Structural, Electrical and Optical Properties of

Copyright © 2014 American Scientific Publishers All rights reserved Printed in the United States of America

Nanoscience and Nanotechnology Letters Vol. 6, 37–43, 2014

Structural, Electrical and Optical Properties of Aluminum Doped Zinc Oxide Spin Coated Films Made Using Different Coating Sols A. Al Kahlout1 2 ∗ † , N. Al Dahoudi1 2 , S. Heusing1 , K. Moh1 , R. Karos1 , and P. W. de Oliveira1 1

Leibniz-Institut für Neue Materialien—INM, Im Stadtwald, Gebaeude 22, 66123 Saarbruecken, Germany 2 Department of Physics, Al-Azhar University-Gaza AUG, P.O. Box 1277, Gaza-Strip, Palestine

Transparent conducting aluminum doped zinc oxide Al:ZnO (AZO) layers have been deposited by spin coating on glass substrates using two different sols, 1-propanolic solution of Zinc and Aluminum salts (conventional sol) and a suspension of already crystalline AZO nanoparticles redispersed in 1-propanol. The coatings have been sintered in air at 600 C for 15 min. and then post annealed in a reducing atmosphere at 400 C for 90 min. The influence of the aluminum content in the coating sol (sol–gel layer) and in the redispersed nanoparticles (nanoparticulare suspension layer) on the optical properties and electrical resistivity have been investigated. A single step spin coated thin layer is obtained, so that multilayers coating have been used to lower the obtained sheet resistance. The visible transmission of both types of layers is high (T > 80%). The influence of the sintering temperature and the optimum doping concentration are investigated. Seven layers synthesized with Al/Zn = 1 mol.% and submitted to reducing treatment in forming gas (N2 :H2 = 92:8) exhibited a Delivered by Publishing−3Technology to: Peter Derycz sheet resistance R = k ( = 79 × On: 10 Mon, · cm) average transmittance of 80% at IP:042 216.185.156.28 24 with Feb an 2014 20:29:16 −1 550 nm for layer depositedCopyright: from conventional sol and 36 k ( = 25 × 10 American Scientific Publishers · cm) for nanoparticles suspension layer.

Keywords: Nanoparticles, Sol–Gel, Spin Coating, Aluminum Doped Zinc Oxide AZO, Transparent Conductive Coating.

1. INTRODUCTION The expanding use of transparent conductive oxide (TCO) coating materials, especially for the production of transparent electrodes for optoelectronic device applications, motivates an intensive investigation of the physics and chemistry of these materials. ITO is the well-known TCO world wide,1–3 but the work in this field is limited by the scarcity and high price of Indium, in addition ITO is relatively soft and scratches easily. The situation drives the search for alternative TCO materials to replace ITO. Doped and un-doped zinc oxides have been extensively studied in the last years as alternatives to ITO for thinfilm transparent electrode applications because of their combined electrical and optical properties associated to their low material cost, resource availability and high thermal mechanical stability.4–8 ZnO has been synthesized via different techniques. A two-step gas/liquid interfaceassisted assembly strategy was used to construct ZnO ∗

Author to whom correspondence should be addressed. Permanent address: Physics Department, Al-Azhar University Gaza, P.O. Box 1277, Gaza-Strip, Palestine. †

Nanosci. Nanotechnol. Lett. 2014, Vol. 6, No. 1

3D-superstructures with ZnO nanorod building blocks which then assemble to form microspheres to decrease their high surface energy. Then these microsphere units can be further connected in a side-by-side link manner undergoing a secondary assembly to generate ultralarge 3D-superstructures.9 Hierarchical shells composed of closely packed ZnO nanorods were synthesiszed via one-pot solvothermal method in which the morphology of ZnO superstructures could be modulated by varying experimental parameters.10 Pai et al. reported bubble-assisted nanofabrication of ZnO macroporous materials with pore size ranging from 100 to 200 nm. They found that selected organic solvents with different reducing ability play key roles in determining the micromorphologies of the asprepared ZnO samples.11 A series of Mgx Zn1−x O (x = 0–1) alloyed powders with different Mg concentrations were crystallized by sol–gel technique. UV-visible (UVVis) absorption spectra indicate that Mg doping significantly increases the band gap of Mgx Zn1−x O alloys.12 The substitution of Zn2+ ions with group III ions (B3+ , Al3+ , Ga3+ , In3+ generates extra electrons and improves ZnO optical, electrical, thermal and magnetic properties.

