A low-temperature ( ) photoluminescence spectrum of high-purity CdTe 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 (

4.2\,\mathrm{K}

) photoluminescence spectrum of high-purity CdTe shows four lines, at

1.596

1.593

1.575

and

1.560\,\mathrm{eV}

. The measured band gap of CdTe at

4.2\,\mathrm{K}

E_g=1.606\,\mathrm{eV}

, and the LO-phonon energy in CdTe is

21.6\,\mathrm{meV}

. Carriers thermalise to the band edges well before recombining, bound excitons sit

1

3\,\mathrm{meV}

below the free-exciton line, and an LO-phonon replica sits one phonon energy below the line it replicates. ((a)) Assign each of the four lines to one of: the free exciton, an exciton bound to a neutral donor or acceptor, an LO-phonon replica of the free exciton, or donor-acceptor pair recombination. ((b)) State the exciton binding energy implied by the free-exciton assignment.

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