Doping in Zinc Oxide Thin Films
Zheng Yang
PhD Defense
When: Tuesday, May 12, 2009
Time: 10:00 AM
Location: Bourns A275
Committee: Dr. Jianlin Liu (Chair), Dr. Alexander Balandin, Dr. Ward Beyermann
Abstract:
Zinc oxide (ZnO) has promising applications in optoelectronics and spintroincs due to its large exciton binding energy (60 meV) and theoretically predicted above-room-temperature TC with transition metal incorporation. Doping in ZnO is indispensable to achieve the device applications, such as n-/p-doping in ZnO light emitting devices and magnetically doping in ZnO spintronic devices. In this dissertation, Ga n-type doping, Sb p-type doping, Co and Mn magnetic doping in ZnO thin films were investigated. In the 9 K photoluminescence (PL) spectra of ZnO:Ga thin films, as the carrier concentration increases from 1.8 × 1018 to 1.8 × 1020 cm-3, the dominant PL line changes from I1 (3.368 - 3.371 eV), to IDA (3.317 - 3.321 eV), and finally to I8 (3.359 eV), which are assigned as ionized Ga donor bound exciton, donor-acceptor-pair transition, neutral Ga donor bound exciton emissions, respectively. A generalized model has been proposed to explain the widely observed 3.30-3.32 eV PL lines in ZnO based on the studies of the IDA lines in our ZnO:Ga thin films. A Haynes factor of ~0.3 was reported for Ga donors in ZnO. Typical behaviors of Sb-doped p-type ZnO were presented. The Sb doping mechanisms and preferences in ZnO were discussed. Diluted magnetic semiconducting (DMS) ZnO:Co and ZnO:Mn thin films with above room-temperature TC were prepared. Transmission electron microscopy and x-ray diffraction studies indicate these DMS thin films are free of secondary phases. The magnetizations of the ZnO:Co and ZnO:Mn thin films show a free electron carrier concentration dependence, which increase dramatically when the free electron carrier concentration exceeds ~1019 cm-3, indicating a carrier-mediated mechanism for ferromagnetism. The anomalous Hall effect is observed in the ZnO:Co and ZnO:Mn thin films. The anomalous Hall coefficient and its dependence on longitudinal resistivity were analyzed. The presence of a side-jump contribution further supports an intrinsic origin for ferromagnetism in these DMS thin films. These observations together with the magnetic anisotropy and magnetoresistance results, support an intrinsic carrier-mediated mechanism for ferromagnetic exchange in ZnO:Co and ZnO:Mn DMS thin films.