|
We report on in-situ studies of the resistivity and the Hall effect in partially oxygen-depleted, metallic YBa2Cu3Ox (YBCO) thin films during illumination with white light. The measurements were performed at temperatures of 100 K, 200 K, and 290 K and showed that the resistivity as well as the Hall coefficient decreased as a function of the illumination time. The photo-induced reduction of both quantities was largest at 290 K. Evaluation of the results of the Hall effect measurements within a simple one-band model shows that both the carrier mobility and the carrier concentration are enhanced by photodoping at 100 K and 200 K, with the enhancement of the carrier concentration dominating. At 290 K, however, the mobility decreases at large illumination times whereas the carrier concentration increases substantially, over-compensating for the loss of mobility. From the qualitatively different time dependencies of the carrier mobility and the carrier concentration, we conclude that two co-existing mechanisms contribute to photodoping: The first mechanism is related to a change of the electronic structure and is tentatively attributed to photo-assisted oxygen ordering. The second mechanism resembles the photogeneration of carriers in semiconductors and is ascribed to a photo-induced charge transfer. At 290 K, only the charge transfer process drives photodoping, whereas oxygen ordering seems to be hampered by thermal disordering. Additional evidence for the co-existence of the two persistent photoconductivity mechanisms is derived from measurements of the spectral efficiency of photodoping at 253 K. We observe a finite photodoping effect at photon energies above and below the charge transfer gap of YBCO. Above the gap energy (approximately equals 1.6 eV), however, the efficiency of photodoping increases remarkably. Our conclusion is that two mechanisms contribute to photodoping in metallic YBCO. One is related to oxygen ordering and does not involve interband transitions. Additionally, photo-induced charge transfer is responsible for the enhanced spectral efficiency of photodoping when the photon energy of the exciting light exceeds the charge transfer gap.
|
