Note: Descriptions are shown in the official language in which they were submitted.
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The present invention relates to a process for the production of
hornogeneously doped p-conductive semiconductor material, and is a divisional
application of Canadian Application Seri&l No. 233,055 filed on August 7
1975-
The doping of semiconductor material is frequently carried out (~orexample, in the case of silicon) during deposition of the semiconductor mate-
rial from the gas phase by the thermal decomposition of a gaseous silicon
compound of silicon on a heated carrier body of the same material. During
this process, doping is effected by mixing a gaseous compound of a dopant
with the gaseous silicon compound, so that this also decomposes on the car-
rier body. Silicon rods produced in this way are polycrystalline and must
be converted into the monocrystalline state by a subsequent zone melting
treatment. In this subsequent treatment, the dopant concentration often
thereby changes uncontrollably and very much higher dopant concentrations
must be used to ensure that the desired concentration of dopant still exists
in the final product.
Germanium is frequently produced by the Czochralski crucible draw-
:ing method, in which a seed crystal is submerged into a germanium melt con-
-taining a suitable dopant and which is located in a crucible, and a mono-
crystalline rod is drawn from the melt by movement of the seed crystal. Hereagain, the dopant is found to evaporate uncontrollably during the crystal
growth process.
In the case of A B compounds, e.g. in the case of gallium
arsenide or gallium phosphide, doping is frequently likewise effected from
a melt contained in a crucible or boat.
These known doping processes are time-consuming and inaccurate.
Consequently, the components produced from this semiconductor material do
not possess optimal values for their electrical properties.
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It is an ob~ect of the present invention to provide a process ~or
the production of p-conductive material by means o~ which it is possible to
obtain a p dopiIIg o~ a semiconductor crystal which is homogeneous throughout
l;he crystal (e.g. in the case of a rod over the rod length and rod cross-
section independent of the diameter of the rod) in a simple and economical
rashion, and by means o~ which, in particular, very high ohmic semiconductor
material can be produced. Previously this could be effected only with di~-
riculty using the convention processes with narrow radial and axial resis-
tance tolerances.
As an example, where the semiconductor material is silicon,
aluminium atoms are produced as doping atoms on irradiation of the silicon
with y-photons, in accordance with the nuclear reaction:-
Si (y, p) ~ ~ 7Al.
From the natural isotope Si contained in the silicon, the stable isotope
7A1 is formed in accordance with the nuclear reaction, during which process
protons are emitted.
As another example, f`or the production of p-doped germanium, gal-
lium is i`ormed as the dopant by irradiation with y-photons in accordance with
the nuclear reaction:-
70Ge (Y' n) Ge -~ 9Ga.
From the natural isotope 7 Ge present in the germanium, the unstable isotope
9Ge is formed, neutrons being emitted. The 9Ge is spontaneously converted
into the stable isotope 9Ga um, nuclear reaction (K) being emitted. ~o ex-
ternal irradiation is required ~or this second stage.
As another example, for the production of p-doped gallium arsenide,
gallium phosphide, or gallium arsenide phosphide, zinc is used as a p-dopant
and is formed from the gallium by irradiating the material with y-photons, in
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accordance with the nuclear react;on:-
69Ga (Y' n) 3Ga ~+ 68z
From the stable isotope 9Ga, the unstable isotope Ga is formed duringwhich process neutrons are emitted. Ga is a ~ radiator with a half-life
period of 1.14 hours, and is converted into the stable isotope Zn. Here
again, no external measures need to be ta~en to effect this transformation.
In accordance with this invention there is provided a process for
the production of substantially homogeneously doped p-conductive semi-
conductor material comprising the step of subjecting semiconductor material
to be doped to irradiation with ~-photons, whereby p-doping atoms are pro-
duced in said material by a nuclear reaction or reactions initiated by the
~-photons, wherein said semiconductor material is gallium arsenide, or gal-
lium phosphide, and wherein zinc atoms are formed as doping atoms from gal-
lium atoms on irradiation of the gallium with ~-photons, in accordance with
the nuclear reaction:
69Ga ( ~ ' n) 68Ga ~ > Zn.
The doping concentration is dependent upon the duration of the
y-radiation and the strength of the photon stream per unit of area (photon
stream density). The product of the two values is referred to as the
"fluence".
Thus, for example, starting with a germanium rod having a specific
resistance of 47 Q cm (i.e. the intrinsic conductivity at 300 K), a desired
resistance of 8.75 Q cm p-type can be produced as follows:-
A 35 MeV electron bearn of 100 ~A current density is arranged tostrike a o.6 cm thick tungsten target. The radiation produced by the retar-
clation of the electrons is used to irradiate the germanium and acts to ini-
tiate the following nuclear reactions therein:-
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1 76Ge (y n) 75Ge ~~ 75As (stable)
T = o2 min
2 70G (y, n) 69Ge ~ ~ 69Ga (stable)
T / = 38 h.
When the irradiation is set to last for 10 mins., and after thecomplete disintegration of the radioactive isotopes produced, the following
dopant concentrations in the germanium beneath the irradiated surface are
obtained :-
1. 1.14 x 10 atoms As/cm3
2. 2.11 x 10 atoms Ga/cm3.Since As leads to n-doping and Ga leads to p~doping, after allowing for com
10 pensation 9.7 x 10 atoms Ga/cm3 remain for p-doping.
In the above example the following dopant production is required :-
Starting value : 2.4 x 10 3 cm 3 ~ 47 ~ cm
Target value : 2.75 x 10 4 cm 3 ~ 3.75 ~ cm
To be produced : 2.56 x 10 cm 3.
The required duration of irradiation is thus about 44 hours.
Various devices for carrying out the irradiation are known. For
example, can de Graaff-generators, cyclotrons, linear accelerators or nuclear
reactors can be used for this purpose.
In order to heal any damage to the crystal lattice caused by ex-
posure to the ~-radiation, the irradiated semiconductor crystals may advan-
tageously be annealed for at least one hour at a temperature above 500 C.
When the semiconductor material is silicon, the annealing can expediently be
carried out in a silicon tube. The annealing step can be dispensed with,
however, if the semiconductor material is to be further processed to form
components and at least one high-temperature process is to be carried out
during the further processing.
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In a particularl~ advantageous embodiment of the invention, the
semiconductor material is in the form of a semiconductor rod, ~Ihich is caused
to rotate about its longitudinal axis during the irradiation. A polycrystal-
line silicon rod having a length of 900 mm and a diameter of 35 mm may, for
example, be used as the starting material. This rod is ~one-melted in vacuum
and subsequently or simultaneously a seed crystal having (111)-orientation
is fused onto it.
In accordance with another embodiment of the invention, the semi-
conductor material is in the form of a crystalline wafer and an x-y scanning
of the wafer with a y-photon beam is effected during irradiation.
The process of the invention makes it possible to provide silicon,
germanium and gallium arsenide or gallium arsenide phosphide crystals with a
homogeneous p-doping. Such crystals are particularly useful for the produc-
1;ion of electronic semiconductor components.
The advantages of the process of the invention in comparison with
the known doping processes are clearly shown by the greater homogeneity o-f
t;he concentration of dopant in the crystal and by the avoidance of the need
for high-temperature processes and their unfavourable consequences.
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