Optoelectronic Characteristics Of Oxygen-deficient La_0.6Ca_0.4MnO_3-δ/Si Films And Heterojunctions

Perovskite-type hole-doped manganites have stimulated much attention due to their exotic magnetic and electronic properties. The interaction of spin, charge, lattice and/or orbital leads to giant responses to small external stimuli, such as magnetic field, electric field and light. The sensitivity of transport to light stimuli promises to open up attractive applications in new optoelectronic devices.In this thesis, oxygen-deficient La0.6Ca0.4MnO3-δ(LCMO) thin films and LCMO films with thickness gradient are grown on Si substrates by pulsed laser deposition (PLD). Both continuous wave (cw) laser and pulse laser with the identical wavelength of 532nm (2.3eV) are used to investigate the optoelectronic characteristics of LCMO/Si films and heterojunctions. Photoinduced switching of resistance by several orders of magnitude is observed in the oxygen-deficient LCMO thin films. The magnitude of transient photoconductivity could achieve 95.9% at room temperature under a 532nm cw laser illumination at the laser intensity of 142mW/cm2. The switching has a fast photoresponsive effect in nanosecond timescale when irradiated by a 532nm laser pulse of 7 ns duration. It is found that the photocurrent and response time depend on the laser intensity and the applied electric field.Large transient photoconductivity is also observed at room temperature in oxygen-deficient LCMO/Si heterojunctions. The increase of conductivity is up to 90% while applying -1.5V reverse bias under 532nm cw laser illumination at the laser intensity of 142 mW/cm2. However, the increase of conductivity is only about 10% with same illumination condition but a forward bias. It is also found that the photoconductivity and response time depend on the laser intensity. Open-circuit photovoltage with rise time of 13 ns is also observed.The LCMO/Si films and the corresponding heterojunctions have interesting characteristics with a thickness gradient. The LCMO/Si films have rectifying characteristics similar to that of p-n junctions. Exotic photovoltaic effects have been observed in LCMO/Si films with thickness gradient. Oxygen deficiency can raise the photovoltaic sensitivity. It is found that the thickness gradient of the film has important effects on the photoelectric characteristics and the photovoltage of the film is due to the thickness difference. These results can be significant for potential applications in the manganite-based optoelectronic devices.