arXiv subjects

Omar Concepción

Publications and source records attributed to Omar Concepción.

4 recordsLinked to original sources

arXiv2026

Epitaxial SiGeSn alloys for CMOS-compatible thermoelectric devices

The integration of thermoelectric devices into mainstream microelectronic technological platform could be a major breakthrough in various fields within the \emph{so-called} Green-IT realm. In this article, the thermoelectric properties of heteroepitaxial SiGeSn alloys, a novel CMOS compatible material system, are evaluated to assess their possible application in thermoelectric devices. To this purpose, starting from the experimentally low lattice thermal conductivity of SiGeSn/Ge/Si layers of about $\sim$1-2 W/m$\cdot$K assessed by means of 3-$ω$ measurements, the figure of merits are calculated through the use of Boltzmann transport equation, taking into account the relevant inter-valley scattering processes, peculiar of this multi-valley material system. Values for the figure of merit $ZT$ exceeding $1$ have been obtained for both p- and n- type material at operating temperatures within the 300---400 K range, i.e. at a typical On-Chip temperatures. In this interval, the predicted power factor also features very competitive values of the order of 20 $\rm{μW/cm\cdot K^2}$. Our finding indicates that this new class of Si-based materials has extremely good prospects for real-world applications, and can further stimulate scientific investigation in this ambit.

cond-mat.mtrl-sci↗

arXiv2026

Anisotropy-driven interfacial magnetism in Ru-deficient SrRuO$_3$ thin films

While stoichiometric SrRuO$_3$ (SRO) is a metallic itinerant ferromagnet with relatively homogeneous magnetization, Ru deficiency provides a powerful route to alter its electronic transport and depth-dependent magnetic properties. Ru-deficient SRO thin films grown by radio-frequency high oxygen pressure sputtering were investigated using a combination of X-ray reflectivity, polarized neutron reflectometry, off-specular neutron scattering, scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy, electrical transport, and magnetometry. Structural and compositional analyses reveal that Ru deficiency is intrinsic to the films, with an enhanced deficiency at the interfaces. As a result, coherent electronic transport is suppressed and the saturation magnetization is reduced, while the Curie temperature remains largely unaffected, placing Ru-deficient SRO in a regime consistent with ferromagnetic insulator-like behavior. Depth- and lateral-resolved magnetic measurements further show that the interfacial regions remain ferromagnetic but exhibit enhanced perpendicular magnetic anisotropy, which constrains the local magnetization to remain predominantly out-of-plane and strongly reduces its in-plane projection. Our results establish Ru deficiency as a key control parameter governing transport, magnetization, and anisotropy in SRO thin films and highlight defect and interface engineering as powerful routes to tailor interfacial magnetism in correlated oxide heterostructures.

cond-mat.mtrl-sci↗

arXiv2024

Thermal expansion and temperature dependence of Raman modes in epitaxial layers of Ge and Ge$_{1-x}$Sn$_{x}$

Temperature dependence of vibrational modes in semiconductors depends on lattice thermal expansion and anharmonic phonon-phonon scattering. Evaluating the two contributions from experimental data is not straightforward, especially for epitaxial layers that present mechanical deformation and anisotropic lattice expansion. In this work, a temperature-dependent Raman study in epitaxial Ge and Ge$_{1-x}$Sn$_{x}$ layers is presented. A model is introduced for the Raman mode energy shift as a function of temperature, comprising thermal expansion of the strained lattice and anharmonic corrections. With support of x-ray diffraction, the model is calibrated on experimental data of epitaxial Ge grown on Si and Ge$_{1-x}$Sn$_{x}$ grown on Ge/Si, finding that the main difference between bulk and epitaxial layers is related to the anisotropic lattice expansion. The phonon anharmonicity and other parameters do not depend on dislocation defect density (in the range $7\cdot 10^6$ - $4\cdot 10^8$ cm$^{-2}$) nor on alloy composition in the range 5-14 at.%. The strain-shift coefficient for the main model of Ge and for the Ge-Ge vibrational mode of Ge$_{1-x}$Sn$_{x}$ is weakly dependent on temperature and is around -500 cm$^{-1}$. In Ge$_{1-x}$Sn$_{x}$, the composition-shift coefficient amounts to -100 cm$^{-1}$, independent of temperature and strain.

cond-mat.mtrl-sci↗

arXiv2023

Local Alloy Order in a Ge1-xSnx/Ge Epitaxial Layer

The local ordering of atoms in alloys directly has a strong impact on their electronic and optical properties. This is particularly relevant in nonrandom alloys, especially if they are deposited using far from the equilibrium processes, as is the case of epitaxial Ge1-xSnx layers. In this work, we investigate the arrangement of Ge and Sn atoms in optoelectronic grade Ge1-xSnx epitaxial layers featuring a Sn content in the 5-14% range by using polarization-dependent Raman spectroscopy and density-functional-theory calculations. The thorough analysis of the polarization-dependent spectra in parallel and perpendicular configuration allowed us to properly tag all the observed vibrational modes, and to shed light on that associated to disorder-assisted Raman transitions. Indeed, with the help of large-scale atomistic simulations, we were able to highlight how the presence of Sn atoms, that modify the local environments of Ge atoms, gives rise to two spectral features at different Raman shifts, corresponding to distortions of the atomic bonds. This analysis provides a valuable framework for advancing the understanding of the vibrational properties in Ge1-xSnx alloys, particularly with regard to the impact of local ordering of the different atomic species.

cond-mat.mtrl-sci↗