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Rare earth Sm3+ co-doped AZO thin films for opto-electronic application prepared by spray pyrolysis
Author’s Accepted Manuscript Rare earth Sm3+ co-doped AZO thin films for optoelectronic application prepared by spray pyrolysis V. Anand, A. Sakthivelu, K. Deva Arun Kumar, S. Valanarasu, A. Kathalingam, V. Ganesh, Mohd Shkir, S. AlFaify, I.S. Yahia www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)30100-7 https://doi.org/10.1016/j.ceramint.2018.01.088 CERI17220
To appear in: Ceramics International Received date: 11 December 2017 Revised date: 10 January 2018 Accepted date: 11 January 2018 Cite this article as: V. Anand, A. Sakthivelu, K. Deva Arun Kumar, S. Valanarasu, A. Kathalingam, V. Ganesh, Mohd Shkir, S. AlFaify and I.S. Yahia, Rare earth Sm3+ co-doped AZO thin films for opto-electronic application prepared by spray pyrolysis, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.01.088 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 galley proof before it is published in its final citable 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.
Rare earth Sm3+ co-doped AZO thin films for opto-electronic application prepared by spray pyrolysis V. Anand1, A. Sakthivelu2, K. Deva Arun Kumar3, S. Valanarasu*3, A. Kathalingam4, V. Ganesh5,6, Mohd Shkir*5,6, S. AlFaify5,6, I.S. Yahia5,6 1
Physics Department, VKS College of engineering and Technology, Karur, India PG and Research Department of Physics, Periyer E.V.R College, Trichy, India 3 PG and Research Department of Physics, Arul Anandar College, Karumathur, Madurai, India 4 Millimeter-Wave Innovation Technology Research Center (MINT), Dongguk University-Seoul, Seoul 04620, Republic of Korea 5 Advanced Functional Materials & Optoelectronic Laboratory (AFMOL), Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia 6 Nanoscience Laboratory for Environmental and Bio-medical Applications (NLEBA), Semiconductor Lab., Metallurgical Lab.2 Physics Department, Faculty of Education, Ain Shams University, Roxy, 11757 Cairo, Egypt. 2
Corresponding author* Dr. Mohd. Shkir Assistant Professor Department of Physics, College of Science, King Khalid University, Abha, SA E-mail: [email protected], [email protected]
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Abstract Samarium co-doped aluminum zinc oxide (Sm:AZO) thin films were deposited on glass substrate by nebulizer spray pyrolysis technique with different Sm doping concentrations (0 at.%, 0.5 at.%, 1 at.% and 1.5 at%). X-ray diffraction patterns confirm the polycrystalline nature of prepared films with hexagonal crystal structure. The average crystallite size was found to be reduced with Sm doping due to increased lattice defects. The Raman spectra exhibited characteristic ZnO wurtzite structure confirmed through the presence of E2-high mode peak at 438 cm−1. The surface topology analysis revealed the uniformly distributed wheat shaped particles without any pinholes for 1 at.% Sm doped ZnO film. Sm:AZO films displayed high transparency which is around 90% and the energy gap of ~ 3.30 eV. Photoluminescence spectra of the thin films showed an UV emission peak at ~ 386 nm corresponds to near band edge (NBE) emission of bulk ZnO. Room temperature Hall Effect measurement showed that all the prepared films possess n-type conducting nature with low electrical resistivity (ρ) 4.31×10-4 Ω.cm for 1 at.% Sm doped film. The high figure of merit (ф) value of ~ 11.9 ×10-3 (Ω/cm)-1 was observed which indicates that the deposited films are highly suitable for opto-electronic device applications. Keywords:
Rare earth, Sm:AZO thin film, Nebulizer spray pyrolysis, Opto-electronic
application 1. Introduction Zinc Oxide (ZnO) with unique optical and electrical properties is an attractive candidate for a wide range of technological applications such as: transparent conducting oxide, solar cells, light emitting diodes, gas sensors and flat panel displays etc. [1-4]. ZnO is a direct optical band gap (i.e. 3.37 eV) semiconductor material and has huge exciton binding energy of about 60 meV [5]. Naturally, ZnO is an n-type semiconductor caused by defect of zinc interstitials and oxygen 2
vacancies. In recent past many researchers studied the ZnO due to their physical and chemical stabilities, nontoxic nature with good optical and electrical properties. In fact, the electrical resistivity of pure ZnO thin films has been high due to low carrier concentration. Most researchers choose doping with group III element like (Al, Ga and In), to reduce the electrical resistivity of ZnO thin films. Especially, Al-doped ZnO (AZO) thin film display low optical loss with good electrical conductivity. Moreover, the (AZO) is an excellent replacement for indium tin oxide (ITO) because of its low cost and chemical stability compared to ITO [6]. But, upon increasing the doping concentration level, the resistivity begins to increase which may be due to the increased solubility limit of the particular dopant in the ZnO structure [7]. So, we must choose a co-dopant element for improving ZnO film conductivity. Commonly, rare earth (Gd, Nd, Eu, Sm, etc.,) metals are co-doped to ZnO to enhance optical and electrical properties. Among the rare earth materials, samarium (Sm) is one of the best due to their unique electrical properties. The ionic radius of Sm3+ (1.04 Å) is higher than the ionic radius of Zn2+ (0.74Ǻ), hence the Zn ions can be easily replaced by Sm ions with some minor change in the lattice parameters. Only a few reports are available on the doping of rare earth Sm with ZnO thin films using chemical method [8]. In recent years, the majority of AZO thin films are prepared using variety of physical and chemical methods such as pulsed laser deposition [9], magnetron sputtering [10], chemical vapor deposition [11], molecular beam epitaxy (MBE) [12], hydrothermal [13], sol–gel coating [14], SILAR method [15], chemical bath deposition [16] and spray pyrolysis [17], etc. Amongst all these methods, spray pyrolysis is a very simple and cost-effective method. In particular, from spray method, we can easily control the film thickness in the nanometer range and maintain the smooth film surface morphology.
