Temperature Dependence Of Raman Scattering In Gesn Films

Temperature dependence of Raman scattering from Ge(0.95)Sn(0.05)and Ge(0.92)Sn(0.08)films, grown on Ge (001) substrates by low temperature molecular beam epitaxy, was studied over the temperature range of 90 to 850 K. Nonlinear temperature dependencies in the Raman shift of Ge-Ge and Ge-Sn modes have been observed. Although they could be well described by an empirical formula, much different values were found in first-order temperature coefficient of the Raman shift for the Ge-Ge mode in bulk Ge, the Ge0.95Sn0.05, and Ge(0.92)Sn(0.08)films, being 1.7, 2.4, and 2.7 x 10(-2)cm(-1)/K, respectively. Two factors, phonon-phonon coupling and thermal expansion, contribute to the temperature dependence of Raman shift. Detailed analysis shows that the thermal expansion contribution increases slightly with the Sn content, whereas the phonon-phonon coupling contribution increases markedly with the Sn content. In other words, the anharmonic decay process is much enhanced by the alloy perturbation in GeSn alloys. In addition, an abrupt change was observed at temperature of 650 K in Raman shift and linewidth of Ge-Ge mode for the Ge(0.92)Sn(0.08)film, which is caused by Sn segregation, concomitant lattice relaxation, and crystallization as well.