Semiconducting Properties of Al Doped ZnO Thin Films

5/20/2018 Semiconducting Properties of Al Doped ZnO Thin Films

Semiconducting properties of Al doped ZnO thin films

Ahmed A. Al-Ghamdi a,, Omar A. Al-Hartomy a,b, M. El Okr c, A.M. Nawar d, S. El-Gazzar d,Farid El-Tantawy d, F. Yakuphanoglu a,e,

a Physics Department, Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabiab Department of Physics, Faculty of Science, Tabuk University, Tabuk 71491, Saudi Arabiac Department of Physics, Faculty of Science, Al-Azhar University, Cairo, Egyptd Department of Physics, Faculty of Science, Suez Canal University, Ismailia, Egypte Department of Physics, Faculty of Science, Firat University, Elazig 23169, Turkey

h i g h l i g h t s

Al doped ZnO (AZO) thin films weresuccessfully deposited via spincoating technique.

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

The AlZnO films can be used foroptoelectronic applications in nearfuture.

g r a p h i c a l a b s t r a c t

a r t i c l e i n f o

Article history:

Received 8 February 2014Received in revised form 2 April 2014Accepted 7 April 2014Available online 18 April 2014

Keywords:

ZnOAl-dopedSolgelThin film

Optical and electrical properties

a b s t r a c t

Aluminum doped ZnO (AZO) thin films were successfully deposited via spin coating technique onto glasssubstrates. 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 arepolycrystalline with a hexagonal wurtzite structure with a preferential orientation according to the direc-tion (002) plane. The crystallite size of ZnO and AZO films was determined from Scherrers formula andWilliamsonHall analysis. The lattice parameters of the AZOfilms were found to decrease with increasingAl content. Energy dispersive spectroscopy (EDX) results indicate that Zn, Al and O elements are presentin the AZO thin films. The electrical conductivity, mobility carriers and carrier concentration of the filmsare increased with increasing Al doping concentration. The optical band gap (Eg) of the films is increasedwith increasing Al concentration. The AZO thin films indicate a high transparency in the visible region

with an average value of 86%. These transparent AZO films may be open a new avenue for optoelectronicand photonic devices applications in near future. 2014 Elsevier B.V. All rights reserved.

Introduction

Nowadays, transition-metal oxides and their alloys have beenfascinating due to their unique physical and chemical properties.Zinc oxide with wide optical band gap (3.37 eV at room tempera-ture), large exciton binding energy (60 meV) and admirable

http://dx.doi.org/10.1016/j.saa.2014.04.020

1386-1425/2014 Elsevier B.V. All rights reserved.

Corresponding authors. Address: Department of Physics, Faculty of Science,Firat University, Elazig 23169, Turkey. Tel.: +90 4242370000x3792; fax: +904242330062 (F. Yakuphanoglu).

E-mail addresses: [email protected](A.A. Al-Ghamdi), [email protected](F. Yakuphanoglu).

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 512517

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electron mobility has attracted great attention in various electron-ics and photonics devices [13]. ZnO is applied for a variety ofimportant 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[49]. How-ever, ZnO thin film is prepared using various techniques such as

spray pyrolysis, radiofrequency sputtering, solgel spin coating,pulsed laser deposition, chemical vapor deposition and lasermolecular beam epitaxy[912]. Today, the solgel chemistry is apromising one for control of chemical components, low cost, lowprocessing temperature, uniform chemically homogenous films,high yield and scalable process [13]. The transparent conductingoxide (TCO) thin film materials with high electrical conductivityand high optical transparency are being heavily used for severalof practical applications, such as flat panel displays, sensors, opticallimiters and switches and a variety of devices that rely on the non-linear optical response of their components [1416]. Increasingconsumption of TCO based on tin-doped indium oxide (ITO), risingcost, low electrical conductivity 104 (X cm)1, toxic indium andinstability in hydrogen plasma has a spur development for newalternative transparent conducting sources. One of the mostimportant obstacles to overcome is how to improve the electricalconductivity and optical transparency of metal oxides. Amongthese approaches, metal oxides can be alloyed with proper ele-ments to improve its optical, electrical, and magnetic performance.Therefore, extrinsic doping like (Al, In, Ga, Cu, Cd, etc) is consideredas one of the most representative dopants and it can serve as adonor in a ZnO lattice and increase wide band gap engineering(i.e. induce defects), which will improve the electrical and opticalproperties of ZnO. In fact, we believe that aluminum doped zincoxide is very useful for the fabrication of optoelectronic devices,heterojunction and superlattices, and detectors [1012]. Itimproves the electrical and optical properties of ZnO films andthese properties also are depended on the production parametersof ZnO films by sol gel. The sol gel method is low cost which offers

several advantages such as easier composition control, controllingthe size, low processing temperature to improve the electronicproperties and product low cost semiconducting materials.

