Remote Silane Plasma Chemistry Effects and their Correlation with a-Si:H Film Properties
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ABSTRACT A remote silane plasma, capable of depositing solar grade a-Si:H at a rate of 10 nm/s and with an up to ten times higher hole drift mobility than standard a-Si:H, has been investigated by means of several plasma diagnostics. The creation of the different reactive species in the plasma and their contribution to film growth has been analyzed and is correlated with the film properties obtained under various conditions. Furthermore, the first results on a n-i-p solar cell with the intrinsic a-Si:H film deposited by this remote plasma are presented.
INTRODUCTION An increase of the a-Si:H growth rate can stimulate its application in thin film solar cells as a more competitive position in the energy market can be obtained. Remote plasmas are one of the candidates to meet this goal as they allow a better and independent optimization of the plasma conditions. The so-called 'expanding thermal plasma' (ETP) is such a plasma which has proven to be able to deposit solar grade a-Si:H at a rate of 10 nm/s. Additionally, the larger freedom in plasma parameters can reveal new insights in a-Si:H film properties as will be shown in this paper. Furthermore, remote plasmas facilitate the study of the plasma and surface chemistry during deposition which can lead to a better understanding of a-Si:H deposition when a correlation with the film properties is made. An onset to the latter will be given in this contribution.
EXPERIMENTAL SETUP The expanding thermal plasma deposition technique (cf. Fig. 1) is based on generation of an Ar-H 2 plasma in a thermal plasma source (cascaded arc), which is subsequently expanded in a low pressure chamber where it dissociates SiH 4. The dc plasma source is operated at a current and voltage of respectively 45 A and about 180 V. The Ar flow used is 55 sccs (standard cubic centimeter per second) while the H2 flow is varied between 0 - 15 sccs and the source pressure is
about 400 mbar. Pure SiH 4 is admixed in the low pressure (0.2 mbar) deposition chamber just behind the source exit by means of an injection ring and at a fixed flow of 10 sces. A substrate holder with accurate substrate temperature control (100 - 500 'C) is positioned at 38 cm from the plasma source. The plasma chemistry has been studied by replacing the substrate holder by a mass spectrometer (cf. Fig. 1), capable of analyzing the flux of neutrals, radicals and ions arriving at the substrate. Furthermore, a Langmuir probe and optical system have been used to obtain absolute and spatially resolved ion fluxes [1] and to monitor the plasma emission respectively. A residual gas analyzer is used to determine the consumption of SiH 4 [2]. Films have been deposited on 2.5x2.5 cm2 p-type Si(l 11) substrates (10 - 20 Qcm) and on Corning 25 Mat. Res. Soc. Symp. Proc. Vol. 557 © 1999 Materials Research Society
Emission Spectroscopy Mono, PMT and CCD 1
Substrate holder or Mass Spectrometer Hid"n Analytical EPIC 300EZ Pumpinq system
Fig. 1: Expanding thermal plasma deposition setup with plasma diagnostics
7059 glass. Refractive index,
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