Note: Descriptions are shown in the official language in which they were submitted.
This invention relates to a method of fabrication
of optoelectronic devices by ion im,plantation.
Optoelectronic devices such as ligh-t emitters or
detectors, for example, are usually fabricated from binary or
ternary semi~conductor alloys. The operating principle of
these devices is based on mechanisms for converting photons
to charge carriers in the case of optoelectronic devices which
operate as light detectors or on mechanisms for converting
charge carriers to photons in the case of optoelectronic
devices which operate as light emitters.
The charge carriers in a semiconductor are electrons
and holes having properties which are directly related to the
nature and the quality of the semiconductor. In particular,
in the case of optoelectronic devices, carrier lifetime is a
very important parameter which governs the quality of
electrical energy to light energy conversion or light energy
to electrical energy conversion. As the case may be, said
lifetime is dirPctly related to the treatments (ion implanta-
tion, annealiny) applied to the semiconductor throughout all
stages of manufacture. During the process of preparation of
the device, it will therefore be sought to ensure that the
lifetime of the charge carriers is not modified to an
excessive degree in order to maintain the intrinsic properties
of the starting material. ~ ~
One of the most important optoelectronic devices is ,,,
the electroluminescent diode in which a light emission is
obtained by means of an injection of charge carriers.
At the present time, practically all existing
electroluminescent diodes have a base of Type III-V semi-
conductors. These are binary or ternary alloys having a base
of gallium. Among the alloys which can be mentioned by way
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of example are Ga P, Ga As P, Ga ~l As and so forth~
Diodes or diode matrices made from alloys are p-_
junctions in which the substrate is usually of type n and in
which an acceptor impurity such as zinc is either diffused or
implanted. Another solution makes use of liquid or gaseous
epitaxial growth of the p-type layer on the -type substrate.
In point of fact, although III-V semiconductors have
reached a very advanced stage of development and fabrication
at the present time, they are nevertheless attended by a
number of disadvantages, viz:
-they have a base of gallium which is a very costly
material,
-the theoretical quantum yields are fairly limited,
-they are well suited to emission in the red or infrared
region but cannot readily be employed for radiative emissions
of shorter wavelengths such as green or blue.
On the other hand, there does exist another class
of alloys having a base of II-VI elements such as the alloys
Zn Te, Zn Se, Cd Te, Mg Zn Te and so forth. Apart from the
fact that their cost price is lower than that of the gallium
compounds, these alloys result in emissions ranging from the
infrared region to the ultraviolet region of the spectrum
with very high theoretical ~uantum yields. However, although
it is relatively easy to fabricate electroluminescent _-n
junctions from the III-V alloys by means of conventional
thermal techniques (diffusion, alloying), the intrinsic
properties of II-VI semiconductors usually make it essential
to have recourse to a method of doping without thermodynamic
equilibrium such as ion implantation.
A number of attempts have been made up to the present
time with a view to doping semiconductors of the II-VI type or
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III-V type by ion implantation in order to produce electro-
luminescent devices. As a general rule, the ~uantum yields
obtained with the devices thus formed are of a low order. This
result is due to the fact that, during ion implantation, a
large quantity of defects is created within the semiconductor
by the penetrating ion beam. These defects produce non-
radiative recombination centers. In consequence, they are
liable on the one hand to inhibit the desired doping action
and, on the other hand, they reduce the quantum yield of the
material. The conventional method of removing these defects
consists in annealing the crystal at a temperature which is
usually lower than the temperature required for gaseous
diffusion. However, in spite of this annealing process, the
result produced by the defects is not wholly eliminated and
the quantum yield of these diodes remains of low value.
The present invention is precisely directed to a
method of fabrication of optoelectronic devices which overcomes
the disadvantages mentioned in the foregoing by making it
possible in particular to fabricate devices of this type from
compounds of the II-VI type while endowing these devices with -
substantially higher yields than those obtained by simple ion
implantation and annealing.
In a first alternative embodiment, the method of
fabrication of optoelectronic devices in accordance with the
invention essentially consists in starting from an at least
binary semiconductor substrate selected from the group
comprising the alloys of the elements of columns II and VI
and the alloys of the elements of columns III and V of the
Periodic Table of El~ments and having a first type of conduct-
ivity and in carrying out on one of the faces of said substrate
an ion implantation with :impurities which are capable of
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endowing said substrate with a second type of conductivity to
a given depth, in removiny a surface layer from -the implanted
face of said substrate to a dep-th which ls sufficient to permit
removal of the greater part of the defects caused by said
implantation hut not to permit removal of the greater part of
the implanted impurities.
