Bias effect on the preparation of YBa2Cu3O 7-δ films by laser ablation

Bias effects on the preparation of superconducting thin films by pulsed laser deposition have been investigated by means of time-of-flight mass spectroscopy. Since a large number of electrons were emitted from the target at the same time as ions during ablation, ions in the plume are shielded from the external electric field by electrons and are not influenced by substrate biasing. Under positively biased conditions, the electron emission was greatly enhanced. Multivalenced ions (Cu²⁺ and Cu³⁺) were also observed in these conditions. It can be understood qualitatively that the rapid extraction of electrons from the target surface by the electric field tends to generate the multivalenced ions. It was also found that the multivalenced ions have an energy of more than 250 eV, which is higher than that of the other atomic ions. Energy differences between the atomic and molecular ions have been observed. While the former have energies of about 200 eV, the latter have energies of a few tens of electronvolt. This indicates that the atomic ions are accelerated in the initial laser ablation process and that the molecular ions are produced in the plasma reactions in the plume. Cluster ions of the mass number of several hundreds were also observed and also had energies of a few electronvolts. It was also found that the laser plume of the YBa₂ Cu₃O_7-δ, target is poor in the CuO⁺ ion compared to the plasma of other metals