CHAPTER 4. CHARACTERIZATION OF ZINC DOPED TIN OXIDE (Zn:SnO 2 ) THIN FILMS

84 CHAPTER 4 CHARACTERIZATION OF ZINC DOPED TIN OXIDE (Zn:SnO 2 ) THIN FILMS 4.1 INTRODUCTION Tin oxide (SnO 2 ) is one of the most important Transparent Conductive Oxide (TCOs) materials, which finds numerous applications in modern technology (Chopra et al 1983, Terrier et al 1997) due to its attractive properties such as high electrical conductivity, high transmittance in the UV- Visible region, and high reflectance. SnO 2 is the prototype of the transparent conductor, being a wide band gap (3.6 ev) material with up to 97% optical transparency in the visible range (for films of thickness.1 to 1 m), yet having a resistivity of 1-4 to 1 6 Ωcm, considerably lower than most semiconductors (1-3 to 1 9 Ωcm). For these reasons SnO 2 and its alloy with In 2 O 3 is widely used as a transparent electrical contact in flat panel displays and in solar cells. Another property of SnO 2 and other TCOs is that they are transparent in visible region and are highly reflective in infrared region. This property is responsible for today s dominant use of SnO 2 as an energy conserving material. SnO 2 coated architectural windows, for instance, allow transmitting light but keeping the heat out in the building depending on the climatic region. Many of the binary TCO S already possess a high conductivity due to intrinsic defects, i.e. oxygen deficiencies. SnO 2 is an insulator in bulk form, whereas it becomes an n-type wide band gap semiconductor with a band gap of 3.7-4 ev (Arai 196), when it is deposited in the form of thin films. Non-stoichiometry, in particular oxygen deficiency, makes it a

85 conductor. Kilic and Zunger (22) showed that the formation energy of oxygen vacancies and tin interstitials in SnO 2 is very low and thus these defects form readily, explaining the often observed high conductivity of pure, but non-stoichiometric SnO 2. In all applications of these materials the charge carrier concentration and thus the conductivity is further increased by extrinsic dopants. The properties of SnO 2 can be enhanced by doping with different dopants such as indium (Ji et al 23), fluorine (Shanthi et al 1998), antimony (Rizzato et al 23). The most commonly observed phases of polycrystalline tin oxide include rutile structure SnO 2 and litharge SnO in crystalline forms corresponding to tin oxidation states +4 and +2, respectively. The rutile tetragonal structure of SnO 2 is shown in Figure 4.1 and its physical properties are summarized in Table 4.1. Figure 4.1 Rutile tetragonal structure of tin oxide (SnO 2 )

86 Table 4.1 Physical properties of the SnO 2 crystal (Batzil and Diebold 25, Samson and Fonstad 1973) Property SnO 2 Mineral name Cassiterite Crystal structure Rutile Tetragonal Lattice constants (nm) a =.474, b =.319 Density (g/cm 3 ) 6.99 Melting point ( o C) >19 Melting point of metal ( o C) 232 Band gap (ev) 3.6 Refractive index at 55 nm 1.8 to 2. Electron effective mass.275 m e Electron Hall mobility at 3 K (cm 2 /Vs) 25 For TCO applications, it is possible to decrease the sheet resistance and increase the n-type conductivity of SnO 2, like other TCOs, by doping with donar impurities. Different groups have extensively studied the wellknown donars such as fluorine as an anion dopant or antimony as a cation dopant, which increase the n-type electrical conductivity to several orders, F 1- substitutes O 2+ and Sb 3+ substitutes Sn 4+ so that an extra electron enters the lattice (Thangaraju 22, Freeman et al 2). However a low valency cation as an acceptor such as Al 3+ in SnO 2 produces a hole and decrease the n-type conductivity (Chopra et al 1983, Freeman et al 2). With the exception of some limited applications, most of the investigations have been focused on increasing the n-type conductivity of TCOs with donars (Lewis and Paine 2), However only very few reports are available on carrier type conversion by high acceptor doping such as Al 3+ and Zn 2+. Also, from our literature survey it has been observed that even though zinc (Zn) doped SnO 2 is an attractive TCO material, its structural, optical and electrical properties

