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We report on the formation of ion beam induced ripples in Si001 wafers when bombarded with Ar ions at an energy of 60 keV. A set of samples varying incidence and azimuthal angles of the ion beam with respect to the crystalline surface orientation was studied by two complementary near surface sensitive techniques, namely atomic force microscopy and depthresolved Xray grazing incidence diffraction GID. Additionally, crosssection TEM investigations were carried out. The ripplelike structures are formed at the sample surface as well as at the buried amorphous–crystalline interface. Best quality of the ripple pattern was found when the irradiating ion beam was aligned parallel to the 111 planes. The quality decreases rapidly if the direction of the ion beam deviates from 111.

We report on the improved assembly and characterization of a small molecule organic field-effect transistor (OFET). Novel α,ω-dicyano substituted β,β′-dibutylquaterthiophene molecules (DCNDBQT) were synthesized and characterized by UV–Vis spectroscopy, differential scanning calorimetry, thermal gravimetric analysis and cyclic voltammetry. The ultra-thin organic film formation on TiO2templates was effectively promoted through the specifically designed bifunctional self assembly molecules (SAM) 5-cyano-2-(butyl-4-phosphonic acid)-3-butylthiophene (CNBTPA). Excellent structural properties were found for up to 9 DCNDBQT molecule thick films prepared through UHV vacuum sublimation as investigated with UHV non-contact atomic force microscopy (nc-AFM) and X-ray diffraction. Both X-ray and nc-AFM data indicate that the DCNDBQT molecules form a well-ordered terraced structure exhibiting step heights of 1.5 nm to 2.0 nm layers. Hence, the DCNDBQTmolecules are linked to the functional SAM interface layer by H-bond interactions (see structure model) standing quasi perpendicular to the TiO2template, and thus providing optimal orbital overlap neigh-bouring thiophene rings.The vacuum sublimated DCNDBQT molecules form a closed packed and dense molecular layer that was used to construct and operate a nanoscopic OFET-structure. The resulting field mobilities of 10–5cm2V–1s–1reflect a high current density in our ultrathin but highly ordered structure. (© 2008 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim)

We measured the thermal expansion coefficient (TEC) of a vertically stacked multi-quantum-well (MQW) structure buried under a thick GaAs top layer before and after lateral patterning of the GaAs top layer. After patterning the TEC of the whole MQW structure differs from that of the planar structure by about 20%. Based on calculations in terms of methods of finite elements the effect is explained by the influence of the strain field originating from the bottom edges of the etched nanostructure. Due to the long range nature of this strain field the strain release within the individual quantum wells changes as a function from the distance from the valley. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Local lattice plane curvature of semiconductor disk laser devices is determined by an X-ray spot mapping technique using white beam synchrotron radiation. This method allows for in-situ studies of the dependence of the lattice plane profile on device temperature and local heating by optical pumping. The influence of different device mounting procedures on the structural and optical device parameters was investigated. This knowledge was used for device technology optimisation. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report on in-situ investigations of a recrystallization process of amorphous and damaged crystalline parts generated during ion-beam induced rippling on a Si(100) surface. The ripple structure was created by 60 keV 40Ar+ irradiation with a dose of ∼5 × 1017 ions/cm2 at ion incident angle of 60° with respect to the surface normal. At this dose the ripples have an average spatial periodicity of about 715 nm and surface undulations with an amplitude of about 40 nm. Structure and morphology of ripples were studied by two types of X-ray scattering (grazing incidence diffraction and amorphous scattering) methods, transmission electron microscopy and atomic force microscopy. X-ray grazing-incidence amorphous scattering pattern were recorded in-situ for a temperature range from 250 to 750 °C. Up to about 500 °C mainly we found a single broad scattering maximum corresponding to the Si(111) inter-planar distances. At higher temperature these peaks become sharp and intense indicating the onset of a re-crystallization process in the amorphous top layer.Two processes were found, a formation of crystalline islands on top of the former amorphous surface ripples and a growth of polycrystalline twins close to the former amorphous-crystalline interface. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Multilayers of an Fe(II)‐polyelectrolyte‐amphiphile complex (Fe‐PAC) were investigated simultaneously by energy‐dispersive X‐ray reflectivity and in‐plane diffraction at the bending‐magnet beamline at BESSY II. By recording spectra between room temperature and about 348 K with a time resolution of about 60 s, two phase transitions were identified and the respective activation energies were determined. Owing to a fixed geometrical set‐up, an energy‐dispersive experiment is suitable for measuring X‐ray reflectivity and in‐plane X‐ray diffraction of thin organic films simultaneously. Installed at a bending magnet at BESSY II and using an energy range of 4 < E< 25 keV, both reflectivity and diffraction spectra can be collected within about 60 s. The performance is demonstrated by temperature‐ and time‐resolved measurements of the phase transition behaviour of multilayers of an Fe‐PAC deposited on a silicon support by means of the Langmuir–Blodgett (LB) technique. By measuring the X‐ray reflectivity while increasing the temperature, it is shown that the original LB phase is not stable and transforms irreversibly into a liquid‐crystalline (LC)‐like phase at about 318 K. At the same temperature the in‐plane diffraction signal vanishes reversibly, reflecting rotational disorder of the hexagonal arrangement of amphiphilic chains. Its activation energy is determined to be about 1.3 eV. At about 338 K a second irreversible phase transition occurs to another LC phase with smaller vertical layer spacing. This transition is reversible between 329 K and 338 K.

