J.A. Freitas

Near-bandedge cathodoluminescence of an AlN homoepitaxial film

Cathodoluminescence experiments were performed on a high-quality AlN epitaxial film grown by organometallic vapor phase epitaxy on a large single crystal AlN substrate. The low-temperature near-bandedge spectra clearly show six very narrow lines. The thermal quenching behavior of these emission lines provides insight on how to assign them to free and bound exciton recombination processes. The binding energy for the free-exciton-A in AlN was found to be nearly twice that in GaN. The observation of the free-exciton-A first excited state permitted us to estimate its reduced effective mass and, by using recent reported values for the hole effective mass in Mg-doped AlN, the electron effective mass in AlN has been deduced.

Zingiberene-mediated resistance to the South American tomato pinworm derived from Lycopersicon hirsutum var. hirsutum

The Lycopersicon hirsutum var. hirsutum accession PI 127826 is recognized as a good source of resistance to arthropod pests due to the action of the allelochemical zimgiberene, a sesquiterpene present in its glandular trichomes. Five genotypes were selected from the F2 generation of the interspecific cross Lycopersicon esculentum ‘TOM-556’ × Lycopersicon hirsutum var. hirsutum ‘PI 127826’, based on their low levels (BPX-368-clone#56) or high levels(BPX-368-clone#92, BPX-368-clone#105,BPX-368-clone#179, BPX-368-clone#250) of zingiberene. The five F2 genotypes were tested for resistance to the South American tomato pinworm Tuta absolutaalong with accession L. esculentum ‘TOM-556’ (pinworm susceptible), and the accessions L. hirsutum var. hirsutum ‘PI 127826’ and L. pennellii ‘LA716’ (resistant). The F2 clones selected for high foliar zingiberene levels showed lower scores for leaflet lesion type(LLT), percent leaflets attacked (PLA) and overall plant damage (OPD) than the low zingiberene genotypes. The results indicated that zingiberene mediates resistance to the South American pinworm, based on feeding and on ovipositing deterrence, in populations derived from the interspecific cross between Lycopersicon. esculentum and Lycopersicon hirsutum var. hirsutum. Indirect selection for high foliar zingiberene content is suggested as an efficient technique for breeding tomatoes for resistance to the South American tomato pinworm.

Structural and optical properties of thick freestanding GaN templates

Structural and optical properties of thick (larger than 160 μm) freestanding hydride vapor phase epitaxy GaN templates have been investigated. AFM measurements showed that flat and smooth surface could be fabricated. High-resolution X-ray diffraction studies carried out with different spectrometer slit for the symmetric and asymmetric diffractions show that the linewidth increases with increasing slits width, indicating that a considerable degree of tilting and twisting of the individual grains are still present in these thick samples. Raman scattering measurements performed in a few samples indicate good crystalline quality and reduced strain. Very sharp and intense exciton related lines (FWHM less than 1 meV) have been observed in the low temperature photoluminescence spectra. Variable-temperature photoluminescence experiments were performed on both the growth surface and interface to identify the nature of the recombination processes observed in the luminescence spectra. FTIR absorption measurements show the presence of at least two donors with binding energy of 30.5 and 33.6 meV.

Properties of Bulk AlN grown by thermodecomposition of AlCl3⋅NH3

Self-nucleated bulk AlN crystals were grown by thermodecomposition of AlCl3⋅NH3 vaporized in the low-temperature zone of a two-zone furnace. X-ray diffraction of the AlN crystals show single lines with a small linewidth indicating high single-crystalline quality. Polarized Raman scattering experiments of these samples confirm the x-ray results based on the detection of a small linewidth for all allowed optical phonons. Low-temperature cathodoluminescence spectra show very sharp emission bands close to the optical band gap, which have been assigned to free-excitons A and B, and exciton-bound to shallow neutral impurity. The latter has a full width at half maximum smaller than 1.0 meV.Self-nucleated bulk AlN crystals were grown by thermodecomposition of AlCl3⋅NH3 vaporized in the low-temperature zone of a two-zone furnace. X-ray diffraction of the AlN crystals show single lines with a small linewidth indicating high single-crystalline quality. Polarized Raman scattering experiments of these samples confirm the x-ray results based on the detection of a small linewidth for all allowed optical phonons. Low-temperature cathodoluminescence spectra show very sharp emission bands close to the optical band gap, which have been assigned to free-excitons A and B, and exciton-bound to shallow neutral impurity. The latter has a full width at half maximum smaller than 1.0 meV.

