Method for the manufacture of light emitting and/or photodetective diodes

Method for the manufacture of light emitting and/or photodetective diodes, wherein it comprises the following operations: PA1 (a) starting with a substrate of the material Mg.sub.x Zn.sub.1-x Te; PA1 (b) a means which will make this material conductive is applied thereto; PA1 (c) a layer of thickness x.sub.j is applied to the surface of this substrate and which is compensated in such a way that it has a high resistivity; PA1 (d) ions are implanted with a sufficient energy to create a trapping zone of thickness x.sub.1 in the semi-conductor surface and above it an insulating zone of thickness x.sub.2 with x.sub.1 <x.sub.j ; PA1 (e) conductive contacts are formed on the substrate surface and on its second face. Light emitting and/or photodetective diodes and diode matrixes obtained by this method.

BACKGROUND OF THE INVENTION 
The present invention relates to a method for the manufacture of diodes 
which have light emitting and detecting properties. More specifically 
these diodes are able to emit light with a clearly defined wavelength when 
an adequate electrical voltage is applied thereto or can form the source 
of a current when subject to the action of light radiation. Certain of the 
diodes manufactured according to this method can have both of these 
properties at the same time. 
More specifically the invention relates to diodes having these two types of 
properties and which are manufactured from a composite semiconductor 
material. 
Light emitting diodes are already known which are constructed by ion 
implantation in a semiconductor substrate of zinc telluride ZnTe. The 
operation of such diodes has been described by Pfister and Marine in Acta 
Electronica 1976, page 166. 
With the aim of improving the properties of such diodes and in particular 
to increase their emission and reception efficiencies and to give them the 
capacity of operating in either emission or reception as required, an 
improved method for the manufacture of diodes by ion implantation has been 
developed and is covered by French Patent Application EN 78 08522 of Mar. 
23, 1978 in the name of the Commissariat a l'Energie Atomique. The 
improved method of this French application is essentially characterised by 
producing a compensated layer having a high resistivity in the ZnTe plate 
surface prior to ion implantation. This layer is more particularly 
obtained by thermal diffusion of the constituent atoms of a conductor 
deposit covering the semiconductor plate. 
However, said diodes produced by ion implantation in zinc telluride have a 
number of shortcomings: 
The emission wavelength is limited and, as the band gap of ZnTe is 2.3 eV, 
it is not possible to obtain a higher energy emission, e.g. a blue 
luminescence. 
The spectral reception region is limited to the band of 2.3 to 2.82 eV, 
also fixed by the band gap of ZnTe. 
In particular, in emission, these diodes have a large number of black lines 
indicating the presence of dislocations in the starting material, said 
dislocation lines being non-radiative recombination zones which enlarge 
with time. Furthermore, at the end of a few hours operation the 
non-radiative zones cover the entire surface and finish up by making the 
diode very inefficient in emission. 
BRIEF SUMMARY OF THE INVENTION 
The present invention relates to a method for the manufacture of light 
emitting and/or photodetective diodes obviating the above disadvantages. 
This invention is based on the following findings observed during testing 
with a large number of samples. The crystalline quality of samples of the 
ternary alloy Mg.sub.x Zn.sub.1-x Te is systematically better than that of 
ZnTe samples. It has been found that a small proportion of magnesium is 
sufficient to decrease in a spectacular manner the number of crystal 
dislocations. For example, a Mg.sub.x Zn.sub.1-x Te crystal in which 
x=0.08 is virtually without defects. When x increases this effect is 
retained and the band gap of the semiconductor widens. These experimental 
results led the Inventors to study a method for the manufacture of diodes 
from the alloy Mg.sub.x Zn.sub.1-x Te. A priori, it appeared that the 
diodes obtained would have a considerably increased service life, that 
their emission wavelength would be modified and could, in particular, be 
located in the blue which is very difficult to obtain with no materials, 
and that it could be adjusted to a given value in the spectral region 
comprising the green and the blue due to the proportion of magnesium. 
Finally the spectal sensitivity range in detection would also be modified 
and adjustable due to the proportion of magnesium. 
However, this research led to a certain number of difficulties which misled 
the expert. 
The preparation of MgZnTe required more precautions due to the very 
considerable reactivity of the magnesium. 
The electrical contacts on MgZnTe are less ohmic due to the much wider band 
gap. 
In low temperature photoluminescence the lines emitted by MgZnTe are wider 
than those of ZnTe making impossible a fine analysis of the nature of the 
electronic transitions. 
The fundamental phenomena appearing in ZnTe are already very complex and 
the incorporation of magnesium adds a supplementary degree of complexity, 
particularly with regard to the native defect. 
Finally and more particularly the MgZnTe compound obtained by conventional 
metallurgical processes is highly insulating making it impossible to 
inject the electrons permitting the electroluminescence of a diode. 
The research carried out by the inventors has shown that it was possible to 
make MgZnTe sufficiently conductive to ensure the operation of a diode, 
that the crystalline quality of the material was not impaired, and that 
the application of ion implantation to this substrate led to an 
electroluminescent and/or photodetective structure being obtained. 
