S. Wachter

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...

Hot exciton transport in ZnSe quantum wells

The in-plane transport of excitons in ZnSe quantum wells is investigated directly by microphotoluminescence in combination with a solid immersion lens. Due to the strong Frohlich coupling, the initial kinetic energy of the excitons is well controlled by choosing the excess energy of the excitation laser. When increasing the laser excess energy, we find a general trend of increasing transport length and more importantly a pronounced periodic quenching of the transport length when the excess energy corresponds to multiples of the LO-phonon energy. Such features show the dominant role of the kinetic energy of excitons in the transport process. Together with the excitation intensity dependence of the transport length, we distinguish the phonon wind driven transport of cold excitons and defect-limited hot exciton transport.

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.

Effect of quantum confinement on exciton-phonon interactions

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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.

Excitation induced shift and broadening of the exciton resonance

The influence of coherent photoexcited excitons on the absorption spectra and the Four Wave Mixing response of ZnSe/ZnMgSSe quantum wells is investigated for various polarization configurations. For purely resonant excitation at the heavy-hole-exciton a blue shift and an increasing homogeneous line width of the exciton absorption peak is found. The clear and stable blue shift shows a linear dependence on excitation fluence both in experiment and theory with different slopes for the different pump and probe polarization configurations. The equivalence of pump and probe, and Four Wave Mixing results is shown experimentally.

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.

Relation between spin and momentum relaxation in ZnSe/ZnMgSSe quantum wells

Abstract First, we measure the coherence time of optically oriented carrier spins in wide gap quantum wells and observe an increase with temperature from less than 10 ps at 20 K to 500 ps at 200 K. Second, we measure the temperature dependence of the momentum relaxation time by four wave mixing and absorption experiments. The spin relaxation time is approximately proportional to the inverse of the momentum relaxation time. In the low-temperature regime, the strong increase of spin lifetime can be caused by motional narrowing due to the faster scattering of excitons with increasing temperature. In the high-temperature regime, quenching of the electron-hole spin flip dephasing due to an increasing ionization of excitons with increasing temperatures can prolongate spin lifetime.

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.

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.