Ultrafast study of phonon transport in isotopically controlled semiconductor nanostructures

Abstract

Isotopically modulated silicon and germanium multilayers are analyzed by means of femtosecond spectroscopy and pulsed X‐ray scattering for determining thermal conductivity and phonon modes. Isotopic modulation decreases thermal conductivity stronger than expected from a band bending model in the coherent phonon transport regime, in particular for silicon. Femtosecond spectroscopy and X‐ray scattering resolve zone‐folded vibration modes, which are located at the edge of the new, smaller Brillouin zone due to the multilayer periodicity. These modes can contribute to the reduction of thermal conductivity by Umklapp processes within the zone‐folded mini‐bands. Color‐coded increase in ultrafast X‐ray scattering in vicinity to the mini‐zone boundary of a germanium multilayer.

Incubation Effect of Pre‐Irradiation on Bubble Formation and Ablation in Laser Ablation in Liquids

“…The bubble size shows almost no incubation and at the same time an overall larger bubble volume as compared to the data with similar pulse energy but nanosecond duration. We explain this observation by a better energy localisation with picosecond pulses at the interface, considering the heat dissipation time scale of some nanoseconds [33] and on the other hand a lower efficiency of plasma heating by the trailing part of the laser pulse at a shorter pulse duration. Therefore, incubation plays a minor role in the case of ps-PLAL.…”

Measurement and analysis of thermal conductivity of isotopically controlled silicon layers by time‐resolved X‐ray scattering

“…Overall, the heat transport in isotope multilayer structures is not only affected by the number and quality of the interfaces but also by the layer ordering . Measurements of similar isotope‐modulated germanium multilayers suggest phonon‐blocking effects beyond the pure acoustic mismatch. Considering a structure of alternating 28 Si and 30 Si layers with individual layer thicknesses that change aperiodically, thermal conductivities below that of 28 Si 50 30 Si 50 alloy are predicted .…”

“…First, the cooling of the Au layer on top of the natural Si epilayer is compared to theoretical predictions with σ as a free fitting parameter. Compared to a Laplace transform approach , this allows for setting and accessing the detailed subsurface temperature profile. Figure demonstrates the attempts to reproduce the cooling of a 25‐nm (red triangles) and 30‐nm (blue circles) thick Au layer.…”

Fluence Threshold Behaviour on Ablation and Bubble Formation in Pulsed Laser Ablation in Liquids

“…We observe ar educed bubble volumew ith faster growth in the range of 30-50 Jcm À2 .I nt he low-fluencer ange bubbles are very small but can still be observed down to 3Jcm À2 .A sd iscussed above, we postulate at hreshold process that accounts for the fast rise of bubble size in the intermediate fluence range.W ith the ansatz from the target-position dependence [Eq. Ac alculation of the heat transfer [41] from a3 0nm-thick heated region inside the silver target produces at hermald ecay time of about 0.14 ns into the silver target (temperature decay to 1/e for short pulses), but ac ooling time towards the water phase of 5ns. Clearly it reproduces the take-off of bubble volume close to the threshold and continues to grow almost linearly until it reaches the experimental data, but with as lightly different slope.…”

“…Due to the differences in thermal conductivity of water and metal,a sw ella ss ubstantial thermal boundaryr esistance at the interface, [40] heat dissipation into the target is much faster than water heating. Ac alculation of the heat transfer [41] from a3 0nm-thick heated region inside the silver target produces at hermald ecay time of about 0.14 ns into the silver target (temperature decay to 1/e for short pulses), but ac ooling time towards the water phase of 5ns. Therefore, dissipation into the metal is faster than the pulse length and prevents boiling or phase explosion occurring on the timescale discussed here.…”