- Email: [email protected]
Combined influence of fluorine doping and vacuum annealing on the electrical properties of ZnO:Ta films
Accepted Manuscript Title: Combined influence of fluorine doping and vacuum annealing on the electrical properties of ZnO: Ta films Authors: K. Subha, K. Ravichandran, S. Sriram PII: DOI: Reference:
S0169-4332(17)30606-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.233 APSUSC 35342
To appear in:
APSUSC
Received date: Revised date: Accepted date:
29-11-2016 2-1-2017 27-2-2017
Please cite this article as: K.Subha, K.Ravichandran, S.Sriram, Combined influence of fluorine doping and vacuum annealing on the electrical properties of ZnO: Ta films, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.02.233 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Full Length Article Combined influence of fluorine doping and vacuum annealing on the electrical properties of ZnO: Ta films K. Subha1, 2, K. Ravichandran1* and S. Sriram3 1
Materials Science Research Laboratory, PG and Research Department of Physics, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur-613 503, Tamil Nadu, India. 2
Research Department of Physics, Kunthavai Naachiyaar Govt. Arts College for Women (Autonomous), Thanjavur – 613 007, Tamil Nadu, India. 3
School of Electrical and Electronics Engineering, SASTRA University, Thanjavur – 613 401, Tamil Nadu, India.
*Corresponding author: Dr. K. Ravichandran, Associate Professor of Physics, Materials Science Research Laboratory, PG and Research Department of Physics, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur-613 503, Tamil Nadu, India. Mobile: (+91) 94435 24180 Fax : +91 4374 239328 Land line: 04362 278602 E. mail: [email protected] [email protected]
Highlights
First report on combined effect of F doping and annealing on resistivity of ZnO:Ta films Various possible incorporation mechanisms of tantalum and fluorine are addressed. Theoretical validation of Ta and F incorporation by DFT analysis is done. Quality factor comparable with those obtained by physical methods is achieved.
Abstract In this study, our main focus is to investigate the effects of F doping and post deposition annealing (air and vacuum) on the optical and electrical characteristics of tantalum doped zinc oxide films (ZnO:Ta). A cost-effective, automated jet nebulizer spray pyrolysis technique is adopted to deposit the ZnO:Ta:F films. The doping level of Ta is kept constant (1 at.%) and that of F is varied from 5 to 20 at.% in steps of 5 at.%. The electrical resistivity of the as-deposited films decreases for 10 at.% of F concentration. The resistance increases thereafter. The same trend is also observed in annealed films. The reasons for these variations are addressed based on the effective F incorporation into the ZnO lattice and annealing atmosphere with the help of XRD, FESEM, AFM and PL studies. The incorporation of the dopants was confirmed from XPS and EDX analysis and the DFT studies show that the incorporation of the dopants does not affect the stability of the ZnO lattice. Vacuum-annealed films show better electrical properties over the as-deposited and air-annealed counterparts, though their transparency is affected marginally. A minimum resistivity of 0.81× 10-3 Ω cm and an enhanced quality factor of 2.265×10-4 (Ω/sq)-1 are achieved for the vacuum-annealed films having Ta+F doping levels as 1+10 at.%. These results make this sample a desirable candidate for transparent electrode applications. ________________________________________________________________________________________________________________
Keywords: ZnO thin films; Annealing; Electrical transport; Transmittance; Figure of merit;
1. Introduction
The electrical resistivity and optical transparency are the chief selection criteria for transparent conducting oxide films to be used as absorber or window layers for the fabrication of solar cells. Among those materials which exhibit both these properties simultaneously, indium tin oxide (ITO) places itself at the top spot and hence it is widely used for these transparent electrode applications. But toxicity, source scarcity and the cost of indium urge the researchers to search for a desirable alternative [1-5].At this juncture, ZnO, the non toxic, abundantly available and most importantly inexpensive material can be considered as one of the most suitable alternatives to ITO films [6-8]. Even though ZnO film has good transparency, its electrical resistivity is not low enough to meet the requirements of a good transparent electrode [9]. To make ZnO films a desirable alternative to the most commonly used indium based thin films, the electrical resistivity of ZnO thin film has to be reduced further. As several researchers in the recent past have showed that ZnO is a tailor- friendly material, a renewed interest is kindled in tuning their optical and electrical properties to make them fit for various applications. Its transport properties can be modified in many ways which includes, doping with appropriate cationic or/and anionic elements, adopting different deposition techniques, depositing over layers, annealing at different ambiences and varying the process parameters like film thickness, doping concentration and substrate temperature [10-14]. Of them, doping with a suitable element is proven to be the most effective way, by which the electrical and optical properties can be modified to a greater extent [15]. Most importantly, according to the literature reports, the resistivity of ZnO thin films can be reduced considerably by appropriate doping [16-18]. At times, doping with a single element is insufficient to achieve a desired low resistance. Therefore, in the past few decades, for improving the electrical properties of ZnO thin films, researches have adopted simultaneous or co-doping tactic, expecting combined or even synergistic effects of the dopants used [19-21]. In this work, keeping this point in mind, we have doped fluorine (an anionic dopant) along with tantalum (a cationic dopant) as a follow-up work of our previous study in which we have reported the effect of the doping concentration of Ta on the characteristics of ZnO films [22].
Fluorine has been recognized as a suitable anionic impurity which improves the electrical properties of ZnO [23-24]. The comparable ionic radius of F- ion (131 pm) with that of O2- ion (138 pm), increases the chances of substitutional incorporation over the other types of incorporations viz. interstitials and anti-sites [25]. Interestingly, it has been accepted as one of the best dopants used to enhance the transparency as well as the conductivity of ZnO films [26]. Moreover, in the present study, using the DFT calculations, we found that, the stability of ZnO system is improved when F is doped with ZnO. As annealing is one of the best ways to improve the quality of TCO films [20], in the present work, the deposited films are annealed at two different ambiences (air and vacuum) and the transparent conducting properties of these films are compared with those of as-deposited films. It is worth mentioning here that reports on the effect of post annealing on the electrical resistivity and transmittance of ZnO:Ta:F thin films is seldom available in the literture. In this juncture, Ta + F doped ZnO thin films at different fluorine concentrations are deposited using nebulizer spray pyrolysis technique and the changes in the properties of annealed films are studied. 2. Materials and methods The automated jet nebulizer spray technique elaborated in our previous study [13] was employed to deposit tantalum and fluorine doped zinc oxide (ZnO:Ta:F) thin films onto borosilicate glass substrates. The source materials used and the process parameters maintained to deposit the four sets of films having different Ta+F doping levels are presented in Table 1. For the confirmation of the reproducibility of the films, the experiment was repeated many times keeping the same deposition parameters. The analytical techniques used for the characterization of the deposited samples are given in Table 2. 3.1 Various possible incorporation mechanisms of Ta and F in ZnO lattice The doping process often causes changes in the crystal lattice and its defects which often affect most of the properties of the host material [20]. Hence, by the better understanding of the related doping mechanism, one can prepare TCO thin films like ZnO-based films with required properties. To facilitate the understanding, various possible incorporation mechanisms regarding the simultaneous doping of Ta and F with ZnO are presented in Table 3 and illustrated using a schematic diagram (Fig. 1.)
3.1 Electrical studies Fig. 2 a-c shows the variation in electrical parameters obtained for the as-deposited, and annealed (air and vacuum) ZnO:Ta:F films with respect to Ta+F doping level. The resistivity of the as-deposited film with minimum F concentration (5 at. %) is 0.81× 10-3 Ω cm (Table 4). When the F doping concentration in the starting solution increases, the resistivity of the film initially decreases up to 10 at.% and then increases for higher doping levels [Fig. 2a]. The two main reasons for the observed decrease in resistivity are (i) the increase in carrier concentration and (ii) the increase in carrier mobility. When F is doped with ZnO:Ta, each of the F-ions that substitutes an O2-ion results in an extra free carrier (electron) generation, as only one electron is needed for the Zn-F bond formation. Hence, the increase in F concentration in the starting solution, results in an increase in the free carriers in the ZnO lattice up to a certain doping level which is 10 at.% for the deposition conditions employed in the present work. It is well established that 10 at.% is the optimum F doping level to obtain low electrical resistivity values, as reported in previous studies [27-28]. When the doping level of F increased, the mobility increases as shown in Fig. 2a. This improved mobility is attributed to the increase in grain size caused by the increase in F doping concentration as evident from the FESEM images. As the size of the grain increases the grain boundary increases, which in turn leads to a reduced scattering [14, 22]. The substitution of Fmainly disturbs the valance band and makes the conduction band fairly free from scattering [29]. Thus the combined effects of increased free carrier concentration and the enhanced mobility caused by F doping leads to a remarkable reduction in the resistivity of Ta doped ZnO films. The increase in the resistivity beyond this level (10 at. %) of F doping concentration may be due to the excess number of anionic dopants present in the ZnO matrix. These excess F- ions probably occupy the interstitial or anti sites in the lattice and thereby causes an adverse effect, in general [22, 30]. It is learnt that the excess F- ions can occupy also at the grain boundaries [31], leading to carrier scattering and the potential barrier thus formed at the grain boundaries plays a significant role in reducing the carrier mobility. Moreover, the fluorine ions at interstitial position can attract free electrons (charge trapping sites) due/attributed to its high electronegativity. Consequently, there is a decrease in carrier concentration and mobility which results in increased resistivity [32].
When the films are air-annealed, all the samples exhibit relatively decreased carrier concentration and a resultant increased resistivity (by 2 orders) compared to their as-deposited counterparts (Fig. 2b). This result can be interpreted on the basis of the following two different effects of chemisorbed oxygen on the free carriers of the system. (i) During air annealing, chemisorption of oxygen can take place (at the existing vacant sites of oxygen and also sites from which F is released during air annealing). Subsequently these chemisorbed oxygen ions induce the formation of trapping states at the grain boundaries, which can trap the electrons and make them immobile, resulting in the reduction of number of free carriers. (ii) The electrically charged traps create potential barriers, which restrict the carrier movement from one crystallite to another, thereby reduce their mobility [33]. We believe that, these two factors cause a drastic increase in the resistivity. Therefore, in spite of the increase in the grain size (as seen from FESEM images) which should improve the mobility, the resistivity of air-annealed films increases. The result clearly suggests that the decrease in carrier concentration caused by the oxygen adsorption which takes a predominant role over the increase in mobility may be the reason for this increase in electrical resistance. In contrast to the air annealing process, desorption of oxygen and substituted fluorine takes place in vacuum annealing [34]. This phenomenon leads to the creation of increased number of oxygen vacancies which in turn results in an enhancement of carrier concentration and a consequent reduction in resistivity (Fig. 2c). It is important to mention here that, as fluorine is more volatile than oxygen, desorption of fluorine can take place easily, creating a large number of oxygen vacancies. Moreover, the growth of larger crystallites and grains during vacuum annealing helps in improving the intra-grain mobility of the charge carriers and thereby reduces the electrical resistivity further [35]. The enhancement in the size of the crystallites and grains is confirmed using the XRD and FESEM/AFM results, (section 3.3 and 3.4) respectively. These combined effects of fluorine doping and vacuum annealing enhance the electrical properties of ZnO: Ta films to a greater extent. The resistivities of vacuum-annealed films are compared with those of their as-deposited counterparts using bar diagrams in Fig. 3. It is seen from the figure that, the lowest resistivity value (0.81× 10-3 Ω cm) is recorded for the vacuum-annealed film with Ta+F doping level as 1+10 at. %. This vacuum annealing induced reduction in the resistivity is illustrated as bar diagram in Fig. 3. This minimum resistivity value obtained in the present study is found to be
comparable or even superior when compared with the related results obtained using chemical deposition techniques by various researchers as seen from Table 5 [36-53]. 3.2 Optical studies The transmittance spectra of the films are shown in Fig 4a-c. From the Fig. 4a, it is observed that, all the as-deposited films are highly transparent in the visible region. The average visible transmittance is nearly 91% for minimum fluorine doping concentration (5 at.%). When we increase the doping level, a slight decrease in the visible transmittance and a notable decrease in IR transmittance are seen from the Fig 4a. This observed decrease in the IR transmittance is a strong evidence for the improvement in the carrier concentration of the corresponding films. It is a known fact that an increase in carrier density leads to a decrease in IR transmittance due to scattering 54]. When the films are annealed under air ambient, the variation in visible transmittance with respect to doping level is found to be in the same trend as that of the as-deposited films, whereas, a different trend is observed in the IR transmittance (Fig. 4b). The transmittance at IR range increases remarkably for all the films indicating the decrease in carrier concentration. This decrease in carrier concentration caused by the chemisorptions of oxygen from the air is discussed in detail in the electrical studies (section 3.1). At the same time, the vacuum-annealed films exhibit (Fig. 4c) slightly diminished transmittance value in visible range which may be due to the metal richness in the films [55]. This metal richness is caused by the oxygen desorption occurred during vacuum annealing. The noticeable decrease in transmittance in the infra-red range may be attributed to an increased IR scattering as discussed in the as-deposited case. This result is consistent with the resistivity obtained for these set of films. Moreover, the decrease in transmittance spectra can be correlated with the increase in surface roughness, as evidenced from FESEM observation [20]. The optical band gap (Eg) values are obtained from the plots drawn between dT/dλ and energy (Fig. 5a–c) and are given in Table 6. From Table 6, it is found that, in all the three cases, the Eg increases for 10 at.% and decreases for further doping. The Moss–Burstein theory can explain this increase in Eg [56]. This result on band gap is agreed well with the discussion on electrical parameters (section 3.1).