1941-4900/2014/6/037/007

doi:10.1166/nnl.2014.1721

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Structural, Electrical and Optical Properties of Aluminum Doped Zinc Oxide Spin Coated Films

Kahlout et al.

The best candidates is doping zinc oxide with AluAl-isopropoxide Al(O–i–Pr)3 is added gradually under minum to get Al:ZnO (AZO) films, which exhibit high stirring, where Al/Zn molar ratio is varied from 0 to transparency and low resistivity. Such materials are 2 mol%). suitable for fabricating transparent electrodes in solar cells, gas sensors, ultrasonic oscillators and as surface 2.2. Nanoparticulate Suspension acoustic devices, optical waveguides and micro-machined 2.2.1. Nanoparticles Preparation actuators.13–15 Several techniques have been employed to Sol with a concentration of 0.25 mol/L with Al/Zn = obtain AZO films: pulsed laser deposition,16 17 chemical 1.0 mol% was prepared by dissolving 5.48 g of zinc vapor deposition,18 sputtering.19 20 Ellmer21 showed that acetate dihydrate reagent grade Zn(CH3 COO)2 · 2H2 O in for ZnO films deposited by various methods, the resis100 mL 1-isopropoxy ethanol. 2.62 g of diethanolamine tivity and mobility were nearly independent of the depowas added. The sol was left for stirring at 50 C for sition method and are limited to about 2 × 10−4 · cm 2 hours where very clear solution was obtained. 0.05 g and 50 cm2 /Vs, respectively.21 22 Recently, AZO films with of Al-isopropoxide Al(O–i–Pr)3 was gradually added. The resistivity ∼ 85 · 10−5 · cm was reported by Agura sol was left under stirring for few hours. A 5 M Na(OH) et al.23 This value is very close to the lowest resistivity of aqueous solution was added dropwise to get white preITO of 77 × 10−5 · cm,24 with a free carrier density of cipitate. The pH value was adjusted to 11–12. The white 25 × 1021 cm−3 . precipitate was left under stirring overnight then treated Most reports in literature regarding wet chemical methhydrothermally at 200 C for 12 hours. The precipitate was ods for preparing AZO films concerns sol–gel method. washed with water 4–5 times and dried in air at 100 C for Sol–gel method is a low cost and attractive wet-chemical 12 hours. Different doping ratios were tested namely Al/Zn approach for TCO films preparation when sheet resistance molar ratio = 0.5, 1.0 and 2.0 mol%. This was achieved is not of prime importance.25 One of the drawbacks of this following the same steps as above but with changing the technique is the need of drying and heat treatment at relamount of added Al-isopropoxide. atively high temperature to convert the gel hydroxide film into oxide phase. This step is energy consuming and not 2.2.2. Coating Solution suitable to many kind of substrates. In addition numer26 The coating sol Derycz was obtained by dispersing AZO ous deposition-calcination cycles are needed. Deposition Delivered by Publishing Technology to: Peter mechanically IP: 216.185.156.28 On: Mon,nanopowders 24 Feb 2014 20:29:16 in a mixture of polyethelyne using colloidal suspension of already crystalline nanopartiCopyright: American Scientific Publishers glycol (PEG cles is another interesting option, in which the crystalliza600 and 3,6,9-Trioxadecanoic acid (TODS) as tion step of the TCO material is separated from the process the dispersing agent where TODS/PEG = 15 wt%. The of film formation.27 It is worth mentioning that synthesis of wetted powder was grinded mechanically in a mortar by transparent conductive coating starting with low cost matehand for 15–20 min until homogenous paste is obtained. rial like crystalline ZnO nanoparticles with simple steps The paste was dissolved in 1 propanol as a solvent. The will be a significant achievement for the industry to coat sol was subjected to ultrasonic path for 5 minutes, then many kinds of substrates especially formed glasses and centrifuged at a speed of around 4000 rpm for 15 minutes polymeric substrates. to separate the large agglomerates. This work represents one of the fewest successful trials to redisperse nanocrystalline AZO particles to form coat2.3. Coatings Deposition ing suspension for producing transparent conductive layers Single and multilayer coatings were spun at a speed of on glass substrate. AZO transparent conductive coatings 1000 rpm for 15 s. The layers were first dried in air for have been obtained by two routes, conventional sol–gel a few minutes then sintered for 15 min at 600 C. For and suspension AZO nanoparticles. The structural, electriobtaining multilayers, the spinning and sintering processes cal and optical properties of the obtained AZO layers by are repeated several times. At the end, a sintering proboth routes were determined, analyzed and compared. cess for 30 min at 600 C was carried out, followed by a post annealing treatment in reducing atmosphere (form2. EXPERIMENTAL WORK ing gas N2 :H2 = 98:2) at 400 C for 90 min. The reducing process is a way to create oxygen vacancies, which In this work two routes to prepare coating sols were used. leave extra free electrons in the lattice. This adds more charge carriers that contributes the improvement of the 2.1. Conventional Sol electrical conductivity of the films. When the effect of the 0.25 mol/L zinc actetate dihydrate Zn(CH3 COO)2 · 2H2 O sintering temperature was studied, fused quartz substrates is prepared by dissolving 1.362 g of zinc acetate were used in temperature range 500 to 900 C. The thickdihydrate in 25 mL of 2-isopropoxy ethanol C5 H12 O2 ness of the layers was measured by the ellipsometry techat room temperature. 0.65 g of diethanolamine DEA nique (Spectroscopic Ellipsometer M-2000, J. A. Woollam HN(CH2 CH2 OH)2 is added as a stabilizer. The sol was left Co., Inc.). under stirring for 1 hour until clear solution is obtained. 38

