Pekka T. Neuvonen

Diffusion and configuration of Li in ZnO

Diffusion of Li into ZnO from an “infinite” surface source under oxygen-rich conditions is studied using secondary ion mass spectrometry. The Li concentration-versus-depth profiles exhibit a distinct and sharp drop, which evolves in position with temperature and time. The sharp drop is associated with an efficient conversion from highly mobile Li-interstitials (Lii) to practically immobile Li-substitutionals (LiZn) via a kick-out mechanism. The characteristic concentration level at which Li drops provides a measure of the active donor concentration in the samples at the processing temperature, and gives evidence of residual impurities being responsible for the commonly observed “native” n-type conductivity. These donors are suggested to arise from different impurities, with Al and Si as the prevailing ones in hydrothermal and melt grown material. Further, evidence of electric field effects on Li diffusion profiles is obtained, and they are considered as a main reason for the slow diffusivity obtained in t...

Infrared upconversion in radio frequency magnetron sputtered Er-doped zinc oxide thin films

Upconversion in radio frequency magnetron sputtered Er-doped zinc oxide thin films on Si substrate has been demonstrated using 1550 nm cw laser excitation. As-sputtered thin films did not show any upconversion emission, and annealing was required to optically activate the Er3+-ions. Emissions at 985, 809, and 665–675 nm were observed in annealed thin films, corresponding to transitions from 4I11∕2, 4I9∕2, and 4F9∕2 to the ground state 4I15∕2, respectively. The emission from 4I11∕2 was the dominant one, whereas emission from 4I9∕2 was the weakest. The highest intensity at 985 nm was obtained with 2.4 at. % of Er by annealing the film at 700 °C. Annealing at higher temperatures causes Er to diffuse and segregate to the Si-ZnO interface between the Si substrate and the ZnO film.

Interaction between Na and Li in ZnO

The interaction between group-Ia elements in ZnO have been studied by implanting Na into hydrothermally grown ZnO samples containing similar to 4x10(17) Li/cm(3) and employing secondary ion mass spectrometry for sample analysis. Postimplantation annealing above 500 degrees C results in a diffusion of Na and concurrently Li is efficiently depleted from the regions occupied by Na. The data show unambiguously that Na and Li compete for the same trapping site and the results provide strong experimental evidence for that the formation energies of Na on Zn site together with that of interstitial Li are lower than those of Li on Zn site and interstitial Na in highly resistive ZnO. This conclusion is also supported by recent theoretical estimates of the formation energies of these species as a function of the Fermi-level position in ZnO.

Transportation of Na and Li in Hydrothermally Grown ZnO

Secondary ion mass spectrometry has been applied to study the transportation of Na and Li in hydrothermally grown ZnO. A dose of 10 15 cm-2 of Na+ was implanted into ZnO to act as a diffusion source. A clear trap limited diffusion is observed at temperatures above 550 °C. From these profiles, an activation energy for the transport of Na of 7 ∼1.7 eV has been extracted. The prefactor for the diffusion constant and the solid solubility of Na cannot be deduced independently from the present data but their product estimated to be ∼ 3 × 1016 cm-1 s-1. A dissociation energy of ∼2.4 eV is extracted for the trapped Na. The measured Na and Li profiles show that Li and Na compete for the same traps and interact in a way that Li is depleted from Na-rich regions. This is attributed to a lower formation energy of Na-on-zinc-site than that for Li-on-zinc-site defects and the zinc vacancy is considered as a major trap for migrating Na and Li atoms. Consequently, the diffasivity of Li is difficult to extract accurately from the present data, but in its interstitial configuration Li is indeed highly mobile having a diffasivity in excess of 10-11 cm2 s-1 at 500 °C .