T. Kümmell

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.

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.

Size control of InAs quantum dashes

Self-organized InAs quantum dashes grown on In0.53Ga0.23Al0.24As∕InP have been investigated by chemically sensitive scanning transmission electron microscopy. The quantum dashes, which consist of pure InAs, exhibit a triangular cross section. Most important, the quantum dash size depends linearly on the nominal InAs layer thickness and can be varied by a factor of 3 without changing the height∕width ratio. Thus, the emission wavelength can be controlled between 1.37 and 1.9μm without modifying shape and composition of the quantum dashes by adjusting a single growth parameter.

Lateral quantization effects in lithographically defined CdZnSe/ZnSe quantum dots and quantum wires

Quantum dots and quantum wires based on CdZnSe/ZnSe single quantum well heterostructures have been achieved using electron beam lithography and wet chemical etching. Photoluminescence spectra of the dot and wire structures show a blue shift due to lateral quantization for lateral dimensions below 40 nm. For the dot ground state, a lateral confinement energy of 16 meV is obtained for 28 nm diameter structures. For wires with widths on the order of 20 nm, lateral confinement energies of about 5 meV are observed. The dot diameter and wire width dependence of the emission energies can be described based on a square well potential and the measured sizes of the structures.

Room temperature single photon emission from an epitaxially grown quantum dot

Single photon emission from an epitaxially grown quantum dot at room temperature is presented. CdSe/ZnSSe quantum dots are embedded into MgS barriers, providing dominant radiative recombination up to 300 K. Under continuous wave optical excitation, the autocorrelation function g(2)(t) exhibits a sharp dip at (t = 0) with g(2)(0) = 0.16 ± 0.15 at T = 300 K, revealing excellent suppression of multiphoton emission even at room temperature.

All-inorganic light emitting device based on ZnO nanoparticles

We report on room-temperature electroluminescence from an all-inorganic light emitting device based on spin-coated ZnO nanoparticles. A tight submicron thick layer was fabricated on a fluorine doped tin oxide glass as a substrate using commercially available ZnO nanoparticles from the gas phase. After the evaporation of the top Al electrode, a diodelike I-V characteristic was obtained. An emission peak at around 390 nm and a broad defect-related electroluminescence in the visible range were observed at voltages below 10 V and ambient air conditions.

Room temperature emission from CdSe∕ZnSSe∕MgS single quantum dots

The authors report on room temperature photoluminescence from single CdSe quantum dots. The quantum dots, realized by self-organized epitaxial growth, are embedded in ZnSSe∕MgS barriers. The integrated intensity of the emission drops by less than a factor of 3 between 4K and room temperature. Microphotoluminescence with a spatial resolution of 200nm exhibits single dot emission that remains visible up to 300K. The linewidth of the single dot emission increases thereby from 340μeVto25meV at room temperature, which the authors attribute to the interaction of excitons with optical phonons.

Buried single CdTe/CdMnTe quantum dots realized by focused ion beam lithography

Buried single CdTe/CdMnTe quantum dots are realized by implantation-induced intermixing using a focused 100 keV Ga+ ion beam. For an implantation dose of 5×1013 cm−2 and an annealing temperature of 390 °C, a lateral potential depth of about 65 meV is obtained. By means of photoluminescence spectroscopy, the formation of zero-dimensional multiexcitons in single quantum dots is investigated, yielding a biexciton binding energy of about 3.5 meV. In addition, the occurrence of an excited biexciton transition in the photoluminescence spectrum gives clear evidence of a suppressed exciton spin-flip process in quantum dots.

p-Si/n-ZnO Nanocrystal Heterojunction Light Emitting Device

ZnO has a high potential for use in light-emitting devices in the visible and UV spectral range. One of the main challenges in an electrically driven device is the low energy of the valence band and, consequently, the difficult injection of holes. Here, we present an approach combining naturally n-type ZnO nanocrystals with intentionally p-doped Si nanoparticles in a solution-processable nanoparticle heterojunction multilayer. The heterojunction device exhibits an efficiency, that is more than one order of magnitude enhanced compared with the ZnO reference device. White electroluminescence with color rendering indices up to 98 is obtained.

Electrically driven single quantum dot emitter operating at room temperature

We present both optically and electrically driven room temperature emission from single CdSe quantum dots, realized by self-organized epitaxial growth. A structure design that embeds the CdSe quantum dots into ZnSSe/MgS barriers results in high carrier confinement and exceptionally large quantum efficiencies at room temperature. Microphotoluminescence with a spatial resolution of 200 nm exhibits single dot emission that remains visible up to 300 K. When integrating these quantum dots into p-i-n diode structures, an electrically driven single dot emitter with pronounced room temperature emission is realized. The linewidth of the single dot emission increases with temperature due to exciton-phonon interaction and reaches 26 meV at 300 K. This value is only slightly larger than the biexcitonic binding energy, opening a way to solid state single photon emitters operating at elevated temperatures.