METHOD OF PRODUCING HOMOGENEOUSLY DOPED SEMICONDUCTOR MATERIAL OF P-CONDUCTIVITY

METHODE DE PRODUCTION DE SEMICONDUCTEURS DE TYPE P DOPES DE FACON HOMOGENE

English Abstract

ABSTRACT
A process for the production of substantially homogeneously doped
p-conductive semiconductor material is described, which comprises subjecting
the semiconductor material to be doped to irradiation with .gamma.-photons wherein
the semiconductor material is germanium and wherein gallium atoms are pro-
duced as doping atoms on irradiation of the germanium with .gamma.-photons.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for the production of substantially homogeneously
doped p-conductive semiconductor material comprising the step of subjecting
semiconductor material to be doped to irradiation with .gamma.-photons, whereby p-
doping atoms are produced in said material by a nuclear reaction or reac-
tions initiated by the .gamma.-photons, wherein said semiconductor material is
germanium, and wherein gallium atoms are produced as doping atoms on irradi-
ation of the germanium with .gamma.-photons, in accordance with the nuclear reac-
tion:
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2. A process as claimed in claim 1, wherein the doping concentration
produced is determined by selection of the duration of irradiation and the
density of the .gamma.-photon stream per unit area.
3. A process as claimed in claim 1, wherein, in order to heal any
damage to the crystal lattice caused by irradiation, the irradiated semi-
conductor material is annealed for at least one hour at a temperature above
500°C.
4. A process as claimed in claim 1, wherein said semiconductor
material is in the form of a rod, which is caused to rotate about its longi-
tudinal axis during the irradiation step.
5. A process as claimed in claim 1, wherein said semiconductor
material is in the form of a crystalline wafer, and wherein an x-y scanning

of the wafer with a.gamma.-photon beam is effected during irradiation.

Description

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Thepresent invention relates to a process for the production of
homogeneously doped p-conductive semiconductor material and is a divisional
application of Canadian Application Serial No. 233,055 filed on August 7,
1975-
The doping of semiconductor material is frequently carried out(for example, in the case of silicon) during deposition of the semiconductor
material from the gas phase by the thermal decornpostion 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.Here again, the dopant is found to evaporate uncontrollably during the crys-
tal 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.
~ hese 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 object of the present invention to provide a process for
the production of p-conductive material by means of which it is possible to
obtain a p-doping o~ a semiconductor crystal which is homogeneous throughout
the 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
fashion, and by means of which, in particular, very high ohmic semiconductor
material can be produced. Previously this could be e~ected only with di~-
ficulty using the convention processes with narrow radial and axial resis-
tance tolerances.
According to the invention, there is provided a process for the
production of substantially homogeneously doped p-conductive semiconductor
material comprising the step of subjecting semiconductor material to be doped
to irradiation with ~-photons, whereby p-doping atoms are produced in said
material by a nuclear reaction or reactions initiated by the ~-photons,
wherein said semiconductor material is germanium, and wherein gallium atoms
are produced as doping atoms on irradiation of the germanium with ~-photons,
in accordance with the nuclear reaction:
7 Ge ( ~' n) 69Ge K~ 69G
For the production of p-doped germanium, gallium is formed as the
dopant by irradiation with y-photons in accordance with the nuclear reac-
tion:-
70Ge (Y~ n) 69Ge ~ 69Ga.
From the natural isotope 7 Ge present in the germanium, the uns-table isotope
9Ge is formed, neutrons being emitted. The 9Ge is spontaneously converted
into the stable isotope 9Ga urn, nuclear reaction (K) being emitted. ~o
external irradiation is required for this second s'cage.
As an example where the semiconductor material is silicon, alumin-
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ium atoMs are produced as dopin~ a-torns on irradiation of the silicon with
y-photons, in accordance with the nuclear reaction:-
28si ( y p ) >27~1 .
From the natural isotope Si contained in the silicon, the stable isotope
7Al is formed in accordance with the nuclear reaction, during which process
protons are emitted.
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 accordance with the nuclear reaction: -
9Ga (y, n) 68Ga ~ 68z
From the stable isotope 9Ga, the unstable isotope Ga is formed during
which 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 taken to effect this transformation.
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 beam of 100 ~A current density is arranged to
strike a o.6 cm thick tungsten target. The radiation produced by the retar-
dation 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 _ ~ ~ 5As (stable)
Tl/2 = 82 min-
2. 70Ge (y n) 69Ge ~ ~ 69Ga (stable)
Tl/2 = 38 h.
When the irradiation is set to last for 10 mins., and after thecomplete disintegration of the radioactive isotopes produced, the following
(lopant 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-
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 3cm 3 - 47 Q cm
Target value : 2.75 x 101 cm 3 ~ 8.75 Q 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 Graff-generators~ cyclotrons, linear accelerators or nuclear
reactors can be used for this pu~pose.
In order to heal any damage to the crystal lattice caused by ex-
posure co the y-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 part:icularly advantageous embodiment of the invention, the
semiconductor material is in the form of a semiconductor rod, which is caused
to rotate about its longitudinal axis during the irradiation. A poly-
crystalline silicon rod having a length of 900 mm and a diameter of 35 mm
may, for example, be used as the starting material. This roa is zone-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 ~-photon beam is effected during irradiation.
r~he 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
production of electronic semiconductor components.
r~he advantages of the process of the invention in comparison with
the known doping processes are clearly shown by the greater homogeneity of
-the concentra-tion of dopant in the crystal and by the avoidance of the need
f'or high-temperature processes and their unfavourable consequences.
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