M. Avella

Luminescent nanostructures based on Ge nanoparticles embedded in an oxide matrix

Ge nanoparticles embedded in an oxide matrix have been obtained by (a) steam thermal oxidation at 650 °C of polycrystalline SiGe layers and (b) deposition of discontinuous Ge films/SiO2 multilayers by low pressure chemical vapour deposition at 390 °C and thermal annealing at 700 °C. These two approaches are compared in terms of the composition and size of the nanoparticles and the luminescence properties of the structures. In both cases violet luminescence peaking at 3.1 eV is detected. The origin of this emission in both types of structures is the same and it will be related to defects at the interface between the nanocrystalline Ge and the dielectric matrix. Thinking about future applications, the second approach is found to be much more attractive from the technological point of view, even considering that the first one gives a more intense luminescence emission, since the structure can be fabricated in a single-run process; these structures are currently being investigated to improve their luminescence emission.

Characterization of GaAs conformal layers grown by hydride vapour phase epitaxy on Si substrates by microphotoluminescence cathodoluminescence and microRaman

Conformal growth of GaAs on Si consists of the confined lateral selective epitaxy of GaAs from GaAs oriented seeds on silicon, the vertical growth being stopped by an overhanging dielectric mask. Low defect GaAs films are obtained due to the absence of direct nucleation of the conformal GaAs epilayers on Si, and to the geometrical hindrance of the propagation of dislocations into the growing layer by the capping surface and by the substrate. GaAs conformal layers grown by hydride vapour phase epitaxy (HVPE) were characterised by microphotoluminescence (MPL), cathodoluminescence (CL) and microRaman. The GaAs conformal layers were found of superior quality since their luminescence emission was enhanced by several orders of magnitude with respect to the seeds directly grown on the Si substrate. CL and MPL images revealed in plane modulation of the luminescence emission. This modulation was associated with residual stress. MicroRaman measurements revealed stress distribution and eventually local symmetry breakdown.

A physical model for the rapid degradation of semiconductor laser diodes

The degradation of AlGaAs based high power laser bars (808 nm) is modeled in terms of the thermal stress gradient induced by the overheating produced at a facet defect by self-absorption and nonradiative recombination. Using a thermomechanical model, the local heating at the defect is shown to induce local stress above the yield strength necessary for plastic deformation. Cathodoluminescence images of the facets show the formation of large facet defects. The role of the packaging stress is also elucidated. The power density dissipation and the local temperature necessary to achieve the plastic deformation are in good agreement with the experimental values reported for laser degradation.

Uniformity and physical properties of semi-insulating Fe-doped InP after wafer or ingot annealing

Semi-insulating Fe-doped InP was annealed under different conditions and investigated by Hall effect, extrinsic photocurrent mapping, chemical etching, and optical microscopy. The resistivity is increased for any treatment, particularly in wafer-annealed InP. This result is probably due to strong losses of shallow donors. Remarkable differences exist between the structural properties of the wafer and ingot annealed material; wafer annealing produces a quick elimination of growth striations and decoration microdefects while ingot-annealed InP still retain these microdefects, both along dislocations and in the matrix. The photocurrent maps indicate that the thermal treatments normally improve the doping uniformity (especially the short-range fluctuations). The best uniformity is achieved for the long annealing time (⩾50 h), while a slow cooling rate improves the mobility.

Gallium-assisted growth of silicon nanowires by electron cyclotron resonance plasmas

The use of gallium droplets for growing Si nanowires (SiNWs) by electron cyclotron resonance plasmas is investigated. First, the relationship between evaporation time and resultant size of the gallium droplets is studied. Through the use of spectroscopic ellipsometry, the dependence of the surface plasmon resonance (SPR) energy on the droplet size is determined. From these gallium droplets, SiNWs were grown at 300 and 550 °C in electron cyclotron resonance plasmas containing SiH(4), Ar, and H(2). Scanning electron microscopy results show that tapered NWs are obtained for a wide range of growth conditions. Besides, it is found that H(2) plays an important role in the parasitic axial growth of the SiNWs. Namely, H(2) inhibits the radial growth and contributes dramatically to increasing the SiNW defects.

