Electrical, structural and optical properties of fluorine-doped zinc oxide thin films: Effect of the solution aging tim

1. Electrical, structural and optical properties of fluorine-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 b Materials Science Department/CENIMAT, Faculty of Sciences and Technology, New University of Lisbon, Campus da Caparica, 2829-516 Caparica, Portugal 1

2. Outline Introduction Experimental details Results and discussion Conclusions 2

3. Introduction TCO films produced using binary compounds namely In2O3, SnO2 and ZnO doped with an impurity are in practical use. ZnO is naturally an n-type semiconductor, due to the presence of native donors inside the lattice, known as defects. The most prominent ones are oxygen vacancies, zinc interstitial atoms, and hydrogen, which are always present in all growth methods and can easily diffuse into ZnO, in large amounts due to its large mobility. In this work we report a study concerning the effect of aging of the starting solution used to fabricate films by spray pyrolysis (SP) technique, on the structural, electrical, optical and morphological properties of ZnO:F thin films. 3

4. Experimental details Fluorine-doped zinc oxide thin films were deposited on glass substrates by the SP technique from a staring 0.4 M solution containing zinc acetate dehydrated (Merck) dissolved in a mixture of double distilled water, methanol (3:7 volume proportion ) and acetic acid. Doping of the films was achieved by adding ammonium fluoride to the starting solution, with a fixed [F]/[Zn] ratio of 2 at.%. Temperature of the 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. 4

5. The glass substrate was kept on a stainless steel (ss) plate that was heated by a 3 kW heater using canthal heating coils. The temperature of the substrate is controlled by through a temperature controller connected to a Chromel�Alomel thermocouple kept at the center of the ss plate. The air produced by the compressor was first filtered and then connected to the glass spray-gun (atomizer) through a sensitive flow-meter to control its flow. The custom glass spray gun having a nozzle diameter of 0.2 mm was positioned at a distance of 25 cm above the substrate. 5

6. Electrical resistivity was measured using two-probe method. The optical transmittance was measured with a spectrophotometer Varian model Cary 100 in the UV�visible region (200�800 nm). The optical band gap (Eg) of the films has been calculated from the dependence of absorption coefficient (a) on the photon energy (h?), taking into account that ZnO is a direct band gap semiconductor. The structural properties were studied by X-ray diffraction measurements (Philips PW1840) using the Cu-Ka radiation with ?=1.5418 �. 6

7. The surface morphology of the films deposited were analyzed by a Scan Electron Microscope (SEM), from Philips, model XL30. The data obtained from spectral transmittance were used to calculate the thickness of the films. Film thickness and refractive index are calculated using unconstrained optimization method. Thicknesses value of around 550 nm was kept constant, for all set of samples analyzed. 7

8. Results and discussion This value decreases with the age of the starting solution, reaching a minimum of about 24 O/?, after 15�21 days of solution ageing. 8

9. These data seem to indicate that during the doping process, a fraction of F atoms move into the ZnO lattice, increasing the incorporation efficiency with the age of the solution, up to a certain aging time, after which the solution may start degrading. 9

10. Zinc Oxide is a tetrahedral coordinated solid that with a wurtzite structure. 10

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

12. The average mean crystallite size was calculated for the (002) diffraction peak, using Scherrer formula[23]. For films deposited using 21 days aging solutions, the average diameter of the crystallites is of the order of 24 nm. 12

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

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

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. 15

16. Hence, transparency in the shorter wavelength region is better for the optimum film than undoped ZnO film that can be considered as an indication of the incorporation of F doping. 16

17. 17

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

19. Conclusions ???????,??????????????????????? (110)????????????????,????????????????? ?SEM???????,???F???????,?????????36nm? ???????,FZO???????????15-21?,???????????550nm,???????????(?24O/?)? ???????????,??????,???????????? 19

20. ???????,???????,??????????????????;????????,????FZO???????1.7-2(400-800nm)? 20

21. 21