Low thermal conductivity and thermal boundary conductance in BiOI crystals and films grown at low temperature

We present measurements of thermal conductivity and thermal boundary conductance (TBC) in the layered crystal BiOI. These measurements are applied to bulk crystals and crystalline films grown at low temperature (623 K) using chemical vapor methods. Such low-temperature growth was enabled by the growth kinetics of halogens in a reduced oxygen environment. The temperature-dependent Raman spectra showed red shifts of −8 × 10−3 and −12 × 10−3 cm−1 K−1for the characteristic A1g peak of the bulk and film samples, respectively. Most peaks also showed broadening with increased temperature, which indicated that phonon lifetimes are limited by anharmonic scattering. Laser heating measurements were combined with thermal models based on the finite-element method to estimate a directionally averaged thermal conductivity of 1.1 W m−1 K−1 in the bulk crystal. This low thermal conductivity was investigated by computing the phonon dispersion from first principles, which revealed extreme hybridization between acoustic and optical modes that is uncommon for layered crystals. The TBC of an exfoliated film and a grown film were measured to be 17 × 106 and 10 × 106 W m−2 K−1, which are at the lower end of solid–solid interfaces at room temperature. These measurements are valuable for applications in thermal isolation and understanding the thermal limits of BiOI electron devices given its semiconducting nature and low-temperature growth that is compatible with three-dimensional integration of electronics.