Microstructural Characterization of SiGe Heterolayers
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MICROSTRUCTURAL CHARACTERIZATION OF SiGe HETEROLAYERS N. David Theodore, Peter Fejes, Mamoru Tomozane and Ming Liaw Motorola Inc., Advanced Technology Center, 2200 W. Broadway Rd., Mesa, AZ 85202 ABSTRACT SiGe is of interest for use in heterojunction-bipolar transistors, infrared detectors and field-effect transistors. In this study, graded SiGe heterolayers grown on Si, and heterolayers grown on SIMOX by CVD, were characterized using TEM. The graded-heterolayers consisted of ten layers of Sil.xGex on substrate silicon. Misfit dislocations were present at interfaces in the bottom 4-5 layers of the heterostructure. This conforms with predictions from qualitative strain-energy considerations. The greatest density of misfit dislocations was present at the Sil. xGex interface between the bottom two layers of the heterostructure. Dislocations were observed to extend out of the interface and up into the heterolayer structure. The defects were found to interact with interfaces in the structure and finally cease extending upwards towards the surface of the wafer. In addition to graded heterolayers, SiGe heterolayers grown on SIMOX were also investigated. The structures consisted of epi-silicon grown on a Si/SilxGex superlattice which was in turn grown on a Si/SiO 2 (SIMOX) structure. The behavior of defects in the layers was of interest. TEM characterization showed a large density of extended-defects present in the layers. Dislocations were observed to originate at the SIMOX oxide/Si interface, propagate up through the SiGe superlattice and into the epi-Si layer. Some dislocations were found to interact with the SiGe superlattice and cease propagating up towards the top of the wafer. SiGe superlattices with a higher concentration of Ge are more effective in reducing defect propagation towards the surface of the wafer. INTRODUCTION SiGe heterolayers are being investigated for use in heterojunction bipolar transistors [1,2]. Infrared detectors [2] and field-effect transistors [3] are potential applications for SiGe/Si superlattices. An advantage of SiGe is that the processing required for use of the material is compatible with existing silicon technology. One of the methods used in silicon technology for electrical isolation of devices (from the substrate) is SIMOX (Separation of Silicon by IMplanted OXygen) [4]. SIMOX could potentially be used for isolation of SiGe structures from the substrate. Before SiGe can be used extensively for devices, there is a need to understand and then control the origin and behavior of defects in the materials. In the case of SIMOX, epitaxial-Si grown over the buried implanted-oxide can have grown-in dislocations that arise due to SIMOX-related damage. If SiGe heterolayers were grown on silicon, dislocations could interact with the strain fields associated with the SiGe layers. Such interaction could lead to a reduction in defect densities in upper layers of the structures [5]. In the present study, the structural quality of, and the behavior of defects in, graded SiGe layers grown by chemical
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