Nanosci. Nanotechnol. Lett. 6, 37–43, 2014

Kahlout et al.


2.4. Characterization The thermal properties of the AZO paste have been determined by differential thermal analysis and thermal gravimetry analysis (DTA/TG) using a Netzsch STA 449 C Jupiter instrument. 62.7 mg of the AZO paste was heated in synthetic air in an Al2 O3 crucible up to 1000 C at a heating rate of 10 K/min. The structural characterization of the deposited films was carried out using an X-ray Diffractometer XPERT-PRO-MPD (Panalytical) diffractometer unit, using Cu anode material operating at 40 kV and 30 mA with wavelength (K: 1.541837 Å, K: 1.392220 Å and beam radius of 240 mm. The AZO coatings were scanned between 2 = 00055 to 89.99 with a 2 scan step size of 0.020 . The surface morphology of the coating was imaged using a high-resolution scanning electron microscopy HR-SEM (JSM6400F, JEOL) using the secondary electron signal excited by a 10 keV primary beam. The sheet resistance R of the films was measured using the four-point probe technique (34401A Multimeter, HEWLETT PACKARD). Transmission of the coating was measured using UV-VIS spectrometer (CARY 5000 from Varian). The measurements were carried out in the spectral range 300–3000 nm.

sol–gel layer exhibited larger crystallite size than that of the nanoparticuate suspension layer. The calculated textured coefficient, (101)/(002), for sol–gel layer is 0.92849 which is much lower than that of nanoparticulate one (1.6458). This shows that layer prepared via sol gel route has strong preferred growth along the (002) plane (c-axis). The higher the c-axis preferred orientation the lower the electrical resistivity due to the reduction in the scattering of the carriers at the grain boundaries28 29 which is confirmed by the results of the electrical properties of this work.