To find the optimum concentration of F for obtaining good quality TCO films, the quality factor (figure of merit) was calculated (Haacke formula ɸ = T10/Rsh [57]) and presented in Table 6. It is seen from Table 6 that, in each of the three sets of films for all the three cases, the quality factor is the highest for the film with F doping level 10 at. %. Among all the three sets of films, the vacuum-annealed film with 10 at. % of F doping level exhibits the highest figure of merit as it has the lowest resistivity and good transmittance. Thus, it can be concluded that 10 at. % of F doping level is the optimum value for preparing good quality spayed ZnO:Ta;F films.. 3.3 Structural studies The XRD profiles of the samples are shown in Fig. 6 a-c. The hexagonal wurtzite structure of the prepared films is verified by comparing the obtained XRD profile with the related JCPDS Card no. 36-1451. The peaks appearing at the 2θ values 31.7 ̊, 34.4 ̊ and 36.2 ̊ are associated with the lattice planes (100), (002) and (101), respectively. Absence of any secondary peaks reveals the purity of all the films. It is obvious from the Fig. 6a that, the intensity of all the peaks increases when F doping level is increased. This may be due to the possible filling of oxygen vacancies already exist in the ZnO lattice by the F- ions which leads to an improvement in the periodicity of the lattice. This result is reflected in the calculated values (using Scherrer’s formula of the crystallite size (Table 6) [58]. This trend continues up to 15 at.%. When the doping level is increased further, the intensity of all the peaks deceases. This may be due to the large number of F- ions incorporated into the ZnO matrix which is beyond the solubility limit of F in the ZnO system. The peak intensities of the annealed films are higher than those of their as-deposited counterparts as seen from the Fig. 6 b and c [59]. Among the three sets of films, the vacuumannealed films have the highest intensity suggesting that, the vacuum annealing enhances the quality of the films to a greater extent. FESEM images (section 3.4.1) also support the above discussion. 3.4 Surface morphological studies and elemental analysis 3.4.1 FESEM, XPS and EDX analysis with mapping The FESEM images of all the samples are shown in Fig. 7 a-l. When the F concentration is minimum (5 at. %), as-deposited film’s surface has spherical grains having nearly uniform size
(approximately 25 nm) as shown Fig. 7a. When the doping concentration of F increases, the grains retain their shape, however, the size of the grains increases marginally [53, 58]. When the film samples are air-annealed, smaller spherical grains are agglomerated together to form linear larger grains of nearly uniform size [42, 46] attributed to Ostwald ripening. It is a spontaneous thermo-dynamical process by which larger grains are formed at the cost of smaller ones. The formation of larger grains makes the system more stable because of the fact that the larger grains have lesser surface energy [60, 61]. The average size of the linear agglomerated grains is found to be increased with F doping as seen from the Fig. 7 e-h. This change is seemed to be appreciable in the case of vacuum-annealed films (Fig. 7 i-l). This increase in grain size may be reason for the improved mobility of the annealed films. The composition of elements of the samples are studied using EDX spectroscopy and as a representative image, the one recorded for the film with Ta + F doping levels as 1+10 at.% is shown in Fig. 8. The peaks at 1.8 and 3.8 keV represent the elements silicon and calcium which are generally present in the substrate. It is observed from the Fig. 8 that, the elements zinc, oxygen, tantalum and fluorine are found on the surface of the film. This result confirms the successful incorporation of both the dopants (Ta and F). The atomic and weight percentages of Zn, O, Ta and F is shown in the table given as inset of Fig. 8. It is observed from the inset table that, the atomic percentage of fluorine in the film is remarkably less than that in the starting solution. This is because of the high volatile nature of the fluorine compounds. Mapping is an effective tool to explore about the positions as well as the proportions of the constituent elements on the surface of the film under study. The images shown in Fig. 9 depict the information about the anticipated elements in the deposited film. The chemical states of the elements in ZnO:Ta:F film are elucidated by XPS analysis. Fig.10 shows the XPS survey spectrum of the ZnO:Ta:F sample. Calibration of the energy scale is done with reference to the peak of carbon (C1s at 284.60 eV). The strong peaks correspond to 2p state of Zn and 1s state of O, as well as the weak chemical signals represent 4f state of Ta and 1s state of F were observed, which revealed the existence of the elements zinc, oxygen, tantalum and fluorine on the surface of the film.
3.4.2 Atomic force microscopy study
The 3D images of the optimized films (10 at.% of F) are obtained using atomic force microscopy (AFM) with scan area 10 x 10 µm and 2.5x 2.5 µm are shown in Fig. 10 a. the corresponding 2D images are presented in Fig. 10b. From the images it is inferred that, annealing causes slight changes in the surface morphology of the samples. On the surface of the air-annealed film, few bigger grains can be seen here and there, whereas, the film annealed under vacuum ambient shows bigger grains almost all over the surface. The annealing induced change in the grain size reveals that the crystallinity is enhanced by annealing (Fig. 10b). The color scale bar supports the above discussion. This result reflects the observations of the XRD study (section 3.3). In general, during the film growth, depending on the deposition parameters employed, uneven nucleation takes place which results in irregularities on the film surface. These irregularities generally create traps, and scattering sites on the surface. Surface roughness representing these finest irregularities is an important parameter which strongly affects the electrical transport properties and optical quality of a film. Surface roughness is a measure of the vertical deviation of surface points from a reference plane and it is commonly represented as root mean square deviation (RMS). It shows the average deviation of each of the grain hillock or pits from the reference plane. The surface roughness of the films (having 1+10 at.% of Ta+F doping) are found to be 0.0156, 2.647 and 6.83 nm, respectively. The RMS roughness increases due to annealing which may be owing to the different reorientations of the grains caused by the annealing treatment. This result is agreed well with the FESEM results. The RMS roughness values obtained in the present work are remarkably low when compared with the reported values [46, 48, 62-64]. This result suggest that these films are suitable for solar cell applications, as the photovoltaic layers are normally very thin, high roughness value may result in local shunts and hence low roughness is desirable for these applications [65]. 3.5 Photoluminescence studies The room temperature PL measurements of all the deposited films were carried out to study the crystalline quality of the films as well as the presence of defects. The photoluminescence spectra of the films (Ta+F doing concentration is 1+10 at.%) are shown in Fig. 11. The PL spectra of the samples exhibit the same set of peaks and the peak positions have not been altered by the increased concentration of F and also due to annealing.
There are many prominent peaks in the figure which are found around 390, 445, 490 and 525 nm. The weak UV emission peak around 390 nm corresponds to near band edge emission [66]. The blue emission peak around 445 nm is attributed to the zinc interstitial (Zni) defect [67]. The emission peaks at 490 nm and 525 nm represent the singly and doubly ionized oxygen vacancies, respectively [15, 68]. Furthermore, an appreciable blue shift of NBE peak maxima is observed for vacuumannealed films whereas, a slight red shift is observed for air-annealed films. The blue shift is an evident for the widening of Eg [69]. This result is a supplementary evidence for the vacuum annealing induced increase in the band gap already observed from the optical transmission data. Table 7 recapitulates the annealing induced changes in the energy gap of the Ta+F doped ZnO films (1+10 at.% ). The annealing induced changes observed in Eg from optical and PL analysis are agreed well with each other. The intensity of the peak associated with oxygen vacancy (525 nm) is found to be increased remarkably after vacuum annealing confirming the oxygen desorption during annealing under vacuum. However during air annealing, peak intensity is found to be less compared with the as-deposited film which should be due to the adsorption of oxygen from the air- atmosphere during air annealing. 3.6 Analysis of stability of the Ta+F doped ZnO structures using DFT studies 3.6.1 Computational Details Structures of ZnO, Ta doped ZnO and Ta+F doped ZnO with different combinations were constructed and optimized through SIESTA package [70]. The exchange correlation effect is estimated using GGA (Generalized Gradient Approximation) with PBE (Perdew-BurkeErnzerhof) parameterization [71]. In SIESTA, the core electron potentials are replaced by effective Troullier-Martins pseudo-potentials in fully separable form. The double zeta plus valance polarization basis sets are chosen for calculations. The 4s and 3d orbitals of Zn, 2p and 2s orbitals of O, 4f, 5d and 6s orbitals of Ta and 2p orbitals of F are considered as valance orbitals. A 444 Monkhorst-Pack k points grid were used for Brillouine zone sampling. The minimization energy approach based structures of undoped and Ta + F doped ZnO are used for geometrical optimization through conjugate gradient (GA) algorithm. The optimized structures are shown in Fig. 13.
3.6.2 Analysis The calculated total energy variations for ZnO-Ta, ZnO-F and ZnO-Ta- F structures with reference to the ZnO structure obtained through structure optimization are given in Table 8. From the Table, it is seen that, the total energy of the system is increased only marginally when Ta is incorporated into the ZnO lattice suggesting that, the structure formation is not disturbed much. Upon the simultaneous addition of Ta and F atoms, the system exhibits total energy which is closer to that of ZnO structure. This is true for both the cases: (i) when Ta and F are placed in adjacent site and (ii) F is placed far away from Ta. Thus the simultaneous doping of Ta+F leads to higher stability than Ta doping. These observations make the present study a fruitful attempt to obtain a stable ZnO:Ta:F structure with desirable transparent conducting properties.
4. Conclusion This study on the ZnO:Ta:F films deposited with different Ta+F doping levels (1+5, 1+10, 1+15 and 1+20 at.%) and annealed at two different ambiences (air and vacuum) clearly shows that, the sample having Ta+F doping levels 1+10 at.% and annealed under vacuum exhibits minimum resistivity of 0.81×10-3 Ω cm due to the combined effect of fluorine doping and vacuum annealing. Even though vacuum annealing causes a marginal decrease in transmittance, this film possesses quality factor of 2.265×10-4 (Ω/sq)-1 which is superior over all the other samples prepared in this study. From the results, we can conclude that 1 and 10 at.% are the optimum doping concentration limits for Ta and F, respectively to obtain nebulizer sprayed ZnO:Ta:F films with desirable transparent conducting properties. It is also found that, all the vacuum annealed films exhibit reduced resistivity compared with their as-deposited and air-annealed counterparts irrespective of their Ta+F doping levels. Acknowledgment: The authors wish to thank the Council for Scientific and Industrial Research, New Delhi, India (No. 03(1321)/14/EMR-I) and the University Grants Commission, New Delhi, India (MRP-5688/15(SERO/UGC) for the financial support.
References
[1] S. F. Tseng, W. T. Hsiao, K. C. Huang, D. Chiang, M. F. Chen, C. P. Chou, Laser scribing of indium tin oxide (ITO) thin films deposited on various substrates for touch panels, Appl. Surf. Sci. 257 (2010) 1487. [2] Y. T. Park, A. Y. Ham, Y. H. Yang, J. C. Grunlan, Fully organic ITO replacement through acid doping of double-walled carbon nanotube thin film assemblies, RSC Adv. 1 (2011) 662. [3] K.A. Sierros, N.J. Morris, K. Ramji, D.R. Cairns, Stress-corrosion cracking of indium tin oxide coated polyethylene terephthalate for flexible optoelectronic devices, Thin Solid Films 517 (2009) 2590-2595. [4] Z. Chen, B. Cotterell, W. Wang, The fracture of brittle thin films on compliant substrates in flexible displays, Eng. Fract. Mech. 69 (2002) 597-603. [5] K. C. Lai, C. C. Liu, C. h. Lu, C.H. Yeh, M. P. Houng, Characterization of ZnO:Ga transparent contact electrodes for microcrystalline silicon thin film solar cells, Sol. Energy Mater. Sol. Cells 94 (2010) 397–401. [6] K. Saravanan, R. Krishnan, S. H. Hsieh, H. T. Wang, Y. F. Wang, W. F. Pong, K. Asokan, D. K. Avasthi, D. Kanjilal, Effect of defects and film thickness on the optical properties of ZnO–Au hybrid films, RSC Adv. 5 (2015) 40813. [7] C. Cachoncinlle, C. Hebert, J. Perriere, M. Nistor, A. Petit, E. Millon, Random lasing of ZnO thin films grown by pulsed-laser deposition, Appl. Surf. Sci. 336 (2015) 103. [8] R. Mimouni, K. Boubaker, M. Amlouk, Investigation of structural and optical properties in Cobalt–Chromium co-doped ZnO thin films within the Lattice Compatibility Theory scope, J. Alloys Compd. 624 (2015) 189. [9] E. Bacaksiz, S. Aksu, S. Yılmaz, M. Parlak, M. Altunbas, Structural, optical and electrical properties of Al-doped ZnO microrods prepared by spray pyrolysis, Thin Solid Films 518 (2010) 4076–4080. [10] X. Zhang, L. Zhu, H. Xu, L. Chen, Y. Guo, Z. Ye, Highly transparent conductive Fdoped ZnO films in wide range of visible and near infrared wavelength deposited on polycarbonate substrate, J. Alloys Compd. 614 (2014) 71–74.
[11] R. Mariappan, V. Ponnuswamy, P. Suresh, Effect of doping concentration on the structural and optical properties of pure and tin doped zinc oxide thin films by nebulizer spray pyrolysis (NSP) technique, Superlattices Microstruct. 52 (2012) 500–513. [12] R. Pandey, S. Yuldashev, H. D. Nguyen, H. C. Jeon, T. W. Kang, Fabrication of aluminium doped zinc oxide (AZO) transparent conductive oxide by ultrasonic spray pyrolysis, Curr. Appl. Phys. 12 (2012) S56-S58. [13] N. S. Kumar, K. V. Bangera, C. Anandan, G.K. Shivakumar, Properties of ZnO:Bi thin films prepared by spray pyrolysis technique, J. Alloys Compd. 578 (2013) 613–619. [14] K. Ravichandran, K. Subha, A. Manivasaham, G. Sridhar, T. Arun, Fabrication of a novel low-cost triple layer system (TaZO/Ag/TaZO) with an enhanced quality factor for transparent electrode applications, RSC Adv. 6 (2016) 63314 - 63324. [15] T. P. Rao, M.C. S. Kumar, N. S. Hussain, Effects of thickness and atmospheric annealing on structural, electrical and optical properties of GZO thin films by spray pyrolysis, J. Alloys Compd. 541 (2012) 495–504. [16] H. Liu, Y. Wang, J. Wu, G. Zhang, Y. Yan, Oxygen vacancy assisted multiferroic property of Cu doped ZnO films, Phys. Chem. Chem. Phys. 17 (2015) 9098-9105. [17] V. Gokulakrishnan, S. Parthiban, K. Jeganathan, K. Ramamurthi, Investigation on the effect of Zr doping in ZnO thin films by spray pyrolysis, Appl. Surf. Sci., 257 (2011) 9068. [18] Y. S. Avadhut, J. Weber, E. Hammarberg, C. Feldmann, J. S. auf der Gunne, Structural investigation of aluminium doped ZnO nanoparticles by solid-state NMR spectroscopy, Phys. Chem. Chem. Phys. 14 (2012) 11610–11625. [19] F. H. Wang, C. L. Chang, Effect of substrate temperature on transparent conducting Al and F co-doped ZnO thin films prepared by rf magnetron sputtering, Appl. Surf. Sci. 370 (2016) 83-91. [20] A. Hadri, M. Taibi, M. loghmarti, C. Nassiri, T. S. Tlemania, A. Mzerd, Development of transparent conductive indium and fluorine co-doped ZnO thin films: Effect of F concentration and post-annealing temperature, Thin Solid Films 601 (2016) 7-12. [21] A. Mallick, S. Sarkar, T. Ghosh, D. Basak, An insight into doping mechanism in Sn-F co-doped transparent conducting ZnO films by correlating structural, electrical and optical properties, J. Alloy. Compd. 646 (2015) 56–62.
[22] K. Ravichandran, K. Subha, N. Dineshbabu, A. Manivasaham, Enhancing the electrical parameters of ZnO films deposited using alow-cost chemical spray technique through Ta doping, J. Alloys Compd. 656 (2016) 332. [23] Z. Pan, P. Zhang, X. Tian, G. Cheng, Y. Xie, H. Zhang, X. Zeng, C. Xiao, G. Hu, Z. We, Properties of fluorine and tin co-doped ZnO thin films deposited by sol–gel method, J. Alloys Compd. 576 (2013) 31–37. [24] Y. Z. Tsai, N. F. Wang C. L. Tsai, Formation of F-doped ZnO transparent conductive films by sputtering of ZnF2, Mater. Lett. 63 (2009) 1621–1623. [25] M. L. Olvera, A. Maldonado, R. Asomoza, O. Solorza, D. R. Acosta, Characteristics of ZnO:F thin films obtained by chemical spray. Effect of the molarity and the doping concentration, Thin Solid Films 394 (2001) 241-48. [26] David S. Ginley and John D. Perkins, Transparent Conductors, in: David S. Ginley, Hideo Hosono, David C. Paine (Eds.), Handbook of Transparent Conductors, Springer, 2011, pp-7 [27] F. Yakuphanoglu, Y. Caglar, S. Ilican, M. Caglar, The effects of fluorine on the structural, surface morphology and optical properties of ZnO thin films, Physica B 394 (2007) 86-92. [28] A. Maldonado, A. G. Santiago, M. de la L. Olvera, R. C. Perez, G. T. Delgado, The role of the fluorine concentration and substrate temperature on the electrical, optical, morphological and structural properties of chemically sprayed ZnO:F thin films, Mater. Lett. 59 (2005) 1146-1151. [29] Y. Guo, L. Zhu, Y. Li, W. Niu, X. Zhang, Z. Ye, Interaction of H and F atoms - Origin of the high conductive stability of hydrogen-incorporated F-doped ZnO thin films, Thin Solid Films 589 (2015) 85-89. [30] B. N. Pawar, D. H. Ham, R. S. Mane, T. Ganesh, B. W. Cho, S. H. Han, Fluorine-doped zinc oxide transparent and conducting electrode by chemical spray synthesis, Appl. Surf. Sci. 254 (2008) 6294 - 6297. [31] S. S. Shinde, P. S. Shinde, S. M. Pawar, A. V. Moholkar, C. H. Bhosale, K. Y. Rajpure, Physical properties of transparent and conducting sprayed fluorine doped zinc oxide thin films, Solid State Sci. 10 (2008) 1209 – 1214.
[32] Y. Y. Bu, Sol–gel production of wrinkled fluorine doped zinc oxide through hydrofluride acid, Ceram. Int. 40 (2014) 14589–14594. [33] Y. R. Sui, Y.P. Song, Y.J. Wu, J.H. Lang, X.W. Meng, S.Q. Lv, B. Yao, J. H. Yang, Ceram. Int. 42 (2016) 15166-15170. [34] X. Yu, J. Ma, F. Ji, Y. Wang, C. Cheng, H. Ma, Thickness dependence of properties of ZnO:Ga films deposited by rf magnetron sputtering, Appl. Surf. Sci. 245 (2005) 310-315. [35] M. Caglar, S. Ruzgar, Influence of the deposition temperature on the physical properties of high electron mobility ZnO films by sol–gel process, J. Alloys Compd. 644 (2015) 101-105. [36] F. Aktaruzzaman, G. L. Sharma, L. K. Malhotra, Electrical, optical and annealing characteristics of ZnO:Al films prepared by spray pyrolysis, Thin Solid Films 198 (1991) 67- 74. [37] Y. Aoun, B. Benhaoua, B. Gasmi, S. Benramache, Effect of annealing temperature on structural, optical and electrical properties of zinc oxide (ZnO) thin films deposited by spray pyrolysis technique, Optik 126 (2015) 5407-5411. [38] J. H. Lee, B. O. Park, Characteristics of Al-doped ZnO thin films obtained by ultrasonic spray pyrolysis: effects of Al doping and an annealing treatment, Mater. Sci. Eng. B 106 (2004) 242–245. [39] H. Gomez, A. Maldonado, M. de la L. Olvera, D.R. Acosta, Gallium-doped ZnO thin films deposited by chemical spray, Sol. Energy Mater. Sol. Cells 87 (2005) 107–116. [40] A. El Manouni, F.J. Manjon, M. Perales, M. Mollar, B. Marı, M.C. Lopez, J. R. R. Barrado, Effect of thermal annealing on ZnO:Al thin films grown by spray pyrolysis, Superlattices Microstruct. 42 (2007) 134–139. [41] J. H. Lee, B. W. Yeo, B. O. Park, Effects of the annealing treatment on electrical and optical properties of ZnO transparent conduction films by ultrasonic spraying pyrolysis, Thin Solid Films 457 (2004) 333–337. [42] S. Benramache, B. Benhaoua, Influence of annealing temperature on structural and optical properties of ZnO: In thin films prepared by ultrasonic spray technique, Superlattices Microstruct. 52 (2012) 1062–1070.
[43] N. S. Kumar, K. V. Bangera, G. K. Shivakumar, Effect of annealing on the properties of Bi doped ZnO thin films grown by spray pyrolysis technique, Superlattices Microstruct. 75 (2014) 303–310. [44] K. Ravichandran, R. Mohan, N. J. Begum, S. Snega, K. Swaminathan, C. Ravidhas, B. Sakthivel, S. Varadharajaperumal, Impact of spray flux density and vacuum annealing on the transparent conducting properties of doubly doped (Sn + F) zinc oxide films deposited using a simplified spray technique, Vacuum 107 (2014) 68-76. [45] K. Ravichandran, R. Mohan, B. Sakthivel, S. Varadharajaperumal, P. Devendran, T. Alagesan, K. Pandian, Enhancing the photocatalytic efficiency of sprayed ZnO thin films through double doping (Sn + F) and annealing under different ambiences, Appl. Surf. Sci. 321 (2014) 310–317. [46] S. M. Lee, Y. H. Joo, C.Kim, Influences of film thickness and annealing temperature on properties of sol–gel derived ZnO–SnO2 nanocomposite thin film, Appl. Surf. Sci. 320 (2014) 494–501. [47] G. Li, X. Zhu, X. Tang, W. Song, Z. Yang, J. Dai, Y. Sun, X. Pan, S. Dai, Doping and annealing effects on ZnO:Cd thin films by sol–gel method, J. Alloys Compd. 509 (2011) 4816–4823. [48] X. Yu, X. Yu, Z. Hu, J. Zhang, G. Zhao, Y. Zhao, Effect of sol–gel derived ZnO annealing rate on light-trapping in inverted polymer solar cells, Mater. Lett. 108 (2013) 50–53. [49] I. Y. Y. Bu, Sol–gel production of ZnO:CO: Effect of post-annealing temperature on its optoelectronic properties, Mater. Sci. Semicond. Process. 41 (2016) 240–245. [50] C. Ravichandran, G. Srinivasan, C. Lennon, S. Sivananthan and J. Kumar, Influence of post-deposition annealing on the structural, optical and electrical properties of Li and Mg co-doped ZnO thin films deposited by sol–gel techniqueSuperlattices Microstruct. 49 (2011) 527–536. [51] X.L. Zhang , K.S. Hui, F. Bin, K.N. Hui, L. Lia, Y.R. Cho, R. S. Mane, W. Zhou, Effect of thermal annealing on the structural, electrical and optical properties of Al–Ni co-doped ZnO thin films prepared using a sol–gel method, Surf. Coat. Technol. 261 (2015) 149– 155.
[52] K. Ravichandran, P.V. Rajkumar, B. Sakthivel, K. Swaminathan, L. Chinnappa, Role of precursor material and annealing ambience on the physical properties of SILAR deposited ZnO films, Ceram. Int. 40 (2014) 12375–12382. [53] P.V. Rajkumar, K. Ravichandran, M. Baneto, C. Ravidhas, B. Sakthivel, N. Dineshbabu, Enhancement of optical and electrical properties of SILAR deposited ZnO thin films through fluorine doping and vacuum annealing for photovoltaic applications, Mater. Sci. Semicond. Process. 35 (2015) 189–196. [54] C. M. Muiva, T. S. Sathiaraj, K. Maabong, Effect of doping concentration on the properties of aluminium doped zinc oxide thin films prepared by spray pyrolysis for transparent electrode applications, Ceram. Int. 37 (2011) 555. [55] J. Kennedy, P. P. Murmu, J. Leveneur, A. Markwitz, J. Futter, Controlling preferred orientation and electrical conductivity of zinc oxide thin films by post growth annealing treatment, Appl. Surf. Sci. 367 (2016) 52-58. [56] X. L. Chen, J. M. Liu, J. Ni, Y. Zhao, X. d. Zhang, Wide-spectrum Mg and Ga co-doped ZnO TCO thin films for solar cells grown via magnetron sputtering with H2 introduction, Appl. Surf. Sci. 328 (2015) 193. [57] G. Haccke, New figure of merit for transparent conductors, J. Appl. Phys. 47 (1976) 4086 – 4089. [58] K. Ravichandrana, N. Dineshbabu, T. Arun, A. Manivasaham, E. Sindhuja, Synergistic effects of Mo and F doping on the quality factor of ZnO thin films prepared by a fully automated home-made nebulizer spray technique, Appl. Surf. Sci. 392 (2017) 624-633. [59] C. A. Gupta, S. Mangal, U. P. Singh, Impact of rapid thermal annealing on structural, optical and electrical properties of DC sputtered doped and co-doped ZnO thin film, Appl. Surf. Sci. 288 (2014) 411– 415. [60] T. Ghosh, P. Karmakar, B. Satpati, Electrochemical Ostwald Ripening and Surface Diffusion in Galvanic Displacement Reaction: Control over Particle Growth, RSC Adv., 5 (2015) 94380. [61] H. Wang, L. Xin, H. Wang, X. Yu, Y. Liu, X. Zhou, B. Li, Aggregation-induced growth of hexagonal ZnO hierarchical mesocrystals with interior space: nonaqueous synthesis, growth mechanism, and optical properties RSC Adv., 3 (2013) 6538.
