Superlattices and Microstructures 70 (2014) 82–90

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Superlattices and Microstructures

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . co m / l o c a t e / s u p e r l a t t i c e s

Effect of annealing on structural, optical,and electrical properties of nickel (Ni)/indiumtin oxide (ITO) nanostructures preparedby RF magnetron sputtering

http://dx.doi.org/10.1016/j.spmi.2014.02.0100749-6036/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors. Tel.: +60 135010455.E-mail addresses: sobri@nitride.org.my (M. Sobri), shuhaimi@um.edu.my (A. Shuhaimi).

M. Sobri a,⇑, A. Shuhaimi a,⇑, K.M. Hakim a, V. Ganesh a, M.H. Mamat b,M. Mazwan a, S. Najwa a, N. Ameera a, Y. Yusnizam a, M. Rusop b

a Low Dimensional Materials Research Centre, Department of Physics, Faculty of Science, University of Malaya, 50603 Kuala Lumpur,Malaysiab NANO-ElecTronic Centre, Faculty of Electrical Engineering, Universiti Teknologi MARA (UiTM), 40450 Shah Alam, Selangor, Malaysia

a r t i c l e i n f o

Article history:Received 7 February 2014Accepted 19 February 2014Available online 3 March 2014

Keywords:Indium tin oxideRF magnetron sputteringAnnealingNanostructureResistivity

a b s t r a c t

Nickel (Ni)/indium tin oxide (ITO) nanostructures were depositedon glass and silicon (111) substrates by RF magnetron sputteringusing nickel and ITO (In–Sn, 90–10%) targets. The post-depositionannealing has been performed for Ni/ITO films in air. The effectof annealing temperature on the electrical, optical and structuralproperties of ITO films was studied. We found the appearance of(622) peak in addition to (400) and (222) major peaks, whichindicates an enhancement of the film crystallinity at high temper-ature annealing of 650 �C. The samples show higher transmittanceof more than 90% at 470 nm after annealing which is suitable forblue light emitting diode (LED) application. The optical energybandgap is shifted from 3.51 to 3.65 eV for the Ni/ITO film afterannealing at 650 �C. In addition, increasing the annealing temper-ature improves the film electrical properties. The resistivity valuedecreases from 3.77 � 10�5 X cm to 1.09 � 10�6 X cm uponincreasing annealing temperature.

� 2014 Elsevier Ltd. All rights reserved.

M. Sobri et al. / Superlattices and Microstructures 70 (2014) 82–90 83

1. Introduction

Tin-doped indium oxide (ITO) nanostructure possesses good electrical conductivity, high opticaltransparency in the visible region, and wide optical bandgap properties (Eg � 3.34 eV) [1]. Nowadays,the ITO nanostructures are widely used in many applications such as solar cells [2], gas sensors [3],electrode in transparent organic light emitting diodes (OLEDs) [4] and ferroelectric photoconductorstorage devices [5]. The ITO nanostructures have been prepared by various deposition techniques suchas sol–gel [6], chemical vapor deposition (CVD) [7], electron beam evaporation [8], radio frequency(RF) sputtering [9], and direct current (DC) sputtering [10]. The RF magnetron sputtering techniquehas been widely used for the deposition of metal and dielectric solid targets such as oxide and nitridebased-compound materials, with introduction of various reactive gases. It is also widely used methodfor the preparation of conductive and transparent thin films due to good film adhesion, large areapreparation, good reproducibility and the simplicity of the growth process required. Moreover, RFsputtering generates relatively low energy particles and uses a high plasma density [11].

It is known that indium oxide crystallizes in two different structures as body centered cubic andhexagonal. The body centered cubic structure is a bixbyite Mn2O3 type, also called C-type rare earthoxide structure [12]. The unit cell consists of 16 molecules and 80 atoms, resulting in a rather complexstructure. This phase is obtained at a low or normal pressure. The hexagonal type is a high-pressuretype phase obtained at 68 kbar and at a temperature 800–1500 �C [13]. When small amount of tinis added to indium oxide all the tin atoms replaces the site of indium cation sub-lattice. The tin dopinghas been observed to induce a change in the lattice constant. Previous literatures obtained ‘a’ higherthan ‘ao’ of undoped indium oxide, which may suggest the incorporation of larger Sn2+ ions at eitherIn3+ sites substitutionally or at interstitial positions [14]. Agnihotri et al. observed a reduction of thelattice constant with increase in tin concentration in ITO films [15]. Mossbauer results have shownthat tin in good quality ITO films are either in Sn2+ state or in Sn4+ state [16]. Thilakan et al. observeda change in preferred orientation between (222) and (400) planes when deposition rate is increased[17].

