Th. Kehagias

Heteroepitaxial growth of In-face InN on GaN (0001) by plasma-assisted molecular-beam epitaxy

The thermodynamic aspects of indium-face InN growth by radio frequency plasma-assisted molecular-beam epitaxy (rf-MBE) and the nucleation of InN on gallium-face GaN (0001) surface were investigated. The rates of InN decomposition and indium desorption from the surface were measured in situ using reflected high-energy electron diffraction and the rf-MBE “growth window” of In-face InN (0001) was identified. It is shown that sustainable growth can be achieved only when the arrival rate of active nitrogen species on the surface is higher than the arrival rate of indium atoms. The maximum substrate temperature permitting InN growth as a function of the active nitrogen flux was determined. The growth mode of InN on Ga-face GaN (0001) surface was investigated by reflected high-energy electron diffraction and atomic force microscopy. It was found to be of the Volmer–Weber-type for substrate temperatures less than 350°C and of the Stranski–Krastanov for substrate temperatures between 350 and 520°C. The number of m...

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.

Akaganeite and goethite-type nanocrystals: synthesis and characterization

The synthesis of iron oxyhydroxides and hydroxides is reported in this work, by the use of a novel, simple and low-cost method. The preparation involves the hydrolysis of aqueous solutions of ferric salts followed by membrane purification and freeze drying of the products. Three different iron precursors have been tested and combined to three different volatile precipitating agents. The obtained products were akaganeite, goethite and iron(III) hydroxide. Irrespective of the starting materials used, all three products, although different in chemical nature, presented some very interesting and unique features; they consisted of nanoparticles with mean sizes ranging from 1 to 10 nm and they had very high surface areas and pore sizes in the meso- and micropore regions. The produced materials were examined by powder X-ray diffraction for crystalline phase identification, TEM and XRD for particle size estimation and nitrogen sorption for surface area, pore volume and pore size distribution measurement.

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.

Internal quantum efficiency of III-nitride quantum dot superlattices grown by plasma-assisted molecular-beam epitaxy

We present a study of the optical properties of GaN/AlN and InGaN/GaN quantum dot (QD) superlattices grown via plasma-assisted molecular-beam epitaxy, as compared to their quantum well (QW) counterparts. The three-dimensional/two-dimensional nature of the structures has been verified using atomic force microscopy and transmission electron microscopy. The QD superlattices present higher internal quantum efficiency as compared to the respective QWs as a result of the three-dimensional carrier localization in the islands. In the QW samples, photoluminescence (PL) measurements point out a certain degree of carrier localization due to structural defects or thickness fluctuations, which is more pronounced in InGaN/GaN QWs due to alloy inhomogeneity. In the case of the QD stacks, carrier localization on potential fluctuations with a spatial extension smaller than the QD size is observed only for the InGaN QD-sample with the highest In content (peak emission around 2.76 eV). These results confirm the efficiency o...

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.

Mechanism of compositional modulations in epitaxial InAlN films grown by molecular beam epitaxy

A mechanism for compositional modulations in InxAl1−xN films is described which considers growth kinetics during molecular beam epitaxy. InAlN crystalline films with various indium contents, grown on GaN or AlN buffer layers to create a variation in lattice mismatch conditions, were studied by transmission electron microscopy. Films comprise of columnar domains which are observed regardless of mismatch, with increasing indium concentration toward domain edges. We propose that indium is incorporated preferentially between adjacent dynamical InAlN platelets, owing to tensile strain generated upon platelet coalescence. The resulting In-rich boundaries are potential minima for further indium adatoms, creating a permanent indium composition gradient.

Misfit accommodation of compact and columnar InN epilayers grown on Ga-face GaN (0001) by molecular-beam epitaxy

The interfacial structural properties of compact InN films and of noncoalesced three-dimensional InN islands, grown by molecular-beam epitaxy on Ga-face GaN/Al2O3 (0001) substrates, were investigated by transmission electron microscopy. Compact film growth was accomplished employing an InN nucleation layer, grown at low substrate temperatures. A 60° misfit dislocation network effectively accommodated the lattice mismatch in the InN/GaN interface in both cases of epilayers. The lattice constants of InN were determined by electron diffraction analysis, revealing a 0.28% larger in-plane parameter of the compact InN film relative to the corresponding lattice parameter of the InN islands. This is attributed to thermal tensile strain developed during post-growth cooling down of the epilayers, which also compensated the remaining compressive strain originating from the in-plane lattice mismatch of InN and GaN.