Th. Karakostas

Properties of GaN Nanowires Grown by Molecular Beam Epitaxy

On Si(1 1 1) and Si(0 0 1), GaN nanowires (NWs) form in a self-induced way without the need for any external material. On sapphire, NW growth is induced by Ni collectors. Both types of NWs exhibit the wurtzite crystal structure and grow in the Ga-polar C-direction perpendicular to the substrate. The NW sidewalls are M-plane facets, although on the Ni-induced NWs also A-plane segments form, if the growth temperature is low. Both self-induced and collector-induced NWs are free of strain and epitaxially aligned to the substrate, but in particular the former show a significant spread in tilt and twist caused by a mostly amorphous interfacial layer of Si-N. The self-induced NWs are virtually free of extended defects, but the collector-induced NWs contain many stacking faults. The photoluminescence of the former is significantly brighter and sharper. The spectra of single, dispersed, self-induced NWs contain extremely sharp excitonic lines. Significant emission is caused by excitons bound to donors close to the surface whose binding energy is reduced compared to the bulk value. In comparison, both the microstructure and optical properties of the self-induced NWs are superior. The limited material quality of the collector-induced NWs can be explained by detrimental effects of the collector.

Axial and radial growth of Ni-induced GaN nanowires

GaN nanowires (NWs) were grown on sapphire by molecular beam epitaxy. NWs form only in the presence of Ni seed particles and only under N-rich conditions. Their length increases linearly with growth time up to about 7.5μm while their diameter remains almost constant. In contrast, a switch to Ga-rich conditions after NW formation results in radial growth, i.e., the NW diameter increases while lengthening is negligible. These results corroborate the fact that the growth of III-V NWs is governed by the accumulation of group-III atoms in the seeds, while group-V species are not preferentially incorporated at the seeds.

Microstructural changes of processed vitrified solid waste products

Abstract Toxic lead-rich solid industrial wastes were stabilized by the vitrification method. Vitrification was attained by the addition of SiO 2 and Na 2 O as vitrifying and melting agent, respectively. The non-toxic, homogeneous, vitreous products studied in the present work, contain 60 wt.% of solid waste. Products with such a high content of solid waste comprise an economically realistic suggestion, but are easily devitrified in conditions of large-scale production due to the difficulty to achieve rapid cooling conditions in the whole volume of a large piece of stabilized product. Thus, it must be ascertained that the loss of homogeneity is not accompanied with the loss of chemical stability. Differential Thermal Analysis (DTA) was applied in order to inspect the prospect to crystal phase separation. The separated crystal phases were characterized by X-ray diffraction and transmission electron microscopy. Possible devitrification processes are investigated in order to interconnect the microstructure with the chemical stability of the devitrified products.

Indium migration paths in V-defects of InAlN grown by metal-organic vapor phase epitaxy

InAlN thin films grown on GaN/Al2O3 (0001) templates by metal-organic vapor phase epitaxy were studied by transmission electron microscopy techniques. V-defects in the form of hexagonal inverted pyramids with {101¯1} sidewalls were observed on the films’ surfaces linked to the termination of threading dislocations. Their origin is explained by the different surface atom mobility of In and Al and the built-in strain relaxation. Indium segregation in the films is influenced by the formation of V-defects, the edges and the apexes of which function as paths of migrating indium atoms diffusing along nanopipes formed at the open-core threading dislocations.InAlN thin films grown on GaN/Al2O3 (0001) templates by metal-organic vapor phase epitaxy were studied by transmission electron microscopy techniques. V-defects in the form of hexagonal inverted pyramids with {101¯1} sidewalls were observed on the films’ surfaces linked to the termination of threading dislocations. Their origin is explained by the different surface atom mobility of In and Al and the built-in strain relaxation. Indium segregation in the films is influenced by the formation of V-defects, the edges and the apexes of which function as paths of migrating indium atoms diffusing along nanopipes formed at the open-core threading dislocations.

Vitrification of lead-rich solid ashes from incineration of hazardous industrial wastes

Lead-rich solid industrial wastes were vitrified by the addition of glass formers in various concentrations, to produce non-toxic vitreous stabilized products that can be freely disposed or used as construction materials. Toxicity of both the as-received industrial solid waste and the stabilized products was determined using standard leaching test procedures. The chemically stable vitreous products were subjected to thermal annealing in order to investigate the extent of crystal separation that could occur during cooling of large pieces of glass. Leaching tests were repeated to investigate the relation between annealing process and chemical stability. X-ray, scanning and transmission electron microscopy techniques were employed to identify the microstructure of stabilized products before and after thermal treatment. Relation between synthesis and processing, chemical stability and microstructure was investigated.

