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We report on the effect of pressure on the relaxation modes of the type-A polymer polyisoprene as a function of the chain length for M/Me ratio’s (Me is the entanglement molecular weight): 15, 12, 23, 2, and 5. We find that the spectral shape of the segmental and normal modes is invariant under temperature (T) and pressure (P) variations, but the time–pressure superposition fails as a consequence of the stronger P dependence of the segmental mode. The segmental mode activation volume for the different molecular weights was found to scale with the temperature difference from the respective glass transition temperature. Pressure is found to slow down both modes in a quantitatively similar way, irrespective of the chain length, implying that Me is largely independent of pressure.

We report on the pressure dependence of the local segmental and normal modes of a type-A polymer (polyisoprene) using dielectric spectroscopy. We find that external pressure exerts a stronger influence on the dynamics of the segmental mode as compared to the longest normal mode which results in the crossing of the two modes at higher pressures. The predictions of the Rouse model are not adequate to explain the different shift factors even at elevated temperatures.

An aliphatic random copolymer ionene (a cationic polyelectrolyte with N + in the repeat unit), was fractionated with the continuous polymer fractionation and investigated for its molecular weight dependent properties. The fractions were analyzed with GPC and capillary viscometry for their molecular weight. Glass transition temperatures were determined with DSC and a strong molecular weight dependence was observed, but no influence in the ionic conductivity was observed. The temperature dependency of ionic conductivity in ionenes is described.

We investigate II–VI optical waveguides consisting of ternary ZnS x Se 1− x and Zn 1− x Mg x Se compounds. The refractive indices are determined experimentally as a function of their composition. A linear interpolation was found to be a good approximation in the case of ZnS x Se 1− x . An empirical relation for Zn 1− x Mg x Se compounds was derived, giving the index of refraction from the band edge to the near infrared for magnesium contents up to 40%. Waveguide modelling is used to optimize waveguides according to the requirements of applications or epitaxial growth. Field distributions and propagation constants of multilayered guiding structures and losses caused by substrate leakage are calculated. Attenuation coefficients of 1–2 cm −1 are measured in waveguides containing ternary II–VI compounds.

The critical thickness anisotropy of the Zn 1-x Mg x Se/GaAs(001) and ZnSe/GaAs(001) systems grown by molecular beam epitaxy (MBE) is investigated by in situ reflection high-energy electron diffraction (RHEED) and transmission electron microscopy (TEM). The increasing FWHM of the specular spot in the RHEED-pattern during growth indicates the initial relaxation stages in strained epitaxial layers as described in an earlier publication [Reisinger et al.. Mater. Sci. Forum 182-184 (1995) 147]. Plan-view TEM-micrographs of ZnSe/GaAs samples show that the perfect 60 -misfit dislocations in the [1 - 10]-direction nucleate prior to the [110]-direction due to their higher mobility. According to the TEM investigation the FWHM increase of the specular spot is connected with the formation of 60 -dislocations perpendicular to the electron beam, whereas dislocations aligned parallel to the beam leave the FWHM unaffected. Finally, a thickness range of the ZnSe-layer from about 225-337 nm is found with misfit dislocations aligned only in the [1 - 1 0]-direction. Furthermore, we present a new in situ RHEED-method to determine the critical thickness of the Zn 1-x Mg x Se/GaAs system separately in both (110)-directions without changing the experimental set-up. The application of this method to various Zn 1-x Mg x Se-samples with different Mg-content x confirms the observation that different critical thicknesses in [110] and [1 - 1 0] are measured related to two different glide systems as observed in ZnSe. Plan-view TEM-micrographs reveal the dislocation structure during the different states of relaxation.

Photoluminescence-excitation (PLE) measurements of wide ZnSe/Zn 1− x Mg x Se single quantum wells (SQW) are presented, performed under high hydrostatic pressure up to 4 GPa. The fully strained samples have been grown by MBE on GaAs(0 0 1) substrates or (1 1 0) substrates. The well transitions 1nH 1s and 1mL 1s with n = 1, 2, 3, 4, 5 and m = 1, 2 were recorded as well as excitonic signals from the barrier material at 2 K and their pressure dependence were determined. A cross-over between the 11L and 13H, as well as between the 12L and 14H transitions, was observed above 1.2 GPa. The transitions were assigned using a modified Kronig Penney model. The exciton binding energy was derived from the energy difference between the 11H 1s and the 11H 2s transitions.

The temperature dependence of the 1s exciton energy has been measured in Zn 1-x Mg x Se epitaxial films at compositions x= 0, 0.07, 0.12, and 0.19 from 2 K up to 280 K. Detailed numerical fits of the individual temperature dependences are provided on the basis of an analytical four-parameter representation developed recently by one of the authors. These are compared with previously used three-parameter models of Vina et al. and Varshni. The x-dependence of the exciton energy, E 1s (T, x), and of the fundamental band gap energy, E g (T, x), is given to very good approximation by linear functions of the composition x for any T from absolute zero up to room temperature. A comparison with recent room temperature band gap energy data by Jobst et al. shows that this linear dependence holds up to x0.7. The magnitudes of the model-dependent empirical parameters, which control the temperature dependence of the band gap energy in different compounds, are found to change significantly with increasing magnesium content. From the magnitude of the effective phonon temperature, particularly in the case of ZnSe, we conclude that the main contributions to the band gap shrinkage effect are due to acoustic phonons.

