Isotope Effects on the Optical Phonons of Cubic Boron Phosphide

Abstract

The ratio of boron isotopes (10B and 11B) affects the thermal conductivity of cubic borides because of the phonon scattering. There are two signals appearing in the Raman spectra of cubic boron phosphide (c‐BP) single crystals with natural boron isotope abundance. One is a sharp peak with high intensity and small linewidth, which has been identified as a TO‐LO‐mixed mode; the other is a small peak in the low‐frequency side with quite low intensity and large linewidth, which is difficult to be observed in isotope‐purified crystals and with an unclear origin. Herein, in order to clarify the isotope effects on optical phonons and to identify the origin of the small peak in c‐BP, the Raman spectra of isotope‐tailored c‐BP single crystals in the range from nearly pure c‐10BP to nearly pure c‐11BP are analyzed in detail. Both the frequency and linewidth of the TO‐LO‐mixed mode are closely related to the boron isotope ratio under the isotopic average mass effect and mass disorder effect. The small Raman peak is also isotope dependent. It originates from LO phonon scattering at the non‐origin of the Brillouin zone under the isotopic mass disorder effect.