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In the present work, Sm co-doped AZO thin films are deposited on microscopic glass substrate by nebulizer spray pyrolysis technique. Nebulizer spray pyrolysis [18, 19] is a key method for preparing device quality semiconducting material. The Sm co-doping effects on the lattice structural, surface morphological, optical and electrical properties for the prepared films are investigated. To the best of our knowledge, this will be the first report on the synthesis and fabrication of Sm co-doped AZO thin films using a simplified nebulizer spray technique. 2. Experimental procedure 2.1. Precursor and film deposition method Analytical grade zinc acetate dihydrate [Zn (CH3COO)2·2H2O], aluminum chloride hexahydrate [AlCl3.6H2O] and samarium oxide [SmO2] purchased from (99.9% purity, SigmaAldrich), (99.9% Alfa Aeser) and (98.9% Alfa Aeser), respectively. All the mentioned materials were used as host, dopant and co-dopant precursors, respectively. Hydrochloric acid (HCl) and Methanol (CH3OH) were used to prepare the precursor solutions. The Sm doped AZO thin films were deposited onto glass substrates by simple nebulizer spray pyrolysis technique. Initially, 0.2M of zinc acetate dihydrate precursor (salt) was dissolved in 10 ml of methanol solvent. Further, initial doping element of (3%) aluminum chloride hexahydrate precursor was mixed in the above zinc solution and the mixed solution was stirrer continuously at room temperature. Additionally, co-doping element of samarium oxide was dissolved in HCl solution with different doping level (0%, 0.5%, 1% and 1.5%), as samarium oxide only dissolves in acid solution. Finally, the prepared Sm solution was added drop by drop to the zinc aluminum solution. The solution was stirred for 15 minutes at room temperature to yield a clear and homogeneous Sm:AZO solution. Before the deposition process, the amorphous glass substrates were respectively cleaned using chromic acid, de-ionized water and acetone. The cleaned glass substrate was kept onto the hot plate and the substrate temperature was maintained 4
at ~ 450°C (±2 oC) using PID temperature controller. The 10 ml prepared Sm:AZO solution was taken in a nebulizer container (nebulizer unit) and it was continuous sprayed onto heated substrate. The nebulizer spray gun was slowly moved on to the substrate to deposit uniform Sm:AZO thin film. The main parameters of nebulizer spray pyrolysis (NSP) technique for depositing the films were maintained as follows:
Host precursor
:0.2 M (Zinc acetate)
Dopant precursor
:3 % (Aluminum chloride)
Co-dopant precursor
:0, 0.5, 1 and 1.5 at. % (Samarium oxide)
Solvent
: methanol (10ml)
Substrate temperature
:450°C
Substrate to nozzle distance :30 mm
Carrier gas pressure
:1.5 kg/cm2
Flow rate
:1 ml/minute
2.2. Characterization of prepared films X-ray diffraction spectra of the prepared films were record using X’PERT 3 POWDER X-ray diffractometer system using CuKα (λ=1.5406 nm). The surface morphology was recorded using AFM-NT-MDT (Type Next, Russia) in a semi-contact mode using attached NT-MDT software. The EDAX spectrum was recorded using energy dispersive X-ray spectroscope equipped with SEM (MODEL: JSM 6360 LA, Japan). The optical absorption and electrical properties study of Sm:AZO thin films were performed in the wavelength range (350-1100 nm) using SHIMADZU 1800 (UV- visible spectrophotometer). A photoluminescence spectrum for the prepared films was observed using Perkin Elmer LS55 florescent spectrophotometer with excitation wavelength is 325 nm. The electrical parameters of the Sm doped AZO films was measured using the four point probe hall effect technique with van der Pauw configuration at room temperature. 5
3. Results and discussions 3.1. XRD Analysis Fig. 1(a) shows the X-ray diffreaction patterns of pure and Sm:AZO thin films with different Sm doping concentration (0, 0.5, 1 and 1.5at.%). From XRD data it can be observe that it exhibits a high intense peak at 34.53° corresponding to (002) plane and other low intensity peaks at 31.8, 36.4, 47.6 and 63.01° corresponding to (100), (101), (102) and (103) planes, respectively for 0% Sm doped AZO film. The presence of these planes indicates only ZnO phase formation with hexagonal crystal structure (JCPDS file number: 89–0510). Moreover, the predominant peak (002) intensity was suddenly decreased when adding Sm as a doping element because the c-axis orientation was reduced with respect to Sm content. Similar type of results was also reported by He et.al [8]. However, the other two major planes such as (100) and (101) peak intensity was increases with increasing Sm doping concentration is may be due to the transformation of c-axis orientation from (002) to (100) and (101). In addition the c-axis orientation of (100) plane was high compared to (101) plane. This indicates that the c axis orientations of the grains becomes uniformly perpendicular to the substrate surface for (100) plane. Only one additional peak was observed at 0.5, 1 and 1.5 at.% Sm doped Sm:AZO films. The observed peak (2 theta) position of 56.5° is related to ZnSm2O4 phase was matched with JCPDS file number (39–0858). This means the introduction of the Sm ions into the hexagonal structure of ZnO resulted in the expansion of the ZnO lattice along c-axis and a reduction in the a-axis to preserve the Wurtzite hexagonal structure of ZnO [20]. Whereas, the film crystallinity was decreases when transformation of c-axis orientation occurs with addition of Sm content. The lattice constants (a) and (c) of the hexagonal phase structure of ZnO was determined by the relation [21] using XRD data:
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1 4 h 2 hk k 2 l 2 2 d2 3 a2 c
(1)
where, d is the interplanar lattice spacing and (hkl) is the miller indices. The calculated lattice parameters and average particle sizes of the AZO:Sm films is shown in Fig. 1(b) and these values are listed in Table 1. The calculated lattice constants are found to be a = 3.247Å and c = 5.1967Å, which is slightly lower than the standard value (JCPDS file: 89.0510) of the hexagonal structure. This may be due to the addition of aluminum and samarium elements caused by local changes in lattice. The lattice constants values are found to increase significantly with Sm content due to larger ionic radius of Sm3+ (1.04 A˚) compared to Zn2+ (0.74 A˚). The average crystallite size (D) were obtained by Scherrer’s equation [22-25]:
D
0.9 cos
(2)
where, λ is the wavelength of X-rays and β the full-width at half-maximum (FWHM) of the X-ray diffraction peaks which is found to be decreases with increasing Sm content. This is attributed to the Sm cations incorporation into the ZnO crystal lattice leading to the crystal lattice distortion due to the lower ionic radius of Zn2+ than that of Sm3+. The film thickness (t) was measured by using stylus-profilometer. Film thickness variation with different Sm doped films was presented in Table 1. 3.2. Morphological and roughness analysis by AFM The surface topology of the Sm co-doped AZO thin films was obtained using atomic force microscopy (AFM) as depicted in Fig. 2 (a-d). From the AFM (2D and 3D) images, it is clear that the AZO film morphology strongly depends on the samarium content. The undoped AZO film has spherical shaped homogeneous grains on the film surface [Fig. 2(a)] similar to reported by R.A. Mereuet.al. [26]. The morphology changes from spherical to wheat shaped 7
particles with the addition of Sm content in ZnO [Fig. 2 (b)]. Further, these wheat shaped particles are uniformly distributed without any pinholes and the porous nature increases with Sm doping [Fig. 2(c)]. Consequently, the surface roughness of the films also increases. This kind of morphology results in the improvement of film conductivity and it is consistent with our electrical result (see sec 3.6). However, the film with higher Sm concentration [Fig. 2(d)] are porous and have clusters with reduced carrier concentration. Fig. 3(a) illustrates the SEM and elemental mapping images of 1% at Sm doped film. Zinc, Oxygen, Aluminum and Samarium are uniformly distributed over the Sm:AZO film surface. Fig. 3(b) shows the EDAX spectrum of Sm: AZO thin film prepared at 1% Sm. The film is stoichiometric with no other chemical species. It is found that Zinc, Oxygen, Aluminum and Samarium elements are present in the film as 47.7%, 49.1%, 2.5 % and 0.7%, respectively. 3.3. Raman analysis Raman spectroscopy is a key device to explore the phase information for the prepared material such as nanoparticles and thin film by using Raman light scattering. Fig. 4 illustrates the Raman spectra of Sm:AZO thin films as a function of wave numbers in the range of 2001400 cm-1. From the Raman spectra, two high intense lead peaks and one wide peak located at 564 cm-1, 1097 cm-1 and 791 cm-1, respectively, are observed. No secondary phase modes of aluminum and samarium are detected which may be due to their interstitial incorporation into the ZnO lattice. In the Raman spectrum, the (E2-high) mode peak is located at 438 cm−1 and it is assigned to ZnO wurtzite structure [6]. The peak observed at 564 cm–1 represents the A1 (LO) and E1 (LO) vibration modes. This vibration mode is attributed to oxygen vacancies defects and free charge carriers. Previously, the A1 (LO) mode at 570 cm−1 was also observed for Sb, Fe and Al-doped ZnO thin film [27]. Another large intense A1 (TO) mode was obtained at 1097 cm−1 is
