Search results for " semi"
showing 10 items of 926 documents
The influence of AlN buffer over the polarity and the nucleation of self-organized GaN nanowires
2015
We experimentally investigate the influence of AlN buffer growth on the nucleation and the polarity of a self-organized assembly of GaN nanowires (NWs) grown on Si. Two complementary growth mechanisms for AlN buffer deposited on Si are demonstrated. Both emphasize the aggregation of Si on the AlN surface and the growth of large cubic crystallites, namely, AlN pedestals. Further growths of GaN NWs assembly reveal that the GaN 2D layer found at the bottom of the NW assembly is the result of the coalescence of Ga-polar pyramids, whereas AlN pedestals are observed as preferential but not exclusive NW nucleation sites. NWs are N-polar or exhibit inversion domains with a Ga-polar core/N-polar she…
Phase segregation in Mg$_{x}$Zn$_{1-x}$O probed by optical absorption and photoluminescence at high pressure
2017
The appearance of segregated wurtzite Mg$_x$Zn$_{1-x}$O with low Mg content in thin films with $x>0.3$ affected by phase separation, cannot be reliably probed with crystallographic techniques owing to its embedded nanocrystalline configuration. Here we show a high-pressure approach which exploits the distinctive behaviors under pressure of wurtzite Mg$_x$Zn$_{1-x}$O thin films with different Mg contents to unveil phase segregation for $x>0.3$. By using ambient conditions photoluminescence (PL), and with optical absorption and PL under high pressure for $x=0.3$ we show that the appearance of a segregated wurtzite phase with a magnesium content of x $\sim$ 0.1 is inherent to the wurtzit…
Photo-electrical and transport properties of hydrothermal ZnO
2016
We performed the studies of optical, photoelectric, and transport properties of a hydrothermal bulk n-type ZnO crystal by using the contactless optical techniques: photoluminescence, light-induced transient grating, and differential reflectivity. Optical studies revealed bound exciton and defect-related transitions between the donor states (at ∼60 meV and ∼240 meV below the conduction band) and the deep acceptor states (at 0.52 eV above the valence band). The acceptor state was ascribed to VZn, and its thermal activation energy of 0.43 eV was determined. A low value of carrier diffusion coefficient (∼0.1 cm2/s) at low excitations and temperatures up to 800 K was attributed to impact the rec…
SIC based solid state protections switches for space applications
2017
Development and technology maturation of Silicon Carbide (SiC) power transistors over the last 15 years has motivated its study in aerospace systems. When compared with Si devices, superior voltage blocking capacity and the capability of operation at higher temperatures, give important advantages in space power electronics applications, similar to what happens in terrestrial electronics. This paper discusses the use of SiC power transistors for Solid State Power Switches especially addressed to the space segment. Two applications will be covered, the first is the Solid State Shunt Switch, widely used in high power Direct Energy Transfer (DET) photovoltaic power regulators and the second is …
Half-Heusler compounds: novel materials for energy and spintronic applications
2012
Half-Heusler compounds are an impressive class of materials with a huge potential for different applications such as future energy applications and for spintronics. The semiconducting Heusler compounds can be identified by the number of valence electrons. The band gap can be tuned between 0 and 4 eV by the electronegativity difference of the constituents. Magnetism can be introduced in these compounds by using rare-earth elements, manganese or ‘electron’ doping. Thus, there is a great interest in the fields of thermoelectrics, solar cells and diluted magnetic semiconductors. The combination of different properties such as superconductivity and topological edge states leads to new multifunct…
Simplified feedback control system for scanning tunneling microscopy
2021
A Scanning Tunneling Microscope (STM) is one of the most important scanning probe tools available to study and manipulate matter at the nanoscale. In a STM, a tip is scanned on top of a surface with a separation of a few \AA. Often, the tunneling current between tip and sample is maintained constant by modifying the distance between the tip apex and the surface through a feedback mechanism acting on a piezoelectric transducer. This produces very detailed images of the electronic properties of the surface. The feedback mechanism is nearly always made using a digital processing circuit separate from the user computer. Here we discuss another approach, using a computer and data acquisition thr…
Optical properties and microstructure of 2.02-3.30 eV ZnCdO nanowires: effect of thermal annealing
2013
International audience; ZnCdO nanowires with up to 45% Cd are demonstrated showing room temperature photoluminescence (PL) down to 2.02 eV and a radiative efficiency similar to that of ZnO nanowires. Analysis of the microstructure in individual nanowires confirms the presence of a single wurtzite phase even at the highest Cd contents, with a homogeneous distribution of Cd both in the longitudinal and transverse directions. Thermal annealing at 550 C yields an overall improvement of the PL, which is blue-shifted as a result of the homogeneous decrease of Cd throughout the nanowire, but the single wurtzite structure is fully maintained.
Reactions in the presence of irradiated semiconductors: Are they simply photocatalytic?
2018
A generally accepted classification distinguishes semiconductor photocatalysis in type A and type B. The first class comprehends reactions where the primary redox products (radical anions and cations) further evolve giving rise to the final reduced and oxidized species. Type B reactions occur when the primary redox products undergo intermolecular bond formation to addition products. Recently, some photocatalytic reactions have been reported wherein the photocatalytically produced compounds are not the final products. In fact, they react in the solution bulk or catalytically on the surface of the semiconductor with other species thus producing the target compounds. These reactions, hereby re…
Rhynchophorus ferrugineus: Behavior, Ecology, and Communication
2017
Red palm weevil (RPW) ecology is characterized by the adults' ability to aggregate on palms. The aggregation process has the functions of protection, feeding, and reproduction for the individuals. Semiochemicals and visual cues strongly influence this behavior at intraspecific and interspecific levels. Adults actively fly over long distances, following chemical cues, such as aggregation pheromone and host plant odor, or visual cues to colonize a new host. The aggregation pheromone of RPW is a male-produced mixture of 4-methyl-5-nonanol (major component) and the related ketone 4-methyl-5-nonanone (minor component). These compounds are largely used in the field to lure males and especially fe…
Paysandisia archon: Behavior, Ecology, and Communication
2017
This chapter on Paysandisia archon behavior, ecology, and communication is presented in three parts. The first part deals with reproductive behavior and the sex pheromone. In this section, mating behavior is described with a consideration of optimal age and diel periodicity. Early observations have shown that mating can occur a few hours after adult molting, from noon to the beginning of the afternoon, according to timing of emergence. The female is described as flying close to perching males and the pair then fly together before mating. The male exhibits a scratching movement with the mid-legs while the female extrudes the abdominal tips. Putative sex pheromone production is discussed in r…