87 are not studied thoroughly and there are still several unanswered questions about the properties of Zn doped SnO 2 films prepared by spray pyrolysis. The objective of this work is to prepare the Zn doped SnO 2 films by spray pyrolysis at optimized conditions and investigate the film properties by different methods. 4.2 LITERATURE SURVEY OF TIN OXIDE AND ZINC DOPED TIN OXIDE Gordillo et al (1994) studied the preparation and characterization of SnO 2 :F thin films deposited by spray pyrolysis using SnCl 2 and SnCl 4 precursors. For fluorine (F) doping they have used two different precursors, namely HF and NH 4 F. The comparative study showed that the samples prepared using SnCl 2 and HF precursors showed lower resistivity than those obtained from SnCl 4 and NH 4 F precursor solutions. The authors obtained SnO 2 films with resistivities of about 2.5 x 1-4 cm and transmittance greater than 9%. X-ray diffraction measurements indicated that SnO 2 films prepared from SnCl 4, have a preferential growth along the (2 ) direction, whereas the samples prepared from SnCl 2 present a tendency to grow preferentially along the (1 1), (2 1 1) and (3 1) directions. The systematic analysis showed that the resistivity of pure SnO 2 films are mainly influenced by the crystallographic orientation of the sample. Kaplan et al (1996) and Ben-Shalom et al (1993) studied the effect of magnetic field, arc current, and film thickness on the surface morphology of SnO 2 films using the scanning tunnelling microscopy/spectroscopy (STM/STS). No correlation was found between cathode magnetic field and the film surface morphology. The surface had a very rough and grainy structure, with a lateral periodicity close to 2 nm. The average crest height of the films depended on film thickness, e.g. 5 and.6 nm for the 5 and

88 35 nm thick films, respectively. Well defined clusters were reported for 5 nm thick films. In addition, Kaplan et al (1996) hypothesized (using STM, I-V measurements) that nanoclusters of non-degenerate semiconductor having an energy band gap larger than 3 ev, which were deposited with a magnetic field of ~.4 mt imposed on the cathode, consisted of Sn 2+, while nanoclusters of degenerate semiconductor consisted of Sn 4+ ions. The authors have also reported that the electrical conductivity of the films are strongly influenced by the deposition pressure. The lowest film resistivity obtained at RT was 3 1-3 Ω-cm. Zhitomirsky et al (25) reported that the conductivity of films increased with increasing thickness and.26 m was found to be the threshold thickness for the lowest resistivity of ~6.4 1 3 Ω-cm. Elangovan and Ramamurthi (25) have reported the properties of fluorine doped SnO 2 thin films prepared by spray-pyrolysis method. The authors have varied the dopant (fluorine) concentration from to 15 wt%. and studied the film properties as a function of F content. From X-ray analysis, it has been observed that the undoped films had a preferential orientation along (2 1 1) axis, and the preferred orientation of the grains from (2 1 1) axis to (2 ) axis due to fluorine doping has also been reported. For 15 wt% F doping, a very low sheet resistance value of 1.75 / has been reported. The influence of Zn doping on the electrical and optical properties of SnO 2 films prepared by sol-gel spin coating method was reported by Bhat et al (26). The authors have varied the Zn concentration up to 1 wt% and focused on the electrical property analysis. They reported an increase in sheet resistance while increasing the Zn doping concentration. Bilgin et al (24) have prepared the Zn doped SnO 2 films by ultrasonic spray pyrolysis method and studied the film property by DC conductivity, XRD and SEM methods. They have demonstrated that due to Zn doping the preferential orientation of