Intensity (a.u.)

3.1.2. SEM SEM images of the surface of spin coated triple layers AZO films prepared by the two routes followed by heating at 600 C are shown in Figure 2. The thickness of the conventional sol film is 80 nm while that of nanoparticulate one is 30 nm. Both films consist of nanocrystalline particles which have a granular shape. The films have a porous morphology due to evaporation of the solvent and dispersion agent during the heating process. The sol gel layer has homogenous denser structure with smooth surface when compared with the nanoparticulate layer which consists of spherical particles dispersed and well separated from each other forming a rough surface on the substrate. This result explains the low conductivity of the nanopar3. RESULTS AND DISCUSSION ticulate layer compared to the sol gel one which will be 3.1. Structural and Morphological Properties Delivered by Publishing Technology Derycz discussedto: inPeter Section 3.3. 3.1.1. XRD IP: 216.185.156.28 On: Mon, 24 Feb 2014 20:29:16 Figure 1 shows the XRD patterns of triple AZO Scientific Publishers Copyright: American 3.2. Thermal Properties layers deposited on borosilicate glass substrate from To ensure the removal of the organic species of the AZO nanopowders redispersed in 1-propanol, and layAZO paste, a differential thermal analysis and thermal ers deposited by sol–gel route. The layers are sintered in gravimetry (DTA/TG) spectra were obtained for an AZO air at 600 C. According to XRD pattern, zincite ZnO paste prepared by wetting AZO nanoparticles in PEG (wurtzite hexagonal structure, pdf no. 01-074-9940) is the and TODs then dissolving in i-propanol (see Fig. 3). only detected crystallographic phase for both coatings. The A very small endothermic peak is observed at temperacrystallite size calculated for the (100), (002) and (101) ture below 150 C. It is accompanied by a mass loss of peaks with the lattice dimensions are shown in Table I. The about 05.83 wt.%. This peak corresponds to the evaporation of the water minorities in the paste. The main feature (002) (101) of the DTA curve is a strong exothermic peak between 250 a-sol–gel b-nanoparticle and 450 C with maximum at 367.5 C. This feature is (100) accompanied by the main mass loss of about 38.24 wt.%. a It may correspond to the degradation and consumption of the organic ligands used in dispersing the particles. No further changes are observed at temperatures higher than b 500 C, so no phase transformation is expected by heating (102) (110) (103) the particles at temperature greater than 500 C. (112)

30

40

50

60

70

2θ (º) Fig. 1. XRD pattern of spin coated triple AZO films deposited from nanoparticulate suspension and from conventional sol–gel solution and sintered at 600 C.


3.3. Electrical and Optical Properties The electrical conductivity of Aluminum doped zinc oxide (AZO) films were characterized as a function of Al doping ratio, sintering temperature and thickness of the coatings. Figure 4 shows the electrical resistivity, , of 5 spin coated layers as a function of Al doping ratio in the coating sol. The layers were sintered in air at 600 C followed by post annealing in forming gas (N2 :H2 = 92:8) at 400 C for 39


Kahlout et al.

Table I. Crystallite size and lattice dimensions of AZO film deposited from AZO nanoparticulate suspension and film deposited from conventional sol. Crystallite size (nm) Coating Nanoparticles Sol–gel

Phase name

(1 0 0)

(0 0 2)

(1 0 1)

Mean value

a (Å)

c (Å)

Textured coefficient (101)/(002)

Zincite Zincite

28 40

36 42

26 35

30 39

3.250360 3.248412

5.209163 5.205542

16458 092849

Fig. 2. SEM image with different magnification, of the surface morphology of spin coated triple AZO layers sintered at 600 C. (b) made with the nanoparticulate suspension; (a) made with the conventional sol.

