Quantitative Analysis of Raman Spectra in Si/SiGe Nanostructures
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Quantitative Analysis of Raman Spectra in Si/SiGe Nanostructures Selina Mala1, Leonid Tsybeskov1, Jean-Marc Baribeau2, Xiaohua Wu2, David J. Lockwood2 1 Department of Electrical and Computer Engineering, New Jersey Institute of Technology, Newark, NJ 07102, U.S.A. 2 Institute for Microstructural Sciences, National Research Council, Ottawa, Ontario, Canada.
ABSTRACT We present comprehensive quantitative analysis of Raman spectra in two-(Si/SiGe superlattices) and three-(Si/SiGe cluster multilayers) dimensional nanostructures. We find that the Raman spectra baseline is due to the sample surface imperfection and instrumental response associated with the stray light. The Raman signal intensity is analyzed, and Ge composition is calculated and compared with the experimental data. The local sample temperature and thermal conductivity are calculated, and the spectrum of longitudinal acoustic phonons is explained. INTRODUCTION
Despite significant lattice mismatch, Si/SiGe nanostructures (NSs) found many applications in electronics and optoelectronics, mainly due to their compatibility with Si-based complementary metal-oxide semiconductor (CMOS) technology [1] and possible applications in CMOS compatible photonics [2]. Therefore, strain, heat management, and thermal properties of SiGe NSs attract enormous attention [3]. Various aspects of strain relaxation, interface abruptness, intermixing, and thermal conductivity of Si/SiGe NSs can be analyzed using Raman scattering, a powerful and nondestructive characterization technique. In this paper, we report the quantitative analysis for Si/SiGe NSs over a wide range of Raman scatterings spectra. Typically, the Rayleigh scattered light is 4 to 6 orders of magnitude stronger than the weaker Raman scattered light [4]. Imperfections on the sample surface could play a major role in the creation of the background in Raman scattering. The correction of the measured Raman spectra can be performed after proper determination and fitting of the corresponding baseline. The Raman peak intensity ratio of different phonon modes carries information which can be used for the determination of the Ge content in Si/SiGe NSs. Several authors have reported Raman scattering in SiGe alloys [5, 6] grown by liquid-phase epitaxy [7], chemical vapor deposition (CVD) [8], low-energy plasma enhanced CVD [9], as well as in Si/SiGe superlattices (SLs) and Si/Ge quantum dots fabricated by molecular beam epitaxy (MBE) [10-15]. We show that relatively simple experimental procedures can be used to estimate the average value of Ge content x in a variety of Si/Si1-xGex NS samples: partially-relaxed SiGe alloy layers, twodimensional (planar) Si/Si1-xGex SLs, and three-dimensional (3D) Si/Si1-xGex cluster multilayers grown by MBE and CVD techniques. In this work, we also demonstrate how temperature affects the Raman measurements in Si/SiGe NSs. Temperature can be measured from the Stokes/antiStokes Raman peak intensity ratio, Raman peak frequency shift, and the Raman spectrum full
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