F. Gindele

CdSe/ZnSe quantum structures grown by migration enhanced epitaxy: Structural and optical investigations

Migration enhanced epitaxy has been applied to induce the change from two-dimensional growth to formation of self-assembling islands in the growth of CdSe on ZnSe. Transmission electron microscopy images from samples with a ZnSe caplayer show a transition from a flat quantum well to an interrupted layer with pronounced thickness fluctuations when the CdSe exceeds its critical thickness. These results are confirmed by high resolution x-ray diffraction measurements, as only for the former sample the 004 rocking curve can be simulated assuming a flat quantum well with abrupt interfaces. Compared to bulk CdSe, the photoluminescence peak is blueshifted by about 0.5 eV. PL excitation experiments indicate that the interrupted layer consists of CdSe islands embedded in Zn1−xCdxSe with a composition gradient. Atomic force microscopy images of uncapped samples show spherical islands with a height of 20 nm and a diameter-to-height ratio of 4:1.

CdSe/ZnSe Quantum Dot Structures: Structural and Optical Investigations

Using migration enhanced epitaxy the self-organized formation of CdSe islands on ZnSe in open and overgrown structures has been studied by transmission electron microscopy, high-resolution X-ray diffraction, atomic force microscopy, photoluminescence and excitation spectroscopy. The transition from homogeneous CdSe quantum wells with flat interfaces to fluctuating CdSe films could be observed when exceeding the critical thickness. These interrupted layers contain CdSe quantum dots embedded in Cd1—xZnxSe with a concentration gradient. Islands observed on open structures are unstable in time. Their chemical nature is still unclear due to the fact that similar features are obtained on pure ZnSe. First results on other highly lattice mismatched II–VI systems like CdTe/ZnSe and ZnTe/ZnSe will be presented.

Optical transitions in CdSe quantum dots: From discrete levels to broad gain spectra

Abstract The optical transition energies have been determined for the lowest electron-hole pair states of CdSe quantum dots embedded in glass. The data obtained by photoluminescence, differential absorption and photoluminescence excitation spectroscopy have been compared and a general size dependence could be established. Based on theoretical calculations, the dominant features in the spectra have been assigned to the different transitions from the ground state to the one-pair states. Within the lowest one pair transition, a fine structure due to exchange interaction has been observed. The change in the optical properties with increasing intensity has been studied and the two-pair states identified in luminescence. Spectrally broad optical gain has been found due to stimulated transitions involving both one- and two-pair states.

Formation of self-assembling II-VI semiconductor nanostructures during migration enhanced epitaxy

We have studied the transition from two- to three-dimensional (3D) growth during migration enhanced epitaxy (MEE) of CdSe on ZnSe by systematically varying the CdSe thickness. Transmission electron microscopy (TEM) images show structures that contain islands within the quantum well (QW) region when the CdSe exceeds a critical thickness. The transition to a 3D interface could be confirmed by high-resolution X-ray diffraction (HRXRD) measurements. (0 0 4) diffraction profiles could be simulated assuming flat interfaces only for CdSe below the critical thickness. Photoluminescence (PL) shows emission energies up to 2.7 eV for flat QWs while samples with CdSe layers above the critical thickness show a broad emission around 2.3 eV. Photoluminescence excitation (PLE) measurements from the latter samples support the assumption that CdSe islands within the QW region act as a final centre of recombination. Heterostructures containing a CdTe or a ZnTe layer have been investigated for comparison.

Optical Properties of AgI Quantum Dots Embedded in a Glass Matrix

We report the fabrication, transmission electron microscopy investigation and linear optical properties of silver iodide quantum dots in an aluminaborosilicate host network structure. The nearly spherical quantum dots with radii ranging from 1.7 to 2 nm exhibit a pronounced excitonic absorption and intensive intrinsic photoluminescence in the intermediate confinement regime. The selective photoluminescence and photoluminescence excitation measurements revealed the fine structure in the spectra due to the splitting of the lowest exciton state by the electron-hole exchange interaction. A considerable enhancement of the exchange energy with decreasing quantum dot size is observed.

Exciton localisation in CdSe islands buried in a quantum well of Zn1−xCdxSe

The optical properties of CdSe/ZnSe quantum structures with nominal thickness of the CdSe layer between 1 and 4 monolayers (ML) have been studied. The transition from two- to zero-dimensional quantum confinement can be monitored by photoluminescence (PL), photoluminescence-excitation (PLE), and time-resolved PL experiments. The influence of different degrees of localisation on the relaxation process of excitons is investigated. Experimental results for the excitonic transition energies are in good agreement with theory when strain-, confinement- and alloy-fluctuation-effects are included. The observation of multiple phonon modes indicates the formation of a Zn1−xCdxSe alloy with varying composition as well as the accumulation of CdSe in small islands.

Exchange Interaction in II–VI Quantum Dots and Wells

By photoluminescence and excitation spectroscopy the size dependence of the exchange-splitting energy has been determined for CdSe quantum dots with sizes below the bulk excitonic Bohr radius and different interdot distances. An enhancement over the bulk value of more than one order of magnitude is observed. In ultrathin CdS/ZnS single quantum wells the exchange splitting is found and explained within the model of a quantum disc. The relaxation process and the temporal development of the polarization degree are studied.

Excited state luminescence and fine structure in CdSe quantum dots

Abstract The lowest and higher excited electron-hole pair states of CdSe quantum dots realized as nanocrystals embedded in glass were investigated for the case of strong confinement. Applying size-selective techniques, a general size dependence of the optical transition energies could be established. For the lowest one-pair state, a fine structure in the luminescence can be explained mainly by a state split off due to the electron-hole exchange interaction. When using time-resolved detection state-filling and luminescence from higher excited states could be detected.

3D versus 1D quantum confinement in coherently strained CdS/ZnS quantum structures

Monolayer fluctuations in ultrathin, coherently strained CdS/ZnS quantum structures result in a very strong localization of excitons. The deepest localized excitons can be considered as individual, decoupled and three-dimensionally confined. Consequently, fingerprints of zero-dimensionality are found in the optical spectra like single, ultranarrow luminescence lines in micro-photoluminescence and spectrally broad optical gain in the deep blue spectral range. The exchange splitting is proven and a strong enhancement over the bulk value is observed.

Gain processes in CdSe quantum dots

The bleaching of the absorption has been studied for CdSe quantum dots in the strong confinement limit by nanosecond pump-and-probe spectroscopy. For high excitation we detect optical gain in a broad spectral region below the absorption edge accompanied by a new peak in the luminescence. The gain arises from stimulated decay of inverted one-electron-hole pair states and of two-electron-hole pair states. These results are confirmed by examination of the time-resolved luminescence.