90 min. The Al doping concentration ranges ratio ofTechnology increase to: in the freeDerycz charge carriers density. The substituDelivered byfrom Publishing Peter 3+ Al2014 ion20:29:16 for Zn2+ ion releases one free electron Al/Zn = 0 to 2 mol.%. Doping ZnO with Al resulted On: first Mon,tion IP: 216.185.156.28 24 of Feb Copyright: American Publishers in the lattice contributing to the electrical conductivity. On in a sharp decrease of the resistivity of the film, , which Scientific the other hand increasing the Al doping ratio in the latthen started to increase by further increase of the doping tice decreases the crystallite size due to the smaller ionic ratio. At doping ratio of 1 mol%, the resistivity reaches a radius of Al when compared to Zn. (This is confirmed minimum value of 102 × 10−2 · cm for sol gel layer and by the XRD of the dispersed AZO particles which is not 26 × 10−1 · cm for nanoparticulate coating. The resistivshown). The decrease in crystallite size results in more ity then increased by further increase of the doping ratio to grain boundaries which in turn leads to higher electron reach 117 × 10−2 · cm for sol gel layer and 4 × 10−1 · scattering where it is well known that the scattering at cm for nanoparticulate coating at Al/Zn = 2 mol.%. The improvement of electrical conductivity by introducing Al in the coating sol is expected to be due to the sol gel nanoparticulate

ρ(Ω·cm)

100

10–1

10–2 0.0

0.5

1.0

1.5

2.0

Al/Zn mol.%

Fig. 3. DTA/TG thermal analysis of the paste prepared by wetting AZO nanoparticles (Al/Zn = 1 mol.%) in PEG and TODs then dissolving in 1-propanol; heating rate 10 K/min in synthetic air.

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Fig. 4. The electrical resistivity, , of spin coated 5 layers (Al/Zn = 1 mol.%) as a function of Al doping ratio for coating prepared from conventional sol and from nanoparticulate suspension. The layers were sintered at 600 C in air then heated at 400 C for 90 min in reducing gas.


Kahlout et al.


T(%)

