Structures and opto-electrical characteristics of ZITO thin films

Abstract

Transparent conductive oxide (TCO) thin film has received extensive attention all the time due to its potential applications in various opto-electronic devices [1][2]. Among various TCO films, indium-doped ZnO (IZO) and indium tin oxide (ITO) thin films have been widely investigated due to their particular optical and electrical properties [3][4]. Regardless of ZnObased or ITO films, the studies were focused on the doping effects and crystallized mechanisms on materials characteristics were rarely explored. To achieve better conductivity, the multicompound films were deposited by sputtering, such as ZnO-SnO 2 -In 2 O 3 , In 2 O 3 -ZnO and In 2 O 3 -SnO 2 films etc [5][6]. However, the investigation of sol-gel derived multi-compound ZITO (ZnO combined ITO) film has never been studied. For these reasons, the ZITO films were synthesized to investigate the effects of ITO contents and the effect of temperature on structural characteristics, the optical transmittance and electrical properties. To obtain the aqueous solution of ZITO, ZnO and ITO solutions were firstly prepared by sol-gel method, respectively. All clear solutions were mixed with various volume ratios (ZnO : ITO = 1:1 and 2:1). Hereafter, the films will be designated according to volume ratio of ZnO and ITO as Z 1 ITO and Z 2 ITO. Subsequently, this resultant solution was deposited onto the silica-glass substrates using a spin coating. After that, the samples were dried at 200 ºC to evaporate the solvent and remove organic residuals (non-stop until to desired thickness of 150 nm). Finally, the samples were performed at 600~700 ºC for 1 hour under O 2 atmosphere. Fig. 1 shows the XRD pattern of Z 1 ITO films at crystallized temperature of 600 ºC. The Z 1 ITO film exhibited a cubic bixbyite structure of ITO according to previous literature [7]. Notably, no additional diffraction peak belong to ZnO structure was detected which indicated the ITO dominated the structural characteristic of Z 1 ITO film. In inset of Fig. 1, the average optical transmittance in the visible range of Z 1 ITO film was about 12.3 % which associated with the roughness of surface morphology and poor crystallization resulted in the increment of optical scattering, and further decreased the transmittance of film [8]. All Z 2 ITO films not only possessed a cubic bixbyite structure of ITO but also exhibited the diffraction peaks of ZnO phase (Fig. 2). With increasing the crystallized temperature, the intensity of major diffraction peaks of ZnO and ITO phases were slightly increased which indicated the crystallizations of Z 2 ITO films were improved. Also, the transmittances of Z 2 ITO films were measured as shown in Fig. 3. Comparing with Z 1 ITO film, the average transmittance of Z 2 ITO film (~16.5 %) was slightly higher than that of Z 1 ITO film under the crystallized temperature of 600 ºC which confirmed that the ITO content and crystallization certainly affected the transmittance of films. Notably, as the crystallized temperature of 650 and 700 ºC, the average transmittance of Z 2 ITO...