A low-temperature photoluminescence spectrum of CdS shows four lines: ,

PHYSICS · Multi-part · Fermi University Physics · Condensed Matter, Materials & Soft Matter · Dielectric and optical properties of solids · Dielectric and optical properties of solids

Solid State Physics, Ch Optical properties: interband absorption, excitons and luminescence

Problem

A low-temperature photoluminescence spectrum of CdS shows four lines:

2.582\,\mathrm{eV}

2.578\,\mathrm{eV}

2.544\,\mathrm{eV}

and

2.506\,\mathrm{eV}

. The free-exciton binding energy of CdS is

E_b=27\,\mathrm{meV}

and its LO-phonon energy is

38\,\mathrm{meV}

. ((a)) Assign each of the four lines (free exciton, exciton bound to a shallow impurity, and LO-phonon replicas), giving the reasoning for each assignment. ((b)) Use the spacing between the lines to check the LO-phonon assignment against the quoted phonon energy. ((c)) Combine the free-exciton line with

E_b

to obtain CdS's band gap

E_g

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