Characterization of boron doped zinc oxide thin films prepared by spray pyrolysis deposition technique

Abstract

Undoped and boron (B) doped zinc oxide (ZnO) thin films are synthesized onto glass substrate by spray pyrolysis technique (SPT) at various substrate temperatures (300 - 450 °C). Concentration of B are varied between 0.50 and 1.50 at% at the step of 0.25 at% during preparation of B doped ZnO (ZnO: B) thin films. The effect of change of substrate temperature and doping concentration of B on the structural, optical and electrical properties of the ZnO thin films is studied. Scanning electron microscopy (SEM) is used to study the surface morphology of ZnO and ZnO: B thin films. The SEM images show that the glass substrates are nicely covered by fibrous and non-fibrous film. Nanofibers of various thicknesses (200- 550 nm) are observed around the nucleation center of the thin films in the SEM images. Energy dispersive X-ray (EDX) analysis illustrates that the ZnO and ZnO: B films are very close to nominal composition. Quantitative analysis shows that at% of B increases with the increase of B concentration in ZnO: B thin films. X-ray diffraction (XRD) patterns of ZnO and ZnO: B thin films show polycrystalline hexagonal wurtzite type of crystal structure. The reflections for (100), (002), (101) and (110) planes are found, among which (002) is the dominant peak. Lattice constants (𝑎 and c) are calculated for the prominent peaks. The values of 𝑎 are found between 3.2365 and 3.2624 Å and those of c are found between 5.1877 and 5.3089 Å. The c/𝑎 ratio is very close to that for an ideal hexagonal structure. Crystallite size of the thin films varies from 27 to 88 nm. Crystallite size decreases, whereas microstrain and dislocation density increase with the increase of B concentration in ZnO: B thin films. Transmittance and absorbance of ZnO and ZnO: B thin films are measured by UV-visible spectroscopy and optical band gap, refractive index, extinction coefficient, dielectric constants and optical conductivity are calculated from those data. Transmittance and optical band gap (3.11 - 3.33 eV) increase and refractive index, extinction coefficient, dielectric constants and optical conductivity decrease with increasing B concentration. Electrical resistivity of ZnO: B thin films are measured at room temperature and in the range of temperature 298 – 423 K by linear four point probe method. Resistivity of ZnO: B thin films decrease both with the increase of B concentration and temperature. Activation energy of ZnO: B thin films are obtained between 0.27 and 0.52 eV.