Investigation on the Effect of Film Thickness on the Surface Morphology, Electrical and Optical Properties of E-Beam Deposited Indium Tin Oxide (ITO) Thin Film ()
1. Introduction
By means of hasty progress of a number of opto-electronic devices, such as solar cells [1] , light emitting diodes [2] , photodiodes [3] , and electrochromic devices [4] and so on, indium tin oxide (ITO) thin film is attracting increasing attention as a low resistance contact material with high transparency. ITO thin films have been manufactured by using a variety of methods including direct current and radiofrequency sputtering [5] [6] , reactive evaporation [7] , pulsed laser ablation [8] , electron beam evaporation [9] , spray pyrolysis [10] , and sol-gel techniques [11] . Along with the different techniques available, electron beam evaporation seems to be the most attractive one for preparing ITO thin films. It has a benefit of less radiation damage to the substrates. The advantages of this technique can be found in the previous work [12] . In that work, the effect of film thickness on surface morphology, electrical and optical properties were investigated. In this study, surface morphology and EDX of the ITO thin film were investigated by atomic force microscopy (AFM) and SEM respectively. The optical spectra of transmittance, T (%) have been measured at wavelength ranges 200 ≤ λ ≤ 800 nm using a “UV-Visi- ble SHIMADZU spectrophotometer (UV-1650PC)”. Also, the results provide evidence that the film thickness greatly affect the surface morphology, electrical and optical properties of ITO thin films. This paper presents the study of surface morphology, electrical and optical characteristics of the post-annealed (600˚C) ITO films of thickness 54 nm, 80 nm and 140 nm.
2. Experimental
ITO thin films were deposited on a glass substrate at room temperature by electron beam evaporation technique using an Edwards E 306A, UK. The commercially available ITO powder (99.99% pure) obtained from Inframat Advanced Materials, USA, was used as the evaporation source material. ITO thin films were deposited onto pre- cleaned optically flat glass substrates with dimensions of 2 × 2 cm held at room temperature. Before loading the glass substrates into the chamber, they were ultrasonically cleaned acetone methanol and finally rinsed in distill water for 10 min. Finally, they were dried by blowing hot air from a drier. Source turret (with source-hearth contained ITO powder) is adjusted and then shutter is placed between the substrate holder and source hearth to protect the substrate from unwanted deposition. Prior to starting evaporation processes, a steady-state chamber pressure of about 2.6 × 10−3 Pa (base pressure) was reached. The working pressure during the film deposition was fixed at 1.5 × 10−2 Pa by adjusting the needle valve for the ultrapure oxygen gas flow, an accelerating voltage of 4 kV and an electron beam current of 10 mA. The source turret is slowly raised and hearth height is adjusted as the previous relevant steps to obtain the best film conduction. The shutter is then removed to allow deposition onto the substrate through the mask windows. When the deposition of film is completed then the shutter is replaced at the proper time. The EBS power supply is switched OFF properly. The high vacuum valve is then closed. The vacuum chamber and the fabricated devices are allowed to cool down for about 20 min before air is admitted. This adopted technique may reduce the probability of oxidation. The films are then taken out and stored them into desiccators for various measurements. During the deposition of ITO, one film is prepared in a single run. After the deposition, the samples were annealed in a thermal annealing furnace (carbolite CWF 12/13) in air at 600˚C for 10 min. The resistivity measurements were carried out at room temperature by using a standard four-probe technique (van der Pauw technique). The optical spectra of transmittance, T (%) have been measured at wavelength ranges 200 ≤ λ ≤ 800 nm using a “UV-Visible SHIMADZU spectrophotometer (UV- 1650PC)”. The surface morphology and EDX for the ITO films of different thickness was studied by AFM and SEM respectively.
3. Results and Discussions
Fizeau [13] interferometer method was used to measure the thickness of the ITO thin films. Figure 1 shows the variation of ITO film thickness as a function of deposition time td.
It is evident from the Figure 1 that the film thickness increased almost linearly with increasing the deposition time td and deposition rate is 0.64 nm/Sec.
Figure 1. Variation of thickness with deposition time td.
A. The Surface Morphology by AFM
The surface morphology has been studied from the image taken by the AFM (XE-70). From Figure 2 represents the crop amplitude AFM images of ITO thin films having thickness of 54 nm, 80 nm and 140 nm can be observed. These images clearly show the nanostructure nature of the films and how the surface morphology changes with film thickness.
