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Cd,Zn, _.Se/ZnSe quantum wires with different Cd-content x and lateral widths down to 13 nm were fabricated by electron beam lithography and wet chemical etching. In order to study the wirewidth dependence of the LO phonon splitting between the barrier and the wire regions, micro-resonance Raman spectroscopy has been performed. Reducing the wire width down to about 30 nm, we found an increasing splitting between the ZnSe LO phonon of the barrier layers and the ZnSe-like LO phonon of the Cd,Zn,_,Se wire regions. The size of the splitting is a function of the Cd-content. This behavior is due to the strain relaxation of the deeply etched, biaxially strained wires. The experimental data are characterized by three different size regimes: for very wide wires (i.e. L, > 100 nm), the wire data are comparable to the 2D reference. An increase of the ZnSe-like LO-phonon energy compared to the unstrained Cd,Zni _,Se bulk material is observed in the biaxially strained Cd,Zn, _,Se layers. Reduction of the wire size down to about 30 nm leads to a size-dependent strain relaxation. This is indicated by a reduction of the LO-phonon energy of the ZnSe-like LO phonon of the Cd,Zni _,Se layer. Reducing the lateral sizes of the wires further down below 30 nm, the splitting between the ZnSe LO phonon of the barrier and the ZnSe-like phonon of the Cd,Zni_,Se remains nearly constant, indicating that no significant further strain release occurs in narrow wires. The experimental data are in good agreement with the photoluminescence measurements showing an increasing red-shift of the photoluminescence energy with decreasing wire width for wire widths between 100 and 30 nm. In order to discuss the Raman data more quantitatively, detailed calculations of the lateral strain distribution in the wires and the strain-induced change of the LO-phonon energy were performed. Good agreement between experiment and theory is obtained. As a result, the importance of partial strain relaxation for the discussion of the vibronic properties of the patterned semiconductor systems is confirmed.

The polarization dependence of 1.5µm wavelength GaInAsP/InP quantum-wire structures fabricated by electron beam lithography and two-step organometallic vapor phase epitaxy growth were measured and compared with quantum-film structures fabricated on the same wafer. As a result, clear anisotropic photoluminescence (PL) properties of the quantum-wire structure corresponding to the wire width (25 nm and 35 nm) were confirmed. This anisotropy was observed to be slightly stronger in the 1% compressively-strained quantum-wire structure than that in the lattice-matched one with the same wire width (35 nm).

Ridge-waveguide laser diodes based on beryllium chalcogenides have been realised. An extremely large temperature coefficient (T0 = 330 K at room temperature) allows device operation up to temperatures of 413 K. Lateral monomode emission is obtained with a ratio between the vertical and the lateral far field pattern of, for example, 1.2:1 for a stripe width of 1 µm.

A flexible and low damage dry etch technology for the fabrication of ZnSe-based nanostructures is presented. Thermally assisted electron cyclotron resonance etching using gas mixtures of chlorine compounds and Ar and N2, respectively, combines plasma etching at low ion energies with process temperatures between 60 and 250 °C. With increasing process temperatures, rising etch rates and reduced surface roughness indicate a thermal activation of the etching process. The etch profile can be controlled by varying the plasma power causing a transition from partially physical to prevailing chemical etch properties. High quantum efficiencies in CdZnSe/ZnSe quantum wires with lateral sizes down to 20 nm were obtained, indicating a significantly reduced etch damage compared to conventionally dry etched II-VI nanostructures. The potential of the etch technology is demonstrated by realizing quantum wires with a blue shift of the photoluminescence signal (e.g., 8 meV for 20-nm-wide wires) caused by lateral carrier con...

We have analyzed the thermal stability of ZnSe‐based single quantum well structures grown on a GaAs substrate by applying a rapid thermal annealing process. The photoluminescence intensity of the quantum well was used as a monitor for the thermal changes induced by the annealing process. X‐ray diffractometry yields information about the crystal quality and the strain condition before and after the thermal treatment. As a main result, we found that the thermal stability of the quantum well photoluminescence signal critically depends on the thickness of the II–VI buffer layer, i.e., the distance between the active layer and the GaAs‐II–VI heterointerface. For a buffer layer thickness of about 38 nm, the quantum well signal is totally quenched after a 1 min annealing step at 500 °C, while clear luminescence signals can be observed in samples with a 1 μm buffer even for a 750 °C process. Additionally, by comparing CdZnSe/ZnSe and ZnSe/ZnSSe quantum wells, we found that the Cd–Zn interdiffusion seems to be mor...

