Intrinsic and Cr-doped single crystals of ZnSe, grown by the seeded physical vapor transport technique, are studied using Raman and photoluminescence spectroscopic techniques. The coupling between the longitudinal–optical (LO) phonon and the hole plasmons, in Cr-doped ZnSe, results in a downward shift of the LO peak with increasing dopant concentration. The ratio of integrated intensities of LO and transverse–optical (TO) modes in ZnSe:Cr shows a systematic increase, with increasing temperature and decreasing excitation wavelength. This occurs due to the interaction of the LO phonon field with the surface electric field in the depletion layer as an enhancement of LO mode intensity. The localized Raman modes of CrZn–Se are seen in Cr-doped samples, which corroborate the photoluminescence findings of the presence of Cr2+ and Cr1+ deep levels. The LO and TO modes of intrinsic ZnSe show upward renormalization in their peak positions with decreasing excitation wavelength. The interaction of discrete phonons wi...

Raman and photoluminescence studies on intrinsic and Cr-doped ZnSe single crystals

Electrochemical etching of pores in as-grown and doped n-type ZnSe substrates is reported. To dope the samples the as-grown semi-insulating substrates were annealed in a Zn melt containing Al impurity at concentrations ranging from 0.1 to 40 at.%. We demonstrate the growth of arrays of parallel pores with diameters ranging from several hundreds of nanometers down to 40 nm. According to the dependence of the anodic current on the applied potential, the pore growth is found to be mediated by oxide formation. LO–phonon–plasmon coupling and the emergence of the Frohlich-type surface phonon mode are studied by Raman spectroscopy of annealed and electrochemically treated samples. The position of the Frohlich mode is found to be identical in porous samples with different diameters of pores and skeleton wall thicknesses, in accordance with the effective medium theory when applied to porous materials with identical semiconductor skeleton relative volume concentration. The photoluminescence analysis of the prepared porous structures is indicative of effective passivation of the porous skeleton surface during anodization while Raman scattering evidences a decrease in the free carrier concentration and neutralization of impurity centers in the porous skeleton walls.

Photoluminescence and vibrational properties of nanostructured ZnSe templates

We present an experimental and theoretical investigation of the Raman line shape of long-wavelength phonons with longitudinal optical (LO) symmetry from both sides of Zn1−xBexSe/GaAs (001) heterojunctions with special emphasis on samples with low Be content (x⩽020) First the built-in p-type LO phonon–plasmon (LO–P) coupled mode at the near-interfacial substrate is used as a sensitive probe to investigate the interfacial quality The corresponding hole gas is reinforced when the ZnBeSe layers are nominally p doped by nitrogen This provides clear evidence for effective hole transfer across the junction, and thereby indicates a minimized density of interfacial defects In the nitrogen-doped layers hole densities as high as 1017 cm−3, in accordance with capacitance–voltage measurements, are directly inferred from clear weakening of the ZnSe-like LO mode due to LO–P coupling Concerning the intrinsic properties of the alloys, we demonstrate that the asymmetric broadening of the ZnSe-like LO mode is determin

Raman study of Zn1−xBexSe/GaAs systems with low Be content (x⩽0.20)

The characterization of semiconductor layer and interface quality with special reference to raman spectroscopy

The Hall coefficient, electrical conductivity and electron mobility are investigated in Ag-doped n-ZnSe single crystals at temperatures between 8 and 500 K immediately after doping the samples and after storing them for 5 years under normal room conditions in darkness. The formation of donor-type Agi interstitial defects stimulated by time is found for the first time. After 5 years storage, the samples show a dramatic increase in the electrical conductivity, free electron concentration and concentration of shallow donors. A model that explains these changes by the displacement of Ag ions into interstitial sites due to lattice deformation forces is proposed. The formation of an Ag-donor impurity band in n-ZnSe:Zn:Ag crystals stored at room temperature is also studied.

Shallow donor states of Ag impurity in ZnSe single crystals

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

T. Marshall

Papers

Phonon coupling to excitations of free and localized electrons in n-type ZnSe.