A. Meaney

P-type nitrogen- and phosphorus-doped ZnO thin films grown by pulsed laser deposition on sapphire substrates

Nitrogen- and phosphorus-doped ZnO thin films were grown by pulsed laser deposition using an electron cyclotron resonance (ECR) nitrogen plasma ion source or a ZnO:P2O5 doped target, as the dopant source, respectively. Both types of films were grown on sapphire substrates first coated at low temperature with a ZnO buffer layer. For the N-doped ZnO thin films, temperature-dependent Van der Pauw measurements showed consistent p-type behavior over the measured temperature range of 200-450 K, with typical room temperature acceptor concentrations and mobilities of 5 x 1015 cm-3 and 5.61 cm2/Vs, respectively. The room-temperature photoluminescence spectrum of a N-doped ZnO thin film featured a broad near band-edge emission at about 3.1 eV photon energy with a width of 0.5 eV. XPS studies confirmed the incorporation of nitrogen in the samples. The ZnO:P layers (with phosphorus concentrations of between 0.01 and 1 wt %) typically showed weak n-type conduction in the dark, with a resistivity of 70 &OHgr;.cm, a Hall mobility of &mgr;n ~ 0.5 cm2V-1s-1 and a carrier concentration of n ~ 3 x 1017 cm-3 at room temperature. After exposure to an incandescent light source, the samples underwent a change from n- to p-type conduction, with an increase in mobility and a decrease in concentration for temperatures below 300K. Electrical measurements showed noticeable differences for both types of doped films when carried out in air or in vacuum. The results are discussed in terms of both the presence of surface conducting channels and the influence of photoconductive effects.

Laterally and vertically grown ZnO nanostructures on sapphire

Lateral growth of ZnO nanowall arrays with subsequent growth of vertical nanowires using a two-step vapour phase transport method on a-plane sapphire are reported. X-ray diffraction and scanning electron microscopy data show that the nanostructures are aligned with c-axis normal to the substrate. Photoluminescence data demonstrate the exceptionally high optical quality of these structures, with intense emission and narrow bound exciton linewidths. We observe high energy excitonic emission at low temperatures close to the band-edge which we assign to the surface exciton in ZnO at ~3.366 eV. This assignment is consistent with the large surface to volume ratio of the nanowire systems and indicates that this large ratio has a significant effect on the luminescence even at low temperatures. The band-edge intensity decays rapidly with increasing temperature compared to bulk single crystal material, indicating a strong temperature-activated non-radiative mechanism peculiar to the nanostructures. No evidence is seen of the free exciton emission due to exciton delocalisation in the nanostructures with increased temperature, unlike the behaviour in bulk material. The use of such nanostructures in room temperature optoelectronic devices appears to be dependent on the control or elimination of such surface effects.