Here, nickel (Ni) is used as seed layer before the deposition of ITO films to improve the structural,optical and electrical properties of ITO nanostructure. Furthermore, Ni has a high diffusion coefficientand it can migrate quickly into the ITO which increases electron concentration of ITO nanostructure[30]. Ni acts as a catalyst during growth process. Fig. 1 shows a schematic of the vapor–liquid-solid(VLS) mechanism. In this mechanism, nickel catalyst forms liquid alloy islands at 200 �C by adsorbingindium, tin and oxygen components (i). The nickel becomes a liquid phase in which the actual concen-tration of the components is higher than the equilibrium concentration. It then drives the precipita-tion of the component at the liquid–solid interface to achieve minimum free energy of the alloysystem (ii). The mechanism continues as long as the indium, tin and oxygen components are supplied(iii).

Indium tin oxide (ITO) is commonly deposited at temperature above 200 �C due to difficulties toobtain a nanostructure with a resistivity below 2 � 10�4 X cm at a lower growth temperature [18].

Fig. 1. Mechanisme of nickel catalyst on indium tin oxide (ITO) growth process.

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However, low-temperature deposition below 100 �C is necessary to deposit ITO nanostructure on plas-tic substrates due to poor heat resistance, such as polycarbonate [19], polyethylene terephthalate [20]and polyethersulfone [21]. In this paper, Ni/ITO films have been deposited on silicon, Si (111) andglass substrates by RF magnetron sputtering method. Subsequently, the samples were annealed inair ambient for 10 min. In general, a post-deposition annealing in air or in an oxygen free environmentfor reactively sputtered ITO coatings from metal alloy target is needed in order to improve the filmelectrical properties [22]. The structural, optical and electrical properties of the Ni/ITO nanostructuresafter annealing were investigated at various temperatures of 450, 500, 550, 600, and 650 �C,respectively.

2. Experimental details

The schematic of deposition system used in this work is shown in Fig. 2. ITO and Ni sputtering tar-gets, each with 100 mm (4 inch) diameter were used as deposition source. Glass and silicon substrateswere pre-cleaned with acetone and propanol before being loaded into the sputtering chamber. Thechamber is pumped with a turbo molecular pump backed by booster and rotary pumps.

The chamber was evacuated to a background pressure of 1 � 10�6 Torr. The Ni is deposited first asseed layer prior to the ITO deposition. The Ni layer was deposited using Ni target at 50 W RF powerand 5 m Torr deposition pressure, with 15 sccm argon flow rate. The deposition of ITO nanostructurewas performed using ITO target at 200 W RF power and 5 m Torr deposition pressure, with 10:5 sccmargon to oxygen flow rate ratio. The deposition time was fixed for Ni and ITO layer for 3 min and60 min respectively. The deposition temperature for both Ni seed layer and ITO layer is maintainedat 200 �C. The distance between target and substrate was fixed to 14 cm. The substrate holder wascontinuously rotated at 8 rotations per minute (RPM) during the sputtering to improve the uniformthickness. Prior to the actual deposition 5 min pre-sputtering was performed to clean the targetssurface.

After the deposition, the films are annealed in a tube furnace for a period of 10 min in air ambientat different temperatures starting from 450, 500, 550, 600 and 650 �C. The properties of the asdeposited and annealed Ni/ITO nanostructures were examined using various methods. The crystallin-ity of the Ni/ITO nanostructures was ascertained using Siemens D5000 X-ray diffraction (XRD) analysis

Fig. 2. Schematic diagram of RF magnetron sputtering.

M. Sobri et al. / Superlattices and Microstructures 70 (2014) 82–90 85

with Cu Ka radiation in 2h mode. The surface morphology and cross-sectional image of the Ni/ITOnanostructures was examined using atomic force microscopy XE-100 Parks System (AFM) and field-emission scanning electron microscopy was carried out using JEOL JSM-7600F JASCO V-670 (FESEM)respectively. The optical transmission spectra of the Ni/ITO nanostructures were measured using aUV–visible spectrometer in the wavelength range of 300–800 nm. Lastly, the resistivity was studiedusing four point-probe measurement.

