Antimony doping effects on the interband transition of cdoprepared by chemical spray pyrolysis

Journal Of College Of Education __________NO.3.Vol.3-2011

Antimony Doping Effects on The Interband Transition of CdO Prepared by Chemical Spray PyrolysisN. F. Habubi , Z. M. Abood, S. S. ChiadAl_Mustansiriyah University, College of Education,

Physics Department

Abstract Optical properties of spray deposited CdO, Sb doped cadmium chloride thin films from CdCl2 precursor, all the films were deposited on microscope glass slide at the optimized substrate temperature 400oC. The transmittance of the film was observed to decrease for 5% Sb doping, the paper investigates the variation of absorption coefficient and optical energy gap of the as deposited films with Sb doping. CdCl2 400oC. %5 . .

Introduction Cadmium oxide (CdO) thin film, was the first film prepared as a transparent conducting Oxide (TCO) in 1907, by thermal Oxidation of sputtered cadmium, these TCOs have attracted increasing attention over the last decades as critical components of flat panel displays, Solar cells, and low emissivity windows [1, 2].

Recently CdO-based TCOs have been of interest due to their simple crystal structure, high carrier mobilites, and sometimes nearly metallic conductivities [3,4]. Epitaxial growth of Sn doped CdO thin film on MgO prepared by pulsed laser deposition have been the most conductive TCO thin films discovered

Thin film of Cd2SnO4, CdIn2O4 and CdO-ZnO thin films have been fabricated with good conductivities and optical transparencies for photovoltaic application [2]. Although the band gap of bulk CdO is only 2.3eV [5] leading to a poor optical transparency in the short wavelength range. A metal doping offers the possibility of tuning the electronic structure and the optical band gap through a carrier density dependent Burstien- Moss shift [6] for all these reasons, CdO with a simple cubic rock-salt crystal structure and small conduction electron effective mass represents an ideal model material in which one can study the effect of doping on TCO band structure crystal chemistry, and charge transport.

Many deposition technique have been adopted successfully in preparing CdO such as reactive evaporation [7], Solution growth [8], spray pyrolysis [9]. Sputtering and MOCVD. The aim of this work is to fabricate thin films using an easy and cheap technique (spray pyrolysis) and to study the effect antimony doping upon the inter band transitions

Experimental

Thin films of CdO and CdO:Sb were deposited using an in-built spray pyrolysis coating unit, the quality of these films when prepared by spray, depends on various process parameters such as spray rate, substrate temperature and the ratio of the various constituents in the solutions, since the deviation from stoichiometry due to oxgen vacancies makes CdO thin films to posses semiconducting nature.

Spray solution was prepared by mixing the appropriate volumes of cadmium chloride (0.1M) dissolved in deionized water. To achieve Sb doping, antimony trichloride (SbCl3) was dissolved in isopropyl alcohol and added to the precursor solution. The doping concentration was 5%, the amount of spray solution was made together 50 ml. For each condition the reproducibility of the films were verified by repeating the experiments several times, microscope glass slides, cleaned with organic solvent, were used as substrates the substrate temperature was fixed at 400oC which was optimized from the experimental work . Film thickness was about 0.5m, the transmission and absorption spectra for as deposited thin films were recorded using uv-visible shimadzu double beam spectrophotometer.

Result and discussions The transmittance spectrum of CdO and CdO:Sb as a function of incident photon energy are shown in figure (1).

