Substitutional carbon inSi1−xGex

Abstract

Local vibrational modes of carbon impurities in relaxed Si 1Ϫx Ge x have been studied with infrared absorption spectroscopy in the composition range 0.05рxр0.50. Carbon modes with frequencies in the range 512-600 cm Ϫ1 are observed in 13 C ϩ -implanted Si 1Ϫx Ge x after annealing at 550°C. Measurements on samples coimplanted with 12 C ϩ and 13 C ϩ show that these modes originate from defects containing a single carbon atom and from the variation of the mode frequencies with composition x, the modes are assigned to substitutional carbon in Si 1Ϫx Ge x . Based on the frequencies obtained from a simple vibrational model, the observed modes are assigned to specific combinations of the four Si and Ge neighbors to the carbon. The intensities of the modes indicate that the combination of the four neighbors deviates from a random distribution. Ab initio local-densityfunctional cluster theory has been applied to calculate the structure and the local mode frequencies of substitutional carbon with n Ge and 4Ϫn Si neighbors in a Si and a Ge cluster. The calculated frequencies are ϳ9% higher than those observed, but the ordering and the splitting of the mode frequencies agree with our assignments.

Interstitial-carbon defects inSi1−xGex

“…A preferential pairing between a substitutional B atom and a substitutional Ge atom rendering the B atom partly immobile has been put forward to explain the retardation. 19 Hoffmann et al 20 have demonstrated by infrared absorption experiments that a substitutional carbon atom in strain relaxed Si 1Ϫx Ge x of xр0.50 prefers a configuration with four Si neighbors. Thus, for C i a pairing with Ge atoms is excluded.…”

Raman Investigation of the Localized Vibrational Mode of Carbon in Strain-Relaxed Si1-xGex:C

“…Several groups have reported on Raman spectroscopy studies of Si 1−x Ge x :C. [9][10][11][12] Two Raman peaks, one at 607 cm −1 and the other at 560 cm −1 , have been identified as carbon LVMs in Si 1−x−y Ge x C y 11) and their frequencies depends only on the Ge composition x when y 0.01, and on both x and y when y 0.01. 12) The microscopic origin of the two peaks at 607 cm −1 and 560 −1 have been revealed by IR absorption studies performed by Hoffmann et al 1,7,8) In this paper we report on Raman investigations of 13 C LVM in * E-mail address: kitoh@appi.keio.ac.jp Si 1−x Ge x :C samples.…”

“…It is possible to estimate the positions of Peak 1 and 2 theoretically using the following assumptions. 1) The positions of Peak 1 and 2 depend only on the composition x of Ge and not on the composition y of C, because the concentration of carbon is very low in our samples (y ≈ 0.002 for our Si 1−x−y Ge x C y ). Peak 1 and 2 are formed predominantly by the T 2 mode of Si 4 Ge 0 -C and the A 1 mode of Si 3 Ge 1 -C, respectively, i.e., the peak positions of Peak 1 and 2 can be estimated simply by calculating the frequencies of the T 2 mode of Si 4 Ge 0 -C and the A 1 mode of Si 3 Ge 1 -C, respectively, as a function of the Ge composition x.…”

“…To facilitate the comparison between our spectra and those obtained previously by IR absorption, 1,7,8) an equivalent series of 13 C-doped Si 1−x Ge x :C samples was studied in this work. Extensive descriptions of sample preparations are given in ref.…”

“…The vibrational spectrum of carbon is one parameter that can be used to characterize the nature and quality of Si 1−x Ge x :C. Because carbon is significantly lighter than Si and Ge, a carbon sitting substitutionally in the Si 1−x Ge x lattice exhibits localized vibrations with higher frequencies than the Si 1−x Ge x phonons. These localized vibrational modes (LVMs) of carbon in Si 1−x Ge x have been investigated systematically with IR absorption by Hoffmann et al 1,7,8) Their studies have shown that substitutional carbon in Si 1−x Ge x binds to its four-neighboring Si and Ge and that the LVM frequency is determined mainly by the specific combination of silicon and germanium neighbors as well as the composition x of the material. However, IR absorption requires relatively thick samples in order to obtain signal intensities that are detectable.…”