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Max-Planck-Institut fur Mikrostrukturphysik, Weinberg 2, 06120 Halle, Germany Received 12 September 2005, revised 15 October 2005, accepted 3 January 2006 Published online 2 February 2006 PACS 78.45.+h, 78.47.+p, 78.55.Et, 78.67.–n By means of time resolved spectroscopy we compare two samples of ZnO nanorods with respect to their suitability as stimulated emitters. In the case of narrow nanorods their wave guiding quality causes a sup-pression of exciton–exciton scattering whereas no laser emission is detectable. Unlike their narrow coun-terparts, wide nanorods not only benefit from a larger overlap of the guided mode with the gain medium but a variation in VLS growth results in gold nanoparticles being present at the bottom of nanorods. Con-sequently, laser emission from single wide rods is evidenced up to 150 K. In addition to experimental studies we carry out 3D numerical simulations of the electric field distribution to evaluate the influence of gold nanoparticles at the nanorod/substrate interface. This finite element analysis confirms that gold leads to an enhancement of confinement within the resonator.

A direct correlation of structural and optical properties of MOCVD-grown ZnCdO-layers with a systematic variation of Cd-content has been achieved on a microscopic scale using highly spatially and spectrally resolved cathodoluminescence. The ZnCdO layer luminescence measured in cathodoluminescence wavelength images reveals strong lateral fluctuations directly visualizing local band gap fluctuation as a consequence of different local Cd incorporation. We give direct evidence for a chemical phase separation into Cd-rich and Cd-poor nanodomains in ZnCdO.

The magnetoresistance of high-quality epitaxial doped ZnO(:Ga) thin films with various electron concentrations ranging from 3.2×1018 to 1.3×1020cm−3 has been measured. All samples investigated exhibit a negative magnetoresistance at low magnetic fields. Its magnitude systematically depends on carrier concentration and temperature. Low-doped samples switch the sign of the magnetoresistance and the conventional positive component dominates at high fields, whereas highly doped degenerate samples only show a negative component up to fields of 14.5 T. Therefore, the data are analyzed as the sum of a positive and negative contribution to the magnetoresistance applying a semiempirical expression to describe the observed behavior. The model takes into account third-order s–d exchange Hamiltonians describing the negative part and a two-band model for the positive contribution. Least-squares fits to the data are presented. Theory and experiment are in excellent agreement.

The need for diluted magnetic semiconductors has stimulated research on Mn-doped ZnO. However, the type of magnetic coupling (ferro/para) in ZnMnO remains an issue of debate. We have investigated the magnetic properties of Mn-doped ZnO layers grown by molecular beam epitaxy. Some samples showed a hysteresis with remnant magnetization on the order of 10−5emu, thus eventually suggesting ferromagnetism. We observed that the critical influence of the substrate substantially affects magnetic property measurements. This has to be taken into account in order to clearly confirm ferromagnetism. In our case, after subtraction of the substrate effect, there is no evidence of a ferromagnetic behavior for the ZnMnO samples.

We studied the effect of the V/III flux ratio and substrate temperature on magnetotransport properties and lattice parameters of Ga0.96Mn0.04As grown by molecular beam epitaxy. For all the substrate temperatures, the conductivities and Curie temperatures of the layers were found to increase as the V/III flux ratio approaches 1. The Curie temperature as high as 95 K was achieved for the Ga0.96Mn0.04As samples grown at 240C and a V/III ratio of about 1.5. The lattice parameter of Ga0.96Mn0.04As increased with decreasing V/III ratio and/or increasing growth temperature. Possible reasons for the effect of V/III ratio on the magnetotransport properties and lattice parameter of GaMnAs are discussed., Comment: 13 pages, 4 figures

The mean inner potential of ZnO was measured by means of electron holography in a transmission electron microscope. To overcome the problem of imprecise knowledge of the sample thickness, ZnO nanorods with well-defined geometry and diameter were used in this study. The phase of the transmitted beam of the image wave yields a mean inner potential of 15.9 ±1.4 V for ZnO. This value is in good agreement with the calculated value of 15.8 V using ab initio density functional theory computations.