[62] I. Burducea, C. Ionescu, M. Straticiuc, L. S. Craciun, P. M. Racolta, A. Jipa, Characterization Of Indium Nitride And Zinc Oxide Thin Films By AFM And RBS, Rom. Journ. Phys. 58 (2013) 345–353. [63] L. Znaidi, T. Touam, D. Vrel, N. Souded, S. B. Yahia, O. Brinza, A. Fischer, A.Boudrioua, ZnO Thin Films Synthesized by Sol–Gel Process for Photonic Applications, Acta Phys. Pol. A 121 (2012) 165-168. [64] F. K. Shan, G. X. Liu, B. C. Shin, W. J. Lee, Annealing E_ects of ZnO Thin Films Deposited on Si (100) by Using Pulsed Laser Deposition, J. Korean Phys. Soc. 54 (2009) 916-920. [65] G. Kenanakis, N. Katsarakis, E. Koudoumas, Influence of precursor type, deposition time and doping concentration on the morphological, electrical and optical properties of ZnO and ZnO:Al thin films grown by ultrasonic spray pyrolysis, Thin Solid Films 555 (2014) 62-67. [66] Y. Zhang, H. L. Lu, T. Wang, Q. H. Ren, H. Y. Chen, H. Zhang, X. M. Ji, W. J. Liu, S. J. Ding, D. W. Zhang, Photoluminescence enhancement of ZnO nanowire arrays with atomic layer deposited ZrO2 coatings and thermal annealing, Phys. Chem. Chem. Phys. 18 (2016) 16377-16385. [67] R. Swapna, M. Ashok, G. Muralidharan, M. C. S. Kumar, Microstructural, electrical and optical properties of ZnO:Mo thin films with various thickness by spray pyrolysis, J. Anal. Appl. Pyrolysis 102 (2013) 68–75. [68] M. A. Sliem, T. Hikov, Z. A. Li, M. Spasova, M. Farle, D. A. Schmidt, M. H. Newen, R. A. Fischer, Interfacial Cu/ZnO contact by selective photodeposition of copper onto the surface of small ZnO nanoparticles in non-aqueous colloidal solution, Phys. Chem. Chem. Phys. 12 (2010) 9858-9866. [69] H. Zhang, T. Zhao, G. Hu, L. Miao, Y. Yang, Role of Mg doping on morphology and photoluminescence features of MgxZn1−xO films prepared by ultrasonic spray pyrolysis, J. Mater. Sci: Mater. Electron. 23 (2012) 1933–1937. [70] J. M. Soler, E. Artacho, J. D. Gale, A. Garcıa, J. Junquera, P. Ordejon, D. S. Portal, The SIESTA method for ab initio order-N materials simulation, J. Phys.: Condens. Matter 14 (2002) 2745–2779.
[71] J. P. Perdew, K. Burke, M. Ernzerhof, Generalized Gradient Approximation Made Simple, Phys. Rev. Lett. 77 (1996) 38.
Figure captions Fig. 1. Schematic diagram for various possible mechanisms involved in the incorporation of Ta and F in to the ZnO lattice. Fig. 2a-c. Variation in electrical resistance, carrier concentration and mobility of ZnO:Ta:F films as a fuction of Ta+F doping levels. Fig. 3a-d. Resistivity of as-deposited, air-annealed and vacuum-annealed films with Ta+ F doping levels a) 1+5, b) 1+10, c) 1+15 and d) 1+20 at. % Fig. 4a-c. Transmittance spectra of ZnO:Ta:F films. Fig. 5a-c. dT/dλ vs. Energy plots of ZnO:Ta:F films. Fig. 6a-c. XRD patterns of ZnO:Ta:F films. Fig. 7a-i.
FESEM images of ZnO:Ta:F films.
Fig. 8. EDX spectrum of ZnO:Ta:F film with Ta+F doping level 1+10 at.%. Fig. 9. Compositional mapping of ZnO:Ta:F film with Ta+F doping level1+10 at.%. Fig. 10. XPS survey spectrum of ZnO:Ta:F film with Ta+F doping level as 1+10 at.%. Fig. 11 a-b. 3D and 2D images of ZnO:Ta:F film with Ta+F doping level 1+10 at.%. Fig. 12. Room temperature PL spectra of ZnO:Ta:F films. Fig. 13. Optimized structures of a) ZnO, b) ZnO-Ta, c) ZnO-Ta-F(Ta-F adjacent) and d) ZnO-Ta-F(Ta-F apart) models. Table caption Table 1.
Source materials and process parameters.
Table 2.
Analytical instruments used for characterization.
Table 3. Various possible mechanisms involved in the incorporation of Ta and F in ZnO lattice. Table 4. Resistivity of as-deposited, air-annealed and vacuum-annealed ZnO:Ta:F films. Table 5. Reported influence of annealing under different ambiences on the properties of ZnO based films deposited by chemical techniques. Table 6. Transmittance, optical energy gap, quality factor and grain size of the as-deposited, air-annealed and vacuum-annealed ZnO:Ta:F films. Table 7. Optical band gap values estimated from optical transmittance data and PL studies. Table 8. Total energy and change in energy of ZnO based structures calculated using DFT.
Fig. 1.
. Fig. 2.
Fig. 3.
Fig. 4.
Fig. 5.
Fig. 6.
Fig. 7.
Fig. 8.
Fig. 9.
Fig. 10
Fig. 11a.
Fig. 11b.
Fig. 12.
Fig. 13.
Table 1 Host precursor Dopant precursors
Solvent Spray volume Nozzle to substrate distance Substrate temperature Spray time and Spray interval Thickness of the films Annealing ambiences Annealing temperature and duration
: Zinc acetate dihydrate (0.1 M) (Sigma Aldrich, 99.999 %,USA) : Tantalum penta chloride (1 at. %) (Sigma-Aldrich, 99.99%, Germany) Ammonium fluoride (5, 10, 15 & 20 at. %) Sigma-Aldrich, 99.99%, Germany) : Doubly de-ionized water, methanol and acetic acid (7:2:1) : 50 Ml : 30 cm : 350 ± 5 ⁰C : Each 10 sec. : 400 nm (approximately) : Air and vacuum 450 ⁰C and 2 hours
Table 2 Analytical instruments used for characterization Electrical Optical Structural
: Linear four-point probe (vander Paw configuration) and Hall probe (ECOPIA HMS-3000) : UV–vis–NIR double-beam spectrophotometer (Perkin Elmer Lambda 35 model) : X-ray diffractometer (PANalytical PW 340/60 X’pert PRO)
Surface morphology and elemental analysis Photoluminescence Chemical state
: Field Emission Scanning Electron Microscope with Energydispersive X-ray Spectrometer (JEOL JSM – 6701F) : Spectro-flurometer (Jovin Yvon-FLUROLOG-FL3-11) : K-AlphaTM+ X-ray Photoelectron Spectrometer
Table 3
Case
Element
Type of incorporation
Effect on carrier concentration
a
Tantalum
Substitution
Increases the number of free electrons by 3, when the nearby oxygen shares two electrons.
b
Fluorine
Substitution
Increases the number of free electrons by 1, when the nearby zinc site is occupied by zinc.
c
Tantalum
Substitution
Increases the number of free electrons by 4, when the nearby oxygen site is occupied by fluorine.
d
Tantalum
Interstitial
Each interstitial tantalum ion donates its unshared 5 electrons to the system.
e
Tantalum
Substitution
Each substitution causes an increase of 5 free electrons when the nearby oxygen site is vacant.
f
Tantalum
Anti site
Adverse effect, (i.e) no contribution of free carrier.
g
Fluorine
Substitution
Increases the number of free electrons by 4, when the nearby zinc site is occupied by tantalum (similar situation as case c).
h
Fluorine
Interstitial
This defect is not common because the negative ions are usually very large and they cannot easily fit into the interstitial sites (rare case).
i
Fluorine
Anti site
Adverse effect, (i.e) no contribution of free carrier. It is unlikely to be present under equilibrium condition (rare case).
Table 4
Doping level of Ta+F
Resistivity (ρ) × 10-3 (Ω cm) As-deposited film
Air-annealed film Vacuum-annealed film
1+5
3.96
121.7
1.92
1+10
2.056
113.7
0.81
1+15
2.72
130.2
1.40
1+20
3.42
142.9
1.85
Table 5
Method Material
Resistivity (Ω cm) Annealing ambience Before annealing
Spray
ZnO:Ga
Air Vacuum H2 Air N2 N2/H2 Vacuum
Spray
ZnO:Al
Air
-
Spray
ZnO
N2 N2/H2
9.91 Ω -
Spray
ZnO:In
Air
-
Spray
ZnO:Bi
Air
-
Spray
ZnO:Sn:F
9.27 × 10−3
Spray
ZnO:Sn:F
Spray Sol-gel Sol-gel Sol-gel Sol-gel Sol-gel Sol-gel
ZnO:In:F ZnO:SnO2 ZnO:Cd ZnO ZnO:Co ZnO:Li:Mg ZnO:Al:Li
Vacuum Air Vacuum Ar gas Air N2 Air Air Air N2/H2 Air Vacuum Vacuum
Spray
ZnO:Ta:F
Spray Spray
ZnO:Al ZnO
Spray
ZnO:Al
SILAR ZnO SILAR ZnO;F
2.056 × 10−3 2.5 0.5 5.88 × 10−1 3.6 × 10
−2
1.34 × 10−3 5.2 × 10-2 8.17 × 102 0.9 91370 8.98× 10−3 1.63×10−3
After annealing 132.9 × 10−3 0.81 × 10−3 4.7 × 10−3 0.42 1.64 × 10−1 1.71 × 10−2 8 × 10−3 Resistance increases with increase in annealing temperature 1.12 1.62 × 10−1 Conductivity increases (Resistivity data not available) Conductivity increases (Resistivity data not available) Resistance decreases 1.06 × 10−3 0.69 × 10−3 6.3 × 10-3 1.12 × 102 0.034 Resistance decreases (data not available) Resistance decreases (data not available) 513 Ω cm Decreases to 1.05× 10−3 6.2× 10−3 4.04× 10−3 1.32×10−3
Transmittance (%) Before After annealing annealing 93 94 90.5 70 85 T increases
Reference This study [36] [37]
80
T increases
[38]
82
No change
[39]
-
-
[40]
80
T decreases
[41]
-
-
[42]
75
T increases
[43]
-
T increases
[44]
-
T increases
[45]
82.1 78 80
80 T increases 82 T decreases
[20] [46] [47] [48] [49] [50] [51]
-
T increases
[52]
-
T increases
[53]
Table 6
Doping level of Ta+F 1+5 1+10 1+15 1+20
Transmittance T
Optical band gap Eg
Quality factor ϕ
(%)
(eV)
×10-4 (Ω/sq)-1
Crystallite size D (nm)
As-
Air-
Vacuum-
As-
Air-
Vacuum-
As-
Air-
Vacuum-
As-
Air-
Vacuum-
deposited
annealed
annealed
deposited
annealed
annealed
deposited
annealed
annealed
deposited
annealed
annealed
film
film
film
film
film
film
film
film
film
film
film
film
94.0 93.5 93.0 89.0
93.5 93.5 91.0 89.0
93.0 92.5 88.0 87.0
3.26 3.27 3.25 3.23
3.24 3.23 3.22 3.21
3.29 3.33 3.32 3.30
0.544 0.993 0.712 0.365
0.017 0.018 0.012 0.009
1.008 2.265 0.796 0.537
48.2 53.0 68.1 58.3
53.1 64.2 72.0 60.5
59.0 70.1 74.1 71.4
Table 7
Film
Optical energy gap value from transmittance study (dT/dλ vs Eg ) (eV)
Change in Eg due to annealing (eV)
As-deposited
3.27
Air-annealed Vacuum-annealed
From PL study
Change in Eg due to annealing (eV)
Wavelength at NBE peak (nm)
Calculated energy (eV)
-
390.6
3.18
-
3.33
0.06
396.0
3.14
0.04
3.37
0.1
378.6
3.28
0.1
Table 8 Structure ZnO ZnO-Ta ZnO-F ZnO-Ta-F(Adjacent) ZnO-Ta-F-Apart
Total Energy (eV)
(% )
-31673.03 -30420.44 -31881.21 -30627.32 -30628.13
100 96.05 100.66 96.70 96.70
Change in Energy ΔE (%) 3.95 -0.66 3.30 3.30
S0169-4332(17)30606-2 http://dx.doi.org/doi:10.1016/j.apsusc.2017.02.233 APSUSC 35342
To appear in:
APSUSC
Received date: Revised date: Accepted date:
29-11-2016 2-1-2017 27-2-2017
Please cite this article as: K.Subha, K.Ravichandran, S.Sriram, Combined influence of fluorine doping and vacuum annealing on the electrical properties of ZnO: Ta films, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2017.02.233 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Full Length Article Combined influence of fluorine doping and vacuum annealing on the electrical properties of ZnO: Ta films K. Subha1, 2, K. Ravichandran1* and S. Sriram3 1
Materials Science Research Laboratory, PG and Research Department of Physics, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur-613 503, Tamil Nadu, India. 2
Research Department of Physics, Kunthavai Naachiyaar Govt. Arts College for Women (Autonomous), Thanjavur – 613 007, Tamil Nadu, India. 3
School of Electrical and Electronics Engineering, SASTRA University, Thanjavur – 613 401, Tamil Nadu, India.