89 the grains changed from (2 1 1) direction to (2 ) direction with increasing Zn concentration. Further amorphization of the films have also been reported. 4.3 DEPOSITION OF ZINC DOPED TIN OXIDE (Zn : SnO 2 ) Tin oxide and zinc doped tin oxide films have been prepared by chemical spray pyrolysis method. Figure 4.2 depicts the preparation steps of Zn doped SnO 2 films by chemical spray pyrolysis method. The spray solution was prepared by the method proposed by Elangovan and Ramamurthi (23, 25). At first 11g of SnCl 2. 2H 2 O was dissolved in 5ml of concentrated hydrochloric acid. A clear transparent solution was obtained upon heating this mixture at 9 C for 15 min. The solution was diluted with 45 ml of anhydrous methanol and then mechanically stirred for 2h to get homogeneous spray solution. For Zn doping, an appropriate amount of zinc acetate (Zn (CH 3 COO) 2 2H 2 O) was dissolved initially together with SnCl 2.2H 2 O in 5ml of HCl and the above procedure was repeated to prepare a spray solution with different zinc concentrations. The Zn concentration was varied up to 25 wt% in the spray solution. The films were deposited on Si(1 ) and quartz substrates at 4 C. Before deposition, the substrates were heated up to 4 º C by using a resistive heating furnace. Compressed air was used as the carrier gas. During deposition, the carrier gas flow rate was maintained at 9 L min 1 by regulating the compressed air flow. The distance between the spray nozzle and the substrate was maintained at 2 cm. The solution was sprayed on the substrates for several spraying cycles of 2 seconds followed by an interval of for 2 minutes no spray in order to avoid fast cooling of the substrate due to continuous spray. The films were sprayed for 2h with the above said systematic steps, which enabled us to prepare films of thickness approximately 27 3 nm. For each dopant concentration, several films

9 were prepared on Si(1 ) substrates and were confirmed that the thickness of the films was uniform in all runs. Tin(II)chloride + Zinc acetate + Concentrated HCl Stir for 15 mins. /at 9 O C Add 5ml methanol Stir for 2h./Room temp Spray solution Deposition/Substrate temperature. 4 o C Figure 4.2 Flow chart of Zn doped SnO 2 thin film deposition steps by spray pyrolysis method 4.4 CHARACTERIZATION STUDIES 4.4.1 EDX Micro Elemental Analysis The elemental analysis of Zn: SnO 2 films was investigated by EDX analysis. Figure 4.3 depicts typical SEM-EDX spectrum recorded for SnO 2 films with different Zn concentrations. The EDX analysis confirms the presence of Sn, O and Zn elements in the deposited films. In addition to this, very weak signals from Na and Ca elements were also observed, which would have appeared from the substrate. In order to avoid the strong Si peak from the Si(1 ) substrates, films deposited on quartz substrates were used for the elemental analysis. Increasing the zinc concentration in the starting solution increases the amount of Zn in the film as well and is clearly seen from the increase in the intensities of the two Zn characteristic peaks observed around

91 1 and 8.6 kev (Figure 4.4). In Figure 4.5 the elemental weight (in wt %) of Zn in the solid films are plotted as a function of Zn concentration in the starting solution. From the EDX analysis, it is concluded that Zn concentration in the solid film is slightly less than that of the solution concentration. 12 8 Sn Sn (a) SnO 2 4 15 O Sn Sn 1 Sn (b) 5% Zn:SnO 2 Intensity (Cps) Cps 5 O Zn Sn Zn Sn 12 (c) 1% Zn:SnO 8 Sn 2 4 O Sn Zn Zn Sn 1 (d) 25% Zn:SnO 2 Sn 5 O Zn Sn Zn 2 4 6 8 1 Energy [kev] Figure 4.3 SEM-EDX of (a) SnO 2, (b) 5% Zn: SnO 2, (c) 1% Zn: SnO 2 and (d) 25% Zn: SnO 2

92 Intensity (Cps) Intensity (Cps) 1 8 6 4 2 1 8 6 4 2 Z n p e a k.8.9 1. 1.1 1.2 5 % Z n :S n O 2 1 % Z n :S n O 2 2 5 % Z n :S n O 2 Z n p e a k [a ] [b ] 8. 8.2 8.4 8.6 8.8 9. Energy [kev] Figure 4.4 SEM-EDX Spectra of Zn peak observed at (a) 1.5 kev and (b) 8.6 kev. The intensity of Zn peak increases with increasing Zn concentration in the starting spray solution 25 (wt %) Zn concentration in the film measured by EDX 2 15 1 5 5 1 15 2 25 Zn concentration in the starting solution (wt %) Figure 4.5 Variation of Zn concentration in the films as a function of Zn concentration in the starting solution as measured by EDX

