The origin of a peak in the reststrahlen region of SiC J.A.A. Engelbrecht a,n , I.J. van Rooyen a , A. Henry b , E. Janze ´n b , E.J. Olivier a a Physics Department, Nelson Mandela Metropolitan University, PO Box 77000, Port Elizabeth 6031, South Africa b Department of Physics, Chemistry and Biology, Link¨ oping University, SE-581 83 Link¨ oping, Sweden article info Available online 24 September 2011 Keywords: Infrared reflectance SiC Reststrahlen region Anomalous peak abstract A peak in the reststrahlen region of SiC is analyzed in order to establish the origin of this peak. The peak can be associated with a thin damaged layer on the SiC wafers, and a relation is found between surface roughness and the height of this peak, by modeling the damaged layer as an additional layer when simulating the reflectivity from the wafers. & 2011 Elsevier B.V. All rights reserved. 1. Introduction It is beneficial to investigate the reststrahlen region of the spectrum of a material, as much information about the material can be obtained from such an investigation [1–6]. In the case of SiC, the reststrahlen band is located in the infrared region between about 600–1200 cm -1 . The presence of additional features in this range of the SiC reststrahlen band has been observed by some researchers [7–26]. Various reasons were proposed for the observed changes, ranging from the influence of free carrier damping, increased ion dose (implanted specimens), roughness of interfaces (for layer on substrate), stress relaxation in layers, porous material and Fr ¨ olich modes. A similar peak was also observed in the case of AlN wafers, where the peak was ascribed to being related to A 2 O 3 [27]. Theoretical models have also been suggested to explain certain features observed in optical spectra [26,28–30]. However, only a few researchers referred to the surface condition/roughness of samples when presenting their results [7,9,20,24–26]. It is the purpose of this paper to propose a possible explana- tion for the additional peak near the reststrahlen band of SiC. 2. Theory The infrared reflectance of a semiconductor material can be simulated by making use of the complex dielectric function e, based on the work by Holm et al. [31]. The complex dielectric function is given by e ¼ e 1 1 þ o 2 L o 2 T o 2 T o 2 þ ioG  o 2 p oðoigÞ " # ¼ðn2ikÞ 2 ¼ e 0 þ ie 00 ð1Þ where o 2 p ¼ 4pNe 2 =m n e 1 and g ¼ e=m n m, o p is the plasma reso- nance frequency, e N the high frequency dielectric constant, g the free-carrier damping constant, o T the transverse phonon fre- quency, o L the longitudinal phonon frequency, G the phonon damping constant, N the free carrier density, e the electron charge, m * the effective mass and m the mobility of carriers. The reflectance R, normally expressed for the bulk as R ¼ ðn1Þ 2 þ k 2 ðn þ 1Þ 2 þ k 2 ð2Þ can thus be expressed in terms of the complex dielectric function as R ¼ ffiffi e p 1 ffiffi e p þ 1   2 ð3Þ The dielectric parameters contained in Eq. (1) can thus be extracted by obtaining a best fit between an experimentally obtained reflectance and the theoretically simulated spectra. In the current study, it was hypothesized that the additional peak observed in the reststrahlen region was due to the effect of a surface layer. The expressions for a layer on a substrate formu- lated by Heavens [32] were hence used as basis to expand the equations of Holm et al. [31], enabling the dielectric parameters of both layer and substrate to be obtained from the reflectance spectrum. In effect, the refractive indices n i and extinction coefficients k i (i ¼ layer or substrate) are expressed and calculated in terms of the relevant complex dielectric functions: n i ¼½½fðe 0 2 þ e 002 Þ 1=2 þ e 0 g 1=2 ð4Þ k i ¼½½fðe 0 2 þ e 002 Þ 1=2 e 0 g 1=2 ¼ e 00 =2n ð5Þ These are subsequently employed in the formulae of Heavens. It should be noted that the layer-substrate system is analyzed assuming uniform layers with an abrupt interface, while any interdiffusion that may have occurred is ignored. Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/physb Physica B 0921-4526/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2011.09.077 n Corresponding author. Tel.: þ27 41 504 2186; fax: þ27 41 504 2573 E-mail address: Japie.Engelbrecht@nmmu.ac.za (J.A.A. Engelbrecht). Physica B 407 (2012) 1525–1528