Partially deuterated Cd−arachidate Langmuir−Blodgett multilayers were subjected to a gas atmosphere of n-pentane and n-hexane at room temperature. The gas−film interaction was studied by in-situ X-ray and neutron specular scattering through a gas-flow box. Both methods show an increase of the small-angle Bragg peak intensities during the gas treatment while the intensities of Kiessig oscillations decrease at the same time. The change of intensities is partially reversible after purging with air. Neutron scattering reveals that the increase of structural Bragg peaks is also accompanied by a decrease of superstructure peaks originated by the scattering length density difference between the sequentially stacked deuterated and nondeuterated chains. This indicates a gas-assisted intermixing of amphiphilic molecules in the vertical direction. The observed effect is much larger than in the case that the short-chain molecules occupy defect sites of the virgin Langmuir−Blodgett film. It can be explained by assuming that the gas molecules temporarily enter the space between the amphiphilic molecules. This locally reduces the van der Waals interaction and increases the mobility of amphiphilic molecules. The gas-assisted mobility improves the quality of internal interfaces by increasing the density at the Cd-ion sites, but it increases the surface roughness by the vertical pile up of molecules.

Si/Si1−xGexwires were grown by local solid source molecular beam epitaxy using three kinds of masks: Si, SiO2and SiO2/Si3N4. Their structural properties were studied by transmission electron microscopy to obtain information on the shape of cross sections, and by high resolution X-ray coplanar diffraction, which yielded the average vertical and in-plane strain. Grazing incidence diffraction was used to measure the depth-dependent in-plane strain distribution in the wires as well as in the substrate, which is compared to results of finite element calculations. Low temperature photoluminescence measurements were performed to investigate the optical properties of the wires.

AbstractThe investigation of organic multilayer structures prepared by means of the Langmuir-Blodgett (LB) technique is found to be complex due to molecular intermixing arising during the preparation procedure. To fully understand the intermixing processes, not only the X-ray reflectivity, but also neutron reflectivity measurement, must be carried out. It has been already reported 1,2] that it is impossible to investigate the chain-chain intermixing process only by X-ray reflectivity. This due to the lack of electron density contrast within the mixed materials. Sufficient contrast, however, can be achieved by applying neutron reflectivity [3,4], because of the possibility to generate a strong contrast between chains by the deuteration process which only slightly changes the material properties. On the other hand, X-ray reflectivity provides sharp layer structure peaks and Kiessig oscillations from which the period of structure and total thickness of film system can be determined.

We report on a GaAs/AlAs, wide-miniband, superlattice autocorrelator for picosecond THz radiation pulses (operated at room temperature); the autocorrelator is based on the THz radiation-induced reduction of current through the superlattice. THz radiation (frequency 7.2 THz) from the FELIX (free-electron laser for infrared experiments) was coupled into the superlattice with an antenna system. We measured the current reduction for two time-delayed pulses and found that the signal decreased when the time delay was smaller than the pulse duration. With this superlattice autocorrelator we were able to resolve laser pulses that had a duration of a few picoseconds.

We report on a self-sustained high-frequency (HF) current oscillation (at a frequency of 9 GHz) in a doped wide-miniband GaAs/AlAs superlattice with negative differential conductance. An analysis of the current transport, making use of the continuity equation and Poisson equation indicates that the oscillation is due to propagating dipole domains.

We demonstrate that a strong THz-field (3.9 THz) can lead to full supression of the dc current through a GaAs/AlAs superlattice that shows, at room temperature, negative differential conductance. The miniband width (Δ=50meV) was much larger than the quantum energy of the radiation (16 meV). Using a semi-classical model we show that the miniband electrons do not relax during one period of the THz-field. In this case the reduction of the dc current can not be explained in terms of classical rectification caused by the non-linear current-voltage characteristic. Rather it is due to dynamic localization of electrons, performing frequency modulated Bloch oscillations.

We use a semiclassical description for an analysis of the detection of THz-photons by electrons performing Bloch oscillations in a semiconductor superlattice. We show that the current responsivity of the superlattice detector can nearly reach the quantum efficiencye/ħω at room temperature if the superlattice is matched to the THz radiation.

We report on a theoretical analysis of terahertz (THz-) field induced nonlinear dynamics of electrons in a semiconductor superlattice that are capable to perform Bloch oscillations. Our results suggest that for a strong THz-field a dc voltage should be generated. We have analyzed the real-time dynamics using a balance equation approach to describe the electron transport in a superlattice miniband. Taking account of both Bloch oscillations of electrons in a superlattice miniband and dissipation, we studied the influence of a strong THz-field on currently available superlattices at room temperature. We found that a THz-field can lead to a negative conductance resulting in turn in a THz-field induced dc voltage, and that the voltage per superlattice period should show, for varying amplitue of the THz-field, a form of wisted plateaus with the middle points being with high precision equal to the photon energy divided by the electron charge. We show voltage to the finite voltage state, and that in the finite voltage state dynamic localization of the electrons in a miniband occurs.

We observed a self-sustained current oscillation in an InGaAs/InAlAs superlattice at room temperature. The oscillation gave rise to microwave generation at a fundamental frequency around 2GHz and higher harmonics. The power at the first harmonic was 0.1 mW, corresponding to an efficiency (i.e. ratio of microwave power to dc power) of 3.5%. We attribute the current oscillation to travelling dipole domains.

We present experimental results showing that the irradiation of a doped short-period GaAs/AlAs superlattice leads to a reduction of the conductance either due to rectification (for fields at frequencies below 1 THz) or due to dynamical localization of the miniband electrons (for fields above 1 THz).

The cover picture of the present issue of physica status solidi (a) shows a reciprocal space map of the strain sensitive Si(400) reflection from a lateral nanostructure with a period of 550 nm, produced by a focused Ge ion beam. Nominal implantation was along K = 0; the dotted line shows that the true implantation direction was about 3 degrees off.The first author, Jörg Grenzer, is now at the Research Center Rossendorf. His main working field is semiconductor nanostructures, particularly the investigation of strain phenomena which can be exploited in device related structures.