Apparent isotropic thermal conductivity of Si (circles), Si:B (squares) and Si0.99Ge0.01 (down triangles) for w0=5, 10 and 25 μm and f=9.8 MHz, as a function of temperature. For Si0.99Ge0.01, ΛA(T) at 1.1 MHz is also shown (up triangles). Solid lines are literature values for the thermal conductivity of bulk Si and a 3-μm thick thin film of Si:B with a 1 × 1019 cm−3 doping level21,31. Dashed lines are the prediction of our RTA model for Si0.99Ge0.01, for which there are no prior low-temperature thermal conductivity measurements. The spot size used to measure ΛA is indicated by colour, red for 25 μm, black for 10 μm and blue for 5μm.