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Fundamental concepts
Published in W. John Rankin, Chemical Thermodynamics, 2019
If a system is not in its most stable state and there are no flows of any kind, it is said to be metastable. For example, diamond is a metastable form of carbon at ambient* temperature and pressure. It can be converted to graphite, the stable form, but only after overcoming an activation energy barrier. Metastability occurs as a result of a local minimum in a system’s energy. A more familiar example is super-heated water, that is, water heated above 100°C at atmospheric pressure but without boiling. Such water is in an unstable state, and something which causes water vapour bubbles to nucleate within the liquid will result in the water starting to boil, even in the absence of additional heat. This is sometimes encountered when water is heated in a container in a microwave oven. When the vessel containing the water is removed or the water is stirred nucleation of water vapour bubbles may occur, and the water begins to boil, sometimes with catastrophic results.
Base Doping Profile Engineering for High-Performance SiGe PNP Heterojunction Bipolar Transistors
Published in Choi Jung Han, Iniewski Krzysztof, High-Speed and Lower Power Technologies, 2018
Guangrui (Maggie) Xia, Yiheng Lin
Unlike the Si-Ge system, silicon and carbon are only miscible to a very small degree under equilibrium conditions [23]. According to the Si-C phase diagram, stoichiometric SiC (silicon carbide) is the only stable compound [47]. Any alloy with a smaller C concentration is thermodynamically metastable. Such alloy layers can be obtained by kinetically dominated growth methods, such as molecular beam epitaxy (MBE) [48], solid-phase epitaxy [49], or CVD [50]. All of these methods generally work under far from thermodynamic equilibrium conditions, allowing a kinetic stabilization of metastable phases. Although the bulk solubility of C in silicon is small, 3 × 1017 cm−3, at the melting point of silicon [51], epitaxial layers with more than 1%, 5 × 1020 cm−3, C can be achieved. However, during an epitaxial growth step, it is not the equilibrium bulk solubility that is important but the “surface solubility” [52].
Carbon Doping of SiGe
Published in John D. Cressler, SiGe and Si Strained-Layer Epitaxy for Silicon Heterostructure Devices, 2017
Epitaxial growth of C-containing Si1–yCy or Si1–x–yGexCy alloys is more complex than SiGe growth. Several additional problems have to be addressed [5,6]: Unlike to the Si/Ge system, silicon and carbon are not miscible. According to the binary Si—C phase diagram, stoichiometric SiC (silicon carbide) is the only stable compound. Any alloy with a smaller C concentration is thermodynamically metastable. Such alloy layers can be achieved by kinetically dominated growth methods, like molecular beam epitaxy (MBE) [5,6], various kinds of chemical vapor deposition (CVD) at relatively low temperatures [7], or solid-phase epitaxy (SPE). The SPE approach is based on recrystallization of amorphous mixtures of Si/Ge/C obtained mainly by ion implantation with well-defined temperature profiles [8,9]. All of these methods generally work under far from thermodynamic equilibrium conditions, allowing a kinetic stabilization of metastable phases. Although the bulk solubility of carbon in silicon is small (3 × 1017 atoms/cm3 at the melting point of Si [10]), epitaxial layers with more than 1% C can be fabricated. Once deposited as a random alloy at low temperatures (500–700°C), the Si1–yCy appears to be able to withstand anneals up to significantly higher temperatures (up to 900°C) [11,12].
Two-dimensional simulation and experiments of helium discharge between parallel-plate electrodes in short gaps at atmospheric pressure
Published in Journal of Nuclear Science and Technology, 2020
Qi You, Yan Zhou, Xingnan Liu, Ni Mo, Zhengang Shi
In gas mixture, for example, He and Xe, penning ionization is defined as ionization of xenon through collision of metastable He [21], however, in the case of pure helium, ionization of metastable helium (an atom or a molecule at its first excited level, with an electron excited from its 1S orbit to the 2S orbit) caused by collision of another metastable atom or molecule is defined as penning mechanism [6]. In our study, it is found that in pure helium discharge at atmospheric pressure, penning ionization is thought to be an important source of electrons. Figure 8(a–d) presents fractions of ground-state ionization and penning ionization through the discharge process with gap length of 0.25 mm at 251V, room temperature and pressure. In early stages, reaction rate of ground-state ionization is much larger than that of penning ionization due to low concentration of metastable atoms at that time. Later, because density of metastable atoms grows at a much faster speed than that of electrons (the excitation reaction coefficient is about two orders of magnitude larger than the ground state ionization coefficient), fraction of penning ionization will increase rapidly and become comparable with that of ground-state ionization when the breakdown is about to take place, especially near the anode. It is also observed that the penning fraction peak moves towards the anode and this is mainly because the metastable helium molecules significantly outnumber metastable atoms at this time and these molecules are very rich at the anode.
Classical trajectory calculations for state-resolved Penning ionisation reactions of polycyclic aromatic hydrocarbon C10H8 in collision with He*(23S)
Published in Molecular Physics, 2019
Penning ionisation A* + M → A + M+ + e− involves the metastable atom A* and the target molecule M in the entrance channel, and produces the ground state atom A and the molecular/atomic ion M+ with the outgoing electron e− in the exit channel. Theory of Penning ionisation has been presented by Nakamura [1], Miller and co-workers [2–4] based on the earlier discrete/continuum transition theory [5,6]. Penning ionisation occurs as autoionising transition from the super-excited discrete potential V*(R) to the ion and continuum potential V+(R) at a certain distance R. The entrance potential V is represented as the complex potential (optical potential), where the real part V*(R) and imaginary part Γ(R) correspond to the interaction energy and the ionisation width, respectively [5,6], The formalisms for photoelectron [7] and electron scattering phenomena [8] developed by Cederbaum and Domcke also relate with the ionisation width of Penning ionisation.
Fluorescence-lifetime-limited trapping of Rydberg helium atoms on a chip
Published in Molecular Physics, 2019
V. Zhelyazkova, M. Žeško*, H. Schmutz, J. A. Agner, F. Merkt
The He* beam is skimmed twice before it enters the photoexcitation region, with the first (second) skimmer having a diameter of 19 mm (3 mm). The short nozzle-opening times and the strong cooling taking place in the supersonic expansion ensure that the gas pulses remain short during their propagation from the nozzle to the detection region. Measurements of the time-of-flight (TOF) distribution of the gas pulse indicate that the half width at half maximum of the gas pulse over the surface electrode decelerator is about 100 μs. This represents the first important advantage over our previous Rydberg-Stark deceleration and trapping experiments [13, 14]: The decelerated atoms are rapidly overtaken by all atoms in the trailing part of the pulse. Consequently, the trapped Rydberg atoms are not exposed to collisions with ground-state and metastable He atoms.