CdS/ZnSe is a quaternary type II system, which allows to tune the luminescence over a considerable fraction of the visible spectrum. We present here theoretical and experimental investigations of the density dependent blue shift of the emission of single quantum wells under cw excitation and on the luminescence dynamics after pulsed excitation as a function of the US well width.

Earlier we reported the investigation of the electrical properties of selectively doped and degenerate CdS/ZnSe quantum heterostructures grown by molecular beam epitaxy [V. Kažukauskas, M. Grün, St. Petillon, A. Storzum, and C. Klingshirn, Appl. Phys. Lett. 74, 395 (1999)]. The maximum Hall mobilities in these heterostructures were found to be less than 400 cm2/Vs. In the present work we analyze in detail the scattering mechanisms in order to increase the carrier mobility and to optimize these quantum structures. We demonstrate that the Hall mobility can reach in the CdS quantum wells at low temperatures 2800 cm2/V s for slightly doped structures, having an effective sheet carrier density 2.6×1011 cm−2. In these structures the mobility is mostly limited by interface alloying scattering. At high doping levels carriers become redistributed between the quantum well and the ZnSe doped layer. This causes the parallel conductivity phenomena, which diminishes the effective mobility. Near room temperature the scattering by optical phonons prevails which is superimposed by dislocation scattering.

We examined the electrical properties of modulation-doped CdS/ZnSe single quantum wells grown by molecular beam epitaxy. The n-doping was performed in the ZnSe barrier material to realize a two-dimensional electron gas (2DEG) in the CdS quantum well (QW). With van de Pauw and Hall bar measurements we determined the specific resistivity, the charge carrier density and the carrier mobility of the beterostructures. The strong dependence of the results of degenerately doped structures on the doping concentration and on the well thickness can be explained by a parallel layer conduction model. Limitations of the charge carrier mobility in these structures are due to scatter ing at the interface roughness and alloy disorder in the QW as well as ionic and piezo scattering in the doped layers.

Photoluminescence of the ground and excited states of type II CdS/ZnSe single quantum wells with different width is investigated for high electron densities and by means of time resolved photoluminescence measurements. For high excitation intensities the filling of states in the QW up to energies of the second excited state can be observed. The PL decay times of the ground state vary from 2-36 ns for different samples and depend strongly on the QW width. This dependence can be explained with the overlap of electron and hole wavefunctions. The PL rise time of the ground state is influenced by inter- and intrasubband relaxation, which is induced by the cooling of hot carriers.