Dennis Clinton Green

Glow discharge preparation of amorphous hydrogenated silicon from higher silanes

Amorphous hydrogenated silicon has been deposited by plasma decomposition of Si2H6 and Si3H8. A major feature of the process is a deposition rate enhancement of over a factor of 20 compared to monosilane. The resulting films are compositionally similar to monosilane‐produced intrinsic a‐Si(H), but films deposited at 300 °C substrate temperature show greater photoconductivity. On the basis of our deposition experiments and the known thermolysis chemistry of the silanes, a conjectural model for the deposition process is presented.

Mesomorphic Substances Containing Some Group IV Elements

Abstract A unique series of organometallic substances has been prepared from 4′-methoxy-4-aminobiphenyl and para-substituted benzaldehydes. Among the substituents investigated are trimethylsilyl, trimethylgermyl, trimethyltin, trimethylsiloxy, and pentamethyldisiloxanyl. The compounds were examined for mesomorphic behavior, and the heats and entropies of the mesophase transitions were determined by differential scanning calorimetry. A majority of these compounds exhibit either a smectic or a nematic mesophase, although analogous organometallic anils derived from p-aminophenyl acetate or p-aminophenyl benzoate are non-mesomorphic. The results, taken together with data obtained from carbon-containing “isologs,” indicate that the bulkiness associated with the organometallic substituent does not preclude liquid crystal formation in long molecules, especially when the electronic properties of the end groups, such as high polarizability and mesomeric interactions, are beneficial.

A voltammetric study of several oxygen, sulfur and selenium heterocyclic carbonium ions in acetonitrile

Abstract Cyclic voltammetric data are reported on seven new oxygen, sulfur and selenium heterocyclic carbonium ions. Each compound undergoes electrochemical reduction to form a dimer. The reaction mechanisms are discussed. The compounds with selenoethoxy substituents formed heterofulvalenes at room temperature.

Low Melting Nematic Derivatives of Phenyl 3-Methyl-Benzoyloxybenzoate

Abstract During recents months, several research groups have focussed their attention on the synthesis of novel derivatives of phenyl 4-benzoyloxybenzoate (1) in an effort to prepare stable nematic liquid crystals for physical, chemical and technological applications. Investigations by Steinstraesser1 and by Young, Haller and Green2 have uncovered stable nematogens of type 2 and 3, respectively, which exhibit wide nematic ranges and melting points in the vicinity of 90°C. Van Meter and Klanderman,3 through the use of laterally-placed chlorine substituents, have prepared nematic derivatives of 1 which melt as low as 39°C.

Deposition and doping of a‐Si:H from Si2H6 plasmas

Compared to SiH4, the plasma deposition of amorphous hydrogenated silicon from Si2H6 results in compositionally similar films, deposited at rates at least an order of magnitude higher. The films also display larger dark and photoconductivties, a result related directly to higher Ef in the intrinsic Si2H6‐prepared material. The effect is structural, not impurity‐dominated. Dopant incorporation is also found to be strongly influenced by the silicon source, as is the doping efficiency. For a given gas phase concentration of n‐type dopant (PH3), the distribution coefficient is Ceff<1 for Si2H6 plasmas, compared to Ceff≳1 for depositions from SiH4, yet film electrical properties are comparable. On the p‐type side, much smaller differences are observed with B2H6 doping of the two sources. Finally, a‐Si:H plasma deposition chemistry is examined within the context of a neutral radical model and hydrogen etching experiments.