arXiv subjects

Jana Lierath

Publications and source records attributed to Jana Lierath.

3 recordsLinked to original sources

arXiv2026

When heat goes astray -- non-local heating in a semiconductor

Heating of semiconductor devices limits their performance and lifetime, which must be addressed by thermal management starting at the heat source. It is a common assumption that the heat source and the resulting heat spot locally coincide, if their size exceeds the mean free paths of the main heat carriers, the phonons. We show that this paradigm of heat locality breaks down on length scales spanning several micrometers. As a consequence, non-local heating occurs in contradiction to Fourier's law. Therefore, we heat laterally structured semiconductor membranes that feature a rising number of interfaces with a well-focussed laser and map-out lattice temperatures by Raman thermometry. Remarkably, the non-local heating can exceed the laser-induced local heating, which we attribute to ballistic phonon transport far above cryogenic temperatures.

cond-mat.mes-hall↗

arXiv2025

Phonon Mean Free Path Spectroscopy By Raman Thermometry

In this work, we exemplify on a bulk silicon sample that Raman thermometry is capable of phonon mean free path (PMFP) spectroscopy. Our experimental approach is similar to the variation of different characteristic length scales $l_{c}$ during thermal reflectance measurements in the time or frequency domain and transient thermal grating spectroscopy. In place of $l_{c}$, we vary the laser focus spot size ($w_{e}$) and the light penetration depth ($h_α$) during one-laser Raman thermometry (1LRT) measurements. For our largest $w_{e}$ values, the derived effective thermal conductivities $κ_{eff}$ converge towards the bulk thermal conductivity $κ_{bulk}$ for silicon. However, towards smaller $w_{e}$ values, we observe a pronounced increase for the $κ_{eff}$ values, which amounts up to a factor of 5.3 at 293K and even 8.3 at 200K. We mainly assign this phenomenon to quasi-ballistic phonon transport. As a result, we can compare our measured $κ_{eff}(w_{e})$ trends with the thermal accumulation function $κ_{cum}$ and its dependence on the phonon mean free path $l_{ph}$, which we derive from ab initio solutions of the linearized phonon Boltzmann transport equation (BTE). Since the variation of $w_{e}$ can be experimentally cumbersome, we also suggest varying $h_α(λ)$ via the applied Raman laser wavelength $λ$ during 1LRT. In this regard, we present proof-of-principle 1LRT measurements, yielding a step-like $κ_{eff}(λ)$ trend for four different $λ$ values, which we also interpret in terms of quasi-ballistic phonon transport. Our results shall seed future PMFP spectroscopy based on 1LRT, which can directly be benchmarked against state-of-art theory by comparison of $κ_{\text{cum}}$ trends and not only $κ$ values, aiming to test our understanding of the intricate phonon transport physics.

physics.optics↗

arXiv2024

Thermal analysis of GaN-based photonic membranes for optoelectronics

Semiconductor membranes find their widespread use in various research fields targeting medical, biological, environmental, and optical applications. Often such membranes derive their functionality from an inherent nanopatterning, which renders the determination of their, e.g., optical, electronic, mechanical, and thermal properties a challenging task. In this work we demonstrate the non-invasive, all-optical thermal characterization of around 800-nm-thick and 150-$μ$m-wide membranes that consist of wurtzite GaN and a stack of In$_{0.15}$Ga$_{0.85}$N quantum wells as a built-in light source. Due to their application in photonics such membranes are bright light emitters, which challenges their non-invasive thermal characterization by only optical means. As a solution, we combine two-laser Raman thermometry with (time-resolved) photoluminescence measurements to extract the in-plane (i.e., $c$-plane) thermal conductivity $κ_{\text{in-plane}}$ of our membranes. Based on this approach, we can disentangle the entire laser-induced power balance during our thermal analysis, meaning that all fractions of reflected, scattered, transmitted, and reemitted light are considered. As a result of our thermal imaging via Raman spectroscopy, we obtain $κ_{\text{in-plane}}\,=\,165^{+16}_{-14}\,$Wm$^{-1}$K$^{-1}$ for our best membrane, which compares well to our simulations yielding $κ_{\text{in-plane}}\,=\,177\,$Wm$^{-1}$K$^{-1}$ based on an ab initio solution of the linearized phonon Boltzmann transport equation. Our work presents a promising pathway towards thermal imaging at cryogenic temperatures, e.g., when aiming to elucidate experimentally different phonon transport regimes via the recording of non-Fourier temperature distributions.

physics.optics↗