Ion Beam Synthesis of Doped Nanocrystals of Si 1-x Ge x Alloys Embedded in SiO 2
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Ion Beam Synthesis of Doped Nanocrystals of Si1-xGex Alloys Embedded in SiO2
A. Chelouche1, G. Schmerber2, G. Ferblantier1, D. Muller1, D. Mathiot1 ICube, CNRS-Université de Strasbourg, UMR 7357, 23 rue du Loess, BP 20 CR, 67037 Strasbourg Cedex 2, France 2 IPCMS, CNRS-Université de Strasbourg, UMR 7504, 23 rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France 1
ABSTRACT As an extension of our previous proving that ion beam synthesis is an efficient route to form doped silicon nanocrystals (nc's) [1, 2], we show here that ion beam synthesis, by coimplantion of the dopant and of the constituents of the alloy, followed by a single high temperature anneal, is also a convenient way to grow more complex structures, such as As doped nc's of Si1-xGex alloys. Rutherford backscattering spectrometry (RBS) is used to measure the impurity profiles and evaluate the average concentration of the various species (Si, Ge, As). The formation of the nc's is evidenced by TEM observation and further confirmed by Raman and XRD analysis, which also allow us to estimate the Ge content of the nc's. The incorporation of As inside the Si1-xGex nc's is attested by the presence of the characteristic Ge-As related line in the Raman spectra of the As-doped samples, and strengthened by the increased conductivity of MOS structure including such doped nc's inside the dielectric film.
INTRODUCTION Semiconductor nc’s (Si, Ge and Si1-xGex) have attracted enormous interest in recent years because of their potential applications in various fields, including the possibility to incorporate efficient optoelectronics devices compatible with the dominant Si technology. However, due to their indirect bandgap, bulk Si and Ge are unable to emit or absorb light with the required efficiency. Several approaches are used to overcome this limitation: Band gap engineering using either quantum-size effects [3–6] or stress effects [7], surface modification by boron and phosphorus co-doping [8], incorporation of impurities (usually rare-earth elements [9]) and defect engineering [10, 11]. Among these different routes, the formation of Si-based nc's (with their pseudo-direct band gap due to quantum confinement) embedded in a SiO2 dielectric matrix offer new promising possibilities for applications in optoelectronics and photovoltaic (PV). In the PV field, the possibility to form Si1-xGex of various compositions (and thus of various band gaps) could be used to fabricate efficient Si-based tandem cells. For this latter application, it is obvious that the doping of the nc's must be mastered to allow an efficient charge collection.
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Up to now, only a few works have been done to study the properties of Si1-xGex nc's embedded in dielectric matrices. Different techniques were used to elaborate the Si1-xGex nc's: Co-evaporation of Ge, Si and
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