Hydrogenation plasma

Veprek S and Sarott F A 1982 Electron-impact-induced anisotropic etching of silicon by hydrogen Plasma Chem. Plasma Proc. 2 233-46... 

Newer coal-based methods of acetylene manufacture under development include the AVCO process, based on the reaction of coal in a hydrogen plasma. Finely divided coal is passed through a hydrogen plasma arc generating temperature gradients of up to 15,000 K. About 67% of the coal is consumed, yielding char and acetylene in concentrations up to 16%. An energy requirement of 9.5 kW h/kg acetylene has been reported (33). 

Hoechst WHP Process. The Hoechst WLP process uses an electric arc-heated hydrogen plasma at 3500—4000 K it was developed to industrial scale by Farbwerke Hoechst AG (8). Naphtha, or other Hquid hydrocarbon, is injected axially into the hot plasma and 60% of the feedstock is converted to acetylene, ethylene, hydrogen, soot, and other by-products in a residence time of 2—3 milliseconds Additional ethylene may be produced by a secondary injection of naphtha (Table 7, Case A), or by means of radial injection of the naphtha feed (Case B). The oil quenching also removes soot. 

Hydrogen plasma process using naphtha. Case A secondary injection of naphtha Case B radial injection of the naphtha feed. Hydrogen plasma process using cmde oil. 

Acetylene traditionally has been made from coal (coke) via the calcium carbide process. However, laboratory and bench-scale experiments have demonstrated the technical feasibiUty of producing the acetylene by the direct pyrolysis of coal. Researchers in Great Britain (24,28), India (25), and Japan (27) reported appreciable yields of acetylene from the pyrolysis of coal in a hydrogen-enhanced argon plasma. In subsequent work (29), it was shown that the yields could be dramatically increased through the use of a pure hydrogen plasma. 

In the ASTER system a data series is measured for an argon and a hydrogen plasma running at 13.56 MHz, in which the power (5-30 W) and pressure (5-50 Pa) are varied [265,295], The probe tip is positioned exactly between powered and grounded electrode, at the center of the discharge. 

FIG. 48. Material properties as functions of ma at 250°C (a) refractive index nvfV- and (b) microstructure parameter R. The closed circles represent pure silane and silane-hydrogen plasmas. The open circles refer to silane-argon plasmas. Lines are to guide the eye. (Adapted from E. A. G. Hamers. Ph.D. Thesis, Universiteit Utrecht. Utrecht, the Netherlands. 1998.)... 

The plasma potential is about 25 V (Figure 63a). This value of the plasma potential is typical for the silane plasmas in the asymmetric capacitively coupled RF reactors as used in the ASTER deposition system, and is also commonly found in argon or hydrogen plasmas [170, 280, 327]. From the considerable decrease of the dc self-bias with increasing frequency (Figure 63a) it is inferred that the... 

It was argued by Saito et al. [509] that a hydrogen plasma treatment as used in the LBL technique causes chemical transport the a-Si H film deposited on the cathode prior to the hydrogen treatment is etched and transferred to the anode. Hence, in the hydrogen plasma silane-related molecules and radicals are present. In fact, material is deposited on the anode under the same conditions as in the case of high dilution of silane with hydrogen. 

An argon-hydrogen plasma is created in a dc thermal arc (cascaded arc) operated at high pressure 0.5 bar) [556, 559. 560] (the cascaded arc is also employed in IR ellipsometry, providing a well-defined source of intense IR radiation see Section 1.5.4 [343]). As the deposition chamber is at much lower pressure (0.1-0.3 mbar), a plasma jet is created, expanding into the deposition chamber. Near the plasma source silane is injected, and the active plasma species dissociate the silane into radicals and ions. These species can deposit on the substrate, which is positioned further downstream. 

Fig. 2. Spreading resistance profiles from (a) Au-diffused n-type Si, (b) same sample after hydrogen plasma-exposure for lh at 200°C and (c) another position of the sample in (b). The bar represents the range of initial resistance values of the Si prior to Au diffusion (Mogro-Campero et al., 1985). 

Fig. 3. Capacitance transient spectra from Au-diffused p-type Si showing passivation of the Au donor level (Ev + 0.35eV) after exposure to a hydrogen plasma.

It is known that hydrogen incorporated into Si subsequently exposed to ionizing radiation inhibits the formation of induced secondary point defect (Pearton and Tavendale, 1982a). For example, in both Si and Ge a number of electron or y irradiation induced defect states appear to be vacancy-related, and exposure of the Si or Ge to a hydrogen plasma (or implantation of hydrogen into the sample) prior to irradiation induces a degree of... 

Fio. 1. Schematic diagram of a remote hydrogen plasma system. 

Additional considerations in the design and operation of a remote hydrogen plasma system include the following (1) convective heat transfer between the downstream gas and the specimen can introduce a significant difference between the heater temperature and the specimen surface temperature (this effect is generally not as severe as for direct immersion in... 

Essential to the identification of H-induced defects in silicon was the use of a remote hydrogen plasma system as described in Section 1.2. The alternative of direct immersion in a plasma introduces charged-particle bombardment and possible photochemical effects that can obscure the purely chemical consequences of hydrogen migrating into silicon. While the evidence presented below strongly argues for the existence of H-induced defects, many issues remain to be resolved. 

In addition to the generation of platelets, hydrogenation of silicon also induces electronic deep levels in the band gap. As in the case of platelet formation, these defects are considered to be unrelated to either plasma or radiation damage because they can be introduced with a remote hydrogen plasma. Comparison of depth distributions and annealing kinetics of the platelets and gap states has been used to a limited extent to probe the relationship among these manifestations of H-induced defects. 

The introduction of electronic deep levels is demonstrated in Fig. 9 with low-temperature photoluminescence spectra for n-type (P doped, 8 Cl cm) silicon before (control) and after hydrogenation (Johnson et al., 1987a). The spectrum for the control sample is dominated by luminescence peaks that arise from the well-documented annihilation of donor-bound excitons (Dean et al., 1967). After hydrogenation with a remote hydrogen plasma, the spectrum contains several new transitions with the most prominent peaks at approximately 0.95, 0.98, and 1.03 eV. These transitions identify... 

In the study by Johnson et al. (1986) it was shown by Hall effect measurements that the sheet carrier density was decreased and the mobility was increased for a thin w-type layer following exposure to a hydrogen plasma at 150°C. To explain the mobility increase it was argued that donor-H complexes were formed and that the concentration of ionized scattering centers was thereby decreased. On the basis of semiempirical calculations, a structural model was suggested for the donor-H complex in... 

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