93 4.4.2 Structural Studies 4.4.2.1 X-ray diffraction Analysis Grazing angle X-ray diffraction (GXRD) studies have been carried out to understand the film structure. Figure 4.6 depicts the GXRD pattern recorded for pure and zinc doped SnO 2 films. It can be seen from this figure that all films are polycrystalline and the SnO 2 grains show the (1 1 ) preferred orientation. Upon increasing the zinc dopant concentration, the intensity of the (1 1 ) peak decreases, while the intensity of (2 ) peak increases, which reveals that the preferred orientation of the grains changes from the (1 1 ) plane to (2 ) plane. The X-ray -rocking curve measurements have also been performed for the (1 1 ) and (2 ) peaks of these samples (Figure 4.7). The rocking curve measurement shows that the increasing zinc concentration in the spray solution suppresses the intensity of the (1 1 ) peak with a simultaneous increase in the intensity of the (2 ) peak, which is an evidence for the change in the preferred orientation of the grains. But, the over-all crystalline quality is found to be decreased due to zinc doping (i.e, the crystallinity of pure SnO 2 is better than that of zinc doped ones), which is clear from Figure 4.6. A similar trend of change in preferred orientation has also been observed for Zn:SnO 2 films prepared by ultrasonic spray pyrolysis (Bilgin et al 24). The experimental peak positions were carefully compared with the standard reported tetragonal SnO 2, wurtzite ZnO and cubic Zn 2 SnO 4 phases and the miller indices were indexed to the peaks. It has been observed that both SnO 2 and Zn:SnO 2 films possess tetragonal SnO 2 structure (JCPDS card: 1-88-287) and no indication for Zn, ZnO and any other mixed phase peaks, which eventually would be expected to form upon mixing SnO 2 with ZnO or vice - versa (Bilgin et al 24, 25). The unit cell parameters were calculated by using a software package, with experimental peak positions (2 ) and their corresponding miller

94 indices (h k l) as the input. The calculated lattice parameter values and cell volume are given in Table.4.2. From this Table, it evidenced that the cell volume increases upon increasing the zinc concentration in the spray solution. For lower Zn concentrations we do not observe any change in the cell volume. Whereas, for higher Zn concentrations we observed a slight increase in the unit cell parameters. The increase of cell volume can be attributed to the difference in the ionic radii of Sn 4+ (r =.71 Å) and Zn 2+ (r =.74 Å). It has been observed that the calculated cell parameter values are almost comparable to the reported theoretical values. 6 4 (11) SnO 2 2 (11) (2) (211) (22) (31) (31) Intensity (Cps) Intensity Cps. 15 75 15 5% Zn:SnO 2 15% Zn:SnO 2 75 15 1 5 2 25 3 35 4 45 5 55 6 65 7 75 2 [ o ] 25% Zn:SnO 2 Figure 4.6 Grazing angle X-ray diffraction patterns recorded for SnO 2 and Zn: SnO 2 films deposited on Si (1 ) substrates

95 (a). (11) peak SnO 2 5% Zn:SnO 2 15% Zn:SnO 2 25% Zn:SnO 2 12.75 13. 13.25 13.5 13.75 14. (b). (2) peak Intensity [arb. units] Intensity [arb. units] 18.5 18.75 19. 19.25 19.5 [ o ] Figure 4.7 X-ray rocking curves performed for (1 1 ) and (2 ) peaks of Zn:SnO 2 films Table 4.2 Lattice parameter and cell volume of Zn doped SnO 2 films calculated from X-ray diffraction Analysis. Sample Cell parameters (Å) a = b c Cell Volume (Å 3 ) Theory 4.737 3.186 71.51 SnO 2 4.7357.2 3.1841.2 71.484.16 5% Zn:SnO 2 4.7437.2 3.185.2 71.6728.17 1% Zn:SnO 2 4.7465.3 3.1799.2 71.6411.117 15% Zn:SnO 2 4.7524.5 3.1773.3 71.7611.2 25% Zn:SnO 2 4.7997.13 3.1624.2 72.8534.32