More specifically the present invention relates to a method for the 
manufacture of light emitting and/or photodetective diodes, wherein it 
comprises the following operations: 
(a) starting with a substrate of the material Mg.sub.x Zn.sub.1-x Te; 
(b) a means which will make this material conductive is applied thereto; 
(c) a layer of thickness x.sub.j is applied to the surface of this 
substrate and which is compensated in such a way that it has a high 
resistivity; 
(d) ions are implanted with a sufficient energy to create a trapping zone 
of thickness x.sub.1 in the semiconductor surface and above it an 
insulating zone of thickness x.sub.2 with x.sub.1 &lt;x.sub.j ; 
(e) conductive contacts are formed on the substrate surface and on its 
second face. 
According to a preferred embodiment of the invention the operation which 
makes the material Mg.sub.x Zn.sub.1-x Te conductive is carried out by 
incorporating a doping element, preferably an alkali metal, e.g. 
potassium. This incorporation can be carried out during the manufacture of 
the material or by diffusion into the existing material. It is preferably 
completed by annealing under a zinc atmosphere. 
The compensated layer is for example obtained by thermal diffusion of the 
material constituting a conductive surface layer which also serves to form 
one of the conductor contacts, said material being advantageously 
aluminium. 
The method of the invention is applied to a substrate of Mg.sub.x 
Zn.sub.1-x Te of type p when it is intended to produce diodes which are 
both electroluminescent and photodetective. 
The invention also relates to a semiconductor material of high crystalline 
quality and from which it is possible to produce light emitting and/or 
photodetective diodes. 
According to the invention this material is in accordance with the formula 
Mg.sub.x Zn.sub.1-x Te in which x is below 0.15. 
Preferably and more specifically in the application to light emitting 
and/or photodetective diodes this material also contains a doping impurity 
such as potassium or any other alkali (lithium, sodium, etc.).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Hereinafter a number of exemplified and non-limitative embodiments are 
given of light emitting and/or photodetective diodes according to the 
present invention. 
Into an internally graphited quartz flask of diameter 23 mm is introduced a 
charge formed from 193.81 grams of tellurium, 65.37 grams of zinc and 3.05 
grams of magnesium corresponding to the composition Zn.sub.0.378 
Mg.sub.0.047 Te.sub.0.575 leading to a crystal Mg.sub.x Zn.sub.1-x Te in 
which x=0.094. To this is added 26 mg of potassium telluride K.sub.2 Te in 
such a way as to obtain a 100 p.p.m. atomic potassium doping. 
The flask is sealed under argon vacuum and introduced into an oven such as 
described in French Pat. No. 75 35997 of Nov. 26, 1975. It is heated to a 
temperature such that the charge is internally liquid, i.e. at a 
temperature above 1160.degree. C. It is then slowly moved at a speed 
between 0.1 and 0.3 mm per hour in such a way as to obtain a 
unidirectional crystallisation. 
Plates are cut from this monocrystal and annealed at 750.degree. C. under a 
zinc atmosphere for one hour. The crystal obtained has a type p 
conductivity and the concentration of free carriers is 10.sup.17 vacant 
sites per cm.sup.3. 
The method described in French patent application EN No. 78 08522 of Mar. 
23, 1978 is applied to this crystal. However, the parameters are adapted 
to the substrate Mg.sub.x Zn.sub.1-x Te which, due to its better 
crystalline quality, has a much greater diffusibility. 
Onto the upper face of a plate is deposited a 2,000 A aluminium coating 
which serves as the upper electrical contact, said coating being engraved 
in such a way as to delimit the elementary diodes. The thus coated plate 
is heated to 400.degree. C. for 20 minutes to create a diffused zone. The 
plate is then subject to implantation of boron ions through the aluminium 
layer, the implantation energy of 57 KeV and the layer thickness of 2,000 
A are such that the majority of the crystalline defects due to the 
implantation are located therein, as described in French Pat. No. 74 27556 
of Aug. 6, 1974. Finally a contact layer is deposited in conventional 
manner on the opposite face of the plate. 
The diode obtained emits light at the wavelength of 5380 A, i.e. in green 
colour. Compared with a diode produced under corresponding conditions in 
ZnTe it has an identical operating voltage (threshold between 5 and 6 V), 
has the same conversion efficiency, supplies a current when it receives 
energy radiation at above 2.32 eV, but there are no black lines and there 
is no limitation to its life. Its operation in emission is the same as 
that described on page 7, line 14 to page 8, line 32 of French patent 
application EN No. 78 08522 of March 23rd 1978 and these paragraphs are 
included in the present description. 
Other diodes have been produced according to the same method and with the 
same parameters, except for the value of x. For x=0.106 a diode emitting 
at a wavelength of 5360 A has been obtained and for x=0.014 a diode 
emitting at a wavelength of 5510 A has been obtained, all the other 
properties being retained. 
The diodes obtained by the method of the invention can be applied more 
particularly to the construction of a screen for the visual display and/or 
reading of data constituted by a matrix of such diodes and equivalent to 
the screen of a cathode ray tube, and/or a picture tube respectively. In a 
same matrix configuration they can also be applied to the construction of 
a system for the writing and/or reading of documents by projecting onto 
photosensitive paper the image of each diode and/or by projection onto the 
matrix of a document to be read. 
The invention is not limited to the embodiments described and represented 
hereinbefore and various modifications can be made thereto without passing 
beyond the scope of the invention.