Improved Stability of Amorphous Silicon by Post-Deposition Fluorine Implantation
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IMPROVED STABILITY OF AMORPHOUS SILICON
BY POST-DEPOSITION FLUORINE IMPLANTATION
RUUD E.I. SCHROPP* Department of Applied Physics, University of Groningen, The Netherlands. Glasstech Solar, Inc. (GSI), 12441 West 49th Avenue, * Present address: Wheat Ridge, Colorado 80033, USA. ABSTRACT In this paper it is demonstrated for the first time that the material properties of amorphous silicon films deposited from a pure silane plasma It is shown that can be improved by an appropriate fluorine implantation. the field-effect density of states is reduced by this treatment, and that the instability induced by different treatments is
reduced.
This technique
is of particular interest for the fabrication of thin film transistors. INTRODUCTION Amorphous silicon alloys are useful because thin films of these alloys can be prepared easily over large areas for application, photovoltaic modules and in liquid crystal display (LCD) A major
drawback
of
amorphous
silicon
is,
among others,
in
panels.
however,
its
inherent
instability, which has found expression in, for instance, photostimulated creation of dangling bonds (the Staebler-Wronski effect)[1,2] and the fieldinduced creation of interface
states in semiconductor/insulator
structures
[3,4]. Early reports [5,6] on fluorinated amorphous silicon films deposited from SiF4 /H 2 mixtures claimed that these films showed no photostructural changes
and that
they
were
in
many
respects
superior
to
silane-based
since the fluorine concentration in a-Si:H:F and a-SiGe:H:F
films.
However,
alloys
generally
is
very small
(0.1
- 6 %, [6,7]),
it
is
questionable
whether the beneficial effects are due to the fluorine actually bonded in the film or to the presence of fluorine-containing species in the plasma. The purpose of this report is
to address the above question and to
present the results on post-deposition implantation of fluorine
in a-Si:H
films deposited from pure silane (SiH4 ). The study of electronic and optical characteristics was performed in a thin-film transistor (TFT). devices,
Being planar
the implantation technique is of particular interest to TFTs.
the following,
we will characterize
the F-implanted
effect density of states and the quantum efficiency
Mat. Rea. Soc. Symp. Proc. Vol. 118. 1988 Materials Research Society
In
films by the field-
(q) - electron mobility
238
We will discuss degradation as caused by field
(r) product.
(p) - lifetime
intense illumination and heat treatment.
stressing,
EXPERIMENTAL The substrates used are highly doped ( p - 1 - 15 mnf cm) n-type singlecrystal
A silicon dioxide layer of 110 nm thickness
silicon wafers.
(100)
grown
thermally
was
Five
insulator.
in
oxygen
dry
F+ ions in
to
as
serve
gate
the
labeled A
hereafter
nm,
to prevent implantation damage in the conducting channel 2 5 2 14 2 14 cm' ; 1 x 101 cmu ; cm- ; 5 x 10 1 x 10
order
The implant doses were
5 x 1015 cm"
2
, for sample A through D, To
implanted.
annealed
430
four samples were implanted with lo
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