Therefore, the aim of this study is to synthesize Al doped ZnOwith different concentrations via solgel approach to obtain highlyconductive and transparency films.

Furthermore, the microstructures and optical properties wereinvestigated in detail, too.

Experimental details

Preparation of AZO thin films

Undoped ZnO precursor solution was prepared by dissolvingzinc acetate dehydrate [ZnAc:(Zn(CH3COO)22H2O)] in 2-proponaland diethanolamine (DEA, C4H11NO2). ZnAc was first dissolved in2-proponal and followed by addition of DEA to increase the solubil-ity. The molar ratio of DEA/ZnAc was chosen as 1 corresponding toa solution with 0.5 M concentration. In addition, to prepare Aldoped ZnO (AZO) precursor solution, aluminum acetate basichydrate [(CH3CO2)AlOHH2O] was used as doping agent. To obtainAZO thin films with different Al doping concentrations, the solu-tions with different Al/Zn molar ratios of Al+3/Zn+2 = 1%, 2%, 3%,4%; were prepared by adding aluminum acetate basic hydrate tothe precursor solution prepared for ZnO. The prepared precursorsolutions were stirred at 70 C for 2 h to yield a clear andhomogenous solution. The glass substrates were cleaned in ethanol

for 10 min each by using an ultrasonic cleaner and then cleanedwith 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 filmswere preheated at 250C for 10 min on hot plate to evaporatethe solvent and remove organic residuals. Finally, the films wereannealed 400 C for 2 h.

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 Cu Ka radiation at 35 kV and 25 mA. The surfacemorphology of the thin films was characterized by atomic forcemicroscopy (AFM) (AFM park system, 212). The element chemicacompositions of the films were investigated by an energy disper-sive X-ray spectrometer (EDX) (ISIS300, Oxford, England). The filmthicknesses of un-doped and Al-doped ZnO thin films with variouwt% Al of 0, 1, 2, 3 and 4 were determined to be about 112, 110106, 107 and 103 nm using AFM. The carrier concentration, mobility and resistivity were performed using the van der Pauw config-uration under direct current ranging from 3 103 to 5 104 Aand 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 normaincident transmittance measured using Jasco (V-570) UV/VIS/NIRspectrophotometer.

Results and discussion

Microstructure analysis of aluminum doped ZnO thin films

The crystal phases and structures of the synthesized Al-dopedZnO were performed using X-ray diffraction. Typical X-ray diffraction spectra for un-doped and Al-doped ZnO thin films withvarious wt% Al of 0, 1, 2, 3 and 3 are shown in Fig. 1. All the diffraction peaks are observed at 31.80, 34.46, 36.19, 47.54, 56.59

62.81 and 67.87 corresponding to (100), (002), (101), (102)(110), (103) and (112) respectively, which belong to a hexagonal

Fig. 1. X-ray patterns of pure ZnO and ZnO-doped Al films with various dopingconcentrations. All films were annealed at 400 C in air for 2 h.

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crystal structure with lattice parameters of a= 3.249 andc= 5.206 and coincide with the peaks JCPDS Card No. 89-7102[11]. No characteristics reflection peaks related to Al and otherrelated impurities alumina phases were detected in the X-ray pat-tern which supports that Al ions were substituted by Zn sites entirethe lattices of ZnO crystal. It is suggested that the synthesized AZO

nanoparticles exhibit (00 2) preferential orientation with the c-axisperpendicular to the substrate [18]. In addition, no significantdifferences were observed for pure ZnO and Al-doped ZnO thinfilms with the exception of the peak position of the (002) and(101) which are slightly shifts due to substitution of zinc ions byaluminum ions into the hexagonal lattice [12]. The sharp peaksand high intensity reflect that the synthesized Al-doped ZnO nano-particles are well crystalline. In fact, the incorporation of Al into ofZn may give rise to the generation of different sorts of stress stem-ming fromthe differences in the ion size between Al and Zn, which

could be the reason behind the modifications observed in thestructure [13,16]. Furthermore, for un-doped ZnO, the observedpeaks become the sharper and higher in intensity indicating theincrease the grain size. These peaks become more broadening withAl concentration which agrees with decreasing the grain size ascalculated by Scherrer equation and Williamson Hall method.The average grain size and lattice stain (c) of the films were calcu-

lated using the full width at half maximum (FWHM) of all peaksfrom the Scherrers equation[3,4]:

Ds kk

b cosh 1

and Williamson Hall method equation:

b cosh

k

k

Dwc sinh

k 2

wherek is the shape factor, k is the wavelength of incident X-ray, bis the FWHM measured in radians and h is the Bragg angle ofdiffraction peak.

