New Insight into Damage-Related Phenomena in Si Implanted Under Extreme Conditions
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spectrometry are used to profile the ion-induced damage. The RBS spectra were acquired using 2.3 MeV He++-ions and a detector positioned to intercept backscattered ions at 160'. RESULTS AND DISCUSSION XTEM micrographs are shown in Fig. 1 from p-type Si(100) crystals implanted at
4500C with different fluences of 1.25 MeV Si+-ions. The resistivity of the Si crystals was
nominally 10 0-cm, and the average ion current density during implantation was < 1 I±A/cm 2. Damage formed near the eor is seen to consist of a band of dislocations centered at -1.5 gm which broadens with fluence. Surprisingly, the 3 x 1017 cm-2 implanted sample is seen in Fig. 1(a) to be free of visible defects to a depth of -1.0 gim. This occurs despite the fact that the average number of displacements per atom (dpa) within this region is calculated at this dose to be -40 by computer simulation using TRIM(9). Clearly, Frenkel pair recombination dominates at 450'C over those pointdefect reactions which result in damage formation. A unique morphology is seen at the higher fluences in Figs. 1(b) and 1(c). A distinct band of dislocations is present within the superficial layer centered near -0.45 gm. The width of the band increases with fluence and eventually expands to intersect both the surface and the advancing eor damage. Similar results have been reported for O+-implantation under nearly identical conditions( 10,11), but the use of self-ions indicates that these effects are intrinsic to these extreme conditions (i.e., high temperature and dose) and are not dependent upon the ion-solid chemistry. In ref. (10), it was shown that these dislocations are formed as a result of yielding of the lattice in response to an accumulation of precursor defects. The similarity of the self-ion results suggests a common mechanism. Si SURFACE
a
m;_;
~ i ;A
A# ._
Fig. 1. XTEM micrographs from Si(100) crystals implanted at 450'C with 1.25 MeV
self-ions to a fluence of (a) 2 x 1017 cm-2, (b) 3 x 1017 cm-2, and (c) 5 x 1017 cm- 2.
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Identification of the precursor defects within the superficial layer prior to dislocation formation was done using positron annihilation techniques which are very sensitive to open volume defects in solids(12 ). The sensitivity of positrons to lattice defects is reflected in a quantity called the S-parameter which measures the Doppler broadening of the 511 keV photon annihilation peak. The S-parameter is calculated by dividing the area of a fixed region in the center of the annihilation peak by the total peak area. Since a positron trapped within an open-volume defect interacts less frequently with the higher momentum, inner shell electrons, the width of the annihilation spectrum from such defects will be narrower (and the S-parameter larger) than from sites in undamaged crystal. The dependence of the S-parameter on the incident positron energy (depth) is shown in Fig. 2 for samples implanted with (a) 2 x 1017 and (b) 5 x 1017 cm- 2 ; a dose range which spans the onset of dislocation growth within the superficial layer. At the low
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