A concept of mixed exciton-trion states is formulated theoretically and proved experimentally for II-VI semiconductor quantum wells with a two-dimensional electron gas. The concept considers the resonances of neutral excitons and charged excitons (trions) as mixed (with each other) via their interaction with free electrons. Reflectivity spectra of modulation-doped ZnSe/(Zn,Mg)(S,Se) and CdTe/(Cd,Mg)Te quantum wells are analyzed. A good qualitative agreement of the experimental results with model calculations is achieved.
An optical method is suggested to determine the concentration of two-dimensional electrons in modulation-doped quantum wells at low and moderate electron densities between 10 9 and 2×10 11 cm -2 . The method is based on an analysis of magneto-reflectivity spectra of charged excitons (trions). The circular polarization degree and the oscillator strength of the charged excitons contain information about the density and spin polarization of two-dimensional electron gas. The method is applied to CdTe/(Cd,Mg)Te and ZnSe/(Zn,Mg)(S,Se) heterostructures.
We demonstrate that the quaternary compound ZnMgSeTe can be grown by molecular beam epitaxy in reasonable quality. For layers with energy gaps as high as 3.1 eV, nitrogen doping leads to free hole concentrations around 1018 cm−3. In combination with n-ZnMgCdSe, this material allows the fabrication of II–VI diodes lattice matched to InP substrate. Light emitting diodes containing a tensile strained ZnCdSe quantum well in ZnMgCdSe emit green light when operated in forward direction. In contrast to diodes with a comparable density of extended defects grown on GaAs substrate, these diodes show no formation of dark line defects and a lifetime which is about three orders of magnitude longer.
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