*Corresponding author: Dr. K. Ravichandran, Associate Professor of Physics, Materials Science Research Laboratory, PG and Research Department of Physics, AVVM Sri Pushpam College (Autonomous), Poondi, Thanjavur-613 503, Tamil Nadu, India. Mobile: (+91) 94435 24180 Fax : +91 4374 239328 Land line: 04362 278602 E. mail: [email protected] [email protected]
Highlights
First report on combined effect of F doping and annealing on resistivity of ZnO:Ta films Various possible incorporation mechanisms of tantalum and fluorine are addressed. Theoretical validation of Ta and F incorporation by DFT analysis is done. Quality factor comparable with those obtained by physical methods is achieved.
Abstract In this study, our main focus is to investigate the effects of F doping and post deposition annealing (air and vacuum) on the optical and electrical characteristics of tantalum doped zinc oxide films (ZnO:Ta). A cost-effective, automated jet nebulizer spray pyrolysis technique is adopted to deposit the ZnO:Ta:F films. The doping level of Ta is kept constant (1 at.%) and that of F is varied from 5 to 20 at.% in steps of 5 at.%. The electrical resistivity of the as-deposited films decreases for 10 at.% of F concentration. The resistance increases thereafter. The same trend is also observed in annealed films. The reasons for these variations are addressed based on the effective F incorporation into the ZnO lattice and annealing atmosphere with the help of XRD, FESEM, AFM and PL studies. The incorporation of the dopants was confirmed from XPS and EDX analysis and the DFT studies show that the incorporation of the dopants does not affect the stability of the ZnO lattice. Vacuum-annealed films show better electrical properties over the as-deposited and air-annealed counterparts, though their transparency is affected marginally. A minimum resistivity of 0.81× 10-3 Ω cm and an enhanced quality factor of 2.265×10-4 (Ω/sq)-1 are achieved for the vacuum-annealed films having Ta+F doping levels as 1+10 at.%. These results make this sample a desirable candidate for transparent electrode applications. ________________________________________________________________________________________________________________
Keywords: ZnO thin films; Annealing; Electrical transport; Transmittance; Figure of merit;
1. Introduction
The electrical resistivity and optical transparency are the chief selection criteria for transparent conducting oxide films to be used as absorber or window layers for the fabrication of solar cells. Among those materials which exhibit both these properties simultaneously, indium tin oxide (ITO) places itself at the top spot and hence it is widely used for these transparent electrode applications. But toxicity, source scarcity and the cost of indium urge the researchers to search for a desirable alternative [1-5].At this juncture, ZnO, the non toxic, abundantly available and most importantly inexpensive material can be considered as one of the most suitable alternatives to ITO films [6-8]. Even though ZnO film has good transparency, its electrical resistivity is not low enough to meet the requirements of a good transparent electrode [9]. To make ZnO films a desirable alternative to the most commonly used indium based thin films, the electrical resistivity of ZnO thin film has to be reduced further. As several researchers in the recent past have showed that ZnO is a tailor- friendly material, a renewed interest is kindled in tuning their optical and electrical properties to make them fit for various applications. Its transport properties can be modified in many ways which includes, doping with appropriate cationic or/and anionic elements, adopting different deposition techniques, depositing over layers, annealing at different ambiences and varying the process parameters like film thickness, doping concentration and substrate temperature [10-14]. Of them, doping with a suitable element is proven to be the most effective way, by which the electrical and optical properties can be modified to a greater extent [15]. Most importantly, according to the literature reports, the resistivity of ZnO thin films can be reduced considerably by appropriate doping [16-18]. At times, doping with a single element is insufficient to achieve a desired low resistance. Therefore, in the past few decades, for improving the electrical properties of ZnO thin films, researches have adopted simultaneous or co-doping tactic, expecting combined or even synergistic effects of the dopants used [19-21]. In this work, keeping this point in mind, we have doped fluorine (an anionic dopant) along with tantalum (a cationic dopant) as a follow-up work of our previous study in which we have reported the effect of the doping concentration of Ta on the characteristics of ZnO films [22].
Fluorine has been recognized as a suitable anionic impurity which improves the electrical properties of ZnO [23-24]. The comparable ionic radius of F- ion (131 pm) with that of O2- ion (138 pm), increases the chances of substitutional incorporation over the other types of incorporations viz. interstitials and anti-sites [25]. Interestingly, it has been accepted as one of the best dopants used to enhance the transparency as well as the conductivity of ZnO films [26]. Moreover, in the present study, using the DFT calculations, we found that, the stability of ZnO system is improved when F is doped with ZnO. As annealing is one of the best ways to improve the quality of TCO films [20], in the present work, the deposited films are annealed at two different ambiences (air and vacuum) and the transparent conducting properties of these films are compared with those of as-deposited films. It is worth mentioning here that reports on the effect of post annealing on the electrical resistivity and transmittance of ZnO:Ta:F thin films is seldom available in the literture. In this juncture, Ta + F doped ZnO thin films at different fluorine concentrations are deposited using nebulizer spray pyrolysis technique and the changes in the properties of annealed films are studied. 2. Materials and methods The automated jet nebulizer spray technique elaborated in our previous study [13] was employed to deposit tantalum and fluorine doped zinc oxide (ZnO:Ta:F) thin films onto borosilicate glass substrates. The source materials used and the process parameters maintained to deposit the four sets of films having different Ta+F doping levels are presented in Table 1. For the confirmation of the reproducibility of the films, the experiment was repeated many times keeping the same deposition parameters. The analytical techniques used for the characterization of the deposited samples are given in Table 2. 3.1 Various possible incorporation mechanisms of Ta and F in ZnO lattice The doping process often causes changes in the crystal lattice and its defects which often affect most of the properties of the host material [20]. Hence, by the better understanding of the related doping mechanism, one can prepare TCO thin films like ZnO-based films with required properties. To facilitate the understanding, various possible incorporation mechanisms regarding the simultaneous doping of Ta and F with ZnO are presented in Table 3 and illustrated using a schematic diagram (Fig. 1.)
3.1 Electrical studies Fig. 2 a-c shows the variation in electrical parameters obtained for the as-deposited, and annealed (air and vacuum) ZnO:Ta:F films with respect to Ta+F doping level. The resistivity of the as-deposited film with minimum F concentration (5 at. %) is 0.81× 10-3 Ω cm (Table 4). When the F doping concentration in the starting solution increases, the resistivity of the film initially decreases up to 10 at.% and then increases for higher doping levels [Fig. 2a]. The two main reasons for the observed decrease in resistivity are (i) the increase in carrier concentration and (ii) the increase in carrier mobility. When F is doped with ZnO:Ta, each of the F-ions that substitutes an O2-ion results in an extra free carrier (electron) generation, as only one electron is needed for the Zn-F bond formation. Hence, the increase in F concentration in the starting solution, results in an increase in the free carriers in the ZnO lattice up to a certain doping level which is 10 at.% for the deposition conditions employed in the present work. It is well established that 10 at.% is the optimum F doping level to obtain low electrical resistivity values, as reported in previous studies [27-28]. When the doping level of F increased, the mobility increases as shown in Fig. 2a. This improved mobility is attributed to the increase in grain size caused by the increase in F doping concentration as evident from the FESEM images. As the size of the grain increases the grain boundary increases, which in turn leads to a reduced scattering [14, 22]. The substitution of Fmainly disturbs the valance band and makes the conduction band fairly free from scattering [29]. Thus the combined effects of increased free carrier concentration and the enhanced mobility caused by F doping leads to a remarkable reduction in the resistivity of Ta doped ZnO films. The increase in the resistivity beyond this level (10 at. %) of F doping concentration may be due to the excess number of anionic dopants present in the ZnO matrix. These excess F- ions probably occupy the interstitial or anti sites in the lattice and thereby causes an adverse effect, in general [22, 30]. It is learnt that the excess F- ions can occupy also at the grain boundaries [31], leading to carrier scattering and the potential barrier thus formed at the grain boundaries plays a significant role in reducing the carrier mobility. Moreover, the fluorine ions at interstitial position can attract free electrons (charge trapping sites) due/attributed to its high electronegativity. Consequently, there is a decrease in carrier concentration and mobility which results in increased resistivity [32].
When the films are air-annealed, all the samples exhibit relatively decreased carrier concentration and a resultant increased resistivity (by 2 orders) compared to their as-deposited counterparts (Fig. 2b). This result can be interpreted on the basis of the following two different effects of chemisorbed oxygen on the free carriers of the system. (i) During air annealing, chemisorption of oxygen can take place (at the existing vacant sites of oxygen and also sites from which F is released during air annealing). Subsequently these chemisorbed oxygen ions induce the formation of trapping states at the grain boundaries, which can trap the electrons and make them immobile, resulting in the reduction of number of free carriers. (ii) The electrically charged traps create potential barriers, which restrict the carrier movement from one crystallite to another, thereby reduce their mobility [33]. We believe that, these two factors cause a drastic increase in the resistivity. Therefore, in spite of the increase in the grain size (as seen from FESEM images) which should improve the mobility, the resistivity of air-annealed films increases. The result clearly suggests that the decrease in carrier concentration caused by the oxygen adsorption which takes a predominant role over the increase in mobility may be the reason for this increase in electrical resistance. In contrast to the air annealing process, desorption of oxygen and substituted fluorine takes place in vacuum annealing [34]. This phenomenon leads to the creation of increased number of oxygen vacancies which in turn results in an enhancement of carrier concentration and a consequent reduction in resistivity (Fig. 2c). It is important to mention here that, as fluorine is more volatile than oxygen, desorption of fluorine can take place easily, creating a large number of oxygen vacancies. Moreover, the growth of larger crystallites and grains during vacuum annealing helps in improving the intra-grain mobility of the charge carriers and thereby reduces the electrical resistivity further [35]. The enhancement in the size of the crystallites and grains is confirmed using the XRD and FESEM/AFM results, (section 3.3 and 3.4) respectively. These combined effects of fluorine doping and vacuum annealing enhance the electrical properties of ZnO: Ta films to a greater extent. The resistivities of vacuum-annealed films are compared with those of their as-deposited counterparts using bar diagrams in Fig. 3. It is seen from the figure that, the lowest resistivity value (0.81× 10-3 Ω cm) is recorded for the vacuum-annealed film with Ta+F doping level as 1+10 at. %. This vacuum annealing induced reduction in the resistivity is illustrated as bar diagram in Fig. 3. This minimum resistivity value obtained in the present study is found to be
comparable or even superior when compared with the related results obtained using chemical deposition techniques by various researchers as seen from Table 5 [36-53]. 3.2 Optical studies The transmittance spectra of the films are shown in Fig 4a-c. From the Fig. 4a, it is observed that, all the as-deposited films are highly transparent in the visible region. The average visible transmittance is nearly 91% for minimum fluorine doping concentration (5 at.%). When we increase the doping level, a slight decrease in the visible transmittance and a notable decrease in IR transmittance are seen from the Fig 4a. This observed decrease in the IR transmittance is a strong evidence for the improvement in the carrier concentration of the corresponding films. It is a known fact that an increase in carrier density leads to a decrease in IR transmittance due to scattering 54]. When the films are annealed under air ambient, the variation in visible transmittance with respect to doping level is found to be in the same trend as that of the as-deposited films, whereas, a different trend is observed in the IR transmittance (Fig. 4b). The transmittance at IR range increases remarkably for all the films indicating the decrease in carrier concentration. This decrease in carrier concentration caused by the chemisorptions of oxygen from the air is discussed in detail in the electrical studies (section 3.1). At the same time, the vacuum-annealed films exhibit (Fig. 4c) slightly diminished transmittance value in visible range which may be due to the metal richness in the films [55]. This metal richness is caused by the oxygen desorption occurred during vacuum annealing. The noticeable decrease in transmittance in the infra-red range may be attributed to an increased IR scattering as discussed in the as-deposited case. This result is consistent with the resistivity obtained for these set of films. Moreover, the decrease in transmittance spectra can be correlated with the increase in surface roughness, as evidenced from FESEM observation [20]. The optical band gap (Eg) values are obtained from the plots drawn between dT/dλ and energy (Fig. 5a–c) and are given in Table 6. From Table 6, it is found that, in all the three cases, the Eg increases for 10 at.% and decreases for further doping. The Moss–Burstein theory can explain this increase in Eg [56]. This result on band gap is agreed well with the discussion on electrical parameters (section 3.1).