(a)(a) (b)
Figure 2. 3D Cropped Amplitude AFM image of the ITO thin films samples of different thickness. (a) 54 nm; (b) 80 nm; (c) 140 nm.
To obtain information of effect of thickness on grain size, the mean area of the grains has been studied. Figure 3 shows the AFM images of grain amplitude of different film thickness. From this figure it can be found that the mean grain area is gradually increased with film thickness. The mean grain area of the films of thickness 54 nm, 80 nm and 140 nm which are annealing at temperature 600˚C also studied.
(a)(b)(c)
Figure 3. AFM images of grain amplitude. (a) 54 nm; (b) 80 nm; (c) 140 nm.
Figure 4 is a plot of mean grain area as a function of film thickness. It is observed that with increasing thickness from 54 nm to 140 nm the average grain area increases from 6.021 µm2 to 10.19 µm2 and this different grain size influences the surface morphology of the films. Figure 4(b) also shows the roughness of ITO thin films increased with a corresponding increased in film thickness. It is observed that the mean grain area and roughness (Ra & Rq) is increased with film thickness. Similar result has also been found in the literatures [14] [15] for ITO thin films prepared by reactive DC magnetron sputtering and activated reactive evaporation.
(a) (b)
Figure 4. Variation of (a) Grain Area (b) Roughness (Ra, Rq) with film thickness.
In AFM images, the average roughness Ra & RMS roughness Rq have been used to describe the surface morphology. Figure 5 Line Amplitude AFM images of different film thickness. From Figure 5 the values of average roughness Ra of values are 1.440 nm, 1.603 nm, and 1.873 nm and also RMS roughness Rq of values are 1.980 nm, 2.117 nm, and 2.538 nm of the films of thickness 54 nm, 80 nm and 140 nm respectively. It can be seen that the average roughness Ra and RMS roughness Rq values increased as the film thickness is increased.
The ITO thin film of thickness 140 nm [Figure 2(c), Figure 3(c) & Figure 5(c)] has larger grains size and becomes rougher, as expected.
(a)(b)(c)
Figure 5. Line amplitude AFM images. (a) 54 nm; (b) 80 nm; (c) 140 nm.
B. Energy Dispersive X-Ray (EDX) Study
EDX studies have been used to analyze the elemental composition of the ITO thin film and their intensity va-ries according to the elements concentration present. The EDX spectrum for the ITO/Glass thin film sample is shown in Figure 6 and it can be seen that the elements concentration increased with film thickness.
(a)(b)
Figure 6. SEM EDX spectrums in the 0 - 10 keV range of ITO thin films. (a) 54 nm; (b) 140 nm.
C. Electro-Optical Properties
The optical spectra of transmittance, T (%) have been measured at wavelength ranges 200 ≤ λ ≤ 800 nm using a “UV-Visible SHIMADZU spectrophotometer” (UV-1650PC). Figure 7(a) show the transmission spectra of ITO films with different thickness. The transmittance in the visible region is found to be almost independent of film thicknesses. But the transmission in the NIR region a little decreases with increase in film thickness.
(a)(b)
Figure 7. (a) Variation of transmittance with thickness; (b) Variation of resistivity with film thickness.
Figure 7(b) shows the variation of the resistivity with the film thickness. The resistivity are 2.01 × 10−6 Ω∙m, 1.47 × 10−6 Ω∙m and 1.25 × 10−6 Ω∙m when the film thickness 54 nm, 80 nm and 140 nm respectively. It was observed that the resistivity is decreased with film thickness and a similar variation has been reported for ITO and SnO2 films [15] -[17] .
4. Conclusions and Future Work
In this paper, the surface morphology and EDX of E-beam deposited ITO thin films of different thickness were studied by the AFM and SEM respectively. Through experimentation it is demonstrated that the mean grain area, average (Ra) and rms (Rq) roughness are increased with film thickness. From the EDX spectrum of the ITO thin film samples, it can be seen that the elements concentration increased with film thickness. The visible transmittance (≥85%) is found to be almost independent of thickness, whereas the transmittance in the NIR region decreases with film thickness. It is also observed that the resistivity with film thickness decreases.
Acknowledgements
The authors gratefully acknowledge the Thin Film Lab, department of applied physics & Electronic Engineering and Central Science Laboratory of Rajshahi University, Bangladesh for experimental support in using the E- Beam vacuum coating unit (Edwards E-306) and the AFM.