In accordance with a first alternative procedure,
thermal annealing of the substrate is carried out prior to
removal of the surface layer.
In accordance with a second alternative procedure,
thermal annealing is carried out after removal of the surface
layer.
In a first mode of execution, the surface layer is
removed by ionic abrasion or by chemical attack.
In a second mode of e~ecution, the ion implantation
is carried out through a surface layer having a thickness
corresponding to the zone of creation of the greater part of
the defects and said deposited layer is removed by chemical
attack.
In a second alternative embodiment, the method of
fabrication of optoelectronic devices in accordance with the
invention essentially consists in starting from at least a
binary semiconductor substrate selected from the group
comprising the alloys of elements of columns II and VI and
alloys of elements of columns III and V of the Periodic Table
of Elements and having a first type of conductivity, in
depositing a layer of material on one of the faces of said
substrate, in carrying out an ion implantation wi-th impurities
which are capable of endowing said substrate with a second
type of conductivity to a predetermined depth, the thickness
of said layer of material being such that the greater part of
the defects is concentrated in sai.d layer at the time of ion
implantation.
Thus in accordance with this second alternative
embodiment, the layer in which the defects are concentrated
is not removed as in the first alternative embodiment. This
layer can be either of metal (aluminum, for example) or of
semiconductor material (tin-doped indium oxide In203, for
example which is both conductive and -transparent) or of
insulating material.
A more complete understanding of the invention will
in any case be gained from the following description of a
number of embodiments of the invention which are given by way
of example and not in any limiting sense, reference being made
to the accompanying drawings, wherein:
Figure 1 provides schematic illustrations of the
different stages of a first embodiment of the method;
Figure 2 provides schematic illustrations of the
different stages of a second embodiment of the method;
Figure 3 is a graph which glves the impurity profile,
the defect profile and the quantum yield as a function o~ the
depth within the substrate in the case of an implanted layer
of Zn Te;
Figure 4 is a graph giving the quantum yield as a
function of the abrasion depth.
In the following description, consideration will be
given to the case of fabrication of electroluminescent diodes
in a substrate of Zn Te. It is readily apparent that the
steps of the method would be the same if use were made of
substances other than those which have been described in the
foregoing.
Fig. 3 shows the curve I which gives the defect,
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profile at the time of ion imp:Lantation and the curve II which
gives the impurity profile as a function of the depth within
the substrate which is plotted as abscissae. An arbitrary
unlt is adopted for the axis of ordinates. These curves have
been plotted in the case of boron implantation into a Zn Te
crystal with an energy of 140 (KeV). It will be noted that
the curve which gives the profile of doping with impurities is
displaced towards the right, that is to say towards the greater
depths with respect to the curve I which gives the concentra-
tion of defects.
Fig. 1 shows the different stages of a first alter- -
native mode of execution of the method. The starting material
consists of a crystal 2 of Zn Te which is polished mechanically
and then chemically on its top face 4. This substrate 2 is
p-type. Boron ions are introduced by ion implantation so as
to form an n-type surface layer 6. In this zone 6, there is
found the defeet profile and the impurity concentration
proEile which is represented on curves I and II of Fig. 3. In
the following stage shown in Fig. lb, the surface layer ~
represented by a ehain-dotted line, is abraded by means of a
well-known ionic machining process.
An annealing operation is then carried out, for
example at a temperature of 550C for a period of 30 minutes.
As mentioned earlier, this annealing operation is not
necessary and can also be performed prior to the abrasion
stage.
In order to complete the fabrication of the electro-
lumineseent diode, a metal is deposited on the implanted faee
in order to form the electrical contacts. By wa~ of example,
this deposit eonsists of indium which results in the formation
of the contact pads 10, 10', 10" after etehing.
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A deposit of gold 12 is then formed on the rear face
of the substrate in order to form the second electrical contact.
Fig. 2 shows a second alternative mode of e~ecution
of the method. In this alternative form, a layer 14 of a
substance which will subse~uently be relatively easy to remove
is deposited on the front face 4 of the substrate before
carrying out the ion implantation in order to form the doped
zone 6. The thickness e' of this deposlt is such that, at the
time of ion implantation, the greater part of the defects
caused by this implantation is localized within the surface
layer 14. In a second stage, said layer 14 is then removed
and this can preferably be carried out by chemical attack.
There are then formed the deposits which are intended to give
rise to the electrical contact as described in connection with
Fig. lc.