To find the optimum concentration of F for obtaining good quality TCO films, the quality factor (figure of merit) was calculated (Haacke formula ɸ = T10/Rsh [57]) and presented in Table 6. It is seen from Table 6 that, in each of the three sets of films for all the three cases, the quality factor is the highest for the film with F doping level 10 at. %. Among all the three sets of films, the vacuum-annealed film with 10 at. % of F doping level exhibits the highest figure of merit as it has the lowest resistivity and good transmittance. Thus, it can be concluded that 10 at. % of F doping level is the optimum value for preparing good quality spayed ZnO:Ta;F films.. 3.3 Structural studies The XRD profiles of the samples are shown in Fig. 6 a-c. The hexagonal wurtzite structure of the prepared films is verified by comparing the obtained XRD profile with the related JCPDS Card no. 36-1451. The peaks appearing at the 2θ values 31.7 ̊, 34.4 ̊ and 36.2 ̊ are associated with the lattice planes (100), (002) and (101), respectively. Absence of any secondary peaks reveals the purity of all the films. It is obvious from the Fig. 6a that, the intensity of all the peaks increases when F doping level is increased. This may be due to the possible filling of oxygen vacancies already exist in the ZnO lattice by the F- ions which leads to an improvement in the periodicity of the lattice. This result is reflected in the calculated values (using Scherrer’s formula of the crystallite size (Table 6) [58]. This trend continues up to 15 at.%. When the doping level is increased further, the intensity of all the peaks deceases. This may be due to the large number of F- ions incorporated into the ZnO matrix which is beyond the solubility limit of F in the ZnO system. The peak intensities of the annealed films are higher than those of their as-deposited counterparts as seen from the Fig. 6 b and c [59]. Among the three sets of films, the vacuumannealed films have the highest intensity suggesting that, the vacuum annealing enhances the quality of the films to a greater extent. FESEM images (section 3.4.1) also support the above discussion. 3.4 Surface morphological studies and elemental analysis 3.4.1 FESEM, XPS and EDX analysis with mapping The FESEM images of all the samples are shown in Fig. 7 a-l. When the F concentration is minimum (5 at. %), as-deposited film’s surface has spherical grains having nearly uniform size
(approximately 25 nm) as shown Fig. 7a. When the doping concentration of F increases, the grains retain their shape, however, the size of the grains increases marginally [53, 58]. When the film samples are air-annealed, smaller spherical grains are agglomerated together to form linear larger grains of nearly uniform size [42, 46] attributed to Ostwald ripening. It is a spontaneous thermo-dynamical process by which larger grains are formed at the cost of smaller ones. The formation of larger grains makes the system more stable because of the fact that the larger grains have lesser surface energy [60, 61]. The average size of the linear agglomerated grains is found to be increased with F doping as seen from the Fig. 7 e-h. This change is seemed to be appreciable in the case of vacuum-annealed films (Fig. 7 i-l). This increase in grain size may be reason for the improved mobility of the annealed films. The composition of elements of the samples are studied using EDX spectroscopy and as a representative image, the one recorded for the film with Ta + F doping levels as 1+10 at.% is shown in Fig. 8. The peaks at 1.8 and 3.8 keV represent the elements silicon and calcium which are generally present in the substrate. It is observed from the Fig. 8 that, the elements zinc, oxygen, tantalum and fluorine are found on the surface of the film. This result confirms the successful incorporation of both the dopants (Ta and F). The atomic and weight percentages of Zn, O, Ta and F is shown in the table given as inset of Fig. 8. It is observed from the inset table that, the atomic percentage of fluorine in the film is remarkably less than that in the starting solution. This is because of the high volatile nature of the fluorine compounds. Mapping is an effective tool to explore about the positions as well as the proportions of the constituent elements on the surface of the film under study. The images shown in Fig. 9 depict the information about the anticipated elements in the deposited film. The chemical states of the elements in ZnO:Ta:F film are elucidated by XPS analysis. Fig.10 shows the XPS survey spectrum of the ZnO:Ta:F sample. Calibration of the energy scale is done with reference to the peak of carbon (C1s at 284.60 eV). The strong peaks correspond to 2p state of Zn and 1s state of O, as well as the weak chemical signals represent 4f state of Ta and 1s state of F were observed, which revealed the existence of the elements zinc, oxygen, tantalum and fluorine on the surface of the film.
3.4.2 Atomic force microscopy study
The 3D images of the optimized films (10 at.% of F) are obtained using atomic force microscopy (AFM) with scan area 10 x 10 µm and 2.5x 2.5 µm are shown in Fig. 10 a. the corresponding 2D images are presented in Fig. 10b. From the images it is inferred that, annealing causes slight changes in the surface morphology of the samples. On the surface of the air-annealed film, few bigger grains can be seen here and there, whereas, the film annealed under vacuum ambient shows bigger grains almost all over the surface. The annealing induced change in the grain size reveals that the crystallinity is enhanced by annealing (Fig. 10b). The color scale bar supports the above discussion. This result reflects the observations of the XRD study (section 3.3). In general, during the film growth, depending on the deposition parameters employed, uneven nucleation takes place which results in irregularities on the film surface. These irregularities generally create traps, and scattering sites on the surface. Surface roughness representing these finest irregularities is an important parameter which strongly affects the electrical transport properties and optical quality of a film. Surface roughness is a measure of the vertical deviation of surface points from a reference plane and it is commonly represented as root mean square deviation (RMS). It shows the average deviation of each of the grain hillock or pits from the reference plane. The surface roughness of the films (having 1+10 at.% of Ta+F doping) are found to be 0.0156, 2.647 and 6.83 nm, respectively. The RMS roughness increases due to annealing which may be owing to the different reorientations of the grains caused by the annealing treatment. This result is agreed well with the FESEM results. The RMS roughness values obtained in the present work are remarkably low when compared with the reported values [46, 48, 62-64]. This result suggest that these films are suitable for solar cell applications, as the photovoltaic layers are normally very thin, high roughness value may result in local shunts and hence low roughness is desirable for these applications [65]. 3.5 Photoluminescence studies The room temperature PL measurements of all the deposited films were carried out to study the crystalline quality of the films as well as the presence of defects. The photoluminescence spectra of the films (Ta+F doing concentration is 1+10 at.%) are shown in Fig. 11. The PL spectra of the samples exhibit the same set of peaks and the peak positions have not been altered by the increased concentration of F and also due to annealing.
There are many prominent peaks in the figure which are found around 390, 445, 490 and 525 nm. The weak UV emission peak around 390 nm corresponds to near band edge emission [66]. The blue emission peak around 445 nm is attributed to the zinc interstitial (Zni) defect [67]. The emission peaks at 490 nm and 525 nm represent the singly and doubly ionized oxygen vacancies, respectively [15, 68]. Furthermore, an appreciable blue shift of NBE peak maxima is observed for vacuumannealed films whereas, a slight red shift is observed for air-annealed films. The blue shift is an evident for the widening of Eg [69]. This result is a supplementary evidence for the vacuum annealing induced increase in the band gap already observed from the optical transmission data. Table 7 recapitulates the annealing induced changes in the energy gap of the Ta+F doped ZnO films (1+10 at.% ). The annealing induced changes observed in Eg from optical and PL analysis are agreed well with each other. The intensity of the peak associated with oxygen vacancy (525 nm) is found to be increased remarkably after vacuum annealing confirming the oxygen desorption during annealing under vacuum. However during air annealing, peak intensity is found to be less compared with the as-deposited film which should be due to the adsorption of oxygen from the air- atmosphere during air annealing. 3.6 Analysis of stability of the Ta+F doped ZnO structures using DFT studies 3.6.1 Computational Details Structures of ZnO, Ta doped ZnO and Ta+F doped ZnO with different combinations were constructed and optimized through SIESTA package [70]. The exchange correlation effect is estimated using GGA (Generalized Gradient Approximation) with PBE (Perdew-BurkeErnzerhof) parameterization [71]. In SIESTA, the core electron potentials are replaced by effective Troullier-Martins pseudo-potentials in fully separable form. The double zeta plus valance polarization basis sets are chosen for calculations. The 4s and 3d orbitals of Zn, 2p and 2s orbitals of O, 4f, 5d and 6s orbitals of Ta and 2p orbitals of F are considered as valance orbitals. A 444 Monkhorst-Pack k points grid were used for Brillouine zone sampling. The minimization energy approach based structures of undoped and Ta + F doped ZnO are used for geometrical optimization through conjugate gradient (GA) algorithm. The optimized structures are shown in Fig. 13.
3.6.2 Analysis The calculated total energy variations for ZnO-Ta, ZnO-F and ZnO-Ta- F structures with reference to the ZnO structure obtained through structure optimization are given in Table 8. From the Table, it is seen that, the total energy of the system is increased only marginally when Ta is incorporated into the ZnO lattice suggesting that, the structure formation is not disturbed much. Upon the simultaneous addition of Ta and F atoms, the system exhibits total energy which is closer to that of ZnO structure. This is true for both the cases: (i) when Ta and F are placed in adjacent site and (ii) F is placed far away from Ta. Thus the simultaneous doping of Ta+F leads to higher stability than Ta doping. These observations make the present study a fruitful attempt to obtain a stable ZnO:Ta:F structure with desirable transparent conducting properties.
4. Conclusion This study on the ZnO:Ta:F films deposited with different Ta+F doping levels (1+5, 1+10, 1+15 and 1+20 at.%) and annealed at two different ambiences (air and vacuum) clearly shows that, the sample having Ta+F doping levels 1+10 at.% and annealed under vacuum exhibits minimum resistivity of 0.81×10-3 Ω cm due to the combined effect of fluorine doping and vacuum annealing. Even though vacuum annealing causes a marginal decrease in transmittance, this film possesses quality factor of 2.265×10-4 (Ω/sq)-1 which is superior over all the other samples prepared in this study. From the results, we can conclude that 1 and 10 at.% are the optimum doping concentration limits for Ta and F, respectively to obtain nebulizer sprayed ZnO:Ta:F films with desirable transparent conducting properties. It is also found that, all the vacuum annealed films exhibit reduced resistivity compared with their as-deposited and air-annealed counterparts irrespective of their Ta+F doping levels. Acknowledgment: The authors wish to thank the Council for Scientific and Industrial Research, New Delhi, India (No. 03(1321)/14/EMR-I) and the University Grants Commission, New Delhi, India (MRP-5688/15(SERO/UGC) for the financial support.
References
[1] S. F. Tseng, W. T. Hsiao, K. C. Huang, D. Chiang, M. F. Chen, C. P. Chou, Laser scribing of indium tin oxide (ITO) thin films deposited on various substrates for touch panels, Appl. Surf. Sci. 257 (2010) 1487. [2] Y. T. Park, A. Y. Ham, Y. H. Yang, J. C. Grunlan, Fully organic ITO replacement through acid doping of double-walled carbon nanotube thin film assemblies, RSC Adv. 1 (2011) 662. [3] K.A. Sierros, N.J. Morris, K. Ramji, D.R. Cairns, Stress-corrosion cracking of indium tin oxide coated polyethylene terephthalate for flexible optoelectronic devices, Thin Solid Films 517 (2009) 2590-2595. [4] Z. Chen, B. Cotterell, W. Wang, The fracture of brittle thin films on compliant substrates in flexible displays, Eng. Fract. Mech. 69 (2002) 597-603. [5] K. C. Lai, C. C. Liu, C. h. Lu, C.H. Yeh, M. P. Houng, Characterization of ZnO:Ga transparent contact electrodes for microcrystalline silicon thin film solar cells, Sol. Energy Mater. Sol. Cells 94 (2010) 397–401. [6] K. Saravanan, R. Krishnan, S. H. Hsieh, H. T. Wang, Y. F. Wang, W. F. Pong, K. Asokan, D. K. Avasthi, D. Kanjilal, Effect of defects and film thickness on the optical properties of ZnO–Au hybrid films, RSC Adv. 5 (2015) 40813. [7] C. Cachoncinlle, C. Hebert, J. Perriere, M. Nistor, A. Petit, E. Millon, Random lasing of ZnO thin films grown by pulsed-laser deposition, Appl. Surf. Sci. 336 (2015) 103. [8] R. Mimouni, K. Boubaker, M. Amlouk, Investigation of structural and optical properties in Cobalt–Chromium co-doped ZnO thin films within the Lattice Compatibility Theory scope, J. Alloys Compd. 624 (2015) 189. [9] E. Bacaksiz, S. Aksu, S. Yılmaz, M. Parlak, M. Altunbas, Structural, optical and electrical properties of Al-doped ZnO microrods prepared by spray pyrolysis, Thin Solid Films 518 (2010) 4076–4080. [10] X. Zhang, L. Zhu, H. Xu, L. Chen, Y. Guo, Z. Ye, Highly transparent conductive Fdoped ZnO films in wide range of visible and near infrared wavelength deposited on polycarbonate substrate, J. Alloys Compd. 614 (2014) 71–74.
