Effects Of Lattice Defects On The Photoresponse Of BNT Ferroelectric Thin Films

Recently, photovoltaic effect observed in ferroelectric thin film has attractedsignificant research attention because it favors device miniaturization and exhibits itspotential applications in optical microsensing, nondestructive optical reading andphotovoltaic devices. However, the photovoltaic mechanism remains controversial.Films with high quality are critical to ensure their excellent photovoltaic performance.However, ferroelectric thin films prepared practically are usually polycrystalline, inwhich lattice defects exist universally. Though various factors that may affect thephotovoltaic output have been investigated, further exploration on the influence oflattice defects on the photovoltaic behavior should be made. In this paper,Bi3.15Nd0.85Ti3O12(BNT) ferroelectric thin films were fabricated with in-planesymmetrical electrodes, and effects of lattice defects on the photoresponse of BNTthin film were investigated. The annealing atmosphere and temperature during filmpreparation were changed, in order to control the density of oxygen vacancies andgrain boundaries, respectively. And effects of these intrinsic lattice defects on thephotoresponse of BNT films were investigated. The evolvement of lattice defectsinduced by surface absorption and carriers injecting was focused to study its relationto the photoresponse of BNT film. The main work and results can be summarized asfollows:1. Fabrication and characterization of the BNT film with in-plane electrodesFirstly, the YSZ and BNT thin films were prepared on the Si substrate insequence using pulse laser deposition and sol-gel process, respectively. Subsequently,symmetrical Pt stripline electrodes were deposited on the BNT thin film by spurring.Then the structure of BNT thin film with in-plane electrodes was formed. Themicrostructure, work-function, ferroelectric property and photoresponse of the BNTthin film were characterized.2. Effects of intrinsic lattice defects on the photoresponse of BNT filmsBNT ferroelectric thin films were fabricated under different annealingatmospheres and temperatures, and the microstructure and photoresponse werecharacterized. The effects of oxygen vacancies and grain boundaries on thephotoresponse of BNT thin films were investigated. The results suggest thatphotoresponse increases with decreasing oxygen concentration in annealingatmosphere. This is because oxygen vacancies in BNT films actually provide paths for transportation of photogenerated carriers. With decreasing annealing temperature, thethreshold voltage of photoconduction as well as the saturation photoconductionincreases. This can be explained as follows. Though grain boundaries hinder themigration of carriers, they promote the separation of photogenerated carriers timely.3. Effects of lattice defect evolution on the photoresponse of BNT filmsThe evolution was realized through exposing the BNT film in the air for longperiods or applying pre-injecting voltage. The effect of these evolutions on thephotoresponse of the BNT film was investigated. The results suggest that both thesurface work function and photoresponse of the BNT film show significant decreasedue to dipole absorption. As the pre-injecting voltage increases, the photoresponse ofthe BNT film increases first, and then decreases. This could be attributed to thechange of defects energy distribution with the pre-injecting voltage. The impact ofsurface absorption and carriers injection was eliminated through heat excitement, andconsequently the sample almost restore its origin state.