Effect of disorder on the Raman scattering of CdS x Se 1-x ﬁlms deposited by laser ablation S. Pagliara a , L. Sangaletti a, * , L.E. Depero b , V. Capozzi c , G. Perna c a Istituto Nazionale per la Fisica della Materia and Dipartimento di Matematica e Fisica, Universita ` Cattolica, Via Musei 41, 25121 Brescia, Italy b Istituto Nazionale per la Fisica della Materia, and Dipartimento di Ingegneria Meccanica, Universita ` di Brescia, Via Branze 38, 25123 Brescia, Italy c Istituto Nazionale per la Fisica della Materia and Dipartimento di Fisica dell’Universita ` di Bari, Via Amendola 173, 70126 Bari, Italy Received 15 March 2000; accepted 15 June 2000 by C. Calandra Abstract We present micro-Raman spectra at room temperature of CdS x Se 1-x ternary ﬁlms deposited on Si(111)-oriented substrates by means of the laser ablation technique. The lineshape of the LO phonon structure is characterized by signiﬁcant asymmetry and broadening induced by disorder in the alloy. The asymmetric line proﬁle can be described as a composite of two phonon modes, one ascribed to zone-center and the other to zone-edge phonons. Attempts to ﬁt the experimental data to the spatial correlation model result in a poor agreement. We discuss the possibility to include into the lineshape analysis the contribution from the disorder-activated phonon density of states.  2000 Elsevier Science Ltd. All rights reserved. Keywords: A. Semiconductors; D. Phonon; D. Optical properties PACS: 63.20.Dj; 63.50. + x; 78.30.Fs 1. Introduction Despite the technological importance of the II–VI alloy semiconductors, many microscopic vibrational properties of these systems are not well understood yet. In particular, the potential ﬂuctuations related to the compositional disorder on a microscopic scale are a characteristic aspect of substi- tutional semiconductor alloys [1]. Since Raman scattering can yield important information about structural order in the lattice on a scale of a few lattice constants, it can be used to study the microstructural properties of the alloy. This is the motivation for the large efforts that have been made to attain theoretical models, based upon Raman scattering para- meters (frequency shift, intensity, lineshape, and linewidth), describing the effects of disorder. A well-known model, largely used to explain the asym- metric lineshape in the Raman spectra, is the spatial correla- tion model (SCM) [2–7]. The basis of SCM is the breakdown of the usual q  0 (momentum) selection rule for the ﬁrst-order Raman scattering due to the destruction of the translational invariance as a consequence of structural disorder. When the q  0 conservation rule is relaxed, modes away from the Brillouin-zone center are allowed to contribute to light scattering. This results in broadening and asymmetry of the Raman lineshape. In particular, the SCM allows us to evaluate the spatial correlation length L, that is the spatial extent of phonon mode correlation function. In fact, in a disordered lattice the mode correlation function of the phonons becomes ﬁnite in extent, whereas in an ideal lattice it is inﬁnite. Namely, L is a spatial dimension into which the phonon is conﬁned: therefore, the size of L can be viewed as the average size of the ordered domains in the system [8]. In order to study the microscopic nature of disorder in the alloy, we carried out micro-Raman measurements of two types of CdS x Se 1-x (x  0:4 and 0.6) ﬁlms. We recently investigated the structural and optical properties of these ﬁlms, showing that the CdS x Se 1-x epilayers grow oriented along the (002)-direction in the hexagonal phase and are of good quality [9]. CdS x Se 1-x is classiﬁed in the so-called Solid State Communications 116 (2000) 115–119 0038-1098/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S0038-1098(00)00278-7 PERGAMON www.elsevier.com/locate/ssc * Corresponding author. Tel.: +39-030-2406-709; fax: +39-030- 2406-742. E-mail address: sangalet@dmf.bs.unicatt.it (L. Sangaletti).