The transmittance value of 28.5 at (1.45eV) for the pure are found to decrease to 21.5 at (1.45eV) on the addition of 0.5 antimony. it is known fact that a material containing an element in two different oxidation states or in a mixed oxidation state (like CdO:Sb) manifest abnormally deep and intense coloration [13]. Figure (2) represents the absorptance of CdO and CdO:Sb as a function of photon energy, it was clearly seen that the two samples shown the same behaviounr, but the value of CdO absorptions is less than its value when doping by antimony for a particular photon energy, this might be due to the formation of localized energy state due to antimony doping which increased numbers of electrons reaching the condition band. In the present study, an analysis of the absorption coefficient spectra shows the total absorption must due to different optical transition. Absorption coefficient data were elucidated from optical transmission data for both CdO and antimony doped CdO at various energies and the result were reported in figure (3) from this figure it was shown that the absorption coefficient () were affected by antimony doping however, the absorption coefficient is slightly affected by doping at lower energy value while the change is observed at higher energy values. The optical band gap was examined using the equation[14]: hv = A(hv - Eg)rwhere r=1/2 and 2 for direct and indirect allowed optical transition respectively, whereas r=3/2 and 3 for direct and forbidden transition respectively, A is the characteristic parameter, independent on photon energy. In an order to know there is one type of optical transition or more that can exist in CdO and CdO:Sb, a graphical representation of (hv)1/r=f(hv) for the as deposited thin film, figures (4), (5), (6) and (7) represents (hv)2=f(hv) and (hv)1/2=f(hv) for the deposited films, it is obvious that the first and second relation yields to a straight line indicating the existence of direct and indirect allowed transition.

The value of the energy band gap have been determine by extrapolation the liner portion of the respective curves to (hv)2=0 and (hv)1/2=0 for direct allowed transition the value of the optical energy gap of CdO and CdO:Sb were 2.4eV and 2.25eV respectively where as for indirect allowed transition these values were 1.15eV and 1.15eV , 0.725eV respectively.

The optical band gap decreases with antimony doping. This may be due to the presence of localized state in the forbidden gap. According to Mott and Devis[15]. The width of the mobility edge depends on the degree of disorder and defects present in the structure. Such defect produce localized states in responsible for the decrease of optical band gap.

Conclusions Thin films of pure and antimony doped CdO are prepared by spray pyrolysis technique from CdCl2 precursor. The transmittance decreases for Sb doping which is attributed to light absorption.

Absorption coefficient and optical band gap are calculated, the decreases in optical band among antimony doping explained on the basis of defect states.References1- Coutts, I. J, Mason, T. O, Porkins, J. D, Ginely, D. S, Electron Chme. Soc. Proc, 1999, 274-288.2- Kawamura K., Takahashi M., Yagihara M., Nakayama. T, Europe Patent Application, 2003, EP 1271561, A2 20030102, Can 138: 81680, AN 2003:4983.3- Yan. M., Lane M., Kannewurt C. R. I Chang R. P, Appl. Phgs Lett.,78, 2001, 2342-2344.4- Zhoo Z., Morel D. L. J. Ferekides C. S., Thin Solid Films, 413, 2002, 203, 211.5- Koffyberg, F.P. Phys Rev., B, 13, 1976,4470.6- Burustien E. Phys Rev, 93, 1954, 6327- Reddyk., T. Sravani C., Miles R., J. Cryst. Growth, 184, 1998, 1031, 1034.8- Matsuura N., Hohanson., Amm D. T., Thin Solid Films, 295, 1997, 260.9- Vigil O., Vaillant L., Curzf, Santana G., Morales A., Puente G., Thin Solid Film , 361, 2000, 53. 10- Subramanyam T. K., Uthanna S., Srinivasulu B., Physical Soripta, 57, 1998, 317. 11- Culino A., Castelli, F, Dapporto P., Rossi P., Fragala L., Chem. Mater, 14, 2002, 704-79.12- S. L. Lawton, R. A. Jacobson, J. Am. Chem. Soc. 88, 1966, 616. 13- E. Elangovan, K. Ramaurthithi, Crys. Res. Technol 38.No. 9, 2003, 779. 14- N. Tigau, G. Rusu, C. Gherorghies, J. Optoelectron. Adv. Mater.4, 943, 2002, 943. 15- N. F. Molt, E.A. Davis, "Electronic Processes In Non-Crystalline Materials", Clarendon Press, Oxford, 1979.

Figure (1): The Transmittance versus photon energy

Figure (2): The Absorptance versus photon energy

Figure (7): (hv)1/2 versus photon energy for

5% Sb doped sample

Figure (3): The Absorption Coefficient versus photon energy

Figure (6): (hv)1/2 versus photon energy for

The undoped sample

Figure (4): (hv)2 versus photon energy for

the undoped sample

Figure (5): (hv)2 versus photon energy for

5% Sb doped sample

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