Rj René Severens

Plasma chemistry aspects of a-Si:H deposition using an expanding thermal plasma

The plasma chemistry of an argon/hydrogen expanding thermal arc plasma in interaction with silane injected downstream is analyzed using mass spectrometry. The dissociation mechanism and the consumption of silane are related to the ion and atomic hydrogen fluence emanating from the arc source. It is argued that as a function of hydrogen admixture in the arc, which has a profound decreasing effect on the ion-electron fluence emanating from the arc source, the dissociation mechanism of silane shifts from ion-electron induced dissociation towards atomic hydrogen induced dissociation. The latter case, the hydrogen abstraction of silane, leads to a dominance of the silyl (SiH3) radical whereas the ion-electron induced dissociation mechanism leads to SiHx (x<3) radicals. In the pure argon case, the consumption of silane is high and approximately two silane molecules are consumed per argon ion-electron pair. It is shown that this is caused by consecutive reactions of radicals SiHx (x<3) with silane. Almost indepe...

Hydrogenated amorphous silicon deposited at very high growth rates by an expanding Ar–H2–SiH4 plasma

The properties of hydrogenated amorphous silicon (a-Si:H) deposited at very high growth rates (6–80 nm/s) by means of a remote Ar–H2–SiH4 plasma have been investigated as a function of the H2 flow in the Ar–H2 operated plasma source. Both the structural and optoelectronic properties of the films improve with increasing H2 flow, and a-Si:H suitable for the application in solar cells has been obtained at deposition rates of 10 nm/s for high H2 flows and a substrate temperature of 400 °C. The “optimized” material has a hole drift mobility which is about a factor of 10 higher than for standard a-Si:H. The electron drift mobility, however, is slightly lower than for standard a-Si:H. Furthermore, preliminary results on solar cells with intrinsic a-Si:H deposited at 7 nm/s are presented. Relating the film properties to the SiH4 dissociation reactions reveals that optimum film quality is obtained for conditions where H from the plasma source governs SiH4 dissociation and where SiH3 contributes dominantly to film ...

Surface reaction probability during fast deposition of hydrogenated amorphous silicon with a remote silane plasma

The surface reaction probability β in a remote Ar–H2–SiH4 plasma used for high growth rate deposition of hydrogenated amorphous silicon (a-Si:H) has been investigated by a technique proposed by D. A. Doughty et al. [J. Appl. Phys. 67, 6220 (1990)]. Reactive species from the plasma are trapped in a well, created by two substrates with a small slit in the upper substrate. The distribution of amount of film deposited on both substrates yields information on the compound value of the surface reaction probability, which depends on the species entering the well. The surface reaction probability decreases from a value within the range of 0.45–0.50 in a highly dissociated plasma to 0.33±0.05 in a plasma with ∼12% SiH4 depletion. This corresponds to a shift from a plasma with a significant production of silane radicals with a high (surface) reactivity (SiHx,x<3) to a plasma where SiH3 is dominant. This has also been corroborated by Monte Carlo simulations. The decrease in surface reaction probability is in line wi...

Characterization of plasma beam deposited amorphous hydrogenated silicon

Fourier transform infrared spectrometry, visual transmission spectroscopy, and in situ ellipsometry have been performed on plasma beam deposited (PBD) amorphous hydrogenated silicon layers. From these measurements refractive index at infrared wavelengths and at 632.8 nm, the optical band gap and the hydrogen content of the layers have been determined. The hydrogen concentration of the layers varies between ∼9 and 25 at. %. It was found that the refractive index decreases more with hydrogen concentration in the layer than predicted by theoretical calculations assuming tetrahedral structures. The band gap of the material remains constant at ∼1.72 eV for the range of hydrogen contents measured. The resonance frequency of the SiH stretching mode (around 2000 cm−1) increases with increased hydrogen content. This is additional evidence to support the assumption that clustered SiH (SiH on voids) does not have its stretching mode near the 2100 cm−1 SiH2 peak. From the results presented it is concluded that PBD la...