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corresponds to the zinc interstitial on the ZnO lattice. A broad band was also covered at 791 cm-1 which is recognized to glass substrate. Manghnani et.al [28] also reported a broad band at around 800 cm-1 assigned to the symmetric bond stretching on the glass plate. 3.4. Optical properties In general, the high optical transparency plays a key role in opto electronic applications of TCO material. Hence, the optical transmittance spectrum with respect to wavelength of Sm:AZO thin films using different samarium doping concentration are plotted in Fig. 5. The thin film optical transmittance is above 75% in the wavelength range of 500–800 nm, which is high enough for optoelectronic and solar cell applications. In our case, the observed transmittance of the prepared films are 90, 88, 86 and 82% for the Sm co-doping level of 0, 0.5, 1 and 1.5%, respectively. From Fig. 5, the entire transmittance spectrum shows steep absorption edges at the wavelength range of 365-375 nm indicative of good crystalline nature of Sm:AZO with low defect density attractive for solar cell (window layer) and optoelectronic devices [29]. Moreover, the transmittance spectrum shows oscillations caused by interference (small maxima and minima) due to good film crystallinity and surface smoothness. Further, the absorption edge is slightly shifted in the direction of higher wavelength with respect to Sm co-doping concentration. It is indicating the reduction in
band gap value for the deposited films. The observed
transmittance values systematically decrease with increase in Sm co-doping concentration. He et.al [8] reported the transmission increase with increase in Sm content for Sn:ZnO films. In our case, the transmission value is in opposite direction, because of already Al element was placed in to the ZnO lattice; so the Sm ions are creating some lattice defects (see XRD). Wurtzite structure of ZnO thin film has a direct energy gap, so that the optical energy gap (Eg) value is able to estimate with Tauc’s formula [24, 30-32]:
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h = B(h - Eg )n
(3)
In the equation (3) used to get the energy gap value, (hν) is the incident photon energy, (α) is the absorption coefficient, (B) is constant value and (Eg) is the direct energy gap. Inset of Fig. 5 and Table 2 as shows the calculated energy gap values of the Sm:AZO thin films are 3.30, 3.27, 3.28 and 3.25 eV correspond to films Sm (0%, 0.5%, 1% and 1.5%), respectively. Observed energy gap value of our prepared Sm:AZO (3.30eV) film is very close to the customary energy gap value of bulk ZnO (3.36eV) material. However, a small reduction in the energy gap values is may be due to the change of different surface structure [33]. The obtained least energy gap (3.25eV) value for the Sm:AZO thin film with 1.5% doping content compared to others, is due to some lattice defect present in the film. Refractive index (n) and extinction coefficient (k) are the main optical parameters for TCO thin film. The (n) and (k) values of Sm:AZO thin films are obtained from the reflectance (R) using the following equations [34-37]:
n
(1 R) 4R k2 (1 R) (1 R) 2
k
4
(4)
(5)
Fig. 6 shows the variation of refractive index as a function of wavelength (380-1000 nm). In general, the refractive index decreases when transmittance increases. Fig.7 shows the variation of extinction coefficient in the same wavelength range. In our case, both the (n) and (k) values increase with Sm content in contrast to the variation of film transmittance. The obtained (n) and (k) values are listed in Table 2, and the variation of extinction coefficient is similar to the variation of refractive index. The observed (n) and (k) values are in good agreement with the
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previously reported AZO thin films prepared by sol-gel method [6]. The calculated values of real 2 2 part ( r n k ) and imaginary part ( i
2nk ) of dielectric constants [38] of Sm:AZO
thin films are presented in the Table 2. Real part (εr) and imaginary part (εi) of dielectric constants are related to the speed of light and absorption energy from the electric field, respectively [39]. It clear from tabulated data (see Table 2) that both εr and εi values increase with increasing Sm co-doping content which may be due to the reduction of film transmittance. In addition, the (εr) and (εi) should also be connected with the increase of refractive index and decrease of film crystallinity. 3.5. Photoluminescence Analysis Room temperature photoluminescence spectra of all the deposited Sm:AZO thin films with 325 nm excited wavelength were recorded as shown in Fig. 8. From the PL spectra, it is found that two high and low intense emission bands are located at 389 and 561 nm corresponding to 3.19 and 2.21 eV for all the Sm:AZO films, respectively. These two bands are subsequent to UV and green emission region. Also a broad blue-green emission band was observed at 502 nm corresponding to 2.47 eV for the Sm:AZO films. Obtained high intensity UV emission peak (386 nm) is due to near band edge (NBE) emission of bulk ZnO [39], which is mainly depends on the film crystalline quality. Similar results have also been reported by Kumar et.al. [6] in AZO thin films prepared by sol-gel method. The broad blue-green emission peak located at 502 nm (2.47eV), is due to single ionized oxygen vacancies (VO+) and also it has some surface defects on the film [40]. Final green emission peak was found at 561 nm (2.21 eV) is caused by zinc interstitial position [41]. 3.6. Electrical studies
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Electrical properties of AZO thin films were done by room temperature Hall Effect measurement. In fact, the AZO thin films have low electrical resistivity and have n-type conducting nature. The electrical resistivity (ρ), carrier concentration (n) and hall mobility (μ) values of deposited films are listed in Table 3 and are also plotted in Fig. 9. As shown in Fig 9, the electrical resistivity decreases up to (at 1% Sm) and then slightly increases with higher Sm doping level (at 1.5%). This kind of resistivity variation with respect to Sm content was also reported by He et.al [8] using chemical solution method. They observed the low resistivity value is 0.28×10-2 Ω.cm. In our case, the film with Sm-content of 1 at.% shows a minimum resistivity (ρ) and maximum carrier concentration (n) of 4.31×10-4 Ω.cm and 6.31×1021 cm-3, respectively. The film carrier concentration continuously increases due to reduction of grain boundary scattering and creation of more free electrons. It is evidenced our smooth morphology (SEM) with pinholes free on the film surface. The observed (ρ) and (n) values are better than the previously (Sm doped) reported values. However, the observed hall mobility values were initially reduced and then increase with respect to Sm doping level is due to the ionized impurity scattering mechanism is reported by Shen et al. [42]. Fig.10 shows the variation of resistivity ln (ρ) versus various inverse temperatures K-1 for the prepared Sm:AZO thin films. The resistivity values systematically reduced with increase of temperature which is confirmed that the prepared films had semiconducting nature. All the prepared films had non-linear behavior due the activation energy. The obtained heated resistivity ln (ρ) value is agreed well with the room temperature Hall Effect resistivity value for all the prepared films due to activation energy. Activation energy (Ea) values was calculated from the resistivity ln (ρ) Vs temperature K-1 plot by using the below equation:
Ea KT
0 exp
(6) 12
Where k is the Boltzmann constant, ρ0 is the exponential parameter and T is the Kelvin temperature. The calculated Ea values are 0.11 eV, 0.05 eV, 0.02 eV and 0.012 eV which is associated to 0%, 0.5%, 1% and 1.5% Sm doped films respectively. The activation energy initially reduced and then increases with the change of different Sm doping concentration due to defect of oxygen deficiency [43]. The opto electric quality of the Sm:AZO film can be determined using figure of merit (ϕ) calculation. The (ϕ) value was estimated from transmittance (T) and sheet resistance (Rsh) by using the below formula [44]: ( )
T 10 R sh
(7)
Generally, the optical and electrical properties must be high as possible for any TCO thin films. In our case, the calculated figure of merit value is found to be higher for 1 at.% Sm doped film compared to other films and the values are listed in Table 3. Hence, we can conclude that the AZO film deposited with samarium element is more suitable for optoelectronic device, because it has good optical transparency and electrical conductivity. 4. Conclusion In this report, for the first time, we have prepared device quality rare earth co-doped AZO thin films via chemical route i.e. nebulizer used spray pyrolysis technique. There are several key conclusions of this study as follows: 1. Samarium (Sm3+) and aluminum (Al3+) elements are successfully dispersed and incorporated, respectively into ZnO lattice. 2. Sm:AZO thin films had multiple interference with high transmittance (90%) and the obtained energy gap is calculated to be ~3.3 eV.
