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S.M. Rozati a , S. Moradi a , S. Golshahi a , R. Martins b , E. Fortunato b

Electrical, structural and optical properties of fluorine-doped zinc oxide thin films: Effect of the solution aging time. S.M. Rozati a , S. Moradi a , S. Golshahi a , R. Martins b , E. Fortunato b a Department of physics, University of Guilan, Rasht 41335, Iran

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S.M. Rozati a , S. Moradi a , S. Golshahi a , R. Martins b , E. Fortunato b

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  1. Electrical, structural and optical properties offluorine-doped zinc oxide thin films:Effect of the solution aging time S.M. Rozatia, S. Moradia, S. Golshahia, R. Martinsb, E. Fortunatob a Department of physics, University of Guilan, Rasht 41335, Iran bMaterials Science Department/CENIMAT, Faculty of Sciences and Technology, New University of Lisbon, Campus da Caparica, 2829-516 Caparica, Portugal Adviser：林克默 Advisee：郭俊廷 Date ：99/03/12

  2. Outline • Introduction • Experimental details • Results and discussion • Conclusions

  3. Introduction • TCO filmsproduced using binary compounds namely In2O3, SnO2 and ZnOdoped with an impurity are in practical use. • ZnO is naturally an n-type semiconductor, due to thepresence ofnative donors inside the lattice, known as defects. • The mostprominent ones are oxygen vacancies, zinc interstitial atoms, andhydrogen, which are always present in all growth methods and caneasily diffuse into ZnO, in large amounts due to its large mobility. • Inthis work we report a study concerning the effect of aging of thestarting solution used to fabricate films by spray pyrolysis (SP) technique,on the structural, electrical, optical and morphological propertiesof ZnO:F thin films.

  4. Experimental details • Fluorine-doped zinc oxide thin films were deposited on glasssubstrates by the SP technique from a staring 0.4 M solution containingzinc acetate dehydrated (Merck) dissolved in a mixture ofdouble distilled water, methanol (3:7 volume proportion ) and aceticacid. • Dopingof the films was achieved by adding ammonium fluoride to thestarting solution, with a fixed [F]/[Zn] ratio of 2 at.%. • Temperature ofthe substrate and the spray rate were fixed at 450 °C and 22 l/min,respectively. • The SP apparatus used in this work consists of a homemade spraying unit, substrate holder with heater and enclosure.

  5. Theglass substrate was kept on a stainless steel (ss) plate that was heatedby a 3 kW heater using canthal heating coils. • The temperature of thesubstrate is controlled by through a temperature controller connectedto a Chromel–Alomel thermocouple kept at the center of the ss plate. • The air produced by the compressor was firstfiltered and then connected to the glass spray-gun (atomizer) througha sensitive flow-meter to control its flow. • The custom glass spray gunhaving a nozzle diameter of 0.2 mm was positioned at a distance of25 cm above the substrate.

  6. Electrical resistivity was measured using two-probe method. • Theoptical transmittancewas measured with a spectrophotometer Varianmodel Cary 100 in the UV–visible region (200–800 nm). • Theoptical band gap (Eg) of the films has been calculated from the dependenceof absorption coefficient (α) on the photon energy (hυ),taking into account that ZnO is a direct band gap semiconductor. • Thestructural properties were studied by X-ray diffraction measurements(Philips PW1840) using the Cu-Kα radiation with λ=1.5418 Å.

  7. Thesurface morphology of the films deposited were analyzed by a ScanElectron Microscope (SEM), from Philips, model XL30. • The data obtainedfrom spectral transmittance were used to calculate the thicknessof the films. Film thickness and refractive index are calculatedusing unconstrained optimization method. • Thicknesses value ofaround 550 nm was kept constant, for all set of samples analyzed.

  8. Results and discussion • This value decreases with the age of the starting solution, reaching aminimum of about 24 Ω/□, after 15–21 days of solution ageing.

  9. These dataseem to indicate that during the doping process, a fraction of F atomsmove into the ZnO lattice, increasing the incorporation efficiencywith the age of the solution, up to a certain aging time, after which thesolution may start degrading.

  10. Zinc Oxide is a tetrahedral coordinatedsolid that with a wurtzite structure.

  11. For films deposited using 15and 24 days aging solutions, it can be seen that the intensity of theXRD peak associated with (100) and (110) planes predominate. • A(002) preferential orientation is observed from a first-day solution inall the cases of chemically sprayed ZnO:F thin films[21,22]. • Smallsignals of the increase of peak intensity, corresponding to the (110)and (101) planes, start appearing as the age solution increases. Weattribute this to fluorine ions replacement by oxygen ions in the ZnOlattice.

  12. The average mean crystallite size was calculated for the (002)diffraction peak, using Scherrer formula[23]. • For films deposited using 21 days agingsolutions, the average diameter of the crystallites is of the order of24 nm.

  13. Fig. 3a shows the porous structure and non-uniform coverageobserved for the undoped ZnO films, while by doping with fluorine,a change in the surface morphology is observed, as can be seen inFig. 3b. Using solutions 9 days aged, thesurface looks to be compact, consisting of small grains as seen in Fig. 3c.

  14. Reaching values below55% for films deposited using solutions aged by 36 days. This behavior is ascribed tothe decrease in the Zn/[O+F] ratio in the films deposited from agedsolutions

  15. There was a shift in the absorption edge to shorter wavelength for the optimum film(15 days aged solution), which was due to Burstein–Moss shift.

  16. Hence, transparency in the shorterwavelength region is better for the optimum film than undoped ZnOfilm that can be considered as an indication of the incorporation of Fdoping.

  17. Fig. 6. plot of refractive index as a function of wavelength for (a) un-doped and fluorine-doped ZnO thin films deposited from (b) first day (c) 6 days aged solution.

  18. Fig. 6 shows the refractive index as a function of wavelength for undoped and fluorine-doped ZnO thin films deposited from fresh and6 days aged solutions. It is clear that the aged solutions raise therefraction index value (between 1.7 and 2). • The calculatedvalues lead to optical gaps ranging between 3.3 and 3.4 eV, forall the films.

  19. Conclusions • 從實驗數據得知，溶液的熟化對薄膜的結構與表面形態有很大的影響。 • (110)波峰強度隨著熟化時間的增加而增強，主要是因為薄膜的結晶性受到了改善。 • 從SEM之表面觀測得知，在摻雜F原子熟化九天後，所獲得之晶粒大小約36nm。 • 從實驗數據得知，FZO溶液最佳的熟化時間約在15-21天，所沉積出的樣品其膜厚約550nm，擁有高穿透率與低片電阻(約24Ω/□)。 • 隨著熟化時間增加到六天，其穿透率最高，但之後穿透率便開始降低。

  20. 從實驗數據得知，在可見光範圍下，熟化效應對薄膜之折射率並無明顯的影響；在溶液熟化六天後，所沉積出FZO薄膜其折射率為1.7-2(400-800nm)。從實驗數據得知，在可見光範圍下，熟化效應對薄膜之折射率並無明顯的影響；在溶液熟化六天後，所沉積出FZO薄膜其折射率為1.7-2(400-800nm)。

  21. Thanks for your attention

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