Effect of substrate temperature on the stability of transparent conducting cobalt doped ZnO thin films

Abstract

Transparent conducting Co doped ZnO thin films have been fabricated by Ultrasonic spray. The thin films were deposited at three different substrate temperatures of 300, 350 and 400 ı C. The obtained films had a hexagonal wurtzite structure with a strong (002) preferred orientation. The maximum crystallite size value of the film deposited at 350 ı C is 55.46 nm. Spectrophotometer (UV-vis) of a Co doped ZnO film deposited at 350 ı C shows an average transmittance of about 90%. The band gap energy increased from 3.351 to 3.362 eV when the substrate temperature increased from 300 to 350 ı C. The electrical conductivity of the films deposited at 300, 350 and 400 ı C were 7.424, 7.547 and 6.743 ( cm) 1 respectively. The maximum activation energy value of the films at 350 ı C was 1.28 eV, indicating that the films exhibit a n-type semiconducting nature.

The crystalline structure, conductivity and optical properties of Co-doped ZnO thin films

“…The slight variation (increase) in the conductivity of the films has been explained by displacement of electrons [28] which are coming from the ions Co 2+ donors in the substitutional sites of Zn 2+ , as a result the carrier density increases. Moreover, the decrease of the electrical conductivity with increasing of the doping level may be explained by a segregated part of cobalt atoms into the grain boundaries which acts in increasing of the potential barriers, this interpretation is consistent with similar results obtained elsewhere [26][27][28][29][30][31].…”

“…As can be seen in Fig. 6, a minimum Urbach energy was reached with Co-doped ZnO thin films at 2 wt%, which means that this doping level (2 wt%) leads to a less disorder; we have as consequence large band gap and vice versa as it was expressed in the literatures [15,26,27]. Fig.…”

Substrate Temperature Effect on Optical property of ZnO Thin Films

“…As clearly seen in Fig. 3, the variations of both Eg and Eu correlate very well, the optical gap energy decreased with increasing of substrate temperature from 3.44 to 3.29 eV, the band gap is narrowing due to the decrease in the transition tail width and shift effect [15,21]. Witch explained by increasing in carrier concentration.…”

“…We are noting that the temperature effect is clearly observed in the layer quality. These results show that the produced ZnO thin films could be used in solar cells due to the low absorbance in the visible region [15,21]. In order to investigate the effect substrate temperatures on ZnO films further, the optical band gap energy g E was measured from the transmission spectra using the following relations [23]:…”

“…There are still many factors affecting the physical properties of ZnO thin films. These factors include ZnO sol concentration [14], substrate temperature [15], preheating temperature [16], post-annealing temperature [17], annealing atmosphere [18] and film thickness [19]. Among these factors, the influence of substrate temperature on structural and optical properties of ZnO thin films (especially undoped ZnO thin films) deposited by ultrasonic spray method was less studied.…”