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Characterization of cadmium doped zinc oxide (Cd : ZnO) thin films prepared by spray pyrolysis method
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IOP PUBLISHING
JOURNAL OF PHYSICS D: APPLIED PHYSICS
J. Phys. D: Appl. Phys. 41 (2008) 245403 (7pp)
doi:10.1088/0022-3727/41/24/245403
Characterization of cadmium doped zinc oxide (Cd : ZnO) thin films prepared by spray pyrolysis method S Vijayalakshmi1 , S Venkataraj1 and R Jayavel1,2 1 2
Crystal Growth Centre, Anna University, Chennai-600025, India Centre for Nanoscience and Technology, Anna University, Chennai-600025, India
E-mail:
[email protected] and
[email protected]
Received 2 July 2008, in final form 12 October 2008 Published 27 November 2008 Online at stacks.iop.org/JPhysD/41/245403 Abstract Thin films of cadmium doped zinc oxide (Cd : ZnO) with different cadmium concentrations have been prepared by the spray pyrolysis method on different substrates at 400 ◦ C. The physical properties of the films were studied as a function of increasing cadmium dopant concentration (up to 25 wt%). The films were characterized by different methods to understand their structural, optical and electrical properties. Energy dispersive x-ray diffraction spectroscopy analysis showed that the Cd concentration in the solid film is slightly lower than that of the starting solution. The x-ray diffraction analysis revealed that the films are polycrystalline in nature having a hexagonal wurtzite type crystal structure with a preferred grain orientation in the (0 0 2) direction. Due to Cd doping, the degrees of polycrystallinity increased simultaneously and the orientation of the grains in the (0 0 2) axis is found to be deteriorated. Atomic force microscopy measurements reveal that the surface morphology of the films changes continuously with a decrease in the grain size due to Cd doping. Both photoluminescence and optical measurements showed that the band gap decreases from 3.12 to 2.96 eV with increasing Cd concentration. Increasing the Cd concentration also leads to the broadening of the emission peak and degrading the crystalline quality. The films are highly transparent in the visible region and the absorption edge showed a red shift. The sheet resistance of pure ZnO has been determined as 650 sq−1 cm−1 and is decreased for higher concentrations of Cd doping. The characterization studies clearly indicate the incorporation of Cd into ZnO; hence the observed decrease in the optical band gap and electrical resistivity can be directly attributed to the effect of Cd ion incorporation into the ZnO lattice.
and easy preparation methods. The pure and mixed oxide films of CdO, SnO2 and ZnO have been used as window materials in many optoelectronic devices and are extensively studied. Among these materials, the properties of cadmium doped zinc oxide (Zn1−x Cdx O) have not yet been studied clearly, though it is one of the promising candidates in the field of optoelectronics and also for the fabrication of ZnO based devices [9]. Cadmium oxide possesses cubic structure and a narrow direct band gap of 2.3 eV, whereas ZnO is a wide band gap semiconducting oxide material (Eg = 3.2 eV) that possesses large exciton binding energy (60 meV) [10, 11]. Hence, it is possible to modify the physical properties of ZnO upon mixing with CdO. Moreover, the incorporation of Cd
1. Introduction Materials with high transparency and electrical conductivity are essential for modern transparent conducting oxide (TCO) applications [1, 2]. Recently, there has been great demand for low cost TCO films (e.g. ZnO, SnO2 , In2 O3 , F : SnO2 , In : SnO2 , etc) due to their applications in various energy efficient purposes, such as window layer, heat mirrors, piezoelectric devices, multilayer photo thermal conversion systems and solid state sensors [3–8]. Even though several materials such as ZnO, SnO2 , F : SnO2 , In : SnO2 are proposed for TCO applications, ZnO has received great attention due to its attractive properties 0022-3727/08/245403+07$30.00
