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IUPAC Prize for Young Chemists - 2001
Honorable Mention

 

  Erwin Kessels receives one of four Honorable Mention awards associated with the IUPAC Prize for Young Chemists, for his Ph.D. thesis work entitled "Remote Plasma Deposition of Hydrogenated Amorphous Silicon: Plasma Processes, Film Growth, and Material Properties"

Current address (at the time of application)

Colorado State University
Department of Chemistry
Fort Collins, CO 80524-1872, USA

Tel: +1 970-491-2670
Fax: +1 970-491-1801
E-mail: [email protected]

Academic degrees

  • Ph.D. Applied Physics, Eindhoven Univ. of Technology, September 2000.
  • Ingenieurs-diploma' Applied Physics (equivalent to M.S.), Eindhoven Univ. Of Technology, August 1996.

Ph.D. Thesis

Title Remote Plasma Deposition of Hydrogenated Amorphous Silicon: Plasma Processes, Film Growth, and Material Properties
Adviser Prof. M.C.M. van de Sanden
Thesis Committee D.C. Schram, Dept. of Applied Physics, Eindhoven Univ. of Technology (NL); W.F. van der Weg, Dept. of Physics and Astronomy, Utrecht Univ. (NL);. E.S. Aydil, Dept. of Chemical Engineering, Univ. of California Santa Barbara (U.S.); N.J. Lopes Cardozo, FOM Institute for Plasma Physics Rijnhuizen (NL); G.J.M. Meijer, FOM Institute for Plasma Physics Rijnhuizen (NL); L.J.F. Hermans, Dept. of Physics, Leiden Univ. (NL); W.J.M. de Jonge, Dept. of Applied Physics, Eindhoven Univ. of Technology (NL); Niemantsverdriet, Dept. of Chemical Engineering, Eindhoven Univ. of Technology (NL).

Essay

Electricity from solar cells is an important renewable energy source to fulfill the worldwide growing energy demands while preserving a healthy environment for future generations. Large-scale introduction of solar cells will, however, require a considerable number of technological breakthroughs, especially to reduce the cost and to improve the efficiency of the cells. Among the different types of cells, hydrogenated amorphous silicon (a-Si:H) solar cells are one of the most promising candidates to enable this large-scale implementation. In a-Si:H solar cells, electricity is generated by the absorption of light in an alloy film of Si and H, which can be very thin (~500 nm) due to the high absorption caused by the film's non-crystalline nature. This makes it possible to apply the cells on flexible foils in a continuous roll-to-roll production process.

One of the main issues in this production process is the increase of the deposition rate of the a-Si:H, which is usually deposited by low-pressure plasma activation of silane (SiH4) into reactive radicals and ions. Although a considerable part of the thesis work has been devoted to an increase of the deposition rate of device quality a-Si:H by a factor of 100 using a newly-developed technique (the so-called "Expanding Thermal Plasma"), the fundamental understanding of the deposition process is at least as important. Insight into the plasma processes and the reactions taking place at the film surface during growth and their relation to the film quality is essential for full optimization of the a-Si:H production. Because of the process's complexity, this thesis work focuses on using a broad and integral approach to cover all the different aspects of the deposition process of a-Si:H. Moreover, it yields very interesting science in which different disciplines of research come together.

The first important aspect studied is the plasma chemistry during deposition. Insight into the reactions taking place during SiH4 dissociation and the species subsequently created is crucial for the understanding of how the SiH4 is converted into a film and how the plasma processes can be controlled by the different plasma settings. These studies have been performed by investigations of the densities of different silane radicals (SiHx, x < or = 3) and cations SinHm+ in the aforementioned Ar-H2-SiH4 plasma. In this remote plasma, SiH4 dissociation and deposition are spatially separated, enabling not only high-rate deposition but also independent parameter control. For the study of the low-density radical species in the plasma several diagnostic techniques have been applied such as optical emission spectroscopy (OES), threshold ionization mass spectrometry (TIMS), and cavity ring down spectroscopy (CRDS). Some of the diagnostics have particularly been adjusted for this purpose to obtain high sensitivity under the harsh conditions of plasma deposition. Among the different results obtained, it has been found that for a considerable flow of ions from the plasma source, the SiH4 dissociation is governed by charge transfer reactions between these ions and the SiH4. The created silane ions subsequently undergo dominantly dissociative recombination with electrons leading to hydrogen-deficient and consequently very reactive silane radicals (SiHy, y < or = 2). Although the deposition process is dominated by radicals under all conditions, a small fraction of the silane ions will undergo sequential ion-molecule reactions with SiH4 forming large hydrogen-poor cationic clusters. It has even appeared that larger clusters could be created in the plasma environment than was theoretically predicted and measured under well-defined low-energy conditions. Working at high H2 flows and therefore low ion densities on the other hand, has revealed that SiH4 dissociation is governed by H abstraction reactions between atomic H and SiH4 leading to SiH3 radicals. It has been found that this is an important reaction for the deposition of high quality a-Si:H as a considerable improvement of the film quality has been observed with increasing contribution of SiH3 (Figure 1). This beneficial contribution of the SiH3 radical in terms of film quality originates mainly from the low reactivity of the radical as has been confirmed by surface reaction probability measurements. SiH3 enables dense and defect-poor film growth as opposed to the very hydrogen-deficient (poly)silane radicals that govern film growth at low H2 flows. Furthermore, this work has revealed the technologically relevant result that device quality a-Si:H can be obtained at very high deposition rates when the a-Si:H film growth is ~90% dominated by SiH3.

Fig. 1: The improvement of the a-Si:H film quality related to the increasing contribution of the SiH3 radical to film growth.

In addition to the insight into the plasma processes, knowledge of the nature of the a-Si:H surface during deposition is crucial because at this interface the plasma species are converted into film. Especially the hydrogen surface coverage and composition is considered to have an important role. Therefore, a new in situ technique for the analysis of the silicon surface hydrides during deposition has been further developed and tested. In this technique, attenuated total reflection infrared spectroscopy is combined with ion-assisted hydrogen desorption to obtain surface selectivity. This part of the thesis work, which was performed at the University of California Santa Barbara, has revealed detailed information on ion-induced reactive surface site creation and a thermally activated decomposition reaction of higher surface hydrides into lower surface hydrides.

Finally, an extension of and improvement in the kinetic growth model of a-Si:H has been proposed to gain a deeper understanding of the reactions during growth on the atomic scale. This model is fully based on surface reactions of SiH3 (Figure 2) as justified by the above-mentioned results. With respect to SiH3's low surface reactivity, the thermally activated decrease in surface roughness, and the substrate temperature independent deposition rate some important new insights are: the presence of mobile SiH3 on the surface in a "weaker" 5-fold coordinated bond in which the SiH3 can subsequently diffuse to a dangling bond for strong bond formation, surface dangling bond creation by direct H abstraction by gaseous SiH3, and a hydrogen elimination process at the surface. Especially the latter insight, which is based on the surface infrared spectroscopy results, is a major accomplishment because the riddle of how SiH3 containing 75% hydrogen can lead to a film with only ~10% hydrogen has not yet been unraveled. The hydrogen concentration has a large influence on the film's opto-electronic properties and the observation that hydrogen elimination decreases at high deposition rates might point out an important clue for high rate deposition of high quality a-Si:H, i.e., finding a non-thermal elimination process. This research has brought us one small step closer to large-scale application of clean solar energy.

 

 

Fig. 2: SiH3-based growth model for a-Si:H.

 

Full-text PDF files can be downloaded from <http://alexandria.tue.nl/extra2/200012840.pdf>


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