arXiv subjects

Marvin Hartwig Zoellner

Publications and source records attributed to Marvin Hartwig Zoellner.

3 recordsLinked to original sources

arXiv2025

Brittle-to-ductile transition and strain relaxation in Si$_{1-x}$Ge$_x$ linearly graded buffers

The strain-relaxation mechanism of a set of Si$_{0.6}$Ge$_{0.4}$ linearly graded buffers (LGBs), grown following different temperature profiles, has been investigated by means of defect-etching and variable-temperature high-resolution X-ray diffraction (VT-HRXRD). Defect-etching experiments demonstrate that a sharp increase of threading dislocation density (TDD) from $3 \times 10^{5}$\,cm$^{-2}$ to $1.2 \times 10^{6}$\,cm$^{-2}$ takes place when the final growth temperature exceeds a critical value T$_c\approx 530^\circ$C. VT-HRXRD measurements show that in low TDD samples extra relaxation takes place for annealing temperatures larger than T$_c$, thanks to the nucleation of new dislocations. These results indicate that, below T$_c$, strain relaxation is driven by the gliding of existing dislocations while above T$_c$ new dislocations are nucleated, suggesting a link with our results and the brittle-to-ductile transition in Si$_{1-x}$Ge$_x$ alloys.

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↗