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© 2008 IOP Publishing Ltd
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J. Phys. D: Appl. Phys. 41 (2008) 245403
S Vijayalakshmi et al
into ZnO is very useful for the fabrication of ZnO/ZnCdO heterojunction and superlattice structures, which are the key elements in ZnO based light emitters and detectors. Most of the recent works have been focused on the preparation of ZnCdO films [12–15] and knowledge of the physical properties of these films is very limited. Hence, an attempt has been made to study the physical properties of cadmium doped zinc oxide (Cd : ZnO) films prepared by the spray pyrolysis technique. ZnO thin films can be prepared by different methods such as pulsed laser deposition [16–18], magnetron sputtering [19–21], molecular beam epitaxy [22], spray pyrolysis [23] and sol–gel spin coating methods [24–26] on different substrates. In our experiments, we used the chemical spray pyrolysis method to prepare thin films of Cd : ZnO, which is a simple, versatile and economical method for the preparation of polycrystalline and amorphous thin films. The spray pyrolysis method is used for the preparation of a large number of pure and doped simple and complex semiconducting and insulating oxide films [27]. The method is based on the preparation of clear solutions from certain salts of the material whose thin films are to be prepared. During the thin oxide film deposition processes, both chemical and thermal reactions are used. Using the spray pyrolysis method, it is easy to grow uniform films with very high growth rates, in the order of several hundred nanometres per minute. Hence spray pyrolysis is an attractive and widely practiced method in industries for large area coating applications.
(d)25% Cd: ZnO
Zn
Cd Cd
O
Intensity.
Zn
Cd
Zn
Zn
Cd
Zn
(a) 5% Cd: ZnO Zn
Zn
0 1
Zn
(b)10% Cd: ZnO
O
O
Zn
(c)15% Cd: ZnO
Zn O
Zn
Cd
2 3
Zn
4 5 6 7 8 9 10 11 12 Energy (keV)
Figure 1. HRSEM–EDX patterns of (a) 5% Cd : ZnO, (b) 10% Cd : ZnO, (c) 15% Cd : ZnO and (d) 25% Cd : ZnO.
The elemental analysis of the films was performed by an energy dispersive x-ray spectrometer (EDX) attached to a Hitachi S-4800 high resolution field emission scanning electron microscope (HR-SEM). The crystalline properties of the films were studied by the x-ray diffraction (XRD) method employing a RINT Rigaku 2500 x-ray diffractometer with Cu Kα radiation. The XRD measurements have been carried out in both grazing angle x-ray diffraction (GXRD) and θ/2θ geometries. It is well known that GXRD is very sensitive to probing the surface structure of thin films. Typical XRD patterns were recorded for the 2θ values ranging from 20◦ to 80◦ . The experimental peak positions were compared with the standard JCPDS files and the Miller indices were indexed to the peaks. The surface morphology of the films has been studied using the atomic force microscopy (AFM) method. The luminescent properties of the films have been studied by photoluminescence (PL) measurements and were performed employing a He–Cd laser light source with an excitation wavelength of 325 nm. In order to understand the optical properties of the films, transmittance spectra were recorded for the wavelengths ranging from 1100 to 200 nm. The band gap of the films has been calculated from the absorption edge of the transmittance spectrum. The electrical resistivity of the films has also been measured using a four-point probe apparatus in the van der Pauw configuration.
