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1. (a) Explain why absorption and luminescence processes in silicon and germanium are weak near the band-edge of these materials. [6 marks] (b) The absorption spectrum of germanium at low temperature is shown below. Explain the main features of this spectrum. 10,000 1000 100 10 a (cm) 1 01 0.0! 075 זזס 079 0.83 065 0.87 0.09 PHOTON ENERGY lev) [4 marks) (c) Show that the dispersion of the dielectric constant resulting from the interaction of light with TO-phonons is given by: NI? 8,(@)=1 +2+ io (22-02- where N is the number of unit cells per unit volume, / is the effective charge per ion, se is the reduced mass and to is the TO phonon angular frequency 17 marks] (d) The reflectance spectrum of AISb in the infra-red region is shown below. Explain why there is a region of large reflectance around 30 microns. Determine the frequencies of the TO and LO phonon modes. [5 marks] 1.0 0.8 AISb 0.6 Reflectivity 0.4 0.2 0.0 16 20 40 24 28 32 36 Wavelength (um)