Atomistic Origin of the Thermally Driven Softening of Raman Optical Phonons in Group III Nitrides

It has long been puzzling regarding the physical origin of the thermally induced red-shift of Raman optical phonons in group III nitride crystals despite some possible mechanisms such as phonon decay, thermal expansion, interfacial effects, or their combinations. Here we show that an extension of the recent approach [Sun, C. Q.; Pan, L. K.; Li, C. M.; Li, S. Phys. Rev. B 2005, 72, 134301] to the functional dependence of the frequency of Raman optical modes on the atomic bonding identities (bond length, bond strength, and atomic coordination numbers) to the temperature domain has enabled us to gain a simple solution with improved understanding of the thermally induced red-shift of Raman optical phonons. Reproduction of the measured temperature dependence of the Raman shift of AlN, GaN, and InN reveals that the thermally driven red-shift arises simply from bond vibration and bond expansion, together with derived information about the mode cohesive energy.