2. Experiment The ZnO and Cd : ZnO thin films were deposited using a homemade spray pyrolysis experimental setup [28, 29]. The spray solution was prepared by mixing an appropriate amount of zinc acetate [Zn(CH3 COO)2 .2H2 O] and cadmium acetate [Cd(CH3 COO)2 .H2 O] dissolved in a mixture of deionized water and ethanol. In order to obtain a homogeneous clear transparent solution, the mixture was mechanically stirred for 2 h. The cadmium concentration was varied up to 25 weight percentage (wt%) in the spray solution. The films were deposited on Corning glass, Si (1 0 0) and quartz substrates. Compressed air was used as the carrier gas. Before real sample preparations, several initial trials were made to optimize the deposition conditions. Under the optimized conditions, during deposition, (i) the solution flow rate and carrier gas flow rate were maintained constant at 10 ml min−1 and 12 l min−1 , respectively; (ii) the spray nozzle to substrate distance was maintained at 30 cm and (iii) the substrate temperature was maintained at 400 ◦ C. In order to avoid the fast cooling of the substrate due to continuous spraying of the solutions, the solution was sprayed on the substrates for several spraying cycles of 3 s followed by an interval of no spray for 2 min. The films were sprayed for a maximum of up to 3 h with the above said systematic steps, which enabled us to have films of thickness approximately 250–300 nm. For each dopant concentration, several films were prepared on Si(1 0 0) substrates and confirmed that the thicknesses of the films are reproducible.
3. Results and discussion 3.1. EDX micro elemental analysis The elemental analysis of Cd : ZnO films with different Cd doping concentrations has been investigated using HRSEM– EDX and is shown in figure 1. Films deposited on quartz substrates have been used for the EDX analysis; this enabled us to avoid the strong silicon background signal that used to appear from the silicon substrate. The EDX analysis confirmed the presence of Zn, O and Cd elements in the deposited films. It has been observed that upon increasing the Cd concentration in the starting solution, the amount of Cd in the solid films was 2
J. Phys. D: Appl. Phys. 41 (2008) 245403
S Vijayalakshmi et al
(112) (201)
25% Cd:ZnO (103)
500 0
4000
Intensity Cps.
Intensity
5000
1000
(110)
6000
(102)
5% Cd: ZnO 10% Cd: ZnO 15% Cd: ZnO 25% Cd: ZnO
(100) (002) (101)
1500
7000
3000 2000 1000 0 2.9
3.0
3.1
3.2
3.3
3.4
15% Cd: ZnO
1000 0
5% Cd:ZnO 1000
3.5
0
Energy (keV)
ZnO
1500
Figure 2. HRSEM–EDX image of Cd peak observed at 3.14 and 3.35 keV. The intensity of the Cd peak increases with increasing Cd concentration in the starting spray solution.
750 0 25 30 35 40 45 50 55 60 65 70 75 2θ [º] Figure 4. GXRD patterns recorded for ZnO and Cd : ZnO films deposited on quartz substrates. Compared with pure ZnO, the Cd doped films showed a greater number of XRD peaks, which indicates the increase in polycrystallinity due to Cd doping.
15
800 5
(0 0 2) (1 0 1)
1200
10
400
(d) 25% Cd:ZnO (1 0 2)
20
(1 0 0)
Cd concentration in the film measured by EDX
25
0
0 0
5
10
15
20
(c) 15% Cd:ZnO
25
Intensity Cps.
Cd concentration in the starting solution
Figure 3. Variation of the Cd concentration in the films as a function of the Cd concentration in the starting solution as measured by EDX.
also found to be increased. From figure 1, it is also interesting to note that the intensity of the Zn characteristic peaks observed at higher energies (8.5 and 9.5 keV) decreases upon increasing the Cd concentration. This clearly indicates the substitution of Cd into ZnO. The Cd characteristic peaks observed between 2.8 and 3.4 keV are plotted in figure 2. From this figure 2, it is clear that the intensity of the Cd peak increases upon increasing the Cd concentration in the starting solution. In figure 3, the elemental weight (wt%) of Cd in the solid films is plotted as a function of the Cd concentration in the starting solution. From the EDX analysis, it has also been observed that the amount of Cd in the solid film is slightly less than that in the starting solution. The Cd concentration in the film did not vary by more than 3% which is expected to be due to the error margin of the EDX setup. Similar kinds of observations were also observed in the case of Zn : SnO2 [29].
