Study of doping in GaAs layers by local probe techniques: micro-Raman, micro-photoluminescence and cathodoluminescence
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Study of doping in GaAs layers by local probe techniques: micro-Raman, micro-photoluminescence and cathodoluminescence Angel M. Ardila1,2, O. Martínez2, M. Avella2, Luis F. Sanz2, J. Jiménez2, B. Gérard3, J. Napierala4 and E. Gil-Lafon4 1 Depto. de Física, Facultad de Ciencias, Universidad Nacional de Colombia, Ciudad Universitaria, Santa Fe de Bogotá, Colombia 2 Física de la Materia Condensada, E.T.S.I.I., Universidad de Valladolid, Valladolid, 47011, Spain 3 THALES, Corporate Research Laboratory, 91404 Orsay Cedex, France 4 LASMEA UMR CNRS 6602, Université Blaise Pascal, Les Cézeaux, 63177 Aubiére Cedex, France ABSTRACT The free carrier concentration of GaAs layers grown by MOCVD either on GaAs or Si substrates, by the conformal method in the last case, was obtained from the micro-Raman spectra using the hydrodynamic approach to fit the LO phonon-plasmon coupled Raman modes. The results on homoepitaxial layers were used as a calibration of the fitting method. The measurements in the selectively doped conformal layers were then compared with data obtained by micro-photoluminescence and cathodoluminescence spectroscopy and imaging. The doping data are compared with those deduced from the room temperature micro-photoluminescence and cathodolumiescence spectra. INTRODUCTION Nondestructive optical techniques for determining the free-carrier concentration in semiconductors have obvious advantages over more traditional techniques such as the Hall effect and spreading resistance, for which the samples must undergo destructive processes such as etching, cleavage, or contact metal fabrication. This difficult the study of samples with reduced dimensions or located in integrated circuits. Optical techniques, including Raman scattering [1,2], infrared reflectivity [3], and photoluminescence [4], have been successfully used in different IV and III-V semiconductors for this purpose with the advantage that they do not require particular sample handling and preparation. In n-type polar materials such as GaAs and InP the free-carrier plasma couples with the LO phonon mode giving two additional peaks in the Raman spectrum, labelled as L+ and L -, with frequencies ω+ and ω-, respectively. The position of the LO phonon-plasmon coupled (LOPC) modes strongly depends on the free-carrier concentration, so Raman scattering constitutes a powerful technique to evaluate their density. The carrier concentration is determined with lineshape calculations of the LOPC modes using the fluctuation-dissipation theory [1] in which the electric susceptibility is evaluated by different models, such as the simple classical Drude and hydrodynamic models, and the Linhard-Mermin (LM) model. Most of the charge density determinations are done with the first two classical models because they allow simple analytical expressions for the electrical susceptibility to be derived. Although, in these approaches important effects are neglected giving rise, in some cases, to greate errors in the determination of
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charge density values. The LM model giv
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