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Journal of Non-Crystalline Solids 352 (2006) 209–215

Electrical, structural and optical properties of SnO2 thin filmsprepared by spray pyrolysis

T. Serin a,*, N. Serin a, S. Karadeniz b, H. Sarı a, N. Tugluoglu b, O. Pakma a

a Department of Engineering Physics, Faculty of Engineering, Ankara University, 06100 Besevler, Ankara, Turkeyb Ankara Nuclear Research and Training Center, 06100 Besevler, Ankara, Turkey

Received 12 April 2005; received in revised form 18 November 2005Available online 20 December 2005

Abstract

This study investigated the effect of the substrate temperature on the structural, optical, morphological, and electrical properties ofundoped SnO2 films prepared by a spray deposition method. The films were deposited at various substrate temperatures ranging from300–500 �C in steps of 50 �C and characterized by different optical and structural techniques. X-ray diffraction studies showed that thecrystallite size and preferential growth directions of the films were dependent on the substrate temperature. These studies also indicatedthat the films were amorphous at 300 �C and polycrystalline at the other substrate temperatures used. Infrared and visible spectroscopicstudies revealed that a strong vibration band, characteristic of the SnO2 stretching mode, was present around 630 cm�1 and that the opti-cal transmittance in the visible region varied over the range 75–95% with substrate temperature, respectively. The films deposited at400 �C exhibited the highest electrical conductivity property.� 2005 Elsevier B.V. All rights reserved.

PACS: 7360.H

Keywords: X-ray diffraction; Electrical and electronic properties; Films and coatings; FTIR measurements; Tin oxide

1. Introduction

Stannic oxide thin films are attractive for many opto-electronic devices [1] due to their unique physical propertiessuch as high electrical conductivity, high transparency inthe visible part of spectrum, and high reflectivity in theIR region. In particular, tin oxide films are stable at hightemperatures, have excellent resistance to strong acidsand bases at room temperature, are resistant to mechanicalwear, and have very good adhesion to many substrates[2,3]. Thus, transparent and electrically conductive stannicoxide films are widely used for a variety of applications.Briefly, these applications include: as electrodes in electro-luminescent displays, imaging devices, protective coatings,

0022-3093/$ - see front matter � 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.jnoncrysol.2005.11.031

* Corresponding author.E-mail addresses: serin@eng.ankara.edu.tr (T. Serin), serdar@taek.

gov.tr (S. Karadeniz).

antireflection coatings, gas and chemical sensors [4], trans-ducers applications based on transparent conductors (e.g.window heaters for aircraft and cars, incandescent lamps,solar cells) and other optoelectronic devices (pin, photo-diodes, optical integrated circuits). Furthermore, tin oxidefilms are more stable than the other transparent conductingoxide (TCO) films such as zinc oxide (ZnO) and Sn-dopedIn2O3 (ITO) [5–7]. Moreover, they have a lower materialcost. Recently, the synthesis of ultra fine tin oxide particlesis of great technological and scientific interest owing totheir superior physical and chemical properties and theiruse as catalysts for the oxidation of organic compoundsand gas sensors, rechargeable Li-batteries, and optical elec-tronic devices [8,9].

All the above properties have led to intense research ofSnO2 coatings over the past few decades. Currently, a largenumber of techniques exist for the preparation of tin oxidefilms. These include r.f. sputtering [10], d.c. glow discharge

Fig. 1. The diagram of the apparatus used for spraying of undoped SnO2.

210 T. Serin et al. / Journal of Non-Crystalline Solids 352 (2006) 209–215

methods [11], reactive sputtering, non-reactive sputter-ing [12], ion-beam sputtering, electron beam evaporation,spray hydrolysis [13–15], chemical vapor deposition(CVD) [16–18], and vacuum evaporation [19]. Among thesemethods, the spraying technique is a simple, economic andcommonly used method [20] and it is performed at rela-tively low temperatures. In addition, the tin oxide preparedby the spraying technique is also physically and chemicallyresistant against environmental effects and adheres stronglyto different substrates. The crystal structure, composition,electrical conductivity and optoelectronic properties ofSnO2 films depend critically on substrate temperature, pre-heating rate of deposition, grain size in the film, sprayingsolution, design of the apparatus for spraying, quality ofthe substrate etc. When the extraneous impurities areadded to the SnO2 lattice [21–24], the electrical conductiv-ity and optical transparency of SnO2 films are increasedwithout altering either the transparency or stability of thefilms.

The aim of this research was to establish a relationshipbetween the substrate temperature and the film properties.In order to realize this goal, tin oxide films were preparedby a spraying method at various substrate temperaturesranging from 300 to 500 �C. The properties of the filmswere characterized by X-ray diffraction analysis (XRD),Fourier transform infrared (FTIR), atomic force micros-copy (AFM), UV–VIS spectroscopy, a two-point conduc-tivity method and the Hall effect experiment.