750 0 1500
(b) 5% Cd:ZnO
750 0 2000
(a) ZnO
1000 0 30
35
40 2θ [º]
45
50
Figure 5. XRD pattern of Cd : ZnO films measured in θ/2θ scan mode. The FWHM of the (0 0 2) peak increased due to Cd doping, which indicates the degradation of grain size and crystalline quality due to Cd doping. ◦
grazing angle of 0.75 are depicted in figure 4. The GXRD measurements revealed that the films are polycrystalline with hexagonal wurtzite structure. It is seen in figure 4 that pure ZnO films show polycrystalline structure with a preferred orientation in the (0 0 2) direction. Compared with pure ZnO film, the Cd doped films showed a greater number of diffraction peaks (figure 4), which indicates the increase in the degree of
3.2. Structure and morphology of the films 3.2.1. X-ray diffraction. The crystalline structure of Cd : ZnO films was analyzed by the XRD method. Typical x-ray diffractograms recorded in the GXRD method for a 3
J. Phys. D: Appl. Phys. 41 (2008) 245403
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Table 1. FWHM, RMS roughness, NBE emission peak position, band gap and sheet resistance values of Cd : ZnO films. Cd concentration (wt%)
FWHM 2θ (deg)
RMS roughness (nm)
NBE emission peak position (nm)
Band gap Eg (eV)
Sheet resistance ( sq cm−1 )
0 5 7 10 15 25
0.2917 0.2895 0.3012 0.3657 0.443 0.502
14.5 — — 11.29 10.64 8.25
378 382 379 389 408 —
3.12 3.11 3.12 3.07 3.02 2.96
650 671 655 613 570 465
polycrystallinity of the films due to Cd doping. In order to confirm this observation, the XRD measurements have also been repeated in θ/2θ scan mode and are shown in figure 5. The θ/2θ scan showed a decrease in the intensity of the (0 0 2) peak along with additional new peaks due to Cd doping. It has also been observed that the intensity of the (0 0 2) peak decreased (figure 5) with the simultaneous increase in the FWHM value (table 1) and indicates the constant decrease in the grain size values. Apart from ZnO characteristic peaks, no peaks that correspond to either cadmium, zinc or their complex oxides could be detected, which are normally expected to appear upon mixing these two oxides. This observation suggests that the films do not have any phase segregation or secondary phase formation. As explained in the experimental section, the experimental peak positions were compared with the theoretical JCPDS files and the Miller indices were indexed. A software package developed in our laboratory has been used to calculate the lattice parameter values. The lattice parameter c is calculated from the (0 0 2) peak of Cd : ZnO films shown in figure 5 (T /2T measurement). From the calculations, it has been observed that the c lattice parameter value of Cd : ZnO films increases linearly from 5.19 (for pure ZnO) to 5.22 Å (for 25% Cd doping). This increase in the c parameter value clearly indicates that the smaller Zn ions (ionic radius 0.74 Å) are substituted by the larger Cd ions (ionic radius 0.97 Å) in the hexagonal wurtzite ZnO structure and hence there is an increase in the lattice parameter values. A similar observation has also been reported by Ye et al for the sputter deposited Cd : ZnO films [30].
0
60.0
200 400 [ nm ] 600 800
(a) ZnO 0
200
400
600
0.0
800 0
50.0
200 400 [ nm ] 600 800
(b) 15% Cd ZnO 0
200
400
600
0.0
800 0
35.0
200 400 [ nm ]
3.2.2. Atomic force microscopy. Figures 6(a)–(c) show the AFM images of Cd : ZnO films with different Cd concentrations. It is observed that pure ZnO films (figure 6(a)) possess larger grains and surface roughness values around 14.5 nm. However, upon increasing the Cd concentration, the grain size of the Cd : ZnO films was found to be decreased, and the surface roughness values were also found to be decreased simultaneously (table 1). The decrease in surface roughness upon increasing the Cd concentration can be attributed to the polycrystallization of the films which has also been confirmed by XRD analysis.
