Characterization of copper oxide, titanium oxide and copper doped titanium oxide thin films prepared by spray pyrolysis technique

Abstract

Copper oxide (CuO), titanium oxide (TiO2) and copper (Cu) doped TiO2 (Cu/TiO2) thin films have been synthesized on to glass substrates for different molar concentrations (MC), different substrate temperatures (Ts) and various Cu doping by a spray pyrolysis technique (SPT) from aqueous solutions using cupric acetate Cu(CH3COO)2.H2O for CuO, titanium chloride (TiCl4) for TiO2 and Cu(CH3COO)2.H2O and TiCl4 for Cu/TiO2 thin films as a precursor. All the CuO thin films, irrespective of Ts, show a monoclinic structure with the main CuO (111) orientation, and the crystallite size (D) is about 9.57 nm for the thin film synthesized with MC of 0.10 M at Ts of 350 °C. The optical transmission of the film is found to increase with the increase of MC from 0.05 to 0.10 M and then that decreases for the film with MC above 0.10 M. The optical band gap (Eg) is found to be 2.40 eV for MC of 0.05 M and then a minimum value 1.60 eV is observed for MC of 0.10 M. The room temperature electrical resistivities (ρ) are 31 and 24 Ω-cm for the films grown with MC of 0.05 and 0.15 M, respectively. The optical transmission of the film for MC of 0.10 M is found to increase with the increase of Ts. The Eg decreases from 1.90 to 1.60 eV and the room temperature ρ varies from 30 to18 Ω-cm for the films grown at Ts from 250 to 350 °C for MC of 0.10M. The higher values of activation energy (Ea) of CuO thin films may suggest that the prepared sample is stoichiometric. The Figure of merit for the CuO thin films deposited with MC of 0.10 M and Ts of 350 °C is found to be 0.1424 Ω−1- cm−1. MC has a significant effect on surface morphology, crystalline quality and crystallite size of the TiO2 thin films. Energy dispersive analysis of X-ray (EDX) results clearly shows that the grains of TiO2 thin films typically comprise both Ti and O. The XRD studies show the formation of tetragonal structure with anatase phase and the D value of the film is about 13 nm when films deposited at 400 °C from MC of 0.10M. The optical transmission of the films is found to increase from 60 to 80 % with the increases of MC. The value of Eg is decreased from 3.65 to 3.40 eV for the film of MC from 0.05 to 0.10 M whereas Eg increases for the films of MC above 0.10 M. The ρ of the TiO2 thin films are 55.67 and 27.50 Ω-cm for MC of 0.05 M and 0.10 M, respectively. Strong diffraction peaks (101) and (200) around at 25° and 48°, respectively, indicate TiO2 of tetragonal structure with anatase phase at different Ts for MC of 0.10 M, except film deposited at Ts of 250 oC which shows amorphous nature. Ea is maximum, 0.028 eV for Ts of 400 oC and MC of 0.10 M of TiO2 thin film. The peaks are found to shift a little from their standard positions at higher Ts, indicating a small deviation in the lattice parameters. The optical transmission of the films is found to increase from 73 to 89 % for the films grown at Ts of 250-400 oC and above these Ts, transmition decreases. Eg decreases with Ts up to 400 oC and above these Ts, Eg does not very much. The room temperature ρ varies from 42 to 27.50 Ω-cm for Ts of 250 to 400 oC. It is observed that with increasing D of TiO2 thin film, the Eg shifts from 3.64 eV to 3.40 eV. Moreover, other possible influences of microstructure of TiO2 thin film, such as, surface roughness and thin film absorption on the modification of Eg may be present. Cu/TiO2 thin films have been prepared by SPT for MC of 0.10 M at Ts of 400 oC by variation of Cu doping (1-10 wt%). The XRD studies show that Cu/TiO2 thin films have only tetragonal structure with anatase phase. The optical transmission of the thin films is found to increase from 88 to 94 % with the addition of Cu up to 8 wt% and then decreases for higher percentage of Cu doping. Due to Cu doping, the Eg is shifted to lower energies and increases further with increasing concentration of Cu. The refractive index (n) decreases up to 8 wt% Cu doping, above 8 wt% Cu doping, n increases. The room temperature ρ of the films decreases with increasing Cu doping and is found to be 27 -23 Ω-cm for Cu doping of 1-8 wt%. Ea increases with Cu doping and it is maximum, 0.0311 eV, for 8wt% Cu doped TiO2 thin film. The Figure of merit of Cu/TiO2 thin films varies from 0.2238 to 0.0.2328 Ω-1-cm-1 for Cu doping from 0 to 10 wt% and it is maximum, 0.2334 Ω-1-cm-1, for 8wt% Cu doped TiO2 thin film. It is evident from the present study that the Cu doping has modified the thin film morphology and so the effect on the structural, optical and electrical properties of the thin films.