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Raman characterization of Si1−xGex alloys at ambient conditions (a) and under high pressure (on example of Si0.974Ge0.026). Pressure shifts of LO (at ∼519 cm−1) and LA (at ∼300 cm−1) phonons are displayed in (b, c) and (d, e) plots, respectively. (c) The inset shows pressure evolution of the intensity of this main peak (b); Ref–data from Ref. 6. (e) The inset shows Raman spectrum of Si0.974Ge0.026 after pressure releasing, i.e., in the Si-III phase.8–10
Pressure dependencies of thermopower of Si1−xGex and Si (a-d) and of σp /σn ratio (e). The curves for Si1−xGex correspond to pressurization cycle; for Si the direction of pressure variation is indicated by the thin arrows. The bulk arrow in (d) points out the transition to the metal. Labels, #1–#4 mark micro-samples cut from the same big sample (a): Photograph of Si0.986Ge0.014 sample is shown in the inset. (b) The inset shows pressure dependence of electrical resistance for Si0.982Ge0.018. (c) The inset shows determination of Seebeck effect from linear slopes of “thermoelectric voltage vs temperature difference” curves. (d) The inset shows enlarged high-pressure region. (e) The σp /σn ratios were calculated from some curves shown in (a–d). Plot (f) illustrates simple application of stress-induced switching of conduction type of p–Si:Ge. Using stresses of ∼0.3–0.6 GPa one can reversibly change the surface conduction from p- to n-type. Using higher stresses one can write either n-type zones (for stresses of ∼0.8–1.5 GPa), or zones with sandwich structure: compensated layer on n-type layer (the latter may be formed because of stress distribution).
Examples of pressure dependences of thermopower demonstrating reversible (a, curves 1 and 2) and irreversible (b, curves 1 and 2) switching between p- and n-types of conduction. The labels 1–4 indicate sequence of pressure runs. The notations are the same as in Fig. 2. Above ∼2 GPa (curves 3) the both samples turn into state that nearly compensated. The inset in (a) shows Raman spectrum on recovered sample #3 of Si0.982Ge0.018 after multiple pressure cycling below 9 GPa; it confirms conservation of the original diamond structure.
Si1−xGex samples and their characteristics.
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