Defect population and electrical properties of Ar + -laser crystallized polycrystalline silicon thin films
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Defect population and electrical properties of Ar+-laser crystallized polycrystalline silicon thin films S. Christiansen1*, M. Nerding1, C. Eder1, G. Andrae2, F. Falk2, J. Bergmann2, M. Ose2, H. P. Strunk1 1
Universität Erlangen-Nürnberg, Institut für Werkstoffwissenschaften-Mikrocharakterisierung, Cauerstr. 6, D-91058 Erlangen, Germany 2
Institut für Physikalische Hochtechnologie, Winzerlaer Strasse 10, D-07745 Jena, Germany
*e-mail: [email protected] Abstract We crystallize amorphous silicon (a-Si) layers (thicknesses: ~300nm and ~1300nm for comparison) that are deposited on glass substrates (Corning 7059) by low pressure chemical vapor deposition using a continuous wave Ar+-laser. We scan the raw beam with a diameter of ~60µm in single traces and traces with varying overlap (30-60%). With optimized process parameters (fluence, scan velocity, overlap) we achieve polycrystalline Si with grains as wide as 100µm. The grain boundary population is dominated by first and second order twin boundaries as analyzed by electron backscattering analysis in the scanning electron microscope and convergent beam electron diffraction in the transmission electron microscope. These twins are known not (or only marginally) to degrade the electrical properties of the material. In addition to twins, dislocations and twin lamellae occur at varying densities (depending on grain orientation and process parameters). The recombination activity of the defects is analyzed by EBIC and according to these measurements crystallization receipts are defined that yield the reduction of electrically detrimental defects. Introduction The preparation of thin polycrystalline silicon (poly-Si) films on low cost amorphous substrates such as glass has been subject of intensive research with a variety of techniques [1-7] over the past few years, motivated by the perspective to use poly-Si thin film transistors (TFTs) in active matrix liquid displays [8] and for thin film solar cells on glass which could be an attractive lowcost alternative to conventional solar cells [9]. For both fields of applications it is the task to realize large grained material, a control of the defect population and especially in the case of TFTs a location control of the extended defects. These defects may affect the electrical characteristics of the material in different ways [10-15]. Thus, knowledge about the nature of the grain boundary and dislocation population in laser crystallized poly-Si is not only an important structural information but is also a prerequisite for a clear understanding of the electrical characteristics of the material. Presently laser crystallization is considered to be one of the most promising approaches to bring a thin crystalline Si layer on the amorphous, temperature sensitive glass substrate without damaging the substrate thermally. Different types of laser annealing procedures are tested, that all base on melting and re-solidification of an amorphous Si layer. The most common and frequently used laser is the short pulse excimer laser [1,2
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