Highly conductive and transparent ZnO thin films prepared by spray pyrolysis technique

Materials Chemistry and Physics 99 (2006) 382–387

Highly conductive and transparent ZnO thin films prepared by spray pyrolysis technique M.T. Mohammad a , A.A. Hashim b,∗ , M.H. Al-Maamory c a

b

Uxbridge College, School of Engineering, Uxbridge, Middx UB8 1NQ, UK Material Engineering Research Institute, Centre for Electronic Devices and Materials, Sheffield Hallam University, City Campus, Pond Street, Sheffield S1 1WB, UK c Department of Physics, College of Science, University of Babylon, Babylon, Iraq Received 16 May 2005; received in revised form 17 October 2005; accepted 11 November 2005

Abstract Thin layers of pure and Al-doped zinc oxide of 0.2 ␮m thickness have been prepared by spray pyrolysis of aqueous solution of ZnCl2 on borosilicate slides at temperature of 430 ◦ C. Doping is achieved by adding AlCl3 to the solution (by weight ratio), which is mixed thoroughly prior to spraying, using the air as the carrier gas. Substrate temperature has been found to be the most important film preparation parameter. The optimum substrate temperature was obtained by looking for maximum electrical conductivity accompanied by good optical properties. This substrate temperature was found to be 430 ◦ C, giving conductivities of 0.023(
cm)−1 and 0.3(
cm)−1 , for pure and 0.3% Al-doped films, respectively, and having optical transmittance at solar maximum of 85 and 70%, for pure and doped samples, respectively. The ZnO films prepared are polycrystalline but retain w¨urtzite structure with preferred orientations of (0 0 2) and (1 0 0). The effect of doping and annealing on the crystalline structure was studied, by investigating the X-ray patterns obtained for films with different doping before and after annealing at 400 ◦ C in Argon for 40 min. The film sensitivity to CO and C4 H10 gases was evaluated by studying the electrical conductivity as a function of gas molar concentrations. © 2005 Elsevier B.V. All rights reserved. Keywords: Zinc oxide films; Spray pyrolysis; Electrical and optical properties and gas sensors

1. Introduction In the recent years, there has been great demand for low cost transparent conducting films (e.g.; ZnO, SnO4 , In2 O3 , Cd2 SnO4 ) [1–6], due to their applications in various energy efficient purposes, such as, window layer in heterojunction solar cells, heat mirrors, piezoelectric devices, multilayer photo thermal conversion systems and solid state gas sensors [7,8]. The availability of ZnO makes it less expensive, and its sharp UV-cut-off makes it desirable in many applications. Moreover, they can be prepared by different techniques, such as magnetron sputtering, reactive evaporation, chemical vapor deposition (CVD), pulsed laser deposition (PLD) and spray pyrolysis (SP) [9–20].

∗

Corresponding author. Tel.: +44 114 2253296; fax: +44 114 2253433. E-mail address: [email protected] (A.A. Hashim).

0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.11.009

Among these methods, the spray pyrolysis technique has several advantages, such as, simplicity, safety, and low cost of the apparatus and raw materials. Several works on the physical properties of intrinsic and Al-doped ZnO films prepared by spray pyrolysis have been published earlier [20–22]. ZnO is a wide band gap metal oxide semiconductor that is well suited as photovoltaic window material. The band gap of 3.3 eV is large enough to transmit most of the terrestrial sunlight, and optical transmission is high over the whole solar spectrum [23]. In the present work, we report on the crystalline structure of ZnO, showing the effect of the impurity and annealing on the grain size and atomic distance. The room temperature conductivity of ZnO films doped with 0.3% Al, is studied as a function of substrate temperature Ts , which is a critical parameter and found to vary in the range of 0.084–0.3 (
cm)−1 , when Ts is changed in the range of 340–430 ◦ C. The film sensitivity to CO and C4 H10 gases is also evaluated by plotting its conductivity against the molar concentration.

