Materials Science Forum Vol. 494

Abstract: The paper summarizes recent developments and implementation of the national science and technology policies. National as well as international environment play an important role, especially regarding threats and opportunities. Small size of the country is advantageous in terms of, for instance, possibility of establishing an uncomplicated research and innovation system, but, on the other side the country’s own resources, both financial and human, are far from being sufficient for the creation of knowledge needed for the development of a knowledge-based society. Consequently, strong embedment in international S&T networks as well as an efficient knowledge transfer system is needed.

Abstract: A review is given of the application of in situ transmission electron microscopy to study various processes associated with the crystallization of amorphous thin films. Solid phase epitaxial regrowth of ion-implanted silicon is compared with nucleation and growth in deposited thin films. The mechanism of metal-mediated crystallization is deduced directly from high resolution recordings, and the kinetics of tantalum oxide devitrefication are obtained. The advantages of direct in situ observation are described

Abstract: We show that it is possible to use high rate co-evaporation of Al and Si onto room temperature substrates to achieve a novel two-phase nanoscale microstructure. These nanocomposites have a hardness as high as 4GPa (Al-23at.%Si), and display noticeable plasticity. Films with compositions of Al-12at.%Si and pure Al (used as baseline) were analyzed using transmission electron microscopy (TEM). The scale of the Al-12at.%Si microstructure is an order of magnitude finer compared to that of pure Al films. It consists of a dense distribution of spherical nanoscale Si particles separating irregularly-shaped small Al grains. These new structures may have a mechanical performance advantage over conventional single phase nanomaterials due to the role of the dispersed hard phase in promoting strain hardening.

Abstract: We present resonant Raman scattering measurements on strained and relaxed InxGa1-xN/GaN multiple quantum wells. The pseudomorphic sample does not show significant deviation of the A1(LO) phonon frequency with respect to GaN value due to a strong compensation of composition and strain effects which makes the frequency of this mode almost independent on In concentration. In contrast, the relaxed sample shows a marked decrease of the Raman frequency. Raman spectra excited in the energy range of sample emission have been recorded at room temperature. The resonant conditions have been attained using tuneable lasers in the blue-green spectral region. Resonant profiles are significantly blue-shifted with respect to the photoluminescence emission as a result of an inhomogeneous In distribution. In relaxed multiple quantum well, the Raman shift of the A1(LO) mode and the maximum of the resonant Raman profile give a direct estimate of the In concentration and its variation range.

Abstract: In this paper a procedure for the global optimization of mid-infrared GaAs/AlGaAs quantum cascade lasers that relies on the method of simulated annealing is presented. We propose a double longitudinal optical phonon resonance design obtained via a ladder of three states, with subsequent pairs separated by optical phonon energy. Addition of an extra level decreases the lower laser level population by enabling an efficient extraction into the injector region. The output characteristics of the optimized structures are calculated using the full self–consistent rate equation model, which includes all of the relevant scattering mechanisms. We also presented the experimentally measured output characteristics of an initial device, which are in agreement with the numerically calculated values, confirming the good design capabilities of the applied procedure.

Abstract: The optical gain in the active region of quantum cascade laser in an external magnetic field is analyzed. When the magnetic field is applied in the direction perpendicular to the plane of the layers, electron dispersion is broken into series of discrete Landau levels. This additional confinement strongly modifies the lifetime of electrons in the upper state of the laser transition, which is controlled by electron-phonon scattering. Landau levels are magnetically tuneable and, depending on their configuration, phonon emission is either inhibited or resonantly enhanced. This translates into a strong modulation of the population inversion, and consequently of the optical gain by varying the magnetic field. Numerical results are presented for a structure previously considered by Smirnov et al. [Phys. Rev B 66 (2002) 125317] which is designed to emit radiation at λ~11.4µm, with the magnetic field varied in the range 10-60T. The effects of band nonparabolicity are taken into account in this model.

Abstract: The intersublevel absorption in n-doped InAs/GaAs self-assembled quantum-dot molecules composed of three quantum dots is theoretically considered. The transition matrix elements and the transition energies are found to vary considerably with the spacer thickness. For s polarized light, decreasing the thickness of the spacer between the dots brings about crossings between the transition matrix elements, but the overall absorption is not affected by the variation of the spacer thickness. For p-polarized light and thick spacers, there are no available transitions in the single quantum dot, but a few of them emerge as a result of the electron state splitting in the stacks of coupled quantum dots, which leads to a considerable increase of the transition matrix elements, exceeding by an order of magnitude values of the matrix elements for s-polarized light.

Abstract: In this paper a new analytical carrier mobility model of a heterostructure unipolar transistor, High Electron Mobility Transistor (HEMT), is presented. The influence of the two dimensional electron gas confined in a HEMT channel on the device carrier mobility, is considered. The mobility dependence on temperature is also included in the model. Advantages of this model are its simplicity and straightforward implementation. Besides, it promises to be applied to quite different types of HEMTs. The model was tested. The results derived from simulations based on the proposed model are in very good agreement with the already known experimental data and theoretically obtained ones, available in literature.

Abstract: Dispersion laws and states (i.e. probability of finding) of Frankel excitons in ultra-thin molecular films are found using a well-known method of Green’s functions. Space boundaries and changes of energetic parameters on boundaries are considered as perturbations. The cubic crystalline system with complex cell consisted of two molecules (a and b), i.e. bimolecular film, was analyzed in harmonic approximation, and then compared with the results obtained for simple cubic cell systems (i.e. monomolecular film). In both cases the energy spectra show sharp discrete levels, although the energy spectra of bimolecular films split into two zones with discrete levels. Probability of finding exciton in the mono- or bimolecular ultra-thin films is significantly influenced by the perturbation and the values of on-site energies of molecules a and b. Obtained conditions of the existence of localized exciton states at boundaries are of special interest.