Study of the Oxidation of Polycrystalline SiGe: Formation of Ge Nanocrystals and their Related Luminescence
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A8.3.1
Study of the Oxidation of Polycrystalline SiGe: Formation of Ge Nanocrystals and their Related Luminescence A. C. Prieto 1, M. Avella 1, J. Jiménez 1, A. Rodríguez 2, J. Sangrador 2, T. Rodríguez 2, A. Kling 3. 1 Dpto. Física de la Materia Condensada, E.T.S.I.I., U. de Valladolid, Valladolid, Spain. 2 Dpto. Tecnología Electrónica, E.T.S.I.T., Universidad Politécnica de Madrid, Madrid, Spain. 3 Instituto Tecnológico e Nuclear, Sacavém, and Centro de Física Nuclear da Universidade de Lisboa, Lisbon, Portugal.
ABSTRACT Polycrystalline SiGe layers have been oxidized in either dry or wet atmospheres. The evolution of the growing oxides and the SiGe layer during the oxidation processes have been characterized using Raman spectroscopy, X-ray diffraction, Rutherford backscattering and Fourier transform infrared spectroscopy. Formation of Ge nanocrystals has been observed for both oxidation atmospheres. Violet luminescence emission has been observed and its relation to the oxidation processes has been studied. The luminescence is unambiguously related to the presence of these Ge nanocrystals. Since it does not exhibit quantum confinement, it must be related to defects linked to the nanocrystals.
INTRODUCTION Ge nanocrystals embedded in a dielectric medium can be used either as storage elements for electronic memories [1] or as light emitters for photonic devices [2]. Ge-nanocrystals have been formed by thermal oxidation, dry and wet, of SiGe films [3-5]. When SiGe films are oxidized in a pure O2 ambient, a layer of SiO2 is grown and almost all the Ge is segregated into the remaining SiGe layer [3-5]; once all the Si of the SiGe layer is oxidized, the segregated Ge starts to be oxidized as well. On the other hand, when SiGe is oxidized in a wet atmosphere, SiOGe oxide is grown at the early stages of the process. The fraction of Ge in the oxide is very close to the Ge fraction in the initial layer [4]. Therefore, only a small fraction of Ge is segregated into the SiGe layer. In both processes, Ge nanocrystals are formed in the final stages of oxidation. The luminescence spectra of the samples show the existence of violet and UV luminescence at 3.1 eV and 4 eV induced by the oxidation [5, 6]. The origin of this luminescence is a matter of controversy: in particular, the role of the Ge nanocrystals is not well understood [6, 7]. We present a study about the influence of the oxidation parameters (oxidation atmosphere and time) on: a) the composition of the grown oxide, b) the segregation of Ge, c) the formation of Ge nanocrystals and d) the characteristics of the luminescence emission. Samples with two different SiGe layer thickness were oxidized in order to clarify the origin of the luminescence. The results of different experimental techniques, X Ray Diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, cathodoluminescence (CL) and Rutherford Backscattering (RBS) have been correlated in order to get a detailed insight of the process.
A8.3.2
EXPERIMENTAL AND SAMPLES Amorphous SiGe la
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