JOURNAL OF OPTOELECTRONICS AND ADVANCED MATERIALS Vol. 9, No. 1, January 2007, p. 178 - 181 Raman study of ZnSe/SiO x multilayers M. J. ŠĆEPANOVIĆ a* , M. GRUJIĆ-BROJČIN a , I. BINEVA b , D. NESHEVA b , Z. ANEVA b , Z. LEVI b , Z. V. POPOVIĆ a a Center for Solid State Physics and New Materials, Institute of Physics, Pregrevica 118, 11080 Belgrade, Serbia b Institute of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko Chaussee Blvd., 1784 Sofia, Bulgaria Multilayers of ZnSe/SiO x having different ZnSe layer thicknesses (2.0, 3.5, 4.0, 5.0, 7.0 and 10 nm) have been prepared by thermal evaporation in vacuum. Raman scattering measurements have been performed at room temperature using the 442 nm line of a He-Cd laser. Two bands, which appear at about 250 and 500 cm -1 , have been attributed to the 1LO and 2LO modes from “pure” ZnSe in multilayers. Both modes show a large homogeneous broadening. The 1LO Raman mode displays an asymmetric shape and redshift, which can be related to the phonon confinement effect, due to the nanometric size of the ZnSe layers. The size effect on the band shape has been simulated by using a one-dimensional phonon confinement model for ZnSe nanolayers. The comparison of experimental and calculated data also implies the existence of surface phonon modes. Moreover, a change in the intensity of Raman scattering with ZnSe layer thickness points to the resonant enhancement of the Raman bands when the photon energy of the exciting light (2.8 eV) approaches the energy band gap of some samples. (Received November 1, 2006; accepted December 21, 2006) Keywords: ZnSe multilayers, Raman spectroscopy, Phonon confinement, Numeric simulation 1. Introduction Zinc-selenide (ZnSe) based nanostructures have great potential for various optical device applications, due to their nonlinear optical properties. The size-dependent effects on the properties of ZnSe nanostructures play a very important role in these applications. Raman spectroscopy has been recognized as an important technique for the study of the vibrational and structural properties of low-dimensional materials. The properties of the first order longitudinal (1LO) Raman mode in ZnSe nanostructures have been studied earlier [1-4], but the size effects on this mode are still tentative. Some authors did not notice any frequency shift [1,3], while others observed either a red- [2], or a blue-shift [4]. The asymmetric broadening was explained either by the phonon confinement model [1] or by the presence of surface phonon modes [2,3]. In this work, we study reduced space dimension effects on the Raman spectra of ZnSe/SiO x multilayers. The influence of the ZnSe layer thickness on the frequency shift and asymmetrical broadening of the 1LO Raman mode is analyzed by applying an one-dimensional (1D) phonon confinement model. The shoulder between the TO and LO modes is attributed to Raman scattering from surface phonon modes. 2. Experimental details Multilayers (MLs) of ZnSe/SiO x (x~1.7, [5]) were prepared by consecutive thermal evaporation of ZnSe (Merck, Suprapure) and glassy SiO from two independent tantalum crucibles at a vacuum of 10 -5 Torr. Corning 7059 glass substrates maintained at room temperature were used. The film thickness and deposition rate (0.2 nm/s for SiO and 1.5 nm/s for ZnSe) were controlled by two quartz crystal monitors (type MIKI-FFV), calibrated in advance. The thickness of the ZnSe layers was varied between 2.0 and 10 nm, while that of the SiO x layers was between 2.5 and 6.0 nm A step-by-step procedure was applied in the deposition of each layer in the multilayers (ML). This procedure allowed the fabrication of nanocrystalline/amorphous MLs with smooth interfaces and good periodicity [6]. Raman measurements were performed in the backscattering geometry using the 442 nm line of a He-Cd laser, a Jobin-Yvon U1000 monochromator and a photomultiplier as the detector. The measurements were performed in air at room temperature. 3. Experimental results The Raman spectra of ZnSe/SiO x MLs of various thickness are shown in Fig. 1. The spectrum of a 100 nm ZnSe film is also shown. Two bands at ~250 and ~500 cm - 1 are seen, attributed to first order (1LO) and second order (2LO) longitudinal optical scattering from “pure” ZnSe in MLs. The changes in the intensity of the LO modes with the ZnSe layer thickness (Fig. 1) imply the presence of resonant Raman enhancement. The resonant Raman effect appears if the energy of the exciting light is close to that of a stationary electronic transition in the sample. The strongest enhancement is observed when the exciting light approaches the optical band gap of the material [7]. A band gap of ~2.67 eV was obtained for the 100 nm ZnSe thin film, from its PL spectrum shown in Fig. 2. Due to a rather strong PL emission from the Corning glass substrate, we were not