Enhancement of the c_{11} elastic constant of Ag/Pd superlattice films as determined from longitudinal guided modes

Abstract

In Brillouin-light-scattering studies of a series of Ag/Pd superlattice films we have observed scattering from longitudinal guided modes (LGMs) whose velocities are determined primarily by the en elastic constant. From the measured velocity of the first-order LGM, we have obtained the first accurate determination of CM for metallic thin films. For the Ag/Pd superlattice films, large increases in both the longitudinal (en) and shear (C55) elastic constants were observed as the superlattice modulation wavelength A was decreased below A -60 A.PACS numbers: 62.20. Dc, 62.30.+d, 68.60.Bs, 78.35,+c The elastic properties of metallic superlattice films have been shown to depend on the superlattice modulation wavelength A. 1_9 Dramatic enhancements of up to 500% in the biaxial modulus YB for A ^ 25 A were inferred from bulge tester measurements. 1 ' 2 More recently, Brillouin light scattering (BLS) has been used to study metallic superlattice films. 3 " 8 Changes of up to 20% in the Rayleigh mode velocity have been observed 3,8 as A was varied between 5 and 100 A.The elastic constants of metallic films can be determined from BLS measurements of the Rayleigh and Sezawa 4 " 6 film-guided acoustic-mode velocity dispersion. For sputtered films which have hexagonal symmetry, 8 the velocities of these modes are determined by four of the five independent elastic constants: en, C13, C33, and C55 -c^-For film thicknesses greater than 3000 A, the Rayleigh mode velocity is determined almost entirely by C55. 8 However, it is very difficult to distinguish the contributions of the other three elastic constants to the Sezawa mode velocities. 4 " 6,10 Because of this, in previous BLS studies, 4 " 6 two elastic constants were fixed to their average values and the remaining two elastic constants were obtained by using a least-squares-fitting procedure. Despite combining such fits to the measured mode velocities with comparisons of the measured and calculated scattering intensities, ambiguities in the elastic constant values were obtained. 10 In the present work, we report the first indentification of light scattering from the longitudinal guided modes (LGMs) of metallic films.LGMs are film-guided acoustic modes which have displacements primarily along the propagation direction in the film plane, and velocities that are strongly dependent on the longitudinal elastic constant c\\. In previous BLS studies, 11,12 LGMs were observed for films that were transparent to the light. For transparent films, strong LGM peaks are observed in the BLS spectra since the large penetration depth of the light £ 0 pt produces a large amount of elasto-optic scattering. 13 However, for metallic films, S opt (=150 A) is much smaller than for transparent films, so that the scattering intensity is due almost entirely to the surface ripple mechanism. 13 For semi-infinite metallic films, theLGMs become the longitudinal resonance, 14 there is no surface ripple, and a dip corresponding to the longitudinal resonance should be observed in BLS spectra. 14 However, ...

Anomalous Elastic Properties of Si/Ge Superlattices: The Role of Interfaces

“…The first was the observation of an increase of the C 11 elastic constant when the modulation wavelength was decreased (due to an increase of the roughness normalized to the layer thickness) [45]. In the second, thermal annealing generated an increase of the roughness (leading even to the formation of an alloy) and a parallel increase of C 11 [46].…”

Elastic Characterization of Transparent and Opaque Films, Multilayers and Acoustic Resonators by Surface Brillouin Scattering: A Review

“…Among the systems successfully investigated we recall here periodic superlattices: Mo/Ta [55], Cu/Nb [56] (quasi-periodic [57]), Ag/Pd [58], Fe/Pd [59], Co./Cu [60]. Co./Au [61], Ag/Ni [62], FeNi/Cu and FeNi/Nb [63] and Ta/Al [64] (quasi-periodic [65]).…”

Influence of interfaces on the phonon density of states of nanoscale metallic multilayers: Phonon confinement and localization

“…However, Raman spectroscopy is sensitive to long wavelength phonons only, and cannot be used to determine g(E). This applies also for Brillouin light scattering, which revealed localized phonon modes due to interface-induced modifications of elastic force constants in metallic multilayer structures [29][30][31]. The classical method of inelastic neutron scattering remains a challenge because of insufficient sensitivity.…”