95 grain boundaries has great effect on the conductivity and the charge mobility.30 It was reported also that the mobility 90 and hence the resistivity of transparent conductive oxides 85 are limited by ionized impurity scattering for high car31 32 80 rier concentrations. In addition to the above mentioned effects that limit the conductivity, high dopant concentra75 tion could lead to clustering of the dopant ions, which 70 increases significantly the scattering rate, and it could also nanoparticles 65 sol–gel produce nonparabolicity of the conduction band, which has to be taken into account for degenerately doped semicon60 ductors with filled conduction bands.33 This is in agree55 ment with Burstein-Moss effect, showing the increase of 50 band gap by increasing the doping ratio (charge carrier).34 500 1000 1500 2000 2500 3000 Although the obtained resistivity for nanoparticulate Wavelength (nm) layer is higher than that obtained by conventional sol gel Fig. 5. The optical transmittance in the wavelength range 300–3000 nm layer, it is still 10 times lower than reported by Tarasov for spin coated 3 layers (Al/Zn = 1 mol.%) produced from the conven35 et al. where resistivity of 2.6 · cm that was reported tional sol and from the nanoparticle suspension sintered in air at 600 C for AZO nanoparticulate layers on glass substrates. The and post annealed in forming gas at 400 C. exhibited lower electrical resistivity of sol gel layer compared with the layer made of nanoparticulate suspension the stronger the blue-shift,35 which explains the greater is expected to be due to the high scattering at the boundblue shift for the nanopartiulte layer when compared with aries in the nanoparticulate layer which has smaller grains sol gel one as it has smaller particles. as confirmed by the XRD pattern shown in Figure 1. XRD pattern showed also that sol gel layer has preferFigure of Merit ential growth in the (002) direction, which enhances the The figure of merit, is a measure of the quality of the electron path and reduces scattering which in turn reduces films andto: it Peter is estimated by the equation Delivered by Publishing Technology Derycz electrical resistivity. IP: 216.185.156.28 On: Mon, 24 Feb 2014 20:29:16 Figure 2 shows that sol gel layer has denser and Copyright: American Scientific Publishers = T 10 /R smoother surface while the nanoparticulate layer has porous structure with loosely packed nanoparticles resultwhere T is the average transmission in the visible range ing in a poor charge percolation. Similar results were and R is the sheet resistance. reported by Tarasov et al.35 where the percolation was The figure of merit calculated for sol–gel layer is 14 × improved via reducing the porosity by depositing a sol– 10−4 which is higher than that of nanoparticulate layer gel layer-by-layer in alternation with layers derived from 037 × 10−4 . the AZO colloid followed by heating leading to a signifiVery thin coating is obtained from both solutions, therecant decrease in resistivity to 13 × 10−2 · cm which still fore multi layers were deposited. Figure 6 shows the elechigher than the result obtained in this work for sol gel trical resistivity and optical transmission of spin coated layer, 102 × 10−2 · cm. AZO multilayers (1, 3, 5, 7) deposited from nanoparticles The optical properties of the AZO layers were studied suspension sol, sintered in air at 600 C, 15 min and furby UV-Vis spectrometer. Figure 5 shows the transmittance ther post annealed in forming gas at 400 C, 90 min as a of the film made with the conventional sol and that of function of number of layers. Single layer has sheet resislayer made with the nanoparticulate sol. Both films, sintance of 760 k and an optical transmission of 90%. The tered in air at 600 C and post annealed in forming gas at deposition of multilayers improves the electrical properties 400 C, exhibit a high transmittance in the visible range where sheet resistance of a 7-layers coating is 36 k a ( = 400–700 nm), close to 90%. However, the transmitfactor of about 14 lower than single layer. Layers obtained tance of sol–gel layer is lower than that of nanoparticulate using conventional sol still have lower resistivity where one in the visible range, this is due to the higher thickness single layer deposited from conventional sol has sheet of sol gel film (80 nm) when compared to the thickness resistance of 8 k while the optical transmittance is as of the nanoparticulate film (30 nm). Both coatings exhibit high as 91%. The resistivity still goes lower for multilayhigher absorption in the near IR range which is due to ers coatings where 7-layers coating has a sheet resistance the absorbance of free carriers. The obtained energy gap of 0.42 k. for both films is higher than the reported value for bulk As seen in Figure 6, the transmittance in the visible ZnO. This blue shift may be attributed to quantum confinerange namely at = 550 nm, decreases by increasing ment effects.36 This effect is appeared as a blue-shift of the the number of layers from 91% for single layer to 77% energy gap in such a way that the smaller the nanoparticles for 7-layers coating. The absorption at near-infrared range


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Kahlout et al.

103

10

2

86 84 82

1

90 88 86 84 82

10

0

80 78

80 101

0

1

2

3

4

5

6

7

T(%)

88

Sheet Resistance (kΩ)

10 90

92

Transmittance Sheet resistance

92

T(%)

Sheet Resistance (κΩ)

Transmittance Sheet resistance

76

8

0

1

2

Number of layers

3

4

5

6

7

8

Number of layers

Fig. 6. Sheet resistance and optical transmission of spin coated AZO multilayers (1, 3, 5, 7) (Al/Zn = 1 mol.%) deposited from nanoparticles suspension, sintered in air at 600 C, 15 min and further post annealed in forming gas at 400 C, 90 min as a function of layer thickness. Left: nanoparticle suspension layer. Right: sol–gel layer. 95

10

90

Sheet Resistance (kΩ)

85

T(%)

80 T(ºC)

75

500

70

600

65

700

8

6

4

Delivered by 800Publishing Technology to: Peter Derycz IP: 216.185.156.28 On: Mon, 242 Feb 2014 20:29:16 Copyright: American Scientific Publishers

60 55 50

0 500

1000

1500

2000

2500

Wavelength (nm)

500

550

600

650

700

750

800

Temperature (ºC)

Fig. 7. Sheet resistance (b) and optical transmittance in the Vis-IR range (a) for spin coated 5 layers (Al/Zn = 1 mol.%) produced from the conventional sol on quartz substrate and sintered in air at different temperatures, then post annealed in reducing gas for 90 min at 400 C.