There can be mentioned by way of explanation the
study which is carried out by cathodoluminescence and photo-
luminescence on a layer of Zn Te implanted with boron donor
ions. To this end, the starting material consists of a ~-type
substrate having a concentration of acceptors of the order of
1016/cm3 (various experiments have been made with concentra-
tions ranging from 10 to 10 atoms/cm ~. A boron implanta-
tion is carried out with an energy of 140 KeV and a dose of
5 x 1014 atoms/cm2. Abrasions of the top layer of the
substrate are carried out by ionic machining down to various
depths. Analogous results have been obtained by chemical
attack with cerium sulphate, for example.
There are shown in Fig. 3 the curves III and IV
which serve to show the results obtained in the luminescence
of the implanted layer described earlier and fabricated by
means of the method according to the invention~ Curve IV
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relates to the alternative procedure in which an additional
annealing opera-tion has been performed prior -to abrasion
whilst curve III relates to the procedure in which no annealiny
has taken place. These curves give the quantity Q of light
emitted by the implanted material as a function of the
abrasion depth d.
It is apparent that curve III has a well-marked
minimum in the regions of small depth. It is found that, if
sufficient abrasion is performed, the quan-tity of light
emitted by the layer is multiplied by a substantial factor
which is greater than 10. The gain obtained with respect to
a layer annealed without abrasion (zero abrasion point of
curve IV) is of smaller value but still very substantial.
The influence of annealing prior to abrasion appears on curve
IV (as shown in Fig. 3).
~ lthough it is possible to revert to a crystal which
is sound from a macroscopic viewpoint as a result of thermal
annealing after ion implantation, complete rearrangement of
the crystal lattice in the presence of impurities nevertheless
appears to be difficult at the atomic level. This mechanism
is characteristic of semiconductor compounds by reason of the
fact that, while in the case of a monoatomic semiconductor
such as silicon, for example, the silicon atom displaced by
ion bombardment can return only to a silicon site, the
annealing mechanism is much more complex in the case of a
binary or ternary compound and is liable to give rise to
associations of defects which are either inherent or induced
by implantation and which, from a macroscopic point of view,
distort the lattice and result in very low quantum yields -
since this latter is directly related to the quality of the
crystal.
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There will now be described a series of experiments
concerned with the fabrication of an electroluminescent diode
in the green region of the spectrum (wavelenyth of 5480 A) by
implantation of boron donor ions into a Zn Te Crystal. To
this end, the starting material consists of a p-type substrate
having a concentration of holes of 3 x 1016 atoms/cm3; boron
ions having an energy of 140 KeV are implanted in respect of
a dose of 5 x 1014 atoms/cm2. There is -then carried out an
ionic or chemical abrasion so as to remove a surface layer of
variable thickness and the metal contacts are deposited, namely
indium or aluminium on the implanted layer and gold on the rear
face of the crystal. There will thus be obtained the curve
shown in Fig. 4 which represents the quantum yield R of the
diode as a function of the abrasion depth d. A very well-
defined maximum appears in the case of an abrasion depth of
o
4000 A.
~n electroluminescent diode for the applica-tion of
the invention will be obtained by abrasion to a depth of
4000 A.
This value is clearly valid under the experimental
conditions described abo~e but would be different under other
conditions of fabrication. ;
Two methods of collective fabrication of electro-
luminescent diodes for the practical application of the
present invention will now be described.
The first of these methods is based on the principle
of the so-called planar technology. A layer of Zn, Te, for
example, is employed as starting material and a uniform
insulating deposit is formed on this layer followed by select-
ive etching of the insulator; implantation is carried out
with suitable ions until junctions are obtained in the holes
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formed in the insulator; in accordance with the method of the
present invention/ a surface abrasion to a precise depth is
carried out in the holes; a metal coat:incJ is deposited, then
etched so as to define the contacts on the top faces of the
diodes and the external connections. It then remains necessary
to cut-out the entire layer in order to mount the individual
electroluminescent diodes if so desired. -
The second of these two methods is even more
straightforward. A layer of Zn Te/ for example, is employed
as starting material; a uniform implantation with suitable
ions is carried out over the entire surface; masking is
effected by means of an ordinary photosensitive resin and this
is followed by etching so as to define the electroluminescent
zones; in accordance wi-th the method of the present invention,
surface abrasion is performed in the unmasked zones to a pre-
determined depth, whereupon the resin is removed; a metal
coating is then deposited and etched so as to define the
contacts on the top faces of the diodes and the external
connections.
It will be noted that the disturbed surface zone is
employed in this case for the purpose of ensuring electrical
insulation between diodes. Selective abrasion defines the
location of the diodes and at the same time increases their
quantum efficiency.
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