Surface Processes Limiting the High Rate Deposition of High Electronic Quality a-Si
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SURFACE PROCESSES LIMITING THE HIGH RATE DEPOSITION OF HIGH ELECTRONIC QUALITY a-Si S. Veprek, 0. Ambacher and M. RuckschloB Institute for Chemistry of Information Recording Technical University Munich Lichtenbergstr. 4, D-W-8046 Garching/Mijnchen, FRG
ABSTRACT Amorphous silicon of a high electronic quality with oa_.u = I0-10, 1 aoh = 1-10-4 (Qcm)and density of gap states of about 0.3 to 3 2-10"6 eV-±cmis deposited at rates up to 17 A/sec. The rate limiting steps are identified and their control via the plasma parameters is explained. This allows one to establish the fundamental scaling parameters for large-area high-rate deposition of a-Si. Long term stability of the films in terms of postoxidation and photodegradation is briefly addressed. Key words:
Amorphous Silicon.
High Rate Deposition,
Scaling Parameters
1.
INTRODUCTION High-rate deposition of a stable, electronic-grade a-Si is of fundamental interest for its applications. For example. at the deposition rates usually achieved in the large-scale fabrication of solar cells of 1-3 A/sec one needs about 80 to 30 min to deposit the photosensitive i-layer. In order to match the much shorter deposition time of the thin n- and p-Si contact layers, this is usually done by the use of several sequentially operating deposition chambers for the i-layer. The corresponding increase of the capital and operating costs is an undesirable consequence. An increase of the deposition rate, yet keeping the high quality of the deposited material, to about 17 A/sec reduces the deposition time of the i-Si layer to less than 5 min. This makes the deposition time comparable or even shorter than the time of the insertion of the substrates into the deposition machine, their heating up to the deposition temperature, outgasing and transport between the individual chambers. In this paper we shall demonstrate that such a deposition rate can be achieved yet keeping the optoelectronic quality of the deposited a-Si equal to the best values ever reported at much lower deposition rates. Comparably high deposition rates were reported by Curtins et al [1] who used a high frequency discharge operating at frequencies in the 40 to 80 Mfz range. Their results have been confirmed by several research groups. However, because of the lack of understanding of the reaction mechanism, scaling of their results has not been possible. The main purpose of our communication is to explain the rate limiting steps which control such a process and to give guidelines how our results, which were obtained in a small laboratory apparatus, can be scaled up to a large production equipment. Therefore, we shall briefly outline also the fundamental plasma and discharge parameters for the scaling of the process. More details will be reported in an extended paper to follow.
Mat. Res. Soc. Symp. Proc. Vol. 219. 01991 Materials Research Society
668
2.
THE REACTION MECHANISM AND GUIDELINES FOR SCALING
The dominant reaction mechanism of the plasma induced deposition of a-Si has been elucidated [2,3] and substantiated by a
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