D. W. Ma

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.

Deposition and characteristics of CdO films with absolutely (200)-preferred orientation

Abstract CdO crystal thin films with (200)-preferred orientation have been prepared on Si and glass substrates by dc reactive magnetron sputtering method. At an optimum substrate temperature of 375 °C, the film has the best crystal quality. By the electrical and transmittance measurements the film shows large carrier concentration of 2.00×10 20 /cm 3 , Hall mobility of 64 cm 2 /V s, resistivity of 4.87×10 −4 Ω cm and a high average transmittance over 80% in the visible region together with a direct band gap of 2.43 eV. In view of the Burstein–Moss (BM) shift, theoretical calculations show that the film has a direct band gap of 2.17 eV, close to its intrinsic band gap of 2.2 eV. The photoluminescence (PL) measurement shows that the pure CdO film has no luminescence behavior, but it can alloy with ZnO to realize its applications in luminescent devices.

Structural and photoluminescent properties of ternary Zn1-xCdxO crystal films grown on Si(111) substrates

The ternary Zn1-xCdxO (0less than or equal toxless than or equal to0.6) alloying films with highly c-axis orientation have been deposited on Si(111) substrates by direct current reactive magnetron sputtering method. X-ray diffraction measurement indicates that the wurtzite-type structure of ZnO can be stabilized up to nominal Cd content x similar to 0.6 without cubic CdO phase separation. The lattice parameter c of Zn1-xCdxO increases almost linearly from 5.229 Angstrom (x = 0) to 5.247 Angstrom (x = 0.6), indicating that Cd substitution takes place on the Zn lattice sites. The photoluminescence spectra of the Zn1-xCdxO thin films measured at 12 K display a substantial red shift (similar to0.3 eV) in the near-band-edges (NBEs) emission of ZnO: from 3.39 eV of ZnO to 3.00 eV of Zn0.4Cd0.6O. The direct modulation of band gap caused by Zn/Cd substitution is responsible for the red shift effect in NBE emission of ZnO

Quasi-aligned ZnO nanotubes grown on Si substrates

Quasi-aligned ZnO nanotubes have been grown on silicon substrates by metalorganic chemical vapor deposition without using any catalyst. Two kinds of ZnO nanotubular structures were found: Nanotubes with single walls and nanotubes with double walls. The nanotubes were grown along the [001] direction. Room-temperature photoluminescence measurements of the ZnO nanotubes indicate strong ultraviolet emission and weak green emission. A new growth mode for these ZnO nanotubes is proposed, which can be used to prepare other nanotubular structures.

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.

Preparation and characterization of ZnO nanotubes

ZnO nanotubes were grown by metalorganic chemical vapor deposition (MOCVD) on silicon substrates without using any catalyst. Scanning electron microscopy measurement revealed that the ZnO nanotubes have hexagonal outer shape and round inner shape, the outer diameter is of 60/spl sim/110 nm and the wall thickness is 10/spl sim/15 nm. Transmission electron microscopy measurement showed that the nanotubes are single crystals with c-axis growth orientation. Low-temperature photoluminescence measurements revealed intense ultraviolet light emission near the band gap of ZnO at 3.37 eV, and show almost no broad green photoluminescence emission band related to native defects.