B. Dal Don

Solid immersion lens-enhanced nano-photoluminescence: Principle and applications

We demonstrate a far-field nano-photoluminescence setup based on the combination of a hemispherical solid immersion lens (SIL) with a confocal microscope. The spatial resolution is confirmed to be 0.4 times the wavelength in vacuum in terms of half width at half maximum. The collection efficiency is found to be about five times higher than the same microscope without SIL, which is consistent with our theoretical analysis. We investigate in detail the influence of an air gap between the SIL and the sample surface on the system performance, and prove both experimentally and theoretically the tolerance of this far-field system to an air gap of several micrometers. These features make the present setup an ideal system for spatially resolved spectroscopy of semiconductor nanostructures. In particular, we show two examples of such applications in which the present setup is clearly suitable: Studies of excitonic transport in quantum wells and spectroscopy of single quantum dots with emphasis on polarization depe...

Spatiotemporal dynamics of quantum-well excitons

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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.

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.

Combining a scanning near-field optical microscope with a picosecond streak camera: statistical analysis of exciton kinetics in GaAs single quantum wells

Combining a low-temperature scanning near-field optical microscope with a picosecond streak camera allows us to measure the complete wavelength-time behavior at one spot on the sample within about 13 min at excitation powers of 100 nW. We use this instrument to measure the variation of relaxation times in disordered single-GaAs quantum wells with sample position.

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.

Influence of the Quantum Island Distribution on Relaxation of Localized Excitons in CdSe/ZnSe Heterostructures

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 (μ-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.

High Excitation Effects and Relaxation Dynamics in CdS/ZnSe Single Quantum Wells

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.

Investigations on phonon-assisted relaxation mechanisms in CdSe/ZnSe quantum islands using time-resolved near-field spectroscopy

We report on relaxation studies of excitons in quantum islands using time-resolved near-field photoluminescence (n-TRPL). This new method allows us to follow the temporal evolution of an ensemble of localized states with a spatial resolution of about 200 nm. The temporal and spectral resolutions are 5 ps and 1.7 meV, respectively. We study the relaxation mechanisms of excitons in two samples containing CdSe/ZnSe quantum islands. In the first sample, where the excitons are weaker localized, the sharp lines of the n-TRPL are repeated at a constant distance of 25 meV. This is not the case in the second sample, where the excitons are stronger localized. We discuss the origin of this correlation, comparing the dynamics of the lines.

Non-classical transport regimes of excitons in semiconductor quantum wells

We monitor directly non-classical transport of quantum-well excitons. The excitons propagate coherently up to four hundreds nanometers, followed by a hot-exciton transport on a length scale of several micrometers at low temperatures.