Blueshift of optical band gap in c-axis oriented and conducting Al-doped ZnO thin films

We have investigated the structural, optical, and electrical properties of alumina-doped zinc oxide (AZO) thin films, grown by pulsed laser deposition. The optical transmittance of the films is over 80% in the visible region, and the absorption edge shifts from about 380 nm of the undoped sample to 320 nm of the AZO film. The calculated optical band gap (E_g) of 2 wt % AZO films shows a widening up to 3.82 eV with respect to the undoped film (3.28 eV). Higher doping concentration (6 wt %) leads to films with larger E_g (4.1 eV), but also epitaxial properties are affected. A further widening of the gap occurs when the AZO films are deposited by lowering the substrate temperature (T_s) from 450 to 250 °C. These blueshifts are respectively attributed to the increase in carrier concentration, induced by Al-donor doping, and also a lower degree of crystalline order. AZO films with doping concentration of 2 wt % show resistivity values of about ^10-3 Ω cm and the local I-V curves, measured by scanning tunneling spectroscopy, show higher tunneling current than ZnO film. The Al-doping route proved to be effective in tailoring the optical and electrical properties without essentially affecting the crystalline structure of the films.