Iron-doped Lithium Niobate thin films on GaN: fabrication and characterization

In recent time ferroelectric thin films have attracted significant atten-tion due to the observation of several phenomena that manifest innew and intriguing ways. This is the case of photogalvanic effect,also known as bulk photovoltaic effect (BPVE), consisting in the spon-taneous onset of a small electrical current when a polar material isilluminated with a uniform light. A precise understanding of thiseffect could possibly lead to new applications, in particular in thefield of photovoltaics. In fact, as the mechanism of charge separationis completely different, those materials are not a priori bounded bythe theoretical Shockley–Queisser limit which applies to standardphotovoltaic devices such as p-n junctions in semiconductors. In spite of this, the use of BVPE for photovoltaic energy conversion islimited by the smallness of the photogenerated current. In an effortto improve this aspect, a proposal has been made to use nano-sizedmaterials because in this case photogenerated charges could be ex-tracted more efficiently from the ferroelectric. In particular, iron-dopedLithium Niobate (Fe:LN) is one of the best known photogalvanic ma-terials and is therefore the ideal playground to test this approach,namely evidence an increase of the BVPE upon reducing the crystalsize to the nanoscale. Unfortunately, lithium niobate has an intrinsically defective structureand its electrical properties are strongly affected both by its com-position and its structural properties, such as strain etc. A preciseknowledge of the film composition and structure is thus mandatory inorder to set the ground for forthcoming investigations. For this reason,in this work we undergo the task of characterizing a set of Fe:LN thinfilms epitaxially grown on GaN substrates on different conditions byRF-Magnetron sputtering. This activity, carried out in collaborationwith Università Ca’ Foscari (Venezia) and Laboratori Nazionali diLegnaro (LNL-INFN) is based on a combination of techniques andprotocols developed here for this specific kind of samples. The tech-niques used are HR-XRD, RBS, NRA and UV-Vis optical absorptionspectroscopy. The results show that the produced samples are charac-terized by a marked Li deficiency and a structural compressive straindetermined by the epitaxial relation with the substrate and provideuseful indications on how to improve the deposition parameters.