96 4.4.2.2 Raman spectroscopy For structural analysis, the samples have also been characterized by Raman spectroscopy. This inelastic light scattering technique probes the eigen frequencies of different vibration modes of Sn-O and Zn-O bonds. In this way, specific frequency values are expected for each stoichiometry and for each crystalline modification, if any (Venkataraj et al 21). Raman measurements were performed for the samples deposited both on quartz and Si(1 ) substrates. It has been reported that rutile SnO 2 is characterized by four fundamental Raman scattering peaks at 123 cm -1 (B 1g ), 472 cm -1 (E g ), 632 cm -1 (A 1g ) and 778 cm -1 (B 2g ) (Scott 197). The B 1g phonon corresponds to a vibrational motion of octahedra (Abello et al 1998). The B 1g phonon mode was first reported by Peercy and Morosin (1973) in SnO 2 single crystals, but its Raman intensity was very low that cannot be normally observed in thin films. Figure 4.8 shows Raman spectra of the films deposited on the quartz substrates. With 1 magnification, we have observed two intense peaks at 634 and 781 cm -1. These two peaks are attributed to A 1g and B 2g vibrational modes of SnO 2. In addition to the expected Raman vibrational modes, additional peaks at 33 cm -1, 435 cm -1, 526 cm -1 and 671 cm -1 have also been observed for the samples deposited on Si(1 ) upon further magnification (using 1 lens). Typical Raman spectrum measured for 1% and 25% Zn: SnO 2 is shown in Figure 4.9. The strong peak observed at 526 cm -1 is attributed to the LO-phonon mode of the Si (1 ) substrate and is not shown in Figure 4.9 due to its very high intensity that suppresses the visibility of other peaks. The peak at 435 cm -1 is in closer agreement with the zinc oxide E 2 mode, which is expected to appear at 442 cm -1 in ZnO. Hence this peak cannot be attributed to the ZnO vibrational mode and the possibility for phase segregation (from the XRD analysis also we did not observe any peaks related to secondary

97 phases or mixed phases). Therefore it is presumable that this peak is originated from doped Zn ion that occupies Sn site in the SnO 2 lattice. This could be one of the possible reasons for the observed amorphization of the films caused by Zn taking the position of Sn that leads to the movement of Sn ions in interstitial sites. The other peaks observed at 33 and 671 cm -1 are not very well understood, namely un-assignable. S n O 2 A 1g B 2 g Raman intensity [arb units] 7 % Z n :S n O 2 1 5 % Z n :S n O 2 2 5 % Z n :S n O 2 2 3 4 5 6 7 8 9 R a m a n s h ift [1 /cm ] Figure 4.8 Raman spectra of Zn: SnO 2 films with 1X magnification From the SEM analysis it was observed that the films possess nanoscale grains and hence the phonon confinement effect in SnO 2 might be the reason for the appearance of these unknown peaks. Wang et al (22), Maguire et al (22) have also observed similar kinds of unknown peaks in SnO 2 and discussed the peaks on the basis of density functional theory calculated by Parlinski, using PHONON software (http://wolf.ifj.edu.pl/ phonon/). From both XRD and Raman studies we did not observe any

98 structural changes upon increasing the zinc concentration in the spray solution. It has been observed that the SnO 2 films retain their tetragonal structure for all compositions. Raman Intensity [arb. units] 33 435 Intense Si peak 631 671 25% Zn:SnO 2 1% Zn:SnO 2 2 3 4 6 7 Raman Shift [1/cm] Figure 4.9 Raman spectra of Zn: SnO 2 films with 1X magnification 4.4.3 Surface Morphology of the Films The surface morphology of pure and zinc doped SnO 2 films were studied by HR-SEM and is shown in Figure 4.1. From Figure 4.1a, it can be seen that pure SnO 2 films possess uniform grains and the grains are almost homogeneously distributed, which indicates the high packing density of the films. Upon increasing the Zn concentration, the surface morphology of the films changes continuously and the grains also deteriorated. Also we did not observe any phase aggregation other than the decrease in grain size due to Zn doping. This observation is in agreement with the XRD results and confirms the transition from the crystalline state to the amorphous state for higher Zn concentrations.