Temperature and growth-rate effects on the hydrogen incorporation in a-Si:H

Hydrogen incorporation in amorphous hydrogenated silicon produced by the expanding thermal plasma is investigated as a function of substrate temperature at three different deposition rates of 0.3, 3 and 11 nm/s. The increase of the refractive index with increasing substrate temperature is attributed to decreasing hydrogen concentration. The latter result is explained by a model which assumes a thermally activated hydrogen cross-linking step immediately after the chemisorption of a silyl radical. The activation energy for this process is about 150 meV. For growth rates larger than 1 nm/s the hydrogen content is significantly growth-rate dependent.

Deposition of a-Si:H and a-C:H using an expanding thermal arc plasma

This paper deals with the deposition of a-C:H and a-Si:H using the expanding thermal arc technique. The method is compared with other deposition techniques. The basics of the technique are explained and recent results on the deposition of high-quality a-C:H and a-Si:H are discussed. It is shown that high rates, for a-Si:H and for a-C:H, are possible without loss of quality.

Dissociative recombination in cascaded arc generated Ar–N2 and N2 expanding plasma

The expanding plasma obtained from a cascaded arc thermal source is analyzed with double probe, mass spectrometric, and Faraday cup measurements. In the argon–nitrogen mixtures a decrease in ion fluence is observed, contrary to pure argon plasmas in which recombination is insignificant. The recombination in argon–nitrogen plasmas is caused by charge exchange between atomic ions and N2 molecules followed by dissociative recombination. Hence, these processes account for the enhanced axial decay of the plasma density and also for the change in the ion mass spectra of the ion beam extracted from the expanding plasma. The total ion beam current density is also governed by charge exchange followed by dissociative recombination and is thus dependent on the recirculating neutral molecules.

Hydrogen atom cleaning of archeological artefacts

For the development of H+ and H0 beams a new method has been developed based on the expansion of a cascade arc plasma. A partial aim was to develop an intense beam of atomic hydrogen. The result was a 100 A equivalent hydrogen atom beam with an energy conversion efficiency of typically 30–40%. The resulting hydrogen plasma has also been used to do preliminary experiments on restoration treatment of archeological artefacts according to the method of Vepřek et al. The present high density atom rich plasma beam with < 0.4 eV energies and temperatures proved to be effective in the treatment of these artefacts. The treated artefacts showed good erosion resistance compared to untreated as well as conventionally treated samples, notwithstanding the short treatment time of 20 minutes at a temperature of 400°C and the provisional character of the trial experiment.

Carbon Nitride thin films prepared by a capacitively coupled RF plasma jet

Abstract Carbon nitride thin films have been downstream deposited from a nitrogen plasma beam sustained by a capacitively coupled discharge generated between a RF powered carbon electrode and a grounded carbon nozzle. The spectral emission of the plasma jet strongly exhibits the CN radical emission indicating that the deposition takes place via a mechanism involving the CN radical. The deposition process is enhanced by DC biasing the powered electrode. The films have been investigated by X-ray diffraction, infrared absorption spectroscopy and X-ray photoelectron spectroscopy. The results show that the films are amorphous and contain in a large extent carbon nitrogen bonds.

Plasma and surface chemistry effects during high rate deposition of hydrogenated amorphous silicon

The plasma chemistry of an expanding thermal plasma in argon/hydrogen mixtures in interaction with silane is discussed. Based on Langmuir probe measurements and (appearance potential) mass spectrometry, it is argued that under optimal conditions in terms of the a-Si:H deposited the fluxes reaching the growing film mainly consist of and atomic hydrogen. The surface chemistry, hydrogen incorporation, growth rate and dependence on substrate temperature are discussed and the results obtained are compared with growth models as proposed by Matsuda, Gallagher and Perrin. It is argued that although atomic hydrogen plays no role in the modification of the bulk properties, atomic hydrogen is probably responsible for the creation of dangling bond sites by hydrogen abstraction. Possible consequences for the substrate temperature dependence of the growth rate are discussed.