We have developed a versatile and low‐damage technology to realize quantum dots and quantum wires based on CdxZn1−xSe/ZnSe heterostructures. Electron beam lithography was used for pattern definition. The pattern transfer into the semiconductor was performed by a wet etch process based on a K2Cr2O7:HBr:H2O solution. Preferential etching along crystallographic planes leads to well‐defined sidewalls of the freestanding dot and wire structures. Dot sizes down to 28 nm and wire sizes down to 13 nm have been achieved. High photoluminescence efficiencies, even in the narrowest structures, allow the study of lateral confinement effects. We observe a size dependent blueshift of up to 20 meV in the photoluminescence emission spectrum of the narrowest nanostructures. The measured energy shifts can be modeled based on a rectangular lateral potential well and the measured sizes of the structures.

CdTe/CdMgTe quantum wells were processed by rapid thermal annealing (RTA) at temperatures between 390° C and 550° C. Due to the interdiffusion of Cd and Mg ions the photoluminescence of the quantum wells exhibits strong blue shifts up to 360 meV at the highest annealing temperature. This energy shift is close to the total discontinuity of the quantum well structures, indicating an almost perfect interdiffusion of barrier and well material. By using a simple model for the quantum well potential changes due to interdiffusion we derive the interdiffusion length as a function of the annealing temperature. The interdiffusion is characterized by an activation energy of 2.8 eV.

Ich verstehe mich als ein Praktiker, der die Vehikel vorzustellen hat, die in Zukunft der Verkundigung als Medien zur Verfugung stehen. Dankenswerterweise wurde ich in der Einladung darauf aufmerksam gemacht, das ich dies nur von der technischen und publizistischen Seite her zu besorgen hatte, ohne Berucksichtigung des astoralen Aspekts. Ich werde mich in meinem Referat vornehmlich auf den engeren Begriff der Massenmedien beschranken, also vor allem auf die elektronischen und dann noch auf die Presse. (...) English In his paper for the meeting of communication responsible people at the European Bishops' Conferences in Schonbrunn, Switzerland, (18th-22nd April, 1983), the Director General of the Austrian Broadcasting dealt with the development of mass media in Europe and the possibilities and conditions of such a development for the proclamation of the Church. Society depends on information, but through new technologies the mass media are going to change. These changes are especially to be expected in three areas, in the broadband cable systems, the home electronics like video recorder and video records, and through direct broadcasting satellites. An oversaturation of the recipients however, despite the many programmes, at least for European countries, is not to be expected. Also print media like newspapers, periodicals and books are not going to suffer. The growing mass media however create cthical problems for journalists and producers. The growing possibilities are not matched with growing responsibilities. The extension of communication possibilities however has also new chances for the proclamation. The Church should however openly speak as Church and not as a social movement or relief organisation. People expect from her not questions, but answers.

Etude de l'influence de l'epaisseur de la barriere sur le refroidissement des porteurs chauds dans les puits quantiques excites par laser, par spectroscopie rapide (picoseconde). Les experiences fournissent des differences remarquables des temperatures de porteurs transitoires pour differentes epaisseurs de la barriere

We have performed picosecond time‐resolved measurements on In0.53Ga0.47As/InP quantum wells with varying barrier thicknesses using 10 ps Nd:YAG excitation. For this excitation, holes and electrons are created in the In0.53Ga0.47As layers. Due to momentum conservation the Nd:YAG excitation accelerates the electrons above the InP barrier where they can diffuse but cannot recombine. By examining the rise time of the quantum well emission, we can show that for samples with thick barriers, the barrier geometry largely controls the dynamic properties of the carriers after Nd:YAG excitation.

The radiative excitonic lifetime in ${\mathrm{In}}_{\mathrm{x}}$${\mathrm{Ga}}_{1\mathrm{\ensuremath{-}}\mathrm{x}}$As/InP quantum wells is studied as a function of the well width. For well thicknesses below \ensuremath{\sim}5 nm, we find an increase of the lifetime for decreasing well width, in striking contrast to the well-width dependence of the lifetime usually observed. We explain the dependence of the lifetime on well width in very thin quantum wells by a reduced transition probability caused by the penetration of the envelope wave function of the electrons into the barrier material.

We demonstrate electrically driven spin injection into a single semiconductor quantum dot. Spin polarized electrons are transferred from a diluted magnetic semiconductor ( ZnMnSe ) into InAs quantum dots embedded into GaAs barriers. The spin information can be extracted directly from the polarization degree of the electroluminescence signal stemming from an individual quantum dot. By slightly modifying the device design, we demonstrate a concept to electrically charge the quantum dot by a spin polarized electron and present a simple way to probe this spin state optically.

