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Preparation and characteristic of low resistive zinc oxide thin films using chemical spray technique for solar cells application
The effect of thickness and temperature substrate
S. Sali1,21, M. Boumaour2 and R. Tala-Ighil2
1Faculté de Physique, Université des Sciences et de la Technologie Houari Boumediène, ‘USTHB’
B.P. 32 El Alia, Alger, Algérie 2Laboratoire des Cellules Photovoltaïques,
Unité de Développement de la Technologie du Silicium, ‘UDTS’
2 Bd F. Fanon, B.P. 399 Alger-Gare, Alger, Algérie
Abstract - In this paper, we present results concerning undoped and indium-doped zinc oxide (ZnO: In) thin films were grown on glass and Si substrates using the chemical spray technique. The effects of thickness (e), as well as the substrate temperature (Ts), were studied. It was revealed by X-Ray diffraction that the preferred orientation of polycrystals is along C-axis, with hexagonal wurtzite structure. Two important facts were calculated from RBS measurements: the dopant concentration throughout the film and the thickness of the films, it was found that the thickness increase with time of deposition. Under optimum deposition conditions a low resistivity and a high optical transmittance of the order of 2.8 × 10−4 Ω cm and 85 %, respectively, were obtained.
Keywords: Spray deposition - ZnO - Doping - XRD - RBS – Transmittance - Resistivity.
1. INTRODUCTION
Due to the excellent electrical, optical and structural properties, Zinc oxide (ZnO) thin films have wide applications as solar cells [1] gas sensors [2], light emitting diodes (LED's), laser systems [3] and transparent electrodes [4]. Moreover, they can be prepared by different techniques, such as magnetron sputtering [5], reactive evaporation [6], chemical vapor deposition (CVD) [7], pulsed laser deposition (PLD) [8] and spray pyrolysis [9].
Among these methods, the spray pyrolysis technique has several advantages, such as, simplicity, safety, and low cost of the apparatus and raw materials.
In the present work, we report on the crystalline structure of ZnO, and In:ZnO showing the effect of the resistivity as function of deposition temperature and thickness of ZnO films.
2. ZINC OXYDE
The principal advantage of ZnO is the fact that its components are not toxic (contrary, for example, with indium in the ITO), and very abundant on Earth. It is an undeniable asset because it makes it possible to reduce the production costs.
ZnO has a crystallographic structure of hexagonal type wurtzite. Its density is 5.72 G.cm-3 which corresponds to a molecular density of 4.21 × 1022 molecules per cm3. ZnO stochiometric is an intrinsic semiconductor having a minimal optical gap of 3.1 eV.
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.
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 325 - 500 °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 480 °C. The thickness effect is also studied.
4. RESULTS AND DISCUSSION 4.1 Pure zinc oxide
The figure 1 shows typical X-ray diffraction of the ZnO films. It was observed that the preferred orientation of polycrystals is along C-axis, with hexagonal wurtzite structure. Whose cell parameters are 3.24992 Å and 5.20658 Å for A and C respectively.
As shown in figure 2, as the RBS spectra, the concentration of ZnO is 51.09 % Zn and 48.01 % O. The observation with the MEB of surface of the layer of ZnO deposited on silicon by spray, The image in figure 3 below shows a surface very Net and homogeneous layer obtained.
The two spectra of figure 4 exhibit the transmittance and reflectance results obtained from the ZnO films deposited on glasses substrate at 480 °C. Both of them show the normal transmittance of more than 90 %.
4.2 indium-doped zinc oxide
Spectrum RBS of a layer of ZnO: In deposited by spray is represented in figure 5.
The chemical composition of the elements is: 50.57 % Zn, 48.33 % O and 1.1 % In. The observation with the MEB of surface of the layer of ZnO: In deposited on silicon deposited by spray it possible to visualize the grains. The image below shows a little disturbed surface of the layer obtained (Fig. 6).
The spectra of transmission (t) and reflexion (r) are represented in Fig. 7. The transmission (t) and reflexion (r) are cancelled below approximately 366 nm. This cut corresponds to the optical gap of ZnO, which is approximately 3.4 eV
Fig. 1: the typical X-ray diffraction of the ZnO thin film
Fig. 2: RBS spectra of the ZnO thin film
Fig. 3: MEB of surface of the layer of ZnO
Fig. 4: the transmittance and reflectance spectra obtained from the ZnO films deposited on glasses substrate at 480 °C
Fig. 5: RBS spectra of the ZnO: In thin film
Fig. 6: MEB of surface of the layer of ZnO: In
Fig. 7: the transmittance and reflectance spectra obtained from the ZnO:In films deposited on glasses substrate at 480 °C 4.3 Effect of the temperature on the resistivity
We were interested in particular in this study the resistivity of the layers as function of the temperature of the substrate. The figure 8 shows the evolution of the resistivity of the layers according to the temperature of the substrate.
The resistivity passes by a minimum for a temperature which increases. This effect were already observed in layers of SnO2 Manifacier et al. allot this effect observed in layers of SnO2 un-doped to an excess of Sn detected in films.
This figure shows that the resistivity of the doped samples decreases compared To ZnO un-doped. This reduction in the resistivity can be interpreted by the increase in the number of the charge carriers (electrons) coming from the ions In3+ donors incorporated in the substitutional or interstitial sites from cation from Zn2+.
The increase in the temperature of deposit produces a rise in the resistivity, which is probably due to a reduction in the mobility of carriers resulting from excess from Zn and In.
Fig. 8: Evolution of the resistivity of the layers according to the temperature of the substrate
4.4 Effect of the thikness on the resistivity
Fig. 9 shows the variation of the resistivity of the layers according to the thickness, the resistivity decreases with the thickness. This reduction can be due to the increase in the mobility and the concentration of the electrons in the layers.
Fig. 9: variation of the resistivity of the layers according to the thickness
5. CONCLUSION
Conductive and transparent ZnO and ZnO:In thin films were deposited by chemical spray starting from a diluted solution of zinc acetate [ Zn (CH3 CO2)2, 2 H2O ], and the corresponding characteristics of the films as a function of the substrate temperature and thickness were reported. As the substrate temperature increases, a decrease in the resistivity value for the films was observed.
Under optimum deposition conditions a low resistivity and a high optical transmittance of the order of 2.8 × 10−4 Ω.cm and 85 %, respectively, were obtained.
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