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2. Preparation technique The film deposition set-up has been described elsewhere [24–26]. Aqueous solution of ZnCl2 (0.05 M) was used for spraying. The doping was achieved by the addition of AlCl3 to the spraying solution, and the whole mixture was sprayed onto microscopic glass slides. Many parameters were found to affect the film preparation, namely substrate temperature, spraying rate, spraying time, the distance between the substrate and the spray nozzle. Most of these parameters were established by trial and error, depending on previous experiences. The effect of the substrate temperature Ts , the most important parameter, was studied by fixing the other parameters, and preparing a number of samples having the same thickness on glass slides at different substrate temperatures in the range of 300–540 ◦ C. The optimal substrate temperature was obtained from the maximum room temperature dark conductivity, and desirable optical properties, and it was found to be around 430 ◦ C with 0.3% Al doping. However, the minimum substrate temperature was obtained also from the room temperature dark conductivity. The conductivity was dropped dramatically lower than 10−3
cm−1 , and the X-ray analysis doesnot shows an acceptable ZnO structure spectrum, which means that the ZnO film was doubtful. These conditions were found to be near optimal, since they provide 70% optical transmission at solar maximum, high conductivity of 0.3
cm−1 , and carrier concentration of 2.14 × 1019 cm−3 [23]. Aluminum electrodes with 99.99% purity were evaporated onto the film at a vacuum of 10−6 Torr, through specially prepared masks, in order to perform I/V and Hall effect measurements. Thin copper wires were attached to the electrodes using silver paint (electrical measurements sample). 3. Results and discussion 3.1. X-ray and conductivity analysis The prepared films were clear and transparent, having glassy appearance. Pure ZnO is an n-type wide gap semiconductor at room temperature [23]. It is generally accepted that in undoped ZnO material, the n-type conductivity is due to oxygen and interstitial zinc deficiencies. X-ray diffraction measurements were used to examine the crystalline structure of the sprayed films. CuK␣; Irradiation ˚ was used in this study. (␭ = 1.54117 A) Fig. 1 shows the typical X-ray diffraction patterns, and reveal that the films are polycrystalline with preferred (0 0 2) orientations together with (1 0 0), (1 0 1), (1 0 2) and (1 0 3) peaks. For doping of less than 0.1% Al, the diffraction peaks were pronounced but reduces slightly as the doping increases to 0.3%, with the peak (1 0 0) showing more pronounced reduction. Diffraction peak (0 0 2) was observed by many researchers [20,27,28], but Nnse et al. [22], reported that (0 0 2) orientation changes to (1 0 1) for doped films. Lee and Park [23], reported that the peaks (1 0 1) and (1 0 2) as well as (0 0 2) and (1 0 3) were observed in 1% Al-doped films, and indicated that the peak of the preferred (0 0 2) orientation

Fig. 1. X-ray diffraction patterns of pure and doped ZnO films (Ts = 430 ◦ C, t = 0.2 ␮m).

was decreased. The present results shows that for films doped with more than 1% Al, the peaks were reduced to such an extent, that it was hard to detect (1 0 2) and (1 0 3) peaks in films doped with 5% Al. The crystalline structure of the films is degraded by an increase in the doping concentration, which may be due to the stress arisen from the difference in the ion radii of zinc and aluminum (rZn +2 = 0.074 and rAl +3 = 0.054 nm). Fig. 2 shows the atomic distances d1 0 0 , d0 0 2 and d1 0 1 , calculated from the X-ray diffraction patterns for different Alcontent films, as shown in Table 1. The values compare well with the ASTM cards, showing hexagonal unit cell having w¨urtzite structure [29]. The introduction of impurities shifted the peaks towards higher values of (2θ), pushing the main principle peak (0 0 2) at 2θ = 34.3◦ for pure ZnO to 2θ = 35◦ for 27.19% Aldoped films. Such an increase in (2θ) will lead to a decrease in the atomic distance due to the substitution of Zn2+ with Al3+ of larger diameter. This is expected to distort the crystalline structure, and reduce the lattice constants to d1 0 0 = 2.880, ˚ d1 0 2 = 2.681 and d1 0 0 = 2.488 A.

Fig. 2. Plots of the atomic distances d1 0 0 , d0 0 2 and d1 0 1 as the function of the Al-content.

