Effects of ion implantation on the mechanical behavior of GaN films

The effects of ion implantation on the response to indentation in epitaxially grown hexagonal GaN films were studied by means of the static microindentation technique, utilizing Knoop and Vickers indenter geometries. Mg, O, Au, Xe and Ar ions were used as projectiles for the implantation process. Heavily damaged polycrystalline epilayers showed enhanced microhardness values and normal indentation size effect (ISE). Amorphised epilayers showed lower microhardness values, while they presented reverse ISE. The shape of the Knoop indentation print as a function of the implanted species revealed that reverse ISE is connected with plastic behavior. Implantation was also found to render films more receptive to fracture. Normal ISE curves were explained using models such as Meyer's law, Hays-Kendall approach, proportional specimen resistance (PSR), modified PSR and elastic/plastic deformation (EPD) models.     

Influence of thermal oxidation on the interfacial properties of ultrathin strained silicon layers

In this work we examine the influence of thermal oxidation on the electrical characteristics of ultra-thin strained silicon layers grown on relaxed Si_0.78Ge_0.22 substrates under moderate to high thermal budget conditions in N₂O ambient at 800 °C. The results reveal the presence of a large density of interfacial traps which depends on the oxidation process. As long as the strained silicon layer remains between the growing oxide and the underlying Si_0.78Ge_0.22 layer, the density of interface traps increases with increasing oxidation time. When the oxidation process consumes the s-Si layer the interface state density undergoes a significant reduction of the order of 40%. This experimental evidence signifies that the strained silicon-Si_0.78Ge_0.22 interface is a major source of the measured interfacial defects. This situation can be detected only when the front SiO₂-strained silicon interface and the rear strained silicon-Si_0.78Ge_0.22 interface are in close proximity, i.e. within a distance of 5 nm or less. Finally, the influence of the material quality deterioration—as a result of the thermal treatment—to the interfacial properties of the structure is discussed.     

Crystallization of amorphous silicon thin films: comparison between experimental and computer simulation results

Polycrystalline silicon obtained by the crystallization of thin amorphous silicon films has been an important material for microelectronics technology during the last decades. Many properties are improved in crystallized amorphous silicon compared to the as-deposited polysilicon such as larger grain size, smoother surface, and higher-carrier mobility. In this work, the crystallization of amorphous silicon is investigated by combining transmission electron microscopy (TEM) observations and molecular dynamics calculations. TEM observations on a series of specimens have shown that the majority of the silicon grains are oriented with a zone axis normal to the surface. In order to understand the crystallization mechanism molecular dynamic simulations were performed. It is found that the c/amorphous interface exhibits the lowest reduced interfacial energy density while the c/amorphous has the lowest reduced energy differences per unit interfacial area. The most energetically unfavorable interface is c/amorphous.     

Direct comparison of catalyst-free and catalyst-induced GaN nanowires

GaN nanowires have been grown by molecular beam epitaxy either catalyst-free or catalyst-induced by means of Ni seeds. Under identical growth conditions of temperature and V/III ratio, both types of GaN nanowires are of wurtzite structure elongated in the Ga-polar direction and are constricted by M-plane facets. However, the catalyst-induced nanowires contain many more basal-plane stacking faults and their photoluminescence is weaker. These differences can be explained as effects of the catalyst Ni seeds.      

Influence of laser annealing on the structural properties of sputtered AlN:Ag plasmonic nanocomposites

We propose a process of deposition of plasmonic nanocomposites comprising magnetron sputtering of AlN:Ag multilayers combined with intermediate steps of flash annealing. When the AlN matrix structure was amorphous, thermal annealing induced the break-up of silver layers and the formation of homogeneously distributed spherical nanoparticles. On the other hand, in the case of a nanocrystalline AlN matrix, the larger nanoparticles were observed to form only at an interfacial and a surface zone. Further treatment by laser annealing was employed in order to photo-modulate the localized surface plasmon resonances (LSPRs) by promoting ripening of the nanoparticles. Using high resolution transmission electron microscopy, it was observed that laser annealing caused nanoparticle enlargement with a concurrent improvement of their separation, while retaining their average spherical shape. Optical reflectance measurements showed that better LSPR was obtained when the AlN matrix was amorphous due to the restrained nanoparticle ripening inside nanocrystalline AlN. Roughening at the film/substrate interface and film degradation after laser annealing at the employed radiation wavelength where reduced compared to similar samples grown by pulsed laser deposition. Based on finite difference time domain simulations and X-ray reflectivity measurements, this was attributed to the improved quality of the AlN matrix.     

Rigid body translation in the (\bar 211) twin boundary in silicon

The rigid body translation accompanying a ( 11) twin boundary in silicon has been studied by transmission electron microscopy. From a detailed analysis of theα-type fringe systems in the 111, 311 and 2 0 common reflections, the following translation vector is deduced: [011], which is equivalent to [411] in the other crystal element. A slight deviation of this orientation is possible.     

Study of InN/GaN interfaces using molecular dynamics

Epitaxial growth of thin films is, in general, based on specific interfacial structures defined by a minimum of interfacial energy and usually influenced by the structural mismatch. In the present study, the structures and energies of (0001) InN/GaN epitaxial interfaces are studied using the Tersoff interatomic potential. The potential describes the metallic and intermetallic interactions sufficiently well and is assembled in order to accurately reproduce the lattice and elastic parameters of wurtzite Ga(In)-Nitrides. Different configurations are examined for each interface depending on polarity and atomic stacking. It is shown that the interfacial structures of InN thin films grown with indium polarity interfaces exhibit lower self-energies than those of N-polarity. Although the substrate and the epilayer were assumed to exhibit the wurtzite crystal structure, both wurtzite and zinc-blende type atomic stackings are possible at the interfacial region since they were found energetically degenerate within the accuracy of our model. Finally, the spatial location of the epitaxial interface is also energetically defined. Epitaxial interfaces in this system can in principle be imagined to pass through so-called single or double atomic bonds, but the former case was energetically more favourable.     

Evaluation of virological quality of sewage from four biological treatment plants by a nested-PCR technique.

In order to determine the virological quality of sewage from four biological treatment plants in Greece (two in the city of Athens and two in the city of Patras), 92 raw sewage samples were analysed for the presence of enteroviruses and adenoviruses during the period from October 2000 to February 2003. A nested-PCR method was used in order to increase the sensitivity of virus detection. Enteroviruses were detected in 43 samples (47%) and adenoviruses in 75 samples (81.5%) of raw sewage by nested PCR. The more frequent isolation of adenoviruses in raw sewage indicated their stability as virological indicators of the pollution of the environment and their increased persistence in sewage.