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Exciton and phonon effects in the absorption spectra of germanium and silicon

Journal of Physics and Chemistry of Solids
Publication Date
DOI: 10.1016/0022-3697(59)90372-5


Abstract Evidence for exciton creation in Ge and Si by indirect and direct transitions has been obtained from high resolution absorption spectra. At low levels of absorption fine structure is observed, which is characteristic of indirect transitions involving the emission and absorption of phonons. For Ge four different phonons with energies of 90°K, 320°K, 350°K and 420°K and for Si four phonons with energies 212°K, 670°K, 1050°K and 1420°K contribute to the absorption. They correspond with the TA, LA, LO, and TO vibrational modes respectively. Each component is interpreted as being due to the formation of excitons and free electron-hole pairs. The exciton binding energy is found to be about 0.0027 eV for Ge and about 0.010 eV for Si. Absorption to the first excited state of the exciton is also observed. This state is estimated to be about 0.0010 eV above the ground state for Ge and 0.0055 eV for Si. The structure of the exciton absorption is smoothed out as the temperature is raised. The effect is explained as due to a temperature dependent relaxation time for the excitons. Direct exciton transitions have been observed as a line structure at high levels of absorption in Ge. By taking into account the coulomb interaction of the electron and hole in the theory of the direct band-to-band absorption, the energy dependence of the absorption above the exciton line can be explained and a value of about 0.0012 eV obtained for the exciton binding energy.

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