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3. Low resistivity (ρ) and high figure of merit (ф) values of 4.31×10-4 Ω.cm and 11.9 ×10-3 (Ω/cm)-1, respectively, are observed for 1% at Sm doped film. Therefore, rare earth doped AZO metal oxide thin film is most suitable for optoelectronic device applications. Acknowledgement The authors express their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through research groups program under grant number R.G.P.2/9/38. Conflict of interest: The authors declares that there is no conflict of interest in the current work.
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[31] M. Shkir, S. AlFaify, I.S. Yahia, M.S. Hamdy, V. Ganesh, H. Algarni, Facile hydrothermal synthesis and characterization of cesium-doped PbI2 nanostructures for optoelectronic, radiation detection and photocatalytic applications, Journal of Nanoparticle Research, 19 (2017) 328. [32] M. Shkir, S. AlFaify, V. Ganesh, I.S. Yahia, Facile one pot synthesis of PbS nanosheets and their characterization, Solid State Sciences, 70 (2017) 81-85. [33] R. Zamiri, A. Lemos, A. Reblo, H.A. Ahangar, J. Ferreira, Effects of rare-earth (Er, La and Yb) doping on morphology and structure properties of ZnO nanostructures prepared by wet chemical method, Ceramics International, 40 (2014) 523-529. [34] T. Moss, Optical process in semiconductors, Butter Worths, London, (1959). [35] Z.D. Luo, Y.D. Huang, Qualitative analysis of relationship between refractive index and atomic parameters of solid materials, Journal Of Rare Earths, 22 (2004) 486-488. [36] M. Shkir, V. Ganesh, S. AlFaify, I.S. Yahia, Structural, linear and third order nonlinear optical properties of drop casting deposited high quality nanocrystalline phenol red thin films, Journal of Materials Science: Materials in Electronics, 28 (2017) 10573-10581. [37] V. Ganesh, M. Shkir, S. AlFaify, I.S. Yahia, H.Y. Zahran, A.F.A. El-Rehim, Study on structural, linear and nonlinear optical properties of spin coated N doped CdO thin films for optoelectronic applications, Journal of Molecular Structure, 1150 (2017) 523-530. [38] M.A. Omar, Elementary solid state physics: principles and applications, Pearson Education India1975. [39] L. Ma, S. Ma, H. Chen, X. Ai, X. Huang, Microstructures and optical properties of Cu-doped ZnO films prepared by radio frequency reactive magnetron sputtering, Applied Surface Science, 257 (2011) 1003610041. [40] N. Tarwal, P. Jadhav, S. Vanalakar, S. Kalagi, R. Pawar, J. Shaikh, S. Mali, D. Dalavi, P. Shinde, P. Patil, Photoluminescence of zinc oxide nanopowder synthesized by a combustion method, Powder Technol., 208 (2011) 185-188. [41] H.S. Kang, J.S. Kang, J.W. Kim, S.Y. Lee, Annealing effect on the property of ultraviolet and green emissions of ZnO thin films, Journal of Applied Physics, 95 (2004) 1246-1250. [42] H.-l. Shen, H. Zhang, L.-f. Lu, F. Jiang, Y. Chao, Preparation and properties of AZO thin films on different substrates, Progress in Natural Science: Materials International, 20 (2010) 44-48. [43] F. Yakuphanoglu, Electrical conductivity, Seebeck coefficient and optical properties of SnO 2 film deposited on ITO by dip coating, Journal of Alloys and Compounds, 470 (2009) 55-59. [44] K.D.A. Kumar, S. Valanarasu, K. Jeyadheepan, H.-S. Kim, D. Vikraman, Evaluation of the physical, optical, and electrical properties of SnO2: F thin films prepared by nebulized spray pyrolysis for optoelectronics, Journal of Materials Science: Materials in Electronics, (2017) 1-9.
