Abstract
Ferroelectric materials are used in actuators or sensors because of their non-volatile macroscopic electric polarization. GeTe is the simplest known diatomic ferroelectric endowed with exceedingly complex physics related to its crystalline, amorphous, thermoelectric, and—fairly recently discovered—topological properties, making the material potentially interesting for spintronics applications. Typically, ferroelectric materials possess random oriented domains that need poling to achieve macroscopic polarization. By using X-ray absorption fine structure spectroscopy complemented with anomalous diffraction and piezo-response force microscopy, we investigated the bulk ferroelectric structure of GeTe crystals and thin films. Both feature multi-domain structures in the form of oblique domains for films and domain colonies inside crystals. Despite these multi-domain structures which are expected to randomize the polarization direction, our experimental results show that at room temperature there is a preferential ferroelectric order remarkably consistent with theoretical predictions from ideal GeTe crystals. This robust self-poled state has high piezoelectricity and additional poling reveals persistent memory effects.
Highlights
In the past decade, a lively interest has been sparked in relation to condensed matter systems with unconventional electronic structures possessing massless Dirac fermions, i.e., systems with a linearly dispersing “Dirac cone” in their band structure
By using Extended X-ray absorption fine structure measurements (EXAFS) and anomalous scanning X-ray diffraction techniques, complemented with piezoresponse force microscopy (PFM), we show that GeTe possesses a robust ferroelectric self-polarized state
High quality epitaxial 200-nm-thick-ferroelectric thin films of Te-terminated α-GeTe were grown by molecular beam epitaxy (MBE) using GeTe and Te sources, following the procedures described in Ref. [26]
Summary
A lively interest has been sparked in relation to condensed matter systems with unconventional electronic structures possessing massless Dirac fermions, i.e., systems with a linearly dispersing “Dirac cone” in their band structure This property is at the heart of a new class of materials called topological insulators [1,2], possibly interesting for technological applications in Crystals 2019, 9, 335; doi:10.3390/cryst9070335 www.mdpi.com/journal/crystals. Crystals 2019, 9, 335 low power electronics Their characteristic two-dimensional (2D) energy momentum relation, or band structure, is similar to the graphene model system and their spin texture was studied in surface derived states for numerous materials [3]. Α-GeTe(111) was predicted to satisfy these criteria by theory [5]
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