3. Results and discussion

3.1. Effect of nickel seed layer for indium tin oxide (ITO) growth

Fig. 3 shows the grain size and cross-sectional FESEM images of ITO with Ni and without Ni seedlayer. A dense triangular grain structure was observed at surface for both the ITO samples, with andwithout Ni seed layer. The grains have uniform shapes and sizes for sample with Ni seed layer as com-pared with sample without Ni seed layer. From the surface image, it can be seen that the grain sizealong the sample surface decreases when the Ni seed layer is added. The grain size of sample withNi seed layer is 35–45 nm while for the sample without Ni seed layer is 45–80 nm. The cross sectionalimages indicate that both of the films have a columnar structure. Image (b) was coated with platinumto reduce charging effect of the sample to obtain a clearer columnar structure. However, the filmprepared with Ni seed layer has a denser columnar structure than that of the film prepared withoutNi seed layer.

The nickel (Ni) content is monitored after the deposition by energy dispersive X-ray (EDXs)analysis. Fig. 4 shows that Ni content was found to be 0.16% on the surface of indium tin oxide(ITO). It may be hypothesized that the Ni is out diffuse to the top of ITO layer during the deposition.It is proved that Ni acts as a catalyst during the deposition of ITO. Ni adsorbs strongly enough to holdand activate the oxygen, indium and tin sputtered from the ITO target. It provides an alternative routefor the reaction with a lower activation energy to speed the growth process of ITO.

Fig. 3. Surface morphology and cross sectional of FESEM for (a) ITO with Ni and (b) ITO without Ni seed layer.

Fig. 4. EDX spectra result of Ni/ITO nanostructures.

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3.2. Effect of Ni/ITO after annealed

Fig. 5 shows the XRD spectra of nickel (Ni)/indium tin oxide (ITO) nanostructures as deposited andannealed at different temperatures from 450 �C to 650 �C. The spectrum of the as-deposited Ni/ITOexhibits the existence of (222) and (400) crystal planes. All the films show the (400) planes texturing.In general, ITO nanostructures prepared by evaporation show the (222) plane texturing [23] andprepared by sputtering show the (400) plane texturing [24]. These different results indicate thatthe ITO nanostructure orientation does not only depend on the deposition parameters but also onthe deposition methods. At high annealing temperature of 650 �C, (622) peak appears in additionto (400) and (222) major peaks. We presume that the thermal energy during annealing processenables atoms to diffuse, therefore improves the lattice structure when annealing temperature isincreased [4]. Ni peak which supposed to appear at 42� is not significant in our results due to the verylow thickness of around 10 nm of the Ni seed layer.

Figs. 6 and 7 shows the FESEM images of ITO and graph between grain size versus different anneal-ing temperature. The grain size of the ITO nanostructure increased with increasing annealing temper-ature. Results indicated that annealing treatment of Ni/ITO nanostructures could increase theactivation energy of diffusion. The heating process could enable atoms to diffuse and to stackaltogether producing a larger grain boundary. The grain boundary scattering must be a main factor

Fig. 5. XRD spectra of Ni/ITO for various annealing temperatures in air ambient.

Fig. 6. FESEM images of Ni/ITO for various annealing temperatures in air ambient.

Fig. 7. Graph of Ni/ITO grain size for various annealing temperatures in air ambient.

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in determining the carrier mobility. It can be expected that carrier mobility will be improved by con-trolling grain size [25]. When the grain size is increased, the grain boundary is reduced over a givendistance. As a result, less carriers are trapped in the grain boundary leading to a higher amount of freecarriers and reduced the resistivity.

Fig. 8a shows the transmittance spectra of the Ni/ITO for various annealing temperature. The sam-ples show higher transmittance of more than 90% at 460 nm which is suitable for application as trans-parent current spreading layer in blue light emitting diode (LED). It can be seen that the fundamentalabsorption edge of Ni/ITO nanostructure shifts to shorter wavelength as the annealing temperature isincreased. Absorption coefficients of the films at different wavelength have been calculated fromtransmission and reflection data. The absorption coefficient for the direct allowed transition can bewritten as [26],

Fig. 8. Optical transmittance and energy gap of Ni/ITO for various annealing temperatures in air.