Infrared dielectric function of wurtzite aluminum nitride

The infrared dielectric function of wurtzite AlN has been determined by fitting an analytic Lorentzian dielectric function to experimentally observed interference fringes in infrared transmission. The analytic model is scaled to agree with recent measurements of the visible refractive index, and the experiment and model extend to the submillimeter range of the infrared. A complete, experimentally verified dielectric function is found from the visible to the submillimeter spectral region for radiation with E⊥c axis, and an analytic model is produced for E‖c axis. Refractive indices and extinction coefficients from the visible to zero frequency are presented.

Anharmonic decay of phonons in strain-free wurtzite AlN

We present Raman scattering measurements on high-quality freestanding AlN single crystals. Polarization studies provide clear identification of all allowed phonons. We report the smallest phonon linewidths observed in AlN, which provide a direct measurement of the anharmonic decay of phonons in this material. Also from the Raman mode frequencies and reported index of refraction values we estimated the ordinary and extraordinary dielectric constant values. The calculated values are considerably different from previously reported values of the dielectric constant of AlN.

INFRARED DIELECTRIC CONSTANT OF CUBIC SIC

The real dielectric constant for chemical vapor deposition 3C‐SiC grown on silicon (Si) has been determined at 300 K and at 5 K from an analytic fit to interference fringes in transmission over the spectral range from the near infrared to the submillimeter region. This technique is capable of high accuracy being limited typically by the sample thickness and accuracy with which the thickness is measured. The resulting real dielectric constant is lower than the values usually attributed to this material. We find: at 300 K e 0=9.52 and e ∞=6.38; at 5 K e0=9.28 and e ∞=6.22. In all cases the estimated error is ±0.8%. The observed ratio e0 /e∞ agrees with the Lyddane–Sachs–Teller relation to 0.1% at 300 K and 0.2% at 5 K.

Effects of high-temperature anneals on 4H–SiC Implanted with Al or Al and Si

Co-implanting Si into 4H–SiC with Al hinders the ability of the Al acceptors to activate electrically at the lower annealing temperatures, but for annealing temperatures>1600°C, the effect is much less, suggesting that the activation energy for incorporating Al as an acceptor no longer controls the rate-determining step in this process. The cathodoluminescence data indicate that Al acceptors are trapped out by the DI defect, and this effect is more pronounced for the higher annealing temperatures. The increase in χmin with the annealing temperature at the higher temperatures can be explained by the nucleation and growth of structural defects, and the transmission electron miscroscopy results show that these defects are stacking faults. The stacking faults can be described as being quantum dots of different polytypes or domain walls with a point-defect periodic structure between the faulted and unfaulted regions, and they could be the source for the peaks associated with the DI defect. Also, we observed th...

Alcobaça ALLELE AND GENOTYPIC BACKGROUNDS AFFECT YIELD AND FRUIT SHELF LIFE OF TOMATO HYBRIDS

Post-harvest shelf life of tomato fruit may be increased by deploying mutant alleles which affect the natural ripening process and/or by a favorable genotypic background. Among the several ripening mutant genes, alcobaca (alc) has proved to be highly efficient in increasing shelf life of commercial tomato fruits, especially in heterozygosis, a state at which no limiting deleterious effects upon fruit color occur. The effects of heterozygosity in the alcobaca locus (alc+/alc) on yield and fruit quality traits of tomato hybrids with three genotypic backgrounds. We evaluated three pairs of hybrids obtained from crosses between the near-isogenic pollen source lines Flora-Dade (alc+/alc+) and TOM-559 (alc/alc), and three maternal lines (Stevens, NC-8276 and Piedmont). The six treatments were factorial combinations of two different status in the alc locus (alc+/alc and alc+/alc+) versus three different genotypic backgrounds (maternal lines). Fruits were harvested at the breaker stage of maturation and stored in shelves at 21oC for 14 days. Yield and fruit quality traits were then evaluated. Regardless of the background, the alc allele in heterozygosis (alc+/alc) did not interfere with the total yield, commercial yield, average mass per fruit, average mass per commercial fruit, fruit shape, or with fruit peduncular scar diameter. The alc+/alc genotype reduced the rate of firmness loss and delayed evolution of the red color of the fruit, thus contributing to an increase of the post-harvest shelf life for all three genotypic backgrounds.

Detection and Identification of Donors in Hydride-Vapor-Phase Epitaxial GaN Layers

Low temperature photoluminescence experiments carried out on HVPE GaN samples produced in two different reactors are consistent with a low background level of donors and an extremely low concentration of compensating shallow acceptors. Infrared absorption experiments on these samples indicated that different shallow donors are incorporated at different rates in each reactor. High sensitivity SIMS was employed to measure the concentration of background impurities of a number of samples produced in both reactors. The comparison between the infrared absorption cross-sections with the SIMS results allows the identification of Si, the shallower, and O as the two dominant residual donors in these examples of HVPE GaN.