Giant Tuning of Electronic and Thermoelectric Properties by Epitaxial Strain in p-Type Sr-Doped LaCrO₃ Transparent Thin Films

Abstract

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 two orders of magnitude, 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 two from ~127 µV K -1 (compressive strain) to 208 µV K -1 (tensile strain). Interestingly, we show that the thermoelectric power factor (PF = S 2 σ) can consequently be tuned by almost two orders of magnitude. The compressive strain yields a remarkable enhancement by a factor of three 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, photo-catalyzers and spintronic memories.

Chemical Synthesis of La0.75Sr0.25CrO3 Thin Films for p-Type Transparent Conducting Electrodes

“…The electrical resistivity of vacuum deposited epitaxial LSCO films is reported to be higher on STO substrates than on LAO due to strain effects. − Indeed, no measurable electrical conductivity was found for our CSD-LSCO films on STO (not shown). Therefore, for the purpose of this study, the electrical properties of ∼45 nm - LSCO films have been measured on LAO substrates.…”

“…In-plane texture of LSCO on LAO has been investigated from a phi-scan, Δϕ (103), LSCO = 1.4° and Δϕ (103), LAO = 0.2°, and the out-of-plane texture from the rocking curve, Δω (003), LSCO = 0.4°, which confirm that films are biaxially textured; see Figures a,b. From the XRD-RSM, Figure c, the in-plane, a , and out-of-plane, c , lattice parameters of LSCO have been extracted considering a pseudocubic symmetry, resulting a = 3.867 ± 0.023 Å and c = 3.896 ± 0.005 Å ( a LSCObulk = 3.876 Å). − The films show subtle in-plane compressive strain of −0.232% and an out-of-plane tensile strain of 0.516%. These results were further confirmed from strain maps by STEM images using geometrical phase analysis (GPA), Figure S7.…”

“…Consequently, closed and quasi-closed shell electronic configurations of cation species ( d 10 , d 6 , d 3 ) are the best candidates to reduce localization of holes in oxygen ions and minimize colorization from the metal d–d excitation. In this line, some new p -type TCOs based on Cr 3+ oxides ( d 3 in octahedral configuration) have been already identified − among which Sr-doped LaCrO 3 appears as a strong candidate with reasonably high transparency and p -type conductivity. − Importantly, the ability to grow these materials as epitaxial films can be used as a lever to tune the physical properties. − In fact, it has been recently demonstrated that electric conductivity of Sr-doped LaCrO 3 films grown by molecular beam epitaxy (MBE) can be finely tuned through epitaxial strain . Nevertheless, when evaluating the suitability of an oxide material to be used as a TCO, along with high electrical conductivity and optical transmittance, it is important to consider cost-effective and sustainable processing techniques.…”

“…In this line, some new p -type TCOs based on Cr 3+ oxides ( d 3 in octahedral configuration) have been already identified 20 − 22 among which Sr-doped LaCrO 3 appears as a strong candidate with reasonably high transparency and p -type conductivity. 23 − 26 Importantly, the ability to grow these materials as epitaxial films can be used as a lever to tune the physical properties. 27 − 29 In fact, it has been recently demonstrated that electric conductivity of Sr-doped LaCrO 3 films grown by molecular beam epitaxy (MBE) can be finely tuned through epitaxial strain.…”

An efficient material search for room-temperature topological magnons

“…In the case of LaCrO 3 thin films, the pseudocubic phase has been grown epitaxially on SrTiO 3 (001) to use strain to tune magnetic and optical properties ( 36 ). Furthermore, substitutions in M x La 1− x CrO 3 (i.e., M = Sr, x < 0.25) has been used to control p-type doping and induce in-plane ≈2% compressive to ≈2% tensile strain ( 37 ). For R FeO 3 ( R = Nd, Ce, Tb, Sm), high-quality crystals on the order of centimeter (length) and millimeter (diameter) have been grown with floating zone techniques ( 38 , 39 ).…”