The pressure dependence of the photoluminescence (PL) transition associated with the fundamental band gap of ZnO nanowires has been studied at pressures up to 15 GPa. ZnO nanowires are found to have a higher structural phase transition pressure around 12 GPa as compared to 9.0 GPa for bulk ZnO. The pressure-induced energy shift of the near band-edge luminescence emission yields a linear pressure coefficient of 29.6 meV/GPa with a small sublinear term of -0.43 meV/GPa{sup 2}. An effective hydrostatic deformation potential -3.97 eV for the direct band gap of the ZnO nanowires is derived from the result.

The spectral response of the lateral optical anisotropy of periodic undoped type-II ZnSe/BeTe heterostructures with nonequivalent interfaces was studied by ellipsometry. The spectra revealed two types of features corresponding to optical transitions with energies lying in the bandgap. The position of features of the first type does not depend on the heterostructure period. Features of the second type shift toward lower energies with decreasing period of the heterostructure. This behavior is explained in terms of a model taking into account the existence of electronic and hole interface states and of a mixed-type interface state.

The realisation of a p/sup +/-BeTe/n/sup +/-ZnSe ESAKI tunnelling diode is reported. Negative differential resistance was observed under forward bias at room temperature with a peak to valley ratio of 1.55:1. In the backward direction a current density of 400 A/cm/sup 2/ was achieved at 3.3 V. These data indicate that p/sup +/-BeTe/n/sup +/-ZnSe tunnel diodes are suitable as buried electron to hole converters in II-VI optoelectronic devices.

The authors report the realisation of an ESAKI-reverse-injection-concept (ERIC) device using Be chalcogenides. A blue light emitting diode is described based on an ERIC electron-to-hole conversion in front of the waveguide, avoiding p-cladding. The I-V-characteristics show a current density of 1 kA/cm/sup 2/ at 12.3 V under forward bias and a negative differential resistance (NDR) with a peak-to-valley ratio (PVR) of 8.6:1 under reverse bias at room temperature. The electroluminescence at room temperature at an energy corresponding to the ZnSe active region proves the applicability of the concept.

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.

The presence of magnetic ions in a dilute semiconductor quantum well is shown to decrease dramatically the low-temperature excitonic dephasing time measured by transient four-wave mixing. The decrease is associated with the magnetic nature of the material. Surprisingly, the application of a magnetic field does not significantly change the dephasing time, which indicates that there must exist a fast dephasing mechanism in addition to the spin–spin interactions responsible for magnetic polaron formation.

We report on an investigation of the growth mechanisms of HgTe using a combination of reflection high energy electron diffraction (RHEED) and scanning tunneling microscopy (STM). The experiments were carried out on untilted (001) CdTe substrates. Growth rates were determined from RHEED intensity oscillations of the (00) specular spot reflection. The amplitude of these oscillations decrease with increasing substrate temperature. Above 178 °C no RHEED oscillations could be measured. Upon reducing the sample temperature below 178 °C these oscillations could again be observed. This cyclic behavior could be induced several times for each sample, indicating a reversible change in the growth mechanism. In order to correlate the surface structure with RHEED observations, several samples have been investigated with STM. Thus, it could be confirmed that a temperature dependent transition occurs during the MBE growth of HgTe from the island growth mode below the critical temperature of 178 °C to a step flow mode abo...

We have investigated the temperature dependence of Shubnikov-de Haas (SdH) oscillations in p-type GaAs/(AlGa)As heterostructures at high magnetic fields and low temperatures (0.35–4.2 K). From both the temperature and the magnetic field dependence of the SdH minima the density of states (DOS) and the activation energies in the localized region between adjacent Landau levels were determined. The background DOS between two Landau levels reaches a substantial fraction of that at zero magnetic field.