16
Table.1. Lattice parameters and crystallite size variation of AZO:Sm thin films Samarium doping level (%)
Lattice parameter (A0)
Crystallite size (nm)
Thickness (nm)
(100)
(002)
(100)
(102)
(103)
Average
a
c
0
16.8
33.4
34.5
29.4
26.2
28.06
3.24086
5.17905
236
0.5
26.0
28.1
42.5
24.6
18.6
27.95
3.24394
5.19254
298
1
33.4
23.4
45.9
17.3
10.2
26.04
3.24538
5.19534
324
1.5
42.9
17.2
48.1
12.6
5.7
25.31
3.24682
5.19636
410
17
Table.2. Optical and dielectric parameters of AZO:Sm thin films Sm doping level (%)
Transmittance (%)
Band gap (eV)
Refractive Extinction Dielectric index coefficient constant (n) (k) (εr) Average visible region (600nm)
Dielectric constant (εi)
0
90
3.30
1.60
0.015
2.50
0.04
0.5
88
3.27
1.69
0.018
2.96
0.06
1
86
3.28
1.72
0.021
3.19
0.07
1.5
82
3.25
1.85
0.037
3.52
0.10
18
Table.3. Electrical parameters of Sm:AZO thin films with different doping level
Sm doping level (%)
Resistivity(ρ) Ω.cm
Carrier concentration cm-3
Hall mobility(μ) ×cm2(VS)-1
0
8.62×10-4
5.63×1020
12.86
1.16×103
7.5
0.5
6.28×10-4
9.05×1020
11.01
1.58×103
8.3
1
4.31×10-4
6.31×1021
2.29
2.32×103
11.9
1.5
9.86×10-4
3.12×1020
20.25
1.02×103
6.1
Conductivity(σ) Figure of (Ω.cm)-1 merit (Փ)×10-3 (Ω/sq)-1
19
Figure captions Fig.1(a). XRD patterns of Sm:AZO thin films with different co-doping level Fig.1(b). Variations of lattice constant values with different Sm doping level Fig.2. AFM 2D and 3D images of Sm:AZO thin films with different co-doping level (a) 0%, (b) 0.5%, (c) 1% and (d) 1.5 % Fig.3(a) SEM-EDS micrograph and maps of distribution of elements on the surface of Sm:AZO thin film for (1% at. Sm) and, 3(b) EDAX spectrum for that film Fig.4. Raman spectra of Sm:AZO thin films with different co-doping level Fig.5. Optical transmittance of Sm:AZO thin films with different co-doping level and (Inset of Fig.5) shows the relation of (αhυ)2 and hυ of Sm:AZO thin films with different co-doping level Fig.6. Refractive index of Sm:AZO thin films with different co-doping level Fig.7. Extinction coefficient of Sm:AZO thin films with different co-doping level Fig.8. Photoluminescence spectra of Sm:AZO thin films with different co-doping level Fig.9. Variations of electrical resistivity, carrier concentration and conductivity of Sm:AZO thin films with with different co-doping level Fig.10. Plot of ln (ρ) versus 1000/T for the Sm:AZO films with different co-doping level
20
Figure 1 (a) and (b)
21
Figure 2 (a)
22
Figure 2 (b)
23
Figure 2 (c) 24
Figure 2 (d) 25
Figure 3(a) 26
Figure 3(b)
27
Figure 4
28
Figure 5
Figure 6
29
Figure 7
Figure 8 30
Figure 9
Figure 10 31
32
PII: DOI: Reference:
S0272-8842(18)30100-7 https://doi.org/10.1016/j.ceramint.2018.01.088 CERI17220
To appear in: Ceramics International Received date: 11 December 2017 Revised date: 10 January 2018 Accepted date: 11 January 2018 Cite this article as: V. Anand, A. Sakthivelu, K. Deva Arun Kumar, S. Valanarasu, A. Kathalingam, V. Ganesh, Mohd Shkir, S. AlFaify and I.S. Yahia, Rare earth Sm3+ co-doped AZO thin films for opto-electronic application prepared by spray pyrolysis, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.01.088 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 galley proof before it is published in its final citable 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.
Rare earth Sm3+ co-doped AZO thin films for opto-electronic application prepared by spray pyrolysis V. Anand1, A. Sakthivelu2, K. Deva Arun Kumar3, S. Valanarasu*3, A. Kathalingam4, V. Ganesh5,6, Mohd Shkir*5,6, S. AlFaify5,6, I.S. Yahia5,6 1
Physics Department, VKS College of engineering and Technology, Karur, India PG and Research Department of Physics, Periyer E.V.R College, Trichy, India 3 PG and Research Department of Physics, Arul Anandar College, Karumathur, Madurai, India 4 Millimeter-Wave Innovation Technology Research Center (MINT), Dongguk University-Seoul, Seoul 04620, Republic of Korea 5 Advanced Functional Materials & Optoelectronic Laboratory (AFMOL), Department of Physics, Faculty of Science, King Khalid University, P.O. Box 9004, Abha, Saudi Arabia 6 Nanoscience Laboratory for Environmental and Bio-medical Applications (NLEBA), Semiconductor Lab., Metallurgical Lab.2 Physics Department, Faculty of Education, Ain Shams University, Roxy, 11757 Cairo, Egypt. 2
Corresponding author* Dr. Mohd. Shkir Assistant Professor Department of Physics, College of Science, King Khalid University, Abha, SA E-mail: [email protected], [email protected]
1
Abstract Samarium co-doped aluminum zinc oxide (Sm:AZO) thin films were deposited on glass substrate by nebulizer spray pyrolysis technique with different Sm doping concentrations (0 at.%, 0.5 at.%, 1 at.% and 1.5 at%). X-ray diffraction patterns confirm the polycrystalline nature of prepared films with hexagonal crystal structure. The average crystallite size was found to be reduced with Sm doping due to increased lattice defects. The Raman spectra exhibited characteristic ZnO wurtzite structure confirmed through the presence of E2-high mode peak at 438 cm−1. The surface topology analysis revealed the uniformly distributed wheat shaped particles without any pinholes for 1 at.% Sm doped ZnO film. Sm:AZO films displayed high transparency which is around 90% and the energy gap of ~ 3.30 eV. Photoluminescence spectra of the thin films showed an UV emission peak at ~ 386 nm corresponds to near band edge (NBE) emission of bulk ZnO. Room temperature Hall Effect measurement showed that all the prepared films possess n-type conducting nature with low electrical resistivity (ρ) 4.31×10-4 Ω.cm for 1 at.% Sm doped film. The high figure of merit (ф) value of ~ 11.9 ×10-3 (Ω/cm)-1 was observed which indicates that the deposited films are highly suitable for opto-electronic device applications. Keywords:
Rare earth, Sm:AZO thin film, Nebulizer spray pyrolysis, Opto-electronic
application 1. Introduction Zinc Oxide (ZnO) with unique optical and electrical properties is an attractive candidate for a wide range of technological applications such as: transparent conducting oxide, solar cells, light emitting diodes, gas sensors and flat panel displays etc. [1-4]. ZnO is a direct optical band gap (i.e. 3.37 eV) semiconductor material and has huge exciton binding energy of about 60 meV [5]. Naturally, ZnO is an n-type semiconductor caused by defect of zinc interstitials and oxygen 2
vacancies. In recent past many researchers studied the ZnO due to their physical and chemical stabilities, nontoxic nature with good optical and electrical properties. In fact, the electrical resistivity of pure ZnO thin films has been high due to low carrier concentration. Most researchers choose doping with group III element like (Al, Ga and In), to reduce the electrical resistivity of ZnO thin films. Especially, Al-doped ZnO (AZO) thin film display low optical loss with good electrical conductivity. Moreover, the (AZO) is an excellent replacement for indium tin oxide (ITO) because of its low cost and chemical stability compared to ITO [6]. But, upon increasing the doping concentration level, the resistivity begins to increase which may be due to the increased solubility limit of the particular dopant in the ZnO structure [7]. So, we must choose a co-dopant element for improving ZnO film conductivity. Commonly, rare earth (Gd, Nd, Eu, Sm, etc.,) metals are co-doped to ZnO to enhance optical and electrical properties. Among the rare earth materials, samarium (Sm) is one of the best due to their unique electrical properties. The ionic radius of Sm3+ (1.04 Å) is higher than the ionic radius of Zn2+ (0.74Ǻ), hence the Zn ions can be easily replaced by Sm ions with some minor change in the lattice parameters. Only a few reports are available on the doping of rare earth Sm with ZnO thin films using chemical method [8]. In recent years, the majority of AZO thin films are prepared using variety of physical and chemical methods such as pulsed laser deposition [9], magnetron sputtering [10], chemical vapor deposition [11], molecular beam epitaxy (MBE) [12], hydrothermal [13], sol–gel coating [14], SILAR method [15], chemical bath deposition [16] and spray pyrolysis [17], etc. Amongst all these methods, spray pyrolysis is a very simple and cost-effective method. In particular, from spray method, we can easily control the film thickness in the nanometer range and maintain the smooth film surface morphology.
