Measurement of the residual macro and microstrain in strained Si/SiGe using Raman spectroscopy
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Measurement of the residual macro and microstrain in strained Si/SiGe using Raman spectroscopy P. Dobrosz*, S.J. Bull*, S.H. Olsen** and A.G. O'Neill** *School of Chemical Engineering and Advanced Materials, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK **School of Electrical, Electronic and Computer Engineering, University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK ABSTRACT The use of laser Raman spectroscopy to assess the residual strain in strained silicon/silicon germanium devices is well established. The peak shift associated with the 520cm-1 silicon peak can be used to directly measure the strain in the cap layer provided that the strained silicon peak can be deconvoluted from the more intense Si in SiGe peak which occurs at slightly lower wavenumbers. However, though peak position gives a measure of the macrostrains in the layer it is not useful for the assessment of microstrains associated with point defects which may also influence device performance; such microstrains influence the intensity of the Raman peaks and can, in principle, be monitored by this method. In this study we have undertaken a study of peak shape as a function of processing conditions for strained silicon on SiGe. Changes in peak position may be correlated with macrostrains and macrostrain relaxation around extended defects such as dislocations. Changes in peak width can be correlated with processes which lead to changes in composition (e.g. germanium build-up in the surface after etching) and microstrain. Such changes are not necessarily correlated with changes in macrostrain but indicate that microstrain could also be an important factor influencing device performance. INTRODUCTION Recent research shows that strained Si/SiGe heterostructures offer a very attractive platform for building high performance electronic devices. Theoretical studies have suggested that the optimum performance of strained Si/SiGe MOSFETs is achieved by the use of a strained Si surface electron channel and a buried hole channel of compressively strained SiGe [1]. Electron mobility is enhanced in tensile strained Si due to modifications to the electronic band structure induced by strain. Using a strained Si layer for n-MOSFET channels can increase device performance [2]. The strain is caused by the lattice mismatch of Si and Ge; the 4.2 % difference in atomic spacing between Si and Ge dictates that pseudomorphic growth of SiGe on bulk Si substrate will lead to compressive strain which enhances hole mobility [3]. In order to derive the greatest benefits from such devices it is essential to accurately know the strain in the SiGe and silicon layers. This is particular important for MOS technology which requires several high-temperature processing stages and can lead to strain relaxation [4]. In the present experiments Raman spectroscopy was used as a contactless technique providing information about strain and strain relaxation in Si/SiGe films. Since the first report of Anastasskis et al. [5] on the sensitivity of Raman peak
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