|
Sa
m
pl
e
Catalyst
Temperature,
о
С
The reaction period,
min
Conversion of
triethoxysilane, %
1
Sodium ethylate of
Sigma
-Aldrich
50
150
95%
2
Sodium ethylate,
freshly prepared
50
30
100%
3
Sodium ethylate,
freshly prepared
Room temp.
30
>
99%,
marks
4
Sodium ethylate, just
prepared
0
90
>
95%
109
conversion was abo
ve 99%. At higher temperatures the homogeneous formation
silicon particles begins, which leads to loss of monosilane.
For indirect estimation of monosilane purity and polycrystalline silicon
quality the silicon substrates for epitaxy were placed in the pyrolysis reactor. The
resistivity of silicon films grown from monosilane at 750
o
C was 0.96 ohm per cm.
Polycrystalline silicon from monosilane can be also obtained in fluidized bed
reactors. In this case, power consumption can be dramatically reduced and the
polycrystalline silicon granules with sizes from 1 to 3 mm can be produced. With a
specially designed reactor, the possibility of obtaining polycrystalline silicon
granules in a continuous mode is shown. Monosilane in a mixture with hydrogen
was supplied
into the reactor from the bottom. The initial particles of silicon with
the size within the range 63
-100 microns were loaded into the reactor from the top.
Due to the sharp increase in the growth surface the rate of polysilicon growth
increases; due to the
continuous removal of ready granules from the fluidized bed
power consumption of product unity was sharply reduced.
Thus, a new technology has allowed for the first time realization of the
process of monosilane synthesis by a chlorine-
free alkoxysilane wa
y in a
continuous mode and production of polycrystalline silicon by its thermal
decomposition. The high selectivity in main product has been achieved (over
95%), as well as the high conversion of the reactants (over 99%). Simplification of
technological eq
uipment has allowed reducing the capital costs for creation of
manufacture according to the proposed technology.
2. Ion-
stimulated methods of creation of silicon structures
Now there is a great tendency of the using a multilayer compositions
consisting of
alternating layers of semiconductors, metals and dielectrics of nano
-
meter thickness. A method of molecular-
beam epitaxy (MBE) is one of the main
methods for creation of such heterostructures; it allows growing thin
continuous
films perfect in structure an
d surface morphology. However, elastic deformation
and defects when the heteroepitaxial films are formed have an influence on a
mechanism of film growth, particularly for great
mismatch of the lattice
parameter
s of film and substrate, which makes it difficult to use MBE for
producing thin thickness-
homogeneous heteroepitaxial films.
The use of ion beams in combination with molecular beams can significantly
change the situation, since ions can transmit energy of the atom, the pulse directly
on the substrate surface during growth.
As known, with the ions it is possible to
significantly decrease epitaxy temperature, to create or destroy structural defects
with well-
controlled parameters of ions in right moment and at right place, to have
effect on a mechanism o
f nucleation and growth of heteroepitaxial films.
Theoretical basis of the ion to stimulate growth processes are based on a
combination of the two aspects of modern condensed matter physics
: theory of
epitaxial structure formation and radiation physics of solid. On the surface the
main atomic process are adsorption, desorption, surface diffusion and inclusion in
a monoatomic chain. It is evident that these known elementary atomic process
110
along with the influence factors provide a variety of the mechanisms of stimulated
growth, which requires, however, development of other methods of analysis
allowing consideration of ion-stimulated growth as a whole.
The typical MBE setups have an electron
-
beam evaporator (EBE), for
example for creation of a silicon flow to
grow the Si
-
Ge structures. As known, an
electron beam used in EBE to heat a working substance loses its energy while
deceleration as a result of different elastic and inelastic processes. Some part of
this energy is taken away by in inversely scattered e
lectrons, which reduces the
EBE efficiency. The coefficient of inversely scattered electrons
η
depends weakly
on primary electron energy and strongly on the atomic number of elements being
in a substance from which the electrons are reflected.
While electr
on-
beam evaporation, the electrons with energy of accelerating
voltage 6
-
10 kV partially ionize the evaporated flow; the cross section of atom
ionization is independent of the flow density and a degree of ionization remains
constant for a given value of el
ectron energy. These ions will be accelerated to the
substrate by applying negative voltage to it. For a traditional arrangement of
evaporator and substrate this ion flow has the same spatial distribution as the main
flow of material and requires no specia
l scanning along the substrate of large
diameter.
2.1.
Application of ions generated under electron
-
beam evaporation as a
tool to activate surface growth processes
A method of separation and control of charged particles and an apparatus for
its realization has been proposed.
The problem to be solved by this development
lies in the fact that the direct current measurement is impossible to determine the
proportion of positive ions and electron
s in mixed streams of particles. For
example, under electron
-beam evaporation there is such a flow when in the
effective space there are both positive ions and electrons reflected from a target;
their maximal energy is equal to the energy of primary electrons
.
In this process
the positive ions appear as a result of collisions of electrons with atoms of
evaporated materials.
The use of these ions as a tool of modification of growing
layer properties requires creation of methods and devices for control of ion flow
parameters.
Ion source
Using the silicon electron gun as a source for silicon ions no additional ion
source is needed and contamination can be prevented. In the area above the
silicon melt the
flux
of evaporated atoms is ionized partially by the interaction
with the electron
beam,
see
Fig.
2.1.
The
probability for a silicon atom to be
ionized by electron impact is given by the product of the
flux
density of the
electron beam
Fe
and the cross section for ionization
a
.
This
probability
multiplied with the density of evaporated silicon atoms
n
Si
gives the number of
ionizations per time and volume
g
ion
,
see
Eq.
(1)
and
Fig.
2.2.
111
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