We extend to longitudinal-optical (LO) phonons the percolation model set for the basic understanding of the atypical transverse-optical (TO) one-bond→ two-mode behavior observed by Raman scattering in the Be-Se spectral range of the random Zn 1−x Be x Se alloy ͑0 ഛ x ഛ 1͒, which opens the class of mixed crystals with contrast in the bond stiffness. The study is supported by contour modeling of the TO and LO Raman line shapes. This is achieved via application of the Hon and Faust treatment to a version of the modified-randomelement-isodisplacement model generalized to multioscillators. While the TO signal clearly discriminates between Be-Se vibrations within the hard Be-rich region and the soft Zn-rich one, complexity arises in the LO symmetry due to vibration coupling via the 1ong-range longitudinal polarization field. In particular this generates a massive transfer of oscillator strength from the low-frequency ͑LO − ͒ (hard, soft)-mixed mode to the high-frequency ͑LO + ͒ one, which results in an apparent LO + single-mode behavior. Moreover the contrasts between the Zn-Se and Be-Se bond lengths and bond stiffness are proposed to force a Verleur and Barker-like (VB) discrete multimode Raman response from each region. Accordingly LO − and LO + intramode transfers of oscillator strength superimpose to the LO − → LO + intermode one. This accounts for the spectacular distortions of the LO + line shape. On the whole, the puzzling LO behavior can be regarded as the result of a cooperative phenomenon between two discrete assemblies of polar LO phonons, driven by the long-range longitudinal polarization field. Also, the Verleur and Barker description accounts for subtle unexplained behaviors in the TO symmetry. More generally it appears to provide a much attractive area for the discussion of the asymmetries of the TO and LO Raman line shapes in random alloys, as a possible alternative to the much debated spatial correlation model or to internal/external strain effects.

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Long wavelength longitudinal optical (LO) and transverse optical (TO) phonons of BeSe and ZnxBe1−xSe layers are identified in a wide composition range by using Raman spectroscopy. A two-mode behavior is clearly evidenced. As predicted by the dielectric model of Hon and Faust, the eigenfrequencies of the BeSe- and ZnSe-like LO modes correspond to the maxima of Im〈−ε(ω, x)−1〉. Excellent agreement is obtained with a model where the calculations are performed by using the equations of motion and polarization derived from the modified random element isodisplacement model. Besides, the TO and LO frequencies for BeSe are determined to be 501 and 579 cm−1, respectively.

We present an experimental and theoretical investigation of the Raman line shape of long-wavelength phonons with longitudinal optical (LO) symmetry from both sides of Zn1−xBexSe/GaAs (001) heterojunctions with special emphasis on samples with low Be content (x⩽0.20). First the built-in p-type LO phonon–plasmon (LO–P) coupled mode at the near-interfacial substrate is used as a sensitive probe to investigate the interfacial quality. The corresponding hole gas is reinforced when the ZnBeSe layers are nominally p doped by nitrogen. This provides clear evidence for effective hole transfer across the junction, and thereby indicates a minimized density of interfacial defects. In the nitrogen-doped layers hole densities as high as 1017 cm−3, in accordance with capacitance–voltage measurements, are directly inferred from clear weakening of the ZnSe-like LO mode due to LO–P coupling. Concerning the intrinsic properties of the alloys, we demonstrate that the asymmetric broadening of the ZnSe-like LO mode is determined by topological disorder only, and not by other possible mechanisms such as structural disorder, nonhomogeneity in the alloy composition, a distribution of tensile strain, or a Fano-type interference. This reveals a high structural quality that parallels the high interfacial quality. We also show that a spatial correlation model with Gaussian distribution applies to Zn1−xBexSe. Most of this study is supported by a quantitative treatment. We extend the phenomenological approach of Hon and Faust to equations of motion and polarization derived from the modified-random-element-isodisplacement model in order to achieve line shape analysis of the alloy-related LO and LO–P modes.

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