2. Experiment

The borosilicate glass substrates prepared in 2 mm ·40 mm · 40 mm dimensions were subsequently cleaned ina boiling solution of chromic acid, deionized water andultrasonic bath. The cooled substrates were placed on hotplate at temperature of 300 �C. The solution consisting of32.21 wt% of ethylalcohol (C2H5OH), 40.35 wt% of deion-ized water (H2O) and 27.44 wt% of stannic chloride(SnCl4 Æ5H2O, having 99.9999 purity) was filled in the pyrexcontainer of vertical home made spraying apparatus shownin Fig. 1. The solution was sprayed through a preheatingfurnace on borosilicate glass substrates by means of carriernitrogen (N2) gas. The reversible endothermic reaction

SnCl4 þ 2H2O ! SnO2 þ 4HCl ð1Þ

occurs and undoped SnO2 films are formed on the surfaceof substrates. During the spraying process the gas flow ratewas controlled by a gas flowmeter at the value of 3.5 l/min.After the completion of spraying process the samples wererapidly let to cool down to room temperature. The samespraying processes were followed for the substrate temper-atures 350, 400, 450, and 500 �C.

The films were characterized by means of structural,optical, morphological and electrical techniques. The X-ray diffraction studies were carried out using a RigakuD/Max 2200 diffractometer to investigate the structure ofthe deposited films, CuKa radiation with a wavelength

0.154 nm was used the scanning range of 2h was restrictedto the range 2–72� (Fig. 2).

The UV–VIS–NIR transmittance measurements wereperformed with Perkin Elmer Lambda 2 in the spectralrange 300–1100 nm (Fig. 3). The FTIR transmission spec-tra were recorded using Mattson 1000 FTIR spectrometerin the spectral range 400–4000 cm�1 (Fig. 4). In order torecord IR patterns, the pellets were prepared by mixingKBr with tin oxide powder collected by scratching thinfilms from glass substrates in the ratio 300:1 and thenpressing powder between two pieces of polished steel.

The surface morphology of the films was observed usinga Topometrix TMX2000 Explorer atomic force microscopywith the aid of AFM, root mean square (RMS) valuesof surface roughness and the crystallite size were esti-mated to study the effect of the deposition temperature(Fig. 5).

The electrical conductivity of the films was determinedby two-point probe method at the temperature range 25–150 �C. The variation of conductivity as a function of tem-perature and deposition temperature were given in Fig. 6.The experimental results showed that the conductivity offilms is almost constant over the temperature range but itis significantly dependent on deposition temperature. Theconcentration and mobility of carriers in the films wereobtained from Hall measurements at room temperatureand were given in Table 1.

3. Results

The X-ray diffraction patterns for undoped SnO2 filmsdeposited at 300, 350, 400, 450, and 500 �C are shown inFig. 2. Films deposited at 300 �C were nearly amorphouswhereas the increase of deposition temperature above350 �C caused the crystallization of the films as mentionedby other researchers [1,25]. The films deposited at 350 �Cshowed only three peaks namely (200), (211), and (301)

Fig. 2. 2h X-ray diffraction pattern of undoped SnO2 film prepared at various substrate temperatures.

T. Serin et al. / Journal of Non-Crystalline Solids 352 (2006) 209–215 211

whereas the films deposited at higher temperatures showedas many as five peaks. Since all the peaks are sharp it is evi-dent that the films deposited at 350, 400, 450, and 500 �Care polycrystalline in nature and are of cassiterite tetrago-nal structure. In all films the half width of the Bragg peaksdecreases and the intensity of the peaks increases withincreasing deposition temperature. Especially, the peaks(110) and (211) dominantly increased with increasingtemperature.

In order to determine the variation of the crystallite sizewith increasing substrate temperatures the size of the crys-tallites oriented along (211) plane is calculated usingScherrer’s formula, neglecting peak broadening due toresidual stresses in the films, D ¼ 0:9k=ðb cos hÞ where D

is the size of crystallite, b is the broadening of diffractionline measured at half its maximum intensity in radiansand k is wavelength of X-rays (1.54 A). The calculated val-ues of crystallite size are given in Table 1. It can be seen

that the crystallite size increases with increasing substratetemperature.