600 800
(c) 25% Cd ZnO 0
200
400
600
0.0
800
Figure 6. AFM picture of (a) ZnO, (b) 15% Cd : ZnO and (c)25% Cd : ZnO. Both surface roughness and grain size decreased with increasing Cd concentration. (This figure is in colour only in the electronic version)
(30 K). Figure 7 depicts the PL spectrum obtained for pure ZnO. From figure 7, it is clear that pure ZnO films show two emission peaks: (i) a narrow UV emission peak centred approximately at 378 nm, which corresponds to the near
3.3. Luminescent properties The PL spectra of Cd : ZnO films deposited at 400 ◦ C on Si (1 1 1) substrates have been measured at low temperatures 4
J. Phys. D: Appl. Phys. 41 (2008) 245403
S Vijayalakshmi et al
NBE emission of Cd:ZnO
378 nm
(c) 10% Cd:ZnO
610 nm PL Intensity
PL Intensity
(d) 15% Cd:ZnO NBE emission of ZnO
(b) 7% Cd:ZnO
350 400 450 500 550 600 650 700 750 Wavelength (nm)
Figure 7. PL spectra of ZnO films. The NBE emission peak appears at 378 nm.
(a) 5% ZnO
band edge (NBE) emission of ZnO due to direct band-toband electronic transition [31], and (ii) a broad shoulder-like peak centred at ∼610 nm, related to the oxygen vacancies in crystals [32]. However, compared with the NBE emission, the intensity of the defect related peak observed at ∼610 nm is very weak, indicating that deposited ZnO films have only a few defects and satisfactory luminescent properties. The NBE emission peaks observed for ZnO with different Cd dopant concentrations are shown in figure 8. Upon increasing the Cd concentration in the spray solution, the NBE emission showed a red shift that indicates the narrowing of the band gap due to Cd incorporation. The observed NBE peak positions are listed in table 1. Due to the incorporation of Cd, the width of this NBE emission peak tends to increase greatly (peak broadening behaviour), with a simultaneous decrease in the intensity of this peak (figure 8). Furthermore, no NBE emission peak is detected for films prepared from the spray solution containing 25 wt% of Cd concentration. This implies the degradation of the grains as well as the crystalline quality with increasing Cd concentration. This observation fairly agrees with the XRD measurements in which the intensity of the (0 0 2) preferred orientation peak has been found to be decreased (please refer to figures 4 and 5).
325
350
375
400
425
450
Wavelength (nm)
Figure 8. The NBE emission peak of Cd : ZnO films deposited on Si(1 0 0) substrates. The NBE emission peak shifted to the higher wavelength side is an indication of band gap narrowing due to Cd doping. The intensity of this emission peak decreased for higher Cd concentration. 1.0
Transmission
0.8
0.6
Red shift of the absorption edge indicating band gap narrowing
0.4 ZnO 10% Cd:ZnO 15% Cd:ZnO 25% Cd:ZnO
0.2
0.0 300
3.4. Transmittance and band gap
400
500 600 Wavelength (nm)
700
Figure 9. Optical transmittance of Cd : ZnO films deposited on silica glass substrates. The films are highly transparent in the visible region. The absorption edge shows red shift as the Cd concentration increases which indicates the decreasing band gap of the films.