Two amines, Et3N and tBuNH2 have been investigated in their role to improve the quality of MOVPE-grown ZnSe layers. A reduction of growth rate is observed in the kinetic limited range, which is shown to be due to an increase of activation energy, independent of the amine used. An improvement of the layer quality is related to the suppression of nonradiative recombination centers. However, 2D growth, controlled by ex situ RHEED and MBE-regrowth experiments, was only observed without Et3N. Under photoirradiation the growth rate increases in the kinetic limited range, where the two amines show remarkable differences. Whereas in this case tBuNH2 leads to a poor optical layer quality, Et3N gives excellent layers at sufficiently high growth rates. The experiments, including RDS growth control, lead to the conclusion that the nitrogen compounds are always adsorbed at Se-terminated surfaces and thus inhibit a quick reaction of Zn and Se on the surface. A model is proposed which describes the presented experiments consistently.

We present optical measurements of freestanding thin Zn1−xMgxSe films and of a ZnSe/Zn0.93Mg0.07Se single quantum well (SQW) under hydrostatic pressure applied by a diamond anvil cell. For the Zn1−xMgxSe compounds the pressure shift of the fundamental gap was determined from absorption measurements at room temperature for X ⩽ 0.37. A hydrostatic deformation potential a(x) = −4.85 + 0.67x eV is derived. The transition pressures Ptr to high pressure phases were determined by visual observation. The pressure dependent absorption and photoluminescence of the ZnSe SQW was investigated at 2 K. The energy shift of two SQW-related transitions was recorded and is reproduced by a simple theoretical model. LO-phonon replica were visible in photoluminescence at the sharp 11hh transition.

MBE-grown ZnCdSe ZnSe and MOVPE-grown ZnSe ZnSSe single quantum wells (SQW) were investigated by photoreflectance (PR) and photoluminescence excitation (PLE) spectroscopy. For various compositions a set of quantum well structures with different well thicknesses was grown. The PR spectra exhibit sharp signatures of which the transition energies have been accurately determined by a line shape fit. The peaks in the PLE spectra and correspondingly the PR signatures can be attributed to ground and excited excitonic states by comparison with the excitonic absorption model of Toyozawa. With an envelope function formalism including all strain effects a fitting procedure was developed which allows the evaluation of the band alignment. This procedure overcomes the problems of the uncertainties in the hole masses by simultaneously fitting all the transitions observed at one composition by varying also the hole masses. It is the accurate knowledge of the structural parameters of the samples and the precise determination of the transition energy which gives the composition dependent offset a high reliability.

ZnCdSe ZnSe quantum wells (QW) were grown with molecular beam epitaxy (MBE) on GaAs(001) substrates cleaned with hydrogen plasma. Reflection high-energy electron diffraction (RHEED) was used for in-situ growth control of the QW structures. A quantitative evaluation of the RHEED oscillations yields an exact value of the growth rate. Furthermore, in comparing the growth rate of the barrier and the well a reasonable estimate of the Cd content is possible. The in-situ RHEED measurements were supplemented by ex-situ HRXRD and HRTEM investigations. The latter method was found to be especially useful to evaluate the Cd-concentration profile of the QWs by digital analysis of lattice images (DALI). X-ray rocking curves of MQWs were recorded which show well-resolved satellite peaks. A comparison with simulations based on dynamical diffraction theory yields the structural parameters such as well width, barrier width and composition of the QWs. The XRD and TEM results are compared with the parameters determined by RHEED. In addition, we performed photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy for optical characterization of the samples. The PLE spectra show an interference pattern which is explained by standing polariton waves.

The diffusion coefficient D(T) of Cd in CdSe ZnSe single quantum well (SQW) structures grown pseudomorphically on GaAs(001) is determined by high resolution transmission electron microscopy of annealed SQWs and subsequent digital analysis of lattice images. SQWs of 2 monolayer (ML) thickness were grown by molecular beam epitaxy. During growth the CdSe quantum wells (QWs) broaden to about 7 ML CdZnSe as measured by reflection high energy electron diffraction (RHEED). We find for the diffusion coefficient of Cd in ZnSe at temperatures between 340 and 400° C D(T) = 1.9 × 10−4 [cm2/s] · exp(− 1.8 [eV]/kT).

Under an excitation density of 0.4 W/cm2, a strong biexcitonic peak was observed in the photoluminescence (PL) spectrum of a single quantum well which was grown in submonolayer deposition mode. Its intensity was found to increase superlinearly with excitation density. The shape of the biexcitonic peak was found to be similar to that of biexcitons in CuCl and CdS crystal and a binding energy of 9.8 meV was obtained. When the temperature increased from 10 to 31 K, the biexcitonic peak decreased considerably whereas the excitonic peak increased slightly.

We investigate the transient four-wave mixing (FWM) response of wide ZnSe/Zn 1- xMg xSe single quantum wells in a two-beam self-diffraction configuration. The spectrally resolved FWM signals show excitonic transitions which are essentially homogeneously broadened, and show dephasing times between 1 and 2 ps at a lattice temperature of 10 K. Simultaneous excitation of several exciton states leads to pronounced quantum beats in the FWM signal. The formation of heavy-hole biexcitons with binding energies of E bXX = 3.5 ± 0.5 meV is identified by polarization-dependent measurements. These measurements also indicate that excitation-induced dephasing gives the main contribution to the FWM process.