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

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

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

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

Damage and strain in epitaxial GexSi1–x films irradiated with Si

The damage and strain induced by irradiation of both relaxed and pseudomorphic GexSi1–x films on Si(100) with 100 keV 28Si ions at room temperature have been studied by MeV 4He channeling spectrometry and x-ray double-crystal diffractometry. The ion energy was chosen to confine the major damage to the films. The results are compared with experiments for room temprature Si irradiation of Si(100) and Ge(100). The maximum relative damage created in low-Ge content films studied here (x=10%, 13%, 15%, 20%, and 22%) is considerably higher than the values obtained by interpolating between the results for relative damage in Si-irradiated single crystal Si and Ge. This, together with other facts, indicates that a relatively small fraction of Ge in Si has a significant stabilizing effect on the retained damage generated by room-temperature irradiation with Si ions. The damage induced by irradiation produces positive perpendicular strain in GexSi1–x, which superimposes on the intrinsic positive perpendicular strain of the pseudomorphic or partially relaxed films. In all of the cases studied here, the induced maximum perpendicular strain and the maximum relative damage initially increase slowly with the dose, but start to rise at an accelerated rate above a threshold value of ~0.15% and 15%, respectively, until the samples are amorphized. The pre-existing pseudomorphic strain in the GexSi1–x film does not significantly influence the maximum relative damage created by Si ion irradiation for all doses and x values. The relationship between the induced maximum perpendicular strain and the maximum relative damage differs from that found in bulk Si(100) and Ge(100)