Ultra-thin strain relaxed SiGe buffer layers with 40% Ge
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Ultra-thin strain relaxed SiGe buffer layers with 40% Ge Klara Lyutovich, Erich Kasper, Michael Oehme University of Stuttgart, Institute of Semiconductor Engineering, Pfaffenwaldring 47, 70569 Stuttgart, Germany ABSTRACT Virtual substrates with ultra-thin SiGe strain relaxed buffers have been grown on Si substrates by a method employing point defect supersaturation in the growing layers. A concept of the point defect influence on the strain relaxation and on defect interactions in layers has been proposed. A method is developed to increase the degree of relaxation in sub-100 nm SiGe buffer layers and to provide a smooth surface morphology. Layer growth has been realized by solid source molecular beam epitaxy in a chamber equipped with an in situ monitoring system. One of the growth stages, performed at a very low temperature, serves the generation of point defects. Strain relaxation tunable up to the high degree and a crosshatch-free surface morphology are demonstrated in 40nm thick SiGe buffers which contain 40-45% Ge. Growth monitoring enables the control of the process window and the layer crystallization by a chosen mechanism. Virtual substrates produced by the described method were successfully tested in nMODFET structures. INTRODUCTION Carriers in Si channels under tensile strain gain high mobilities caused by lower effective mass and reduced intervalley scattering. The consequently enhanced performance of strained-Sibased devices excites considerable interest in such systems (catchword: strained Si) [1]. To yield the necessary tensile strain in the Si channel, a virtual substrate consisting of a standard Si substrate with a strain-relaxed SiGe buffer layer on top of it offers an appropriate solution. The ability of virtual substrates to adjust the strain between the Si substrate and the layers in heterostructures is now widely acknowledged but their quality and integration ability still do not meet all requirements. We report about ultra-thin SiGe layers with 40%Ge emphasizing their high degree of relaxation and surface smoothness. These parameters as well as ultra-low thickness are crucial for bufferlayer application in virtual substrates for the adjustment of high strain values (up to 1.7% strain) in integrated circuits on Si [2]. To produce such buffer layers, molecular beam epitaxy (MBE) method employing point defect generation by a very low temperature of 140-160°C at the first growth stage is elaborated [3]. At low temperatures, generation and accumulation of point defects takes place. On the second growth stage at conventional MBE temperature, point defects form dislocation loops which support the early relaxation and enable a high degree of relaxation in thin layers. Also, interaction of point defects with dislocations can further the threading-dislocation climbing and increase their annihilation improving the layer quality.
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EXPERIMENTAL DETAILS Virtual substrates with ultra-thin strain-relaxed SiGe buffers have been grown using a solid source MBE system [4] with in situ growth monit
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