K. Leonardi

Single zero-dimensional excitons in CdSe/ZnSe nanostructures

Zero-dimensional excitons (0DXs) in CdSe/ZnSe nanostructures have been studied by time- and spatially resolved photoluminescence spectroscopy. The three-dimensional confinement is confirmed by an exciton lifetime up to 550 ps, independent of temperature up to 130 K. By preparing mesa structures with diameters down to 50 nm as local probes, an extremely high spatial resolution is achieved, giving experimental access to single 0DXs. A splitting of the ground state into a linearly polarized doublet with an energy spacing up to 1.5 meV is found, varying from dot to dot in sign and magnitude. This indicates a noncircular shape with no preferential orientation of the dots. The dot density is estimated to increase from 5×1010 to 1.5×1011 cm−2, when changing the nominal CdSe layer thickness from 1 to 3 ML, i.e., close to the critical thickness.

Stark effect and polarizability in a single CdSe/ZnSe quantum dot

The quantum-confined Stark effect in a single self-assembled CdSe/ZnSe quantum dot was studied by means of highly spatially resolved photoluminescence spectroscopy. A nanotechnological approach making use of a capacitor-like geometry enabled us to apply a well-defined lateral electric field on the quantum dots. Stark shifts of up to 1.1 meV were obtained, which can be well fitted by a purely quadratic dependence on an electric field. In quite good agreement with theoretical calculations, an exciton polarizability of 4.9×10−3 meV/(kV/cm)2 can be extracted, while the permanent dipole moment in the lateral direction is found to be negligible.

Spectral diffusion of the exciton transition in a single self-organized quantum dot

We report on reversible spectral shifts in the emission spectra of self-organized CdSe single quantum dots on a time scale of seconds. Energy shifts of up to 3.5 meV have been observed and can be attributed to the Stark effect caused by fluctuating local electric fields. Most surprisingly, the energy shift turns out to be quasi-periodic with time constants between 70 and 190 s.

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.

Quantum dot formation by segregation enhanced CdSe reorganization

The influence of the growth conditions during capping of CdSe/ZnSe quantum structures grown on GaAs(001) by molecular-beam epitaxy (MBE) were systematically investigated by high-resolution x-ray diffraction, transmission electron microscopy, and temperature dependent, partly time-resolved photoluminescence spectroscopy. The results clearly indicate formation of quantum wells with potential fluctuations if conventional MBE is used for capping the CdSe by ZnSe. In contrast, quantum dot formation occurs using migration enhanced epitaxy for this growth step. In the latter case, quantum dots can be obtained without formation of stacking faults.

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.

Nondestructive detection of stacking faults for optimization of CdSe/ZnSe quantum-dot structures

CdSe/ZnSe quantum structures were systematically investigated by high-resolution x-ray diffraction. The samples were grown at different growth temperatures on GaAs(001) substrates by molecular-beam epitaxy. A model is presented enabling the simulation and quantitative analysis of x-ray diffraction profiles influenced by stacking faults. This yields a fast and nondestructive method for the determination of stacking fault densities after calibration by transmission electron microscopy. A steep increase of the stacking fault density above a critical thickness was found. The critical thickness decreases with increasing growth temperature. Above this critical thickness, the amount of incorporated CdSe remains apparently constant.

Growth of self-assembled (Zn)CdSe nanostructures on ZnSe by migration enhanced epitaxy

The growth of CdSe on ZnSe by migration enhanced epitaxy has been studied in situ by means of reflection high-energy electron diffraction (RHEED) and reflection anisotropy spectroscopy (RAS). The change from a streaky to a spotty RHEED pattern during CdSe deposition indicates the transition to a three-dimensional growth mode. This change is accompanied by a damping of RHEED intensity oscillations and a decrease in the overall intensity. RAS spectra show a pronounced change at a photon energy of about 2.2 eV with increasing CdSe thickness. To investigate the role of lattice mismatch, ternary ZnCdSe layers of different compositions have been grown. With decreasing Cd concentration the critical thickness for the growth mode transition increases and for Cd concentrations below 30% no growth mode transition could be observed.

Manipulating single quantum dot states in a lateral electric field

Abstract We demonstrate measurements of the quantum confined Stark effect on single self-assembled CdSe/ZnSe quantum dots. For this purpose, a nano-scaled capacitor was developed being capable of exerting lateral electric fields up to 15 kV / cm on single quantum dots. Stark shifts of up to 1.1 meV have been obtained for the single exciton emission accompanied by a line width broadening due to field-induced carrier tunneling into the barrier. Evaluating the redshift of the luminescence signal as a function of the electric field enables us to extract the polarizability as well as information on the permanent dipole of a single quasi-zero-dimensional exciton.

Comparative study of molecular beam and migration-enhanced epitaxy of ZnCdSe quantum wells: influence on interface and composition fluctuations

Optical and structural properties of ZnCdSe quantum wells (QWs) grown by either migration-enhanced epitaxy (MEE) or molecular beam epitaxy (MBE) have been compared. Firstly the QWs are grown by depositing Zn and Cd at the same time during MEE. These samples exhibit significantly lower Cd concentrations than samples grown by MBE with similar Cd fluxes. Compared to MBE grown samples with similar low Cd concentrations neither photoluminescence (PL) nor high-resolution transmission electron microscopy (HRTEM) reveal significant differences in the optical and structural properties, respectively. Secondly the QWs are grown as a digital alloy by depositing CdSe and ZnSe by turns. In this case typical PL spectra show line widths as narrow as 16 meV which means a reduction of about 10 meV compared to typical values from MBE grown samples. By means of HRTEM this could be correlated to improved interfaces and a better composition homogeneity.