Strain Shift Coefficients for Phonons in Si-Ge Heterostructures
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STRAIN SHIFT COEFFICIENTS FOR PHONONS IN Si-Ge HETEROSTRUCTURES J.-M. BARIBEAU AND DJ. LOCKWOOD Institutefor MicrostructuralSciences, National Research Council Canada, Ottawa, KIA OR6, CANADA. ABSTRACT Strain shift coefficient measurements for longitudinal optical phonons in molecular beam epitaxy grown metastable pseudomorphic Sil-xGex layers on (100) Si (0 < x < 0.35) and Ge (0.80 < x < 1) are reported. Strain in partially relaxed annealed specimens was obtained by double-crystal x-ray diffractometry and the corresponding strain phonon shift was measured by Raman scattering spectroscopy. For epilayers grown on Si it was found that the epilayer Si-Si phonon frequency varies linearly with strain. The magnitude of the strain shift coefficient b however showed a small composition dependence varying from b -= -700 cm- 1 at x = 0 to b - -950 cm- 1 at x = 0.35, corresponding to a stress factor - 0.40 + 0.57x + 0.13x 2 cm- 1/kbar. For the Ge-Ge vibration mode in epilayers grown on Ge, b decreased from --425 cm-1 at x = 1 to -- 500 cm-1 at x = 0.8, corresponding to a stress factor T - 0.52 - 0.14x - 0.08x 2 cm- 1/kbar. INTRODUCTION
Measurement of the frequency of optical phonons by Raman scattering spectroscopy is a fast and non-destructive means to determine the composition in SilxGex epilayers. For a given composition, strain variation in an epilayer can also be obtained from the shift of the phonon frequencies. This however requires accurate knowledge of the strain coefficient b (or stress factor T) which relates this shift to the strain (stress) in the epilayer. In pure Si (Ge) layers only one Raman peak is observed due to the optic phonon at 520 cm- 1 (301 cm- 1) and the corresponding shift coefficients have been obtained from a number of uniaxial stress experiments. 1-6 In SiI-xGex three peaks are found near 300, 400 and 500 cm-1 due largely to vibrations of the Ge-Ge, Si-Ge, and Si-Si bonds, repectively. 7 Although the frequency dependence of these modes on composition is known, 7 - 1 1 their strain shift coefficient is less well known. 12. 13 Such information, however, is essential if Raman scattering is to be used routinely for accurate strain measurements in Si-Ge heterostructures with arbitrary strain partition. We measured the strain, using x-ray diffraction, and the optic phonon frequencies in Sil.xGex strained epilayers grown on (100) Si and Ge for Ge concentrations in the range 0 < x < 0.35 and 0.8 < x < 1, respectively. The biaxial compressive strain in the layers was varied by annealing the samples. Analysis of the data on Si has enabled us to deduce bsi-si over the whole concentration range and bsi-Ge and bGe-Ge at higherx. The value of bGe-Ge for epilayers on Ge was also obtained. EXPERIMENT
The epitaxial layers were grown by molecular beam epitaxy in a VG Semicon V80 14 at a temperature of -450'C on lightly doped 100 mm (100) Si and 50 mm (100) Ge wafers. The thickness of the various layers was chosen to maintain pseudomorphicity at the growth temperature. Physical data from the various samples are g
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