Accepted Manuscript Semiconducting properties of Al doped ZnO thin Films Ahmed A. Al-Ghamdi, Omar A. Al-Hartomy, M. El Okr, A.M. Nawar, S. ElGazzar, Farid El-Tantawy, F. Yakuphanoglu PII: DOI: Reference:

S1386-1425(14)00574-5 http://dx.doi.org/10.1016/j.saa.2014.04.020 SAA 11984

To appear in:

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy

Received Date: Revised Date: Accepted Date:

8 February 2014 2 April 2014 7 April 2014

Please cite this article as: A.A. Al-Ghamdi, O.A. Al-Hartomy, M. El Okr, A.M. Nawar, S. El-Gazzar, F. El-Tantawy, F. Yakuphanoglu, Semiconducting properties of Al doped ZnO thin Films, Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy (2014), doi: http://dx.doi.org/10.1016/j.saa.2014.04.020

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Semiconducting properties of Al doped ZnO thin Films Ahmed A. Al-Ghamdi1, Omar A. Al-Hartomy1,2, M. El Okr3, A. M. Nawar4, S. El-Gazzar4, Farid El-Tantawy4, F. Yakuphanoglu1,5 1 2

Physics Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia Department of Physics, Faculty of Science, Tabuk University, Tabuk 71491, Saudia Arabia 3 Department of Physics, Faculty of Science, Al-Azhar University, Cairo, Egypt 4 Department of Physics, Faculty of Science, Suez Canal University, Ismailia, Egypt 5 Department of Physics, Faculty of Science, Firat University, Elazig 23169, Turkey

Abstract Aluminum doped ZnO (AZO) thin films were successfully deposited via spin coating technique onto glass substrates. Structural properties of the films were analyzed by X-ray diffraction, atomic force microscopy (AFM) and energy dispersive X-ray spectroscopy. X-ray diffraction results reveal that all the films are polycrystalline with a hexagonal wurtzite structure with a preferential orientation according to the direction (002) plane. The crystallite size of ZnO and AZO films was determined from Scherrer’s formula and Williamson-Hall analysis. The lattice parameters of the AZO films were found to decrease with increasing Al content. Energy dispersive spectroscopy (EDX) results indicate that Zn, Al and O elements are present in the AZO thin films. The electrical conductivity, mobility carriers and carrier concentration of the films are increased with increasing Al doping concentration. The optical band gap (Eg) of the films is increased with increasing Al concentration. The AZO thin films indicate a high transparency in the visible region with an average value of 86 %. These transparent AZnO films may be open a new avenue for optoelectronic and photonic devices applications in near future. Keywords: ZnO, Al-doped, sol-gel, thin film, optical and electrical properties Corresponding author: [email protected] (F.Yakuphanoglu), [email protected] (Ahmed. A. Al-Ghamdi) Tel:+904242370000-3792 Fax:+904242330062

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1. Introduction Nowadays, transition-metal oxides and their alloys have been fascinating due to their unique physical and chemical properties. Zinc oxide with wide optical band gap (3.37 eV at room temperature), large exciton binding energy (60 meV) and admirable electron mobility has attracted great attention in various electronics and photonics devices [1-3]. ZnO is applied for a variety of important potential applications, such as chemical and gas sensors, optical and magnetic memory devices, UV-light emitting diodes, solar cells, piezoelectric transducers, photodiodes, photodetectors, transparent conductive oxides, biomedical and more [4-9]. However, ZnO thin film is prepared using various techniques such as spray pyrolysis, radiofrequency sputtering, sol–gel spin coating, pulsed laser deposition, chemical vapor deposition and laser molecular beam epitaxy [9-12]. Today, the sol–gel chemistry is a promising one for control of chemical components, low cost, low processing temperature, uniform chemically homogenous films, high yield and scalable process [13]. The transparent conducting oxide (TCO) thin film materials with high electrical conductivity and high optical transparency are being heavily used for several of practical applications, such as flat panel displays, sensors, optical limiters and switches and a variety of devices that rely on the non-linear optical response of their components [14-16]. Increasing consumption of TCO based on tin-doped indium oxide (ITO), rising cost, low electrical conductivity

∼ 10−4 (Ohm cm)-1, toxic indium and instability in hydrogen plasma has a spur

development for new alternative transparent conducting sources. One of the most important obstacles to overcome is how to improve the electrical conductivity and optical transparency of metal oxides. Among these approaches, metal oxides can be alloyed with proper elements to improve its optical, electrical, and magnetic performance. Therefore, extrinsic doping like (Al, In, Ga, Cu, Cd, etc) is considered as one of the most representative dopants and it can serve as a donor in a ZnO lattice and increase wide band gap engineering (i.e. induce defects), which will improve the electrical and optical properties of ZnO. In fact, we believe that aluminum doped zinc oxide is very useful for the fabrication of optoelectronic devices, heterojunction and superlattices, and detectors [10-12].It improves the electrical and optical properties of ZnO films and these properties also are depended on the production parameters of ZnO films by sol gel. The sol gel method is low cost which offers several advantages such as