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Table 1 X-ray investigation of ZnO film of 0.2 ␮m thick prepared at substrate temperature of Ts = 430 ◦ C with different Al-doping Doping % Al Mole

2θ

˚ d (A)

hkl

˚ Grain size (A)

1

0

31.60 34.30 36.13 47.40 62.70

2.831 2.614 2.484 1.918 1.462

100 002 101 102 103

2

0.05

31.70 34.38 36.18 47.55 –

2.822 2.602 2.482 1.912 –

3

0.1

31.90 34.60 36.40 47.64 62.86

4

0.3

5

I/Io

˚ ao (A)

˚ co (A)

206.55 332.84 238.91 255.30 332.49

04.1 100 19.0 09.1 13.0

3.263

5.228

100 002 101 102 –

330.50 298.70 278.70 299.58 –

23.2 100 17.4 6.13 –

3.238

5.205

2.805 2.592 2.468 1.907 1.478

100 002 101 102 103

350.50 297.10 278.90 202.10 216.19

08.6 100 25.8 08.6 11.8

3.232

5.209

32.40 34.72 36.55 47.60 63.05

2.793 2.591 2.453 1.903 1.474

100 002 101 102 103

319.10 277.50 219.00 222.90 155.40

07.0 100 15.2 09.4 14.1

3.219

5.197

0.9

32.07 34.78 36.53 47.85 63.10

2.791 2.579 2.456 1.901 1.473

100 002 101 102 103

275.70 277.70 277.10 289.90 233.20

03.6 100 20.7 14.4 17.0

3.215

5.193

6

1

32.08 34.80 36.60 47.90 63.25

2.790 2.578 2.455 1.894 1.470

100 002 101 102 103

– – – – –

3.218

5.182

7

2

32.08 34.80 36.60 47.90 63.25

2.790 2.578 2.455 1.899 1.470

100 002 101 102 103

275.70 308.50 279.10 289.90 212.20

03.60 100 21.15 11.39 18.84

3.218

5.182

8

3

32.09 34.80 36.60 47.85 63.10

2.789 2.578 2.455 1.901 1.473

100 002 101 102 103

275.70 277.70 239.20 152.50 133.20

04.03 100 11.29 04.80 05.64

3.215

5.194

9

5

32.10 34.85 36.65 47.90 63.20

2.788 2.574 2.452 1.899 1.471

100 002 101 102 103

275.70 253.00 190.30 124.70 155.50

04.16 100 11.45 07.29 06.25

3.212

3.107

10

16

32.18 34.90 36.70 – 63.20

2.872 2.571 2.440 – 1.471

100 002 101 – 103

165.40 277.70 209.40 – 199.60

03.70 100 02.30 – 01.60

3.206

5.173

Sample

Fig. 3(a and b) introduces the effect of the annealing on the ˚ and 20.02 A ˚ thick films with crystalline structure for 13.43 A different Al-content. The two films were annealed at 400 ◦ C in Argon for 40 min, leading to an increase in the diffraction peaks especially for the thicker sample. The annealing of the 2% Al-doped sample had little effect on the crystallinity as

– – – – –

were observed by the small peak changes in Fig. 3(a). However, the crystalline structure of the highly doped (27.19%) film was affected considerably, as shown in the change of the same peaks in Fig. 3(b). Chopra and Rath et al. [30,31], indicated that annealing can reduce the surface energy of the film, which leads to the growth

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Fig. 4. Electrical conductivity vs. substrate temperature for pure and doped films of 0.2 ␮m thick.

Fig. 6b shows that α calculated at photon energy of 3.2 eV increases with the amount of doping reaching a maximum value of 15 × 104 cm−1 at 0.3% Al-content, and then decreases to a minimum value of 4 × 104 cm−1 at 5% Al-content. The decrease in the electrical resistivity of the film due to annealing was observed. This was attributed to the increase in the number of carriers by the ionization of the vacancies followed by oxygen evolution from the ZnO crystals, and by absorption of oxygen in the grain boundaries, which act as carrier traps [32]. 3.2. Gas sensing measurements

˚ thick ZnO film showing the Fig. 3. (a) X-ray diffraction pattern for 1343 A ˚ thick ZnO effect of the annealing and (b) X-ray diffraction pattern for 202 A film showing the effect of annealing.