For hexagonal crystal structure, the lattice constants, (a) and (c)are expressed by the following relation,

1

d2

hkl

4h2 hk k2

3a2

l2

c2 3

where hkl are the miller indices, d is the lattice spacing parameter

1d2hkl

4sin2 h

k2

and k is the wavelength of the X-ray source. The

obtained crystallite size (Ds) and (Dw), lattice strain (c), a and clattice constants for the undoped ZnO and AZO hexagonal structureare shown in Fig. 2. It is clear that the crystallite size, latticeconstants and lattice strain are decreased, as the Al dopant concen-tration increases in ZnO lattice. The decrease in crystallite size, lat-tice constants and lattice strain with increasing Al concentration inZnO is attributed to the interstitial substitution of Al ions in Zn sitesinto ZnO lattice as confirmed by EDS results [10,19].

To assess the elemental composition of the synthesizedun-doped ZnO and AZO (contains 2 wt% Al) thin films, the EDXanalysis was done and the result is shown inFig. 3a and b, respec-tively. In EDX spectrum, numerous well-defined peaks wereevident concerned to Zn, O, and Al which clearly support that theAZO nanoparticles are made of Zn, O, and Al. No other peaks relatedto impurities were detected in the spectrum, which furtherconfirms that the synthesized nanoparticles are Al doped ZnO.

In order to understand structural properties of the films, themorphology of undoped ZnO and two samples was examined by

0

5

10

15

20

0 1 2 3 4

Al concentration (wt %)

D(nm)andlatticestrain

0

0.1

0.2

0.3

0.4

0.5

0.6

Latticeparametersaandc(nm)

DAFM

Ds

Dw

lattice strain

c-axix

a-axis

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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one-dimensional and three dimensional AFM. The AFM images of

un-doped ZnO and AZO films are shown inFig. 4ac, respectively.As seen inFig. 3a, the nanoparticles are grown in very high densityand well homogenously distributed along the surface of the filmsshowing no voids. The typical diameters of the undoped ZnO andAZO nanoparticles are evaluated and the results are depicted inFig. 2. From the cross section view in Fig. 4a, the AZO nanoparticlesgrow normal to the ZnO film. The surface roughnesses of film forundoped ZnO and ZnO doped Al are found to be 11, 9, 7, 6, and4, respectively. It is clear that the surface roughness is decreasedwith increased Al concentration into ZnO lattice.

Electrical and optical properties

For the applicability of the synthesized AZO films to optoelec-

tronic devices, the knowledge of electrical parameters like electri-cal conductivity (r), mobility carriers (l) and carrier

concentrations (N) are highly important and necessary. Electrica

conductivity, mobility carriers and carrier concentration oundoped ZnO and ZnO doped-Al thin films as a function of Aconcentration are shown inFig. 5. It is seen that the conductivitycarrier concentration and mobility are increased with increasingAl concentration. The electrical conductivity of Al doped ZnO filmsare in agreement with the electrical conductivity of Al doped ZnOfilms prepared by RF magnetron sputtering [20]. From electricameasurements, one can conclude that diffusion of Al effectivelyis taking the penetration of impurities is deeper as diffusion of Alconcentration increases, and mobility is affected by impurity scat-tering [1619]. The augmentation increase of electrical parameterwith increasing Al concentration of AZO thin filmcan be evoked fotwo 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 increasesThis implies that the Al ions in the ZnO lattice are acting as a

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.

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charge carriers reservoir and acceptor impurities. Second, withincreasing Al concentration, the Al atoms get in more neck contactinto Zn sites leading to the acceleration of driving force of chargecarriers transport and therefore conductivity increases[11,12].

