Thermally Induced Structural Transformations on Polymorphous Silicon
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J. Farjasa) and P. Roura GRMT, Departament de Física, Universitat de Girona, Campus Montilivi, Edif. PII, E17071-Girona, Catalonia, Spain
F. Kail and P. Roca i Cabarrocas LPICM (UMR 7647 CNRS), Ecole Polytechnique, 91128 Palaiseau Cedex, France
E. Bertran FEMAN, Departament de Física Aplicada i Optica, Universitat de Barcelona, E08028, Barcelona, Catalonia, Spain (Received 2 February 2005; accepted 15 June 2005)
Polymorphous Si is a nanostructured form of hydrogenated amorphous Si that contains a small fraction of Si nanocrystals or clusters. Its thermally induced transformations such as relaxation, dehydrogenation, and crystallization have been studied by calorimetry and evolved gas analysis as a complementary technique. The observed behavior has been compared to that of conventional hydrogenated amorphous Si and amorphous Si nanoparticles. In the temperature range of our experiments (650–700 °C), crystallization takes place at almost the same temperature in polymorphous and in amorphous Si. In contrast, dehydrogenation processes reflect the presence of different hydrogen states. The calorimetry and evolved gas analysis thermograms clearly show that polymorphous Si shares hydrogen states of both amorphous Si and Si nanoparticles. Finally, the total energy of the main Si–H group present in polymorphous Si has been quantified.
I. INTRODUCTION
Hydrogenated amorphous silicon (a-Si:H) plays a crucial role in several applications such as solar cells and flat panel displays. Because of its its technological interest, the production techniques have evolved with the aim of improving material properties (electrical conductivity, photovoltaic efficiency, stability, etc.) and deposition rate. As a result, several kinds of nanostructured silicon materials have been synthesized. The ones having the most interesting properties are microcrystalline and polymorphous silicon (pm-Si:H). Pm-Si:H is produced by plasma-enhanced chemical vapor deposition (PECVD) in a variety of plasma conditions that allow the formation of hydrogenated silicon clusters in the plasma. Before these clusters grow into powders, they are incorporated in the growing amorphous silicon film, along with SiHx radicals.
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2005.0322 2562
http://journals.cambridge.org
J. Mater. Res., Vol. 20, No. 9, Sep 2005 Downloaded: 13 Mar 2015
The result is a nanostructured material consisting of silicon clusters or nanocrystals embedded in an amorphous matrix.1 Polymorphous silicon has much better transport properties than the conventional amorphous material.2 For instance, its hole mobility, recently measured by time-of-flight experiments,3 is 2–3 times higher. Additionally, its density of defects is one order of magnitude lower,4 which allows fabrication of p-i-n photodiodes with very low dark current densities.5 Even after light soaking, the defect density remains lower than in conventional a-Si:H.2 Consequently, enhanced photovoltaic conversion efficiency is expected.6 These p
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