Micro-Raman Characterization of Arsenic-Implanted Silicon: Interpretation of the Spectra
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The second section describes the experimental details and the third section introduces typical Raman spectra for high-dose arsenic implants. The spectra are compared to calculations for the lineshapes. The final section discusses the proposed explanation for the observed Raman spectra. EXPERIMENTAL PROCEDURES Lightly-doped, , 100-mm silicon wafers were implanted with 150 keV arsenic ions through a 0.06-jim thick thermal oxide. The arsenic dose was varied from 2 x 1013 to 5 x 1015/cm 2 and the rate was about 1.4 x 1013 As/s-cm 2. The macro-Raman spectroscopy is done with a SPEX 1403 double monochromator. The samples are held in a holder such that there is a 60' angle between the incident beam and the normal to the sample surface. A 900 geometry is used for the collection of the scattered light and the incident power level is 300 mW.
Micro-
Raman spectroscopy is done in the back-scattering mode with the incident polarization fixed and an incident power level of 5 mW. Spectra can be collected with a polarizing filter parallel or perpendicular to the incident polarization. The sample can be rotated about the normal to the silicon surface, so Polarization /Orientation (P/O) spectra can be collected. OBSERVATIONS AND MODELS Figure 1 shows the macro-Raman spectra recorded when the exciting laser wavelength is 514.5 nm for a range of arsenic implant doses. Similar spectra are seen with 488 nm excitation and by Balkanski et al. [2]. The characteristic silicon optical phonon mode at 520 cm-' gives way to the disorder-induced broad response below 500 cm-' that is indicative of amorphous silicon (ct-Si) [6]. In this case, the oa-Si arises from the arsenic implant, which occupies the first 0.1 to 0.15 ptm of the silicon. The residual 520 cm- line comes from the crystalline silicon below the implant-damaged silicon. Representative micro-Raman results with 514.5 nm excitation are shown in Fig. 2 for the implant dose of 5 x 10 5 As/cm 2. The nearly featureless spectrum, labeled 1, has the signature of ot-Si around 470 cm1. The other three spectra of Fig. 2 are from regions that show features associated with crystalline silicon (c-Si). Optical microscopy reveals these regions show contrast with the surrounding silicon when viewed with reflected white light. Their origin may be related to the long room-temperature anneal of the samples or the implantation conditions and this is discussed further in the next section. More extensive measurements over a wide range of implant doses [5,14] show the 510 cm-I feature in Fig. 2 has a tendency to grow in intensity with dose. Figures 3 and 4 illustrate this for the polarization (analyzer)/crystal orientation of parallel/0' and excitation wavelengths of 514.5 and 488 nm, respFectively. This P/O setting minimizes substrate silicon contributions [15], so the silicon 520 cm- line is suppressed with 514.5 nm light and absent with 488 rm light when the arsenic dose exceeds 5 x 10 3/cm 2 . One point to note is that the 510 cm- 'line is present only at, and above, a dose of 2 x 10' 4/cm 2, which is
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