Structural, Optical and Electrical Properties of Spray Pyrolysed Ti-Doped ZnO Films

Pure ZnO has excellent conductivity as it contains high concentration of native defect (oxygen vacancy or zinc interstitials). At high temperature, pure ZnO thin films are not stable chemically and electrically. The performance of ZnO thin films can be greatly modified and improved by appropriate impurity doping. Modification of ZnO with transient metals (Al, In, Ga, Ti) provides a successful and cost effective alternative. In order to create free electrons and enhance n-type conductivity, two criteria should be satisfied: (i) the doping ion should be smaller than, or equal to, the diameter of the host ion; (ii) the ion of the dopant should have higher valency than that of the host atom. The ionic radius of Ti⁴⁺ (0.068 nm) should be smaller than that of Zn²⁺ (0.074 nm), and hence Ti⁴⁺ ions can replace Zn²⁺ ions at substitutional sites. In the present study, ZnO films doped with small Ti concentration were deposited on glass substrate by spray pyrolysis.

Both un-doped and Ti-doped ZnO films were deposited on microscopic glass substrates using spray pyrolysis technique systematically by controlling the deposition parameters. For deposition, 0.1 M of zinc acetylacetonate Zn(AcAc) was dissolved in ethanol and sprayed onto microscopic glass substrates with dimensions of 75 x 25 mm² at fixed substrate temperature; T_s = 400°C. In order to dope ZnO with Ti, titanyl acetylacetonate Ti(AcAc) solution was added to the starting solution. The solution was sprayed using different doping concentrations ranging from 0.1% to 0.9%.

The strongest lines in the entire pattern are the principle lines (100), (002) and (101) of the hexagonal wurzite ZnO, in agreement with JCPDS standard card No. 75-0576. On doping with Ti up to 0.7%, no additional peaks are observed. This is attributed to the fact that the concentration of the dopant Ti is low. When the doping concentration is further increased above 0.7%, it is observed that the crystallinity becomes lower. The crystallite size determined using Scherrer’s relation decreased from 39 nm to 12 nm after doping with Ti. Parameters like dislocation density and strain were also estimated.

Composition of the films was studied by EDAX. Peaks corresponding to Ti, Zn and O elements were presented along with Si peaks coming from the glass substrate.

The value of the band gap of the films is obtained from Tauc’s plot indicate that the band gap gradually increases with increasing Ti concentration. Similar result is reported in the literature. The band gap value decreases from 3.20 eV to 3.32 eV.

Room temperature electrical resistivity of the films doped with different concentrations of Ti was studied. As deposited undoped films exhibited resistivity around 10⁵ ohm cm. After annealing in vacuum, the resistivity decreased by three orders to 100 ohm cm. As the doping concentration increased, the resistivtiy decreases further to 8 ohm cm for 0.9 % Ti doping. Ti⁴⁺ ions substituted Zn²⁺ ions within the crystal lattice induce positive Ti_Zn charges in the material. In order to maintain electrical neutrality, two negative electrons are induced to compensate the excess positive charges. Hence, the resistivity slightly decreases due to the increase of the free electrons in the film. The mobility of the films increases from 10 cm²V^-1s^-1 to 22 cm²V^-1s^-1 with increase of Ti concentration. The carrier density increases from 6.24 x 10¹⁵ cm^-3 to 3.54 x 10¹⁶ cm^-3. The decrease of resistivity can be attributed to the increase of carrier concentration.

Preliminary studies on Dye sensitized solar cells (DSSC), indicated that Ti doped ZnO films exhibited higher photo output compared to undoped ZnO. The results show that the films can be used in DSSC.