99 (a) SnO2 (b) 1%Zn:SnO2 (c) 25%Zn:SnO2 2nm Figure 4.1 HR-SEM images of (a) SnO2, (b) 1% Zn: SnO2 and (c) 25% Zn: SnO2 films. The SEM pictures clearly show the grain size of SnO2 decreases with the increasing Zn doping 4.4.4 Optical Transmittance and Band Gap of the Film Figure 4.11 shows the transmittance spectra obtained for the Zn: SnO2 films. The transmittance measurements reveal that the films are highly transparent in the visible region with the transmittance value ranges from ~75% to ~85%. The absorption edge is found to be shifted towards the higher

1 wavelength region (red shift), which is an indication of a decrease in the optical band gap of the films upon increasing the zinc concentration. The fundamental absorption corresponding to the optical transition of the electrons from the valence band to the conduction band can be used to determine the nature and values of the optical band gap E g of the films. Depending on the characteristic property of the material, different theoretical equations are used to calculate the absorption coefficient ( ) value as a function of photon energy (hν). In our case, since the films are polycrystalline, the ( hν) curves correspond to crystalline material with direct allowed transitions (direct band gap), which can be described by the following equation: K( h E ) n 2 g (4.1) h where is the absorption coefficient, K is a constant and E g is the band gap of the material. The exponent n is equal to 1 or 4 for the direct or indirect transition, respectively. Many groups have used the above formula to calculate the band gap of SnO 2 films and reported that SnO 2 is a direct band gap material (Arai 196, Gu et al 24, De Souza et al 1997 and Leite et al 24). The band gap can be deduced from a plot of ( hν) 2 versus photon energy (hν). Better linearity of these plots suggests that the films have direct band transition. The intercept of the tangent to the plot gives the band gap value of the films (Tsunekawa et al 2).

11 1 Transmittance [%] 8 6 Band gap decrease 4 SnO 2 7%: Zn: SnO 2 2 15%: Zn: SnO 2 25%: Zn: SnO 2 25 3 35 4 45 5 55 6 65 7 Wavelength [nm] Figure 4.11 Optical transmittance of the Zn: SnO 2 films deposited on silica glass substrates Figure 4.12 shows the ( hν) 2 versus photon energy (hν) plot for pure and zinc doped SnO 2 films. The linear fits obtained for these plots are also depicted in this figure. The E g value for pure SnO 2 has been reported to be in the range of 3.8-4..2 ev, which depends on the preparation method and deposition conditions (Gu et al 24, De Souza et al 1997, Leite et al 24, Tsunekawa et al 2, Shanthi et al 1999). In our experiment, for pure SnO 2 films a band gap of ~3.86 ev, has been obtained, which is in good agreement with the previously reported values (Arai 196, Gu et al 24, De Souza et al 1997, Leite et al 24, Tsunekawa et al 2, Shanthi et al 1999). From the Figure 4.11 it is also clear that the band gap of the films decreases upon increasing zinc concentration in the precursor solution. The band gap is plotted as a function of increasing Zn concentration in Figure 4.13 and the values are given in Table 4.3. The decrease in band gap of the films might be due to the large difference in the band gap values of ZnO (E g = 3.2 ev) and SnO 2 (E g =3.8-4 ev). A Similar trend has also been observed by Bilgin et al (24) for the Zn: SnO 2 films prepared by ultrasonic spray pyrolysis.

