Note: Descriptions are shown in the official language in which they were submitted.
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This inventlon relates to cadmium telluridecompensated with magnesium or wi-th beryllium, said telluride
being employed for the purpose of forming infrared windows or
quantum detectors. This product may in some cases be doped.
It is known that, in order to employ cadmium
telluride as an infrared window or as a quantum detector, it
is necessary to make use of material which has high
resistivity, that ls, which contains a very low free-carrier
concentration.
When windows which are transparent to infrared
radiation are employed for high-power C02 lasers, it is
essential to ensure that these windows absorb as little
radiation as possible : the absorption is directly related to
the free-carrier concentratior. which must accordingly be of
very low value since even very weak absorption would result
in a temperature build-up and in destruction of these windows
at the high power levels of modern pulsed carbon-dioxide
lasers.
In regard to quantum detectors, for example the
solid-state ionization chambers in which a semiconductor
crystal is sandwiched between two electrodes, it is necessary
to employ a semiconductor having a high specific weight and
high resisti~ity in order to ensure that the leakage current
is not very high and that electron-hole pairs are formed
under the influence of certain radiations such as X- or gamma-
rays, for example, thus resulting in a current variation
between the electrodes, on condition that the semiconductor
is of sufficiently good quality to ensure that the electron-
hole pair is not destroyed by recombination at the time of
migration through the crystal.
Cadmium telluride can be prepared by crystallization
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in solvent tellurium by the method of zone transport or by the
method of depletion of solution. Under these conditions r the
material obtained is ~-type ; its concentration of free
carriers, namely of cadmium vacancies, is still too high for
the applications which are c~ontemplated.
Fræ~lc
In an earlier~patent No 73 17261 filed on May 11th,
1973 in the name of C.E.A~, there was given a description
which showed how it was possible to compensate for the free-
carrier concentration by making use of suitable doping agents,
especially chlorine introduced into the crystal-growth bath
in the form of cadmium chloride.
The concentration of doping agent in the crystal is ~-~
then of the same order as the vacancy concentration of cadmium
to be compensated, namely about 1017 atoms per cm3. This ;~
concentration increases as the production temperature is
higher. However, it is an advantage to introduce a lower
concentration of doping agents in order to improve certain
electron characteristics of the material. For example, it is
possible to carry out the crystallization at low temperature,
thus reducing the cadmium vacancy concentration and conse~
quently the necessary concentration of doping agent. However,
this is not very advantageous from the point of view of
growth kinetics by reason of the low crystallization tempera~
ture (low rate of crystallization?.
There is praduced in accordance with the invention
a novel material having a base-of cadmium telluride and of
another substance which is either magnesium or beryllium, the
metallic vacancy concentration of said material being lower
than in the case of cadmium telluride alone.
For the applications mentioned in the foregoing, the
material in accordance with the invention consists of cadmium
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telluride compensated by at least one metal selected from the
group of beryllium or magnesium, the maximum concentration of
said metals being equal to 5 x 102 atoms per cm3. The
material in accordance with the invention can also contain at
least one doping agent selected from the group consisting of
chlorides of cadmium, zinc, magnesium, beryllium and aluminum.
The concentration of doping agents which is
necessary in order to complete the compensation process is
very much lower than that which is required for compensation
of cadmium telluride alone, all other things being equal. In
the case of cadmium telluride compensated with magnesium or
with beryllium, a low concentration of magnesium or of
~eryllium considerably reduces the metallic vacancy concentra-
tion if this ternary compound is prepared in solvent tellurium
under conditions similar to those required for Gbtaining the
binary compound alone.
The ionic radius of magnesium which is isoelectronic
with cadmium is shorter than that of cadmium, thus facilitating
the introduction of magnesium into the cadmium telluride
lattice. The resultant low free-carrier concentration means
that the "solidus" surface which determines the range of
existence of the cadmium-magnesium-tellurium ternary compound
on the tellurium side in the phase-equilibrium diagram comes
close to the plane of the pseudo-binary section of cadmium-
tellurium-magnesium-tellurium which is strictly stoichiometric;
a similar result can be obtained when replacing magnesium by
beryllium which is also isoelectronic with cadmium, the ionic
radius of which is even shorter than that of magnesiumO
In the product in accordance with the invention,
the maximum concentration of magnesium or of beryllium is
5 x 102 at/cm3. In the method 6 preparation of this product,
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the solution depletion technique can be employed, the charge
being made up of cadmium, magnesium (or beryllium) or of a
magnesium-cadmium alloy (or a cadmium-beryllium alloy), of
tellurium and of doping agent.
It is also possible to adopt the technique of trans-
port from a tellurium zone containing the doping agent and
magnesium (or beryllium), the polycrystalline ingot traversed
by the solvent zone being in turn constituted by the ternary
alloy consisting of cadmium-magnesium-tellurium. This
ternary alloy can undergo a preliminary preparation by means
of a conventional technique which makes use of a ~ridgman
furnace.
In the case in which the solutlon depletion method
is employed or in the case of the method of solvent zone
transport, the magnesium (or beryllium) can be employed
either in the pure state or in the form of an alloy with
cadmium ; purification of this alloy can readily be carried ;~
out by zone melting. The doping agent is a halide and more
especially a chloride of cadmium, zinc, magnesium, beryllium
and in some cases aluminum.
EXAMPLE I
In a first practical example, there is introduced
into an internally graphitized quartz vessel a charge
composed of :
543 grams of cadmium
942 grams of tellurium
10 grams of magnesium-cadmium alloy containing 70 at ~ Cd
500 mg of cadmium chloride.
The vessel is sealed in an argon vacuum, then intro-
duced into a Bridgman furnace of known type.
The vessel is brought to a temperature such that the
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charge is en-tirely llquld, that is to say at a temperature
above 967C. The vessel is then displaced slowly at a rate of
0~3 mm per hour so as to obtain unidirectional crystallization
from one end of the vessel and to bring back the excess
solvent, namely tellurium, to the other end.
The crystal obtained has a resistivity which is
higher than 106 ohms/cm Said crystal is wholly suitable for
the fabrication of a nuclear detector, for example, and also
an infrared window. The transmission of photons is in the
vicinity of lO0 % in the band of 2.5 to 17 microns, which
justifies the use of the material as an infrared window.
There is noted a localized mode of infrared
absorption at 20 microns which is characteristic of the
presence of magnesium in the crystal.
EXAMPLE II
In a second example of crystallization by transport
of a tellurium zone, a polycrystalline ingot is formed in a
first stage.
To this end, there is introduced into an internally
graphitized quartz vessel a charge composed of :
255.2 g of Te
222.5 g of Cd
0.5 g of Mg
corresponding to the composition :
C~0.49s~ Mgo.005~ 0-5
which is entirely liquid and at a temperature slightly higher
than 1090C. The vessel has a diameter of 25 mm and a length
of 1250 mm, is sealed under an argon vacuum, then introduced
into a furnace. The charge is brought to a temperature which
is higher than 1090C then cooled.
In a second stage, recrysLallization is performed by
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zone transport in which the polycrystalline ingot obtained
in the first stage is withdrawn from its quartz vessel, then
introduced into a second vessel. A quantity of 30 g of
tellurium to which were added 10 mg of magnesium chloride
has previously been placed at the end of said second vessel.
The vessel is sealed in an argon vacuum, whereupon the solvent
zone is transferred from one end of the rod to the other under
the combined action of the furnace and of a relative displace-
ment of the furnace and of the vessel. The growth rate is
0.3 mm/h and the temperature of the furnace is 900C.
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