E. Kurtz

Formation and properties of self-organized II-VI quantum islands

Abstract The formation and optical properties of CdSe based, self-assembled quantum dot (QD) like nano-structures embedded in ZnSe have been studied. Self-assembling growth was achieved under both standard molecular beam epitaxy (MBE) and low temperature atomic layer epitaxy (230°C) with a subsequent annealing step. While in the case of standard MBE the competition between the relaxation via misfit dislocations and the desired dot formation leads to a low reproducibility, the latter method allows a more controlled formation of the QDs, which is clearly indicated by reflection high energy electron diffraction. In particular, a capping of the structures with ZnSe usually recovers a 2D surface thus allowing a stacking of several sheets of QDs. Small dots with a lateral diameter of 5–6 nm, which corresponds to the bulk exciton Bohr radius, and a height of 5–6 ML could be obtained as confirmed by transmission electron microscopy. The optical and structural properties of the QDs were studied by means of time resolved, resonant photoluminescence and were compared with a series of quantum wells (QW). Because of the high bandgap difference of ZnSe and CdSe, deep potential fluctuations exist within the QWs. These are caused by local interdiffusion and interface roughness and can act like low dimensional traps. However, because of their nature, they are not necessarily laterally isolated and can interact via tunneling and phonon assisted hopping. This leads to a very typical red shift of the emission peak with time in time-resolved photoluminescence (PL). In the case of self-assembled QDs, the potentials defined by the QDs are spatially well separated, as the typical dot densities observed are in the mid 1010–1011 cm−2 range. The interaction between these potentials is thus strongly suppressed, which clearly shows in the temporal evolution of both maximum position and the half width of the emission peaks. For the first time we were also able to demonstrate that CdSe can be grown with a CdS compound and additional Se flux. Sulfur seems to act as a surfactant that leads to surface smoothing and a reduced inhomogeneous broadening of the PL emission. The results are quite promising as the layers grown can be thermally activated to reorganize to coherently strained islands.

Statistical analysis of near-field photoluminescence spectra of single ultrathin layers of CdSe/ZnSe

The statistical analysis of thousands of near-field photoluminescence spectra of single ultrathin CdSe layers at 20 K exhibits a strong positive correlation peak around 20 meV energy with a width of 5 meV. Our data are consistent with individual spectra which consist of sets of many pairs of lines. In each pair, the two lines must have comparable strength. We speculate about the origin of these pairs.

Polarized luminescence in CdS/ZnSe quantum-well structures

The photoluminescence from type II CdS/ZnSe quantum-well structures is found to be polarized with respect to the 〈110〉 directions with polarization degrees up to 20%. The absolute polarization direction is related to the interface bond directions in samples with differently prepared interfaces. The observations are explained by the detailed analysis of the epitaxial growth process and polarization sensitive luminescence experiments.

Properties and self-organization of CdSe:S quantum islands grown with a cadmium sulfide compound source

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.

Cd Distribution and Defects in Single and Multilayer CdSe/ZnSe Quantum Dot Structures

Conventional and high-resolution transmission electron microscopy (TEM) was applied to a study of the Cd distribution and structure of defects in single and multiple CdSe layers in a ZnSe matrix. The samples containing nominal CdSe layer thicknesses tn between 1.7 and 3.5 monolayers (ML) were grown by molecular beam and atomic layer epitaxy under different conditions. In all samples ternary CdZnSe wetting layers with Cd-rich regions with sizes of less than 10 nm (small islands: SIs) and a density of ∼1011 cm—2 are observed. The Cd concentration in the wetting layer and the SIs increase with tn. In addition, regions with sizes of 20–30 nm (large islands: LIs) and Cd concentrations >40% occur in specimens with tn > 2.5 ML. In the vicinity of the LIs stacking faults are preferably generated leading to a “coffee-bean” contrast in plan-view TEM images. The multilayer structures with tn ≈ 2.5 ML display a predominant vertical correlation of the SIs at a ZnSe spacer thickness of 12 ML.

Spectroscopic Evidence for the Exciton Percolation Threshold in Low‐Dimensional ZnCdSe Solutions with Nano‐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-rich 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 E ME only a resonant excitation of island related states is possible.

CdSe quantum islands in ZnSe: a new approach

Abstract 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.

Suppression of lateral fluctuations in CdSe-based quantum wells

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.

Correlation in Vertically Stacked CdSe Based Quantum Islands

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.

MBE grown high-quality CdSe-based islands and quantum wells using CdS compound and Se

Abstract 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.