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.

The photoluminescence (PL) and PL excitation (PLE) spectra of quantum wells (QWs) formed by CdSe insertions in ZnSe matrix reveal the states of heavy and light excitons localized in CdSe-rich islands, and-the energy E-ME which is associated with the percolation threshold over the entire lateral plane of QW. The model calculations are performed which result in evaluation of the island mean size and composition of ZnCdSe solid solution within and outside of islands.

The absorption spectra of CuBr, CuCl, CuI, and AgI nanocrystals (NC) embedded in a glass matrix have been investigated in a wide temperature range between T=6 and 860K. The change of the absorption in the vicinity of the exciton resonance for CuBr and CuCl reveals a pronounced reduction of the melting temperature of NCs compared to the bulk value and shows a strong hysteresis between melting and solidification temperatures. The melting temperature and the hysteresis width depend on the sizes of NCs. For CuBr NCs a second hysteresis around the bulk melting point is observed. For large NCs the temperature dependence of the exciton peak energies is analogous to its counterpart in bulk crystals. For CuI NCs the shrinkage effect of the gap for both cubic and hexagonal modifications is observed and the temperature coefficients of the energy gaps are deduced. For AgI NCs a rather unusual temperature dependence of energy gap is observed. This dependence is nonmonotonic and shows a change of the energy gap slope from positive to negative.

We use a spatially integrating sphere to measure the absolute external luminescence quantum efficiency of GaAs/Al0.3Ga0.7As multiple quantum well (MQW) structures with and without GaAs substrate. The substrate reduces the external quantum efficiency by a huge amount depending on structure design and absorption coefficients, thus making it impossible to obtain information about the internal efficiency of the structures with substrate. From the temperature dependence of the efficiency we calculate with a thermal activation model the ratios between radiative and nonradiative lifetimes and extract the activation energies. The individual efficiencies of the various emission bands (i.e. hh, lh and defects) are extracted separately. Furthermore we present a simple model for the efficiency gain after the removal of the substrate. With this model we estimate the internal efficiencies of the samples.

The temperature dependence of exciton peak energies in large CuI quantum dots embedded in a glass matrix has been measured. The gap shrinkage effect for both cubic and hexagonal CuI phases was observed. The temperature coefficients of the energy gaps and the effective phonon energies are deduced.

We report on relaxation of excitons in II-VI heterostructures containing CdSe islands embedded in a ZnSe matrix, and showing different localization properties. We perform spatially resolved photoluminescence (mu-PL), time-resolved PL (TRPL), and time-resolved near-field spectroscopy (n-TRPL) to study the differences between two samples. In the sample showing the weaker localization, we see a correlation between the localized states, which is not obvious in the sample with the stronger localization. We consider as possible explanations for these repeated lines phonon-assisted transport as well as relaxation from excited states into a lower energetic state.

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.

Vertically correlated islands of a quality comparable to the model system InAs/GaAs have been fabricated using modified molecular beam epitaxy. While island widths range from 20-50 nm, island heights are very homogeneous, resulting in narrow photoluminescence (PL) emission line width of 20-40 meV even for stacks of 50 layers. Island centers contain Cd concentrations up to more than 80%. Distinct PL double peaks can be assigned to correlated island stacks with large islands and the uncorrelated region, containing much smaller, laterally interacting islands.

We have studied photoluminescence (PL), PL excitation (PLE), and micro-PL spectra of single quantum wells (QWs) formed by CdSe insertions in ZnSe matrix with different nominal Cd thickness (1-3 monolayers (ML)). The PL spectra are considerably red-shifted with respect to the position expected for homogeneous Cd distribution over the QW and can be attributed to the luminescence of CdSe-ricli islands. It has been found that PLE spectra of different points of the PL band show a characteristic divergence at excitation below some characteristic energy E-ME. This energy is identified with the percolation threshold above which the exciton is able to move over the whole lateral plane of QW whereas below the EME only a resonant excitation of island related states is possible.

While providing a general overview over the current status of self-organization of quantum islands in the II–VI semiconductor system, with the main focus on CdSe embedded in ZnSe, this paper shall give an introduction to the possibilities opened by a modification of the standard growth technique. In molecular beam epitaxy, we have substituted the generally used Cd-elemental source with a CdS-compound source. The sulfur is usually not included in the growing layer. However, its presence can be surfactant-like while the elevated Cd-temperature of the dissociated CdS leads to changed thermodynamic conditions on the growth front. Using migration enhanced epitaxy, nearly perfect quantum wells with respect to lateral homogeneity can be obtained by suppressing the inherent Cd segregation and clustering. These processes are generally responsible for the formation of small islands (SI) (lateral diameter 3–5 nm) even when not attempting to grow island like structures. The suppression of these SI was a first step to gain control over the island formation. Larger islands with central Cd concentrations above 40% are more of interest for device applications, since a population at room temperature is necessary. In particular, high-density systems are required. Using the modified growth mode, well correlated, stacked island systems were obtained. Their outstanding structural and optical properties will be discussed in detail. The absence of a closed wetting layer in the CdSe/ZnSe system and the appearance of island like structures, even at submonolayer nominal deposition, further corroborate the assumption that island formation does not readily occur in a standard Stranski–Krastanow growth mode, which is assumed for InAs/GaAs.

