A gas-phase reaction mechanism is proposed for the chemical vapor deposition (CVD) of amorphous silicon from silane or disilane at atmospheric pressure. The gas stream in the CVD reactor is populated by silanes, silylenes, and disilenes in a variety of sizes. Silylenes form by the decomposition of silanes, and they rapidly insert into other silanes to form larger silanes. Although silylenes are expected to stick to growth surfaces to which they diffuse, they are too reactive in the gas phase to deliver a large flux onto the growth surface. Larger silylenes (SiH3SiH and larger) also isomerize to form less reactive disilenes, which we propose to be principally responsible for film growth. Film profiles observed in depositions from silane and disilane are presented, and computed film profiles are compared to these observations. Deposition from silane is explained quite well by the mechanism, as are some qualitative features of deposition from disilane.
The definition of the mole in the current SI and the draft definition for the "new SI" are reviewed. Current textbook treatments of the mole are compared to these official definitions. For historical perspective, the treatment of the mole and amount of substance in textbooks before and after those quantities were introduced into the SI in 1971 is reviewed. Textbook definitions have not always matched the official definitions, but they reflect the common usage of chemists. Textbook definitions will likely resemble the official definition more closely if the new SI is adopted, because the draft definition is closer than the current official definition to what is found in many textbooks. The SI base quantity amount of substance, however, will likely continue to pose problems for chemistry educators and to be widely ignored by practitioners of chemistry if it is retained. Official definitions and expert usage of some terms related to the mole (particularly amount of substance) do not always coincide.
Including tales of error along with tales of discovery is desirable in any use of history of science to teach about science. Tales of error, particularly when they involve justly well-regarded historical figures, serve to avoid two pitfalls to which use of historical material in science teaching is otherwise susceptible. Acknowledging the false steps of great scientists avoids putting those scientists on a pedestal and illustrates that there is no automatic or mechanical scientific method. This paper lists five kinds of error with examples of each from the development of chemistry in the 18th and 19th centuries: erroneous theories (such as phlogiston), seeing a new phenomenon everywhere one seeks it (e.g., Lavoisier and the decomposition of water), theories erroneous in detail but nonetheless fruitful (e.g., Dalton's atomic theory), rejection of correct theories (e.g., Avogadro's hypothesis), and incoherent insights (e.g., J. A. R. Newlands' classification of the elements).
Gas‐phase reaction mechanisms are proposed for the chemical vapor deposition (CVD) of silicon dioxide false(SiO2false) from silane or disilane with nitrous oxide at atmospheric pressure. Observed SiO2 growth profiles are presented, and computed profiles are compared to these observations. The deposition of silicon dioxide from silane and excess nitrous oxide is hypothesized to be a chain reaction initiated by the decomposition of N2O . SiH3 attack on N2O , and SiH3O attack on SiH4 , are the propagating reactions. SiH3OH is posited to be the film precursor, which is rapidly oxidized and dehydrogenated on the growth surface. SiH3OH is also posited as an intermediate in the formation of other (non‐depositing) oxidized by‐products. The proposed mechanism accounts for a weak dependence of the peak growth rate on initial silane concentration and a strong dependence on nitrous oxide. The decomposition of Si2H6 is proposed to initiate the deposition of silicon dioxide from disilane in a large excess of nitrous oxide. Rapid reaction of the decomposition product, SiH2 , with N2O suppresses the formation of larger silicon hydrides, generating the oxide film precursor, silanone false(SiH2Ofalse) . Besides the film, oxidized by‐products are also formed from SiH2O . This second mechanism accounts for a strong dependence of the peak growth rate on initial disilane concentration and a weak dependence on nitrous oxide. At lower N2O concentrations, both of the above mechanisms, as well as silicon hydride reactions, participate to a significant extent, resulting in silicon‐rich oxide films, SiOx . Under these conditions, oxidized species containing more than one silicon atom are also suspected of participating in the deposition.
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