Strain Relaxation in Si 1-x Ge x Thin Films on Si (100) Substrates: Modeling and Comparisons with Experiments
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Strain Relaxation in Si1-xGex Thin Films on Si (100) Substrates: Modeling and Comparisons with Experiments Kedarnath Kolluri1, Luis A. Zepeda-Ruiz2, Cheruvu S. Murthy3, and Dimitrios Maroudas1 1 Department of Chemical Engineering, University of Massachusetts, Amherst, MA 01003, U.S.A. 2 Chemistry & Material Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94550, U.S.A. 3 IBM Semiconductor Research & Development Center, Hopewell Junction, NY 12533, U.S.A. ABSTRACT Strained semiconductor thin films grown epitaxially on semiconductor substrates of different composition, such as Si1-xGex/Si, are becoming increasingly important in modern microelectronic technologies. In this paper, we report a hierarchical computational approach for analysis of dislocation formation, glide motion, multiplication, and annihilation in Si1-xGex epitaxial thin films on Si substrates. Specifically, a condition is developed for determining the critical film thickness with respect to misfit dislocation generation as a function of overall film composition, film compositional grading, and (compliant) substrate thickness. In addition, the kinetics of strain relaxation in the epitaxial film during growth or thermal annealing (including post-implantation annealing) is analyzed using a properly parameterized dislocation mean-field theoretical model, which describes plastic deformation dynamics due to threading dislocation propagation. The theoretical results for Si1-xGex epitaxial thin films grown on Si (100) substrates are compared with experimental measurements and are used to discuss film growth and thermal processing protocols toward optimizing the mechanical response of the epitaxial film. INTRODUCTION Strained Si devices on Si1-xGex virtual substrates enhance electron and hole mobility compared to unstrained substrates of the same material [1]. When an alloyed Si1-xGex layer is grown on a Si substrate, biaxial strain develops due to lattice mismatch between the substrate and the grown film. Possible mechanisms of strain relaxation include misfit dislocation generation at the film/substrate interface [2] beyond a critical film thickness, as well as film surface morphological transitions [3]. In practice, large numbers of threading dislocations are nucleated which, after gliding a short distance, become immobilized, resulting in a high dislocation density in the film. Device-quality materials, however, need to have a high degree of strain relaxation, low threading dislocation densities, and smooth surfaces. Recently, it has been reported that He ion implantation and subsequent annealing at temperatures (T) over the range 1023 K ≤ T ≤ 1123 K can result in thin Si1-xGex layers possessing a high degree of strain relaxation, as well as relatively low densities of threading dislocations [4]. In this paper, we report a hierarchical approach for computational analysis of the mechanical response of Si1-xGex films on Si substrates. We use continuum elasticity and dislocation theory to study the critical thickness of Si1
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