Dispersive Phonon Linewidths: TheE2Phonons of ZnO

Abstract

Phonon linewidths can exhibit a large variation when either pressure or isotopic masses are changed. These effects yield detailed information about the mechanisms responsible for linewidths and lifetimes, e.g., anharmonicity or isotopic disorder. We report Raman measurements of the linewidth of the upper E2 phonons of ZnO crystals with several isotopic compositions and their dependence on pressure. Changes by a factor of 12 are observed at a given temperature. Comparison with calculated densities of one-phonon states, responsible for isotope scattering, and of two-phonon states, responsible for anharmonic decay, yields a consistent picture of these phenomena. Isotopic disorder broadening by 7 cm(-1) is found in samples with mixed 16O-18O content, whereas the anharmonic processes involve decay into sums and differences of two phonons.

Identifying the influence of the intrinsic defects in Gd-doped ZnO thin-films

“…Therefore, we assume that E 2 (high) depends on oxygen concentration and related defects. 51 This assertion is supported by the PL measurements. Figure 5(b) shows the effect of P d , where a slight shift of E 2 (high) peak to the lower frequencies for samples deposited at 50 mTorr compared to that deposited at 5 and 100 mTorr can be noted.…”

“…The possible mechanisms that can be responsible for such a shift are (i) the biaxial strain, 48 (ii) phonon localization, 49,50 and (iii) laserinduced heating in materials and tensile strain during Raman measurements. 49,50 As E 2 (high) peak is expected to shift to the higher frequency value as the compressive biaxial stress increases (and c-parameter value decreases), we assume the unusual shift of E 2 (high) peak to lower frequencies as c-parameter decreases in our samples can be due the second assumption, which is phonon localization by defects, such as those related to oxygen deficiency, 51 zinc excess or surface impurities, are in line with the PL and XRD measurements, suggesting that defects related to oxygen deficiency such as V O accompanied Gd dopants. Furthermore, we excluded the explanation related to laser-induced heating in materials and tensile strain during Raman as all samples were measured at the same time using the same conditions, and the laser power on the samples was $0.07 mW (to avoid the heating effect).…”

Isotopic effects on phonon anharmonicity in layered van der Waals crystals: Isotopically pure hexagonal boron nitride

“…4(b), where the broadened E high 2g line due solely to the isotopic-disorder-induced phonon self-energy is plotted. Similar asymmetric line shapes associated with ridgelike structures in the phonon density of states have been reported in other semiconductors [26][27][28][29].…”

Growth of isotopically enriched ZnO nanorods of excellent optical quality

“…The changing FWHM for different Zn masses results from a change in the overlap of the two-phonon density of states due to changes in phonon frequency. Ab initio calculations performed by Serrano et al [22] show that the E 2 high phonon mode is near a sharp 'ridge' in the two-phonon density of states, the interaction with which results in the variation elsewhere and demonstrates the ability to grow isotopically enriched ZnO materials with excellent optical quality using the straightforward carbothermal reduction VPT method, with mg quantities of source material. [36] Note that the PL emission is associated with the VPTgrown nanorods and there is not a significant emission from the underlying buffer layer.…”

“…[14][15][16][17][18][19] Reports using isotopically enriched ZnO samples have included studies of bandgap energies, phonon positions and linewidths and heat capacity, but in all cases using bulk single crystal samples with quite poor optical quality. [20][21][22][23] In this work, we report a relatively fast, easy and reliable method of producing Zn-isotopically enriched ZnO nanorods of very high structural quality, as well as excellent optical quality as determined by low temperature photoluminescence (PL) studies, requiring small quantities of source materials. 64 Zn, 66 Zn and 68 Zn, as well as samples with engineered isotopic abundances.…”