Lithographic band gap tuning in photonic band gap crystals

We describe the lithographic control over the spectral response of three-dimensional photonic crystals. By precise microfabrication of the geometry using a reproducible and reliable procedure consisting of electron beam lithography followed by dry etching, we have shifted the conduction band of crystals within the near-infrared. Such microfabrication has enabled us to reproducibly define photonic crystals with lattice parameters ranging from 650 to 730 nm. In GaAs semiconductor wafers, these can serve as high-reflectivity (> 95%) mirrors. Here, we show the procedure used to generate these photonic crystals and describe the geometry dependence of their spectral response

Ultra-high External Efficiency From Surface Textured Thin-film LEDs

Dire need for high external quantum efficiency (η_(ext)) visible light-emitting-diodes (LED's) is clearly seen as the flat panel display technology is rapidly evolves. However, there is an enormous gap between the theoretical efficiency of LED's and their actual efficiency. It has been known that good quality III-V double heterostructures can have over 90% internal quantum yields (η) for direct band-gap compounds and over 99% for AlGaAs/GaAs/AlGaAs, as we have demonstrated recently. On the other hand, run-of-the-mill commercial LED's are usually only a few percent efficient (externally)

Wet oxidation of GeSi at (700)C

About 500-nm-thick films of Ge0.36Si0.64 and Ge0.28Si0.72 grown epitaxially on (100)Si have been oxidized at 700-degrees-C in wet ambient. A uniform GexSi1-xO2 oxide layer forms with a smooth interface between it and the unoxidized GexSi1-x layer below. The composition and structure of that layer remains unchanged as monitored by backscattering spectrometry or cross-sectional transmission electronic microscopy. The oxide of both samples grows as square root of oxidation duration. The parabolic rate constant increases with the Ge content and is larger than that for wet oxidation of pure Si at the same temperature. The absence of a regime of linear growth at this relatively low temperature indicates a much enhanced linear rate constant

Structural characterization of Si(m)Ge(n) strained layer superlattices

SimGen strained layer superlattice (SLS) structures were grown by molecular beam epitaxy on GexSi1-x buffer layers on Si substrates to determine the effects of buffer layer composition, SLS thickness ratio, and superlattice periodicity, on the overall quality of these structures. X-ray diffraction methods were used to determine how closely actual periodicities and compositions met targeted values, and to evaluate the quality of these samples. In most instances the as-grown structures matched the targeted values to within 10%, though in some instances deviations of 20-25% in either the period or composition were observed. The quality of the SLS structures was greatly dependent on the composition of the buffer layer on which it was grown. SimGen SLS structures grown on Si- and Ge-rich buffer layers were of much higher quality than SimGem SLSs grown on Ge0.50Si0.50 layers, but the x-ray rocking curves of the SimGen samples indicated that they were far from perfect and contained moderate levels of defects. These results were confirmed by cross sectional transmission electron microscopy, which showed that the SimGem structures contained significant numbers of dislocations and that the layers were nonuniform in thickness and wavy in appearance. SimGen structures, however, displayed fewer defects but some dislocations and nonparallelism of layers were still observed

Superconducting niobium cavities, a case for the film technology

Evidence is presented for niobium film cavities performing as well as niobium bulk cavities, at variance with a widespread belief that their much smaller grain size should be a fundamental limitation preventing high quality factors to be maintained over a wide range of accelerating fields. By comparing the relative merits of the bulk and film technologies, a strong case is presented in favour of the latter

Ultra-high External Efficiency From Surface Textured Thin-film LEDs

Dire need for high external quantum efficiency (η_(ext)) visible light-emitting-diodes (LED's) is clearly seen as the flat panel display technology is rapidly evolves. However, there is an enormous gap between the theoretical efficiency of LED's and their actual efficiency. It has been known that good quality III-V double heterostructures can have over 90% internal quantum yields (η) for direct band-gap compounds and over 99% for AlGaAs/GaAs/AlGaAs, as we have demonstrated recently. On the other hand, run-of-the-mill commercial LED's are usually only a few percent efficient (externally)

Observation of inverse Compton emission from a long gamma-ray burst

Long-duration gamma-ray bursts (GRBs) originate from ultra-relativistic jets launched from the collapsing cores of dying massive stars. They are characterized by an initial phase of bright and highly variable radiation in the kiloelectron volt-to-mega electronvoltband, which is probably produced within the jet and lasts from milliseconds to minutes, known as the prompt emission(1,2). Subsequently, the interaction of the jet with the surrounding medium generates shock waves that are responsible for the afterglow emission, which lasts from days to months and occurs over a broad energy range from the radio to the gigaelectronvolt bands(1-6). The afterglow emission is generally well explained as synchrotron radiation emitted by electrons accelerated by the external shock(7-9). Recently, intense long-lasting emission between 0.2 and 1 teraelectronvolts was observed from GRB 190114C(10,11). Here we report multifrequency observations of GRB 190114C, and study the evolution in time of the GRB emission across 17 orders of magnitude in energy, from 5 x 10(-6) to 10(12) electronvolts. We find that the broadband spectral energy distribution is double-peaked, with the teraelectronvolt emission constituting a distinct spectral component with power comparable to the synchrotron component. This component is associated with the afterglow and is satisfactorily explained by inverse Compton up-scattering of synchrotron photons by high-energy electrons. We find that the conditions required to account for the observed teraelectronvolt component are typical for GRBs, supporting the possibility that inverse Compton emission is commonly produced in GRBs

Instability of a GexSi1−xO2 film on a GexSi1−x layer

The stability of an amorphous GexSi1−xO2 in contact with an epitaxial (100)GexSi1−x layer obtained by partially oxidizing an epitaxial GexSi1−x layer on a (100)Si substrate in a wet ambient at 700 °C is investigated for x=0.28 and 0.36 upon annealing in vacuum at 900 °C for 3 h, aging in air at room temperature for 5 months, and immersion in water. After annealing at 900 °C, the oxide remains amorphous and the amount of GeO2 in the oxide stays constant, but some small crystalline precipitates with a lattice constant similar to that of the underlying GeSi layer emerge in the oxide very near the interface for both x. Similar precipitates are also observed after aging for both x. The appearance of these precipitates can be explained by the thermodynamic instability of GexSi1−xO2 in contact with GexSi1−x. In water at RT, 90% of GeO2 in the oxide is dissolved for x=0.36, while the oxide remains conserved for x=0.28