Tuning of opto-electrical properties of hematite thin films using Co²⁺ doping

Hematite thin films have attracted widespread interest in recent years because of their advanced electronic and optical properties. Optical and electronic properties of hematite thin films can be enhanced/tuned using doping or additive-based strategies. An application oriented sol–gel method is used for the synthesis of cobalt (Co) doped hematite sol with variation in Co concentration in the range of 0–10 wt%. Hematite phase is observed in undoped thin films annealed at 300 °C under 500 Oe magnetic field for 60 min. Strengthening of hematite phase is observed with increase in Co concentration up to a value of 8 wt%. Bond angle with +ive tilt (i.e. ~ 19.74°) was observed in refined structural parameters for thin films prepared with Co concentration in the range of 0–8 wt%. Higher Co concentration, i.e. 10 wt%, results in decrease in crystallinity of the films along with smaller +ive tilt in bond angle (i.e. ~ 8.82°). High transmission (~ 88%) is observed for thin film prepared using dopant concentration of 8 wt% in the visible and infrared regions. The energy band gap varies from 2.42 to 2.25 eV with variation in Co concentration from 0 to 10 wt%. Relatively smaller band gap values are correlated with defect induced states in the band gap. Spectroscopic ellipsometry is used for calculation of refractive index and high values are indication of high density of thin films. Relatively higher value of dielectric constant (~ 183, log f = 5.0) along with lower value of tangent loss is observed at Co concentration of 8 wt%. Higher grain boundary resistance (1.88 × 10⁵ Ω) was observed at 8 wt% Co concentration. Variation in d.c. conductivity with dopant concentration is studied in detail using Jonscher’s power law. The value of frequency exponent (n) lies in the range of 0.88–0.98 (< 1) with variation in dopant concentration signifying that motion of charge carriers involves translational motion along with sudden hopping process. It is important to mention here that combined tuning of optical and electrical properties are observed in the present study with no change in phase pure hematite crystallographic structure.