Figure 9 shows the transmittance spectra obtained for the Cd : ZnO films. The transmittance measurements reveal that the films are highly transparent in the visible region. Depending on the Cd concentration in the starting solution, the transmittance value varies between 75% and 85%. The increase in the Cd concentration, up to 7 wt% in the starting solution, did not show any remarkable changes in the transmittance value. Above this dopant concentration level, we observed coloration (transparent to pale yellowish colour) in the films with a decrease in the transmittance values, i.e. the average transmittance of these films decreased for higher Cd concentrations. It has also been observed that the absorption edge showed a red shift for higher Cd concentrations, which indicates that the decrease in the Eg values is due to Cd doping. Both ZnO and CdO are considered as materials having direct band gap energy [33]. The band gap values of these
materials can be determined using the following relation: α=
K(hν − Eg )n/2 , hν
(1)
where α is the absorption coefficient, K is a constant and Eg is the band gap of the material. The linearity in the plot of α 2 versus hν indicates that the Cd : ZnO films possess direct interband transitions. The extrapolation of the linear portion of the graph to the energy axis gives the band gap energy values (figure 10). The calculated Eg values are listed in table 1. The Eg values for ZnO vary between 3.20 and 3.4 eV depending on the preparation methods and is 2.29 eV for CdO [10, 11, 33]. 5
J. Phys. D: Appl. Phys. 41 (2008) 245403
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films possess polycrystalline hexagonal wurtzite structure with a partial preferred orientation along the (0 0 2) direction. With the increase in Cd concentration, the polycrystalline nature of the films has increased. Simultaneously, the grain orientation in the (0 0 2) axis decreased with an increase in the FWHM value (indication of smaller grains). The AFM measurements showed that upon increasing the Cd concentration, the surface morphology of the films changed continuously with a decrease in both the grain size and surface roughness values. Both PL and optical transmittance measurements showed a substantial red shift of the band gap, which can be interpreted in terms of band gap modulation due to Cd doping. With the increase in Cd concentration, the band gap of the films decreased from 3.12 to 2.96 eV. The electrical resistivity measurements show that the sheet resistance of the films decreases for higher Cd concentrations, which is attributed to the low resistance value of CdO. From our experiments we have demonstrated that the physical properties of ZnO can be well modified by cadmium doping.
16
2.0x10
ZnO 25% Cd: ZnO 16
(αhν)2 (eV cm-1)2
1.5x10
16
1.0x10
15
5.0x10
0.0 2.00 2.25 2.50 2.75 3.00 3.25 3.50 3.75 4.00 Energy (eV)
Figure 10. Photon energy (eV) dependence of (αhν)2 of ZnO and 25% Cd : ZnO films, where α is the absorption coefficient. Linear parts of the curves were extrapolated to obtain the band gap value of the films.
In our experiments, for pure ZnO we obtained a Eg value of 3.12 eV, which is fairly comparable to the previously reported values [10, 11, 34]. As the Cd dopant concentration increases up to 10 wt% in the starting solution, there is no notable change in the Eg value. Further increase in the dopant concentration leads to band gap narrowing (refer to table 1) which can be attributed to the large difference in the Eg values of ZnO and CdO [10, 11, 33]. The observed values are in good agreement with the values reported by Vigil et al [35].