(not shown) increases as thickness of the layers increases. These changes of the optical properties are consistent with the changes observed in the electrical resistivity, which is attributed to the increase of the free charge carriers concentration. The influence of heat treatment temperature on electrical conductivity of five AZO layers prepared from conventional sol (Al/Zn = 1 mol%) was tested in the temperature range 500–900 C. No significant changes were observed in the temperature range 500–600 C, R = 700 at both temperatures. However the resistivity increased gradually by increasing the sintering temperature to reach 8.8 k at 800 C (see Fig. 7(b)). The increase of electrical resistivity for layers sintered at temperatures higher than 600 C seems to be due to the formation of pores through the film resulting from the decomposition reaction of precursors at high temperatures and the evaporation of residual organics. Beside that micro-voids can then coalesce to form bigger pores. The same result was reported by Kim et al.37 for sol gel AZO layers where it was attributed to the decrease of 42

electrons mobility due to scattering by Al2 O3 segregating at the grain boundaries. The effect of sintering temperature on the optical properties of the layer was tested by UV-Vis spectroscopy. The transmittance of the AZO films annealed at 500 C is higher than 90% for wavelengths larger than 400 nm. As the heat treatment temperature increases the transmittance decreases due to segregated Al2 O3 and pores expected to be formed in the AZO films sintered at high temperature.37 The decrease in absorption in FIR range of spectra of films sintered at higher temperature indicates the decrease of the charge carriers concentration by increasing the sintering temperature which is consistent with the measured decrease in electrical resistivity.

4. CONCLUSION Transparent conducting coatings made using two different routes: nanoparticulate AZO suspension and sol–gel method. Seven layers synthesized with Al/Zn = 1 mol.% Nanosci. Nanotechnol. Lett. 6, 37–43, 2014

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and submitted to reducing treatment in forming gas exhibited a sheet resistance R = 042 k with an average transmittance of 80% at 550 nm for layer deposited from conventional sol and 36 k for nanoparticles suspension layer. The results achieved for nanoparticulate AZO layer is still lower than any results reported using similar technique. UV-Vis spectroscopy showed a blue shift which is considered as an indication of the incorporation of Al in ZnO.