The optical transmission spectrum for undoped SnO2

films deposited at 300, 350, 400, 450, and 500 �C are shownin Fig. 3. The absolute transmittances (i.e. when the sub-strate contribution was deduced) of these films vary overthe range 75–95% in the region 400–1100 nm. This regionis the transparent region of films. The variations in trans-mission are due mainly to interference phenomena. Theoptical interferences in the transmittance curves in Fig. 3,can be only observed in very homogeneous films are com-parable to those obtained by sophisticated techniques likeRF magnetron sputtering, CVD, etc. In the transparentregion a minimum for normal incidence of the impinginglight is given by Tmin ¼ 4n2f ns=ðn2f þ nsÞ2 where nf and nsare the index of refractions of the film and substrate respec-tively. The value of nf can be deduced for the wavelength atthe minimum of transmission. Knowing nf we can use the

0

20

40

60

80

100

250 500 750 1000 1250λ (nm)

T%

- 300 °CX - 350 °C

- 400 °C◊ - 450 °C

- 500 °C

Fig. 3. The transmittance spectra of undoped SnO2 films prepared atvarious substrate temperatures.

212 T. Serin et al. / Journal of Non-Crystalline Solids 352 (2006) 209–215

pattern of transmission in the transparent region with suc-cessive minima and maxima to evaluate the thickness of theundoped SnO2 films. The thickness of the layer can be cal-culated from two maxima or two minima [26]

d ¼ Mk1k2=2½ðk1nfðk2Þ � k2nfðk1ÞÞ� ð2Þ

Here M is the number of oscillations between the twoextrema occurring at k1 and k2, nf(k1) and nf(k2) being thecorresponding refractive indices, respectively. Usually inthis region nf is nearly constant: nf � nf (k1) = nf (k2) andthe value of nf (�2) can be deduced for the wavelength atthe minimum of transmission (Tmin)

nf ¼ f½nsð2� TminÞ þ 2nsð1� TminÞ1=2�=Tming1=2 ð3Þ

Fig. 4. IR spectra of undoped SnO2 films pre

for samples prepared at substrate temperatures 300, 350,400, 450, and 500 �C. d values were determined as 295–370 nm by means of Eq. (2). The optical band gap of thematerial of the films has been determined on the basis ofUV–VIS transmission measurements (Fig. 3). For this thefundamental absorption coefficient (a) was evaluated usinga = (lnT�1)/d where d is the film thickness and T is thetransmittance. Fig. 7 shows the variation of (ahm)2 versushm for all the films deposited at 300, 350, 400, 450, and500 �C. The nature of the plots indicates the existence of di-rect optical transitions. The band gap (Eg) is determined byextrapolating the straight line portion of the plot to the en-ergy axis. The intercept on energy axis gives the value ofband gap energy Eg for all the samples and the values liein the range 3.94–3.96 eV.

IR spectra of the samples are shown in Fig. 4. It is seenthat the spectrum for sample deposited at 300 �C comprisessix transmission bands at 468 cm�1, 630 cm�1, 1040 cm�1,1385 cm�1, 1620 cm�1 and 3430 cm�1. These bands werealso observed in the films deposited above 300 �C.

The surface morphologies and roughness values at dif-ferent deposition temperatures are shown in Fig. 5 andTable 1, respectively. It can be seen that the crystallinityof the films improves and the crystallite size become largerwith increasing substrate temperatures which is shown byXRD analysis, also the degree of surface roughnessincreases.

The electrical conductivity of undoped SnO2 films wascalculated from ‘two-point probe’ method as a functionof the measurement temperature and substrate temperatureFig. 6. At the same time the free electron concentration n

and Hall mobility were found as a function of substratetemperature by Hall effect measurements at room temper-ature (Table 1). It was seen that the conductivity of oursamples is almost constant over the measurement but sig-nificantly depends on substrate temperature. The electricalconductivity and the free electron concentration firstlyincreased with increasing substrate temperature thendecreased again with increasing substrate temperature.

pared at various substrate temperatures.

Fig. 5. Atomic force microscope (AFM) surface images of undoped SnO2 film prepared at various substrate temperatures.

50

150

250

0 50 100 150 200

T (oC )

σ (O

hm

-cm

)-1

- 300 °CX - 350 °C

- 400 °C ◊ - 450 °C

- 500 °C

Fig. 6. The electrical conductivity of undoped SnO2 films prepared atvarious substrate temperatures versus temperature.

T. Serin et al. / Journal of Non-Crystalline Solids 352 (2006) 209–215 213

However, the Hall mobility first decreases with increasingsubstrate temperatures then increases again.

4. Discussion

The X-ray results were compared with the results in lit-erature [13,27–30]. Similar results like (i) the films consistof SnO2 phase only and (ii) the increase of the crystalliza-tion of the films and the crystallite size with the substratetemperature were obtained. But there are some differencesin the preferred orientation of the crystallite. These differ-ences were also attributed to the film preparation method,design of apparatus, used solution and its molarities.