3
In the present work, Sm co-doped AZO thin films are deposited on microscopic glass substrate by nebulizer spray pyrolysis technique. Nebulizer spray pyrolysis [18, 19] is a key method for preparing device quality semiconducting material. The Sm co-doping effects on the lattice structural, surface morphological, optical and electrical properties for the prepared films are investigated. To the best of our knowledge, this will be the first report on the synthesis and fabrication of Sm co-doped AZO thin films using a simplified nebulizer spray technique. 2. Experimental procedure 2.1. Precursor and film deposition method Analytical grade zinc acetate dihydrate [Zn (CH3COO)2·2H2O], aluminum chloride hexahydrate [AlCl3.6H2O] and samarium oxide [SmO2] purchased from (99.9% purity, SigmaAldrich), (99.9% Alfa Aeser) and (98.9% Alfa Aeser), respectively. All the mentioned materials were used as host, dopant and co-dopant precursors, respectively. Hydrochloric acid (HCl) and Methanol (CH3OH) were used to prepare the precursor solutions. The Sm doped AZO thin films were deposited onto glass substrates by simple nebulizer spray pyrolysis technique. Initially, 0.2M of zinc acetate dihydrate precursor (salt) was dissolved in 10 ml of methanol solvent. Further, initial doping element of (3%) aluminum chloride hexahydrate precursor was mixed in the above zinc solution and the mixed solution was stirrer continuously at room temperature. Additionally, co-doping element of samarium oxide was dissolved in HCl solution with different doping level (0%, 0.5%, 1% and 1.5%), as samarium oxide only dissolves in acid solution. Finally, the prepared Sm solution was added drop by drop to the zinc aluminum solution. The solution was stirred for 15 minutes at room temperature to yield a clear and homogeneous Sm:AZO solution. Before the deposition process, the amorphous glass substrates were respectively cleaned using chromic acid, de-ionized water and acetone. The cleaned glass substrate was kept onto the hot plate and the substrate temperature was maintained 4
at ~ 450°C (±2 oC) using PID temperature controller. The 10 ml prepared Sm:AZO solution was taken in a nebulizer container (nebulizer unit) and it was continuous sprayed onto heated substrate. The nebulizer spray gun was slowly moved on to the substrate to deposit uniform Sm:AZO thin film. The main parameters of nebulizer spray pyrolysis (NSP) technique for depositing the films were maintained as follows:
Host precursor
:0.2 M (Zinc acetate)
Dopant precursor
:3 % (Aluminum chloride)
Co-dopant precursor
:0, 0.5, 1 and 1.5 at. % (Samarium oxide)
Solvent
: methanol (10ml)
Substrate temperature
:450°C
Substrate to nozzle distance :30 mm
Carrier gas pressure
:1.5 kg/cm2
Flow rate
:1 ml/minute
2.2. Characterization of prepared films X-ray diffraction spectra of the prepared films were record using X’PERT 3 POWDER X-ray diffractometer system using CuKα (λ=1.5406 nm). The surface morphology was recorded using AFM-NT-MDT (Type Next, Russia) in a semi-contact mode using attached NT-MDT software. The EDAX spectrum was recorded using energy dispersive X-ray spectroscope equipped with SEM (MODEL: JSM 6360 LA, Japan). The optical absorption and electrical properties study of Sm:AZO thin films were performed in the wavelength range (350-1100 nm) using SHIMADZU 1800 (UV- visible spectrophotometer). A photoluminescence spectrum for the prepared films was observed using Perkin Elmer LS55 florescent spectrophotometer with excitation wavelength is 325 nm. The electrical parameters of the Sm doped AZO films was measured using the four point probe hall effect technique with van der Pauw configuration at room temperature. 5
3. Results and discussions 3.1. XRD Analysis Fig. 1(a) shows the X-ray diffreaction patterns of pure and Sm:AZO thin films with different Sm doping concentration (0, 0.5, 1 and 1.5at.%). From XRD data it can be observe that it exhibits a high intense peak at 34.53° corresponding to (002) plane and other low intensity peaks at 31.8, 36.4, 47.6 and 63.01° corresponding to (100), (101), (102) and (103) planes, respectively for 0% Sm doped AZO film. The presence of these planes indicates only ZnO phase formation with hexagonal crystal structure (JCPDS file number: 89–0510). Moreover, the predominant peak (002) intensity was suddenly decreased when adding Sm as a doping element because the c-axis orientation was reduced with respect to Sm content. Similar type of results was also reported by He et.al [8]. However, the other two major planes such as (100) and (101) peak intensity was increases with increasing Sm doping concentration is may be due to the transformation of c-axis orientation from (002) to (100) and (101). In addition the c-axis orientation of (100) plane was high compared to (101) plane. This indicates that the c axis orientations of the grains becomes uniformly perpendicular to the substrate surface for (100) plane. Only one additional peak was observed at 0.5, 1 and 1.5 at.% Sm doped Sm:AZO films. The observed peak (2 theta) position of 56.5° is related to ZnSm2O4 phase was matched with JCPDS file number (39–0858). This means the introduction of the Sm ions into the hexagonal structure of ZnO resulted in the expansion of the ZnO lattice along c-axis and a reduction in the a-axis to preserve the Wurtzite hexagonal structure of ZnO [20]. Whereas, the film crystallinity was decreases when transformation of c-axis orientation occurs with addition of Sm content. The lattice constants (a) and (c) of the hexagonal phase structure of ZnO was determined by the relation [21] using XRD data:
6
1 4 h 2 hk k 2 l 2 2 d2 3 a2 c
(1)
where, d is the interplanar lattice spacing and (hkl) is the miller indices. The calculated lattice parameters and average particle sizes of the AZO:Sm films is shown in Fig. 1(b) and these values are listed in Table 1. The calculated lattice constants are found to be a = 3.247Å and c = 5.1967Å, which is slightly lower than the standard value (JCPDS file: 89.0510) of the hexagonal structure. This may be due to the addition of aluminum and samarium elements caused by local changes in lattice. The lattice constants values are found to increase significantly with Sm content due to larger ionic radius of Sm3+ (1.04 A˚) compared to Zn2+ (0.74 A˚). The average crystallite size (D) were obtained by Scherrer’s equation [22-25]:
D
0.9 cos
(2)
where, λ is the wavelength of X-rays and β the full-width at half-maximum (FWHM) of the X-ray diffraction peaks which is found to be decreases with increasing Sm content. This is attributed to the Sm cations incorporation into the ZnO crystal lattice leading to the crystal lattice distortion due to the lower ionic radius of Zn2+ than that of Sm3+. The film thickness (t) was measured by using stylus-profilometer. Film thickness variation with different Sm doped films was presented in Table 1. 3.2. Morphological and roughness analysis by AFM The surface topology of the Sm co-doped AZO thin films was obtained using atomic force microscopy (AFM) as depicted in Fig. 2 (a-d). From the AFM (2D and 3D) images, it is clear that the AZO film morphology strongly depends on the samarium content. The undoped AZO film has spherical shaped homogeneous grains on the film surface [Fig. 2(a)] similar to reported by R.A. Mereuet.al. [26]. The morphology changes from spherical to wheat shaped 7
