Microdiffraction Studies of Oxygen in Silicon - A New SiO 2 Phase?
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MICRODIFFRACTION STUDIES OF OXYGEN IN SILICON - A NEW SiO2 PHASE? J.C.H. SPENCE, N. LONG**, W. BERGHOLZ*, Y. KIM and M. O'KEEFFE*** Department of Physics, Arizona State University, Tempe, Arizona 85287 USA *Siemens Research, Otto Hahn Ring 6, 8000 Munchen 83, F.R.G. **CSSS, Arizona State University, Tempe, Arizona 85287 USA ***Department of Chemistry, Arizona State University, Tempe, AZ 85287 USA ABSTRACT Evidence for a new microcrystalline precipitate in CZ Si annealed for 256 hrs at 635 C is presented by electron microdiffraction. This may be a precursor phase for the formation of amorphous platelets [9]. Multiple scattering microdiffraction calculations which distinguish the symmetries of two models for the thermal donor are also given. INTRODUCTION The high oxygen content of Czochralski silicon has both beneficial and harmful effects on semiconductor devices. The study of the atomic and electronic structure of the various point defects, precipitates and line and planar defects associated with oxygen in CZ silicon has a long history (see Bourret [E] for a review). In addition to isolated intersitial oxygen in as-grown CZ material, various aggregates and precipitates are observed in the two main annealing temperature ranges 300-500C and 650-1100 C. The electrically active thermal donor formed at the lower temperatures has been extensively studied [2], and we present here some electron microdiffraction pattern calculations for two T.D. models in order to determine if the e might be distinguished from experimental patterns. In the range 600-700 C dislocation dipoles, coesite rods or ribbons, amorphous precipitates and stacking faults are found, while at higher temperatures amorphous platelets and polyhedra predominate. We present here also evidence for a new phase in CZ silicon annealed for 256 hrs at 635 C. EXPERIMENTAL Boron-doped with an
initial
3 inch CZ [110] silicon wafers oxygen concentration
from Wacker Chemitronic
of 8xlO17 cm-3 and a carbon concen-
tration of les8 than 2x1016cm'3 were used. Small samples were annealed for 256 hrs at 635 C (no pre-anneal) in an nitrogen atmosphere. Fragments were glued (at low temperature) to a glass block and ground by hand to 30 micron thickness before ion-milling for TEM observation. Our first experiments were aimed at confirming the coesite structure using video recordings of coherent microdiffr ction patterns obtained on a VG HB5 STEM instrument (probe size about 53). High speed parallel detection was used to minimize radiation damage. Despite the observation of weak non-silicon precipitate reflections, we were unable to obtain indexable patterns by this technique. Figure 1 shows the microdiffraction pattern obtained from the precipitate shown inset using a Philips EM 400ST transmission electron microscope fitted with field-emission electron source. The electron probe diameter is about 100. Lower magnification images (see Fig. 2) showed thg presence of dipoles and platelets. In addition, many approximately 300A diameter particles similar to that shown in figure 2
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