We present a simple and inexpensive method to implement a Griess-Saltzman-type reaction that combines the advantages of the liquid phase method (high specificity and fast response time) with the benefits of a solid implementation (easy to handle). We demonstrate that the measurements can be carried out using conventional RGB sensors; circumventing all the limitations around the measurement of the samples with spectrometers. We also present a method to optimize the measurement protocol and target a specific range of NO2 concentrations. We demonstrate that it is possible to measure the concentration of NO2 from 50 ppb to 300 ppm with high specificity and without modifying the Griess-Saltzman reagent.

The influence of different CdTe substrate preheats prior to II‐VI molecular beam epitaxial growth on surface stoichiometry and oxygen contamination has been studied using x‐ray photoelectron spectroscopy. For 15 min preheats with temperatures ranging from 100 to 450 °C, the cadmium to tellurium ratio and the oxide overlayer thickness of (100) CdTe surfaces was determined. A preheat temperature of 200 °C is found to produce optimum stoichiometry. For lower temperatures the CdTe surface is still tellurium rich, as left after etching with bromine‐methanol. For higher temperatures, cadmium evaporates faster than tellurium, leaving again a tellurium‐rich surface. The oxygen contamination remains nearly unchanged for temperatures below 250 °C. Oxygen starts to blow off for preheat temperatures above 250 °C, with a steep decrease between 250 and 350 °C. For preheat temperatures higher than 350 °C, the oxygen contamination drops below the detection limit.

New nano-illumination microscopy instrumentation will be presented together with the road to overcome the diffraction limit in order to achieve super-resolution. The simplicity of this technique would make it suitable in any low-resource scenario.

We report on the comparative studies of linearly polarized photoluminescence (PL) in a ZnO epitaxial film and ZnO nanorods. At low temperatures the PL spectrum of both samples included a number of narrow lines attributed to donor‐bound excitons and a peak of free A excitons. An additional line observed in the nanorod sample was assigned to the excitons bound to some defects introduced during the sample post‐growth history and located near the nanorod tips. The emission of mixed longitudinal — transverse exciton polariton modes was observed at elevated temperatures in both samples.

We report on the fabrication of p-type ZnO:N layers using radical-source molecular beam epitaxy and post-growth annealing of the samples. Plasma-activated oxygen and nitrogen fluxes are supplied via a single plasma cell. The combination of low growth temperature (350 − 400 ◦C), slightly O-rich conditions, and post-growth annealing in the range of 650 − 800 ◦C results in efficient nitrogen pdoping with Hall hole concentration 3 × 10 cm−3. The details of the structural and the electrical characterizations of the films are discussed.

We have investigated the influence of As substrate passivation on the electronic level alignment of the heterovalent BeTe/Si(111) interface. Employing photoelectron spectroscopy, we have performed k-resolved measurements at selected excitation energies in the uv range to maximize the contribution from the \ensuremath{\Gamma} point for a correct determination of the valence band maximum. These results are compared with k-integrated data using a density-of-states function from density functional theory. For BeTe(100) and BeTe(111), we find a negligible influence of surface orientation on the position of the valence band maximum. The As passivation increases the valence band offset by 0.24 eV (\ifmmode\pm\else\textpm\fi{}0.13 eV) compared to BeTe on an unpassivated Si substrate, thus leading to a significantly increased step in the valence band $(0.37\ifmmode\pm\else\textpm\fi{}0.13\mathrm{eV}).$ The results are discussed with respect to the barriers for electron and hole injection in heterostructures and with respect to the interface structure.

Effective g factors of electrons and heavy holes in CdTe/CdMgTe quantum wells have been measured with high accuracy as a function of the angle between the magnetic field and the structure growth axis. As a result, we have observed the electron g factor anisotropy in A 2 B 6 -based heterostructures. The dependences of the g factor tensor components on the quantum-well width and the solid solution composition have been calculated within the k · p method and compared with the available experimental data.

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.

