Electron concentration dependence of optical band gap shift in Ga-doped ZnO thin films by magnetron sputtering

• The effects of electron concentration on optical band gap were analyzed.
• The measured optical band gap corresponded well with the calculated ones.
• The Burstein–Moss (BM) and band gap renormalization (BGR) effects were considered.
• Nonparabolic conduction band parameters were used in theoretical analysis.
• The BM effect was offset by the BGR effect in both conduction band and valence band.

Ga-doped ZnO (GZO) thin films were deposited on glass substrates by a radio frequency magnetron sputtering technique. The optical properties of the deposited GZO films were evaluated using an optical transmission measurement. The optical band gap increased from 3.32 eV to 3.45 eV with the increasing carrier density from 2.0 × 1020 cm− 3 to 3.24 × 1020 cm− 3. Based on the experimental results, the optical band gap as a function of carrier density is systematically investigated with four available theoretical models taken into consideration. The blueshift of the optical band gap in GZO films can be well interpreted with a complex model which combines the Burstein–Moss effect, the band gap renormalization effect and the nonparabolic nature of conduction band. In addition, the BM contribution is almost offset by the BGR effect in both conduction band and valence band due to the approximate equality between electron and hole effective masses in GZO films with a nonparabolic conduction band. The tunability of optical band gap in GZO thin films by carrier density offers a number of potential advantages in the development of semiconductor optoelectronic devices.