The annealing effects on structure and magnetism for Co-doped ZnO films under air, Ar, and Ar/ H 2 atmospheres at 250°C have been systematically investigated. Room-temperature ferromagnetism has been observed for the as-deposited and annealed films. However, the saturation magnetization ͑M s ͒ varied drastically for different annealing processes with M s ϳ 0.5, 0.2, 0.9, and 1.5 B / Co for the as-deposited, air-annealed, Ar-annealed, and Ar/ H 2-annealed films, respectively. The x-ray absorption spectra indicate all these samples show good diluted magnetic semiconductor structures. By comparison of the x-ray near edge spectra with the simulation on Zn K edge, an additional preedge peak appears due likely to the formation of oxygen vacancies. The results show that enhancement ͑suppression͒ of ferromagnetism is strongly correlated with the increase ͑decrease͒ of oxygen vacancies in ZnO. The upper limit of the oxygen vacancy density of the Ar/ H 2-annealed film can be estimated by simulation to be about 1 ϫ 10 21 cm −3 .

In this study, we present a photodetector based on a topological insulator Sb2Te3 thin film, which shows an obvious photoresponse to near infrared light with good responsivity, photoconductive gain and detectivity.

In this work, an inverted device was fabricated using titania (TiO 2 ) as the electron collecting layer (ECL) and sulfonated poly(diphenylamine) (SPDPA) as the hole collecting layer (HCL). Smooth TiO 2 film with good electron collecting ability was easily formed using the spin-coating process. The power conversion efficiency (PCE) was 3.91%, the same as that of a conventional device. This inverted device is ascertained to maintain 2.82% PCE after 400 h of air-storage. Because of the appropriate work functions of ECL and HCL, the interfaces at the active layer have the ohmic contacts those approach the ideal value of open circuit voltage. SPDPA helps improve the interfacial dipole effect between the active layer and the metal, as verified by in-situ ultraviolet photoelectron spectroscopic data.

The annealing effects on magnetism, structure, and ac transport for Co:ZnO films have been systematically investigated. The room temperature saturation magnetization ͑M s ͒ varies drastically with Ar or Ar/ H 2 annealing processes. By using the impedance spectra, the change in grain boundary and grain defects of these films can be analyzed. The results demonstrate that Ar annealing produces mainly the grain boundary defects which cause the enhancement of M s. Ar/H 2-annealing creates not only grain boundary defects but also the grain defects, resulting in the stronger enhancement of M s. Ferromagnetism for Co:ZnO films is influenced by both grain boundaries and grain defects.

Epitaxial growth of (0001) oriented ͓ZnO͑20 Å͒ /Co 0.7 Fe 0.3 ͑x Å͔͒ 25 multilayers (MLs) with nominal thickness x = 1, 2 and 5 has been prepared on ␣-Al 2 O 3 (0001) substrate by ion-beam sputtering. The magnetic properties over a temperature range of 6-350 K and structures probing by x-ray absorption spectroscopy (XAS) are reported. Above room-temperature ferromagnetism has been observed for x = 1 and x = 2 MLs, while superparamagnetic behavior dominates for x = 5 ML. The field-cooled magnetization-temperature M͑T͒ curves of x = 1 and x = 5 MLs can be fitted by a standard three-dimensional (3D) spin-wave and a Curie-Weiss model, respectively. For x = 2 ML, however, neither a 3D spin-wave nor a Curie-Weiss model, but a combination of the two fits the M-T curve. The XAS studies together with the magnetic measurements further reveal that x =1 sample behaves as a diluted magnetic semiconductor (DMS) ML, while x = 2 ML shows a mixed structure consisting of a minor component of DMS and a major component of CoFe clusters. A predominant clustering phase appears for x = 5 ML.

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High-quality Co-doped ZnO single crystalline films with a wide range of carrier concentration and good reproducibility have been grown by molecular beam epitaxy. After the systematic studies of the magnetic and transport properties of the films, we suggest that there are two distinct ferromagnetic mechanisms in different conductivity regimes. In the insulating regime, carriers tend to be localized, favoring the formation of bound magnetic polarons, which leads to ferromagnetism. In the metallic regime, however, most carriers are weakly localized and the free carrier-mediated exchange is dominant. Our experimental observations are well consistent with the recent theoretical description of magnetism in Co-doped ZnO and helpful for understanding the ferromagnetic mechanism in oxide-based diluted magnetic semiconductors.

Metallic (Co, Ga)-codoped ZnO single crystalline films have been grown by molecular beam epitaxy. Besides room temperature ferromagnetism, the anomalous hall effect (AHE) due to spin-orbit interaction was also found. The small AHE signals match quantitatively with the magnetic hysteresis and can be correspondent to the intrinsic ferromagnetism in a true diluted magnetic oxide with charge carrier spin polarization. Both the saturation magnetization and AHE can be significantly enhanced by additional carrier doping, revealing that the ferromagnetism is carrier mediated in (Co, Ga)-codoped ZnO films.