Anomalous LO Phonon Lifetime in AlAs

Abstract

The temperature dependence of the frequencies and linewidths of the Raman-active longitudinal optical (LO) phonons in GaAs and AlAs have been measured. The low-temperature lifetime of the LO phonon in AlAs is found to be 9.7 ps, very close to the corresponding GaAs value of 9.5 ps. This contradicts early theoretical predictions. The agreement between theory and experiment can be restored when the accidental degeneracy between the AlAs LO phonon frequency and a feature in the two-phonon density of states is taken into account. DOI: 10.1103/PhysRevLett.88.215502 PACS numbers: 63.20.Kr, 78.30.Fs The lifetime of the Raman-active longitudinal optical (LO) phonon in III-V semiconductor compounds is of critical importance for the coupled dynamics of carriers and phonons in these materials. Accordingly, an intense effort has been devoted to the study of this quantity for the past 20 years [1]. The recent availability of ab initio predictions of phonon lifetimes [2-5] represents a significant breakthrough, since they pinpoint the relevant LO phonon decay channels that may open the door for the engineering of phonon lifetimes.In spite of the technological relevance of the Al-Ga-As system, the lifetime t of LO phonons in AlAs has not been reported thus far. From time-domain measurements on an Al 0.3 Ga 0.7 As alloy sample, Tsen and Morkoç find t 7 6 1 ps (at 10 K) for both the GaAs-like and AlAs-like LO vibrations [6]. These results are quite puzzling since, as pointed out by Kash and Tsang [1], the phonon dispersion curves for GaAs and AlAs (which determine their possible LO decay channels) are quite different. A comparison of these curves for several semiconductors suggests that t AlAs should be significantly longer than t GaAs . The first ab initio estimates of phonon lifetimes added to the puzzle [3], since they predict t AlAs 12.7 ps . t GaAs 9.2 ps, in agreement with Kash and Tsang's qualitative argument. The reliability of the theoretical prediction was underscored by the good agreement with the existing experimental data for GaAs [7][8][9][10][11].The above discrepancies between theory and experiment could be dismissed by arguing that the GaAs-like and AlAs-like LO vibrations in AlGaAs alloys are not equivalent to the LO phonons in binary GaAs and AlAs crystals, so that a direct comparison is not legitimate. In this Letter, however, we report Raman measurements of the LO phonon frequencies and lifetimes in GaAs and AlAs. Our results show that the low-temperature lifetimes of LO phonons in GaAs and AlAs are indeed very similar. But we also find that these experimental results do not imply a failure of the ab initio method. Instead, they arise from a breakdown -in the case of AlAs -of the standard formalism used to compare theory with experiment. This breakdown is caused by the accidental degeneracy between the frequency of the AlAs LO phonon and a Van Hove singularity in the two-phonon density of states. Such degeneracies have been proposed in the past -most notably in the case of the transverse-optic phonon in ...

Theoretical investigation of decoherence channels in athermal phonon sensors

“…As expected, we find that the shapes of the distribution of the lifetimes are very similar between GaAs and Si owing to their similar crystal structure and phonon structure (see SI section I for more details). We find that the optical modes (shaded grey in Figure 2a) have relatively short anharmonic decay lifetimes of 10 1 − 10 2 ps, which agrees with previous experiments 58 , and suggest that the anharmonic interactions are the dominant decay process for optical modes in these materials. As the phonon energy decreases into the acoustic regions (shaded red and purple), we find that the lifetimes increase, including extremely large anharmonic lifetimes (> 1 s) for transverse acoustic modes at 7 − 9 meV for GaAs (shaded purple), and 15 − 20 meV for Si.…”

Anharmonic phonon decay in cubic GaN

“…Cowley's theory was successfully applied to analyze phonon decay in several temperature-dependent Raman scattering studies of Si, Ge, α-Sn [10], and InN [13], and has revealed subtle effects in the temperature behavior of Raman scattering of compound semiconductors such as AlAs [28] and ZnO [12,29].…”

Dispersive Phonon Linewidths: TheE2Phonons of ZnO

“…Considerable progress has been made in recent years in the understanding of the mechanisms which determine phonon linewidths [1][2][3]. The relevant experimental database has been obtained mainly by Raman spectroscopy on samples with diverse isotopic compositions and under hydrostatic stress [4].…”

“…The line through the points represents the DOS for sums of two phonons with equal q vectors calculated from the ab initio lattice dynamics [8,13]. Raman phonons, at q ' 0, are known to decay mainly into such combinations [1][2][3][4][5]. The frequency range of Zn , where M Zn is the average mass of the zinc atoms, and is nearly independent of the oxygen mass, a conjecture that was checked by changing the isotopic masses in the ab initio calculation.…”