The homogeneous linewidths of excitons in wet-etched ZnSe quantum wires of lateral sizes down to 23 nm are studied by transient four-wave mixing. The low-density dephasing time is found to increase with decreasing wire width. This is attributed mainly to a reduction of electron-exciton scattering within the wire due to the electron trapping in surface states and exciton localization. The exciton-exciton scattering efficiency, determined by the density dependence of the exciton dephasing, is found to increase with decreasing wire width. This is assigned to the reduced phase space in a quasi-one-dimensional system, enhancing the repulsive interaction between excitons due to Pauli blocking.

Laser diodes and heterostructures based on beryllium chalcogenides have been characterized optically by means of photoluminescence and photoluminescence excitation spectroscopy in the temperature range from 1.6 to 300 K. The study was performed on real lasing structures. An efficient photoluminescence at room temperature was observed in heterostructures with (ZnCd)Se and (BeZnCd)Se quantum wells ( L z = 100 A). The decrease of the luminescence intensity with increasing temperature is similar for both quantum well materials. Analyzing the temperature-dependent photoluminescence excitation spectra of laser structures, we have determined the efficiency of carrier collection into the active layer. At temperatures above 150 K all carriers from the 2000 A thick (BeZn)Se waveguide are collected into the active layer.

Lateral patterning techniques have been applied to control the transversal and longitudinal mode structure in ZnSe-based laser diodes. For ridge wave-guide lasers with a stripe width L x of less than 5 μm, predominantly lateral single-mode emission was observed in the near-field and the far-field emission patterns. For the ratio of the vertical and the lateral far-field angle a factor of about 4.4 was obtained. Longitudinal mode control was realized using first-order distributed Bragg reflectors with periods between 95 and 102 nm for an emission in the blue and green spectral range. A side-mode suppression of up to 25 dB could be achieved.

The growth mode of CdSe layers grown by migration enhanced epitaxy between ZnSe barriers has been investigated. In situ reflection high-energy electron diffraction shows a gradual transition to a three-dimensional growth mode which, however, is not accompanied by a change of the surface lattice constant. High-resolution transmission electron micrographs reveal a strong Cd diffusion, leading to ternary ZnCdSe quantum wells. Furthermore. composition fluctuations perpendicular to the growth direction on a nanometer scale are found already prior to the beginning of the growth mode transition. In the case of heterostructures containing a CdSe layer that has undergone the growth mode transition, micrographs show Cd-rich quantum dots with diameters of around 8 nm and heights of around 1.5 nm within a ternary quantum well. By spatially resolved photoluminescence the emission from single quantum dots could be observed. The polarization dependence of the emission from single dots indicates an asymmetric shape of the dots with certain preferential orientations along the [110] and [110] directions.

We have investigated the influence of the barrier thickness on hot carrier cooling in In0.53Ga0.47As/InP quantum wells by picosecond time resolved spectroscopy. Striking differences of the cooling of hot electrons and holes are observed for different barrier thicknesses. For small barrier thicknesses the initial carrier temperatures are significantly higher than for large barrier thicknesses. We can explain this behaviour by microscopic calculations of the transient variation of the carrier densities and the thermalization of hot carriers in the quantum well.

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.

Some recent highlights of our optical studies on single and coupled semiconductor quantum dots are reviewed. In the first part, we concentrate on the role of spins and spin-spin interaction in nonmagnetic and magnetic single quantum dots. In the case of strictly resonant excitation of the ground state in self-assembled CdSe/ZnSe quantum dots, we find an exciton spin relaxation time, which exceeds the recombination lifetime significantly. Linear polarization has to be used for these experiments, as the electron-hole exchange interaction lifts the spin degeneracy and the eigenstates are linear combinations of spin-up and spin-down excitons. In a magnetic quantum dot, the exchange interaction between carrier spins and the spins of magnetic ions is shown to be responsible for giant magneto-optical effects. We demonstrate the formation of zero-dimensional magnetic polarons and we succeeded in measuring the magnetization on a scale of a few nanometers using the characteristic photoluminescence signal of individual quantum dots as experimental monitor. The second part is devoted to pairs of single quahtum dots. On one hand, single exciton tunneling within an individual quantum dot pair is demonstrated studying single pairs of vertically correlated strain-induced and self-organized quantum dots. On the other hand, we show that in a pair of lithographically defined single dots with strongly different g-factors the energy spacing between the dot ground states can be tuned in an external magnetic field by about 10 meV, giving access to a controlled coupling between two individual quantum dots.

The decay characteristics of excitons and biexcitons in one single semiconductor quantum dot (QD) are directly monitored using time- and spatially resolved photoluminescence spectroscopy. The experiments are performed on a CdSe/ZnSe QD, occupied by either one or two excitons at a time, allowing a direct comparison between the radiative lifetime of a biexciton and an exciton confined in the same QD. The rather surprising result of comparable recombination rates for both states is related to the spatial wave function distribution and the spin structure of the particles and their coupling to the photon field, i.e., the superradiance effect.