The microscopic origin of the near-band-edge emission in low-temperature aqueous chemicallygrown ZnO nanorods is analyzed. For the as-grown nanorods, a broad and almost featureless main emission band at 3.366 eV is observed due the presence of a high donor and defect density. After the nanorods have been annealed at 800 ◦C for 1 hour in an oxygen atmosphere, a remarkable reduction of the linewidth of the near-band-edge emission from 10 meV to 4 meV is observed. Sharp and distinguishable excitonic transitions are visible. Time-resolved photoluminescence measurements show very fast recombination times of 11 ps for the as-grown sample while the radiative recombination time increases to 80 ps after the sample is annealed in oxygen atmosphere.

The prevailing design approaches of semiconductor gas sensors struggle to overcome most of their current limitations such as poor selectivity, and high power consumption. Herein, a new sensing concept based on devices that are capable of detecting gases without the need of any external power sources required to activate interaction of gases with sensor or to generate the sensor read out signal. Based on the integration of complementary functionalities (namely; powering and sensing) in a singular nanostructure, self-sustained gas sensors will be demonstrated. Moreover, a rational methodology to design organic surface functionalization that provide high selectivity towards single gas species will also be discussed. Specifically, theoretical results, confirmed experimentally, indicate that precisely tuning of the sterical and electronic structure of sensor material/organic interfaces can lead to unprecedented selectivity values, comparable to those typical of bioselective processes. Finally, an integrated gas sensor that combine both the self-powering and selective detection strategies in one single device will also be presented. © 2015 Published by Elsevier Ltd.

We present a comprehensive investigation of molecular beam epitaxial (MBE) grown InAs quantum dots (QD) on silicon (001) and (111) by reflection high energy electron diffraction (RHEED) and Raman spectroscopy in UHV environment and ex-situ by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Two different ways were developed to prepare up to 10 11 cm -2 InAs QDs on Si(001). One is the conventional mode by exceeding a critical thickness of deposition at which 2D growth changes towards a 3D growth mode. A second way is a dewetting transition, induced by cooling an approximately I ML thin 2D InAs layer from growth temperature below a critical temperature at which RHEED indicates the formation of nanoislands. Samples grown in both manners show significant differences in morphology and shape though RHEED, TEM and Raman studies correspondingly indicate strain relaxation. On Si(111) InAs grows in the common temperature range for InAs growth (∼400°C) in flat clusters separated by deep trenches. A previous passivation of the Si(111) surface with arsenic at ∼700 °C on the other hand leads to the formation of large InAs nanocrystals.

We investigate by photoluminescence (PL) nominally undoped, commercially available Zinc Oxide substrates (from Eagle Picher) grown by seeded chemical vapor transport technique in order to identify residual donors and acceptors. In low temperature PL spectra the dominant emission comes from the decay of bound exciton lines at around 3.36 eV. Zeeman measurements allow the identification of the two strongest lines and some weaker lines in-between as donorrelated. From the associated two-electron satellite lines binding energies of the major donors of 48 meV and 55 meV, respectively, can be deduced.

We present an investigation of the vibrational properties and the elastic constant c 11 of BeTe. For this purpose we applied Raman spectroscopy on folded acoustic phonons (FAPs) and confined optical phonon modes in BeTe/ZnSe superlattices. This method yields basic BeTe vibrational properties as well as the interface properties at the BeTe/ZnSe junctions. The exact values of the superlattice periodicity were obtained from X-ray diffraction, while transmission electron microscopy data supplied the individual thicknesses of the ZnSe and BeTe layers. The acoustic phonon dispersion in BeTe was determined from the FAP frequencies in the Raman spectra, together with the periodicity and the thickness ratio of the SL as well as the well known elastic constants of the ZnSe layers. We obtained for the BeTe sound velocity the value of 4180 ± 200m/s. This value was used to calculate the BeTe elastic constant c 11 , yielding 8.9 ± 0.9 × 10 10 N/m 2 .

This work presents a compact low-cost and straightforward shadow imaging microscopy technique based on spatially resolved nano-illumination instead of spatially resolved detection. Independently addressable nano-LEDs on a regular 2D array provide the resolution of the microscope by illuminating the sample in contact with the LED array and creating a shadow image in a photodetector located on the opposite side. The microscope prototype presented here is composed by a GaN chip with an 8x8 array of 5μm-LEDs with 10 μm pitch light sources and a commercial CMOS image sensor with integrated lens used as a light collector. We describe the microscope prototype and analyze the effect of the sensing area size on image reconstruction.

