Physicochemistry of point defects in fluorine doped zinc tin oxide thin films
Introduction
Due to their combined high optical transparency in the visible region and their good electrical conductivity, transparent conducting oxides (TCO) have numerous technological applications such as transparent electrodes in photovoltaic cells, liquid crystal displays, light emitting diodes, heat reflecting mirrors and gas sensors [1], [2], [3]. In the last decades, different metal oxide semiconductors like SnO2, ZnO, In2O3 and TiO2 have been extensively used as TCO thin films [4], [5]. Doping these oxides with specific elements like F, Al, B and Cl could increase the electrical conductivity while maintaining the high optical transparency in the visible range [6], [7]. Therefore they could be more attractive for many optoelectronic applications. Among the different transparent conducting oxides, zinc tin oxide (ZTO) and fluorine doped zinc tin oxide (FZTO) films are promising candidates for many applications. They have low electrical resistivity, high optical transmittance, good thermal stability, high mechanical strength and low processing cost [8], [9].
Several techniques have been used for preparing ZTO films including pulsed laser deposition [10], sputtering [11], atomic laser deposition [12] and spray pyrolysis [13], [14] among others. The latter technique is known to be simple, reproducible, cheap, and adaptable to large-scale production. Yet only few reports were devoted to study spray deposited ZTO films and even fewer were dedicated to their electrical and optical properties [13], [14]. Moreover, the question of physicochemistry of point defects in ZTO and FZTO films has not been investigated previously. In a recent work, El Akkad et al. [15], [16] have obtained experimental evidence that oxygen vacancies are neutral in SnO2 films at room temperature. Their results are in good agreement with recent theoretical predictions [17], but in contrary to what had long been believed to be the case [18]. Additionally, detailed analysis of the optical properties revealed the presence of optical transitions involving un-identified defects in F:SnO2 thin films [16]. This calls for a close investigation of the role of point defects and their influence on the physical properties of hybrid systems involving SnO2 such as ZTO. Radheshyam Rai [19] reported that doping SnO2 with transition metal oxides influences dramatically the defect chemistry behavior of SnO2. In addition, they found that the substitution of tin ions by zinc ions create more oxygen vacancies. Concerning FZTO films, only three reports were found in the literature that is devoted to study this TCO. Pandey et al. [9] investigated the effect of annealing temperature on the structural, electrical and optical performance of amorphous FZTO thin films prepared by radio-frequency magnetron sputtering technique. Jun-Hyuck et al. [20] described in details the preparation procedure of FZTO from aqueous solution and provided a brief description of their properties. Park et al. [21] studied the electrochemical characteristics of the FZTO films prepared using MOCVD. Yet, no investigations were reported on the physical properties of spray deposited FZTO films.
In this paper, a comprehensive study of the structural, electrical, optical and chemical properties of ZTO and FZTO films prepared by the spray pyrolysis method is introduced. The overall results are correlated to the preparation conditions in an attempt to throw more light on the role of point defects and to provide information that may help in optimizing the properties of this transparent conducting oxide for different device applications.
Section snippets
Experimental procedures
ZTO and FZTO films were prepared by chemical spray pyrolysis technique. The starting solution was a mixture of SnCl4: propanol: H2O with molar ratio 1:9:2. Solutions containing up to 50% Zn were prepared by adding the desired amount of Zn in the form of ZnCl2. Doping with F was accomplished by adding the required amount of F in the form of NH4F to the spray solution. The thin films were prepared by spraying the solution on well cleaned borosilicate glass substrates which were maintained at
Results and analysis
The fluorine and zinc concentrations in the starting solution were varied in a wide range. The concentration of the fluorine and zinc in the thin films were determined by analyzing the XPS spectra as will be discussed in Section 3.2. The investigated thin films were divided into four categories TO, ZTO, FTO and FZTO, and they were labeled as given in Table 1, Table 2.
Discussion
In this section, we evaluate the concentration of oxygen vacancies and of substitutional fluorine (FO) and compare those with the electron concentration measured using Hall effect in order to understand the role of point defects responsible for the optoelectronic properties in FZTO films.
Conclusions
The structural, chemical, optical and electrical properties of ZTO and FZTO films prepared by spray pyrolysis technique are studied. XRD measurements showed that the films are polycrystalline with tetragonal rutile structure and grain size in the range (15–30) nm. The analysis of the XPS results taking into account the formation of oxygen bonds with Sn, Zn and adventitious carbon revealed the presence of oxygen vacancy concentration about 100 times higher than the free electron concentration in
Acknowledgments
This work was supported and funded by research administration of Kuwait University (Project No. SP04/14). We thank Mr. Maneesh Mathai for his help in the electrical and optical measurements. We also acknowledge the support of the general facility of the Faculty of Science (Projects GS 02/08, GS03/01 and GS01/10) and Semiconductor research facility, research administration (project GE01/08).
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- 1
On leave from the Department of Applied Physics, Tafila Technical University, Tafila, Jordan.
- 2
On leave from Department of Physics, The Hashemite University, Zarqa, Jordan.