On the basis of the experimental data obtained one can state that the films of
both
p
-type and
n
-type deposited on the substrates with opposite type of
conductivity have the following characteristic features.
1. For uniform heating with no temperature gradient on these film structures
there is idling dark voltage; its value is
~5-10
mV for temperature
~800K.
2. Effective generation of carriers is observed for T
>500K and when the
temperature increases up to 9
00K there is dark current; its density is ~1,2
-
2,5
mkA
/
cm
2
for different samples.
3. Changes
in dark voltage and short
-circuit current with the raise in
temperature and cooling take place smoothly and along one curve with discrepancy
of results not higher
than 5%.
The results obtained experimentally confirm a role of the deep energy levels
in appearing the thermovoltaic effect, namely its manifestation in structures formed
under vacuum conditions with exclusion of any sources of deep levels but when
the concentration of the indicated levels is sufficient for their appearing.
It should be noted that the high Zeebek coefficient of TEC can be caused not
only by the high concentration of deep levels but also by the presence of
microlayers of oxide through which
the charge carriers are tunneled. Therefore, an
increase in this parameter and in other thermovoltaic parameters of film-based
TECs can be connected with purposeful creation of specially doped areas in the
process of film growth and, as shown by this work, the ion
-stimulated methods are
more suitable for this goal.
2.6.
Ion stimulated electron beam physical vapour deposition
Electron beam physical vapour deposition (EB
-
PVD) is an effective process
for dense coatings,
high thermal efficiency, and relatively
high deposition rates.
Additional benefits can be gained in this process with the simultaneous usage of
ion-assisted deposition. Ion bombardment of the substrate provides a better control
of the deposition process and the resulting adhesion of the films,
their morphology
and chemical composition. By controlling the current density and energy of ions,
porous, columnar, textured and epitaxial coatings may be obtained.
From common reasons it can be confirmed that the
increase of the degree of
ionisation of pa
rticles depositing on the substrate, is possible by magnification of
the electron current of the evaporator (at constant
power) and accelerating potential
on the substrate. Thus,
the current of generated ions and efficiency of the
extraction them on the su
bstrate accordingly increases.
Discussing of other physical factors of this process the following must be
mentioned:
Atoms re-
evaporation (scattering) process due to the ions bombarding the
substrate: in the literature it is shown that by
U
b
= 1 keV the sc
attering coefficient
is ~
1. In our experiments
the degree of ionization is
~
0.15%. Therefore this
scattering has not been taken into account because it is only ~10-3 part of the flow.
The estimation of the electrons effect on the ion current: (
a
)
In the case of reflected
electrons from the e-beam evaporator - special construction design of the detector
123
allows us to separate the ionic and electronic constituents in the field of a parallel-
plate capacitor and they are detected separately at different electrodes.
(
b
)
In the
case of secondary processes including ion-electron emission — the ion detecting
electrode is constructed from pure Ta. In so doing the assessment of the
instrumental error by measuring the ion current is not more than 10%.
In many cases accelerated gas ions are used for ion- assisted deposition.
However,
after neutralizing under the
surface,
the
gas ions can cause
bubbles,
which can tear and damage the structure of the film. Usage of the accelerated
‘self’ ions, generated as
a result of ionization of evaporated
atoms,
overcomes
this problem. In the
EB
-
PVD
process,
used in the present
work,
the
ions are
generated by the collision of
electrons,
which are present in the field of the
evaporator,
with the evaporated atoms.
This ionization
takes place in the area near
to the surface of evaporated
material,
because in this area there is a high
concentration of evaporated atoms and a high current density of electrons.
There
are
three groups of
electrons,
whose contributions are
approximately
the same
order of magnitude in the process of
ionization:
-
electrons of the e-
gun,
which have energy 10
keV;
-
back-scattered electrons with energies 100
eV
– 10
keV;
-
secondary electrons with energies 10 – 100 e
V
.
A vacuum chamber
equipped
with a turbomolecular pump and a
6
kW
electron beam evaporator with a 270
o
beam deflection by a permanent magnet
and 10
kV
acceleration voltage were used for the deposition of metal films and
to measure the density of ionic and electronic current with the ion detector.
The
substrates
used were nickel-based
alloys,
which are an important structural
material for turbine
blades,
tubes and
pipes,
etc.
The
thickness of the
Cr
films
were from
6.5
µm up to 9.5 µm. A specially designed ion detector was installed
in the vacuum chamber in the
position,
where the substrates are usually
mounted.
The
operation of the electrostatic detector is based on the separation
of positive ions and electrons in the field of a parallel - plate capacitor.
The
construction of the detector and the measuring tract allows delivery of ions to it
of a bias potential of up to y1.2
kV
simultaneously
with the application of a
corresponding potential to the substrate.
Structural
elements of the detector and
the potentials on its plates are calculated by assuming that the complete
collection of all ions and
electrons,
acting on its input window is realised.
The
dimensions of the detector
are:
35
х
3
5х
40 mm
3
,
the area of the input
windows 1cm
2
.
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