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a ¼ ðhv � EgÞ1=2 ð1Þ

where hv and E are the photon energy and transition energy gap of the film. The absorption coefficientdata are used to determine the energy bandgap Eg. Since ITO belongs to a direct bandgap semiconduc-tor, the bandgap can be determined by the following formula,

ðhvaÞ2 ¼ Aðhv � EgÞ ð2Þ

where A is the constant, and hv is the photon energy. The plot of (ahv)2 as a function of the photonenergy hv is shown in Fig. 8b. The optical energy bandgap, Eg could be obtained through extrapolatingthe linear part of the curve to ahv = 0. The optical energy bandgap for the as-deposited Ni/ITOnanostructures is 3.51 eV, while the bandgap for the film after annealing at 650 �C is 3.65 eV. Thebandgap shifted due to difference characteristics between the as-deposited and annealing samples.The bandgap widening with the increase of carrier concentration can be explained by Burstein–Mosseffect [27]. It is occurs during the annealing process where, the carrier concentration pushed Fermilevel inside the conduction band and the states near the bottom of the conduction band are filled.Therefore, the absorption edge shifts to higher energy.

The effect of annealing temperature on the electrical property of Ni/ITO nanostructures wasmeasured by four-point probes measurement. Fig. 9 shows the current–voltage graph as a functionof annealing temperature. An Ohmic characteristic was obtained in our entire sample. The measuredresistivity can be calculated using formula [28],

q ¼ VI

W:CF ð3Þ

where V is voltage, I is current, W is thickness of the structure and CF is correlation factor of measure-ment. The resistivity of the as-deposited film is 3.77 � 10�5 X cm. It decreases with temperature,where the resistivity has the lowest value of 1.09 � 10�6 X cm at 650 �C. Fig. 10 shows the result ofresistivity for various annealing temperatures.

It is well known for the ITO films that the electrons are liberated from the substitution entered Snatoms in the cation sub-lattice and from the doubly charged oxygen vacancies. Tin can exist as eitherSnO (valence 2) or SnO2 (valence 4). Since indium has valence 3 in In2O3, the presence of SnO2 wouldresult in n-doping of the lattice because the dopant would add electrons to the conduction band. Incontrast, the presence of SnO would lower the electron density in the conduction band. Some authorsbelieve that, at low substrate temperature, tin is present in the ITO films as SnO resulting in low carrierdensities and annealing the films will transfer SnO into SnO2 and results in the formation of an n-typesemiconductor with high carrier density and low resistance [29].

Furthermore, the resistivity decreases at 650 �C due to the oxygen content in ITO films decreaseswith the effect of annealing temperature. The Ni has a high diffusion coefficient and it can migratequickly into the ITO film during heat process. The nickel produces Ni2+ ions and oxygen will generate

Fig. 9. Current–voltage graph of Ni/ITO nanostructure for various annealing temperatures in air ambient.

Fig. 10. Resistivity of Ni/ITO for various annealing temperatures in air atmosphere.

M. Sobri et al. / Superlattices and Microstructures 70 (2014) 82–90 89

O2�. When Ni diffuses into the ITO film during thermal process it will recombine with oxygen in theITO structure. The process will produce NiO and leaves an oxygen vacancy in the ITO sample. Trans-formation of Ni to NiO during annealing was reported for the achievement of the low resistivity [30].The oxygen vacancies create free electrons in the film because one oxygen vacancy generates two ex-tra electrons. An increase in the number of oxygen vacancies leads to an increase in carrierconcentration.

4. Conclusions

Ni/ITO nanostructures were successfully deposited on the silicon (111) and glass substrates by RFreactive magnetron sputtering technique using Ni and ITO targets. After the deposition, the films wereannealed in air for about 10 min. The annealing temperature was varied from 450 �C to 650 �C with an

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interval of 50 �C. All the films show a preferred orientation along (400) crystal plane. Ni peak is notsignificant in our films due to small amount of Ni in the sample. The grain size along (400) directionincreases as the annealing temperature is increased. The film surface becomes rougher as the anneal-ing temperature is increased. As the annealing temperature is increased, the electron which is liber-ated from the donor sites trapped at the dislocations, results in the increase of the electronconcentration and optical bandgap. The film electrical resistivity decreased as the annealing temper-ature is increased. The resistivity of as deposited film is about 3.77 � 10�5 X cm and decreases downto 1.09 � 10�6 X cm as the annealing temperature is increased up to 650 �C.

Acknowledgements

The work was partially supported by High Impact Research grant (HIR) F000006-21001,Exploratory Research Grant Scheme (ERGS) ER012-2011A, Fundamental Research GrantScheme (FRGS) FP009-2013A, Peruntukan Penyelidikan Pascasiswazah (PPP) PG058-2012B andUniversity of Malaya Research Grant (UMRG) RG141-11AFR.

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