The impact of epitaxial strain on the structural, electronic, and thermoelectric properties of p-type transparent Sr-doped LaCrO3 thin films has been investigated. For this purpose, high-quality fully strained La0.75Sr0.25CrO3 (LSCO) epitaxial thin films were grown by molecular beam epitaxy on three different (pseudo)cubic (001)-oriented perovskite oxide substrates: LaAlO3, (LaAlO3)0.3(Sr2AlTaO6)0.7, and DyScO3. The lattice mismatch between the LSCO films and the substrates induces in-plane strain ranging from −2.06% (compressive) to +1.75% (tensile).
The electric conductivity can be controlled over 2 orders of magnitude, with σ ranging from ∼0.5 S cm–1 (tensile strain) to ∼35 S cm–1 (compressive strain). Consistently, the Seebeck coefficient S can be finely tuned by a factor of almost 2 from ∼127 μV K–1 (compressive strain) to 208 μV K–1 (tensile strain). Interestingly, we show that the thermoelectric power factor (PF = S2σ) can consequently be tuned by almost 2 orders of magnitude. The compressive strain yields a remarkable enhancement by a factor of 3 for 2% compressive strain with respect to almost relaxed films. These results demonstrate that epitaxial strain is a powerful lever to control the electric properties of LSCO and enhance its thermoelectric properties, which is of high interest for various devices and key applications such as thermal energy harvesters, coolers, transparent conductors, photocatalyzers, and spintronic memories.
Oxides for new-generation electronics
Giant Tuning of Electronic and Thermoelectric Properties by Epitaxial Strain in p-Type Sr-Doped LaCrO3 Transparent Thin Films
Dong Han, Rahma Moalla, Ignasi Fina, Valentina M. Giordano, Marc d’Esperonnat, Claude Botella, Geneviève Grenet, Régis Debord, Stéphane Pailhès, Guillaume Saint-Girons, and Romain Bachelet*
Hexagonal manganites, such as h-LuMnO3, are ferroelectric with its polar axis along the hexagonal axis and have a narrow electronic bandgap (≈1.5 eV). Using Pt electrodes, h-LuMnO3 single crystals display a strong rectification, characteristic of a Schottky diode, and a large photoresponse. It is found that the short circuit photocurrent density Jsc along the polar axis is modulated (up to 25%) by the direction of the ferroelectric polarization P, leading to a short circuit photocurrent loop that mimics the ferroelectric polarization. However, a non-switchable Jsc persists. Diffusion photocurrent is shown to dominate current-in-plane measurements and contributes to the non-switchable Jsc.
We have investigated the multiferroicity and magnetoelectric (ME) coupling in HoFeWO6. With a noncentrosymmetric polar structure (space group Pna21) at room temperature, this compound shows an onset of electric polarization with an antiferromagnetic ordering at the Néel temperature (TN) of 17.8 K. The magnetic properties of the polycrystalline samples were studied by DC and AC magnetization and heat capacity measurements.
Epitaxial growth of Hf0.5Zr0.5O2 (HZO) thin films allows for the stabilization of the metastable orthorhombic phase with robust ferroelectric properties. So far, the ferroelectric phase is most commonly stabilized on perovskite substrates upon insertion of a buffer layer of La2/3Sr1/3MnO3 (LSMO).
We present a comprehensive structural study of the charge-orbital ordering and magnetic phase transitions observed in the A-site ordered SmBaMn2O6 perovskite combining synchrotron radiation x-ray powder diffraction and symmetry-adapted modes analysis. InSmBaMn2O6, successive phase transitions in charge, spin, and lattice degrees of freedom take place with decreasing temperature at TCO1≈380K,TCO2≈190K, and TN≈250K.
Stabilization of the orthorhombic phase of HfO2 with La allows very high polarization and endurance. However, these properties have not been confirmed yet in films having thickness of less than 10 nm. We have grown (111)-oriented La (2 at%) doped epitaxial HfO2 films on SrTiO3(001) and Si(001) substrates, and we report on the thickness dependence of their ferroelectric properties.