1. Introduction
Zinc oxide (ZnO) in thin film form shows a wide variety of controlled properties, such as piezoelectricity, gas chemisorption, photo-catalysis, high conductivity, and transparency in the visible spectrum, that can be exploited in the design and manufacture of optoelectronic and electronic devices as well as in catalysis and biology fields [
1,
2,
3].
According to the required application, the deposition of ZnO thin films can be done successfully by different physical techniques, as is the case of sputtering [
4], reactive evaporation [
5], pulsed laser deposition [
6], and chemical techniques—sol-gel [
7], chemical bath [
8], chemical vapor deposition (CVD) [
9], and chemical spray (CST) [
10]. Among these deposition techniques, CST has been refined over the years for industrial applications due to the direct implementation of large area deposition using low cost equipment [
11]. Additionally, for transparent conductors, chemically sprayed as-grown films do not require an extra annealing step and it can also be considered as a key technique for future developments in nano-scale manufacturing [
12].
Focusing only on CST, there have been extensive studies on the effect of deposition variables on the physical characteristics of ZnO thin films [
13,
14,
15,
16,
17,
18]. However, all this time CST has not remained the same, since substantial modifications in the set up have been successfully tried in order to enhance the characteristics of the films. The atomization process can play a key role in the deposition of ZnO thin films with high transmittance and conductivity, as well as textured surface, adequate for transparent electrodes in thin film solar cells. As a matter of fact, the ultrasonic atomization process generates smaller droplets than the conventional pneumatic process, which in turn yield films with a smoother surface and enhanced conductivity.
The effect of deposition variables on the characteristics of undoped [
19,
20] and doped ZnO films [
21,
22,
23,
24,
25,
26,
27,
28,
29] deposited with ultrasonic spray pyrolysis has been reported. It is worthy to note that the number of reports of ZnO thin films based on ultrasonic process is increasing, although not yet at the same depth of knowledge than those based on pneumatic process. In this respect, some home-made adapted equipment, including commercial nebulizers, have been used for depositing conductive and transparent ZnO thin films, as is noted in different reports. Unfortunately, this influences the number of reports regarding ZnO dependence on deposition variables. Therefore, a lack of knowledge about some deposition variables, affecting the performance of ZnO thin films as transparent and conductive materials is still present in the literature. The effect of solvent composition, which is relevant in the deposition process as the gaseous species formed affect the final structure of films, has been pointed out previously by Smith and Rodriguez Clemente [
30]. Finding some correlation between deposition conditions and physical characteristics of the ZnO thin films was expected. The main goal of our study was the optimization of the deposition conditions in order to manufacture highly conductive and transparent ZnO thin films on glass substrates by ultrasonic chemical spray, avoiding extra steps such as a previous synthesis, complex reactor design and annealing on vacuum, or reducing atmospheres.
In this work the effect of water content variation in the starting solution on the electrical, structural, morphological, and optical characteristics of indium doped zinc oxide thin films, deposited by chemical spray at 430°C, is studied.
2. Experimental Details
2.1. Fabrication Of Indium Doped Zinc Oxide [ZnO:In] Thin Films
ZnO:In thin solid films were prepared using the ultrasonic spray pyrolysis (USP) technique (
Figure 1), which is a versatile technique that can be used to produce nanoscale sized powders and thin solid films. With this method the particle’s size can be easily controlled by changing the concentration in the starting solution and the atomization parameters. The deposition system used for depositing the ZnO:In thin films presented in this work includes a piezoelectric transducer operating at variable frequency, which was set to 1.2 MHz and the ultrasonic power at 120 W.
ZnO:In thin films were deposited on 2.54 cm × 2.54 cm soda-lime glass substrates from five different starting solutions. The starting solutions were prepared from a 0.2 M solution of zinc (II) acetate ([Zn(O2CCH3)2] from Alfa, 98%), dissolved in a mix of deionized water, acetic acid ([CH3CO2H] from Baker, 95%), and methanol ([CH3OH] from Baker, 98%). For a 1 L solution the acetic acid quantity was fixed to 50 mL/L in all cases. Separately, a 0.2 M solution of indium (III) acetate ([In(CH3CO2)3] from Alfa, 98 %) dissolved in a mix of deionized water and acetic acid (1:1, volume proportion) was prepared in order to be used as doping solution. A constant [In]/[Zn] ratio of 3.0 at.% was used. The five starting solutions were prepared with different water contents, namely, (M1) 50 mL/L, (M2) 100 mL/L, (M3) 150 mL/L, (M4) 200 mL/L, and (M5) 300 mL/L. Finally methanol was added until 1 L was completed. The substrates were cleaned prior to deposition. The cleaning process of the substrates is as follows: (i) sonication for five minutes in trichloroethylene ([C2HCl3] from Baker, 98%) for degreasing the substrates; followed by (ii) sonication in methyl alcohol ([CH3OH] from Aldrich, 98%); (iii) sonication in acetone ([CH3COCH3] from Baker, 98%), and finally, (iv) the substrates are dried by a jet of pure and dry nitrogen ([N2] from PRAXAIR, 99.997%). Then the substrates are placed on a fused tin bath, whose temperature measured just below the substrate using a chromel-alumel thermocouple, which is contained in a stainless steel metal jacket. The substrate temperature (Ts) was kept constant at 430 °C, within an accuracy of ±0.5 °C. Pure N2 (from PRAXAIR, 99.997%) was used as solution carrier and director gas, with flow rates of 3.5 and 0.5 L·min−1, respectively.
Figure 1.
Schematic diagram of the experimental set up used for depositing the ZnO:In thin films (ultrasonic spray pyrolysis: USP).
Figure 1.
Schematic diagram of the experimental set up used for depositing the ZnO:In thin films (ultrasonic spray pyrolysis: USP).
2.2. Indium-Doped Zinc Oxide Thin Solids Films Analysis
The thickness of the ZnO:In thin films was measured by a KLA profilometer (Tencor model P15 with a resolution of 1.5 nm) on a step formed during deposition. All the samples were grown with a film thickness value around 800 nm. The microcrystalline structure was studied from X-ray diffraction analysis of the samples made in a Siemens D5000 diffractometer by using the Cu-K1 (λ = 0.154056 nm) radiation and the θ-2θ technique. A JEOL scanning electron microscope (SEM) was used for morphological and composition evaluation of the thin solid films. The optical transmittance spectra at normal incidence were obtained by a double-beam Shimadzu 2401 PC spectrophotometer, in the UV-Vis region (350–1000 nm) without glass substrate correction. Electrical sheet resistance of samples was measured by the conventional four-aligned probe method (Veeco equipment) with the appropriate geometric correction factors.
4. Conclusions
The role of water content on the characteristics of ZnO:In thin films has been shown. Microstructure, morphology and electrical characteristics of the films can be achieved solely by solvent composition. Optimum deposition temperature conditions were obtained at 430 °C. Some changes in the preferential growth, from random to highly oriented along the (002) planes in samples deposited at low and high temperatures, respectively, were observed. It is worth mentioning that by using the USP method, there is a great saving of reactants as compared with the pneumatic spray pyrolysis process. Based on these results, the ZnO:In thin films processed by ultrasonic spray pyrolysis show a great potential to be applied as transparent conducting electrodes in thin film solar cells with electrical conductivity close to doped In2O3, [ITO], and with appropriate high optical transmission in the near-UV, VIS and NIR. Further studies on the deposition of ZnO:In films and optimization of the implemented CST parameters should also be performed in order to improve the nonlinear optical -response of the samples and film quality.