Phonon properties and unconventional heat transfer in quasi-2D $Bi_2O_2Se$ crystal

Abstract

Bi2O2Se belongs to a group of quasi-2D semiconductors that can replace silicon in future high-speed/low-power electronics. However, the correlation between crystal/band structure and other physical properties still eludes understanding: carrier mobility increases non-intuitively with carrier concentration; the observed $T^2$ temperature dependence of resistivity lacks explanation. Moreover, a very high relative out-of-plane permittivity of about 150 has been reported in the literature. A proper explanation for such a high permittivity is still lacking. We have performed infrared (IR) reflectivity and Raman scattering experiments on a large perfect single crystal with defined mosaicity, carrier concentration and mobility. Five of the eight phonons allowed by factor group theory have been observed and their symmetries determined. The IR spectra show that the permittivity measured in the tetragonal plane is as high as ${\epsilon}_r{\approx}500$, and this high value is due to a strong polar phonon with a low frequency of ~34 $cm^{-1}$ (~1 THz). Such an unusually high permittivity allows the screening of charge defects, leading to the observation of high electron mobility at low temperatures. It also allows effective modulation doping providing a platform for high performance 2D electronics. DFT calculations suggest the existence of a very low frequency acoustic phonon ~14 $cm^{-1}$ (~0.4 THz). Both the low frequency phonons cause anomalous phonon DOS, which is reflected in the unconventional temperature dependence of the heat capacity, $c_M{\approx}T^{3.5}$. The temperature-dependent, two-component group velocity is proposed to explains the unusual temperature dependence of the thermal conductivity, ${\kappa}{\approx}T^{1.5}$