Phonon coupling to excitations of free and localized electrons inn-type ZnSe

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Abstract
The coupling mechanisms between longitudinal-optical phonons and electronic excitations in n-type ZnSe layers were investigated with Raman spectroscopy in conjunction with transport measurements. The layers were grown by molecular-beam epitaxy and were intentionally doped below the Mott criterion for the insulator-metal transition. The nature of the electron-phonon interaction is determined by the degree of electron localization, which was effectively changed by temperature and donor concentration. The longitudinal-optical phonons couple to plasmons when electrons are thermally excited into the conduction band and to a continuum of electronic excitations when electrons are localized in an impurity band or at donor sites. In both cases unbound phonons are observed. From the renormalized phonon frequencies at high temperature, values of free-electron concentration as a function of temperature were established. They are in excellent agreement with Hall-effect determinations. At low temperatures the phonon Raman profiles are asymmetric and show Fano-type line shapes. The electronic continuum responsible for the phonon self-energies at low temperature was identified as Raman scattering by bound electrons.