Nanostructure and strain in InGaN/GaN superlattices grown in GaN nanowires

The structural properties and the strain state of InGaN/GaN superlattices embedded in GaN nanowires were analyzed as a function of superlattice growth temperature, using complementary transmission electron microscopy techniques supplemented by optical analysis using photoluminescence and spatially resolved microphotoluminescence spectroscopy. A truncated pyramidal shape was observed for the 4 nm thick InGaN inclusions, where their (0001¯) central facet was delimited by six-fold {101¯l} facets towards the m-plane sidewalls of the nanowires. The defect content of the nanowires comprised multiple basal stacking faults localized at the GaN base/superlattice interface, causing the formation of zinc-blende cubic regions, and often single stacking faults at the GaN/InGaN bilayer interfaces. No misfit dislocations or cracks were detected in the heterostructure, implying a fully strained configuration. Geometrical phase analysis showed a rather uniform radial distribution of elastic strain in the (0001¯) facet of the InGaN inclusions. Depending on the superlattice growth temperature, the elastic strain energy is partitioned among the successive InGaN/GaN layers in the case of low-temperature growth, while at higher superlattice growth temperature the in-plane tensile misfit strain of the GaN barriers is accommodated through restrained diffusion of indium from the preceding InGaN layers. The corresponding In contents of the central facet were estimated at 0.42 and 0.25, respectively. However, in the latter case, successful reproduction of the experimental electron microscopy images by image simulations was only feasible, allowing for a much higher occupancy of indium adatoms at lattice sites of the semipolar facets, compared to the invariable 25% assigned to the polar facet. Thus, a high complexity in indium incorporation and strain allocation between the different crystallographic facets of the InGaN inclusions is anticipated and supported by the results of photoluminescence and spatially resolved microphotoluminescence spectroscopy.

Contacts of titanium nitride to n- and p-type gallium nitride films

Abstract The electrical characteristics of titanium nitride (TiN x ) contacts to n- and p-type GaN films, deposited by reactive magnetron sputtering at room temperature, are investigated. The contacts of TiN x to n-type GaN are ohmic and to p-type GaN are rectifying, while their properties are strongly dependent on the stoichiometry of the deposited titanium nitride layer. It is shown that the ohmic behavior of the contacts is associated with the presence of a high density of interface states and not to the low Schottky barrier.

Incineration of tannery sludge under oxic and anoxic conditions: study of chromium speciation.

A tannery sludge, produced from physico-chemical treatment of tannery wastewaters, was incinerated without any pre-treatment process under oxic and anoxic conditions, by controlling the abundance of oxygen. Incineration in oxic conditions was performed at the temperature range from 300°C to 1200°C for duration of 2h, while in anoxic conditions at the temperature range from 400°C to 600°C and varying durations. Incineration under oxic conditions at 500°C resulted in almost total oxidation of Cr(III) to Cr(VI), with CaCrO4 to be the crystalline phase containing Cr(VI). At higher temperatures a part of Cr(VI) was reduced, mainly due to the formation of MgCr2O4. At 1200°C approximately 30% of Cr(VI) was reduced to Cr(III). Incineration under anoxic conditions substantially reduced the extent of oxidation of Cr(III) to Cr(VI). Increase of temperature and duration of incineration lead to increase of Cr(VI) content, while no chromium containing crystalline phase was detected.

Dislocation movements and deformation twinning in zinc

Abstract Deformation twinning is a predominant deformation mode for many h.c.p. metals. In order to contribute to the understanding of the mechanism of this deformation, the (1012) [1011] deformation twin mode has been examined for the case of zinc. TEM observations indicate the possible existence of two families of intrinsic dislocations on the (1012) twin plane (named “twinning” and tilt dislocations), which glide occasionally on that plane. A dislocation-type mechanism is, therefore, proposed as a possibility for deformation twinning.

MISFIT DISLOCATIONS AND ANTIPHASE DOMAIN BOUNDARIES IN GAAS/SI INTERFACE

The interaction of the antiphase boundaries that are formed at the early stage of growth with the interfacial misfit dislocations is studied by transmission electron microscopy using contrast criteria. Experimental analysis has shown that the shifting of the misfit dislocation families, by half of their periodicity, is due to their intersection with antiphase boundaries emanating from demisteps on the Si substrate. The observed discontinuity of dislocation lines is attributed to dynamical contrast conditions. The antiphase boundaries do not interrupt the continuity of the network of dislocations. The dichromatic theory of interfacial defects is applied in order to illustrate the geometrical features of the pattern. The disymmetrization mechanism of the pattern obeys the principle of symmetry compensation. A symmetry analysis of the GaAs/Si interface justifies the agreement of the observations with the structural model.