Photoconductivity and diode effect in Bi rich multiferroic BiFeO_3 thin films grown by pulsed-laser deposition

摘要

Bismuth ferrite, BiFeO_3, is almost the only material that is simultaneously magnetic and a strong ferroelectric at room temperature. As a result it is the most investigated multiferroic material. In this study, bismuth ferrite thin films were deposited on silicon wafer (100) and glass by pulsed-laser deposition and their structural, optical, and electrical properties were measured. Our study indicates that Bi richness in these films can stimulate formation of oxygen vacancy in the system which in its turn leads to delocalization of carriers and a more intensified photoconductivity response. X-ray diffraction analysis revealed formation of BiFeO_3 (BFO), but it also showed formation of Bi_2O_3 and Bi_2O_2.3 as well as BFO. Energy dispersive spectrum (EDS) also showed higher atomic concentration of Bi with respect to Fe. It also disclosed Bi depletion through the films during post-growth heat treatment. Atomic force microscopy showed a homogeneous nano structure with columnar grains. It also disclosed that higher substrate temperature can improve smoothness of the films. Scanning electron microscopy depicted the thickness of about 200 nm. Transmission spectrum illus trated band gap of about 2 eV. Dark-light IV characteristics were conducted on the films which were subjected to post-growth heat treatment at 0.01 and 760 Torr O_2. Dark conductivities increased by an order of magnitude in comparison between films which were subjected to post- growth heat treatment at 0.01 and 760 Torr O_2. Dark-light IV characteristics of the films also uncovered a remarkable increase in conductivity under illumination in comparison to dark one. Diode behavior of the films was investigated by IV characteristics as well.