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

Nanostructuring is considered to be an efficient way to tailor phonon scattering and to reduce the thermal conductivity while keeping good electronic properties. This can be ideally realized by mass modulation of chemical identical elements. In this work, we report measurements of the crossplane thermal conductivity of isotopically modulated Si-28/Si-30 multilayer structures and of isotopically pure Si-28 layers by means of time-resolved X-ray scattering. Compared to earlier investigations, an improved measurement technique has been applied to determine the cooling behavior of a top gold metal layer after laser excitation with picosecond time resolution until thermal equilibration is established. Detailed analysis of the cooling behavior not only confirms a reduced thermal conductivity of Si-28/Si-30 multilayer structures compared to natural and isotopically enriched Si-28 layers but also provides evidence of direct laser heating of the Si layer. This and extrinsic effects affecting the cooling behavior of the gold layer are taken into account to determine the thermal conductivity by means of the pump-and-probe measurement technique. (C) 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim