C. R. Li

Blue-violet photoluminescence from amorphous Si-in-SiNx thin films with external quantum efficiency in percentages

Bright blue-violet photoluminescence centered at 428nm was obtained in amorphous Si-in-SiNx thin films prepared in a cyclic growth mode on cool substrates by plasma-enhanced chemical vapor deposition, in which the typical size of the silicon particles has been brought down to ∼1.80nm and the number density amounts to 1.07×1013cm2. A maximum external quantum efficiency over 3.0% was measured in the as-deposited samples. Time-resolved photoluminescence spectra revealed decay times within nanosecond even at room temperature, disclosing a fast recombination dynamics in this amorphous system. The excellent efficiency can be attributed to a better passivation of the silicon particles—hence, the conventional rapid thermal annealing has a negligible effect on the photoluminescence intensity—and also to an improved transparency of the film by cyclic growth. The high-efficiency, tunable wavelength and nanosecond decay time at room temperature, achieved via a low-temperature process without invoking any posttreatmen...

Precision methylome characterization of Mycobacterium tuberculosis complex (MTBC) using PacBio single-molecule real-time (SMRT) technology

Tuberculosis (TB) remains one of the most common infectious diseases caused by Mycobacterium tuberculosis complex (MTBC). To panoramically analyze MTBCs genomic methylation, we completed the genomes of 12 MTBC strains (Mycobacterium bovis; M. bovis BCG; M. microti; M. africanum; M. tuberculosis H37Rv; H37Ra; and 6 M. tuberculosis clinical isolates) belonging to different lineages and characterized their methylomes using single-molecule real-time (SMRT) technology. We identified three m6A sequence motifs and their corresponding methyltransferase (MTase) genes, including the reported mamA, hsdM and a newly discovered mamB. We also experimentally verified the methylated motifs and functions of HsdM and MamB. Our analysis indicated the MTase activities varied between 12 strains due to mutations/deletions. Furthermore, through measuring ‘the methylated-motif-site ratio’ and ‘the methylated-read ratio’, we explored the methylation status of each modified site and sequence-read to obtain the ‘precision methylome’ of the MTBC strains, which enabled intricate analysis of MTase activity at whole-genome scale. Most unmodified sites overlapped with transcription-factor binding-regions, which might protect these sites from methylation. Overall, our findings show enormous potential for the SMRT platform to investigate the precise character of methylome, and significantly enhance our understanding of the function of DNA MTase.

Ni–Al diffusion barrier layer for integrating ferroelectric capacitors on Si

We report on the use of amorphous Ni–Al film (a-Ni–Al) as conductive diffusion barrier layer to integrate La0.5Sr0.5CoO3∕PbZr0.4Ti0.6O3∕La0.5Sr0.5CoO3 capacitors on silicon. Cross-sectional observation by transmission electron microscope reveals clean and sharp interfaces without any discernible interdiffusion/reaction in the sample. The physical properties of the capacitors are vertically characterized as the parameters of memory elements. Excellent ferroelectric properties, e.g., large remnant polarization of ∼22μC∕cm2, small coercive voltage of ∼1.15V, being fatigue-free, good retention characteristic, imply that amorphous Ni–Al is an ideal candidate for diffusion barrier for the high-density ferroelectric random access memories integrated with silicon transistor technology.

Effect of critical thickness on structural and optical properties of InxGa1−xN/GaN multiple quantum wells

InGaN/GaN multiquantum-well (MQW) structures grown by metalorganic chemical-vapor deposition on n-type GaN and capped by p-type GaN were investigated by cross-sectional transmission electron microscopy, double crystal x-ray diffraction, and temperature-dependent photoluminescence. For the sample with strained-layer thicknesses greater than the critical thicknesses, a high density of pure edge type threading dislocations generated from MQW layers and extended to the cap layer was observed. These dislocations result from a relaxation of the strained layers when their thicknesses are beyond the critical thicknesses. Because of indium outdiffusion from the well layers due to the anneal effect of Mg-doped cap layer growth and defects generated from strain relaxation, the PL emission peak was almost depressed by the broad yellow band with an intensity maximum at 2.28 eV. But for the sample with strained-layer thicknesses less than the critical thicknesses, it has no such phenomenon. The measured critical thicknesses are consistent with the calculated values using the model proposed by Fischer, Kuhne, and Richter

Generation and behavior of pure-edge threading misfit dislocations in InxGa1-xN/GaN multiple quantum wells

The relaxation of the misfit strain by the formation of misfit dislocations in InxGa1-xN/GaN multiple quantum wells grown by metal-organic chemical-vapor deposition was investigated by the cross-sectional transmission electron microscopy, double crystal x-ray diffraction, and temperature-dependent photoluminescence. It is found that the misfit dislocations generated from strain relaxation are all pure-edge threading dislocations with burgers vectors of b=1/3 . The misfit dislocations arise from the strain relaxation due to the thickness of strained layer greater than the critical thickness. The relaxation of strained layer was mainly achieved by the formation of dislocations and localization of In, while the dislocations changed their slip planes from {0001} to {10 (1) over bar0}. With the increasing temperature, the efficiency of photoluminescence decrease sharply. It indicates that the relaxation of the misfit strain has a strong effect on optical efficiency of film

Effect of oxygen inclusion on microstructure and thermal stability of copper nitride thin films

Copper oxynitride thin films with a minor oxygen content were prepared on silicon wafers at 100 °C by reactive magnetron sputtering using a gas mixture of nitrogen and oxygen. Addition of oxygen immediately improves the compactness of the deposits, which otherwise comprise ragged Cu 3 N nanocrystallites. With an oxygen content <10.0 at.%, the deposits reveal some sporadic Cu 2 O nanocrystals under transmission electron microscopy, but their x-ray diffraction (XRD) patterns exhibit reflections only from the Cu 3 N phase. The decomposition temperature, at which the sample after prolonged annealing shows Cu reflections on its XRD pattern, can be raised from 300 °C for stoichiometric Cu3N to 360 °C. The decomposition product after annealing at 450 °C is pure copper having an electrical resistivity of 8.94 × 10 �8 ·m at room temperature, which can be taken as a good conductor and stands in strong contrast with the oxynitride matrix with an electrical resistivity of 6.87 × 10 �2 ·m. These results constitute progress in the search of directly writable copper nitride-based materials.

Barrier performance of ultrathin Ni–Ti film for integrating ferroelectric capacitors on Si

Ultrathin amorphous Ni–Ti film is investigated as conductive diffusion barrier layer to integrate La0.5Sr0.5CoO3∕PbZr0.4Ti0.6O3∕La0.5Sr0.5CoO3 (LSCO/PZT/LSCO) capacitors on silicon. X-ray photoelectron spectroscopy results demonstrate that Ni in LSCO∕Ni–Ti∕Si heterostructure is not oxidized after 550°C annealing in oxygen. The structural properties of LSCO∕PZT∕LSCO∕Ni–Ti∕Si are characterized by x-ray diffraction and transmission electron microscopy. It is found that Ni–Ti film is still amorphous and that there are no discernible reactions at the interfaces of the sample. LSCO/PZT/LSCO capacitor, measured at 5V, possesses very good ferroelectric properties, such as low coercive field (∼1.28V), high remnant polarization (∼27.9μC∕cm2), and good fatigue-free characteristic, implying that ultrathin amorphous Ni–Ti film can be used as barrier layer for fabricating high-density ferroelectric random access memories.

Self-assembly of Ag∕SiOx spherules in triangular pattern on strained surface of primary particles

Spontaneous emergence of nanoscaled structures provides a potential alternative to the conventional bottom-up and top-down fabrication techniques. By a two-step thermal evaporation of mixed Ag2O and SiO2 powder onto sapphire substrate, concentric Ag∕SiOx spherules were assembled into a triangular pattern upon cooling on the spherical surface of large particles, which, of generally a few microns in dimension, were primarily grown with the substrate held at 1270K. The occasional presence of pentagon or heptagon defect, as required by the spherical geometry of the primary particle, was also recognized. Strain field in the oxide shell arising from expansion mismatch with the silver core seeds the subsequent nucleation events. The triangular pattern of the secondary nanoparticles can be accounted for with a minimized strain energy in the spherical surface. The self-assembled structure of striking uniformity is promising for the generation of a large family of building blocks for device production.

Nanoindentation and photoreflectance study on polycrystalline ternary Al–C–N thin films

Berkovich hardness and optical gap were determined by nanoindentation and photoreflectance measurement for ternary Al–C–N thin films of various compositions along with structural characterization. The deposits were fabricated by reactive magnetron sputtering of aluminum target onto Si(100) substrates, with the mixture of argon, nitrogen, and methane as precursor. High-resolution transmission electron microscopy and selected-area electron diffraction revealed a polycrystalline structure with rich defects in the film, and x-ray diffraction displays only one reflection at 2θ≈36° from the basal planes. The hardness is over 26GPa for all the deposits, it measures 53.4GPa in the sample Al47C20N33. The optical gap, defined as the photon energy where (1∕R)(dR∕dE) maximizes, R is the photoreflectance, lies within 5.2to5.6eV. These results indicate the wide-gap nature of aluminum carbonitrides, implying diverse potential applications of their thin films as protective coating of an ultraviolet detector, dielectric b...

Orogenic movement mechanism for the formation of symmetrical relief features in copper nitride thin films

Nonequilibrium growth of crystals has revealed an astonishing variety of structures, resulting from the interplay of relevant factors such as atomic mobility, thermal effect, dynamic heterogeneity, etc. In preparing thin films of the thermally unstable copper nitride Cu3N, growth proceeds with simultaneous nitrogen reemission, which both bestows a preferred, Cu-terminated {111} facet on the Cu3N crystallites and limits their size to about 45nm. The authors observed an orogenic movement of Cu3N nanocrystals via glide along the {111} planes, resulting in a hollow, relief morphology primarily of round caps and later of mesoscale rosettes which often demonstrate a fivefold symmetry at the center. The area expansion in the thin film by a few percent is sustained through the rearrangement of nanocrystals into ragged steps and terraces, while the ongoing fast growth prevents the relief features from cracking. The results bear broad significance for the film growth of thermally unstable materials.