Second-order Raman scattering in II-VI semiconductors: Relative intensities and trends

Multiphonon probe in ZnS quantum dots

“…2, can be understood under off‐resonance conditions. Low value of DP, reported for 1‐TO mode of cubic ZnS (25 eV),5 may be correlated with the disappearance of 1‐TO mode under off‐resonance conditions for clusters below the confinement size. However, a strong DP for two‐phonon process (2470 eV)5 of cubic ZnS (two orders of magnitude higher than that of single‐phonon process) along with the k ‐independence of electron–phonon interaction for 0‐D system4 are made responsible for the increase in the intensity of 2‐TO mode (∼573 cm −1 ) even under off‐resonance conditions.…”

“…Low value of DP, reported for 1‐TO mode of cubic ZnS (25 eV),5 may be correlated with the disappearance of 1‐TO mode under off‐resonance conditions for clusters below the confinement size. However, a strong DP for two‐phonon process (2470 eV)5 of cubic ZnS (two orders of magnitude higher than that of single‐phonon process) along with the k ‐independence of electron–phonon interaction for 0‐D system4 are made responsible for the increase in the intensity of 2‐TO mode (∼573 cm −1 ) even under off‐resonance conditions. In the absence of detailed phonon density of states specific to nanoclusters, it may be specifically noted that the DP values with no exception to higher order processes can only increase with decreasing cluster size and other possible surface disorder 14.…”

“…In the present study, we examine the validity of selection rules in 0‐D systems away from resonance, as discussed above, for Raman spectra of ZnS quantum dots. To understand scattering mechanism related to DP at reduced dimension, ZnS is chosen for having large DP value at least in the two‐phonon process 5. The band‐gap of bulk cubic ZnS is ∼337 nm (3.68 eV) and the Bohr exciton radius is ∼1.25 nm 6.…”

Coherent phonon oscillations excited by second-order Raman scattering in CdZnTe multiple quantum wells and cubic ZnTe

“…It is well known that the strong peak near 117 cm Ϫ1 is attributed to the second-order Raman modes such as the difference mode of TO-TA (119 cm Ϫ1 ) 15,16 and/or the overtone mode of 2TA (109 cm Ϫ1 ), coincident to the peak of the calculated density of two-phonon overtones states, at the point X of Brillouin zone. [17][18][19] As the concentration of substitutional impurity atoms is increased in the mixed crystal, the density of states with the dominant peak being identified as 2TA is enhanced due to the relaxed zone-center selection rule, at which they may exhibit one-phonon Raman scattering due to acoustic and optical phonons with all possible q vectors spanning the Brillouin zone. 15 This strongly suggests that the 120-cm Ϫ1 mode observed in the ZnCdTe MQW sample is from the two-phonon mode of the ZnCdTe well enhanced by the alloy fluctuation, and the LO phonon mode is not seen because the overall sample thickness is much smaller than the bulk ZnTe.…”

“…In general, the ⌫ 1 component appears most strongly, while the ⌫ 15 component is somewhat weaker and ⌫ 12 component negligible in all tetrahedral semiconductors. 19 Defining the pump-polarization angle from the ͓1 10͔ axis of ZnTe, the phonon amplitude Q(t) of the two-phonon mode is represented as follows:…”