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easier composition control, controlling the size, low processing temperature to improve the electronic properties and product low cost semiconducting materials. Therefore, the aim of this study is to synthesize Al doped ZnO with different concentrations via sol-gel approach to obtain highly conductive and transparency films. Furthermore, the microstructures and optical properties were investigated in detail, too.

2. Experimental details 2.1 Preparation of AZO thin films

Undoped ZnO precursor solution was prepared by dissolving zinc acetate dehydrate

(ZnAc:(Zn(CH3COO)2·2H2O)]

in

2-proponal

and

diethanolamine

(DEA,C4H11NO2). ZnAc was first dissolved in 2-proponal and followed by addition of DEA to increase the solubility.

The molar ratio of DEA/ZnAc was chosen as 1

corresponding to a solution with 0.5 M concentration. In addition, to prepare Al doped ZnO (AZO) precursor solution, aluminum acetate basic hydrate [CH3CO2)AlOH.H2O] was used as doping agent.

To obtain AZO thin films with different Al doping

concentrations, the solutions with different Al/Zn molar ratios of Al+3/Zn+2 = 1, 2, 3, 4 %; were prepared by adding aluminum acetate basic hydrate to the precursor solution prepared for ZnO. The prepared precursor solutions were stirred at 70 ◦C for 2 h to yield a clear and homogenous solution. The glass substrates were cleaned in ethanol for 10 min each by using an ultrasonic cleaner and then cleaned with deionized water and dried. The gel solution was deposited onto glass substrate at 3000 rpm for 30 s using a spin coater (Laurell EDC-650-23B). After the deposition by spin coating, the films were preheated at 250 oC for 10 min on hot plate to evaporate the solvent and remove organic residuals. Finally, the films were annealed 400 oC for 2 h.

2.2 Characterization Tools of AZO Thin Films

X-ray diffraction (X-ray) was used for crystal phase identification. X-ray patterns were obtained with a Philips PW3710 diffractometer using CuKα radiation at 35 kV and 25 mA. The surface morphology of the thin films was characterized by atomic force microscopy (AFM) (AFM park system, 212). The element chemical compositions of the films were investigated by an energy dispersive X-ray spectrometer (EDX) (ISIS300, Oxford, England). The film thicknesses of un-doped and

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Al-doped

ZnO thin films with various wt % Al of 0, 1, 2, 3 and 4 were determined to be about 112, 110, 106, 107 and 103 nm using AFM. The carrier concentration, mobility and resistivity were performed using the van der Pauw configuration under direct current ranging from 3x10-3 to 5x10-4 A and the applied magnetic field was 0.37 T. The equipment used for this purpose was a Keithley source-meter (model-6517 A). Optical properties of the films were examined with the normal incident transmittance measured using Jasco (V-570) UV/VIS/NIR spectrophoto- meter.

3. Results and discussion 3.1. Microstructure Analysis of Aluminum Doped ZnO Thin Films

The crystal phases and structures of the synthesized Al- doped ZnO were performed using X-ray diffraction. Typical X-ray diffraction spectra for un-doped and Al-doped ZnO thin films with various wt % Al of 0, 1, 2, 3 and 3 are shown in Fig. (1). All the diffraction peaks are observed at 31.800 , 34.460 , 36.190 , 47.540 , 56.590 ,

62.810 and 67.870 corresponding to (100 ) , ( 002 ) , (101) , (102 ) , (110 ) , (103 ) and

(112 ) respectively, which belong to a hexagonal crystal structure with lattice parameters of a = 3.249 ˚A and c = 5.206 ˚A and coincide with the peaks JCPDS Card No. 89-7102 [11]. No characteristics reflection peaks related to Al and other related impurities alumina phases were detected in the X-ray pattern which supports that Al ions were substituted by Zn sites entire the lattices of ZnO crystal. It is suggested that the synthesized AZO nanoparticles exhibit (002) preferential orientation with the c-axis perpendicular to the substrate [18]. In addition, no significant differences were observed for pure ZnO and Al-doped ZnO thin films with the exception of the peak position of the ( 002 ) and (101) which are slightly shifts due to substitution of zinc ions by aluminum ions into the hexagonal lattice [12]. The sharp peaks and high intensity reflect that the synthesized Al-doped ZnO nanoparticles are well crystalline. In fact, the incorporation of Al into of Zn may give rise to the generation of different sorts of stress stemming from the differences in the ion size between Al and Zn, which could be the reason behind the modifications observed in the structure [13,16]. Furthermore, for undoped ZnO, the observed peaks become the sharper and higher in intensity indicating the increase the grain size. These peaks become more broadening with Al concentration

