Physica B 340–342 (2003) 416–420 Generalized infrared ellipsometry study of thin epitaxial AlN layers with complex strain behavior V. Darakchieva a, *, M. Schubert b , J. Birch a , A. Kasic a , S. Tungasmita a , T. Paskova a , B. Monemar a a IFM, Link . oping University, S-581 83 Link . oping, Sweden b Fakult . at f . ur Physik and Geowissenschaften, Iniversit . at Leipzig, 04103 Leipzig, Germany Abstract The effect of film thickness on the strain and structural properties of thin epitaxial AlN films has been investigated, andasub-layermodelofthedegreeofstrainandrelateddefectsforallfilmsissuggested.Thevibrationalpropertiesof thefilmshavebeenstudiedbygeneralizedinfraredspectroscopicellipsometry.Theproposedsub-layermodelhasbeen successfullyappliedtotheanalysisoftheellipsometrydatatroughmodelcalculationsoftheinfrareddielectricfunction. r 2003 Elsevier B.V. All rights reserved. PACS: 63.20.Dj; 78.20.Ci; 78.30.Fs Keywords: AlN; Phonons; Strain; Infrared ellipsometry Recently, much attention has been focussed on AlN because of its attractive physical properties. AlN also forms a continuous alloy system with GaN, exhibiting tunable band gaps, which are suitable for optical devices operating from blue to the ultraviolet region [1]. In addition, AlN thin filmshaveappearedtobeoneofthemostsuitable buffer layers for good-quality GaN growth on foreign substrates [2]. Although the material has been extensively studied [Ref. [3] and references therein], there is a lack of a detailed study of the strain effect on the infrared (IR) vibrational properties of thin AlN layers. We have recently reported on the experimental determination of the E 1 ðTOÞ deformation potentials in AlN [3,4]. However, more information about the parameters oftheIRactivemodesisneeded,especiallyforthe purpose of modelling the IR response of com- plex III–V nitride heterostructures, which very often contain a thin AlN buffer layer strained to a different extent [5,6] or/and an AlN-based alloy [7]. In this work, thin epitaxial AlN layers with different thicknesses are studied by high- resolution X-ray diffraction (HRXRD) and gen- eralized IR specroscopic ellisometry (gIRSE). A model of the strain evolution in the films with increasing thickness is proposed and the strain in the films is correlated with the frequencies and broadening parameters of the IR active phonon modes. ARTICLE IN PRESS *Corresponding author. Department of Physics and Mea- surement Technology, Link . oping University, S-581 83 Link- . oping, Sweden. Fax: +46-13-142337. E-mail address: vanya@ifm.liu.se (V. Darakchieva). 0921-4526/$-see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2003.09.059