particles with the addition of Sm content in ZnO [Fig. 2 (b)]. Further, these wheat shaped particles are uniformly distributed without any pinholes and the porous nature increases with Sm doping [Fig. 2(c)]. Consequently, the surface roughness of the films also increases. This kind of morphology results in the improvement of film conductivity and it is consistent with our electrical result (see sec 3.6). However, the film with higher Sm concentration [Fig. 2(d)] are porous and have clusters with reduced carrier concentration. Fig. 3(a) illustrates the SEM and elemental mapping images of 1% at Sm doped film. Zinc, Oxygen, Aluminum and Samarium are uniformly distributed over the Sm:AZO film surface. Fig. 3(b) shows the EDAX spectrum of Sm: AZO thin film prepared at 1% Sm. The film is stoichiometric with no other chemical species. It is found that Zinc, Oxygen, Aluminum and Samarium elements are present in the film as 47.7%, 49.1%, 2.5 % and 0.7%, respectively. 3.3. Raman analysis Raman spectroscopy is a key device to explore the phase information for the prepared material such as nanoparticles and thin film by using Raman light scattering. Fig. 4 illustrates the Raman spectra of Sm:AZO thin films as a function of wave numbers in the range of 2001400 cm-1. From the Raman spectra, two high intense lead peaks and one wide peak located at 564 cm-1, 1097 cm-1 and 791 cm-1, respectively, are observed. No secondary phase modes of aluminum and samarium are detected which may be due to their interstitial incorporation into the ZnO lattice. In the Raman spectrum, the (E2-high) mode peak is located at 438 cm−1 and it is assigned to ZnO wurtzite structure [6]. The peak observed at 564 cm–1 represents the A1 (LO) and E1 (LO) vibration modes. This vibration mode is attributed to oxygen vacancies defects and free charge carriers. Previously, the A1 (LO) mode at 570 cm−1 was also observed for Sb, Fe and Al-doped ZnO thin film [27]. Another large intense A1 (TO) mode was obtained at 1097 cm−1 is
8
corresponds to the zinc interstitial on the ZnO lattice. A broad band was also covered at 791 cm-1 which is recognized to glass substrate. Manghnani et.al [28] also reported a broad band at around 800 cm-1 assigned to the symmetric bond stretching on the glass plate. 3.4. Optical properties In general, the high optical transparency plays a key role in opto electronic applications of TCO material. Hence, the optical transmittance spectrum with respect to wavelength of Sm:AZO thin films using different samarium doping concentration are plotted in Fig. 5. The thin film optical transmittance is above 75% in the wavelength range of 500–800 nm, which is high enough for optoelectronic and solar cell applications. In our case, the observed transmittance of the prepared films are 90, 88, 86 and 82% for the Sm co-doping level of 0, 0.5, 1 and 1.5%, respectively. From Fig. 5, the entire transmittance spectrum shows steep absorption edges at the wavelength range of 365-375 nm indicative of good crystalline nature of Sm:AZO with low defect density attractive for solar cell (window layer) and optoelectronic devices [29]. Moreover, the transmittance spectrum shows oscillations caused by interference (small maxima and minima) due to good film crystallinity and surface smoothness. Further, the absorption edge is slightly shifted in the direction of higher wavelength with respect to Sm co-doping concentration. It is indicating the reduction in
band gap value for the deposited films. The observed
transmittance values systematically decrease with increase in Sm co-doping concentration. He et.al [8] reported the transmission increase with increase in Sm content for Sn:ZnO films. In our case, the transmission value is in opposite direction, because of already Al element was placed in to the ZnO lattice; so the Sm ions are creating some lattice defects (see XRD). Wurtzite structure of ZnO thin film has a direct energy gap, so that the optical energy gap (Eg) value is able to estimate with Tauc’s formula [24, 30-32]:
9
h = B(h - Eg )n
(3)
In the equation (3) used to get the energy gap value, (hν) is the incident photon energy, (α) is the absorption coefficient, (B) is constant value and (Eg) is the direct energy gap. Inset of Fig. 5 and Table 2 as shows the calculated energy gap values of the Sm:AZO thin films are 3.30, 3.27, 3.28 and 3.25 eV correspond to films Sm (0%, 0.5%, 1% and 1.5%), respectively. Observed energy gap value of our prepared Sm:AZO (3.30eV) film is very close to the customary energy gap value of bulk ZnO (3.36eV) material. However, a small reduction in the energy gap values is may be due to the change of different surface structure [33]. The obtained least energy gap (3.25eV) value for the Sm:AZO thin film with 1.5% doping content compared to others, is due to some lattice defect present in the film. Refractive index (n) and extinction coefficient (k) are the main optical parameters for TCO thin film. The (n) and (k) values of Sm:AZO thin films are obtained from the reflectance (R) using the following equations [34-37]:
n
(1 R) 4R k2 (1 R) (1 R) 2
k
4
(4)
(5)
Fig. 6 shows the variation of refractive index as a function of wavelength (380-1000 nm). In general, the refractive index decreases when transmittance increases. Fig.7 shows the variation of extinction coefficient in the same wavelength range. In our case, both the (n) and (k) values increase with Sm content in contrast to the variation of film transmittance. The obtained (n) and (k) values are listed in Table 2, and the variation of extinction coefficient is similar to the variation of refractive index. The observed (n) and (k) values are in good agreement with the
10
previously reported AZO thin films prepared by sol-gel method [6]. The calculated values of real 2 2 part ( r n k ) and imaginary part ( i
2nk ) of dielectric constants [38] of Sm:AZO
thin films are presented in the Table 2. Real part (εr) and imaginary part (εi) of dielectric constants are related to the speed of light and absorption energy from the electric field, respectively [39]. It clear from tabulated data (see Table 2) that both εr and εi values increase with increasing Sm co-doping content which may be due to the reduction of film transmittance. In addition, the (εr) and (εi) should also be connected with the increase of refractive index and decrease of film crystallinity. 3.5. Photoluminescence Analysis Room temperature photoluminescence spectra of all the deposited Sm:AZO thin films with 325 nm excited wavelength were recorded as shown in Fig. 8. From the PL spectra, it is found that two high and low intense emission bands are located at 389 and 561 nm corresponding to 3.19 and 2.21 eV for all the Sm:AZO films, respectively. These two bands are subsequent to UV and green emission region. Also a broad blue-green emission band was observed at 502 nm corresponding to 2.47 eV for the Sm:AZO films. Obtained high intensity UV emission peak (386 nm) is due to near band edge (NBE) emission of bulk ZnO [39], which is mainly depends on the film crystalline quality. Similar results have also been reported by Kumar et.al. [6] in AZO thin films prepared by sol-gel method. The broad blue-green emission peak located at 502 nm (2.47eV), is due to single ionized oxygen vacancies (VO+) and also it has some surface defects on the film [40]. Final green emission peak was found at 561 nm (2.21 eV) is caused by zinc interstitial position [41]. 3.6. Electrical studies