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which agrees with decreasing the grain size as calculated by Scherrer equation and Williamson Hall method. The average grain size and lattice stain ( γ ) of the films were calculated using the full width at half maximum (FWHM) of all peaks from the Scherrer's equation [3,4]:

Ds =

kλ β cos θ

(1)

and Williamson hall method equation:

β cos θ γ sin θ k = + λ Dw λ

(2)

where k is the shape factor, λ is the wavelength of incident X-ray, β is the FWHM measured in radians and θ is the Bragg angle of diffraction peak. For hexagonal crystal structure, the lattice constants, ( a ) and (c ) are expressed by the following relation, 1 2 d hkl

=

4 ( h 2 + hk + k 2 ) 3a 2

+

l2 c2

(3)

 1 4sin 2 θ  where hkl are the miller indices, d is the lattice spacing parameter  2 =  λ2   d hkl

and λ is the wavelength of the x-ray source. The obtained crystallite size ( D s ) and ( Dw ) , lattice strain ( γ ) , a and c lattice constants for the undoped ZnO and AZO hexagonal structure are shown in Fig. (2). It is clear that the crystallite size, lattice constants and lattice strain are decreased, as the Al dopant concentration increases in ZnO lattice. The decrease in crystallite size, lattice constants and lattice strain with increasing Al concentration in ZnO is attributed to the interstitial substitution of Al ions in Zn sites into ZnO lattice as confirmed by EDS results [10,19]. To assess the elemental composition of the synthesized un-doped ZnO and AZO (contains 2 wt % -Al) thin films, the EDX analysis was done and the result is shown in Fig. (3a,b), respectively. In EDX spectrum, numerous well-defined peaks were evident concerned to Zn, O, and Al which clearly support that the AZO nanoparticles are made of Zn, O, and Al. No other peaks related to impurities were detected in the spectrum, which further confirms that the synthesized nanoparticles are Al doped ZnO.

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In order to understand structural properties of the films, the morphology of undoped ZnO and two samples was examined by one-dimensional and three dimensional AFM. The AFM images of un-doped ZnO and AZO films are shown in Fig. 4 (a, b and c), respectively. As seen in Fig. (3a), the nanoparticles are grown in very high density and well homogenously distributed along the surface of the films showing no voids. The typical diameters of the undoped ZnO and AZO nanoparticles are evaluated and the results are depicted in Fig. (2). From the cross section view in Fig. (4a), the AZO nanoparticles grow normal to the ZnO film. The surface roughnesses of film for undoped ZnO and ZnO doped Al are found to be 11, 9, 7, 6, and 4, respectively. It is clear that the surface roughness is decreased with increased Al concentration into ZnO lattice.

3.2. Electrical and Optical Properties

For the applicability of the synthesized AZO films to optoelectronic devices, the knowledge of electrical parameters like electrical conductivity (σ ) , mobility carriers

(µ )

and carrier concentrations

(N )

are highly important and necessary. Electrical

conductivity, mobility carriers and carrier concentration of undoped ZnO and ZnO doped -Al thin films as a function of Al concentration are shown in Fig. (5). It is seen that the conductivity, carrier concentration and mobility are increased with increasing Al concentration. The electrical conductivity of Al doped ZnO films are in agreement with the electrical conductivity of Al doped ZnO films prepared by RF magnetron sputtering [20]. From electrical measurements, one can conclude that diffusion of Al effectively is taking the penetration of impurities is deeper as diffusion of Al concentration increases, and mobility is affected by impurity scattering [16-19]. The augmentation increase of electrical parameters with increasing Al concentration of AZO thin film can be evoked for two reasons. First, with increasing Al concentration into ZnO lattice, the Al ions creates an abundance number of free electrons in the ZnO lattice, and in turn, the electrical conductivity increases. This implies that the Al ions in the ZnO lattice are acting as a charge carriers reservoir and acceptor impurities. Second, with increasing Al concentration, the Al atoms get in more neck contact into Zn sites leading to the acceleration of driving force of charge carrier's transport and therefore conductivity increases [11,12]. 6

