1. Introduction
Zinc oxide is a direct, wide band gap semiconductor material with many promising properties for blue/UV optoelectronics, transparent electronics, spintronic devices, antireflection coatings, transparent electrodes in solar cells and gas sensor applications[1-6]. ZnO has been commonly used in its polycrystalline form for over a hundred years in a wide range of applications: facial powders, ointments, sunscreens, catalysts, lubricant additives, paint pigmentation, piezoelectric transducers, varistors, and as transparent conducting electrodes, thin film[7-10]. Its research interest has waxed and waned as new prospective applications revive interest in the material, but the applications have been limited by the technology available at the time.
ZnO has numerous attractive characteristics for electronics and optoelectronics devices. It has a direct bandgap energy of 3.37 eV, which makes it transparent in visible light and operates in the UV to blue wavelengths a high exciton binding energy of 60 meV[1, 2]. This material can be deposited by various methods like reactive evaporation, molecular beam epitaxy (MBE)[11], magnetron sputtering technique[12], pulsed laser deposition (PLD)[13], the sol–gel technique, chemical vapor deposition, electrochemical deposition[14] and spray pyrolysis[15], have been reported to prepare thin films of ZnO. Among these, we will focus more particularly in this paper on the spray ultrasonic technique that is a low method suitable for large-scale production, it has several advantages in producing nanocrystalline thin films, such as a relatively homogeneous composition, a simple deposition on glass substrate because of the low substrate temperatures involved, easy control of film thickness and fine and porous microstructure. It is possible to alter the mechanical, electrical, optical and magnetic properties of ZnO nanostructures[9, 10].
The aim of this paper is study the possibility of the correlation between the optical and electrical properties of ZnO thin films with precursor molarity. Recently Ivill et al.[16] established the inverse correlation between the magnetization of Mn doped film with carrier density. They identified the Mn oxide precipitation even at
In this paper, ZnO thin films were deposited on a glass substrate at various precursor molarity between 0.05 to 0.125 mpl/l-1 at a substrate temperature of 350
2. Experimental and methods
2.1 Experimental procedure
The spray solutions were prepared by dissolving (Zn(CH
The optical properties of the deposited films was measured in the range of 300–800 nm using an ultraviolet–visible spectrophotometer (UV, Lambda 35) and the electrical conductivity of the films was measured in a coplanar structure obtained with evaporation of four golden stripes on the film surface. All spectra were measured at room temperature (RT).
2.2 Methods and model
The aim was to study the correlation between the electrical and optical properties of undoped ZnO thin films with the precursor molarity. The reported relationships between the band gap and the lattice parameters (
The correlation between the electrical and optical properties were studied for the electrical conductivity (
{σ(∗)=σ(e)σ(e)Max,Eg(∗)=Eg(e)Eg(e)Max,M(∗)=M(e)M(e)Max, |
(1) |
where
3. Results and discussion
3.1 Electrical conductivity
Figure 1 shows the variation of the electrical conductivity
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3.2 Optical properties
Figure 2 shows the absorbance spectra of ZnO samples obtained at different precursor molarities. It is clear that, the absorbance at photon energy smaller than 3.1 eV is low. This indication is confirmed with the absorbance value and the tail height of ZnO films were obtained by increasing the precursor molarity and reached a minimum at 0.1 M, followed by an increase with the precursor molarity further increasing. We note that the concentration of the spray effect is clearly observed in the layer quality. From Fig. 2, it can be seen that the extrapolation of the linear portion of the graph to the energy axis at
ZnO is a semiconductor with a large direct band gap; the optical gap energy
A=αd=−lnT, |
(2) |
(Ahν)2=C(hν−Eg),A=αd, |
(3) |
where
A=A0exphνEu, |
(4) |
where
Figure 3 shows the variation of the band gap energy
3.3 Correlation between the electrical and optical properties
We have previously described the experimental data; one can see from this data that the electrical conductivity of the ZnO thin film is varied nonlinearly to the precursor molarity. The model proposed for the ZnO thin film with precursor molarity is discussed.
We have estimated the relationships between the electrical conductivity and the band gap energy and the precursor molarity in all films. We found the following empirical relationships:
σ(c)=abEg(∗)Mc(∗), |
(5) |
where
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The variation experimental electrical conductivity and correlation of ZnO thin films as a function of optical and gap precursor molarity is shown in Figs. 4 and 5, respectively. One can see that this correlation indicates that the measurement in the electrical conductivity of the films from the optical band gap and the precursor molarity was equal; it is predominantly influenced by the transition tail width of undoped ZnO thin films.
In our experience there was no evidence for significant changes in electrical conductivity with correlation upon varying the band gap energy; the main effects have been observed upon variation of modifying the precursor molarity. This difference could be caused by the different compositions. The correlation between the electrical conductivity and the band gap with the precursor molarity was investigated.
4. Conclusion
In summary, high-quality transparent ZnO thin films were deposited on glass substrates by the ultrasonic spray technique. The description of correlation between electrical conductivity and optical gap energy with precursor molarity were discussed. The ZnO films exhibit higher electrical n-type semiconductors, in which band gap energy increased from 3.08 to 3.37 eV with an increasing of precursor molarity of 0.05 to 0.1 M. The maximum value of electrical conductivity of the films is 7.96 (