The band gap energy values of the films at differentdeposition temperatures lie in the range 3.94–3.96 eV,which is quite comparable with the reported values[31,32]. It is noticed that band gap energy value is mini-mum (3.94 eV) for sample deposited at 300 �C, owing tolower carrier concentration. The band gap attains maxi-mum (3.96 eV) for deposited at 400 �C, carrier concentra-tion being higher for this sample. The shift in the

0

1E+10

2E+10

3E+10

4E+10

5E+10

6E+10

7E+10

8E+10

9E+10

1E+11

3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5hν (eV)

αh

ν2 (e

V.c

m-1

)2

- 300 °CX - 350 °C

- 400 °C- 450 °C- 500 °C

Fig. 7. The plots of (ahm)2 versus hm curves of undoped SnO2 filmsprepared at various substrate temperatures.

Table 1Dependence of the width of crystallite size, the roughness parameter RMS, electrical conductivity, electron mobility, and free electron concentration versusvarious substrate temperatures

Substrate temperature (�C) Crystallite size (nm) RMS (nm) r (X cm)�1 l (cm2 V�1 s�1) n (·1019) (cm�3)

300 – 11.6 79.7 ± 7.4 35.0 ± 1.1 1.4 ± 0.2350 11.6 ± 0.4 19.5 212.6 ± 7.4 22.0 ± 1.1 6.0 ± 0.2400 20.1 ± 0.2 22.3 289.1 ± 9.7 13.3 ± 0.7 13.5 ± 0.4450 20.6 ± 0.2 28.3 209.2 ± 7.7 9.8 ± 0.5 13.3 ± 0.4500 47.6 ± 0.7 30.5 144.9 ± 6.4 10.7 ± 0.6 8.5 ± 0.4

Experimental errors in temperature DT = ±1 �C, DRMS = ±0.1 nm.

214 T. Serin et al. / Journal of Non-Crystalline Solids 352 (2006) 209–215

fundamental absorption edge can be attributed to theMoss–Burstein shift, which occurs owing to filling up oflow lying energy levels by the conduction electrons [21].

The broad band at 630 cm�1 observed by FTIR spectracorresponds to Sn–O2 stretching. This result is in agree-ment with data from literature [31]. The bands at1040 cm�1 and 1385 cm�1 can be assigned to chloride.The bands at 1631 cm�1 and 3430 cm�1 due to H–OHstretching and physisorbed water bonds respectively andthe peak at 468 cm�1 which correspond to Si–O–Si bendingvibration modes. The presence of Si bending vibrationmode is based on mixing of glass particles into SnO2 mate-rial, which is prepared from the grinding of SnO2 on theglass. Infrared spectra exhibit the occurrence of chloridecontamination, which arises from SnCl4 and from thehydration bonds which arising from the use of high molar-ity solution (1 M).

The morphological development of the films observedby AFM and XRD analysis with increasing depositiontemperature is generally related with surface thermalenergy and mass diffusion of constituent elements. As

shown in Fig. 5 at lower deposition temperature such as300 �C the fine grains forms because of a relatively lowerthermal energy. However at higher temperatures, the sur-face mobility of constituent elements increases, and thegrowth of face grains are preferred. The atomic forcemicroscope images undoped SnO2 are compared with thepublished results [17,25,33–38]. It is observed that the crys-tallization processes in undoped SnO2 with increasing sub-strate temperature followed similar character, which isshown by XRD analysis, also the degree of surface rough-ness increases.

When our conductivity and electron concentrationresults were compared with published results it was seenthat substrate temperature dependence of conductivityshowed similar behavior [33,34]. In our case, the maximumof conductivity value and the maximum electron concen-tration were observed around the 400 �C substrate temper-ature whereas theirs was seen at 450 �C and ourconductivity values are smaller than theirs but carrier con-centration values are almost the same. The conductivity ofour films varies only slightly with measurement tempera-ture since the tunnel effect played a major role in carriertransport. Similar results have been reported for chemicalvapor deposition tin oxide films by Kajima et al. [34], forspray pyrolysis technique by Patil et al. [31] and for com-mercial films by Napo et al. [39]. Our Hall mobility valuesare lower than the values measured in single crystallinesamples having carrier density which is as high as our car-rier density. This points out to the importance of the grainboundaries in limiting the carrier transport.

5. Conclusion

Undoped SnO2 films prepared at various substratetemperatures exhibited electrical conductivity characteris-tics, atomic force microscope (AFM) images, infrared spec-tra, UV spectra and 2h X-ray diffraction patterns thatconfirmed that the crystallinity in undoped SnO2 filmscan be significantly effected by substrate temperature. Itwas concluded that this in turn significantly affected theelectrical conductivity and optical transparency of undopedSnO2. It was observed that in order to grow films havinggood electrical conductivity and optical quality thesubstrate temperature should be controlled at around400 �C.

T. Serin et al. / Journal of Non-Crystalline Solids 352 (2006) 209–215 215

Acknowledgement

Authors would like to thank to Phys. Eng. O. Cakır for2h X-ray diffraction and E. Tan for atomic force micro-scope images.

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