Acoustic phonons make a dominant contribution to thermal transport in insulators and semiconductors, scatter electrons and holes, and participate in the non-radiative carrier recombination processes. Acoustic phonons are important in certain types of the Auger recombination processes where they are needed to satisfy the momentum conservation. A possibility of engineering the acoustic phonon spectrum provides a tuning capability for changing thermal conductivity and electron – phonon interactions . Until recently, the tuning of the phonon spectrum has been associated with the nanostructured materials, where the phonon dispersion undergoes modification due to the periodic or stationary boundary conditions. In this talk, we describe a drastically different approach for changing the acoustic phonon spectrum of the materials, which does not rely on nanostructuring . We change the phonon spectrum in bulk crystalline materials via introduction of a small concentration of dopant atoms that have a substantially different size and mass from those of the host atoms. We report results of Brillouin – Mandelstam spectroscopy (BMS) of transparent Al2O3 crystals with Nd, Cr and other atoms used as substitutional dopants. The ionic radius and atomic mass of Nd atoms are distinctively different from those of the host Al atoms. Our results show that even a small concentration of Nd atoms incorporated into the Al2O3 samples produces a profound change in the acoustic phonon spectrum. The frequency and velocity of the transverse acoustic phonons decrease by ~4 GHz and ~600 m/s, respectively, at the Nd density of only ~0.1 %. In contrast to Nd dopants, both the ionic radius and atomic mass of Cr atoms are closer to those of the host Al atoms. The BMS results show that the phonon group velocity does not significantly change when the substitutional dopants are similar in size and mass to the host atoms. Our findings confirm that even a small concentration of dopants with strongly dissimilar size and mass can result in a profound change in the bulk phonon spectrum. The difference in atomic size can result in the crystal lattice distortion, i.e. increased inter-atomic plane distance associated with the incorporation of larger atoms. The obtained results, demonstrating a possibility of fine-tuning the phonon spectrum in bulk materials, have important implications for a range of electronic and optoelectronic devices.
This work was supported, in part, by the Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences (BES) under Award # SC0012670. AAB acknowledges support from the Defense Advanced Research Projects Agency (DARPA) project W911NF18-1-0041 Phonon Engineered Materials for Fine-Tuning the G-R Center and Auger Recombination. JEG acknowledges support from the High Energy Laser - Joint Technology Office (HEL-JTO) administered by the Army Research Office for development of over-equilibrium doped alumina.
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 F. Kargar, E.H. Penilla, E. Aytan, J.S. Lewis, J.E. Garay, A.A. Balandin, “Acoustic phonon spectrum engineering in bulk crystals via incorporation of dopant atoms,” Applied Physics Letters, 112, 191902 (2018).