13. P. Jood, R. J. Mehta, Y. Zhang, G. Peleckis, X. Wang, R. W. Siegel, T. Borca-Tasciuc, S. X. Dou, and G. Ramanath, Nano Lett. 11, 4337 (2011). 14. Y. S. Jung, H. W. Choi, and K. H. Kim, J. Korean Phy. Soc. 55, 1945 (2009). 15. J. F. Chang, H. H. Kuo, I. C. Leu, and M. H. Hon, Sens. Actuators. B 84, 258 (2002). 16. F. K. Shan, G. X. Liu, W. J. Lee, and B. C. Shin, J. Appl. Phys. 101, 053106 (2007). 17. S. L. Ou, D. S. Wuu, S. P. Liu, Y. C. Fu, S. C. Huang, and R. H. Horng, Opt. Express. 19, 16244 (2011). 18. W. S. Lau and S. J. Fonash, J. Electron. Mater. 16, 141 (1987). Acknowledgment: This work has been financially sup19. J. N. Duenow, T. A. Gessert, D. M. Wood, T. M. Barnes, M. Young, ported by Leibniz Institute for New Material (INM) B. To, and T. J. Coutts, J. Vac. Sci. Technol. A 25, 955 (2007). Saarbreucken, Germany. A. Alkahlout is thankful to 20. T. Minami, H. Nanto, and S. Takata, Jpn. J. Appl. Phys. 23, L280 Dr. P. Oliveira for inviting her to join his group as visiting (1984). 21. K. Ellmer, J. Phys. D: Appl. Phys. 33, R17 (2000). researcher. N. Al-Dahoudi is thankful to DAAD scholar22. K. Ellmer, J. Phys. D: Appl. Phys. 34, 3097 (2001). ship. The authors thank technicians at INM for their help 23. H. Agura, H. Suzuki, T. Matsushita, T. Aoki, and M. Okuda, Thin and support. Solid Films 445, 263 (2003). 24. H. Ohta, M. Orita, M. Hirano, H. Tanji, H. Kawazoe, and H. Hosono, Appl. Phys. Lett. 76, 2740 (2000). References and Notes 25. M. A. Aegerter, J. Puetz, G. Gasparro, and N. Al-Dahoudi, Optical 1. L. Castañeda, Mater. Sci. Appl. 2, 1233 (2011). Mater. 26, 155 (2004). 2. N. Asakuma, T. Fukui, M. Toki, and H. Imai, J. Sol–Gel Sci. Technol. 26. S. S. Shinde, P. S. Shinde, S. M. Pawar, A. V. Moholkar, C. H. 27, 91 (2003). Bhosale, and K. Y. Rajpure, Solid State Sci. 10, 1209 (2008). 3. N. Al-Dahoudi and M. A. Aegerter, J. Sol–Gel Sci. Technol. 27, 8 27. N. Al-Dahoudi, Wet chemical deposition of transparent conducting (2003). coatings made of redispersable crystalline ITO nanoparticles on glass 4. S.-M. Hyun, K. Hong, and B.-H. Kim, J. Korean Ceram. Soc. and polymeric substrates, Saarbrücken University (2003). 33, 149 (1996). 28. V. Musat, B. Teixeira, E. Fortunato, R. C. C. Monteiro, and 5. P. Nunes, E. Fortunato, P. Tonello, F. B. Fernandes, P. Vilarinho, and P. Vilarinho, Surf. Coat. Technol. 180–181, 659 (2004). R. Martins, Vacuum 64, 28 (2002). 29. M. Ohyama, H. Kozuka, and T. Yoko, J. Am. Ceram. Soc. 81, 1622 Delivered by Publishing Technology to: Peter Derycz 6. S. B. Majumder, M. Jain, P. S. Dobal, and R. S. Katiyar, Mater. Sci. IP: 216.185.156.28 On: Mon, 24(1998). Feb 2014 20:29:16 Eng. B 103, 16 (2003). 30. S. Seki, Y. Sawada, and T. Nishidi, Thin Solid Films 388, 22 (2001). Copyright: American Scientific Publishers 7. K. Ellmer, A. Klein, and B. Rech, Transparent Conductive Zinc 31. H.-M. Zhou, D.-Q. Yi, Z.-M. Yu, L.-R. Xiao, and J. Lih, Thin Solid Oxide: Basics and Applications in Thin Film Solar Cells, Springer, Films 515, 6909 (2007). Berlin (2008). 32. B. Thangaraju, Thin Solid Films 402, 71 (2002). 8. W. Junshu and X. Dongfeng, Sci. Adv. Mater. 3, 127 (2011). 33. T. Pisarkiewicz, K. Zakrzewska, and E. Leja, Thin Solid Films 9. L. Pai and X. Dongfeng, Nanosci. Nanotechnol. Lett. 3, 429 (2011). 174, 217 (1989). 10. W. Junshu and X. Dongfeng, Nanosci. Nanotechnol. Lett. 3, 371 34. F. K. Shan and Y. S. Yu, J. Eur. Ceram. Soc. 24, 1869 (2004). (2011). 35. K. Tarasov and O. Raccurt, J. Nanopart. Res. 13, 671 (2011). 11. L. Pai and X. Dongfeng, Nanosci. Nanotechnol. Lett. 3, 394 (2011). 36. P. M. Aneesh, K. A. Vanaja, and M. K. Jayaraj, Proc. SPIE 12. L. Keyan, W. Junshu, and X. Dongfeng, Nanosci. Nanotechnol. Lett. 6639, 66390J (2007). 37. Y.-S. Kim and W.-P. Tai, Appl. Surf. Sci. 253, 4911 (2007). 3, 417 (2011).

Received: 29 May 2013. Accepted: 30 August 2013.


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