Using CdS and Se, we have grown CdSe/ZnSe quantum structures with improved optical and structural properties exploiting an exchange reaction which leads to the substitution of sulfur by selenium. Depending on the growth mode it is possible to obtain quantum wells without strong lateral localization or as quantum island structures of good homogeneity. The differences in the exciton relaxation behaviour in these structures is demonstrated by photoluminescence measurements.

The photoluminescence quantum efficiency of the yellow series 1s orthoexciton in Cu2O, including its phonon sidebands, was measured in an Ulbricht sphere. The obtained efficiency values between 10(-4) and 10(-6) are remarkably low. The nonmonotonous temperature dependence is analyzed.

In this study we report on the fabrication and investigation of high quality CdSe-based quantum structures embedded in ZnSe. The structures were grown by molecular beam epitaxy using a novel growth technique in which the commonly used Cd-elemental source is substituted by a CdS compound source. An exchange reaction between the sulfur and elemental Se leads to the growth of CdSe with a minor sulfur contamination of the order of 1% depending on the growth conditions. Quantum structures grown with the compound show a better homogeneity and narrower photoluminescence linewidth than conventionally grown structures even though high resolution transmission electron microscopy indicates that intermixing of CdSe and ZnSe still occurs. The properties and implications of the structures grown by the new method will be discussed.

Using CdS compound and elemental Se, we have grown CdSe/ZnSe quantum structures with improved optical and structural properties exploiting an exchange reaction which leads to the substitution of sulfur by selenium. Typical S contamination is below 2%. A possibly enhanced surface diffusion of adatoms caused by the high CdS oven temperature and a surfactant-like effect of the S–Se exchange lead to a suppression of Cd segregation in the case of migration enhanced epitaxy with long Se exposure times. The new growth method leads to CdSe quantum wells with outstanding optical quality. Their properties are compared to CdSe island structures obtained in a non-migration-enhanced growth mode.

ZnTe/Zn1-xMgxSeyTe1-y single quantum wells (SQW) were investigated by means of linear and nonlinear optical spectroscopy. The composition of the quaternary barrier was chosen to achieve an almost lattice matched growth on ZnTe substrates. The good quality of the samples is reflected in the photoluminescence with sharp excitonic features of about 5 meV FWHM. Pulsed ns excitation energetically above the barrier provokes lasing of the barrier with a low threshold of 32 kW/cm(2). Resonant excitation in the well leads to the creation of an electron hole plasma in the SQW. The observed optical gain reaches from 180 cm(-1) for the SQW up to 400 cm(-1) for the barrier.

We report on localization dynamics of excitons in ensembles of self-organized CdSe islands embedded in ZnSe. The experimental methods employed are temperature dependent, spatially-resolved photoluminescence (mu -PL), spatially-integrated PL (macro-PL), as well as time-resolved PL (TRPL). We see the often observed non-monotonous shift of the FL maximum with temperature which we can explain by a redistribution of the excitons amongst the islands. The measured shift is compared with the exact shift of the bandgap deduced from mu -PL measurements and is found to depend strongly on island size and distribution. These transport processes are recovered in the temporal evolution of the FL. The decay time of the spectrally integrated PL reaches its maximum at exactly the same temperature at which the redshift of the macro-FL turns into a blueshift.

We report a reduction of inhomogeneous broadening in CdSe-related quantum wells in ZnSe by employing a growth technique that uses a CdS-compound source instead of the standard Cd elemental source for molecular-beam epitaxy. Assisted by the low sticking coefficient of sulfur and possibly an exchange reaction between S and Se, only a small S contamination is observed. A comparison with standard layers reveals an increase in quality and homogeneity by a strong reduction of the photoluminescence (PL) linewidth. Samples obtained by our method show extremely little lateral confinement as indicated by a lack of sharp single dot emission lines in micro PL and the absence of the extensive redshift observed in temperature dependent PL of fluctuating well potentials.

We report the linear absorption spectra of relatively large copper iodide nanocrystals embedded in an alumina borosilicate host network structure. The spectra reveal pronounced exciton lines of both the zincblende and the layered hexagonal structures. In the approximation of the weak-confinement regime, the translational masses for the Z(12) and Z(3) excitons, as well as the anisotropy of the Z(12)-exciton band, i.e., the exciton Luttinger parameters, are deduced from the spectral positions of the exciton lines.

We demonstrate a new technique to grow high-quality CdSe quantum films and islands with a very small sulfur contamination by using a cadmium sulfide compound source as Cd supply and additional Se flux. By monitoring the lattice constant with reflection high-energy electron diffraction, it is shown that the sulfur is almost completely substituted by Se and CdSe with a contamination below 5% sulfur is formed. The quantum structures obtained by the new method are generally of higher quality than those obtained by more conventional growth methods using elemental sources, even if migration enhanced methods were employed. With a brief growth interruption or post-growth annealing step the initially smooth CdSe layer can be reorganized into islands. The duration of this step as well as the initial amount of deposition allows a rather good control over the island formation. A strongly enhanced growth rate is observed for the first few monolayers of the ZnSe capping layer, which indicates a partial dissolution of the islands in the ZnSe growth front and Cd segregation.