Excitonic Properties of ZnS Quantum Wells in ZnMgS

Abstract

The excitonic properties of cubic ZnS quantum wells in ZnMgS are studied by reflectivity and magneto-optics. A remarkable improvement in the quality of the samples grown by MBE on GaP substrates has allowed the observation of heavy and light hole exciton transitions with values for the FWHM as narrow as 5 meV. The 2s state of the heavy hole exciton is identified and exciton binding energies as high as 55 meV are deduced, indicating that for quantum wells narrower than 3.5 nm the exciton-LO phonon scattering can be suppressed. Zeeman splittings of the order of 10 meV for both the light and heavy hole exciton transitions appear in magneto-reflectivity spectra in magnetic fields up to 54 T. Large light hole exciton g-values of the order of four for all quantum wells are obtained due to the light hole being the uppermost valence band in these tensile strained quantum wells. A strong reduction in the diamagnetic shifts for narrow wells is observed due to increasing quantum confinement.

Mn2+-induced substitutional structural changes in ZnS nanoparticles as observed from positron annihilation studies

“…This argument is based on the fact that the grain size of 2.5 nm of the samples used here is significantly less than the typical thermal diffusion lengths of positrons (50-100 nm [13]) and hence the thermalized positrons can diffuse out to the surface of grains prior to annihilation. Another reason for the larger values of the obtained positron lifetimes is that the grain size of 2.5 nm is smaller than the excitonic Bohr diameter of ZnS (∼5 nm [14]). This means that quantum confinement effects have already set in and given rise to the average one electron energy level split and an effective increase in the band gap in the atoms of these grains.…”

Positron annihilation studies of defects and interfaces in ZnS nanostructures of different crystalline and morphological features

“…8͑a͒ and 8͑b͒, further clarifies the arguments cited above. 17 So the samples with average grain sizes of 1.5 nm could exhibit such effects, as reflected by the remarkable changes observed in all the positron annihilation parameters when the average grain sizes were reduced to this size. The other trapping rate 2 also decreases.…”

Visible emission characteristics from different defects of ZnS nanocrystals

“…The value of the ratio was found to increase with the size of ZnS nanoparticles. Although our smallest sample is still larger than the exciton radius (2.5 nm) of the ZnS bulk material, 43 we report here the correlation of the increasing electron-phonon interaction with increasing nanocrystal size. This phenomenon is observed in pure ZnS particles for the first time.…”