an acceptor. The former may be caused by the spraying process being done in a fume chamber where o xygen is uncontrolled. Consequently, the decrease of carrier concentration may be caused by either extinction of o xygen vacancies or free electron trapping due to the adsorption of oxygen on the grain boundary and/or the surface of films. The latter, carrier co mpensation due to doped Pd acting as an acceptor, may be exp lained by the doping of Pd into SnO 2 which may cause carrier compensation when a Pd 2+ ion occupies the site of an SnO 4+ ion. Although the decrease of carrier concentration with increases of F content up to 7.30at% can be explained by a mechanis m where Pd acts as an acceptor, it may be difficu lt to apply this explanation for higher fluorine doping concentrations. In addition, if Pd acts as an acceptor, it is difficult to account for the increase of carrier concentration and its high value observed for films prepared with F contents above 7.30at%, as shown in Figure 7. We suggest that this observed increased carrier concentration with a high value is related to the format ion of ternary co mpounds, for example, F 2 SnO co mposed of 2F and Sn O [18] and PdSnO co mposed of Pd and SnO [12]. It has been reported that F: SnO 2 films prepared by spray pyrolysis exhib it a high carrier concentration [20]. As described above, the drastic increase of resistivity may be exp lained by both oxygen enhancement and Pd acceptor mechanis ms.

Conclusions
Highly transparent and conductive F-co-doped PTO thin films (1.4~2.67×10 2 Ω -1 cm -1 ) were prepared with a fluorine concentration of 0 -23.96at% by spray pyrolysis technique fro m Sn Cl 4 p recursor solution. The resistivity of the co-doped films init ially increased fro m 5.14×10 -2 Ωcm with increase in fluorine (7.3at% F:PTO) to attain a maximu m value of 6.92×10 -2 Ωcm and d rastically decreased to 3.74×10 -3 Ωcm fo r higher level doping (13.61at% F:PTO) before gradually increasing again with increase in fluorine concentration. The resistivity achieved for the films doped with 13.61at% F:PTO is the lowest for these films fro m SnCl 4 precursor. Initial codoping of PTO with Fluorine leads to the widening of the bandgap (3.969 -4.014 eV ) which decreases on further increase of fluorine concentration to 3.945 eV. This is due to the increase in dopant concentration. The average transmittance of the F:PTO films in the visible region was 83%. Increase in fluorine concentration also leads to decrease in transmittance in the visible wavelengths with high transmittance of the 19.11at% F:PTO found to be 88%.