Thermomechanical model for the plastic deformation in high power laser diodes during operation

A thermomechanical model for the mechanism of rapid degradation of AlGaAs based high power laser bars (808 nm) is presented. Thermal stresses induced in the device by local heating around a facet defect by nonradiative recombination and self-absorption of photons are calculated, as well as the conditions for the beginning of plastic deformation, when these thermal stresses overcome the yield strength. The values of the power density and of the local temperature at which the yield limit is surmounted are in agreement with the threshold values for the degradation of Al based lasers given in the literature. The present model can also elucidate the role played by the packaging stress, being able to explain how this stress reduces the optical power density threshold for failure of these lasers.

Compositional and optical uniformity of InGaN layers deposited on (0001) sapphire by metal–organic vapour phase epitaxy

InGaN/GaN heterostructures grown by metal–organic vapour phase epitaxy were investigated by micro-Raman spectroscopy, photoluminescence and spectrally-resolved cathodo-luminescence (CL). These methods allowed the precise determination of the indium distribution at the microscale and macroscale. Owing to the axial symmetry of the used vertical reactor, the In molar fraction in the films normally tends to increase from the centre to the edge of the 2-inch wafers. It is also observed that for increasing In content, some additional modes appear in the Raman spectra. They are tentatively associated with In clustering phenomena, most probably occurring around bunches of threading dislocations. This hypothesis is further justified by CL spectral maps.

Temperature dependence of the Raman shift in GaAs conformal layers grown by hydride vapor phase epitaxy

Raman spectra between room temperature and 350 °C were measured in GaAs layers grown by hydride vapor phase epitaxy on Si substrates using the selective conformal growth method. The contributions of the thermal expansion, anharmonic phonon decay, and strain are considered in order to analyze the Raman data. The tensile strain in the conventional GaAs/Si seed and in the conformal GaAs layers was determined from the Raman spectra. It is shown that the thin SiO2 layer between the GaAs and the Si substrate is a compliant layer that plays an important role in the reduction of the dislocation density in the conformal layers. The tensile strain in conformal layers was higher than in the conventional GaAs/Si layers, in which strain is relieved by the high density of dislocations.

Uniformity of semi-insulating InP wafers obtained by Fe diffusion

As-cut InP wafers with residual carrier concentration <5times1015cm-3, submitted to Fe-diffusion at high temperature, became semi-insulating, with resistivity above 107 Omegacm and mobility in the range 3000-4000 cm2 /Vs. The photoconductivity and photoluminescence mapping showed that the uniformity is improved with respect to typical as-grown Fe-doped InP whilst the overall Fe content is about 1/3 of that typical of SI LEC indium phosphide

Optical properties of epitaxial lateral overgrowth GaN structures studied by Raman and cathodoluminescence spectroscopies

The properties of epitaxial lateral overgrowth (ELO) GaN layers were studied by means of cathodoluminescence (CL), micro-Raman spectroscopies, and transmission electron microscopy (TEM). CL shows a strong enhancement of the luminescence emission in the ELO regions, where TEM showed the absence of dislocations. The CL enhancement observed is mostly due to the yellow luminescence (YL) band, which contrasts with the good crystal quality observed in the ELO regions by TEM and Raman spectroscopy. Local CL spectra in the ELO regions showed a different behavior of the near band edge emission in relation to the buffer layers and central part of the vertical growth region. Donor-acceptor pair recombination is enhanced in the ELO regions. The Raman spectra indicate a low strain level in the ELO regions. Plasmon-coupled modes are not observed in these areas, indicating the absence of free carriers. CL and Raman observations could thus be explained in terms of the good crystalline quality of the ELO regions, in which...