F. Phillipp

Ordered semiconductor ZnO nanowire arrays and their photoluminescence properties

Ordered semiconductor ZnO nanowire arrays embedded in anodic alumina membranes (AAM) were fabricated by generating alumina templates with nanochannels, electrodepositing Zn in them, and then oxidizing the Zn nanowire arrays. The polycrystalline ZnO nanowires with the diameters ranging from 15 to 90 nm were uniformly assembled into the hexagonally ordered nanochannels of the AAM. Photoluminescence (PL) measurements show a blue PL band in the wavelength range of 450–650 nm caused by the singly ionized oxygen vacancy in ZnO nanowires.

Direct measurement of local lattice distortions in strained layer structures by HREM

Abstract We present a method which allows the direct measurement of local displacements within a crystal lattice by high-resolution electron microscopy (HREM). The basic idea of this method is the formation of moire structures when two two-dimensional lattices are superimposed. The first lattice is given by the experimental micrograph and the second one is calculated from an undisturbed area of the first. From the analysis of the resulting moire structure the local displacements of the crystal lattice with respect to the calculated lattice are deduced. We thus measure strain (which is characteristic for each element embedded in the host lattice) and thickness of the strained layers independently. Simultaneously, the positions of the interfaces are identified, without relying on the contrast difference between the constituent materials. Therefore, this novel method represents a chemical mapping which is inherently free of contrast interpretations.

New high-voltage atomic resolution microscope approaching 1 Å point resolution installed in Stuttgart

Abstract The basic characteristics of the new high-voltage atomic resolution microscope (JEOL JEM-ARM1250) recently installed in Stuttgart are described. Owing to its high electrical stability and proper installation conditions this instrument reaches its theoretical point resolution of 0.105 nm at an accelerating voltage of 1250 kV. The defocus spread due to chromatic aberrations was determined quantitatively by extensive diffractogram tests. An information transfer limit between 0.08 and 0.09 nm can be deduced from the results of these tests. First application are presented, which demonstrate the performance of the instrument in structural studies of various materials.

Structural and optical properties of vertically aligned InP quantum dots

Stacked layers of self-assembled InP quantum dots embedded in Ga0.52In0.48P have been prepared by solid source molecular beam epitaxy. Thereby the distance between the dot layers has been varied from 2 to 16 nm. Cross sectional transmission electron microscopy shows that the InP dots are aligned in the growth direction [100]. As the distance between the dot layers is reduced, each dot of the first dot layer is reproduced in the upper layers, and this leads to an improvement of the dot size homogeneity of the stacked InP dot system. This is confirmed by photoluminescence (PL) measurements, which demonstrate a very narrow linewidth of 26 meV for a triple layer with 2 nm separation between the dot layers in comparison with a linewidth of 41 meV for a single layer sample. At the same time, the PL peak of the dots is shifted by 72 meV to lower energies which is ascribed to a reduced strain and strong electrical coupling between the densely stacked InP dots.

Synthesis and characterization of nanowires and nanocables

Abstract The synthesis and characterization of nanowires and nanocables of carbides and nitrides were reported. The carbothermal reduction of SiO 2 xerogel containing carbon nanoparticles in argon was used to prepare SiC nanowires and SiC nanowires sheathed with amorphous SiO 2 coatings (SiC-cored nanocables). Si 3 N 4 nanowires were also made by the carbothermal reduction, but the reaction should be carried out in N 2 atmosphere. The gas reaction of Ga 2 O vapor with flowing NH 3 was used to form GaN nanowires on a porous alumina template.

Red-light-emitting injection laser based on InP/GaInP self-assembled quantum dots

Red-light-emitting quantum dot injection lasers have been prepared by solid-source molecular beam epitaxy. The separate confinement heterostructure contains densely stacked layers of self-assembled InP quantum dots embedded in Ga0.51In0.49P waveguide and Si/Be-doped Al0.53In0.47P cladding layers. Edge-emitting laser diodes are processed, which show quantum dot lasing at 90 K. Thereby, the threshold current density is 172 A/cm2. The energy of the laser line is at 1.757 eV, which is very close to the peak energy of subthreshold electroluminescence.