The optical transmittance is an important optical parameter for transparent conducting oxides. Optical transmission spectra of AZO films annealed at 400 °C in air for 2 h are depicted in Fig. (6). All films have an average optical transparency over 86% in the visible range. This implies that the AZO thin films transparent are largely preserved. Moreover, a weak fluctuation in the spectra is mainly due to interference phenomenon between thin film layers [18,19]. As clearly seen in the figure, the edge of absorbance is observed in the region of 343-350 nm for all the studied films. The intensity of the absorption spectra is observed in the wavelength region, λ < 350 nm. When the Al doping is increased, the film’s transmission is decreased remarkably. According to the X-ray result (Fig. 1), the AZO films had weaker crystallization. This fact may be due to the optical scattering by the grain boundaries [9,10]. Optical absorbance of as synthesized AZO thin films as a function of Al concentration is depicted in Fig. (7). It is seen that, UV absorption edge is shifted with increasing Al doping concentration, indicating the broadening of the optical band gap. In order to calculate the optical band-gap energy ( E g ) of the AZO thin films, we assume that the absorption coefficient (α ) is given by the following relation [14,15]: 1

1

α =   ln    t  T 

(4)

where T is the transmittance and t is the film thickness. In fact, wurtzite structure ZnO has a direct band gap, and in this case the absorption edge for a direct band transition is analyzed by the following relation [16-19]: 1/2

(α hυ ) = A ( hυ − E g )

(5)

where hυ is the photon energy and A is an energy-independent constant. 2

The dependence of (α hυ ) on the photon energy ( hυ ) for the AZO with different concentration of Al is depicted in Fig. (8). The values of the direct band gap were determined by extrapolations of the linear regions of the plots to zero absorption

((α hυ )

2

)

= 0 . The obtained Eg values of Al doped ZnO films are in agreement with the

Eg values of Al doped ZnO films prepared by RF reactive magnetron sputtering technique [20,21]. The obtained values of direct band gap of AZO films as a function of Al concentration are inset in Fig. (8). It is clear that, the direct band gap of the AZO

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films is increased with increasing Al content, which is explained by Burstein–Moss effect [10]. The increase in the optical band gap with increasing Al concentration is due to the fact that Al ions tends to occupy among ZnO lattice planes which leads to an increase of the transport path of charge carriers into ZnO lattice as confirmed by electrical parameters above. Furhermore, when Al doped ZnO, donor electrons are formed at the bottom of the conduction band. The doubly occupied states is prevented by Pauli principle and therefore, the valence electrons is excited to higher energy level in the conduction band with required extra energy. This causes a broadening in the optical band gap with Al content.

4. Conclusions

Al doped ZnO (AZO) thin films of nominal 112 nm thickness were successfully prepared using sol-gel spin coating approach. Structural analysis based on X-ray measurement has revealed that both ZnO and AZO films have the same crystal structures which belong to hexagonal wurtzite system. In addition, it is observed that Al doping ZnO promotes the growth in ( 002 ) orientation. Based on recorded AFM images, it has been observed that a spherical nanoparticles form on the surface of AZO film with diameter of 14 nm. The conductivity, mobility carriers and carrier concentration are increased with increasing Al concentration. Transmission measurement results indicate that both ZnO and AZO thin films have an average transmittance over 86% for the 350900 nm range. The optical band gap energies of thin films were determined based on transmission spectra and it is found that the band gap energy is increased with increasing Al doping concentration.

Acknowledgments

This study was supported by TABUK University under projects No:S-0195-1434 and S0196-1434. Authors thank to Tabuk University for supporting. Also, this work was supported by Tunisian Ministry of High Education. The authors gratefully acknowledge and thank the Deanship of Scientific Research, King Abdulaziz University (KAU), Jeddah, Saudi Arabia, for the research group “Advances in composites, Synthesis and applications“. This work is as a result of international collaboration of the group with Prof. F. Yakuphanoglu.

