(001)-Textured Laser-Crystallized Silicon thin Films on Glass Substrates
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(001)-textured laser-crystallized silicon thin films on glass substrates M. Nerding1, S. Christiansen1, G. Esser2, U. Urmoneit2, A.Otto2, H.P. Strunk1 1
Universität Erlangen-Nürnberg, Institut für Werkstoffwissenschaften, Lehrstuhl für Mikrocharakterisierung, Cauerstr. 6, D-91058 Erlangen 2 Universität Erlangen-Nürnberg, Lehrstuhl für Fertigungstechnologie, Egerlandstr. 11, D-91058 Erlangen ABSTRACT We investigate the microstructure of polycrystalline silicon films (grain size, texture and grain boundary population) on glass substrates. These films are produced from amorphous silicon precursor layers by scanning the raw beam of a continuous wave Ar+- ion laser operated at a wavelength of 514 nm over the amorphous silicon thereby crystallizing it. The materials applicability for devices in large area electronics strongly depends on the orientation of the surface normal, the average grain size and the defect density and population. Transmission electron microscopy together with electron back-scattering diffraction analysis of the crystallized layers UHYHDOJUDLQZLGWKVRIDERXW
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conditions a preferred (001)-surface normal orientation (texture) forms. The grain boundary population is dominated in the textured films by coincidence boundaries, essentially twin ERXQGDULHVRIILUVWDQGVHFRQGRUGHUDVZHOODV
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INTRODUCTION Polycrystalline silicon (poly-Si) fabricated at low temperatures is of great interest for large area electronics on temperature sensitive, cheap substrates. Possible devices are thin film transistors (TFTs) in active matrix liquid crystal displays [1] and thin film solar cells [2] on glass. Succesful large area electronics applications base on the homogenity of the material. The requirements for the polycrystalline silicon can therefore essentially be characterized by two aspects: 1) uniform and low defect density (grain boundaries and extended intra-grain defects) for homogeneous optimum electrical properties 2) uniform surface normal of the grains to ensure a homogeneous processing of the material (e. g. epitaxy, etching, oxidation...) Over the past few years a variety of techniques [3-6] have been applied to improve the performance of the films i.e. to enhance the grain size and to control defect population, density and location. It is only recently, that some investigations resulted in texture formation in lasercrystallized material [7-9]. All these references describe short pulse laser experiments that can be considered as standard processes and most often a prominent (111) orientation of the surface normal is observed. While a homogeneous (111)-texture might be acceptable from the point of view of TFTs, it is clearly not for thin film solar cells. In thin film solar cells, laser crystallized films are used as seeding layers for a subsequent epitaxial low%temperature thickening process. The low temperature epitaxy is extremely anisotropic with respect to defect formation and growth velocity. The defect density pro
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