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Domains play an integral role in linguistic theories. This paper combines locality domains with current models of the computational complexity of phonology. The first result is that if a specific formalism – strictly piecewise grammars – is supplemented with a mechanism to enforce first-order definable domain restrictions, its power increases so much that it subsumes almost the full hierarchy of subregular languages. However, if domain restrictions are based on linguistically natural intervals, we instead obtain an empirically more adequate model. On the one hand, this model subsumes only those subregular classes that have been argued to be relevant for phonotactic generalisations. On the other hand, it excludes unnatural generalisations that involve counting or elaborate conditionals. It is also shown that strictly piecewise grammars with interval-based domains are theoretically learnable, unlike those with arbitrary, first-order domains.
The present study is dealing with the basic physics for a novel way to generate a free-formed ceramic body, not like common layer by layer, but directly by Selective Volume Sintering (SVS) in a compact block of ceramic powder. To penetrate with laser light into the volume of a ceramic powder compact it is necessary to investigate the light scattering properties of ceramic powders. Compared with polymers and metals, ceramic materials are unique as they offer a wide optical window of transparency. The optical window typically ranges from below 0.3 up to 5 µm wave length. In the present study thin layers of quartz glass (SiO2) particles have been prepared. As a function of layer thickness and the particle size, transmission and reflection spectra in a wave length range between 0.5 and 2.5 µm have been recorded. Depending on the respective particle size and by choosing a proper relation between particle size and wave length of the incident laser radiation, it is found that light can penetrate a powder compact up to a depth of a few millimeters. With an adjustment of the light absorption properties of the compact the initiation of sintering in the volume of the compact is possible.
Low energy plasma enhanced chemical vapour deposition (LEPECVD) is a deposition technique developed for the epitaxy of Si and SiGe at ultra-high deposition rates. Due to a high current plasma discharge composed of low energy particles, a high plasma enhancement can be obtained without any accompanying plasma induced damage of the wafer surface. The most important application of LEPECVD so far is for compositionally graded relaxed SiGe buffer layers. Such relaxed buffer layers are demonstrated with end composition up to pure Ge and with a growth time below 1 hour. A p-type hetero-MOSFET formed in a SiGe channel compressively strained to a Si0.5Ge0.5 relaxed buffer layer, is demonstrated as one example where the high growth rates of LEPECVD allows the synthesis of devices which cannot be produced with an acceptable throughput with conventional deposition methods. The room temperature effective hole mobility of 760 cm2/Vs obtained on such devices demonstrates a high structural and electrical quality of the LEPECVD material.
We discuss a new method for plasma enhanced chemical vapor deposition, applied to the epitaxial growth of Si and of Si-Ge heterostructures. Growth rates up to 5 nm/s become possible at substrate temperatures below 600°C, by utilizing very intense but low energy plasmas to crack the reactive gases, SiH4 and GeH4, and to speed up the surface kinetics. The method is applied to the synthesis of step-graded Si-Ge buffer layers, exhibiting the well known cross-hatched surface morphology.
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