Carrier Compensation Induced by Thermal Annealing in Al-Doped ZnO Films

This study investigated carrier compensation induced by thermal annealing in sputtered ZnO:Al (Al₂O₃: 0.25, 0.5, 1.0, and 2.0 wt %) films. The films were post-annealed in a N₂ atmosphere at low (1 × 10-23 atm) and high (1 × 10-4 atm) oxygen partial pressures (PO2). In ZnO:Al films with low Al contents (i.e., 0.25 wt %), the carrier density (n) began to decrease at annealing temperatures (Ta) of 600 °C at low PO2. At higher PO2 and/or Al contents, n values began to decrease significantly at lower Ta (ca. 400 °C). In addition, Zn became desorbed from the films during heating in a high vacuum (i.e., <1 × 10-⁷ Pa). These results suggest the following: (i) Zn interstitials and Zn vacancies are created in the ZnO lattice during post-annealing treatments, thereby leading to carrier compensation by acceptor-type Zn vacancies; (ii) The compensation behavior is significantly enhanced for ZnO:Al films with high Al contents.