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11. L.H. Van, M.H. Hong, J. Ding, (Structural and magnetic property of Co-doped– ZnO thin films prepared by pulsed laser deposition), Journal of Alloys and Compounds, 449(1) (2008) 207-209. 12. M.H. Aslan, A.Y. Oral, E. Mensur, A. Gul, E. Basaran, (Preparation of c-axisoriented zinc-oxide thin films and the study of their microstructure and optical properties), Solar Energy Materials and Solar Cells, 82, (2004), 543-552. 13. H. Kim, A. Pique´ , J.S. Horwitz, H. Murata, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey, (Effect of aluminum doping on zinc oxide thin films grown by pulsed laser deposition for organic light-emitting devices), Thin Solid Films, 377, (2000), 798-802. 14. Hakan Karaagac, Emre Yengel, M. Saif Islam, (Physical properties and heterojunction device demonstration of aluminum-doped ZnO thin films synthesized at room ambient via sol–gel method), Journal of Alloys and Compounds, 521, (2012) 155- 162. 15. N. Serpone, D. Lawless, R. Khairutdinov, (Size Effects on the Photophysical Properties of Colloidal Anatase TiO2 Particles: Size Quantization versus Direct Transitions in This Indirect Semiconductor?), J. Phys. Chem., 99, (1995), 1664616654. 16. A. A. Al-Ghamdi, F. Al-Hazmi, F. Alnowaiser, R. M. Al-Tuwirqi, Att. A. AlGhamdi, O. A. Alhartomy, F. El-Tantawy, F. Yakuphanoglu, (A new facile synthesis of ultra fine magnesium oxide nanowires and optical properties), J. Electroceram, 29, (2012), 198-203. 17. G.N. Dar, Ahmad Umar, S.A. Zaid, Ahmed A. Ibrahim, M. Abaker, S. Baskoutas, M.S. Al-Assiri, (Ce-doped ZnO nanorods for the detection of hazardous chemical), Sensors and Actuators B, 173, (2012), 72-78. 18. Shisheng Lin, Haiping Tang, Zhizhen Ye, Haiping He, Yu Jia Zeng, Binghui Zhao, Liping Zhu, (Synthesis of vertically aligned Al-doped ZnO nanorods array with controllable Al concentration), Materials Letters, 62(4), (2008), 603-606. 19. Chia-Lung Tsai, Yow-Jon Lin, Chia-Jyi Liu, Lance Horng, Yu-Tai Shih, Mu-Shan Wang, Chao-Shien Huang, Chuan-Sheng Jhang, Ya-Hui Chen, Hsing-Cheng Chang, (Structural, electrical, optical and magnetic properties of Co 0.2AlxZn0.8−xO films), Applied Surface Science, 255, (2009), 8643–8647.

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20. S.H. Jeong, J.W. Lee, S.B. Lee, J.H. Boo, Deposition of aluminum-doped zincoxide films by RF magnetron sputtering and study of their structural, electrical and optical properties, Thin Solid Films 435 (2003) 78–82 21. J.J. Ding,S.Y.Ma, H.X.Chen, X.F.Shi, T.T.Zhou, L.M.Mao, Influence of Al-doping on the structure and optical properties of ZnO films, Physica B 404 (2009) 2439– 2443

Fig. (1): X-ray patterns of pure ZnO and ZnO – doped Al films with various doping concentrations. All films were annealed at 400 °C in air for 2 h.

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20

0.6

D (n m ) and lattice strain

15 0.4

10

0.3

0.2

DAFM Ds Dw lattice strain c-axix a-axis

5

0.1

0

0 0

1

2

3

Al concentration (wt %)

12

4

Lattice param eters a and c (nm )

0.5

Fig.(2): Particle size, lattice parameters, lattice strain of AZO thin films.

Fig.(3): EDX of (a) undoped ZnO and (b) 4 wt % Al doped ZnO films annealed 400 °C in air for 2 h.

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Fig. (4): AEM images of (a) Undoped ZnO films annealed at 400 °C in air for 2 h, (b) AZO film contains 2 wt % - Al and (c) AZO film contains 5 wt % - Al.

200

100

1.E-01 50

σ µ N 1.E-02

2

1.E+00

Mobility carrier (cm /V s) and N x 10

Conductivity (Ohm cm)

-1

150

15

-3

(cm )

1.E+01

0

0

1

2

3

4

Al concentration (wt %)

Fig. (5): Electrical conductivity, mobility carriers and carrier concentration of AZO as a function of Al concentration.

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Fig. (6): Optical transmittance spectra of AZO films with various Al doping concentrations. All films were annealed at 400 °C in air for 2 h.

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Fig. (7): Optical absorbance spectra of AZO films with various Al doping concentrations. All films were annealed at 400 °C in air for 2 h.

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Fig. (8): Plots of (α hυ ) against ( hυ ) for undoped ZnO and AZO films with various Al doping concentrations and the inset is the estimated value of optical energy gap ( E g ) versus Al concentration.

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1.

Al doped ZnO (AZO) thin films were successfully deposited via spin coating technique.

2.

The optical band gap of the films is decreased with increasing Al content.

3.

The AlZnO films can be used for optoelectronic applications in near future.

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Semiconducting properties of Al doped ZnO thin films.

Aluminum doped ZnO (AZO) thin films were successfully deposited via spin coating technique onto glass substrates. Structural properties of the films w...
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