RESUMEN
The resistive switching in metal-oxide thin films typically occurs via modulation of the oxygen content in nano-sized conductive filaments. For Ta2O5-based resistive switching devices, the two current models consider filaments composed of oxygen vacancies and those containing metallic Ta clusters. The present work tries to resolve this dispute. The filaments in Ta2O5 were formerly shown to exhibit the same electrical transport mechanisms as TaOx thin films with xâ¼ 1.0. In this paper, sputtered thin films of pure ß-Ta and of TaOx with different oxygen concentrations are studied and compared in terms of their structure and electrical transport. The structural analysis reveals the presence of Ta clusters in the TaOx films. Identical electrical transport characteristics were observed in the TaOx films with xâ¼ 1.0 and in the ß-Ta film. Both show the same transport mechanism, a carrier concentration on the order of 1022 cm-3 and a positive magnetoresistance associated with weak antilocalization at T < 30 K. It is concluded that the electrical transport in the TaOx films with xâ¼ 1.0 is dominated by percolation through Ta clusters. This means that the transport in the filaments is also determined by percolation through Ta clusters, strongly supporting the metallic Ta filament model.
RESUMEN
Solar cells containing a polycrystalline Cu(In,Ga)Se2 absorber outperform the ones containing a monocrystalline absorber, showing a record efficiency of 22.9%. However, the grain boundaries (GBs) are very often considered to be partly responsible for the enhanced recombination activity in the cell and thus cannot explain the registered record efficiency. Therefore, in the present work, we resolve this conundrum by performing correlative electron beam-induced current-electron backscatter diffraction investigations on more than 700 grain boundaries and demonstrating that 58% of the grain boundaries exhibit an enhanced carrier collection compared to the grain interior. Enhanced carrier collection thus indicates that GBs are beneficial for the device performance. Moreover, 27% of the grain boundaries are neutral and 15% are recombination-active. Correlation with microstructure shows that most of the ∑3 GBs are neutral, whereas the random high-angle grain boundaries are either beneficial or detrimental. Enhanced carrier collection observed for a big fraction of high-angle grain boundaries supports the "type-inversion" model and hence the downward band bending at GBs. The decrease in current collection observed at one of the high-angle grain boundaries is explained by Cu being enriched at this GB and hence by the upward shift of the valence band maximum.