12 3x1 16 SnO 2 25% Zn:SnO 2 ( h ) 2 (ev cm -1 ) 2 2x1 16 1x1 16 2.5 2.75 3. 3.25 3.5 3.75 4. 4.25 4.5 Energy (ev) Figure 4.12 Photon energy (ev) dependence of ( hν) 2 of SnO 2 and 25% Zn:SnO 2 films, where is the absorption coefficient Band gap E g [ev] 4. 3.95 Band gap of Zn doped SnO 2 3.9 3.85 3.8 3.75 3.7 3.65 3.6 3.55 3.5 5 1 15 2 25 3 Concentration of Zn in SnO 2 spray solution (wt.%) Figure 4.13 Variation of band gap (E g ) as a function of increasing Zn concentration in the spray solution

13 Table 4.3 Optical Band gap of the Zn:SnO 2 films. Optical band gap value decreases upon increasing Zn concentration in spray solution Zn Concentration Band gap calculated from Optical transmittance (ev) 3.86 5 3.81 7 3.83 1 3.78 15 3.71 2 3.67 25 3.58 4.4.5 Electrical Properties The electrical properties of Zn doped SnO 2 thin films were measured using the four point probe method in the Van der pauw configuration. The sheet resistance R s (Ω/ ) of the films were determined as a function of increasing Zn concentration and the values are plotted in Figure 4.14. From this figure it is clear that the resistivity of SnO 2 films increases with increasing Zn concentration. The observed increase in sheet resistance can be explained by two mechanisms as follows: (i) due to increasing Zn doping level, the concentration of free charge carriers in SnO 2 decreases, this is because the Zn dopant has two less valance electrons than Sn, and (ii) we may also consider Zn substitutes the Sn atom or occupies the interstitial sites, evidenced by XRD and Raman measurements. In both cases Zn should act as an acceptor dopant and therefore reduce the n-type conductivity of SnO 2. In our experiments, the minimum sheet resistance value is obtained for pure SnO 2, which showed high degree of crystallization (refer XRD Figure 4.6). Upon increasing the Zn concentration the sheet resistance

14 value was found to increase, this behaviour can be attributed to the amorphization behaviour of the films as observed by XRD (Figure 4.6). It is well known fact that smaller the grain size, the larger will be the grain boundary area which leads to higher probability for the scattering of the charge carriers at the grain boundaries. This would result for decrease in carrier mobility. Hence, the observed increase of sheet resistance with increasing Zn concentration is mainly due to the increase of amorphization (decrease in grain size due to amorphization) of the films. Sheet resistance (K.Ohm/square) Sheet resistance (k.ohm/square) 4 3 2 1 Thickness of of the the films film:~ : ~ 25m nm 5 1 15 2 25 Zn concentration (Wt%) Figure 4.14 Variation of sheet resistance as a function of increasing Zn doping concentration observed for Zn doped SnO 2 films

15 4.5 CONCLUSION Thin films of tin oxide (SnO 2 ) and zinc doped tin oxide (Zn:SnO 2 ), with different Zn doping concentrations, were prepared on Si(1 ) and silica glass substrates by the spray pyrolysis technique with a substrate temperature of 4 C. The effect of Zn doping on the properties of SnO 2 films has been systematically studied as a function of increasing Zn concentration.. From the EDX elemental analysis it has been observed that the Zn concentration in the solid film is slightly less than that of the starting solution. The XRD measurements confirmed that pure SnO 2 films possess tetragonal crystalline structure with a preferred orientation along the (1 1 ) plane. Upon increasing the zinc concentration the preferred orientation changes from the (1 1 ) plane to the (2 ) plane, and the crystalline quality was found to be deteriorated. This has also been confirmed by the rocking curve measurements. The Raman measurements revealed the tetragonal structure of the films. From the Raman measurements, in addition to the SnO 2 vibrational modes, the additionally observed peaks at 33 and 671 cm 1 are attributed to the phonon confinement effect of SnO 2 grains. The HR-SEM measurements showed that upon increasing the Zn concentration, the surface features of the films changed continuously and the grains also deteriorated, which is attributed to the amorphization of the films upon increasing the Zn dopant concentration. The optical transmittance measurements confirmed that the films are fully transparent in the visible region. Upon increasing the Zn concentration the band gap was found to decrease from 3.86 to 3.58 ev. The sheet resistance measurements revealed an increase in sheet resistance value upon increasing the Zn concentration. The increase of sheet resistance due to Zn doping is attributed to the amorphization of films. The studies indicate that the properties of SnO 2 films can be finely tuned for a desired application with specific values.