T. Passow

Single-photon emission of CdSe quantum dots at temperatures up to 200 K

We report on the generation of triggered single photons obtained from epitaxially grown self-assembled CdSe/Zn(S,Se) quantum dots for temperatures up to 200 K. At low temperatures (T 40 K) an increasing multi-photon emission probability due to spectrally overlapping acoustic phonon sidebands of neighboring quantum dots is observed. We found that the multi-photon emission probability of a bare quantum dot (background subtracted) is strongly suppressed at 200 K if compared to a Poissonian light source of the same average intensity. Our results demonstrate the large potential of self-assembled CdSe/Zn(S,Se) quantum dots for nonclassical light generation at temperatures up to 200 K.

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.

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.

500–560 nm Laser Emission from Quaternary CdZnSSe Quantum Wells

ZnSe-based laser diodes with emission wavelength from 500 to 560 nm are studied. The long wavelength operation of these laser diodes requires careful optimization of the CdZnSSe quantum well material. It is shown that under stoichiometric growth conditions quantum wells with high optical and structural qualtity can be realized. Employed in laser structures room-temperature cw-operation around 560 nm is obtained. In comparison with laser diodes emitting at 505 nm it is found that the high Cd content of the quantum well does not degrade the operational characteristics of the devices. In pulsed mode more than 1100 mW output power at 560 nm is achieved.

Single-electron charging of a self-assembled II-VI quantum dot

We have studied single-electron injection into individual self-assembled CdSe/ZnSe quantum dots. Using nanostructured contacts to apply a vertical electric field, excess electrons are promoted to the single-quantum-dot ground state in a controlled fashion. Spatially-resolved photoluminescence spectroscopy is applied to demonstrate single-quantum-dot charging via the formation of single zero-dimensional charged excitons with a binding energy on the order of 10 meV.

Electro-Optical Characterization of CdSe Quantum Dot Laser Diodes

We report the electroluminescence characteristics at room-temperature of CdSe quantum dot laser diodes. In comparison with quantum well laser diodes emitting in the same wavelength region a different dynamical behaviour under pulsed operation is found. Under short pulsed (0.1 μs) current injection a blue shift of the emission and an intensity increase are found with increasing driving current. Electrically pumped lasing is realized for 50 ns pulse width.

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.

High-resolution x-ray diffraction investigations of highly mismatched II-VI quantum wells

High-resolution x-ray diffraction (HRXRD) was used to systematically investigate CdSe and ZnTe quantum wells one to three monolayers thick sandwiched between a ZnSe buffer and cap layer grown at different substrate temperatures. For comparison high-resolution transmission electron microscopy (HRTEM) measurements were performed which were evaluated by digital analysis of lattice images. The x-ray diffraction profiles show typically two main layer peaks. Their intensity ratio depends critically on the quantum well thickness and varies only weakly with the thickness of the ZnSe layers. The total Cd or Te content determined from comparisons of experimental and simulated (004) -2 scans is well confirmed by the results from digital analysis of HRTEM lattice images. For quantum well thicknesses larger than 1.5 (ZnTe) or 2.0 (CdSe) monolayers, no simulation parameters could be found to achieve good agreement between theoretical and measured diffraction profiles. This transition is more clearly visible in diffraction profiles of asymmetrical reflections. By HRTEM measurements, this could be correlated to the occurrence of stacking faults at these thicknesses. The formation of quantum islands detected by HRTEM was not reflected in the HRXRD -2 scans.

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.

Optical Gain of CdSe Quantum Dot Stacks

Systematic gain measurements of CdSe quantum dot stacks were carried out using the variable-stripe-length method at various pump densities and temperatures. The fivefold quantum dot stacks show an effective optical gain at double threshold of the order of 125 cm -1 (150 cm -1 ) for the doped (undoped) sample. A maximum optical gain of 400 cm -1 was achieved at a pump density of 980 kW/cm -2 and a temperature of 10 K. High-resolution transmission electron microscope and micro-photoluminescence measurements were carried out. Temperature depending measurements yield a strong increase of the threshold densities for temperatures above 100 K.