Understanding and Controlling Transient Enhanced Dopant Diffusion in Silicon
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extremely low levels ( in Fig. 9 show that the interstitial diffusion profile in sample SPE1 is significantly steeper than in the as-grown MBE sample, indicating a lower interstitial diffusivity. This suggests that the SPE-regrowth step following amorphization has raised the level of interstitial traps in the superlattice, for instance through the introduction of vacancy-type defects [4]. Figure 8(b) shows that the 1018 C/cm 3 in sample SPEI has redistributed under the injection of interstitials and has decorated the B-rich regions. This could indicate that C is gettered at the B clusters, or that Si self-interstitials drive pairing of some B and C during annealing [34]. The substitutional C which was incorporated into superlattice CSPE1 consists of a peak of 5xl0 18/cm 3 at a depth of -3000 A [Fig. 8(b)]. Figure 8(a) shows that the injection of interstitials in sample CSPE1 also enhances B diffusion, but the broadening of the B peaks in the C-rich region is much less pronounced than in reference sample SPEl. Figure 9 demonstrates that the interstitial-enhanced diffusion of B is reduced by a factor of -10 in the presence of C and approaches the equilibrium level. This unambiguously demonstrates that substitutional C acts as a trap for the Si self-interstitials injected from the near-surface implantation damage. For
315
790 °C, 10 min anneal 1000
0 MBE + damage A SPE1 + damage CSPE + damage
11 01
_ SPEI (no damage)
N.100
,E
10-15
•
__
m 1016
10
o 1017 0
2000
4000
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- Figure 9 Depth-dependence
of the
intrinsic
B
diffusivity showing the effect of solid phase
epitaxy and C incorporation on the diffusion profile of ion-generated interstitials. Data were obtained by analyzing the B diffusion profiles in Figs. 2(a) and 8.
1
Depth (A)
instance, C and interstitials might pair to form mobile CI complexes [35], followed by the nucleation and growth of larger carbon-interstitial agglomerates [36]. The strong interaction between C and interstitials suggests that C is the dominant impurity responsible for the occurrence of trap-limited interstitial diffusion in Si, as identified in the previous section. The trapping enthalpy of 2-2.5 eV derived from those experiments would then reflect the enthalpy with which interstitials are effectively bound to C-related traps. Carbon incorporation for suppressing TED In this section we will demonstrate that incorporating substitutional C in Si can also be used to suppress TED of a subsequently implanted dopant. Wafers of FZ Si(100) were amorphized to a depth of 2 jim at -77 K by a series of Si implants (as above). Part of the wafers was implanted with C at energies of 10, 28, 65, 130, 230, and 400 keV to doses of 1.3, 3.2, 6.0, 8.7, 11.3, and 13.3x10 13/cm 2, respectively, to uniformly raise the C level. Samples were annealed in vacuum at 500 "C for 1 h, 600 "C for 2 h, and 900 "C for 15 min to induce SPE and dissolve the 3 t 8 18 implanted C in the c-Si lattice. The obtained C level varies between 4x10 and 6x10 /cm throughout 1 jim of the sample closest to
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