11
Electrical properties of AZO thin films were done by room temperature Hall Effect measurement. In fact, the AZO thin films have low electrical resistivity and have n-type conducting nature. The electrical resistivity (ρ), carrier concentration (n) and hall mobility (μ) values of deposited films are listed in Table 3 and are also plotted in Fig. 9. As shown in Fig 9, the electrical resistivity decreases up to (at 1% Sm) and then slightly increases with higher Sm doping level (at 1.5%). This kind of resistivity variation with respect to Sm content was also reported by He et.al [8] using chemical solution method. They observed the low resistivity value is 0.28×10-2 Ω.cm. In our case, the film with Sm-content of 1 at.% shows a minimum resistivity (ρ) and maximum carrier concentration (n) of 4.31×10-4 Ω.cm and 6.31×1021 cm-3, respectively. The film carrier concentration continuously increases due to reduction of grain boundary scattering and creation of more free electrons. It is evidenced our smooth morphology (SEM) with pinholes free on the film surface. The observed (ρ) and (n) values are better than the previously (Sm doped) reported values. However, the observed hall mobility values were initially reduced and then increase with respect to Sm doping level is due to the ionized impurity scattering mechanism is reported by Shen et al. [42]. Fig.10 shows the variation of resistivity ln (ρ) versus various inverse temperatures K-1 for the prepared Sm:AZO thin films. The resistivity values systematically reduced with increase of temperature which is confirmed that the prepared films had semiconducting nature. All the prepared films had non-linear behavior due the activation energy. The obtained heated resistivity ln (ρ) value is agreed well with the room temperature Hall Effect resistivity value for all the prepared films due to activation energy. Activation energy (Ea) values was calculated from the resistivity ln (ρ) Vs temperature K-1 plot by using the below equation:
Ea KT
0 exp
(6) 12
Where k is the Boltzmann constant, ρ0 is the exponential parameter and T is the Kelvin temperature. The calculated Ea values are 0.11 eV, 0.05 eV, 0.02 eV and 0.012 eV which is associated to 0%, 0.5%, 1% and 1.5% Sm doped films respectively. The activation energy initially reduced and then increases with the change of different Sm doping concentration due to defect of oxygen deficiency [43]. The opto electric quality of the Sm:AZO film can be determined using figure of merit (ϕ) calculation. The (ϕ) value was estimated from transmittance (T) and sheet resistance (Rsh) by using the below formula [44]: ( )
T 10 R sh
(7)
Generally, the optical and electrical properties must be high as possible for any TCO thin films. In our case, the calculated figure of merit value is found to be higher for 1 at.% Sm doped film compared to other films and the values are listed in Table 3. Hence, we can conclude that the AZO film deposited with samarium element is more suitable for optoelectronic device, because it has good optical transparency and electrical conductivity. 4. Conclusion In this report, for the first time, we have prepared device quality rare earth co-doped AZO thin films via chemical route i.e. nebulizer used spray pyrolysis technique. There are several key conclusions of this study as follows: 1. Samarium (Sm3+) and aluminum (Al3+) elements are successfully dispersed and incorporated, respectively into ZnO lattice. 2. Sm:AZO thin films had multiple interference with high transmittance (90%) and the obtained energy gap is calculated to be ~3.3 eV.
13
3. Low resistivity (ρ) and high figure of merit (ф) values of 4.31×10-4 Ω.cm and 11.9 ×10-3 (Ω/cm)-1, respectively, are observed for 1% at Sm doped film. Therefore, rare earth doped AZO metal oxide thin film is most suitable for optoelectronic device applications. Acknowledgement The authors express their appreciation to the Deanship of Scientific Research at King Khalid University for funding this work through research groups program under grant number R.G.P.2/9/38. Conflict of interest: The authors declares that there is no conflict of interest in the current work.
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16
Table.1. Lattice parameters and crystallite size variation of AZO:Sm thin films Samarium doping level (%)
Lattice parameter (A0)
Crystallite size (nm)
Thickness (nm)
(100)
(002)
(100)
(102)
(103)
Average
a
c
0
16.8
33.4
34.5
29.4
26.2
28.06
3.24086
5.17905
236
0.5
26.0
28.1
42.5
24.6
18.6
27.95
3.24394
5.19254
298
1
33.4
23.4
45.9
17.3
10.2
26.04
3.24538
5.19534
324
1.5
42.9
17.2
48.1
12.6
5.7
25.31
3.24682
5.19636
410
17
Table.2. Optical and dielectric parameters of AZO:Sm thin films Sm doping level (%)
Transmittance (%)
Band gap (eV)
Refractive Extinction Dielectric index coefficient constant (n) (k) (εr) Average visible region (600nm)
Dielectric constant (εi)
0
90
3.30
1.60
0.015
2.50
0.04
0.5
88
3.27
1.69
0.018
2.96
0.06
1
86
3.28
1.72
0.021
3.19
0.07
1.5
82
3.25
1.85
0.037
3.52
0.10
18
Table.3. Electrical parameters of Sm:AZO thin films with different doping level
Sm doping level (%)
Resistivity(ρ) Ω.cm
Carrier concentration cm-3
Hall mobility(μ) ×cm2(VS)-1
0
8.62×10-4
5.63×1020
12.86
1.16×103
7.5
0.5
6.28×10-4
9.05×1020
11.01
1.58×103
8.3
1
4.31×10-4
6.31×1021
2.29
2.32×103
11.9
1.5
9.86×10-4
3.12×1020
20.25
1.02×103
6.1
Conductivity(σ) Figure of (Ω.cm)-1 merit (Փ)×10-3 (Ω/sq)-1
19
Figure captions Fig.1(a). XRD patterns of Sm:AZO thin films with different co-doping level Fig.1(b). Variations of lattice constant values with different Sm doping level Fig.2. AFM 2D and 3D images of Sm:AZO thin films with different co-doping level (a) 0%, (b) 0.5%, (c) 1% and (d) 1.5 % Fig.3(a) SEM-EDS micrograph and maps of distribution of elements on the surface of Sm:AZO thin film for (1% at. Sm) and, 3(b) EDAX spectrum for that film Fig.4. Raman spectra of Sm:AZO thin films with different co-doping level Fig.5. Optical transmittance of Sm:AZO thin films with different co-doping level and (Inset of Fig.5) shows the relation of (αhυ)2 and hυ of Sm:AZO thin films with different co-doping level Fig.6. Refractive index of Sm:AZO thin films with different co-doping level Fig.7. Extinction coefficient of Sm:AZO thin films with different co-doping level Fig.8. Photoluminescence spectra of Sm:AZO thin films with different co-doping level Fig.9. Variations of electrical resistivity, carrier concentration and conductivity of Sm:AZO thin films with with different co-doping level Fig.10. Plot of ln (ρ) versus 1000/T for the Sm:AZO films with different co-doping level
20
Figure 1 (a) and (b)
21
Figure 2 (a)
22
Figure 2 (b)
23
Figure 2 (c) 24
Figure 2 (d) 25
Figure 3(a) 26
Figure 3(b)
27
Figure 4
28
Figure 5
Figure 6
29
Figure 7
Figure 8 30
Figure 9
Figure 10 31
32