The optical transmittance is an important optical parameter fortransparent conducting oxides. Optical transmission spectra ofAZO films annealed at 400 C in air for 2 h are depicted inFig. 6.All films have an average optical transparency over 86% in the

visible range. This implies that the AZO thin films transparent arelargely 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 343350 nm for all the studied films.The intensity of the absorption spectra is observed in the wave-length region, k< 350 nm. When the Al doping is increased, thefilms transmission is decreased remarkably. According to theX-ray result (Fig. 1), the AZO films had weaker crystallization. Thisfact may be due to the optical scattering by the grain boundaries[9,10]. Optical absorbance of as synthesized AZO thin films as afunction of Al concentration is depicted inFig. 7. It is seen that,UV absorption edge is shifted with increasing Al doping concentra-tion, indicating the broadening of the optical band gap.

In order to calculate the optical band-gap energy (Eg) of the AZOthin films, we assume that the absorption coefficient (a) is given bythe following relation[14,15]:

a 1

t

ln

1

T

4

1.E-02

1.E-01

1.E+00

1.E+01

Al concentration (wt %)

Conductivity(Ohmc

m

)-1

0

50

100

150

200

Mobilitycarrier(cm

2/Vs)andNx

10

15

(cm

-3)

N

0 1 2 3 4

Fig. 5. Electrical conductivity, mobility carriers and carrier concentration of AZO asa function of Al concentration.

Fig. 6. Optical transmittance spectra of AZO films with various Al dopingconcentrations. All films were annealed at 400 C in air for 2 h.

Fig. 7. Optical absorbance spectra of AZO films with various Al doping concentra-tions. All films were annealed at 400 C in air for 2 h.

Fig. 8. Plots of (aht)2 against (ht) for undoped ZnO and AZO films with various Aldopingconcentrations andthe inset is theestimated value of optical energygap (Eg)versus Al concentration.

516 A.A. Al-Ghamdi et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 131 (2014) 512517


where Tis the transmittance and t is the film thickness. In fact,wurtzite structure ZnO has a direct band gap, and in this case theabsorption edge for a direct band transition is analyzed by thefollowing relation[1619]:

aht Aht Eg1=2

5

where ht is the photon energy and A is an energy-independent

constant.The dependence of (aht)2 on the photon energy (ht) for the AZOwith different concentration of Al is depicted in Fig. 8. The values ofthe direct band gap were determined by extrapolations of the lin-ear regions of the plots to zero absorption ((aht)2 = 0). Theobtained Egvalues of Al doped ZnO films are in agreement withthe Eg values of Al doped ZnO films prepared by RF reactivemagnetron sputtering technique[20,21]. The obtained values ofdirect band gap of AZO films as a function of Al concentration areinset inFig. 8. It is clear that, the direct band gap of the AZO filmsis increased with increasing Al content, which is explained by Bur-steinMoss effect[10]. The increase in the optical band gap withincreasing Al concentration is due to the fact that Al ions tendsto occupy among ZnO lattice planes which leads to an increase of

the transport path of charge carriers into ZnO lattice as confirmedby electrical parameters above. Furthermore, when Al doped ZnO,donor electrons are formed at the bottom of the conduction band.The doubly occupied states is prevented by Pauli principle andtherefore, the valence electrons is excited to higher energy levelin the conduction band with required extra energy. This causes abroadening in the optical band gap with Al content.

Conclusions

Al doped ZnO (AZO) thin films of nominal 112 nm thicknesswere successfully prepared using solgel spin coating approach.Structural analysis based on X-ray measurement has revealed thatboth ZnO and AZO films have the same crystal structures which

belong to hexagonal wurtzite system. In addition, it is observedthat Al doping ZnO promotes the growth in (002) orientation.Based on recorded AFM images, it has been observed that a spher-ical nanoparticles form on the surface of AZO film with diameter of14 nm. The conductivity, mobility carriers and carrier concentra-tion are increased with increasing Al concentration. Transmissionmeasurement results indicate that both ZnO and AZO thin filmshave an average transmittance over 86% for the 350900 nm range.The optical band gap energies of thin films were determined basedon transmission spectra and it is found that the band gap energy isincreased with increasing Al doping concentration.

Acknowledgments

This study was supported by Tabuk University under ProjectsNos.: S-0195-1434 and S-0196-1434. Authors thank to TabukUniversity for supporting. Also, this work was supported byTunisian Ministry of High Education. The authors gratefullyacknowledge and thank the Deanship of Scientific Research, KingAbdulaziz University (KAU), Jeddah, Saudi Arabia, for the researchgroup Advances in composites, Synthesis and applications. Thiswork is as a result of international collaboration of the group withProf. F. Yakuphanoglu.

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Semiconducting Properties of Al Doped ZnO Thin Films