Temperature dependence of exciton–phonon coupling and phonon anharmonicity in ZnTe thin films

Abstract

AbstractIn ZnTe, longitudinal optical (LO) phonons are strongly coupled with free exciton via long‐range order Froehlich interactions. In this study, we ex situ and in situ monitored the effect of annealing temperature on exciton–phonon (e‐ph) coupling in vacuum evaporated ZnTe thin films by resonance Raman scattering experiment. The 350°C air‐annealed thin film exhibited strong e‐ph coupling and nLO phonons up to sixth order were observed in the Raman spectrum. Moreover, a series of (n − 1)LO + TO and (n + 1)LO‐TO combination phonon modes appeared at low and high wavenumber shoulders of the nLO phonon modes, respectively. The phenomenon of e‐ph coupling was in situ monitored by temperature‐dependent Raman scattering experiment for 350°C air‐annealed thin film. During the in situ measurements, the energy of free exciton was decreased and the condition of outgoing resonance was achieved with nLO phonon. The observation of strong overtones phonon scattering and combination phonon modes allowed us to study anharmonic phonon decay of the zone center nLO phonons. The decay of the fundamental 1LO and its overtone 2LO phonon was mainly dominated by three‐phonon process. For higher order LO phonons with n ≥ 3, the four‐phonon process contribution was increased in the decay process with increasing order.