[11] R. Mariappan, V. Ponnuswamy, P. Suresh, Effect of doping concentration on the structural and optical properties of pure and tin doped zinc oxide thin films by nebulizer spray pyrolysis (NSP) technique, Superlattices Microstruct. 52 (2012) 500–513. [12] R. Pandey, S. Yuldashev, H. D. Nguyen, H. C. Jeon, T. W. Kang, Fabrication of aluminium doped zinc oxide (AZO) transparent conductive oxide by ultrasonic spray pyrolysis, Curr. Appl. Phys. 12 (2012) S56-S58. [13] N. S. Kumar, K. V. Bangera, C. Anandan, G.K. Shivakumar, Properties of ZnO:Bi thin films prepared by spray pyrolysis technique, J. Alloys Compd. 578 (2013) 613–619. [14] K. Ravichandran, K. Subha, A. Manivasaham, G. Sridhar, T. Arun, Fabrication of a novel low-cost triple layer system (TaZO/Ag/TaZO) with an enhanced quality factor for transparent electrode applications, RSC Adv. 6 (2016) 63314 - 63324. [15] T. P. Rao, M.C. S. Kumar, N. S. Hussain, Effects of thickness and atmospheric annealing on structural, electrical and optical properties of GZO thin films by spray pyrolysis, J. Alloys Compd. 541 (2012) 495–504. [16] H. Liu, Y. Wang, J. Wu, G. Zhang, Y. Yan, Oxygen vacancy assisted multiferroic property of Cu doped ZnO films, Phys. Chem. Chem. Phys. 17 (2015) 9098-9105. [17] V. Gokulakrishnan, S. Parthiban, K. Jeganathan, K. Ramamurthi, Investigation on the effect of Zr doping in ZnO thin films by spray pyrolysis, Appl. Surf. Sci., 257 (2011) 9068. [18] Y. S. Avadhut, J. Weber, E. Hammarberg, C. Feldmann, J. S. auf der Gunne, Structural investigation of aluminium doped ZnO nanoparticles by solid-state NMR spectroscopy, Phys. Chem. Chem. Phys. 14 (2012) 11610–11625. [19] F. H. Wang, C. L. Chang, Effect of substrate temperature on transparent conducting Al and F co-doped ZnO thin films prepared by rf magnetron sputtering, Appl. Surf. Sci. 370 (2016) 83-91. [20] A. Hadri, M. Taibi, M. loghmarti, C. Nassiri, T. S. Tlemania, A. Mzerd, Development of transparent conductive indium and fluorine co-doped ZnO thin films: Effect of F concentration and post-annealing temperature, Thin Solid Films 601 (2016) 7-12. [21] A. Mallick, S. Sarkar, T. Ghosh, D. Basak, An insight into doping mechanism in Sn-F co-doped transparent conducting ZnO films by correlating structural, electrical and optical properties, J. Alloy. Compd. 646 (2015) 56–62.
[22] K. Ravichandran, K. Subha, N. Dineshbabu, A. Manivasaham, Enhancing the electrical parameters of ZnO films deposited using alow-cost chemical spray technique through Ta doping, J. Alloys Compd. 656 (2016) 332. [23] Z. Pan, P. Zhang, X. Tian, G. Cheng, Y. Xie, H. Zhang, X. Zeng, C. Xiao, G. Hu, Z. We, Properties of fluorine and tin co-doped ZnO thin films deposited by sol–gel method, J. Alloys Compd. 576 (2013) 31–37. [24] Y. Z. Tsai, N. F. Wang C. L. Tsai, Formation of F-doped ZnO transparent conductive films by sputtering of ZnF2, Mater. Lett. 63 (2009) 1621–1623. [25] M. L. Olvera, A. Maldonado, R. Asomoza, O. Solorza, D. R. Acosta, Characteristics of ZnO:F thin films obtained by chemical spray. Effect of the molarity and the doping concentration, Thin Solid Films 394 (2001) 241-48. [26] David S. Ginley and John D. Perkins, Transparent Conductors, in: David S. Ginley, Hideo Hosono, David C. Paine (Eds.), Handbook of Transparent Conductors, Springer, 2011, pp-7 [27] F. Yakuphanoglu, Y. Caglar, S. Ilican, M. Caglar, The effects of fluorine on the structural, surface morphology and optical properties of ZnO thin films, Physica B 394 (2007) 86-92. [28] A. Maldonado, A. G. Santiago, M. de la L. Olvera, R. C. Perez, G. T. Delgado, The role of the fluorine concentration and substrate temperature on the electrical, optical, morphological and structural properties of chemically sprayed ZnO:F thin films, Mater. Lett. 59 (2005) 1146-1151. [29] Y. Guo, L. Zhu, Y. Li, W. Niu, X. Zhang, Z. Ye, Interaction of H and F atoms - Origin of the high conductive stability of hydrogen-incorporated F-doped ZnO thin films, Thin Solid Films 589 (2015) 85-89. [30] B. N. Pawar, D. H. Ham, R. S. Mane, T. Ganesh, B. W. Cho, S. H. Han, Fluorine-doped zinc oxide transparent and conducting electrode by chemical spray synthesis, Appl. Surf. Sci. 254 (2008) 6294 - 6297. [31] S. S. Shinde, P. S. Shinde, S. M. Pawar, A. V. Moholkar, C. H. Bhosale, K. Y. Rajpure, Physical properties of transparent and conducting sprayed fluorine doped zinc oxide thin films, Solid State Sci. 10 (2008) 1209 – 1214.
[32] Y. Y. Bu, Sol–gel production of wrinkled fluorine doped zinc oxide through hydrofluride acid, Ceram. Int. 40 (2014) 14589–14594. [33] Y. R. Sui, Y.P. Song, Y.J. Wu, J.H. Lang, X.W. Meng, S.Q. Lv, B. Yao, J. H. Yang, Ceram. Int. 42 (2016) 15166-15170. [34] X. Yu, J. Ma, F. Ji, Y. Wang, C. Cheng, H. Ma, Thickness dependence of properties of ZnO:Ga films deposited by rf magnetron sputtering, Appl. Surf. Sci. 245 (2005) 310-315. [35] M. Caglar, S. Ruzgar, Influence of the deposition temperature on the physical properties of high electron mobility ZnO films by sol–gel process, J. Alloys Compd. 644 (2015) 101-105. [36] F. Aktaruzzaman, G. L. Sharma, L. K. Malhotra, Electrical, optical and annealing characteristics of ZnO:Al films prepared by spray pyrolysis, Thin Solid Films 198 (1991) 67- 74. [37] Y. Aoun, B. Benhaoua, B. Gasmi, S. Benramache, Effect of annealing temperature on structural, optical and electrical properties of zinc oxide (ZnO) thin films deposited by spray pyrolysis technique, Optik 126 (2015) 5407-5411. [38] J. H. Lee, B. O. Park, Characteristics of Al-doped ZnO thin films obtained by ultrasonic spray pyrolysis: effects of Al doping and an annealing treatment, Mater. Sci. Eng. B 106 (2004) 242–245. [39] H. Gomez, A. Maldonado, M. de la L. Olvera, D.R. Acosta, Gallium-doped ZnO thin films deposited by chemical spray, Sol. Energy Mater. Sol. Cells 87 (2005) 107–116. [40] A. El Manouni, F.J. Manjon, M. Perales, M. Mollar, B. Marı, M.C. Lopez, J. R. R. Barrado, Effect of thermal annealing on ZnO:Al thin films grown by spray pyrolysis, Superlattices Microstruct. 42 (2007) 134–139. [41] J. H. Lee, B. W. Yeo, B. O. Park, Effects of the annealing treatment on electrical and optical properties of ZnO transparent conduction films by ultrasonic spraying pyrolysis, Thin Solid Films 457 (2004) 333–337. [42] S. Benramache, B. Benhaoua, Influence of annealing temperature on structural and optical properties of ZnO: In thin films prepared by ultrasonic spray technique, Superlattices Microstruct. 52 (2012) 1062–1070.
[43] N. S. Kumar, K. V. Bangera, G. K. Shivakumar, Effect of annealing on the properties of Bi doped ZnO thin films grown by spray pyrolysis technique, Superlattices Microstruct. 75 (2014) 303–310. [44] K. Ravichandran, R. Mohan, N. J. Begum, S. Snega, K. Swaminathan, C. Ravidhas, B. Sakthivel, S. Varadharajaperumal, Impact of spray flux density and vacuum annealing on the transparent conducting properties of doubly doped (Sn + F) zinc oxide films deposited using a simplified spray technique, Vacuum 107 (2014) 68-76. [45] K. Ravichandran, R. Mohan, B. Sakthivel, S. Varadharajaperumal, P. Devendran, T. Alagesan, K. Pandian, Enhancing the photocatalytic efficiency of sprayed ZnO thin films through double doping (Sn + F) and annealing under different ambiences, Appl. Surf. Sci. 321 (2014) 310–317. [46] S. M. Lee, Y. H. Joo, C.Kim, Influences of film thickness and annealing temperature on properties of sol–gel derived ZnO–SnO2 nanocomposite thin film, Appl. Surf. Sci. 320 (2014) 494–501. [47] G. Li, X. Zhu, X. Tang, W. Song, Z. Yang, J. Dai, Y. Sun, X. Pan, S. Dai, Doping and annealing effects on ZnO:Cd thin films by sol–gel method, J. Alloys Compd. 509 (2011) 4816–4823. [48] X. Yu, X. Yu, Z. Hu, J. Zhang, G. Zhao, Y. Zhao, Effect of sol–gel derived ZnO annealing rate on light-trapping in inverted polymer solar cells, Mater. Lett. 108 (2013) 50–53. [49] I. Y. Y. Bu, Sol–gel production of ZnO:CO: Effect of post-annealing temperature on its optoelectronic properties, Mater. Sci. Semicond. Process. 41 (2016) 240–245. [50] C. Ravichandran, G. Srinivasan, C. Lennon, S. Sivananthan and J. Kumar, Influence of post-deposition annealing on the structural, optical and electrical properties of Li and Mg co-doped ZnO thin films deposited by sol–gel techniqueSuperlattices Microstruct. 49 (2011) 527–536. [51] X.L. Zhang , K.S. Hui, F. Bin, K.N. Hui, L. Lia, Y.R. Cho, R. S. Mane, W. Zhou, Effect of thermal annealing on the structural, electrical and optical properties of Al–Ni co-doped ZnO thin films prepared using a sol–gel method, Surf. Coat. Technol. 261 (2015) 149– 155.
[52] K. Ravichandran, P.V. Rajkumar, B. Sakthivel, K. Swaminathan, L. Chinnappa, Role of precursor material and annealing ambience on the physical properties of SILAR deposited ZnO films, Ceram. Int. 40 (2014) 12375–12382. [53] P.V. Rajkumar, K. Ravichandran, M. Baneto, C. Ravidhas, B. Sakthivel, N. Dineshbabu, Enhancement of optical and electrical properties of SILAR deposited ZnO thin films through fluorine doping and vacuum annealing for photovoltaic applications, Mater. Sci. Semicond. Process. 35 (2015) 189–196. [54] C. M. Muiva, T. S. Sathiaraj, K. Maabong, Effect of doping concentration on the properties of aluminium doped zinc oxide thin films prepared by spray pyrolysis for transparent electrode applications, Ceram. Int. 37 (2011) 555. [55] J. Kennedy, P. P. Murmu, J. Leveneur, A. Markwitz, J. Futter, Controlling preferred orientation and electrical conductivity of zinc oxide thin films by post growth annealing treatment, Appl. Surf. Sci. 367 (2016) 52-58. [56] X. L. Chen, J. M. Liu, J. Ni, Y. Zhao, X. d. Zhang, Wide-spectrum Mg and Ga co-doped ZnO TCO thin films for solar cells grown via magnetron sputtering with H2 introduction, Appl. Surf. Sci. 328 (2015) 193. [57] G. Haccke, New figure of merit for transparent conductors, J. Appl. Phys. 47 (1976) 4086 – 4089. [58] K. Ravichandrana, N. Dineshbabu, T. Arun, A. Manivasaham, E. Sindhuja, Synergistic effects of Mo and F doping on the quality factor of ZnO thin films prepared by a fully automated home-made nebulizer spray technique, Appl. Surf. Sci. 392 (2017) 624-633. [59] C. A. Gupta, S. Mangal, U. P. Singh, Impact of rapid thermal annealing on structural, optical and electrical properties of DC sputtered doped and co-doped ZnO thin film, Appl. Surf. Sci. 288 (2014) 411– 415. [60] T. Ghosh, P. Karmakar, B. Satpati, Electrochemical Ostwald Ripening and Surface Diffusion in Galvanic Displacement Reaction: Control over Particle Growth, RSC Adv., 5 (2015) 94380. [61] H. Wang, L. Xin, H. Wang, X. Yu, Y. Liu, X. Zhou, B. Li, Aggregation-induced growth of hexagonal ZnO hierarchical mesocrystals with interior space: nonaqueous synthesis, growth mechanism, and optical properties RSC Adv., 3 (2013) 6538.
