Characterization of the In-plane Thermal Conductivity of Sub-10 nm Ir Films on a Flexible Substrate

Abstract

The in-plane thermal conductivity (k) of ultrathin films is of great scientific and engineering importance as the ultrafine thickness will cause remarkable energy carrier scattering. However, the in-plane k is extremely difficult to measure as the in-plane heat conduction is highly overshadowed by the substrate. To date, very rare experimental data and understanding have been reported. Here we report an advanced differential transient electro-thermal (TET) technique to characterize the in-plane k of supported nm-thin Iridium films down to < 2 nm thickness. The ultrathin (500 nm) organic substrate and its low k makes it possible to distinguish the in-plane k of the film with high confidence. The radiation effect is rigorously treated and subtracted from the measured k. Also measurements under different temperature rise levels allow us to determine the k at the zero temperature rise limit. All these physics treatments lead to high accuracy determination of the in-plane k, and understanding of the strong structural effects. The k of ultrathin Ir films supported on polyethylene terephthalate is determined to be 11.7 W·m⁻¹·K⁻¹, 20.1 W·m⁻¹·K⁻¹, 23.5 W·m⁻¹·K⁻¹, and 34.3 W·m⁻¹·K⁻¹ for thicknesses of 1.83 nm, 3.11 nm, 5.86 nm, and 9.16 nm, respectively. This is more than one order of magnitude reduction from the bulk’s k of 147 W·m⁻¹·K⁻¹. The film’s electrical conductivity is found to have more than two orders of magnitude reduction from that of bulk Ir (1.96 × 10⁷ Ω⁻¹·m⁻¹). The Lorenz number of the studied Ir films increases significantly with decreased film thickness, and is upto 14-fold higher (3.97 × 10^–7 W·Ω·K⁻²) than that of bulk Ir (2.54 × 10^–8 W·Ω·K⁻²). It underscores the significant and deviated influence of structure and film dimension on heat and electrical conductions and provides invaluable knowledge for future applications in nanoelectronics.