F. Z. Wang

ZnO p-n homojunctions and ohmic contacts to Al–N-co-doped p-type ZnO

ZnO p-n homojunctions were fabricated on quartz substrates by depositing Al-doped n-type ZnO layer on Al–N-co-doped p-type ZnO layer through reactive magnetron sputtering. In/Zn metal contacts to as-grown ZnO show ohmic behavior, and the ohmic contacts can be improved by annealing in an Ar ambient. The optimal annealing temperatures for Al–N-co-doped ZnO and Al-doped ZnO are 550 °C and 600 °C, respectively. The p-n junction characteristic is confirmed by current-voltage measurements. The turn-on voltage is 2 V, with a low leakage current under reverse bias. Series resistances of the ZnO p-n junctions can be lowered by optimizing the annealing temperature, increasing the grain size of the ZnO, or increasing the hole concentration of the p layer.

Strain and its effect on optical properties of Al-N codoped ZnO films

The dependence of lattice strain in Al-N codoped p-type ZnO films on Al content and growth temperature was investigated. With increasing Al content, the compressive strain in the film first increases and then decreases. We suggest that the strain decrease is due to the occupation of more substitutional sites by Al at relatively high Al content, which partially compensates the compressive strain. Reversion of conduction type at high Al content and high temperature was also observed. By studying the strain and electrical properties of the codoped films, we conclude that ZnO film should be grown at intermediate temperatures and with low Al content to achieve both good p-type conduction and reasonable crystal quality. The compressive strain results in increase of the optical band gap, and a linear relationship between them was obtained.

Photoluminescence properties of quasialigned ZnCdO nanorods

We report on the photoluminescence (PL) properties of quasialigned ZnCdO single-crystal nanorods prepared by thermal evaporation of Zn and CdCl2 on a Si substrate using Au as a catalyst. The temperature-dependent PL spectra of the ZnCdO nanorods show that each spectrum displays an intense near-band-edge (NBE) emission around 3.1eV, as well as a weak defect-related band around 2.5eV. At low temperatures, the NBE band consists of three peaks at 3.23, 3.16, and 3.06eV. From time-integrated and time-resolved PL analyses, we suggest that the peak at 3.06eV is associated with carriers localized at potential minima induced by the Cd spatial inhomogeneous distribution in ZnCdO alloys. The peak at 3.16eV is attributed to the excitonic emission of ZnCdO, while the peak at 3.23eV is ascribed to bound excitons in ZnO.

Raman scattering and photoluminescence of quasi-aligned ternary ZnCdO nanorods

Ternary ZnCdO single-crystal nanorods with a high Cd incorporation of about 16 at% were synthesized onto Au-catalyzed Si substrate using a physical vapour deposition method. Field emission scanning electron microscopy observation, x-ray diffraction analysis and Raman spectrum showed that the ZnCdO nanorods were well-aligned along the c-axis. Raman spectrum revealed that the E2 (high) mode of the ZnCdO nanorods shifted towards the higher frequency side by 5 cm−1 with respect to that of pure bulk ZnO, corresponding to biaxial compressive stress of 1.14 GPa. The near-band-edge emission of ZnCdO red-shifted to 3.04 eV due to Cd substitution. The defect-related emission exhibited anomalous phonon replicas with an energy of 102 meV, which was attributed to the disorder-activated B1 phonon mode.

Synthesis and characterization of quasi-aligned ZnCdO nanorods

Quasi-aligned ZnCdO single-crystal nanorods were prepared for the first time by using thermal evaporation of Zn and CdCl2 on a Si substrate with the presence of Au catalyst. The maximum Cd content was up to about 16.7at.%, which was significantly larger than the thermodynamic solid solubility limits. The ZnCdO nanorods have uniform flat hexagonal crystallographic planes with diameters of about 150nm. Notably, with the Cd content increasing, the ultraviolet near-band-edge emission was redshifted to 407nm(3.04eV) from 386nm(3.21eV). The direct modulation of the band gap caused by Cd substitution is responsible for the redshift. The possible growth mechanism of the ZnCdO nanorods was discussed.

Recombination property of nitrogen-acceptor-bound states in ZnO

The recombination property of nitrogen (N)-related acceptor-bound states in ZnO has been investigated by photoluminescence (PL), time-resolved PL, and selective PL. Several possible recombination processes were discussed by analyzing the relaxation and recombination properties under large Coulomb interaction. It is strongly suggested that bound exciton emission dominates the recombination process related to the N acceptor. The recombination lifetime is 750 ps and the binding energy is 67 meV for N-acceptor-bound exciton at low temperature. (c) 2006 American Institute of Physics.