Electron-hole recombination at the interfaces formed by Zn-Te and Be-Se chemical bonds has been investigated in type-II ZnSe/BeTe heterostructures at low and high levels of excitation power by means of polarized light spectroscopy with applying of electric field across the structure. A more than fourfold variation of the matrix element squared has been reached by changing the electric field for the spatially indirect optical transitions at the interface. The radiative time of electron-hole transitions at the Se-Be interface is found to be at least 50 times longer than that at the Zn-Te interface. The contribution of Be-Se interface emission in the structures with nonequivalent $(\mathrm{Te}\ensuremath{-}\mathrm{Zn}\ensuremath{\cdot}\ensuremath{\cdot}\ensuremath{\cdot}\mathrm{Se}\ensuremath{-}\mathrm{Be})$ interfaces is negligible in the whole range of applied fields. The emission at the Te-Zn interface is carefully investigated. The ratio of matrix-elements squared along and perpendicular to the Zn-Te bonds has been found to be nearly constant in the range of carrier density ${10}^{11}/2\ifmmode\times\else\texttimes\fi{}{10}^{12}{\mathrm{cm}}^{\ensuremath{-}2}$ and equal to 7:1.

Isolated pairs of antiferromagnetically coupled Mn2+ ions in Mn-based semimagnetic semiconductors possess a series of energy levels whose separation is determined by their nearest-neighbour exchange interaction. Raman transitions were observed between theS=0 ground state and the magnetically splitS=1 levels of these pairs in Cd1-xMnxTe/Cd1-yMgyTe quantum well structures comprising semimagnetic quantum wells (Cd1-xMnxTe) of different well widths and Mn concentrations. The Raman shift of theS=0S=1,M=0 transition shows a non-negligible dependence on the Mn concentration, which is interpreted as an effect due to interaction of the Mn2+ pair with more distant neighbours. A nearest-neighbour exchange constantJNN=-(6.11±0.15) K for Cd1-xMnxTe is determined. Further, a model is presented for the scattering mechanism, where band-edge states act as intermediate states in the scattering process owing to the strong exchange interaction between the carrier spin and the localized Mn2+ ions. Confinement and localization effects in the quantum well, which produce strong gradients of the carrier wavefunction, play a decisive role for the intensity of the Mn2+ pair Raman signals.

The reduction of extended defects in ZnSe based II-VI heterostructures grown by molecular beam epitaxy on (001) GaAs is reported, using BeTe buffer layers as a novel approach. After defect selective chemical etching three different types of etch pits could be observed by optical microscopy. By the application of a thin BeTe buffer layer the density of paired Frank type stacking faults could be strongly reduced to values below 103 cm−2. The role of Se in the background pressure for the defect nucleation at the II-VI/GaAs interface is significant. It has been found that BeTe can form a smooth interface to GaAs and ZnSe which is reflected in high resolution x-ray diffraction data.

The physical laws of diffraction limit the spatial resolution of optical systems. In contrary to most superresolution microscopy approaches used today, in our novel idea we are aiming to overcome this limit by developing a spatially resolved illumination source based on semiconductor nanoscale light emitting diode (nanoLED) arrays with individual pixel control. We present and discuss the results of optical simulations performed for such nanoLED emitter arrays and analyze the theoretical limits of this approach. As possible designs we study arrays of GaN nanofins and nanorods (obtained by etching nanofin arrays), with InGaN/GaN multi quantum wells embedded as active regions. We find that a suitable choice of the array dimensions leads to a reasonably directed light output and concentration of the optical power in the near field around an activated pixel. As a consequence, the spatial resolution for this type of microscopy should only be limited by the pixel pitch, and no longer by the optical diffraction. Realization of optimized nanoLED arrays has a potential to open new field of chip based superresolution microscopy, making super-high spatial resolution ubiquitously available.