F. Zhuge

p-type conduction in N–Al co-doped ZnO thin films

p-type ZnO thin films have been realized by the N–Al co-doping method. Secondary ion mass spectroscopy demonstrated that the N incorporation was enhanced evidently by the presence of Al in ZnO. The lowest room-temperature resistivity was found to be 57.3Ωcm with a Hall mobility of 0.43cm2∕Vs and carrier concentration of 2.25×1017cm−3 for the N–Al co-doped p-type ZnO film deposited on glass substrate. The results were much better than those for the N-doped p-type ZnO. Moreover, the co-doped film possesses a good crystallinity with c-axis orientation and a high transmittance (90%) in the visible region.

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.

ZnO light-emitting diodes fabricated on Si substrates with homobuffer layers

ZnO homojunction light-emitting diodes (LEDs), comprised of N–Al codoped p-type ZnO and Al-doped n-type ZnO layers, were fabricated on Si substrates with homobuffer layers. The current-voltage measurements showed typical diode characteristic with a threshold voltage of about 3.3V. The electroluminescence (EL) bands at 110K consisted of a near-band-edge emission at 3.18eV and a deep level emission at 2.58eV. The EL emissions were assigned as radiative recombinations, presumably of donor-acceptor pairs, in the p-type layer of the LED. The quenching of EL with temperature was attributed to the degradation of p-type conducting of the ZnO:(N,Al) layer.

Electrical characterization of ZnO-based homojunctions

Electrical characteristics have been studied for ZnO p-n and p-i-n homojunctions, with optimization of device structures for improved performance. Capacitance-voltage measurements confirm the formation of abrupt junctions. The current-voltage characteristics exhibit their inherent electrical rectification behavior. The p-ZnO:(N,Al)∕n-ZnO:Al homojunctions fabricated on sapphire substrates combining with the intrinsic ZnO buffer layer have acceptable p-n diode characteristics, with the forward turn-on voltage of 1.4V and the reverse breakdown voltage of 5.3V. By introduction of an intrinsic (Zn,Cd)O layer, the resultant p-ZnO:(N,Al)∕i-(Zn,Cd)O∕n-ZnO:Al homojunction exhibits a high reverse breakdown voltage of ∼18V.

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.