Influence of domain size on optical properties of ordered GaInP2

Using dark‐field transmission electron microscopy images of ordered GaInP samples, we show how the ordering domain size depends on the growth temperature. Samples with different average domain sizes are compared with regard to their photoluminescence (PL) and excitation spectra. We find a close correlation between the size of the ordered domains and the relative intensity of the PL peak from band–band recombination compared with the rapidly shifting, below‐band‐gap luminescence emission.

Photoluminescence and optical absorption caused by the F+ centres in anodic alumina membranes

Anodic alumina membranes (AAMs) with highly ordered nanochannel arrays were prepared by anodizing aluminum in acid solutions. X-ray diffraction reveals the amorphous nature of AAMs. The photoluminescence (PL) excitation spectra of AAMs consist of a broad band centred at 235 nm and a narrow band peaking around 360 nm under a monitoring wavelength of 470 nm. There are five prominent absorption bands (or edges) at 370, 294, 254, 220 and 204 nm in the absorption spectra of AAMs prepared in oxalic solution. With the increase of the annealing temperature, the intensities of PL and absorption bands increase first, then decrease, and a blue-shift is found. The photoluminescence and optical absorption in the wavelength range of 200 nm to 500 nm are caused by the F+ centres in AAMs. The energy levels of F+ centres in AAMs are split and a new energy level of 3.35 eV appears due to the amorphous nature of AAMs.

Growth of self-assembled InP quantum islands for red-light-emitting injection lasers

In order to achieve laser emission in the visible part of the spectrum, we have investigated the growth of self-assembled InP quantum dots on GaInP by low-pressure metal-organic vapor phase epitaxy (MOVPE) using the Stranski-Krastanow growth mode. Unlike the well-established InAs-GaAs system, when InP is deposited on GaInP, typically, two types of coherently strained islands with different sizes are formed. A high density of small islands is favored when using growth conditions with a reduced surface diffusion, i.e., low temperatures, high growth rates, and substrates with high misorientation angles. After the deposition of 3.4 monolayers of InP at 580/spl deg/C on GaAs-substrates with a surface angle of 15/spl deg/ to the next [111]B-plane, 2.10/sup 10/ InP dots per square centimeter with an average height of 4 nm were assembled. The emission of these InP islands at 1.72 eV (4.2 K) shows an inhomogeneous broadening of 42 meV because of the size fluctuation of the quantum dots. At 90 K, lasing from self-assembled InP quantum islands was observed above a threshold current density of 288 A/cm/sup 2/. The detected laser line is located at 1.8 eV, about 80 meV higher than is the ground-state transition energy. We attribute this behavior to lasing from excited states in agreement with power-dependent photoluminescence experiments. For temperatures above 150 K, the threshold current density increases dramatically because of a thermally activated escape of carriers up to 4.9 k17/cm/sup 2/ at room temperature, where the characteristic temperature is 35 K. Injection lasers containing stacked InP quantum islands and AlGaInP barrier layers with a higher band offset may exhibit an improved temperature dependence.

Strain distribution in self-assembled InP/GaInP quantum dots

Local strain on an atomic scale, as well as the size and the shape of InP quantum dots (QDs) embedded in GaInP, have been measured directly from high-resolution electron microscopy images. There is a strong spatial variation in strain within a QD. A large part of the misfit strain is distributed in the GaInP matrix surrounding the QDs, especially in the case of small spacing between the QD layers. The strain distribution varies significantly with the thickness of the GaInP spacer layers between QD layers. The compressive strain within the QDs decreases with decreasing spacing between the QD layers. This contributes to an increasing red shift of photoluminescence energy peak positions of QDs of multi-layers with decreasing thickness of the spacer layers between the QD layers in comparison with that of single layer QDs.