Study of the structural, optical and electrical properties of indium doped zinc oxide thin films synthesized by spray pyrolysis technique

Abstract

ZnO and ZnO:In thin films with 0.6 at % to 1.4 at % In doping (with an interval of 0.2 at %) are synthesized on to glass substrate by spray pyrolysis technique (SPT) at two substrate temperatures of 400 °C and 450 °C. The effect of variation of substrate temperature and doping concentration on the structural, optical and electrical properties of the ZnO thin films are studied elaborately. The Field Emission Scanning Electron Microscopy micrographs illustrates that sprayed particles formed clusters as the primary stage of nucleation on to the substrate and appears as spheroid shape. In higher magnifications, clusters appear as nanofiber around the nucleation center. The average thickness of the nanofibers in ZnO and ZnO:In thin films vary from 220 nm to 550 nm. Energy dispersive X-ray analysis of ZnO and ZnO:In films shows that at % of In increases with the increase of In concentration in ZnO:In thin films. The XRD patterns reveal that the films are polycrystalline in nature with a hexagonal wurtzite structure with reflections from (100), (002), (101), (102) and (110) planes. All the films show preferred orientation along (002) axis. The crystallite sizes are found to vary from 20.23 nm to 26.41 nm. The maximum crystallite size is obtained for 1.4 at % and 1.2 at % of In at Ts of 400 C and 450 C, respectively. All the films show good transperancy from 400 nm to 1100 nm with an absorption cut-off at almost 375 nm. The value of transmittance varies from 74 % to 89 %. The maximum tranmittance is found for 0.8 % In doped ZnO, the value of which are 87 % and 89 % at the substrate temperatures of 400 C and 450 C, respectively. The value of optical band gap varies between 3.19 eV and 3.27 eV. The trend of the variation of band gap with In concentration is almost similar for both 400 C and 450 C. An abrupt increase in refractive index (n) is found from 320 nm and becomes maximum within 370 nm to 385 nm and then decreases gradually with the increase of wavelength. n varies from 1.74 to 2.24. The value of the extinction coefficient (k) is found lower for 1.0 at % of In and higher at 0.6 at % of In for both 400 C and 450 C. The optical conductivity (opt) is maximum for 0.8 at % In doping. The minimum value of real part of complex dielectric constant, εr is obtained for 1.0 at % and 1.4 at % of In at 400 C and 450 C, respectively and the minimum value of imaginary part of complex dielectric constant, εi is obtained for 0 at % and 1.0 at % of In at 400 C and 450 C, respectively. The linear nature of I-V characteristics of the ZnO and ZnO:In thin films implies that the conduction process is ohmic. The resistivity () decreases with increasing temperature, which indicates the semiconducting nature of the materials.  decreases initially with the increase of In concentration, then increases slightly for 0.8 at % of In and finally decreases with further increase of In concentration at Ts of 400 °C. At Ts of 450 °C,  is found to increase with In concentration up to 0.6 at %, then reduces at 0.8 at % of In and finally increases with further increase of In concentration. The minimum value of  is found at 2.65 × 106 ohm-m and 1.94 × 106 ohm-m at Ts of 400 C and 450 C respectively. The activation energy (∆E) increases with In doping concentration and becomes maximum at 0.8 at % and minimum at 1.2 at % of In. The maximum ∆E value is higher at lower substrate temperature.