of grains due to their merging in thick samples and, consequently, the expansion of the peaks. In thinner samples, the process of the grain growth may lead to the formation of discrete islands, which stops the growth of crystallites and the peak expansions are less pronounced. Fig. 4 shows the variation of room temperature electrical conductivity with substrate temperature for pure and doped 0.2 ␮m thick films. It was found that films prepared at a substrate temperature of 430 ◦ C and doped with 0.3% Al, provide desirable optical and electrical properties, having conductivity of 0.3 (
cm)−1 , and transmittance not less than 70%, as shown in Fig. 5. Fig. 6(a and b) shows the variation of the absorbance and absorption coefficient α with Al-doping. Absorbance in Fig. 6a increases with doping to reach maximum value at 0.3% Al doping, and this is accompanied with a decrease in the energy band gap from 3.2 eV for a pure sample, which indicate direct transition, to 2.88 eV at 0.5% Al doping. It also shows that any doping beyond 0.3% Al will decrease the absorbance and pushes the energy gap back to 3.2 eV.

Fig. 7 shows the plot of the conductivity versus CO and C4 H10 gas concentration, when introduced reluctantly into the vacuum containing the sample. The samples were kept under a vacuum of 3 × 10−3 mbar during the electrical measurements, to keep the samples free from any other effects. It was found that the conductivity increases linearly with the amount of the CO gas, where the sensitivity increased

Fig. 5. Spectral normal transmittance curves for pure and doped ZnO films of 0.2 ␮m thick prepared at Ts = 420 ◦ C.

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Fig. 6. (a) The absorbance spectrums of pure and doped ZnO film of 0.2 ␮m thick and Ts = 420 ◦ C and (b) the variation of the absorption coefficient with Al-doping.

from 0.3 × 10−2 for pure sample to 1.6 × 10−2 (
cm)−1 per mole for doped sample. The sensitivity to C4 H10 was found to be 0.3 × 10−2 (
cm)−1 per mole for pure sample, increased slightly to 0.3 × 10−2 (
cm)−1 per mole due to doping. The sample sensitivity for CO was increased, therefore, by more than five times at 0.3% Al-doping. It seems that some of these results indicate that ZnO films can be used as gas sensing elements [33]. This increase in the conductivity is due to the fact that C4 H10 and CO gases characterized as a strong reduction impurity. On the other hand, the oxygen chemisorptions create a surface barrier at the grain boundaries, acting as electron traps, and consequently leading to a conductivity decrease [30]. In contrast, the introduction of CO and C4 H10 , will reduce the chemisorbed oxygen or some internal oxygen, and any oxygen reduction will lead to a conductivity increase. The structures and sensing properties of ZnO:Al films as a CO gas with various thickness obtained by rf magnetron sputtering system were investigated [33]. The sensitivity of the gas sensor also increased as the concentration of CO gas was increased. The sensitivity was increased as the film thickness decrease; the sensitivity as well as the response time was improved by increasing the preparation temperature. The maximum sensitivity of 61.6% in this case was obtained for 65 nm film at the preparation temperature of 400 ◦ C [33]. The sensitivities to CO and C4 H10 gases of miniaturized layer on SnO2 -based bead sensors comparing with non-coated sensor were investigated by Dougami [34]. The sensitivity V was increased from being 13 to 19 mV for CO gas, and from 58 to 74 mV for C4 H10 . The sensitivity V is defined by V = Vg − Va , where Va and Vg denote bridge output voltages in air and in sample gas, respectively [34]. Bi-layered TiO2 /Al-doped ZnO (TiO2 /AZ) film showed high sensitivity with about three-orders change in the resistance during the relative humidity variation of 30–90% [35]. 4. Conclusion Spray pyrolysis is a cheap technique used in preparing highly conductive Al-doped ZnO films, having high transparency. The impurity had direct effect on the crystalline stricture as indicated by shifting the X-ray diffraction peaks towards larger values of 2θ, where the atomic distances were reduced. Thin layers of 0.2 ␮m thick showed maximum conductivity of 0.3 (
cm)−1 , and transmittance of 70%, when prepared at substrate temperature of 430 ◦ C and 0.3% Al-doping. The future work will involve some investigations on the stability of the films as gas sensing elements. References

Fig. 7. The conductivity as a function of molar concentration of CO and C4 H10 gases for pure and 0.3% Al-doped films.

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