Mechanism for the Reduction of Threading Dislocation Densities in Si 0.82 Ge 0.18 Films on Silicon on Insulator Substrat
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Mechanism for the Reduction of Threading Dislocation Densities in Si0.82Ge0.18 Films on Silicon on Insulator Substrates E.M. Rehder, T.S. Kuan1, and T.F. Kuech Materials Science Program, University of Wisconsin-Madison Madison, WI 53706, U.S.A. 1 University at Albany, State University of New York New York, Albany, NY 12222, U.S.A. ABSTRACT We have made an extensive study of Si0.82Ge0.18 film relaxation on silicon on insulator (SOI) substrates having a top Si layer 40, 70, 330nm, and 10µm thick. SiGe films were deposited with a thickness up to 1.2µm in an ultrahigh vacuum chemical vapor deposition system at 630°C. Following growth, films were characterized by X-ray diffraction and a dislocation revealing etch. The same level of relaxation is reached for each thickness of SiGe film independent of the substrate structure. Accompanying the film relaxation is the development of a tetragonal tensile strain in the thin Si layer of the SOI substrates. This strain reached 0.22% for the 1.2µm film on the 40nm SOI and decreases with SOI thickness. The Si thickness of the SOI substrate also effected the threading dislocation density. For 85% relaxed films the density fell from 7x106 pits/cm2 on bulk Si to 103pits/cm2 for the 40, 70, and 330nm SOI substrates. The buried amorphous layer of the SOI substrate alters the dislocation dynamics by allowing dislocation core spreading or dislocation dissociation. The reduced strain field of these dislocations reduces dislocation interactions and the pinning that results. Without the dislocation pinning, the misfit dislocations can extend longer distances yielding a greatly reduced threading dislocation density. INTRODUCTION Relaxed SiGe films on Si substrates increase the lattice constant of the material allowing strain engineering of subsequent layers. For example, Si films can then be grown having a tensile strain. This strain modifies the band structure allowing for high carrier mobilities and electron confinement that is not found in the traditional relaxed Si and compressive SiGe structures. The SiGe film relaxation occurs by the formation of large numbers of dislocations in the material. Threading dislocations, which terminate on the film surface, can remain and disrupt the operation of electronic devices fabricated in this material. A variety of growth methods have been employed to reduce the threading dislocation density. High and low temperature compositional grading, as well as the use of surfactants, has reduced the threading dislocation density (TDD) to the mid 104cm-2 for films having 20%Ge [1,2,3,4]. Additionally, SOI substrates have been shown to reduce the TDD in relaxed SiGe films by 107 [5,6,7]. In this work we have studied the relaxation process by varying the thickness of the SiGe layer and the top Si layer of the SOI, referred to as Si-SOI. X-ray diffraction was used to determine the strain state of the SiGe and Si layers. The film TDD was measured with a dislocation decorating etch.
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EXPERIMENT The bonded SOI substrates possessed a thin Si layer having a th
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