References [1] Ginley D S and Bright C (ed) 2000 Mater. Res. Bull. 25 15 and articles therein [2] Kammler D R, Edwards D D, Ingram B J, Mason O, Palmer G B, Ambrosini A, Poeppelmeier K R 1999 Photovoltaics for the 21st Century, Electrochem. Soc. Proc. (Seattle, WA, May) ed V A Kapur et al vol 68 p 99 [3] Minami T, Nanto H and Takata S 1982 J. Appl. Phys. Lett. 41 958 [4] Hamberg I and Granquist C G 1986 J. Appl. Phys. 60 R123 [5] Major S, Kumar S, Bhatnagar M and Chopra K L 1986 J. Appl. Phys. Lett. 49 394 [6] Jin Z C, Hamberg I and Granvist C G 1988 J. Appl. Phys. 64 5117 [7] Mohammad M T and Abdul-Gafor W A 1989 Solid State Commun. 72 1043 [8] Yoo J B, Fahrenbruch A L and Bube R H 1990 J. Appl. Phys. 68 4694 [9] Wang F, Ye Z Z, Ma D, Zhu L and Zhuge F 2005 J. Cryst. Growth 283 373 [10] Dong L F, Cui Z and Zhang Z K 1997 Nanostruct. Mater. 8 815 [11] Cao H, Xu J Y, Zhang D Z, Chang S H, Ho S T, Seeling E W, Liu X and Chang R P H 2000 Phys. Rev. Lett. 84 5584 [12] Sakurai K, Kubo T, Kajita D, Tanabe T, Takasu H, Fujita S and Fujita S 2000 Japan. J. Appl. Phys. 39 L1146 [13] Fons P, Iwata K, Niki S, Yamada A, Matsubara K and Watanabe M 2000 J. Cryst. Growth 209 532 [14] Ye Z Z, Ma D W, He J H, Huang J Y, Zhao B H, Luo X D and Xu Z Y 2003 J. Cryst. Growth 256 78 [15] Wan Q, Li Q H, Chen Y J, Wang T H, He X L, Gao X G and Li J P 2004 Appl. Phys. Lett. 84 3085 [16] Kang H S, Kang J S, Kim J W and Lee S Y 2004 J. Appl. Phys. 95 1246 [17] Lee S Y, Li Y, Lee J S, Lee J K, Nastasi M, Crooker S A and Jia Q X 2004 Appl. Phys. Lett. 85 218 [18] Matsubara K, Fons P, Iwata K, Yamada A, Sakurai K, Tampo H and Niki S 2003 Thin Solid Films 431–432 369 [19] Takeda S and Fukawa M 2004 Thin Solid Films 468 234 [20] Zhu F, Zhang K, Guenther E and Jin C S 2000 Thin Solid Films 363 314 [21] Das R, Jana T and Ray S 2005 Sol. Energy Mater. Sol. Cells 86 207 [22] Sano M, Miyamoto K and Kato H 2004 J. Appl. Phys. 95 5527
3.5. Sheet resistance The sheet resistance values of Cd : ZnO films deposited on quartz glass substrates have been measured using a four probe method and the values are given in table 1. Initially for both pure ZnO and up to 7 wt% of Cd concentration, we observed almost a constant sheet resistance of ∼650 sq−1 cm−1 . As the concentration of cadmium in the initial spray solution has been increased further, the sheet resistance value decreased steadily. A similar kind of observation and tendency has also been reported [36, 37]. The observed tendency of decrease in the sheet resistance with increasing Cd concentration can be ascribed to the low electrical resistance value of CdO. 3.6. Summary In this work, we have demonstrated a simple method to deposit high quality Cd doped ZnO films by spray pyrolysis. Thin films of ZnO and Cd : ZnO, with different Cd concentrations (up to 25 wt%), were successfully prepared on Corning glass, Si (1 0 0) and silica glass substrates by the spray pyrolysis technique with a substrate temperature of 400 ◦ C. The physical properties of these films have been studied in detail as a function of increasing Cd doping (up to 25 wt%) concentration. The elemental analysis of the films measured by energy dispersive x-ray (EDX) spectroscopy showed that the Cd concentration in the solid film is slightly less than that of the starting solution. The XRD analysis showed that pure ZnO 6
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[31] Bagnall D M, Chen Y F, Zhu Z, Yao T, Koyama S, Shen M Y and Goto T 1997 Appl. Phys. Lett. 70 2230 [32] Vanheusden K, Seager C H, Warren W L, Tallant D R and Voigt J A 1996 Appl. Phys. Lett. 68 403 [33] Tabet-Derraz H, Benramdane N, Nacer D, Bouzidi A and Medles M 2002 Sol. Energy Mater. Solar Cells 73 249 ¨ ur U, ¨ Alivov Y I, Liu C, Teke A, Reshchikov M A, [34] Ozg¨ Dogan S and Avrutin V 2005 J. Appl. Phys. 98 041301 [35] Vigil O, Vaillant L, Cruz F, Santana G, Morales-Acevedo A and Contreras-Puente G 2000 Thin Solid Films 361 53 [36] Vigil O, Cruz F, Santana G, Vaillant L, Morales-Acevedo A and Contreras-Punte G 2000 Appl. Surf. Sci. 161 27 [37] Choi Y S, Lee C G and Cho S M 1996 Thin Solid Films 289 153
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