Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
In the recent past, the field of photo-ferroelectrics has been revitalized by the reports of photovoltaic (PV) effect in BiFeO3 (BFO). Unlike traditional semiconductors the open circuit voltages in non-centrosymmetric materials, such as BFO, are not limited by band gap, they can exceed the band gap of the material by orders of magnitude. The microscopic origins of this effect are still poorly understood and largely under debate. This talk will address available tools that might allow understanding of the electronic origin of the anomalous bulk photovoltaic effect in ferroelectrics. We will firstly discuss AFM-based local measurements of photoelectric and photovoltaic effects. We will show that photoelectric atomic force microscopy (Ph-AFM) not only allows investigation of the photovoltaic/photoconductive effects at nanoscale, but shows interesting tip-enhancement of photo-effects.[1] A novel time-resolved spectroscopic scanning probe method, called photo-induced transient spectroscopy (PITS) scanning probe microscopy (SP-PITS), which allows rapid evaluation and local mapping of the generation and recombination rates will be also shown.[2] Additionally, we will discuss complementary characterization methods, such as thermally stimulated current and variable temperature photoelectric characterization, able to provide valuable information on electronic structure of material under investigation, respectively shallow/deep levels in the band gap, which may affect the dark and photo-conduction mechanism. We will show that the existence of the abnormal photovoltaic effect in BFO single crystals is directly correlated to the presence of photoelectric active sub-band gap levels situated around 2.2 eV and that the PV effect can be largely tuned by modifying the occupancy (pumping) of these levels.
[1] M. Alexe and D. Hesse , Nature Communications 2:256, DOI: 10.1038/ncomms1261.
[2] M. Alexe, Nanoletters, Nano Letters 12, 2193, (2012), DOI: 10.1021/nl300618e
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