[62] I. Burducea, C. Ionescu, M. Straticiuc, L. S. Craciun, P. M. Racolta, A. Jipa, Characterization Of Indium Nitride And Zinc Oxide Thin Films By AFM And RBS, Rom. Journ. Phys. 58 (2013) 345–353. [63] L. Znaidi, T. Touam, D. Vrel, N. Souded, S. B. Yahia, O. Brinza, A. Fischer, A.Boudrioua, ZnO Thin Films Synthesized by Sol–Gel Process for Photonic Applications, Acta Phys. Pol. A 121 (2012) 165-168. [64] F. K. Shan, G. X. Liu, B. C. Shin, W. J. Lee, Annealing E_ects of ZnO Thin Films Deposited on Si (100) by Using Pulsed Laser Deposition, J. Korean Phys. Soc. 54 (2009) 916-920. [65] G. Kenanakis, N. Katsarakis, E. Koudoumas, Influence of precursor type, deposition time and doping concentration on the morphological, electrical and optical properties of ZnO and ZnO:Al thin films grown by ultrasonic spray pyrolysis, Thin Solid Films 555 (2014) 62-67. [66] Y. Zhang, H. L. Lu, T. Wang, Q. H. Ren, H. Y. Chen, H. Zhang, X. M. Ji, W. J. Liu, S. J. Ding, D. W. Zhang, Photoluminescence enhancement of ZnO nanowire arrays with atomic layer deposited ZrO2 coatings and thermal annealing, Phys. Chem. Chem. Phys. 18 (2016) 16377-16385. [67] R. Swapna, M. Ashok, G. Muralidharan, M. C. S. Kumar, Microstructural, electrical and optical properties of ZnO:Mo thin films with various thickness by spray pyrolysis, J. Anal. Appl. Pyrolysis 102 (2013) 68–75. [68] M. A. Sliem, T. Hikov, Z. A. Li, M. Spasova, M. Farle, D. A. Schmidt, M. H. Newen, R. A. Fischer, Interfacial Cu/ZnO contact by selective photodeposition of copper onto the surface of small ZnO nanoparticles in non-aqueous colloidal solution, Phys. Chem. Chem. Phys. 12 (2010) 9858-9866. [69] H. Zhang, T. Zhao, G. Hu, L. Miao, Y. Yang, Role of Mg doping on morphology and photoluminescence features of MgxZn1−xO films prepared by ultrasonic spray pyrolysis, J. Mater. Sci: Mater. Electron. 23 (2012) 1933–1937. [70] J. M. Soler, E. Artacho, J. D. Gale, A. Garcıa, J. Junquera, P. Ordejon, D. S. Portal, The SIESTA method for ab initio order-N materials simulation, J. Phys.: Condens. Matter 14 (2002) 2745–2779.
[71] J. P. Perdew, K. Burke, M. Ernzerhof, Generalized Gradient Approximation Made Simple, Phys. Rev. Lett. 77 (1996) 38.
Figure captions Fig. 1. Schematic diagram for various possible mechanisms involved in the incorporation of Ta and F in to the ZnO lattice. Fig. 2a-c. Variation in electrical resistance, carrier concentration and mobility of ZnO:Ta:F films as a fuction of Ta+F doping levels. Fig. 3a-d. Resistivity of as-deposited, air-annealed and vacuum-annealed films with Ta+ F doping levels a) 1+5, b) 1+10, c) 1+15 and d) 1+20 at. % Fig. 4a-c. Transmittance spectra of ZnO:Ta:F films. Fig. 5a-c. dT/dλ vs. Energy plots of ZnO:Ta:F films. Fig. 6a-c. XRD patterns of ZnO:Ta:F films. Fig. 7a-i.
FESEM images of ZnO:Ta:F films.
Fig. 8. EDX spectrum of ZnO:Ta:F film with Ta+F doping level 1+10 at.%. Fig. 9. Compositional mapping of ZnO:Ta:F film with Ta+F doping level1+10 at.%. Fig. 10. XPS survey spectrum of ZnO:Ta:F film with Ta+F doping level as 1+10 at.%. Fig. 11 a-b. 3D and 2D images of ZnO:Ta:F film with Ta+F doping level 1+10 at.%. Fig. 12. Room temperature PL spectra of ZnO:Ta:F films. Fig. 13. Optimized structures of a) ZnO, b) ZnO-Ta, c) ZnO-Ta-F(Ta-F adjacent) and d) ZnO-Ta-F(Ta-F apart) models. Table caption Table 1.
Source materials and process parameters.
Table 2.
Analytical instruments used for characterization.
Table 3. Various possible mechanisms involved in the incorporation of Ta and F in ZnO lattice. Table 4. Resistivity of as-deposited, air-annealed and vacuum-annealed ZnO:Ta:F films. Table 5. Reported influence of annealing under different ambiences on the properties of ZnO based films deposited by chemical techniques. Table 6. Transmittance, optical energy gap, quality factor and grain size of the as-deposited, air-annealed and vacuum-annealed ZnO:Ta:F films. Table 7. Optical band gap values estimated from optical transmittance data and PL studies. Table 8. Total energy and change in energy of ZnO based structures calculated using DFT.
Fig. 1.
. Fig. 2.
Fig. 3.
Fig. 4.
Fig. 5.
Fig. 6.
Fig. 7.
Fig. 8.
Fig. 9.
Fig. 10
Fig. 11a.
Fig. 11b.
Fig. 12.
Fig. 13.
Table 1 Host precursor Dopant precursors
Solvent Spray volume Nozzle to substrate distance Substrate temperature Spray time and Spray interval Thickness of the films Annealing ambiences Annealing temperature and duration
: Zinc acetate dihydrate (0.1 M) (Sigma Aldrich, 99.999 %,USA) : Tantalum penta chloride (1 at. %) (Sigma-Aldrich, 99.99%, Germany) Ammonium fluoride (5, 10, 15 & 20 at. %) Sigma-Aldrich, 99.99%, Germany) : Doubly de-ionized water, methanol and acetic acid (7:2:1) : 50 Ml : 30 cm : 350 ± 5 ⁰C : Each 10 sec. : 400 nm (approximately) : Air and vacuum 450 ⁰C and 2 hours
Table 2 Analytical instruments used for characterization Electrical Optical Structural
: Linear four-point probe (vander Paw configuration) and Hall probe (ECOPIA HMS-3000) : UV–vis–NIR double-beam spectrophotometer (Perkin Elmer Lambda 35 model) : X-ray diffractometer (PANalytical PW 340/60 X’pert PRO)
Surface morphology and elemental analysis Photoluminescence Chemical state
: Field Emission Scanning Electron Microscope with Energydispersive X-ray Spectrometer (JEOL JSM – 6701F) : Spectro-flurometer (Jovin Yvon-FLUROLOG-FL3-11) : K-AlphaTM+ X-ray Photoelectron Spectrometer
Table 3
Case
Element
Type of incorporation
Effect on carrier concentration
a
Tantalum
Substitution
Increases the number of free electrons by 3, when the nearby oxygen shares two electrons.
b
Fluorine
Substitution
Increases the number of free electrons by 1, when the nearby zinc site is occupied by zinc.
c
Tantalum
Substitution
Increases the number of free electrons by 4, when the nearby oxygen site is occupied by fluorine.
d
Tantalum
Interstitial
Each interstitial tantalum ion donates its unshared 5 electrons to the system.
e
Tantalum
Substitution
Each substitution causes an increase of 5 free electrons when the nearby oxygen site is vacant.
f
Tantalum
Anti site
Adverse effect, (i.e) no contribution of free carrier.
g
Fluorine
Substitution
Increases the number of free electrons by 4, when the nearby zinc site is occupied by tantalum (similar situation as case c).
h
Fluorine
Interstitial
This defect is not common because the negative ions are usually very large and they cannot easily fit into the interstitial sites (rare case).
i
Fluorine
Anti site
Adverse effect, (i.e) no contribution of free carrier. It is unlikely to be present under equilibrium condition (rare case).
Table 4
Doping level of Ta+F
Resistivity (ρ) × 10-3 (Ω cm) As-deposited film
Air-annealed film Vacuum-annealed film
1+5
3.96
121.7
1.92
1+10
2.056
113.7
0.81
1+15
2.72
130.2
1.40
1+20
3.42
142.9
1.85
Table 5
Method Material
Resistivity (Ω cm) Annealing ambience Before annealing
Spray
ZnO:Ga
Air Vacuum H2 Air N2 N2/H2 Vacuum
Spray
ZnO:Al
Air
-
Spray
ZnO
N2 N2/H2
9.91 Ω -
Spray
ZnO:In
Air
-
Spray
ZnO:Bi
Air
-
Spray
ZnO:Sn:F
9.27 × 10−3
Spray
ZnO:Sn:F
Spray Sol-gel Sol-gel Sol-gel Sol-gel Sol-gel Sol-gel
ZnO:In:F ZnO:SnO2 ZnO:Cd ZnO ZnO:Co ZnO:Li:Mg ZnO:Al:Li
Vacuum Air Vacuum Ar gas Air N2 Air Air Air N2/H2 Air Vacuum Vacuum
Spray
ZnO:Ta:F
Spray Spray
ZnO:Al ZnO
Spray
ZnO:Al
SILAR ZnO SILAR ZnO;F
2.056 × 10−3 2.5 0.5 5.88 × 10−1 3.6 × 10
−2
1.34 × 10−3 5.2 × 10-2 8.17 × 102 0.9 91370 8.98× 10−3 1.63×10−3
After annealing 132.9 × 10−3 0.81 × 10−3 4.7 × 10−3 0.42 1.64 × 10−1 1.71 × 10−2 8 × 10−3 Resistance increases with increase in annealing temperature 1.12 1.62 × 10−1 Conductivity increases (Resistivity data not available) Conductivity increases (Resistivity data not available) Resistance decreases 1.06 × 10−3 0.69 × 10−3 6.3 × 10-3 1.12 × 102 0.034 Resistance decreases (data not available) Resistance decreases (data not available) 513 Ω cm Decreases to 1.05× 10−3 6.2× 10−3 4.04× 10−3 1.32×10−3
Transmittance (%) Before After annealing annealing 93 94 90.5 70 85 T increases
Reference This study [36] [37]
80
T increases
[38]
82
No change
[39]
-
-
[40]
80
T decreases
[41]
-
-
[42]
75
T increases
[43]
-
T increases
[44]
-
T increases
[45]
82.1 78 80
80 T increases 82 T decreases
[20] [46] [47] [48] [49] [50] [51]
-
T increases
[52]
-
T increases
[53]
Table 6
Doping level of Ta+F 1+5 1+10 1+15 1+20
Transmittance T
Optical band gap Eg
Quality factor ϕ
(%)
(eV)
×10-4 (Ω/sq)-1
Crystallite size D (nm)
As-
Air-
Vacuum-
As-
Air-
Vacuum-
As-
Air-
Vacuum-
As-
Air-
Vacuum-
deposited
annealed
annealed
deposited
annealed
annealed
deposited
annealed
annealed
deposited
annealed
annealed
film
film
film
film
film
film
film
film
film
film
film
film
94.0 93.5 93.0 89.0
93.5 93.5 91.0 89.0
93.0 92.5 88.0 87.0
3.26 3.27 3.25 3.23
3.24 3.23 3.22 3.21
3.29 3.33 3.32 3.30
0.544 0.993 0.712 0.365
0.017 0.018 0.012 0.009
1.008 2.265 0.796 0.537
48.2 53.0 68.1 58.3
53.1 64.2 72.0 60.5
59.0 70.1 74.1 71.4
Table 7
Film
Optical energy gap value from transmittance study (dT/dλ vs Eg ) (eV)
Change in Eg due to annealing (eV)
As-deposited
3.27
Air-annealed Vacuum-annealed
From PL study
Change in Eg due to annealing (eV)
Wavelength at NBE peak (nm)
Calculated energy (eV)
-
390.6
3.18
-
3.33
0.06
396.0
3.14
0.04
3.37
0.1
378.6
3.28
0.1
Table 8 Structure ZnO ZnO-Ta ZnO-F ZnO-Ta-F(Adjacent) ZnO-Ta-F-Apart
Total Energy (eV)
(% )
-31673.03 -30420.44 -31881.21 -30627.32 -30628.13
100 96.05 100.66 96.70 96.70
Change in Energy ΔE (%) 3.95 -0.66 3.30 3.30